Reactive Transport and Its Implications on Heavy Oil HTGC Analysis – A Coupled Thermo-Hydro-Chemical (THC) Multiphysics Modelling Approach

*Diana Margarita Hernandez-Baez, Alastair Reid, Antonin Chapoy, Bahman Tohidi, Roda Bounaceur and François Montel*

## **Abstract**

This chapter provides an insight into the reactive transport in a capillary column which heavy-oil hydrocarbons undergo when analysed by high temperature gas chromatography (HTGC), and their implications on characterisation outcomes, namely thermal cracking of the injected sample; and incomplete or non-elution of heavy components from the column, by using a coupled Thermo-Hydro-Chemical (THC) multiphysics modelling approach. For this purpose, a computational coupled THC, multicomponent, multi-physics model is developed, accounting for: multiphase equilibrium using an in-house, extended thermodynamics distribution factors dataset, up to nC98H198; transport and fluid flow in COMSOL and MATLAB; and chemical reactions using kinetics and mechanisms of the thermal cracking, in CHEMKIN. The determination of the former extended dataset is presented using two complementary HTGC modes: i) High-Efficiency mode, with a long column operated at low flow rate; and ii) true SimDist mode, with a short column operated at high flow rate and elution up to nC100H202.

**Keywords:** Reactive Transport, Thermo-Hydro-Chemical (THC) processes, coupled THC modelling, coupled multi-physics, multiphase equilibrium, solvation thermodynamics, transport and fluid flow, chemical reactions, kinetics and mechanism of thermal cracking, pyrolysis, Heavy n-alkanes thermodynamics distribution factors, High-temperature gas chromatography (HTGC), heavy-oil characterisation, Gas Chromatography modelling, coupled multiphysics modelling, CHEMKIN, COMSOL, MATLAB

## **1. Introduction**

The objective of this chapter is to understand the reactive transport occurring during the High Temperature Gas Chromatography (HTGC) analysis of heavy oil hydrocarbons at common conditions and thereby quantify the implications on the final characterisation results in terms of: (i) the degree of thermal cracking of the original sample; and (ii) the non-elution of heavy components from the HTGC column by using a combined Thermo-Hydro-Chemical (THC) coupled multiphysics modelling approach.

For this endeavour, a synergy between experimental and computational coupled multi-physics approaches, has been carried out to account for three physicochemical processes: thermodynamic equilibrium fluid-flow; transport of chemical species; and chemical reactions.

An outline is given of the experimental approach used, with explanation of the methodology for extending the distribution factors data-set, necessary to describe the first process.

On the computational side, an in-house coupled multi-physics model has been developed using MATLAB [1] as language host, to couple the above three processes. The former is described, using as input to the multi-physics model the extended, experimental distribution factors dataset: and the latter two processes are described using: COMSOL Multi-physics [2] and MATLAB, and CHEMKIN [3] respectively.

Finally, the implication of the inter-related, multi-physics, physicochemical processes is discussed, based on the results from the coupled THC multi-physics model.

## **2. Experimental overview**

### **2.1 Outline of HTGC methodology**

Detailed accounts of the experimental procedures have been published previously [4], covering the generation of isothermal and temperature-programmed retention data for n-alkanes and polywax mixtures, on poly-dimethyl-siloxane HTGC columns, up to 430°C. (i.e. based on well-established SimDist techniques). This database then enabled the distribution factors of heavy n-alkanes to be derived up to nC98H198, which were unavailable in the literature.

## **2.2 Methodology for extending distribution factors up to nC98H198**

In the absence of a database of distribution factors of heavy n-alkanes, it was necessary to obtain insight into their behaviour inside the HTGC column, requiring development of a comprehensive methodology to extend the existing, limited amount of data up to around nC98H196 [4].

Hernandez et al. [4] derived a temperature-dependent function of distribution factors which has been applied to a series of n-alkanes spanning (nC12H26 nC98H196) by combining Eq. (1) and numerous isothermal experiments carried out using two *SGE* HT5 GC capillary columns [5] of different lengths, and under two HTGC methods as follows:


*Reactive Transport and Its Implications on Heavy Oil HTGC Analysis – A Coupled… DOI: http://dx.doi.org/10.5772/intechopen.98614*

In both columns, the standard samples (ASTM D5442) was used and at least 3 isothermal GC measurements were carried out from 80 to 420°C at 20°C intervals, and lastly at 430°C. Further details can be found in [4].

## **3. Theory**

## **3.1 Physicochemical HTGC workflow**

In order to understand the Reactive Transport (inter-related Thermo-Hydro-Chemical multi-physics processes) occurring during HTGC analysis of heavy oils, it is necessary to consider them step-wise, within the column.


## **3.2 Thermo-hydro-chemical processes**

Thus, the three physicochemical processes in a heavy-oil HTGC analysis considered in this work are:

• Multiphase equilibrium:

Solvation thermodynamics using experimental data.

• Transport and fluid flow:

Convection in MATLAB

• Chemical reactions of thermal cracking:

Kinetics and mechanisms of simplified mixture in CHEMKIN, which fall within the classification *Thermo-Hydro-Chemical multi-physics* [8].

### *3.2.1 Multiphase equilibrium: solvation thermodynamics*

The basis of the gas chromatography separation process centres on the nonisothermal multiphase equilibrium of each of the chemical species in the mixture sample between the stationary phase and the gas phase (transported by the carrier gas) taking advantage of their distinct boiling points.

This equilibrium is established in multiple stages throughout the length of the capillary column. The analysis mixture sample is dissolved and retained in the stationary phase at low initial temperatures and each component comprising the mixture is evaporated and separated from the sample mixture once its boiling point temperatures and pressure is reached. Thus, solvation thermodynamics is used to describe the gas–liquid equilibrium of each chemical species inside the GC column.

The temperature-dependent expression for the distribution factor, *K,* is described in Eq. (1) and was obtained by solving the thermodynamic equilibrium of the solvation of a solute in the bulk solvent [9] expressed in terms of the Gibbs free energy at a given temperature and by the logarithm of the solute molecule's numeral density ratio in both phases [10, 11] or the ratio between the molar concentration of the two phases.

The mass transfer is assumed to be governed only by the interaction between the solute and the stationary phase, while the interactions between solute-solute and solute-carrier gas are assumed to be negligible as the interfacial and extra-column effects leading to non-equilibrium conditions [12].

A semi-empirical model [13, 14] developed by Castells et al. [15] for the determination of the isothermal retention times as function of the hold-up time, *tM* and the solvation time expressed by the Gibbs free energy is expressed in the terms of *ΔH* and *ΔS*, which represent the changes in enthalpy and entropy associated with the transfer of solute from the stationary phase to the mobile phase at a given temperature *T*.

$$K(T(t)) = \beta \left[\frac{t\_r}{t\_M} - 1\right] = \exp\left[a\_0 + a\_1 \frac{1}{T(t)}\right]$$

$$a\_0 = \frac{\Delta S(T)}{R}; a\_1 = -\left(\frac{\Delta H(T)}{R}\right)$$

$$\beta = \frac{\left(2r\_o - 2w\right)^2}{2r\_o^2 - \left(2r\_o - 2w\right)^2}\tag{1}$$

*Reactive Transport and Its Implications on Heavy Oil HTGC Analysis – A Coupled… DOI: http://dx.doi.org/10.5772/intechopen.98614*

In Eq. (1), *K* corresponds to the distribution factor, with *tr* and *tm* representing the retention time of the solute and the hold-up time, respectively. *β* is the phase ratio of the column, *ro* and *w* correspond to the inner radius of the column, and the film thickness of the stationary phase. *ΔH* and *ΔS*, correspond to the delta changes in enthalpy and entropy associated with the transfer of solute from the stationary phase to the mobile phase.

Aldaeus [16] has proposed two retention mechanisms according to the nature of the separation hold between the analyte and the stationary phase, based on the semi-empirical values of the thermodynamic properties of Eq. (1).

#### *3.2.2 Transport and fluid flow: convection*

The Snijders [17] method for calculating the retention times in gas chromatography is based on the peak position determination which is not affected by the diffusion effects but by the convection process only [16].

