**2.4 Stir bar sorptive extraction (SBSE)**

Stir bar sorptive extraction (SBSE) is used for the extraction of trace amounts of organics from aqueous food, environmental, and biological samples. A stir bar has been covered with a sorbent phase and placed into the sample solution to separate the analyte of interest. Although SBSE procedures are not exhaustive, more quantitative extractions can be achieved than those of SPME procedures.

The coated stir bar is usually used to stir the sample solution for a specialized period of time, depending on the sample volume and stirring speed, until approaching equilibrium.

SBSE improves on the low concentration capability of in-sample solid-phase microextraction (IS-SPME). Also, SBSE can be applied to headspace sorptive extraction (HSSE) [2].

Some of SBSE applications with GC analysis are discussed below.

Nakamura et al. studied simultaneous determination of alkylphenols and bisphenol A in river water by stir bar sorptive extraction with in situ acetylation and thermal desorption-gas chromatography-mass spectrometry. In this study, SBSE was used for the sample enrichment of seven alkylphenols and bisphenol A in river water. Also, in situ derivatization in aqueous samples was performed with acetic acid anhydride as acetylation reagent [18].

The extraction phase on the stir bar in SBSE is critical for the performance of both extraction and thermal desorption. The sol-gel coating technology possesses the potential to prepare thermally stable coatings [19].

Guan et al. studied determination of organophosphorus pesticides in cucumber and potato by stir bar sorptive extraction. In this study, organophosphorus pesticides (OPPs) in vegetables were determined by SBSE and capillary GC with thermionic specific detection (GC-TSD). Hydroxy-terminated polydimethylsioxane (PDMS) prepared by sol-gel method was used as extraction phase [19].

#### **2.5 Soxhlet extraction**

Soxhlet extraction was accepted as a standard method for the extraction of semivolatile and nonvolatile organics by the US Environmental Protection Agency (EPA 3540C0) and also the extraction of fat in cacao products by the Association of Official Analytical Chemists (AOAC 963.15). Soxhlet extraction was *introduced by Franz Ritter von Soxhlet in 1879*. It had been the most extensive applied technique

**Figure 1.** *The schematic diagram of Soxhlet apparatus.*

till the other modern extraction methods were developed in the 1980s. Nowadays, Soxhlet is still applied for the extraction of semivolatile organic compounds from solid samples. Soxhlet extraction is a classical method which is operated under atmospheric pressure, in high temperature or under ultrasonic irradiation. In this technique, relatively large volumes of organic solvents are usually used, and it is a time-consuming technique [2].

Soxhlet apparatus has three components, and its schematic diagram is shown in **Figure 1** [2]:


The properties of Soxhlet extraction are as follows [2]:


**55**

*Sample Preparation Techniques for Gas Chromatography DOI: http://dx.doi.org/10.5772/intechopen.84259*

4.Soxhlet is a rugged and well-established technique.

5.Relatively large solvent consumption (its drawback).

more rapid, extraction here is faster than in Soxhlet.

tional Soxhlet and takes usually 60 min.

GC-electron capture detector and MS detector [20].

separate Soxhlet units.

in three stages:

for 10–20 min.

**2.6 Ultrasonic extraction**

samples should be investigated.

cleanup is needed before analysis [2].

compounds.

3.The sample size is often *10 g* or more, and multiple samples can be extracted on

An automated Soxhlet extraction (Soxtec) was approved by the EPA (EPA 3541) in 1994 for the extraction of semivolatile and nonvolatile organic compounds [2]. Automated Soxhlet extraction is relatively faster than Soxhlet extraction, with lower consuming organic solvents [2]. In this method, the extraction is performed

• In the first stage, a thimble containing the sample is immersed in the boiling solvent for about 60 min. Since the contact between the solvent and the sample is more vigorous and the mass transfer in a high-temperature boiling solvent is

• In the second stage, the sample thimble is placed above the boiling solvent. Then, the condensed solvent drips into the sample and extracts the organics and falls back into the solvent reservoir as well. This stage is similar to tradi-

• In the third stage, the solvent is evaporated, and a concentration step happens

Li et al. studied the determination of organochlorine pesticide residue in ginseng

root by orthogonal array design Soxhlet extraction and gas chromatography. In this study, a method involving four-factor-three-level orthogonal array design was developed. The orthogonal array designs included extracting solvent component, particle size, solvent overflow recycle, and time needed for the optimization of extracting nine organochlorine pesticides from ginseng root, followed by capillary

Ultrasonic extraction, also known as sonication, uses ultrasonic vibration to ensure intimate contact between the sample and the solvent. Sonication is relatively fast, but the extraction efficiency is not as high as some other techniques. Also, ultrasonic irradiation may decompose some of organophosphorus

Before the sonication is used for real sample, the selected solvent system and optimum conditions for adequate extraction of the target analytes from reference

A typical sonication device can be equipped with a titanium tip. The sample is usually dried with anhydrous sodium sulfate and mixed with a certain volume of selected solvent. The disruptor horn tip is positioned just below the surface of the solvent, yet above the sample. Extraction can be carried out in duration as short as 3 min. After extraction, the extract is filtered or centrifuged, and also some form of

The ultrasonic extraction (USE) is a very versatile technique due to the possibility of selecting the solvent type or solvent mixture that allows the maximum extraction efficiency and selectivity. In USE, several extractions can be done

*Gas Chromatography - Derivatization, Sample Preparation, Application*

till the other modern extraction methods were developed in the 1980s. Nowadays, Soxhlet is still applied for the extraction of semivolatile organic compounds from solid samples. Soxhlet extraction is a classical method which is operated under atmospheric pressure, in high temperature or under ultrasonic irradiation. In this technique, relatively large volumes of organic solvents are usually used, and it is a

Soxhlet apparatus has three components, and its schematic diagram is shown in

2.The middle part is a thimble holder with a siphon device and a side tube.

3.The bottom part is a round-bottomed flask which connects to the thimble

4.A porous cellulous sample thimble is filled with sample solution and inserted into the sample thimble holder. Usually, 300 ml of solvents is introduced to flask for 10 g of a sample. The flask is heated slowly on a heating mantle, and the solvent vapor goes toward the reflux condenser and, after condensing, drips back to the thimble chamber. When the analyte reaches the top of the sample thimble holder, it is transferred back into the bottom flask via a siphon device. This cycle is repeated many times for a predetermined period of time. Since the boiling points of analytes are usually higher than those of solvents, the analytes accumulate in the flask and the solvents recirculate. Finally in each

1.In Soxhlet extraction, the extraction is slow and can take between 6 and 48 h. On the other hand, it is a time-consuming technique (its drawback). It is mainly due to the analyte that is extracted with cooled condensed solvent.

