*2.3.1. GC/MS analysis*

Chromatographic separation of the compounds investigated and registration of their "electron impact" mass spectra (EIMS) was done by use of a gas chromatograph HP 6890 Series GC System (Hewlett‐Packard, Palo Alto, CA, USA) equipped with HP 5973 Network quadrupole mass selective spectrometric detector (Agilent Technologies, Palo Alto, CA, USA). Each sample was dissolved in CHCl3 to 5,0% solution. Then 0.1 μL of that solution was injected by using Hamilton microsyringe to the split/splitless injector (split mode 100:1) kept at 350°C. The fused silica capillary column HP 50+ (30.0 m length, internal diameter 0.2 mm and 0.2 μm phase film thickness) was heated in the range of 70–290°C with programmed temperature ramp 7°C/min. As a carrier gas helium (ultra‐pure, 99,999%) was used.

GC/MS was applied to analyze the synthesized *cis* and *trans* isomers of di(n‐ and isononyl) esters of 1,2‐cyclohexanedicarboxylic acid in order to get their good chromatographic separa‐ tion enabling to record their electron impact (EI) mass spectra and also to calculate their arithmetic retention indices *IA* on the basis of their retention times *tr*.

**Figure 1** shows an example of a chromatogram of *cis* (compound *a*1) and *trans* (compound *a*2) of the di(3,5,5‐trimethylhexyl) esters of cyclohexane‐1,2‐dicarboxylic acid as the reaction products of di(3,5,5‐trimethylhexyl)phthalate hydrogenation. Values of the retention times *tr* of the *cis* and *trans* isomers of the esters are always lower than those of the corresponding phthalates.

**Figure 1.** GC/MS chromatogram of *cis* (*a*1) and *trans* (*a*2) isomers of di(3,5,5‐trimethylhexyl)cyclohexane‐1,2‐dicarboxy‐ late.

On the basis of the retention times *tr* of all analyzed *cis* and *trans* isomers and the retention times of a standard mixture of *n*‐alkanes C20–C40, the arithmetic retention indices (*IA*) were calculated using the following formulae (1) [13]:

$$I\_A = 100z + 100 \frac{T\_i - T\_z}{T\_{z+1} - T\_z} \tag{1}$$

where *Ti* , *Tz*, and *Tz* + 1 are the retention times of the analyzed component and neighboring *n*‐alkanes containing *z* and *z* + 1 carbon atoms, wherein *Tz* < *Ti* < *Tz* + 1

The obtained values of *IA* for the analyzed compounds are given in **Table 1**.

The linear relationship was found between the values of arithmetic retention indices *IA* of di(n‐ alkyl(C4–C9))phthalates and the number of carbon atoms present in the alkyl substituents of esters obtained during the hydrogenation of appropriate phthalates [12].

The values of *IA* of di(n‐alkyl(C4–C9)) phthalates are higher than those of their hydrogena‐ tion products. In both cases there is also a regularity according to which the esters with the longer alkyl substituents have the greater values of the arithmetic retention indices *IA*. Whereas in the case of the esters with branched alkyl chains of the substituents, their reten‐ tion times *tr* and arithmetic retention indices *IA* have lower values compared to those of the corresponding esters with straight chain substituents. The values of their retention times are arranged in the following order: *tra*1,*<sup>a</sup>*2 < *trb*1,*<sup>b</sup>*2 < *trc*1,*<sup>c</sup>*2 and, similarly, the arithmetic retention indices are arranged in the following order: *IA*1,*<sup>A</sup>*2 < *IB*1,*<sup>B</sup>*2 < *IC*1,*<sup>C</sup>*2.

of the *cis* and *trans* isomers of the esters are always lower than those of the corresponding

**Figure 1.** GC/MS chromatogram of *cis* (*a*1) and *trans* (*a*2) isomers of di(3,5,5‐trimethylhexyl)cyclohexane‐1,2‐dicarboxy‐

On the basis of the retention times *tr* of all analyzed *cis* and *trans* isomers and the retention times of a standard mixture of *n*‐alkanes C20–C40, the arithmetic retention indices (*IA*) were

100 100

The obtained values of *IA* for the analyzed compounds are given in **Table 1**.

esters obtained during the hydrogenation of appropriate phthalates [12].