Eq. (2) expresses the convection process in terms of the effective velocity veff of the analyte in the carrier gas. Discretized into finite time-steps of Eq. (2) allows tracking of the position of the analyte at every *x* position through the GC column at every time step until the peak reaches the column outlet [18, 19] at the final time step which cumulated represents the retention time for that analyte as explained in [4]. And lastly, *K* and *β* are the distribution factor and phase ratio of the column described in Eq. (1) and *vm* is the velocity of the mobile phase.

$$\upsilon\_{\mathfrak{H}^{\overline{\mathfrak{f}}},\mathfrak{i}}(\mathbf{x},t) = \frac{\upsilon\_{\mathcal{M}}(\mathbf{x},t)}{\mathbf{1} + \frac{K\_i(T)}{\beta}} \tag{2}$$

Integrating the differential form of the Hagen-Poiseuille fluid mechanics Equations [10, 18] through the length of the column allows calculation of *vm* as described in Eq. (3). This expression relates the carrier gas velocity to the pressure gradient at any position in the column [18] by a proportional constant. The latter depends of the geometry of the cross-section which in this case is for a column of circular crosssection [20]:

$$\upsilon\_{\mathcal{M}}(\mathbf{x},t) = \frac{r\_o^2 \cdot \left(P\_{in}^{\ 2} - P\_{out}^{\ 2}\right)}{16 \cdot \eta(T(t)) \cdot L \cdot P(\mathbf{x})} \tag{3}$$

In Eq. (3), *η(T(t))* corresponds to the viscosity of the carrier gas [21, 22]. (See summarised details in [19]), *Pin* and *Pout* are the inlet and outlet pressures of the GC column. *ro* is the inner radius of the column and *P(x)* is the pressure at position *x*


**Table 1.** *Summary of size of the mechanistic kinetics models developed.* described with Eq. (4), which is obtained by integrating the Hagen-Poiseuille equation between the inlet and outlet position, of a differential element and assuming incompressibility of the gas in each element at position *x,* due to the extremely low pressure-drop in gas chromatography [10].

$$P(\mathbf{x}) = \sqrt[2]{P\_{in}{}^2 - \left(P\_{in}{}^2 - P\_{out}{}^2\right)\frac{\mathbf{x}}{L}}\tag{4}$$

*Comparison of free radical model and "class" molecular model for heavy n-alkanes mixtures. (simulation of a closed reactor at 1 MPa).*

*Reactive Transport and Its Implications on Heavy Oil HTGC Analysis – A Coupled… DOI: http://dx.doi.org/10.5772/intechopen.98614*

## *3.2.3 Chemical reactions: kinetics and mechanism of thermal cracking*

The large number of species in the reduced free-radical pyrolysis model developed in [23] has imposed a need to develop a reduced molecular pyrolysis model, comprising 11 n-alkanes (nC14H30, nC16H32, nC20H42, nC25H52, nC30H62, nC35H72, nC40H82, nC50H102, nC60H122, nC70H142, and nC80H162).

In this work, a "class" molecular mechanism has been obtained after applying the following three rearrangements to our reduced molecular mechanism model [7]:


Refer to [23], to understand the thermal cracking original kinetic and mechanism model development, and refer to [7] to understand the detailed explanation of the kinetics and mechanism reduction procedure from molecular mechanism model to a "class" molecular mechanism.

The optimised reduced "class" molecular mechanism used in this work is composed of 127 molecular reactions and 17 species (11 n-alkanes, and 6 "class" molecular pyrolysis products) which has been obtained after applying to the whole mechanism the above rearrangement and its corresponding kinetic data [7].

Thus, the final reduced molecular mechanism, accounts for:


Finally, as summarised in **Table 1** the number of reactions of the original freeradical pyrolysis model has been reduced from 7055 to 127, and the number of species from 336 to 17, whilst still yielding very good accuracy as depicted in **Figure 1**.

## **4. Computational multi-physics**

An understanding of the Thermo-Hydro-Chemical (THC) processes occurring inside an HTGC column during the analysis of heavy oil hydrocarbons was obtained through detailed study with an in-house coupled THC model.

The coupling of the physico-chemical processes is sequential due to the complexity of the system, and the level of detail with which each process has been described. Hence, a fully coupled model is prohibited while a sequential coupling can handle effectively the following processes:


• the convection process is described using the Hagen-Poiseuille fluid mechanics equations [10, 18].

The sequence of these processes was arranged using short time intervals where the temperature was constant during the temperature ramp, and with a batch size as described using Golay's theory [24] for diffusion and convection processes.

## **4.1 Computational HTGC workflow**

From a computational modelling perspective, the multi-physics processes can be simplified and described as follows:


## **4.2 Coupled multi-physics workflow**

Finally, the coupling of the three physics involved is made in a sequential order as follows:


*Reactive Transport and Its Implications on Heavy Oil HTGC Analysis – A Coupled… DOI: http://dx.doi.org/10.5772/intechopen.98614*


#### **4.3 Discretization methods**

This work uses the discretization method introduced by Snijders [17], which predicts the peak width of the solute zone as the space that a solute migrating through the column occupies [25]. This approach of the convection model was successfully coupled to the reduced molecular pyrolysis model from [7].

Equal time segments are used to discretize the simulation as proposed by Snijders [17] for enabling isothermal properties to be used at every time-step according to the ramp of temperature used. Thus, a sufficiently small time-step permits a uniform pressure to be assumed in the column segments traversed by the solute.

The local plate height (H) is calculated at every time-step based on the Golay [24] expression for open tubular columns, as shown in Eq. (5), where *k* is the retention factor.

$$H(\mathbf{x},t) = 2 \cdot \frac{D\_{\mathcal{M}}(\mathbf{x},t)}{\nu\_{\mathcal{M}}(\mathbf{x},t)} + \nu\_{\mathcal{M}}(\mathbf{x},t) \left\{ \left[ \frac{\mathbf{1} + \mathbf{6} \cdot k(T(t)) + \mathbf{11} \cdot k(T(t))^2}{2\mathbf{4} \cdot [1 + k(T(t))]^2} \cdot \frac{r\_o^2}{D\_{\mathcal{M}}(\mathbf{x},t)} \right] \right. \tag{5}$$
 
$$+ \left[ \frac{2 \cdot k(T(t))}{\mathbf{3} \cdot [1 + k(T(t))]^2} \frac{w^2}{D\_{\mathcal{r}}(\mathbf{x},t)} \right] \}$$

Note here that *k* is dimensionless, being derived from the distribution factor, K, and the phase ratio of the column, *β* namely *K*/*β,* with K corresponding to the ratio between the (moles/volume) in stationary phase to the (moles/volume) in gas phase. ro and *w* correspond to the inner radius of the column and the film thickness of the stationary phase *Ds,* and *Dm* correspond to the diffusion constant respectively in the stationary and mobile phase. *vm* corresponds to the velocity of migration of the carrier gas.

The local zone variance (*σx*<sup>2</sup> ) in the distance of a solute from the zone centroid at a given position *x*, can be calculated using Eq. (6), representing the solute band's spreading.

$$
\sigma\_{\mathfrak{x}}\,^2(\Delta \mathfrak{x}\_n) = H(\mathfrak{x}\_n, t\_n) \cdot \Delta \mathfrak{x}\_n \tag{6}
$$

Eq. (7) describes the increment in the zone variance length, and can be obtained by the summation of all the local contributions of zone variances. Giddings [26]

explained that at every time step, the correction is applied for the expansion of the solute zone due to the reduction in pressure (*P*) along the column.

$$
\sigma\_{\mathbf{x}}\,^2(\mathbf{x}\_n) = \left[\sum\_{i=1}^{n-1} \sigma\_{\mathbf{x}}\,^2(\Delta \mathbf{x}\_i)\right] \cdot \left\{\frac{P(\mathbf{x}\_{n-1})}{P(\mathbf{x}\_n)}\right\} + \sigma\_{\mathbf{x}}\,^2(\Delta \mathbf{x}\_n) \tag{7}
$$

This approach, has been programmed in MATLAB, and has been compared in [7] with the solution yielded by the COMSOL-MATLAB model developed in [19], which solves the diffusive-convective equation by finite elements.

The comparison study confirmed excellent agreement in predictions of the zone's centroid (average relative error of 1.1%) and of the zone's standard deviations (average relative error of 3%), as depicted in **Figure 2**.

Thus, the analytical model implemented in MATLAB has been coupled to the reduced molecular pyrolysis model described above, and as detailed in [7], by calling CHEMKIN at every time step iteration, and using feedback between the two models until each component elutes from the GC column.

#### **4.4 Coupled thermo-hydro-chemical processes**

Both the reduced molecular pyrolysis model and the analytic iterative GC model derive from the prior high-performance improvement process required for ultimately attaining an efficient coupled physics-chemical model.

The latter can predict the zone's centroid, the standard deviation and the pyrolysis decomposition of every solute studied for both as a mixture and as a single component according to the position of every solute related to the batch width at every time-step.

In order to maintain a constant temperature at every time-step, a constant timestep has been implemented, permitting an increment of 1°C every 4 seconds (corresponding to the ramp of 15°C/min, used).

Initially, for every component studied, the position of the zone's centroid in the next time step (xi+1), is calculated, using Snijders [23, 27] approach Eq. (8) (see ref. [19]), the distribution factor (*K*), and the phase ratio (*β*).

#### **Figure 2.**

*Comparison of zone standard deviation and zone centroid of nC12H26, predicted using an iterative analytic approach<sup>11</sup> using MATLAB and solving the diffusive-convection equation by finite element in COMSOL. (Column dimensions Table 3 and temperature programming Table 2).*

*Reactive Transport and Its Implications on Heavy Oil HTGC Analysis – A Coupled… DOI: http://dx.doi.org/10.5772/intechopen.98614*

*Algorithm of the pyrolysis-GC coupled model.*

$$\mathbf{x}\_{i+1} = \mathbf{x}\_i + \frac{\nu\_M(\mathbf{x}\_i, t\_i)}{\mathbf{1} + \frac{K\_i(T(t\_i))}{\beta}} \cdot \Delta t \tag{8}$$

**Figure 3**, shows the algorithm explaining the global calculation carried out by the coupled THC model for an heavy oil analysis by HTGC, using the above models as explained previously. For more detail refer to [7].