2.The extract volume is relatively large (its drawback). So, the evaporation step

is usually needed to concentrate the analytes before the analysis.

time-consuming technique [2].

*The schematic diagram of Soxhlet apparatus.*

1.The top part is a solvent vapor reflux condenser.

cycle, the analyte can be extracted with fresh solvents.

The properties of Soxhlet extraction are as follows [2]:

**Figure 1** [2]:

**Figure 1.**

holder.

**54**


An automated Soxhlet extraction (Soxtec) was approved by the EPA (EPA 3541) in 1994 for the extraction of semivolatile and nonvolatile organic compounds [2]. Automated Soxhlet extraction is relatively faster than Soxhlet extraction, with lower consuming organic solvents [2]. In this method, the extraction is performed in three stages:


Li et al. studied the determination of organochlorine pesticide residue in ginseng root by orthogonal array design Soxhlet extraction and gas chromatography. In this study, a method involving four-factor-three-level orthogonal array design was developed. The orthogonal array designs included extracting solvent component, particle size, solvent overflow recycle, and time needed for the optimization of extracting nine organochlorine pesticides from ginseng root, followed by capillary GC-electron capture detector and MS detector [20].

### **2.6 Ultrasonic extraction**

Ultrasonic extraction, also known as sonication, uses ultrasonic vibration to ensure intimate contact between the sample and the solvent. Sonication is relatively fast, but the extraction efficiency is not as high as some other techniques. Also, ultrasonic irradiation may decompose some of organophosphorus compounds.

Before the sonication is used for real sample, the selected solvent system and optimum conditions for adequate extraction of the target analytes from reference samples should be investigated.

A typical sonication device can be equipped with a titanium tip. The sample is usually dried with anhydrous sodium sulfate and mixed with a certain volume of selected solvent. The disruptor horn tip is positioned just below the surface of the solvent, yet above the sample. Extraction can be carried out in duration as short as 3 min. After extraction, the extract is filtered or centrifuged, and also some form of cleanup is needed before analysis [2].

The ultrasonic extraction (USE) is a very versatile technique due to the possibility of selecting the solvent type or solvent mixture that allows the maximum extraction efficiency and selectivity. In USE, several extractions can be done

simultaneously, and no specialized laboratory equipment is required (advantage). But it is not easily automated (disadvantage) [21].

Goncalves et al. studied the assessment of pesticide contamination in soil samples from an intensive horticulture area, using ultrasonic extraction and gas chromatography-mass spectrometry. In this study, the application of an USE method combined with GC and GC-MS for the analysis of some pesticides in soil samples was investigated. The USE technique was used to separate the pesticides from the soil samples [21].

#### **2.7 Supercritical fluid extraction (SFE)**

In supercritical fluid extraction (SFE), supercritical fluids possess specific properties which make them facilitate the extraction of organics from solid samples. Two configuration of SFE operations are on- or off-line mode. In the online operation, SFE is matched directly to an analytical instrument like GC, supercritical fluid chromatography (SFC), and HPLC. Off-line SFE, as its name implies, is a standalone extraction method independent of the analytical method to be applied. Offline SFE is more flexible and easier to perform than that of the online procedure. It allows the Extract to be available for analysis by different techniques [2].

A supercritical fluid (SF) is a substance above its critical temperature and pressure. Also, it is an interface between gas and liquid. In fact it is not a liquid and or a gas, it is a SF.

CO2 has a low supercritical temperature (31°C) and pressure (73 atm). It is nontoxic and nonflammable and also is available at high purity. So, carbon dioxide has become the solvent of interest for most SFE applications. Supercritical CO2 is nonpolar and without permanent dipole moment; therefore, it can be utilized to extract nonpolar and moderately polar compounds from matrices. For the extraction of polar compounds, supercritical N2O and CHClF2 are more efficient. But these SFs are not environmentally friendly and they are not used in routine analysis [2].

SFE has gained increased attention as a good candidate instead of conventional liquid solvent extraction. This is mainly due to significant properties of supercritical fluids (SFs) such as their high diffusivity and low viscosity which make them extract selectively different chemicals without additional cleanup steps and so use little sample amounts [22].

Rissato et al. studied the supercritical fluid extraction for pesticide multiresidue analysis in honey and determination by gas chromatography with electron-capture and mass spectrometry detection. In this study, SFE procedure was used to separate some pesticides from honey samples, and it was compared with liquid-liquid extraction method [22].

#### **2.8 Accelerated solvent extraction (ASE)**

The other names of accelerated solvent extraction (ASE) are pressurized fluid extraction (PFE) and pressurized liquid extraction (PLE). Conventional solvents are used in ASE at high temperature (100–180°C) and pressure (1500–2000 psi) to increase the extraction percentage of organic compounds from solid samples.

Supercritical fluid extraction is matrix dependent and usually needs the addition of organic modifiers. ASE was developed to overcome these limitations. Although it was expected that conventional solvents would be less efficient than supercritical fluids, the results turned out to be quite the opposite. In many cases, extraction was faster and more complete with organic solvents at elevated temperature and pressure than with SFE [2].

The elevated pressure and temperature used in ASE affect the solvent and sample properties and their interactions as well. ASE properties include the following [2]:

**57**

samples.

*Sample Preparation Techniques for Gas Chromatography DOI: http://dx.doi.org/10.5772/intechopen.84259*

dipole, hydrogen, and van der Waals interactions.

ASE process has some steps mentioned below:

2.Then, the solvent is entered in.

1500 psi and a total time of 10 min [23].

**2.9 Microwave-assisted extraction**

1.The extraction cell is filled with the sample medium.

enhances solvent penetration into the matrix medium.

from the matrix.

ASE procedures.

to 200°C).

enhanced.

1.Under higher pressure, the extraction would be performed at higher temperature values. This is mainly due to the increase of the solvent boiling point.