*A*

*n*‐alkanes containing *z* and *z* + 1 carbon atoms, wherein *Tz* < *Ti*


*T T I z*

1

*i z*

*z z*

, *Tz*, and *Tz* + 1 are the retention times of the analyzed component and neighboring

*T T* (1)

< *Tz* + 1

+

The linear relationship was found between the values of arithmetic retention indices *IA* of di(n‐ alkyl(C4–C9))phthalates and the number of carbon atoms present in the alkyl substituents of

The values of *IA* of di(n‐alkyl(C4–C9)) phthalates are higher than those of their hydrogena‐ tion products. In both cases there is also a regularity according to which the esters with the longer alkyl substituents have the greater values of the arithmetic retention indices *IA*. Whereas in the case of the esters with branched alkyl chains of the substituents, their reten‐ tion times *tr* and arithmetic retention indices *IA* have lower values compared to those of the corresponding esters with straight chain substituents. The values of their retention times are

calculated using the following formulae (1) [13]:

230 New Advances in Hydrogenation Processes - Fundamentals and Applications

phthalates.

late.

where *Ti*

The obtained *tr* and *IA* reference GC parameters for the analyzed *cis* and *trans* isomers of the di(n‐ and isononyl)cyclohexane‐1,2‐dicarboxylates can be used for the unambiguous deter‐ mination of the chemical structure of this type of organic compounds, particularly when both are present in the reaction product of the hydrogenation of di(n‐ and isononyl)phthalic acid esters. They may also be very useful for ongoing optimization of the technological pa‐ rameters of the hydrogenation process using only the GC method.


**Table 1.** Retention times *tr* and arithmetic retention indices *IA* of di (n‐ and iso‐nonyl)cyclohexane‐1,2‐dicarboxylates.

The good chromatographic separation in the GC/MS analysis of the *cis* and *trans* isomers of di(n‐ and isononyl)cyclohexane‐1,2‐dicarboxylates enabled recording of their low‐resolution EI mass spectra, which are different for each isomer, and as such could be used for unambig‐ uous identification of their chemical structures.

**Table 2** gives the relative intensities of the most characteristic peaks of the molecular and fragment ions of all analyzed *cis* and *trans* isomers of di(n‐ and isononyl)cyclohexane‐1,2‐ dicarboxylates. In all mass spectra of these isomers there are low intensity peaks of their molecular ions [M]+• present at *m*/*z* 424, which allow to unambiguously determine the mo‐ lecular weights of these compounds. Also, their mass spectra have a number of peaks re‐ ferring to the characteristic fragment ions. The most intense characteristic peak of the fragmentation ion is easily recognized in the mass spectra of all analyzed esters (except the *cis* and *trans* isomers of di(3,5,5‐trimethylhexyl)cyclohexane‐1,2‐dicarboxylates—com‐ pounds *a*1 and *a*2). It occurs at *m*/*z* 155 and corresponds to the structure of the protonated anhydride of cyclohexane‐1,2‐dicarboxylic acid [11, 12]. However, in the mass spectra of the *cis* and *trans* isomers of compounds *a*1 and *a*2, in which isoalkyl substituents are more branched, as compared to other isomers—compounds *b*1, *b*2, *c*1, and *c*2 (**Tables 1** and **2**), the main peak corresponding to the ion at *m*/*z* 57 has the structure of [C(CH3)3] + . It is formed as a result of a homolytic cleavage of the single C–C bond located at the branched carbon atom of the alkyl substituent. The cleavage of the weaker C–C bond occurs more easily than in the other molecular ions [M]+• of the *cis* and *trans* isomers of the esters investigat‐ ed, and it is the reason of the formation of the less intense ion at *m*/*z* 155 (also formed from the [M−9H19] + ion as a result of the elimination of alkoxy radical •OC9H19 from the molecular ion [M]+• of these compounds). In general, the peak at *m*/*z* 155 may be used as a diagnostic peak for the unique identification of such type of esters, similarly as the peak at *m*/*z* 149, the main ion peak of di(alkyl)phthalates (except di(methyl)phthalate). The ion at *m*/*z* 149 has the structure of the protonated anhydride of phthalic acid [14–16].