### **5. Implications: results**

The implications of the THC processes during an HTGC heavy oil analysis can be summarised under two headings:

Thermal cracking risk of the original sample.

Non-elution or incomplete elution of the sample.

A detailed analysis of these implications is presented, based on the results of the in-house developed THC multiphysics model, described above.

#### **5.1 Thermal cracking risk of the original sample**

The cumulative conversion due to pyrolysis of the 11 n-alkanes is studied in [4], in order to analyse their risk, as depicted in **Figure 4**. For each component the ratio is calculated of the cumulative mass lost (**Figure 5**) due to thermal cracking, compared to the mass injected.

#### **Figure 4.**

*Accumulative mass lost due to thermal cracking for n-alkanes (nC14, nC16, nC20, nC25, nC30, nC35, nC40, nC50, nC60, nC70, nC80) at a common HTGC temperature programming (Table 2) in a HT5 column with dimension summarised in (Table 3).*

*Reactive Transport and Its Implications on Heavy Oil HTGC Analysis – A Coupled… DOI: http://dx.doi.org/10.5772/intechopen.98614*

#### **Figure 5.**

*Cumulative conversion due to thermal cracking for n-alkanes (nC14, nC16, nC20, nC25, nC30, nC35, nC40, nC50, nC60, nC70, nC80) at a common HTGC temperature programming (Table 2) in a HT5 column with dimension summarised in (Table 3).*


#### **Table 2.**

*Temperature programming.*

As would be expected, no pyrolysis reaction occurs in the case of nC14H30 and nC16H34 with the temperature program used (**Table 2**), and their associated short residence times inside the GC column. Similarly, within the range nC20H42 to nC40H82, insignificant conversion occurs, whereas in the case of nC50H102 the maximum mass loss through thermal decomposition before elution is 0.003%.

Low but detectable mass loss occurs with the heaviest n-alkanes. nC60H122 has a significant loss in the stationary phase where only 2.43�10�<sup>12</sup> g are released to the gas phase with the remainder trapped in the stationary phase. Further, pyrolysis loss begins at 373°C with a 0.001% cumulative mass conversion. nC70H142, presents a cumulative conversion of 0.001% at 385°C with only 2.32�10�<sup>10</sup> g released in the gas phase and the rest trapped in the stationary phase.

It should be noted that at the time-step when nC60H122 decomposition starts, nC50H102 is virtually totally eluted (99.9%) eluted, and hence the pyrolysis products present no risk of co-elution with the latter. Rather, the pyrolysis products of nC60H122 are released gradually, evidenced by a slowly increasing baseline signal.

Similarly, nC70H142 starts to decompose when located 1.02 m away from the GC inlet, and 0.68 minutes after nC60H122 is essentially fully eluted (99.99%).

Therefore, the pyrolysis products present no risk of co-elution with, nor distortion of the peak for nC60H122.

Lastly, nC80H162 starts to decompose at 0.41 m from the column inlet, while nC70H142 is located 1.64 m from the inlet. Thus, when nC70H142 is essentially fully eluted (99.99%), at 7.83 m from the column inlet, nC80H162 has undergone a cumulative conversion of 0.52% mass loss by pyrolysis, relative to mass injected. That equates to 3.97.10�<sup>11</sup> g of nC80H162 converted into pyrolysis products, and which co-elutes with nC70H142, resulting in unreliable quantification.

## **5.2 Non-elution of heavy components from the column**

For the determination of non/incomplete elution of heavy n-alkanes, the data set of distribution factors of the n-alkanes spanning the range from nC12H26 to nC98H198, [4] was used as main input for the calculation of the degree of elution of each of the n-alkanes studied.

The *degree of elution* has been introduced in order to determine the non/incomplete elution of heavy n-alkanes (as explained in [19]) as depicted in **Figure 6**.

Alkanes heavier than nC60H122 elute during the isothermal plateau of the temperature programmed at 430°C. Therefore, constant distribution factors apply for the re-equilibration period, when characteristic peak broadening is observable. (c.f. the essentially symmetrical peaks associated with temperature programmed GC analyses).


**Table 3.**

*Column dimensions of in-house HTGC.*

#### **Figure 6.**

*Degree of elution vs. transit time of each component "i": n-alkanes in the range of C14H30 to nC80H162. Degree of elution = moles of "i" inside the GC column at time (t) /moles injected of "i".*

*Reactive Transport and Its Implications on Heavy Oil HTGC Analysis – A Coupled… DOI: http://dx.doi.org/10.5772/intechopen.98614*

nC70H142 starts to elute at 29 minutes with a 99.99% degree of elution at 31.3 minutes, and 100% at 31.5 minutes. nC70H142 takes 2.5 minutes to elute completely.

nC80H162 starts to elute at 33.8 minutes, with a degree of elution of 99.99% at 40.9 minutes and 100% after 42.3 minutes. nC80H162 takes 7.1 minutes to elute and 8.5 minutes to completely elute.

In this simulated study, components from nC70H142 and above, elute so slowly that peak resolution for the group cannot be assessed. Rather, in practice, a continuum is observed, in the form of a gradually increasing baseline, rising to a plateau which gradually reduces during the final isothermal period of the oven temperature program.

It is interesting to note that 99.99% of nC80H162 requires to elute 12.9 minutes at the isothermal conditions at the maximum temperature (430°C) of the analysis. of 99.99%. This means that this component is not normally taken into account in the GC calculations, due to the shorter period of time and stationary phase bleeding.

## **6. Conclusions**

This chapter provides an insight into the analysis of the Reactive Transport process occurring during the analysis of heavy oil hydrocarbons inside a High Temperature Gas Chromatography column, and the implication that those interrelated physicochemical processes generate, by application of a Thermo-Hydro-Chemical (THC) coupled multiphysics approach.

The number of species in the reduced free-radical pyrolysis model developed in [19] has imposed a need to develop a reduced molecular pyrolysis model, comprising 11 n-alkanes (nC14H30, nC16H32, nC20H42, nC25H52, nC30H62, nC35H72, nC40H82, nC50H102, nC60H122, nC70H142, and nC80H162). The number of reactions has been reduced from 7055 to 127, and the number of species from 336 to 17, whilst still yielding very good accuracy.

THC multi-physics model has been implemented to resolve the HTGC limitations. The cumulative pyrolysis conversion of the 11 n-alkanes studied in this work, suggests that 0.52% of the mass injected of nC80H162, thermally decomposed before nC70H142. Therefore, co-elution of nC70H142 and the pyrolysis product of nC80H162 makes the GC analysis of nC70H142 and heavier n-alkanes no longer reliable.

The degree of elution of the 11 n-alkanes studied in the chapter confirms that alkanes heavier than nC70H142 take progressively longer to elute completely from the column, viz. nC70H142 takes 2.3 minutes and nC80H162 takes 7.1 minutes, with co-elution of decomposition products in each case compromising their analyses.

Finally, nC80H162 takes 12.9 minutes to completely elute during the isothermal plateau, resulting in no distinct peak being observable. Consequently, the eluting component will be masked in the FID plateau signal, in combination with column bleed products. As a result the nC80H162 analysis may not be utilised under these HTGC conditions.

### **Acknowledgements**

The authors wish to thank the members of our JIP: Marathon Oil Corporation, Schlumberger and Total for both their technical and financial support during this project.

## **List of symbols**


## **Greek letters**


*Reactive Transport and Its Implications on Heavy Oil HTGC Analysis – A Coupled… DOI: http://dx.doi.org/10.5772/intechopen.98614*

## **Author details**

Diana Margarita Hernandez-Baez<sup>1</sup> \*, Alastair Reid2†, Antonin Chapoy<sup>2</sup> , Bahman Tohidi<sup>2</sup> , Roda Bounaceur<sup>3</sup> and François Montel4,5†

1 SIMULEX Limited, International House, Edinburgh, Scotland, United Kingdom

2 Hydrates, Flow Assurance and Phase Equilibria Research Group, Institute of GeoEnergy Engineering, Heriot-Watt University, Edinburgh, Scotland, United Kingdom

3 Laboratory of Reactions and Process Engineering, Université de Lorraine, LRGP site ENSIC, NANCY Cedex, France

4 Fluids and Organic Geochemistry Department, Fluids Thermodynamics, Exploration and Production, Geosciences Technologies, TOTAL S.A, Avenue Larribau, France

5 Laboratoire des Fluides Complexes et leurs Réservoirs, LFCR, UMR 5150, Université de Pau et des Pays de l'Adour, E2S UPPA, CNRS, TOTAL, Pau, France

\*Address all correspondence to: dianahernandez@gmail.com

† Retired.

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[15] Castells RC, Arancibia EL, Nardillo AM. Regression against temperature of gas-chromatographic retention data. J Chromatogr. 1990;504 (1):45–53.