2.At higher pressures, the solvent penetration into the sample medium would be increased, and so the extraction of the interested analyte may be facilitated

3.At higher temperatures, the mass transfer and solubility of the analyte are

4.The elevated temperature can reduce the power of analyte-sample bonds like

5.High temperature decreases the solvent viscosity and surface tension and so

6.Therefore, faster extractions and better analyte recoveries can be achieved by

3.And, the cell temperature and pressure are increased to the desired level. The necessary time to enhance the temperature can be between 5 and 9 min (for up

The above steps are referred to the prefill method. If before addition of solvent the sample is warmed, the process is mentioned as preheat method. In comparison of the two procedures with each other, the prefill method is usually preferred [2]. A. Pastor et al. studied the determination of PAHs in airborne particles by accelerated solvent extraction and large-volume injection-gas chromatographymass spectrometry. The procedure included extraction of some PAHs by accelerated solvent extraction (ASE) followed by gel permeation chromatography (GPC) cleanup and GC-MS detection of PAHs. In this study, the hexane-acetone mixture (1:1 v/v) gave the best recoveries when ASE parameters were fixed at 125°C and

It should be noted that microwave-assisted extraction (MAE) is different from microwave-assisted acid digestion. The former uses organic solvents to extract organic compounds from solids, while the latter uses acids to dissolve the sample for elemental analysis with the organic contents being destroyed. MAE is applied for the extraction of semivolatile and nonvolatile compounds from solid

In general, organic extraction and acid digestion use different types of microwave apparatuses, as these two processes require different reagents and experimental conditions. The basic components of a microwave system include a microwave generator (magnetron), a waveguide for transmission, a resonant cavity, and a

power supply. There are two types of laboratory microwave units:

*Gas Chromatography - Derivatization, Sample Preparation, Application*

But it is not easily automated (disadvantage) [21].

**2.7 Supercritical fluid extraction (SFE)**

gas, it is a SF.

little sample amounts [22].

extraction method [22].

pressure than with SFE [2].

**2.8 Accelerated solvent extraction (ASE)**

simultaneously, and no specialized laboratory equipment is required (advantage).

USE technique was used to separate the pesticides from the soil samples [21].

allows the Extract to be available for analysis by different techniques [2].

In supercritical fluid extraction (SFE), supercritical fluids possess specific properties which make them facilitate the extraction of organics from solid samples. Two configuration of SFE operations are on- or off-line mode. In the online operation, SFE is matched directly to an analytical instrument like GC, supercritical fluid chromatography (SFC), and HPLC. Off-line SFE, as its name implies, is a standalone extraction method independent of the analytical method to be applied. Offline SFE is more flexible and easier to perform than that of the online procedure. It

A supercritical fluid (SF) is a substance above its critical temperature and pressure. Also, it is an interface between gas and liquid. In fact it is not a liquid and or a

SFE has gained increased attention as a good candidate instead of conventional liquid solvent extraction. This is mainly due to significant properties of supercritical fluids (SFs) such as their high diffusivity and low viscosity which make them extract selectively different chemicals without additional cleanup steps and so use

Rissato et al. studied the supercritical fluid extraction for pesticide multiresidue analysis in honey and determination by gas chromatography with electron-capture and mass spectrometry detection. In this study, SFE procedure was used to separate some pesticides from honey samples, and it was compared with liquid-liquid

The other names of accelerated solvent extraction (ASE) are pressurized fluid extraction (PFE) and pressurized liquid extraction (PLE). Conventional solvents are used in ASE at high temperature (100–180°C) and pressure (1500–2000 psi) to increase the extraction percentage of organic compounds from solid samples.

Supercritical fluid extraction is matrix dependent and usually needs the addition of organic modifiers. ASE was developed to overcome these limitations. Although it was expected that conventional solvents would be less efficient than supercritical fluids, the results turned out to be quite the opposite. In many cases, extraction was faster and more complete with organic solvents at elevated temperature and

The elevated pressure and temperature used in ASE affect the solvent and sample properties and their interactions as well. ASE properties include the following [2]:

CO2 has a low supercritical temperature (31°C) and pressure (73 atm). It is nontoxic and nonflammable and also is available at high purity. So, carbon dioxide has become the solvent of interest for most SFE applications. Supercritical CO2 is nonpolar and without permanent dipole moment; therefore, it can be utilized to extract nonpolar and moderately polar compounds from matrices. For the extraction of polar compounds, supercritical N2O and CHClF2 are more efficient. But these SFs are not environmentally friendly and they are not used in routine analysis [2].

Goncalves et al. studied the assessment of pesticide contamination in soil samples from an intensive horticulture area, using ultrasonic extraction and gas chromatography-mass spectrometry. In this study, the application of an USE method combined with GC and GC-MS for the analysis of some pesticides in soil samples was investigated. The

**56**


ASE process has some steps mentioned below:

1.The extraction cell is filled with the sample medium.

2.Then, the solvent is entered in.

3.And, the cell temperature and pressure are increased to the desired level. The necessary time to enhance the temperature can be between 5 and 9 min (for up to 200°C).

The above steps are referred to the prefill method. If before addition of solvent the sample is warmed, the process is mentioned as preheat method. In comparison of the two procedures with each other, the prefill method is usually preferred [2].

A. Pastor et al. studied the determination of PAHs in airborne particles by accelerated solvent extraction and large-volume injection-gas chromatographymass spectrometry. The procedure included extraction of some PAHs by accelerated solvent extraction (ASE) followed by gel permeation chromatography (GPC) cleanup and GC-MS detection of PAHs. In this study, the hexane-acetone mixture (1:1 v/v) gave the best recoveries when ASE parameters were fixed at 125°C and 1500 psi and a total time of 10 min [23].

## **2.9 Microwave-assisted extraction**

It should be noted that microwave-assisted extraction (MAE) is different from microwave-assisted acid digestion. The former uses organic solvents to extract organic compounds from solids, while the latter uses acids to dissolve the sample for elemental analysis with the organic contents being destroyed. MAE is applied for the extraction of semivolatile and nonvolatile compounds from solid samples.

In general, organic extraction and acid digestion use different types of microwave apparatuses, as these two processes require different reagents and experimental conditions. The basic components of a microwave system include a microwave generator (magnetron), a waveguide for transmission, a resonant cavity, and a power supply. There are two types of laboratory microwave units:


In the liquid and solid states, molecules do not rotate freely in the microwave field, despite of gaseous molecules; therefore, no microwave spectra can be observed. Liquid- and solid-state molecules respond to the radiation differently, and this is where microwave heating comes in. During microwave heating procedure, electromagnetic energy would be changed to heat. This is mainly due to the ionic conduction and dipole rotation of the molecules which are imposed. Ionic conduction is concluded from the ion mobility in a solution under an electromagnetic field, and then, the heat is produced. Dipole rotation means that the directions of dipole rotations are changed under microwave irradiation. When a polarized molecule is imposed in an electromagnetic field, it can rotate around its axis at a rate of 4.9 × 109 times per second. So, with the larger molecular dipole moments, the more vigorous oscillations of molecules are obtained under a microwave field.

The proper choice of solvent is the key to successful extraction in MAE. In general, three types of solvent system can be used in MAE:

1.Solvent(s) of high dielectric coefficient.