the *cis* and *trans* isomers of di(3,5,5‐trimethylhexyl)cyclohexane‐1,2‐dicarboxylates—com‐ pounds *a*1 and *a*2). It occurs at *m*/*z* 155 and corresponds to the structure of the protonated anhydride of cyclohexane‐1,2‐dicarboxylic acid [11, 12]. However, in the mass spectra of the *cis* and *trans* isomers of compounds *a*1 and *a*2, in which isoalkyl substituents are more branched, as compared to other isomers—compounds *b*1, *b*2, *c*1, and *c*2 (**Tables 1** and **2**), the

as a result of a homolytic cleavage of the single C–C bond located at the branched carbon atom of the alkyl substituent. The cleavage of the weaker C–C bond occurs more easily than in the other molecular ions [M]+• of the *cis* and *trans* isomers of the esters investigat‐ ed, and it is the reason of the formation of the less intense ion at *m*/*z* 155 (also formed

molecular ion [M]+• of these compounds). In general, the peak at *m*/*z* 155 may be used as a diagnostic peak for the unique identification of such type of esters, similarly as the peak at *m*/*z* 149, the main ion peak of di(alkyl)phthalates (except di(methyl)phthalate). The ion

> **Isomer** *trans a***<sup>2</sup>**

at *m*/*z* 149 has the structure of the protonated anhydride of phthalic acid [14–16].

[M]+• 424 0.01 0.19 0.07 0.04 0.09 0.28

**Isomer** *cis a***1**

**Fragment ion** *m***/***z* **Relative intensities of fragment ions (%)**

+ ion as a result of the elimination of alkoxy radical •OC9H19 from the

**Isomer** *cis b***1**

281 11.20 4.35 3.28 3.62 20.74 9.88

252 0.42 3.47 0.03 0.23 1.18 10.00

155 49.83 5.70 100.00 100.00 100.00 100.00

**Isomer** *trans b***<sup>2</sup>** +

**Isomer** *cis c***1**

. It is formed

**Isomer** *trans c***<sup>2</sup>**

main peak corresponding to the ion at *m*/*z* 57 has the structure of [C(CH3)3]

232 New Advances in Hydrogenation Processes - Fundamentals and Applications

from the [M−9H19]


**Table 2.** Relative ion intensities [A1] of *cis* and *trans* isomers of di(n‐ and iso‐nonyl)cyclohexane‐1,2‐dicarboxylates in their mass spectra.

Other peaks present in all the MS spectra of the analyzed *cis* and *trans* isomers—compounds *a*1, *a*2, *b*1, *b*2, *c*1, and *c*2—correspond to the characteristic fragment ions formed by the cleavage of single C—C and CO bonds of their molecular ions [M]+•. These fragmentation reactions are accompanied by the transfer of hydrogen atoms in the McLafferty rearrangement together with an elimination of the neutral molecules of H2O and CO (**Table 2**).

**Figures 2a** and **2b** show the mass spectra of isomers *cis* and *trans* of di(3,5,5‐trimethylhex‐ yl)cyclohexane)‐1,2‐dicarboxylic acid.

The differences between EI mass spectra observed in all the analyzed compounds result both from different structures of the alkyl substituents of carboxyl groups of cyclohexane‐1,2‐ dicarboxylic acids and from *cis* and *trans* isomerization being the result of the presence of cyclohexane ring in these esters.

**Figure 2a.** Mass spectrum of compound *a*<sup>1</sup> (**Figure 1**): di(3,5,5‐trimethylhexyl)cyclohexane‐1,2‐dicarboxylate – *cis* iso‐ mer*.*

**Figure 2b.** Mass spectrum of compound a2 (**Figure 1**) di(3,5,5‐trimethylhexyl)cyclohexane‐1,2‐dicarboxylate – *trans* iso‐ mer.

#### *2.3.2. ESI/MS analysis*

Electrospray ionization, being a "soft" ionization technique, was used in mass spectrometry (ESI/MS) for the identification of volatile samples of di(3,5,5‐trimethylhexyl)‐, di(2‐methyloc‐ tyl)‐, and di(n‐nonyl)cyclohexane‐1,2‐dicarboxylates. Each ESI mass spectrum of the com‐ pound investigated represents a mixture of *cis* and *trans* isomers of the same compound, and for this reason it cannot be used for their individual identification.

In ESI/MS analysis the samples of di(n‐ and isononyl)cyclohexane‐1,2‐dicarboxylates were dissolved in methanol (HPLC grade, J.T. Baker) and diluted with the same solvent to 1:40,000 (which corresponds to ca. 10 μM). All measurements were performed on an AB Sciex Q‐TRAP® 4000 series hybrid quadrupole mass spectrometer equipped with electro‐ spray ion source. Collision gas was nitrogen at the nominal pressure 3.2 × 10−5 Torr.