[16] Aldaeus F. New tools for trapping and separation in gas chromatgorahpy and dielectrophoresis (Improved performance by aid of computer simulation). Dr Thesis Anal Chem Stockh Univ. 2007;

[17] Snijders H, Janssen HG, Cramers C. Optimization of temperature-programmed *Reactive Transport and Its Implications on Heavy Oil HTGC Analysis – A Coupled… DOI: http://dx.doi.org/10.5772/intechopen.98614*

gas chromatographic separations .1. Prediction of retention times and peak widths from retention indices. J Chromatogr A. 1995;718(2):339–355.

[18] Aldaeus F, Thewalim Y, Colmsjo A. Prediction of retention times of polycyclic aromatic hydrocarbons and n-alkanes in temperature-programmed gas chromatography. Anal Bioanal Chem. 2007 Oct;389(3):941–950.

[19] Hernandez-Baez DM, Reid A, Chapoy A, Tohidi B, Bounaceur R. Establishing the Maximum Carbon Number for Reliable Quantitative Gas Chromatographic Analysis of Heavy Ends Hydrocarbons. Part 2. Migration and Separation Gas Chromatography Modeling. Energy Fuels. 2013;27(4): 2336.

[20] Davankov VA. The true physical meaning of the corrected retention volumes in GC. Chromatographia. 1997; 44(5–6):279–282.

[21] Kestin J, Knierim K, Mason EA, Najafi ST, Ro ST, Waldman M. Equilibrium and trasport properties of the noble gases and their mixtures at low density. J Phys Chem Ref Data. 1984;13(1):229.

[22] Hawkes SJ. Viscosities of carrier gases at gas-chromatograph temperatures and pressures. Chromatographia. 1993 Oct;37(7–8): 399–401.

[23] Hernandez-Baez DM, Tohidi B, Chapoy A, Bounaceur R, Reid A. Establishing the Maximum Carbon Number for Reliable Quantitative Gas Chromatographic Analysis of Heavy Ends Hydrocarbons. Part 1: Low-Conversion Thermal Cracking Modeling. Energy Fuels. 2012 May 17;26 (5):2600–2610.

[24] Golay MJE. Theory of Chromatography in Open and Coated Tubular Columns with Round and Rectangular Cross-Sections. 1958. 36 p.

[25] Blumberg LM. Temperature-Programmed Gas Chromatography. 2010.

[26] Giddings JC, Seager SL, Stucki LR, Stewart GH. Plate Height in gas chromatography. Anal Chem. 1960;32 (8):867.

[27] Snijders H, Janssen HG, Cramers C. Optimization of temperatureprogrammed gas chromatographic separations 0.1. Prediction of retention times and peak widths from retention indices. J Chromatogr A. 1995;718(2): 339.

## **Chapter 3**

## Features and New Examples of Gas Chromatographic Separation of Thermally Unstable Analytes

*Igor G. Zenkevich*

## **Abstract**

The processes of thermal decomposition of analytes in gas chromatographic (GC) columns are classified and two new examples of them are considered in details. First of them is monomolecular decomposition of monoalkyl esters of benzene-1, 2-dicarboxylic (phthalic) acid (monoalkyl phthalates). This process has the analogy in chemical reactions in solutions and it may be responsible for the toxicity of phthalates. The second example is decomposition of non-substituted hydrazones of both aliphatic and aromatic carbonyl compounds. The analytes of the second sub-group present the first example of bimolecular (second order) decomposition in a GC column: two molecules of hydrazones form stable azines and hydrazine. Besides that this process presents the particular interest, because it is accompanied by secondary chemical reactions not in an injector, but within GC column, when a by-product of decomposition is involved into secondary interaction with other constituents of the samples. It was confirmed, that visual images of all these decomposition processes on the chromatograms are rather identical and coincide with the manifestations of interconversion of isomers or tautomers. The most often expressed features of chromatographic profiles in such cases are the presence of peaks of an initial analyte and a product of its decomposition or isomerization, connected with more or less expressed diffused "plateau" or "train" between them. The decomposition processes during sample preparation prior to chromatographic separation or in the heated injector of GC instrument are not accompanied by such features. Despite of the rather "exotic" character of the examples considered, the knowledge of them seems to be useful for better revealing the analogous situations in chromatographic practice. Thermal instability of analytes is the principal restriction of GC separation of reactive compounds and we cannot eliminate it for objective reasons. However, in some cases we can evaluate the temperature limits of chromatographic columns, which should not be exceeded during GC separation of instable compounds. The simplest (low boiling) homologs of thermally unstable compounds are often characterized by "normal" boiling point at atmospheric pressure (*T*b, °C) without decomposition, that means the possibility of their GC analysis unambiguously. Therefore, we can select such *T*b values as GC and/or GC–MS temperature limit (*T*lim) for other members of series of thermally unstable homologs. If GC separation is carried out not in isothermal, but in temperature programming conditions, so-called retention temperature (*T*R) of unstable analytes should not exceed the evaluated *T*lim value.

**Keywords:** gas chromatography, GC-MS, thermally unstable analytes, new examples of thermal instability, monoalkyl phthalates, non-substituted hydrazones, secondary reactions in GC column, stability of analytes criterion

## **1. General comments (introduction)**

Possible instability of analytes prior or during their separation seems to be the key restriction of chromatographic methods, because it distorts the analytical results and can make them even inacceptable at all due to their irreproducibility and/or incorrectness. The influence of instability due to the different reasons can manifest itself at different stages of analytical procedures, hence it may be considered as a way of classifying them.

*The instability of components in samples prepared for analysis* (**reason I**). Besides the immanent chemical instability of analytes, it can be possible due to their oxidation by air oxygen or hydrolysis by impurities of water in air or in a solvent. These reasons can be avoided by special sample protection that was demonstrated for chemically high reactive fluoro- and chlorosilanes (hydrolysis), boranes, germanes (oxidation), etc. [1]. Gas Chromatographic Retention Indices (**GC RI**) of such labile compounds determined even at the late 1970s [1] remained unique up to present. The instability during GC separation can be caused by *thermal degradation of unstable analytes in a heated injector* (**reason II**), as well. The similar **reason III** of instability seems to be the *mutual interaction of most reactive components of the samples with each other at the temperatures of GC injection*, if even their mixtures are stable in the samples prepared at the ambient conditions. However, the decomposition processes in the injector can be, if not eliminated, then controlled and sometimes minimized by varying its temperature, as well the application of so-called on-column injection. On the other side, *the decomposition processes within chromatographic columns* (**reason IV**) are usually much more difficult to manage and they represent the most difficulties in chromatographic practice. Often enough, in the result of such processes the analytical parameters (e.g., RIs) of decomposition products are attributed to the initial unstable analytes that are the typical examples of misidentification.

In high performance liquid chromatography (HPLC) the main reason of instability is the interaction of analytes with components of an eluent (most often hydrolysis).

Sometimes the revealing of analytes' instability seems to be not so simple. The main signs of analytes' instability are inconstancy of absolute or relative areas of some chromatographic peaks (not all of them) depending on variations of analytical parameters (at first, injector or column temperatures), appearance of additional peaks, distortions of peaks' shapes, loss of separation efficiency, etc. The lack of reference RI values for any reactive compounds in contemporary mass spectrometric (**MS**) and gas chromatographic (**GC**) databases (e.g., [2]) is often due to just their instability. So-called analytical artifacts (when the results do not match the analytes containing in the samples) summarized by Middleditch [3] are often caused by the mentioned reasons.

## **2. Different kinds of analytes' instability complicating gas chromatographic analysis (like their classification)**

As the typical examples of the manifestation of instability in the result of high reactivity of analytes (**reason I**) such semi-volatile compounds as **(3-aminopropyl) trimethoxy-** (boiling point, *T*b, 194°C) and **(3-aminopropyl)triethoxysilane**

*Features and New Examples of Gas Chromatographic Separation of Thermally Unstable… DOI: http://dx.doi.org/10.5772/intechopen.94229*

(*T*b 220 ± 3°C) [4, 5] can be mentioned. It is important to note that these compounds have the normal boiling points at atmospheric pressure, but they are as active as silanization agents that can react even with glass surfaces of chromatographic syringes used for injection of samples and the silica surfaces of injector liners and chromatographic columns. Due to this reason these compounds remain uncharacterized by GC RIs up to date [2]. Another example of the same kind is so "exotic" compound as **dimethyl thionitrosamine**, **(CH3)2N-N=S** that is so unstable as the individual substance or constituent of concentrated solutions at the ambient temperature due to its easy polymerization, that it exists only in dilute solutions [6]. At the same time, this compound seems relatively stable within a chromatographic column, so that its RI on standard non-polar stationary phase has been successfully determined (992 ± 2)<sup>1</sup> [6].

The examples of analytes' thermal decomposition in a heated injector (**reason II**) are numerous, as well. Thus, over a dozen publications presenting RI values of **N-nitrosodiphenylamine**, **(C6H5)2N-NO**, are known up to present, but only one of them contains the correct RI value 1865 [7], while others belong to the decomposition product – diphenylamine, **(C6H5)2NH** (RI 1587 ± 13 [2]). For such decomposition to take place a source of active hydrogen atom is required; it can be using of hydroxylcontaining solvents, or water residues in a sample.