Zhou et al. studied the microwave-assisted extraction followed by gas chromatography-mass spectrometry for the determination of endocrine-disrupting chemicals in river sediments. In this study, the most efficient extraction (>74%) of the analyte was achieved by choosing methanol as the solvent, 110°C and 15 min, as the extraction temperature and time, respectively. The cleanup step was performed by passing the extracts through a non-deactivated silica gel column [24].

#### **2.10 Headspace extraction**

From an analytical point of view, volatile organic compounds (VOCs) are organic materials whose vapor pressures are greater than or equal to 0.1 mmHg at 20°C. Many VOCs are environmental pollutants, and in most cases of their analyses, the analytes are transferred to a gas-vapor phase and then analyzed by GC techniques [2].

Generally, the analysis of pure volatile compounds is simple, and the volatile analyte can be injected directly into a GC column [25]. However, the challenge is to extract the analytes from the matrix samples such as soil, food, cosmetics, polymers, and pharmaceutical raw materials. Headspace extractions are approaches to this and are divided into two categories: static headspace extraction (SHE) and dynamic headspace extraction (purge and trap) [2].

Static headspace extraction is known as equilibrium headspace extraction or simply as headspace. This technique has been available more than 30 years, so its instrumentation is both mature and reliable. In this technique, the extraction method includes the following [2]:

**59**

*Sample Preparation Techniques for Gas Chromatography DOI: http://dx.doi.org/10.5772/intechopen.84259*

volatile analytes diffuse into the headspace vessel.

drop between the vessel and the GC inlet pressure.

be repeated by the same or the next vial.

good sensitivity and accuracy.

form of the matrix itself.

1.A purge vessel.

2.A sorbent trap.

3.A six-port valve.

4.Transfer lines.

solid sample and to disperse it into a liquid.

2.The sample vial is brought to a constant temperature and pressure, and the

3.When the analyte concentration in the headspace part of the vessel reaches to an equilibrium level with respect to its concentration in the sample, the vial is connected to the GC column head, and then, a portion of the headspace is introduced into a GC for detection. This analyte transfer is due to a pressure

4.The vial is again isolated. For automated systems, this sampling procedure can

The advantage of static headspace extraction is the ease of initial sample preparation. Usually for qualitative analysis, the sample can be placed directly into the headspace vial and analyzed with no additional preparation procedures. But for quantitative analysis, it may be vital to know the optimized matrix effects to gain

For large solid samples, it may be needed to change the physical state of the sample matrix. Two approaches in differentiating the sample state are to powder the

By crushing the solid sample, the surface area available for the volatile solute to distribute into the headspace phase is enhanced. So, the solute is distributing between a solid and the headspace phases. But in the second procedure, dispersing the solid into a liquid is preferred because the analyte partitioning process into the headspace often reaches the equilibrium faster. Therefore, by choosing a suitable solvent with high affinity toward the volatile analytes, the problems with sample and standard transfer from volumetric flask to headspace vials can be eliminated [2]. Some experimental factors affecting SHE should be optimized to improve extraction efficiency, sensitivity, quantitation, and reproducibility. These experimental variables include vial and sample volume, temperature, pressure, and the

For the analysis of trace amounts of analytes, or where an exhaustive extraction of the analyte is required, purge and trap or dynamic headspace extraction (DHE) is more preferred than SHE. This technique is used for both solid and liquid samples. The samples can be biological, environmental, industrial, pharmaceutical, and agricultural. In DHE, there is no equilibrium between its concentration in the gas and matrix phases. Instead, they are removed continuously from the sample by a gas flow. This provides a concentration gradient between two mentioned phases

A purge and trap cycle consists of several steps: (1) purge, (2) dry purge, (3) desorb preheat, (4) desorb, and (5) trap bake. Each step is synchronized with the operation of the six-port valve and the GC [or GC-MS (mass spectrometer)]. The

mentioned steps in a purge and trap cycle can be explained as follows:

which makes the exhaustive extraction of the volatile analytes. A typical purge and trap system consists of the following:

1.A sample, either solid or liquid, is put in a headspace autosampler (HSAS) or vial.

*Gas Chromatography - Derivatization, Sample Preparation, Application*

In the liquid and solid states, molecules do not rotate freely in the microwave

the more vigorous oscillations of molecules are obtained under a microwave field. The proper choice of solvent is the key to successful extraction in MAE. In

3.A microwave transparent solvent used with a sample of high dielectric

by passing the extracts through a non-deactivated silica gel column [24].

Zhou et al. studied the microwave-assisted extraction followed by gas chromatography-mass spectrometry for the determination of endocrine-disrupting chemicals in river sediments. In this study, the most efficient extraction (>74%) of the analyte was achieved by choosing methanol as the solvent, 110°C and 15 min, as the extraction temperature and time, respectively. The cleanup step was performed

From an analytical point of view, volatile organic compounds (VOCs) are organic materials whose vapor pressures are greater than or equal to 0.1 mmHg at 20°C. Many VOCs are environmental pollutants, and in most cases of their analyses, the analytes are transferred to a gas-vapor phase and then analyzed by GC

Generally, the analysis of pure volatile compounds is simple, and the volatile analyte can be injected directly into a GC column [25]. However, the challenge is to extract the analytes from the matrix samples such as soil, food, cosmetics, polymers, and pharmaceutical raw materials. Headspace extractions are approaches to this and are divided into two categories: static headspace extraction (SHE) and

Static headspace extraction is known as equilibrium headspace extraction or simply as headspace. This technique has been available more than 30 years, so its instrumentation is both mature and reliable. In this technique, the extraction

1.A sample, either solid or liquid, is put in a headspace autosampler

times per second. So, with the larger molecular dipole moments,

field, despite of gaseous molecules; therefore, no microwave spectra can be observed. Liquid- and solid-state molecules respond to the radiation differently, and this is where microwave heating comes in. During microwave heating procedure, electromagnetic energy would be changed to heat. This is mainly due to the ionic conduction and dipole rotation of the molecules which are imposed. Ionic conduction is concluded from the ion mobility in a solution under an electromagnetic field, and then, the heat is produced. Dipole rotation means that the directions of dipole rotations are changed under microwave irradiation. When a polarized molecule is imposed in an electromagnetic field, it can rotate around its axis at a

1.Closed extraction vessels under elevated pressure.

general, three types of solvent system can be used in MAE:

2.A mixture of solvents of high and low dielectric coefficient.

1.Solvent(s) of high dielectric coefficient.

dynamic headspace extraction (purge and trap) [2].

method includes the following [2]:

(HSAS) or vial.