The typical ESI mass spectrum of these compounds is shown in **Figure 3**. It presents a mix‐ ture of the fragment ion peaks of *cis* and *trans* isomers of di(3,5,5‐trimethylhexyl)cyclohex‐ ane‐1,2‐dicarboxylates and only has fewer peaks of mass fragmentation ions in comparison with the individual EI mass spectra (**Figures 2a** and **2b**).

In all the ESI mass spectra of these types of compounds, the peaks of quasi‐molecular ions [M+H]+ are present at *m*/*z* 425 and they are formed by the addition of hydrogen cation. They arise from both *cis* and *trans* isomers. More information about the fragment ions could be obtained from the interpretation of ESI mass spectra in which they derived from mass transitions: from [M+H]+ ions at *m*/*z* 281, after the cleavage of one of the unbranched or branched alkyl(C9) substituents and following neutral loss of H2O molecule leading to a fragment ion of *m*/*z* 155. The first transition is the most sensitive, and therefore it may be used to quantify di(n‐ and isononyl)cyclohexane‐1,2‐dicarboxylates. The second transition was used only to confirm the results of the first one.

**Figure 2a.** Mass spectrum of compound *a*<sup>1</sup> (**Figure 1**): di(3,5,5‐trimethylhexyl)cyclohexane‐1,2‐dicarboxylate – *cis* iso‐

234 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 2b.** Mass spectrum of compound a2 (**Figure 1**) di(3,5,5‐trimethylhexyl)cyclohexane‐1,2‐dicarboxylate – *trans* iso‐

Electrospray ionization, being a "soft" ionization technique, was used in mass spectrometry (ESI/MS) for the identification of volatile samples of di(3,5,5‐trimethylhexyl)‐, di(2‐methyloc‐ tyl)‐, and di(n‐nonyl)cyclohexane‐1,2‐dicarboxylates. Each ESI mass spectrum of the com‐ pound investigated represents a mixture of *cis* and *trans* isomers of the same compound, and

In ESI/MS analysis the samples of di(n‐ and isononyl)cyclohexane‐1,2‐dicarboxylates were dissolved in methanol (HPLC grade, J.T. Baker) and diluted with the same solvent to 1:40,000 (which corresponds to ca. 10 μM). All measurements were performed on an AB

for this reason it cannot be used for their individual identification.

mer*.*

mer.

*2.3.2. ESI/MS analysis*

The choice of selected characteristic ions for this type of compounds, for example, with an *m*/*z* of 155, present in other MS spectra of di[n‐ and isoalkyl(C4–C9)]cyclohexane‐1,2‐dicar‐ boxylates, may be very helpful for their detection during ESI/MS analysis of one or more of the investigated compounds in complex matrices (e.g., PVC) and may also be very useful for quantitative assessment of the level of their actual impact on the human health.

**Figure 4** shows a general mass fragmentation scheme of molecular ions [M]+ of di(n‐ and isononyl)cyclohexane‐1,2‐dicarboxylates developed on the basis of the data obtained from their mass transitions. It describes a specific type of fragmentation reactions for this type of compounds as contrasted to the phthalic acid esters. The basic knowledge about their behavior during ionization of this type of compounds will make the interpretation of their mass spectra easier and will enable optimization of the methods of their quantification during the analysis of their release from polymers.

The good chromatographic separation during GC/MS analysis of *cis* and *trans* isomers of di(n‐ and isononyl)cyclohexane‐1,2‐dicarboxylates investigated enables the recording of their low‐ resolution EI mass spectra, which are different from each other and thus can be used for unambiguous identification of their individual chemical structures. Also, chromatographic data, such as the values of retention times *tr* and arithmetic retention indices *IA*, are very useful in their identification, even when the reference substances are not available. The ESI/MS mode was shown to be successful in the determination of *cis* and *trans* isomers of the analyzed esters present in complex matrices of a polymer.

The presented GC, GC/MS, and ESI/MS results for the representatives of the *cis* and *trans* isomers of some of the DINCH group of compounds may also be helpful in the determination of the chemical structures of their metabolites.

**Figure 3.** ESI mass spectrum of *cis* and *trans* isomers of di(3,5,5‐ trimethylhexyl)cyclohexane‐1,2‐dicarboxylate.

**Figure 4.** General mass fragmentation scheme of molecular ions [M]+ of di(n‐ and isononyl)cyclohexane‐1,2‐dicarboxy‐ lates.