Sometimes the revealing of structural features of analytes' molecules responsible for their decomposition appears the important problem in organic chemistry. Thus, such heterocyclic compounds as 4-acyl-1,3,4-oxadiazolines do not differ principally from other organic compounds by their stability. Nevertheless, if these heterocycles contain no substituents in the position 2, they decompose at the injector temperatures above 150°C with formation of monoacyl hydrazones [8] in the result of decarbonylation. Within temperature range 150–190°C both initial compounds and decomposition products are detected, but above 190°C no peaks of initial analytes are registered on the chromatograms:

The similar decomposition is observed for substituted high reactive 3,4-dihydroformazans (trivial name "azohydrazines", but the limit of their thermal stability is less, approx. not more than 130°C [9]. The decomposition products are disubstituted hydrazones:

As an example of erroneous determination of retention indices in the result of thermal decomposition of analytes let us mention the RI value for trichloroacetic acid (600) on standard non-polar polydimethylsiloxane stationary phases published in Sadtler Retention Index Library [10]. It looks like obviously erroneous, but its proving appeared to be not so simple, because it requires comparing RI values for series of structural analogues of acetic acid containing up to two chlorine atoms (congeners), as it is illustrated by data in **Table 1**:

<sup>1</sup> Here and afterwards all RI values, if it is not mentioned specially, presented for standard non-polar stationary phases (polydimethyl siloxanes), i.e., RInon-polar.


*RI values for chloro- and dichloroacetic acids are taken from author's data collection.*

*\*\*Tb and RI values are extrapolated using the data for previous congeners and the recurrent algorithm [11].*

#### **Table 1.**

*Comparing the retention indices (RInon-polar) for chloroacetic acids.*

Known RI data for substituted acids Cl0-Cl2 (three points) allows approximating RIs using recurrent relation **RI(***n***Cl + 1) =** *a***RI(***n***Cl) +** *b* [11] (*a* = 0.767, *b* = 385.6), hence for trichloroacetic acid (Cl3) RI = 0.767 × 1048 + 385.6 ≈ 1189. Thus, the value 600 from [10] is erroneous and obviously belongs to the chloroform CHCl3 (reference RI value is 605 ± 4 [2]) formed in the result of decomposition of the trichloroacetic acid within heated GC injector:

$$\text{Cl}\_3\text{C}-\text{CO}\_2\text{H} \rightarrow \text{CHCl}\_3 + \text{CO}\_2\text{H}$$

Thermal instability caused by mutual interaction of constituents of the samples in an heated injector (**reason III**) can be illustrated by features of chromatographic determination of the impurity of 1,2-propanediol (propylene glycol) in the high boiling polar aprotic organic solvent – 4-methyl-1,3-dioxolan-2-one (trivial name – propylene carbonate, *T*b 242°C) [12]. The real content of propylene glycol in this solvent at the ambient temperature is strongly distorted in the result of its hydrolysis by the residual amounts of water in a heated injector during injection:

The last source of instability of analytes during GC separation – decomposition of analytes in a chromatographic column (**reason IV**) – is not equivalent to their decomposition in an injector (**reason II**). In a column analytes are usually exist under the influence of a lower temperature, but a longer time than in an injector. We cannot change the influence of column's temperature without changing the retention time (*t*R), so far as these parameters are connected by the known twoparameters Antoine-like equation:

$$\log\left(\mathbf{t}\_{\mathbf{R}} - \mathbf{t}\_{\mathbf{0}}\right) = \mathbf{a} \;/\; \mathbf{T} + \mathbf{b} \tag{1}$$

where *t*0 is hold-up time of chromatographic system, *T* is the absolute temperature (in Kelvins), coefficients *a* and *b* are calculated by Least Squares Methods.

The temperature dependence of the rate of first-order decomposition reaction (rate constant *k*) is approximated with known Arrhenius relation that is twoparameter Antoine-like equation as well:

$$
\ln k = -E\_\text{a} \,/\, \text{RT} + \ln A \,\tag{2}
$$

*Features and New Examples of Gas Chromatographic Separation of Thermally Unstable… DOI: http://dx.doi.org/10.5772/intechopen.94229*

where *E*a is activation energy for the reaction, *R* is universal gas constant, *A* is pre-exponential factor.

Thus, the influence of column's temperature on the areas of chromatographic peaks of unstable compounds is simultaneously determined by four coefficients, *a*, *b* (Eq. 1), *E*a/*R*, and ln*A* (Eq. 2). Just these relationships confirm that analytes' decomposition in GC column seem to be difficult to eliminate, because if we decrease the column's temperature, we increase the analyte's residence time in the heated area. Besides the examples listed above, the decomposition inside GC column (in addition to decomposition in an injector) is valuable for diazocarbonyl compounds, decomposed with the formation of substituted ketene in the result of so-called Wolff rearrangement [13]:

It is interesting to note that resulted ketenes are unstable compounds as well; they cannot be isolated as individual substances or components of concentrated solutions due to easy polymerization, and, hence, do not form distinct chromatographic peaks. In the result, their mass spectra were registered successfully [13], but GC RIs cannot be determined excepting the most volatile simplest member of this series – dimethyl ketene (RInon-polar 484, RIpolar 1215 [2]).

## **3. The influence of the decomposition of analytes in chromatographic columns on contours of chromatograms**

The last mentioned mode of analytes' transformations during GC separation (**reason IV**, decomposition in a chromatographic column) is often manifested in the appearance of specific profiles of chromatograms. **Figure 1** presents a schematic image of typical chromatogram of unstable analyte "*X*" which is converted into decomposition product "*Y*" within a chromatographic column in the result of the process *X* → *Y*. Usually (at least, in GC) the decomposition product "Y" is more

#### **Figure 1.**

*Schematic image of the typical chromatogram of thermally unstable analyte "X" converted into more volatile decomposition product "Y" within chromatographic column during separation. The appearance of a diffuse zone "Z" ("train" or "plateau") between peaks "X" and "Y" is the sign of decomposition (X* → *Y).*

volatile simpler compound. The peaks of both compounds "*X*" and "*Y*" rather often can be registered on the chromatograms; and retention times of decomposition products "*Y*" are naturally less than those of initial analytes "*X*". However, besides these two regular peaks there is a specific diffuse signal between them (*Z*), named as "anomalous profile", "plateau", "train", "plume", or something similar (no generally accepted terminology exists). In the case of monomolecular decomposition of *X*, this "train" consists exclusively of decomposition product "*Y*".

The "idealistic" profile like that on **Figure 1** in many cases can be more or less distorted that complicates revealing the decomposition processes in a column. Some examples of such distortions are presented on Figures below. For example, decomposition of **1-diazo-4-phenylbutan-2-one**, **C6H5CH2CH2COCH=N2** within a column leads to the formation of "train" (*Z*) of ketene **C6H5CH2CH2CH=C=O** prior the peak of initial diazocompound, but this ketene does not form a separate chromatographic peak [13].

The similar profiles of chromatograms are observed not only in cases of decomposition of analytes (irreversible reactions), but in cases of reversible interconversion of their isomeric or tautomeric forms. It should be noted that similar profiles may be observed both in GC, and HPLC. Such chromatograms were registered for keto- and enolic tautomers of 1,3-diketones (GC) [14], β-ketoesters (GC) [15, 16], *syn*- and *anti*-isomers of 2,4-dinitrophenyl hydrazones of carbonyl compounds (HPLC) [17, 18], derivatives of hydroxyquinones (HPLC) [19], and so on. The fragment of a GC– MS TIC-chromatogram illustrating the separation of keto- and enolic tautomers of ethyl acetoacetate CH3COCH2CO2C2H5 is presented at **Figure 2**. The "train" observed here between the peaks of tautomers is similar to a schematic profile at **Figure 1**.

Comparing the examples mentioned we can conclude that the decomposition of unstable or reactive analytes prior to or during the chromatographic separation restricts the possibilities of this analytical technique and complicates the interpretation of results. Due to this reason any new examples of such decomposition should be reliably revealed and discussed to avoid difficulties in the subsequent data interpretation. According with this viewing, two new examples are considered here in details, namely unusual thermal instability of monoalkyl esters of benzene-1,2-dicarboxylic acid (monoalkyl phthalates) and decomposition of non-substituted hydrazones of carbonyl compounds.

#### **Figure 2.**

*The fragment of the Total ion current (***TIC***) chromatogram (WCOT column with standard non-polar polydimethyl siloxane stationary phase) containing the peaks of keto and enol tautomers of ethyl acetoacetate*  **CH3COCH2C2H5***. The existence of "plateau" between peaks of tautomers confirms their interconversion within GC column during separation. Reproduced from [16] with permission.*

*Features and New Examples of Gas Chromatographic Separation of Thermally Unstable… DOI: http://dx.doi.org/10.5772/intechopen.94229*

## **4. Materials and methods**

Most of chemicals (alcohols, carbonyl compounds, and ethyl acetoacetate) of reagent or chromatographic grade were from "Reakhim" (Moscow, Russia). 2-Propanol and methylene chloride of chemical grade (solvents) were provided from "Vekton" (St. Petersburg, Russia). Hydrazine hydrate of reagent grade was purchased from Acros Organics, Belgium.