2.Open vessels under atmospheric pressure.

rate of 4.9 × 109

coefficient.

**2.10 Headspace extraction**

techniques [2].

**58**


The advantage of static headspace extraction is the ease of initial sample preparation. Usually for qualitative analysis, the sample can be placed directly into the headspace vial and analyzed with no additional preparation procedures. But for quantitative analysis, it may be vital to know the optimized matrix effects to gain good sensitivity and accuracy.

For large solid samples, it may be needed to change the physical state of the sample matrix. Two approaches in differentiating the sample state are to powder the solid sample and to disperse it into a liquid.

By crushing the solid sample, the surface area available for the volatile solute to distribute into the headspace phase is enhanced. So, the solute is distributing between a solid and the headspace phases. But in the second procedure, dispersing the solid into a liquid is preferred because the analyte partitioning process into the headspace often reaches the equilibrium faster. Therefore, by choosing a suitable solvent with high affinity toward the volatile analytes, the problems with sample and standard transfer from volumetric flask to headspace vials can be eliminated [2]. Some experimental factors affecting SHE should be optimized to improve extraction efficiency, sensitivity, quantitation, and reproducibility. These experimental variables include vial and sample volume, temperature, pressure, and the form of the matrix itself.

For the analysis of trace amounts of analytes, or where an exhaustive extraction of the analyte is required, purge and trap or dynamic headspace extraction (DHE) is more preferred than SHE. This technique is used for both solid and liquid samples. The samples can be biological, environmental, industrial, pharmaceutical, and agricultural. In DHE, there is no equilibrium between its concentration in the gas and matrix phases. Instead, they are removed continuously from the sample by a gas flow. This provides a concentration gradient between two mentioned phases which makes the exhaustive extraction of the volatile analytes.

A typical purge and trap system consists of the following:


A purge and trap cycle consists of several steps: (1) purge, (2) dry purge, (3) desorb preheat, (4) desorb, and (5) trap bake. Each step is synchronized with the operation of the six-port valve and the GC [or GC-MS (mass spectrometer)]. The mentioned steps in a purge and trap cycle can be explained as follows:


The trap is usually a stainless steel tube 3 mm in inside diameter (ID) and 25 mm long packed with multiple layers of adsorbents, and it should do the following steps:


The sorbents are often arranged in layers to increase the trapping capacity. During purging process, the purge gas reaches the weaker sorbent at first, and only less volatile organics are retained. But more volatile compounds just pass through this layer and then are trapped by the other stronger adsorbent layers. During

**61**

*2.10.1 Interfacing purge and trap*

*Sample Preparation Techniques for Gas Chromatography DOI: http://dx.doi.org/10.5772/intechopen.84259*

adsorbents, and so, the reversible adsorptions can be achieved.

desorption process, the trap is heated and back-flushed with the GC carrier gas. However, the less volatile compounds have never been in contrast with the stronger

To trap volatile organic compounds, the substances such as Tenax, silica gel, activated charcoal, graphitized carbon black (GCB or Carbopack), carbon molecular sieves (Carbosieve), and Vocarb are usually used [2]. Tenax is not only a porous but also a hydrophobic polymer resin based on 2,6-diphenylene oxide, with low affinity for water. So, highly volatile and polar compounds are seldom adsorbed on Tenax. Tenax should not be heated to temperatures upper than 200°C, because of its decomposition under high temperatures. The two types of Tenax are Tenax TA and Tenax GC. The former has higher purity and is more preferred for trace analysis. Silica gel is hydrophilic and is an excellent candidate for trapping polar compounds. Also, it is a stronger sorbent than Tenax. The problem is that water can be retained on the gel. Charcoal, as another stronger sorbent than Tenax, is hydrophobic and is mainly used to trap very volatile compounds such as dichlorodifluoromethane, a.k.a. Freon 12. These compounds can break through Tenax and silica gel. Conventional traps like Tenax, silica gel, and charcoal are usually used in series. If the boiling points of the analytes are above 35°C, Tenax itself will be suitable, and so, silica gel and charcoal can be ignored. Graphitized carbon black (GCB), as an alternative sorbent to charcoal and silica gel, has both the hydrophobic property and the trapping capacity similar to Tenax. Also, it is often used along with carbomolecular sieves and can trap highly volatile compounds. Vocarb is a highly hydrophobic activated carbon which can diminish water trapping and be purged fast. Vocarb is usually operated with an ion-trap mass spectrometer, which can be affected by trace levels of water or methanol. GCB, carbon molecular sieves, and Vocarb possess high thermal stability and can be operated at higher desorption temperatures than those that Tenax can be done [2]. The transfer line between the trap and the GC column is often made of nickel, deactivated fused silica, and silica-lined stainless steel tubing. By using these inert materials, the active sites which can interact with the analytes are eliminated. On the other hand, the transfer line is kept at a temperature higher than 100°C to avoid the condensation of water and the volatile organics. Also, the six-port valve which controls the gas flow path is also heated above 100°C to avoid condensation.

As noted above, the operational conditions of purge and trap must be adaptable with the GC system configuration. For example, megabore capillary columns (0.53 mm ID or larger) are typically used at a flow rate of 8–15 ml/min. Since desorption process is slower at such flow rates, the column is usually cooled to 10°C or less temperatures at the stating of the GC run to retain the very volatile compounds. Sub-ambient cooling may be eliminated by using a long column (60–105 m) with a thick film stationary phase (3–5 μm). However, this flow rate is still too high for a GC-MS analyzer. So, a GC-MS interface like a jet separator or an open-split interface should be applied to decrease the carrier flow rate in the mass detector. However, an open-split interface makes a reduction in the analytical

Narrow-bore capillary columns (0.32 mm ID or smaller) with MS detector are commonly operated at lower flow rates (less than 5 ml/min). There are two ways to

1.To desorb the trap at a high flow rate and, then, with a split injector, split the flow into the GC instrument. So, a fast injection is obtained without signifi-

sensitivity due to entering just a portion of analyte into the detector [2].

couple purge and trap with this type of columns:

cant loosing of the analytical sensitivity.

#### *Sample Preparation Techniques for Gas Chromatography DOI: http://dx.doi.org/10.5772/intechopen.84259*

*Gas Chromatography - Derivatization, Sample Preparation, Application*

1.An aqueous sample is introduced into the purge vessel.

atmosphere.

quent desorption faster.

calibration standards.