## **4.1 Preparation of reaction mixtures**

*Monoalkyl phthalates.* Approx. 6 mg of phthalic anhydride (melting point 130–132°C, approx. 50 μmol) was added to the 2-mL portions of 1-alkanols CnH2n + 1OH (*n* = 1–8) and heated in the presence of catalytic amounts of phosphorous acid at the boiling point of the alcohol (for alcohols C1 – C4) or not more than 110°C (for alcohol C5 – C8) during 30 min to complete the dissolution of the phthalic anhydride. For GC–MS analysis 5 μL of reaction mixtures were diluted with 0.5–2.5 mL of methylene chloride by a factor of 100–500. All monoalkyl phthalates are characterized without their isolation from reaction mixtures. The list of RI values for monoalkyl phthalates is presented in **Table 2**.

*Non-substituted hydrazones of carbonyl compounds.* Hydrazine hydrate (2 mL) were mixed with 50 μL of carbonyl compounds (molar excess from 60:1 to 120:1) and 2 mL of 2-propanol at the ambient temperature. To increase the yields of azines, if necessary in some experiments, the molar ratio of hydrazine was decreased to 30:1 and 15:1. After 10 min 50 μL of obtained mixtures were diluted with 2 mL of 2-propanol, followed by addition of 2 μL of the reference *n*-alkanes mixture. All hydrazones were characterized without isolation from reaction mixtures. Besides hydrazones these mixtures contained variable amounts of azines. The list of RI values for non-substituted hydrazones and corresponding azines is presented in **Table 3**.


#### **Table 2.**

*Gas chromatographic retention indices of some monoalkyl phthalates on semi-standard non-polar polydimethylsiloxane stationary phases (95% methyl and 5% phenyl groups).*


*\* The minor constituents with mass spectra identical to spectra of principal (E,E)-isomers tentatively belong to (E,Z)-isomers of azines.*

#### **Table 3.**

*Gas chromatographic retention indices of non-substituted hydrazones and corresponding azines on semi-standard non-polar polydimethylsiloxane stationary phases [24].*

#### **4.2 Instrumentation and data processing**

GC–MS analyses of reaction mixtures were performed using Shimadzu QP 2010 SE gas chromatograph – mass spectrometer with electron ionization (70 eV) equipped with RTX-5 MS column of the length 30 m, internal diameter 0.32 mm, and stationary phase film thickness 0.25 μm. The conditions of analysis were as follows: temperature programming regime, initial temperature 70°C (phthalates) or 50°C (hydrazones), and ramp 6 K/min (phthalates) or 10 K/min (hydrazones) up to 200°C. Helium was used as carrier gas, flow rate was 1.8 mL/min, split ratio 1:10–1:12, injector temperatures were 200–250°C, interface and ion source temperatures were 200°C. Volumes of injected samples were 0.5 μL (phthalates) or 2 μL (hydrazones). To determine retention indices, mixtures of C7 – C20 *n*-alkanes (in different combinations) were added to the samples before analysis for determination of retention indices (RI) (calculations of linear or linear-logarithmic RIs were provided using home-made QBasic program).

Statistical data processing and plotting of results were carried out using the Origin software (versions 4.1 and 8.2). The task considered requires the specific mode of results' presentation, preferably in the form of chromatogram images. Most of original chromatograms besides the peaks of target analytes and decomposition products contain peaks of reference *n*-alkanes, which were required for calculation of retention indices and confirmation of the appropriate effectiveness of a column.

### **5. Results and discussion**

#### **5.1 Features of GC separation of monoalkyl phthalates**

Contemporary GC–MS data bases (e.g., like [2]) provide important information for identification of analytes including standard electron ionization (EI) mass spectra and GC retention indices (**RI**) on standard non-polar and polar phases. However, besides

*Features and New Examples of Gas Chromatographic Separation of Thermally Unstable… DOI: http://dx.doi.org/10.5772/intechopen.94229*

that, considering the large collection of reference data allows revealing both individual compound and their various taxonomic sub-groups (homologous series, multitudes of homologs and/or congeners, etc.) insufficiently characterized by analytical parameters up to present. One of such groups appeared to be the acidic esters of polycarboxylic acids, including alkyl esters of benzene-1,2-dicarboxylic acid (monoalkyl phthalates) that stimulated determination of their MS and GC analytical parameters [20] in comparison with the data for their much better characterized structural analogues – dialkyl phthalates. Rather unexpectedly it was found that monoalkyl phthalates appeared to be unstable at standard conditions of GC separation.

Monoalkyl phthalates can be easily synthesized from phthalic anhydride and corresponding alcohols at acid catalysis in accordance with the following scheme:

**Figure 3.**

*The fragment of the TIC-chromatogram of reaction mixture of phthalic anhydride with methanol at the comparable scale of peak intensities. Peak (***I***) – Phthalic anhydride, peak (***II***) – Monomethyl phthalate, C11 and C13 – Peaks of reference n-alkanes. Reproduced from [20] with permission.*

#### **Figure 4.**

*The same fragment of the chromatogram of reaction mixture of phthalic anhydride with methanol as those at Figure 3 in SIM-regime on a larger scale; peak (***I***) – Phthalic anhydride, peak (***II***) – Monomethyl phthalate, m/z 148 – Molecular mass of phthalic anhydride, m/z 104 – Maximal signal in the mass spectrum of phthalic anhydride. Reproduced from [20] with permission.*

The large molar excess of the alcohols in the reaction mixtures allowed us to hope for a high degree of conversion of phthalic anhydride into monoalkyl phthalates. Surprisingly all the chromatograms of reaction mixtures indicate the predominant amounts of phthalic anhydride. For instance, the fragment of the TIC-chromatogram of the reaction mixture of phthalic anhydride with methanol is presented on **Figure 3**; the ratio of peak areas of anhydride (**I**) to monomethyl phthalate (**II**, RI is 1566 ± 18, retention temperature of 152°C) is approx. 100:1. To explain such an anomaly, the original chromatogram was reconstructed into SIMmode using values *m*/*z* 148 (molecular mass of phthalic anhydride) and *m*/*z* 104 (maximal peak in its mass spectrum); the result is shown at **Figure 4**.

From **Figure 4** it is easy to notice that both signal with *m*/*z* 104 and *m*/*z* 148 besides the peak of phthalic anhydride itself indicate the second local maxima for monomethyl phthalate (**II**). However, the most important information from this SIMchromatogram is revealing the registered "train" between peaks (**I**) and (**II**). It is the unambiguous indication of thermal instability of monomethyl phthalate at conditions of GC separation, and its main decomposition product is phthalic anhydride. The same process can be assumed for other monoalkyl phthalates having the higher boiling points than monomethyl ester, and, hence, the higher retention temperatures [20]:

A similar process of cyclization with participation of two carbonyl groups in the *ortho*-position in benzene ring is known for chemical reaction in solution. It explains, for instance, the formation of 6-chloro-3-methoxyphthalide from 2-acetyl-5-chlorobenzoic acid [21]:

The additional independent confirmation of the thermal decomposition of monomethyl phthalate during gas chromatographic separation its mass spectrum from the database [2] should be considered. At **Figure 5** the mass spectrum of monomethyl phthalate (a) is compared with that of phthalic anhydride (b). The similarity of positions and intensities of main peaks of these spectra with *m*/*z* 104, 76, and 50 looks noteworthy. Most intensive peak of monomethyl phthalate itself belongs to the ions [M – CH3O]+ with *m*/*z* 149, but its relative intensity in mass spectrum [2] is about 40%. Moreover, the library search (reversed mode) for monomethyl phthalate [2] gives only two results with matching factor Q > 0.800, namely the same ester (another mass spectrum) with Q = 911, and phthalic anhydride (Q = 807). The similar paradox is observed even for phthalic anhydride itself; one of the results of the library search for its mass spectrum (in reversed mode) is monoethyl phthalate with Q = 854.

Nevertheless, more preferable mass spectrum of monomethyl phthalate can be obtained in the result of the accurate subtracting of background and/or overlapping signals. So as not to overload the text with figures, let us list it in the numerical form, *m*/*z* ≥ 39 (*I*rel ≥ 2%), **M** is the symbol of molecular ions:

180(2)**M**, 163(2), 150(9), 149(100) [M – CH3O], 148(10), 137(4), 136(31), 135(18), 122(5), 121(20), 118(2), 106(5), 105(52) [M – CH3O – CO2], 104(65)

**Figure 5.**

*Comparison of mass spectra of monomethyl phthalate (a) and phthalic anhydride (b) from database [2]. The mass spectrum of monomethyl phthalate is distorted by mass spectrum of phthalic anhydride.*

[C7H4O], 94(2), 93(29), 92(34), 91(16), 78(4), 77(33), 76(76) [C7H4O – CO ≡ C6H4], 75(14), 74(21), 73(5), 71(3), 66(5), 65(37), 64(5), 63(5), 59(3), 58(2), 57(2), 53(3), 52(6), 51(16), 50(46) [C4H2], 49(4), 44(3), 43(3), 41(2), 39(24).

As it can be seen, this mass spectrum strongly differs with that from database [2]; the maximal peak appears to be *m*/*z* 149 as it should be for esters of phthalic acid [22]. However, the presence of the signals with *m*/*z* 104, 76, and 50 do not allow the excluding it's at least partial decomposition in a GC column or at any point between column and ion source of mass spectrometer.

The instability of monoalkyl phthalates – the products of partial hydrolysis of dialkyl phthalates – widely used plasticizers of polymeric compositions – permits us to suggest the novel interpretation of endocrinic toxicity of these esters. Their decomposition in small extent can take place not only at the heating, but at the ambient conditions, as well. The product of such decomposition – phthalic anhydride – is an active acylation reagent which can react with some targets inside the living cells (e.g., peptides or nucleic acids) [20].

## **5.2 Anomalous chromatographic properties of non-substituted hydrazones of carbonyl compounds**

The considering of database [2] allows revealing another series of simple organic compounds that are characterized in enough extent neither mass spectra, nor GC retention indices. It is the products of the nucleophylic addition of hydrazine to carbonyl compounds – non-substituted hydrazones. These compounds are used in practice of organic synthesis since 19th century:

$$\mathsf{n} = \bigvee\_{\mathsf{n}\_{\mathsf{R}\_{\mathsf{2}}}} \mathsf{n} \to \mathsf{n}\_{\mathsf{R}\_{\mathsf{2}}} \to \mathsf{n}\_{\mathsf{R}\_{\mathsf{2}}} \to \mathsf{n} \stackrel{\mathsf{\mathsf{T}\_{\mathsf{R}\_{\mathsf{2}}}}}{\mathsf{n}}$$

Such hydrazones can be synthesized from carbonyl compounds in one stage; they are intermediates in so-called Wolff-Kishner reduction of carbonyl

**Figure 6.**

*The fragment of the TIC-chromatogram of reaction mixture of 4-methyl-2-pentanone with hydrazine hydrate; peak (***III***) – 4-methyl-2-pentanone hydrazone, peak (***IV***) – 4-methyl-2-pentanone azine, C8 – C10 – Reference n-alkanes. Reproduced from [24] with permission.*

compounds into hydrocarbons with the same carbon skeletons [23]. Under such circumstances it seems nearly paradoxical why these synthetically important compounds remained not characterized both by mass spectra and by gas chromatographic analytical parameters until last time. The database [2] contains mass spectrum and RI value for only one simplest member of this series – acetone hydrazone **(CH3)2C=N-NH2**.

Such inconsistency explains the necessity to carry out GC–MS analysis of reaction mixtures of carbonyl compounds with hydrazine hydrate [24]. Surprisingly, instead of "normal" (more or less sharp) chromatographic peaks of hydrazones the very "blurry" signals were recorded for them, as it can be seen from fragment of TIC-chromatograms of the reaction mixtures of hydrazine hydrate with 4-methyl-2-pentanone (**Figure 6**). The chromatograms of reaction mixtures of other carbonyl compounds look similar (see ref. [24]). Any attempts to improve the shapes of chromatograms by varying separation conditions or by dilution of samples remained unsuccessful.

All the chromatograms contain two diffuse peaks with variable relative intensities connected by "trains" between them. Strong broadening the peaks explains us the low accuracy of determining the retention indices of reaction products (and their low interlaboratory reproducibility, as well). From mass spectrometric data it is easy to conclude that molecular masses of the first eluted peaks correspond to non-substituted hydrazones, while those of the latter peaks – to azines of carbonyl compounds – stable products of the bimolecular decomposition of non-substituted hydrazones:

Besides this way of formation, more stable azines are the "normal" by-side reaction products even at the large excess of hydrazine. Mass spectra registered in various points between hydrazones and azines indicate that the "trains" between them are formed both by hydrazones and azines in variable proportions. It is the principal difference of the nature of such "trains" for mono-molecular decomposition processes in a GC column, when these areas are formed by decomposition products only.

*Features and New Examples of Gas Chromatographic Separation of Thermally Unstable… DOI: http://dx.doi.org/10.5772/intechopen.94229*

The decomposition of just non-substituted hydrazones in a GC column is confirmed by analysis of reaction mixtures containing solely azines. For instance, the chromatograms of the reaction mixtures of cyclohexanone with hydrazine hydrate in 10 minutes after mixing the reagents and after one week storage this sample at the ambient temperature look rather different. On the first chromatogram the intensive peak of hydrazone is observed, whereas prior to the peak of azine there is the "train" confirming decomposition process in a GC column. One week later when the sole reaction product appeared to be the azine; the "train" before its peak is disappeared completely.

The decomposition of monoalkyl phthalates considered in the Section 3.1 like other known decomposition processes in a chromatographic column is the first order reactions (monomolecular). On the contrary, the decomposition of non-substituted hydrazones with formation of azines is the first revealed example of second order (bimolecular) reaction in gas chromatographic column. It should be specially noted that the observed features of chromatograms ("trains" or "plateau") in the case of bimolecular decomposition of non-substituted hydrazones are the same, as those in the cases on monomolecular reactions.

#### **5.3 Decomposition followed by secondary chemical reactions in GC column**

After examples presented above we can consider the most unusual example of analytes' conversion in a GC column. **Figure 7** contains the fragment of the TICchromatogram of aromatic carbonyl compound – acetophenone – with hydrazine hydrate. Similarly to the previous examples we observe the peaks of acetophenone hydrazone (**IX**, *t*R approx. 11.5 min), acetophenone azine (**X**, *t*R approx. 19.0 min) connected by a "train" between them (zone **Z1**), as well the peak of initial acetophenone (*t*R approx. 7.5 min). However, besides the expected zone **Z1**, the second anomalous area **Z2** is observed prior to the peak of hydrazone (**IX**). Mass-spectra registered in the different points of this area **Z2** indicated that it composed exclusively of acetophenone hydrazone with the following mass spectrum in the numerical form, *m*/*z* ≥ 39 (*I*rel ≥ 2%):

135(10), 134(100)**M**, 133(21), 120(4), 119(42) [M – CH3], 118(5), 117(20) [M – NH3], 104(3), 103(12), 102(3), 93(9), 92(10), 91(4), 90(2), 89(2), 79(2), 78(10), 77(88) [C6H5], 76(7), 75(3), 74(3), 66(4), 65(6), 63(4), 57(9), 56(5), 52(3), 51(19).

The paradox observed is the follows: the area **Z2** corresponds to the range of retention times which are less than retention time of "normal" acetophenone hydrazone. In other words, *some part of acetophenone hydrazone is eluted from GC column* 

#### **Figure 7.**

*The fragment of the TIC-chromatogram of the reaction mixture of acetophenone with hydrazine hydrate; peak (***IX***) – Acetophenone hydrazone, peak (***X***) – Acetophenone azine, C8 – C10 – Peaks of reference n-alkanes. Reproduced from [24] with permission.*

#### **Figure 8.**

*The scheme of formation of two "trains" on chromatograms. Zone* **Z1** *– Area of acetophenone hydrazone (***IX***) decomposition with the formation of acetophenone azine (***X***) and secondary hydrazine; zone* **Z2** *– Area of interaction of secondary hydrazine with residues of initial acetophenone in reaction mixture with formation of secondary acetophenone hydrazone been eluted before "normal" acetophenone hydrazone.*

*"before" acetophenone hydrazone (!)*. Even the very formulation of this effect looks nearly paradoxical. It is the first known example of so anomalous chromatographic behavior of analytes.

To explain this anomaly let us reconstruct schematically the TIC-chromatogram of reaction mixture of acetophenone with hydrazine hydrate in the reversed scale, like it is shown at **Figure 8**. At this scheme an increase in retention times corresponds to a direction to the left, while an increase in the rate of chromatographic zones within column – to a direction to the right. The decomposition of acetophenone hydrazone (**IX**) with formation of acetophenone azine (**X**) (according with the scheme of reaction above) is occurs in zone **Z1** and it is accompanied by formation of secondary hydrazine that is most volatile constituent comparing with other reaction products (*T*b 114°C). Thus, the rate of chromatographic zone of secondary hydrazine exceeds the rates of chromatographic zones of other components of reaction mixtures. This hydrazine zone can "catch up" the zone of initial acetophenone and reacts with it, that leads to the formation of the "train" of secondary acetophenone hydrazone (**Z2**) located *between* the peaks of acetophenone itself and the "normal" peak of acetophenone hydrazone (i.e., before it). Of course, this reaction of nucleophylic addition of hydrazine to the carbonyl compound proceeds not in the gaseous, but in the condensed phase, namely in the stationary phase film.

Despite of the highly "exotic" character of the examples considered, the knowledge of them seems to be useful for better revealing the analogous situations in routine chromatographic practice.

## **5.4 Criterion of applicability of GC / GC-MS analysis for thermally unstable compounds**

Obviously, thermal instability of analytes seems to be the principal restriction of gas chromatographic separation of highly reactive compounds, thus we cannot eliminate it for objective reasons. However, in some cases we can evaluate the temperature limits of chromatographic columns, which should not be exceeded when analyzing certain unstable analytes.