2.The valve is set to the purge position. A purge gas (typically, helium) breaks through the sample continuously and sweeps the volatile organics to the trap, where they are retained by the sorbents. Then, the gas is vented to the

3.The purging step consists of purge, dry purge, and preheating. However, the purge step takes about 10–15 min, and the flow rate of helium is about 40 ml/ min. After purging, while the trap is at the ambient temperature, the purge gas is transferred directly into the trap without passing through the sample. This step is called dry purge. The main objective of this step is to remove the water which has been accumulated on the trap. Dry purging often takes place between 1 and 2 min. Then, the purge gas is turned off, and the trap is heated to about 5–10°C below the desorption temperature. Preheat makes the subse-

4.When the purging step is complete, the trap is heated, from 180 to 250°C, to desorb the analytes into the GC column to be analyzed. On the other hand, it is back-flushed with the GC carrier gas. So, the preheat temperature is reached, and the six-port valve is rotated to the desorb position to initiate the desorption step. Desorption time is about 1–4 min and depends on the carrier flow rate in GC instrument. For instance, the trap desorption time is short at the high flow rate, and so, a narrowband injection is achieved. The flow rate of the desorb gas should be selected in accordance with the type of GC column used. On the other hand, the operational conditions of the purge and trap must be compatible with configuration of GC system. With a packed GC column, higher carrier gas (desorb gas) flow rates can be applied. Usually, the optimum flow rate is about 50 ml/min. Capillary columns require lower flow rate and are

5.In the trap baking step, after the desorption step, the valve is readjusted in the purge position. The trap condition is adjusted at desorption temperature, or 15°C upper than it, for 7–10 min. The objective of this step is to remove pos-

6.After the trap baking step, the trap temperature is diminished and the next sample can be extracted. In each step, the conditional parameters such as temperature, time, and flow rate should be the same for all of the samples and

The trap is usually a stainless steel tube 3 mm in inside diameter (ID) and 25 mm long packed with multiple layers of adsorbents, and it should do the follow-

The sorbents are often arranged in layers to increase the trapping capacity. During purging process, the purge gas reaches the weaker sorbent at first, and only less volatile organics are retained. But more volatile compounds just pass through this layer and then are trapped by the other stronger adsorbent layers. During

1.Retain the analytes of interest, but do not introduce impurities.

2.Allow rapid injection of analytes into the GC column.

often preferred over the packed one for better resolution.

sible contaminants and eliminate sample transport.

**60**

ing steps:

desorption process, the trap is heated and back-flushed with the GC carrier gas. However, the less volatile compounds have never been in contrast with the stronger adsorbents, and so, the reversible adsorptions can be achieved.

To trap volatile organic compounds, the substances such as Tenax, silica gel, activated charcoal, graphitized carbon black (GCB or Carbopack), carbon molecular sieves (Carbosieve), and Vocarb are usually used [2]. Tenax is not only a porous but also a hydrophobic polymer resin based on 2,6-diphenylene oxide, with low affinity for water. So, highly volatile and polar compounds are seldom adsorbed on Tenax. Tenax should not be heated to temperatures upper than 200°C, because of its decomposition under high temperatures. The two types of Tenax are Tenax TA and Tenax GC. The former has higher purity and is more preferred for trace analysis. Silica gel is hydrophilic and is an excellent candidate for trapping polar compounds. Also, it is a stronger sorbent than Tenax. The problem is that water can be retained on the gel. Charcoal, as another stronger sorbent than Tenax, is hydrophobic and is mainly used to trap very volatile compounds such as dichlorodifluoromethane, a.k.a. Freon 12. These compounds can break through Tenax and silica gel. Conventional traps like Tenax, silica gel, and charcoal are usually used in series. If the boiling points of the analytes are above 35°C, Tenax itself will be suitable, and so, silica gel and charcoal can be ignored. Graphitized carbon black (GCB), as an alternative sorbent to charcoal and silica gel, has both the hydrophobic property and the trapping capacity similar to Tenax. Also, it is often used along with carbomolecular sieves and can trap highly volatile compounds. Vocarb is a highly hydrophobic activated carbon which can diminish water trapping and be purged fast. Vocarb is usually operated with an ion-trap mass spectrometer, which can be affected by trace levels of water or methanol. GCB, carbon molecular sieves, and Vocarb possess high thermal stability and can be operated at higher desorption temperatures than those that Tenax can be done [2].

The transfer line between the trap and the GC column is often made of nickel, deactivated fused silica, and silica-lined stainless steel tubing. By using these inert materials, the active sites which can interact with the analytes are eliminated. On the other hand, the transfer line is kept at a temperature higher than 100°C to avoid the condensation of water and the volatile organics. Also, the six-port valve which controls the gas flow path is also heated above 100°C to avoid condensation.

#### *2.10.1 Interfacing purge and trap*

As noted above, the operational conditions of purge and trap must be adaptable with the GC system configuration. For example, megabore capillary columns (0.53 mm ID or larger) are typically used at a flow rate of 8–15 ml/min. Since desorption process is slower at such flow rates, the column is usually cooled to 10°C or less temperatures at the stating of the GC run to retain the very volatile compounds. Sub-ambient cooling may be eliminated by using a long column (60–105 m) with a thick film stationary phase (3–5 μm). However, this flow rate is still too high for a GC-MS analyzer. So, a GC-MS interface like a jet separator or an open-split interface should be applied to decrease the carrier flow rate in the mass detector. However, an open-split interface makes a reduction in the analytical sensitivity due to entering just a portion of analyte into the detector [2].

Narrow-bore capillary columns (0.32 mm ID or smaller) with MS detector are commonly operated at lower flow rates (less than 5 ml/min). There are two ways to couple purge and trap with this type of columns:

1.To desorb the trap at a high flow rate and, then, with a split injector, split the flow into the GC instrument. So, a fast injection is obtained without significant loosing of the analytical sensitivity.

#### *Gas Chromatography - Derivatization, Sample Preparation, Application*

2.To desorb or refocus the analytes on a second trap and use a low desorb flow rate. At this flow rate, the time of desorption is too long to achieve a narrow bandwidth injection. A cryogenic trap is often used as a second trap and made of a short piece of uncoated fused silica capillary tube. It is cooled to −150°C by liquid nitrogen to refocus the analytes. After refocusing the analytes, the cryogenic trap is heated quickly to 250°C to desorb the analytes into the GC column.

A moisture control device is another interface which must be used. The purge gas, which is coming from purge vessel, is saturated with water, and so water can be collected on the trap and later released into the GC column during trap heating. Water decreases column efficiency and interferes with some certain detectors such as PID and MS. The column can also be plugged by ice if cryogenic trapping is used. Therefore, water requires to be removed before entering the GC. Two water management techniques are ordinarily applied [2]:


Lacorte et al. studied an automated technique based on purge and trap coupled to gas chromatography with mass spectrometric detection for the trace determination of five of the most important water odorants. Analytes were purged from 20 ml of water sample containing sodium chloride at ambient temperature and trapped on a Tenax sorbent by a flow of He. The desorption step was done with helium, purge gas, and temperature programming. The desorbed analytes were directly transferred to a gas chromatograph with a mass spectrometer detector for separation and determination [26].

### **2.11 Membrane extraction**

Among a wide variety of separation methods, membrane extraction and/or transport of analytes through the membranes is a powerful technique for their concentration, separation, and recovery. In this method, the sorption and desorption steps are combined into a one-step process, and, because of its simplicity, low cost, and high efficiency, it possesses an important role in biology, chemistry, and separation sciences; therefore, the efforts for developing of these types of sample treatment methods are increased [5]. In membrane transport, the sample is in contact with one side of the membrane, which is referred to the feed (or donor) phase. Also, the membrane phase serves as a selective barrier. The analytes pass through the membrane phase toward its other side, which is referred to the permeation (or acceptor) phase. Sometimes, the permeated analytes are swept by another phase like either a gas or a liquid. Its schematic diagram is shown in **Figure 2**.

A membrane can be accompanied with an instrumental analysis for online analysis (its advantage). Specially, a mass spectrometer or gas chromatograph can be applied as the detector device. Once a membrane is coupled to the mass spectrometry (MIMS), the membrane can be put in the vacuum compartment of the mass spectrometer. The permeated analytes are directly introduced into the

**63**

*Sample Preparation Techniques for Gas Chromatography DOI: http://dx.doi.org/10.5772/intechopen.84259*

*The schematic diagram of membrane extraction.*

into the GC column [27].

**Figure 2.**

thousand (ppt) to parts per billion (ppb) range.

partial pressure gradient for mass transfer.

membrane, which is the driving force for diffusion [2, 5].

ionization chamber of the MS instrument. In membrane introduction gas chromatography, a sorbent trap is interfaced between the membrane and the GC. Then, the permeated analytes are carried by a gas stream to the trap for preconcentration step. After completing the trap or preconcentration step, the trap is quickly heated to desorb the analytes into the GC column, as a narrowband injection. For instance in a GC connection, an aqueous sample from the loop of a multiport injection valve is injected into the hollow fiber membrane module by a N2 stream. The gas pushes the sample through the membrane fibers, and so the organic analytes permeate to the acceptor phase. Then, they are swept to a micro-sorbent trap by a countercurrent nitrogen stream. After completing the extraction of the analytes on the trap, during a predetermined period of time, the trap is electrically heated to desorb the analytes

For matrix samples, GC has gained a good potential of choice, because of its excellent separation ability. Tandem MS has been introduced as a faster alternative technique to GC separation, but such these instruments make higher costs. In membrane-based methods, limit of detections are especially in the parts per

The main drawback in membrane extraction coupled with a GC instrument is the slow permeation through the polymeric membrane and the aqueous boundary layer. This problem is much less than it in membrane introduction mass spectrometer (MIMS). The reason is that the vacuum in the mass spectrometer makes a high

The time needed to complete the permeation process is mentioned as lag time. Another disadvantage of membrane extraction is that the lag and/or transport time can be significantly longer than the time of sample residence in the membrane phase. This is mainly due to the boundary layer effects. When the carrier fluid is an aqueous stream, a static boundary layer is formed between the membrane and the aqueous phase. Since the analytes are being stuck in the boundary layer, the gradient for mass transfer decreases and the transport time enhances. Sample dispersion is another cause of the long lag time in flow injection techniques where an aqueous carrier fluid is used. Axial mixing of the sample with the carrier stream causes dispersion. So, the sample volume increases, and longer residence time in the membrane phase is obtained. Dilution reduces the concentration gradient across the

Membrane pervaporation (permselective "evaporation" of liquid molecules) is the term used to describe the extraction of volatile organics from an aqueous matrix to a gas phase through a semipermeable membrane. The extraction of volatiles from a gas sample to a gaseous acceptor across the membrane is called permeation, which is the mechanism of extraction from the headspace of an aqueous or solid sample. In pervaporation process, the organic analytes of interest move from the bulk aqueous

*Sample Preparation Techniques for Gas Chromatography DOI: http://dx.doi.org/10.5772/intechopen.84259*

**Figure 2.**

*Gas Chromatography - Derivatization, Sample Preparation, Application*

agement techniques are ordinarily applied [2]:

condenser is heated and the water is vented.

column.

determination [26].

**2.11 Membrane extraction**

2.To desorb or refocus the analytes on a second trap and use a low desorb flow rate. At this flow rate, the time of desorption is too long to achieve a narrow bandwidth injection. A cryogenic trap is often used as a second trap and made of a short piece of uncoated fused silica capillary tube. It is cooled to −150°C by liquid nitrogen to refocus the analytes. After refocusing the analytes, the cryogenic trap is heated quickly to 250°C to desorb the analytes into the GC

A moisture control device is another interface which must be used. The purge gas, which is coming from purge vessel, is saturated with water, and so water can be collected on the trap and later released into the GC column during trap heating. Water decreases column efficiency and interferes with some certain detectors such as PID and MS. The column can also be plugged by ice if cryogenic trapping is used. Therefore, water requires to be removed before entering the GC. Two water man-

1.To have a dry purge step prior to the desorption process. However, by this approach, some hydrophilic sorbents such as silica gel are not compatible.

2.To use a condenser between the trap and the GC instrument. The condenser is made of inert materials such as a piece of nickel tube. During desorption, the condenser is maintained at ambient temperature, and water is condensed and removed from the carrier gas. After completing the desorption process, the

Lacorte et al. studied an automated technique based on purge and trap coupled to gas chromatography with mass spectrometric detection for the trace determination of five of the most important water odorants. Analytes were purged from 20 ml of water sample containing sodium chloride at ambient temperature and trapped on a Tenax sorbent by a flow of He. The desorption step was done with helium, purge gas, and temperature programming. The desorbed analytes were directly transferred to a gas chromatograph with a mass spectrometer detector for separation and

Among a wide variety of separation methods, membrane extraction and/or transport of analytes through the membranes is a powerful technique for their concentration, separation, and recovery. In this method, the sorption and desorption steps are combined into a one-step process, and, because of its simplicity, low cost, and high efficiency, it possesses an important role in biology, chemistry, and separation sciences; therefore, the efforts for developing of these types of sample treatment methods are increased [5]. In membrane transport, the sample is in contact with one side of the membrane, which is referred to the feed (or donor) phase. Also, the membrane phase serves as a selective barrier. The analytes pass through the membrane phase toward its other side, which is referred to the permeation (or acceptor) phase. Sometimes, the permeated analytes are swept by another phase

like either a gas or a liquid. Its schematic diagram is shown in **Figure 2**.