It is known that the low boiling simplest homologs of thermally unstable compounds can often be distilled at ambient conditions without their thermal decomposition and, therefore, they are characterized by "normal" boiling point at *Features and New Examples of Gas Chromatographic Separation of Thermally Unstable… DOI: http://dx.doi.org/10.5772/intechopen.94229*

atmospheric pressure (*T*b, °C). The existence of boiling point means the possibility of gas chromatographic analysis of such compounds using contemporary fused silica WCOT columns of high inertness. Heating the higher members of the series to their boiling points at atmospheric pressure appeared to be impossible because of their decomposition. In practice of organic synthesis, distillation of such compounds under reduced pressure is used. Thus, we can select the boiling point of simplest homolog of the series under consideration at atmospheric pressure as the GC and/ or GC–MS analyses temperature limit (*T*lim) for other members of these homologous series. If GC separation is carried out not in isothermal, but in temperature programming conditions, instead of fixed column's temperature we must operate with so-called retention temperature (*T*R) which should not exceed *T*lim value:

$$T\_{\mathbb{R}} = T\_0 + r t\_{\mathbb{R}} \tag{3}$$

where *T*0 is the initial temperature, *t*R – retention time, *r* – ramp (centigrade per time unit).

To illustrate this criterion, let us consider the boiling point of the series of thermally unstable alkyl azides, **R-N3** in comparison with data for dialkyl diimides **R-N=N-R** (**Table 4**). In both series these data are available for homologs with R ≤ C5H11. Boiling points of homologs with R ≥ C6H13 at atmospheric pressure remain unknown at present due to instability of such compounds. Therefore, GC and/or GC–MS analysis of alkyl azides should be possible up to their retention temperature (*T*R) not exceeding approx. *T*lim ≈ 130–135°C (it has been confirmed experimentally [25]), and approx. *T*lim ≈ 180°C for dialkyl diimides. This conclusion is confirmed by experimental RI values known just for some homologs of these series. On the other hand, within the alkyl hypochlorites series, **R-OCl**, *T*b values are known only for members with R = CH3 (9.2°C), R = C2H5 (27–36°C), and R = *tert*-C4H9 (77–78°C), meaning that *T*lim for this series is not more than approx. 75–80°C. However, it is enough for separation of simplest homologs and determining their RI values, namely 502 (ethyl hypochlorite) and 605 (*tert*-butyl hypochlorite).

Within the series of aliphatic diazocarbonyl compounds with structural fragment **-CO-CHN2**, ethyl diazoacetate, **N2CHCO2C2H5**, is one of the most often used reagents. Hence, the physicochemical properties available for this compound (including *T*b 140–143°C) appear to be the most reliable comparing with data for other homologs [26]. We could not find in the literature the normal *T*b values for higher alkyl diazoacetate homologs (with R ≥ C2H5) just due to their instability, but we can conclude that GC and/or GC–MS analysis of other diazocarbonyl


#### **Table 4.**

*Boiling points of some alkyl azides R-N3 and dialkyl diimides R-N=N-R at atmospheric pressure and their GC retention indices on standard non-polar polydimethylsiloxane stationary phases.*

compounds should be possible up to a temperatures of GC column approx. *T*lim ≈ 140°C. The applicability of this criterion has been verified during analysis of aryl substituted diazocarbonyl compounds [13]. For instance, such analyte as 1-diazo-4-phenylbuten-2-one, **C6H5CH2CH2COCHN2**, was analyzed under conditions ensuring

its retention temperature 145°C, slightly exceeding *T*lim ≈ 140°C. Exceeding the limit naturally leads to the appearance of a "plume" prior to the chromatographic peak of diazocompound that belongs to decomposition product, namely 4-phenyl-2-buten-1-one, **C6H5CH2CH2CH=C=O**.

Thus, the sense of the chemical criterion for GC and/or GC–MS analysis of thermally unstable compounds is revealing their simple homologs for which the reference information on their normal boiling points (at atmospheric pressure) is available. The general recommendation to avoid the decomposition of unstable analytes in a GC column is not to exceed the column's temperature above this limiting value.

The criterion considered can be re-formulated, if necessary. If some homologs of any class of potentially unstable compounds indicate stability at sufficiently high retention temperatures, we can consider these *T*R values as the limiting *T*lim values for other homologs of the same series, or their structural analogues. Such viewing allows evaluating the really anomalous thermal stability of organic hydroperoxides, **R-OOH**, and, especially, peroxides, **RO-OR**.

Improvements of contemporary fused quartz capillary columns in GC (increasing of their inertness) permits us to use them, for example, for separation of obviously unstable hydroperoxides formed from monoterpene hydrocarbons in plant essential oils [27], cyclohexyl- and cycloheptylhydroperoxides and corresponding dicyclohexyl- (**I**) and dicycloheptylperoxides (**II**) [28]. Most exotic structures with peroxide fragments which can be separated using GC without decomposition are 3,3,6,6-tetramethyl-1,2,4,5-tetraoxocyclohexane (trivial name "diacetone diperoxide", **III**), 3,3,6,6,9,9-hexamethyl-1,2,4,5,7,8-hexaoxocyclononane ("triacetone triperoxide", **IV**), and even 3,3,6,6-tetrapropyl-1,2,4,5-tetraoxocyclohexane (**V**) [29]:

The most notable are the high retention temperatures of mentioned compounds: *T*R value for dicyclohexylperoxide (**I**) is approx. 129°C, for dicycloheptylperoxide (**II**) ~ 180°C, for compound (**IV**) – 110-160°C (in different regimes), and for compound (**V**) – 185-260°C (**!**). This corresponds to the possibility of peroxides separation in almost any temperature regimes without risk of thermal decomposition of analytes and no needs for special control of separation conditions.

This conclusion was applied in the analysis of the unusual impurity found in the sample of benzyl alcohol **C6H5CH2OH** [30] with the retention index 1894 ± 10 (semi-standard non-polar stationary phase RTX-5 MS) and the following mass spectrum, *m*/*z* ≥ 39 (*I*rel ≥ 2%), **M** is the symbol of molecular ions:

230(1)**M**, 213(2), 198(2), 197(8), 107(14), 105(6), 92(23), 91(100), 79(3), 77(6), 51(2), 39(2).

*Features and New Examples of Gas Chromatographic Separation of Thermally Unstable… DOI: http://dx.doi.org/10.5772/intechopen.94229*

Attempts to identify this compound using the database [2] appeared to be unsuccessful. Nevertheless, the detailed interpretation of this mass spectrum together with GC retention index permits us to establish its structure unambiguously. The maximal signal with *m*/*z* 91 confirms the presence of benzyl fragment in the molecule, the peak with *m*/*z* 213 belongs to the ions [M – 17] ≡ [M – OH]+ , and the peak with *m*/*z* 197 – to the ions [M – 33] ≡ [M – OOH]<sup>+</sup> . Combining the available chemical and spectral information, we can attribute solely the structure of dibenzyl ether hydroperoxide for this impurity [30]:

The existence and stability of such hydroperoxide seem rather unusual. At first, the presence of two functional groups (one of them with active hydrogen) at one carbon atom looks like very "exotic" structure of unstable hydroperoxide of semiacetal. The second feature is high retention temperature of this impurity (approx. 190°C). Nevertheless, in accordance with criterion mentioned above so high *T*R value does not restrict GC separation of this compound without its thermal decomposition.

It is interesting to note that hydroperoxide of similar structure, C2H5OCH(OOH) CH3, formed from diethyl ether, (C2H5)2O, was detected by gas chromatographic analysis (RI 794) without its decomposition [31].

### **6. Conclusions**

Two new examples of thermal decomposition of analytes during GC separation within a gas chromatographic column were revealed and considered. First of them is monomolecular decomposition of monoalkyl phthalates with the formation of phthalic anhydride. The second process – decomposition of non-substituted hydrazones of carbonyl compounds – seems like the first example of bimolecular reactions of analytes in a GC column. However, the visual manifestations of all these processes on the chromatograms are identical to the manifestations of interconversion of isomers, for example, keto-enol transformations of ethyl ester of acetoacetic acid in a GC column. The profiles of chromatograms in most cases contain a peak of initial analyte, a peak of a product of its decomposition or isomerization, and more or less expressed diffused "plateau" or "train" between them. Besides that, the first example of secondary chemical reaction in a GC column, when the decomposition by-product reacts with other constituents of samples, is revealed for hydrazones of alkyl aryl ketones.

### **Acknowledgements**

Experimental data discussed in this work were determined by Lilia N. Fakhretdinova [20], Nikita E. Podol'ski [24], and Valentina M. Lukina [16] using the equipment of Resource Educational Centre "Chemistry" at the Institute for Chemistry of St. Petersburg State University. The author is grateful to the staff of this Center for assistance.

*Recent Advances in Gas Chromatography*

## **Author details**

Igor G. Zenkevich St. Petersburg State University, Institute for Chemistry, Universitetskii Prospect 26, St. Petersburg 198504, Russia

\*Address all correspondence to: izenkevich@yandex.ru

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Features and New Examples of Gas Chromatographic Separation of Thermally Unstable… DOI: http://dx.doi.org/10.5772/intechopen.94229*

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## **Chapter 4**