A membrane can be accompanied with an instrumental analysis for online analysis (its advantage). Specially, a mass spectrometer or gas chromatograph can be applied as the detector device. Once a membrane is coupled to the mass spectrometry (MIMS), the membrane can be put in the vacuum compartment of the mass spectrometer. The permeated analytes are directly introduced into the

**62**

*The schematic diagram of membrane extraction.*

ionization chamber of the MS instrument. In membrane introduction gas chromatography, a sorbent trap is interfaced between the membrane and the GC. Then, the permeated analytes are carried by a gas stream to the trap for preconcentration step. After completing the trap or preconcentration step, the trap is quickly heated to desorb the analytes into the GC column, as a narrowband injection. For instance in a GC connection, an aqueous sample from the loop of a multiport injection valve is injected into the hollow fiber membrane module by a N2 stream. The gas pushes the sample through the membrane fibers, and so the organic analytes permeate to the acceptor phase. Then, they are swept to a micro-sorbent trap by a countercurrent nitrogen stream. After completing the extraction of the analytes on the trap, during a predetermined period of time, the trap is electrically heated to desorb the analytes into the GC column [27].

For matrix samples, GC has gained a good potential of choice, because of its excellent separation ability. Tandem MS has been introduced as a faster alternative technique to GC separation, but such these instruments make higher costs. In membrane-based methods, limit of detections are especially in the parts per thousand (ppt) to parts per billion (ppb) range.

The main drawback in membrane extraction coupled with a GC instrument is the slow permeation through the polymeric membrane and the aqueous boundary layer. This problem is much less than it in membrane introduction mass spectrometer (MIMS). The reason is that the vacuum in the mass spectrometer makes a high partial pressure gradient for mass transfer.

The time needed to complete the permeation process is mentioned as lag time. Another disadvantage of membrane extraction is that the lag and/or transport time can be significantly longer than the time of sample residence in the membrane phase. This is mainly due to the boundary layer effects. When the carrier fluid is an aqueous stream, a static boundary layer is formed between the membrane and the aqueous phase. Since the analytes are being stuck in the boundary layer, the gradient for mass transfer decreases and the transport time enhances. Sample dispersion is another cause of the long lag time in flow injection techniques where an aqueous carrier fluid is used. Axial mixing of the sample with the carrier stream causes dispersion. So, the sample volume increases, and longer residence time in the membrane phase is obtained. Dilution reduces the concentration gradient across the membrane, which is the driving force for diffusion [2, 5].

Membrane pervaporation (permselective "evaporation" of liquid molecules) is the term used to describe the extraction of volatile organics from an aqueous matrix to a gas phase through a semipermeable membrane. The extraction of volatiles from a gas sample to a gaseous acceptor across the membrane is called permeation, which is the mechanism of extraction from the headspace of an aqueous or solid sample. In pervaporation process, the organic analytes of interest move from the bulk aqueous

sample solution into the membrane phase and dissolve into it. Then, the analytes diffuse across the membrane phase and permeate into the acceptor or permeate phase and evaporate into the gas phase, as well. An additional step is occurred in headspace sampling mode, and the analytes transport into the headspace phase from the bulk aqueous phase. In both cases, the concentration gradient across the membrane is the driving force for the analyte transport across the membrane. Its schematic diagram is shown in **Figure 3**.

## *2.11.1 Membrane modules*

Membranes can be categorized both based on its structure in two kinds, porous and nonporous, and based on its geometry in two types, flat sheet and hollow fiber. Membranes which are applied in pervaporation and gas permeation are especially hydrophobic and nonporous silicone (polydimethylsiloxane or PDMS) membranes. Aqueous organics dissolve into the membrane phase and are extracted, while the aqueous contaminants are unextracted into the membrane. The microporous membranes in pervaporation are usually made of polypropylene, cellulose, or Teflon. The disadvantage of this membrane is to permit the passage of large quantities of water. Usually, water must be removed before it enters the analysis instrument.

As understood the name, flat-sheet membranes are flat, like a sheet of paper, and can be made as thick as less than 1 mm. However, the typical holders are necessary to hold them in place. In-tube hollow fiber membranes are 200–500 mm in diameter and also allow fluids to flow both inside and outside. Hollow fibers are selfsupported and offer the advantage of larger surface area per unit volume and high packing density. A large number of parallel fibers can be packed into a small volume.

## *2.11.2 Optimization of membrane extraction*

Several factors, which affect the extraction efficiency and sensitivity by the membrane, such as temperature, membrane surface area, membrane thickness, geometry, sample volume, and sample flow rate, should be optimized for specific applications. Higher temperature has two opposite effects on the extraction efficiency. On the other hand, it facilitates mass transfer by increasing diffusion coefficient and, on the other hand, decreases analyte partition coefficient in the membrane. So, the temperature of the membrane module should be controlled to avoid fluctuation extraction efficiency and sensitivity. Another effective parameter

**65**

*Sample Preparation Techniques for Gas Chromatography DOI: http://dx.doi.org/10.5772/intechopen.84259*

flow rate makes the extraction efficiency increase.

tion should stop before the solution reaches dryness.

be observed in the retention time of the analyte.

raphy, which are discussed in the following step [3].

the SPE application examples are as follows [29]:

**4. Cleanup techniques**

front of the GC column [28].

is the membrane thickness. Faster mass transfer is achieved by using thinner membranes, and in the case of hollow fibers, using longer membranes and multiple fibers is better. Also by using the larger volume of the sample, higher sensitivity can be obtained. However, larger volumes take longer to extract, but this lower sample

**3. Concentration techniques for reducing the solvent volume**

Once the analytes are diluted in the presence of a large volume of solvents during the extraction processes, they should be concentrated to analyze by instrumental methods as GC. If the amount of solvent to be removed is not very high and the analyte is nonvolatile, the solvent can be vaporized by a gentle stream of nitrogen gas flowing either across the surface or through the solution. But when a large volume of solvent should be removed, a rotary vacuum evaporator is used. In this case, the solution is placed in a round-bottomed flask which put in a heated water bath. A water-cooled condenser is attached at the top of flask to condense the evaporated solvent, and it distils into a separate container. Then, the flask is rotated continually to expose maximum liquid surface to evaporation. It should be noted that evapora-

For achieving smaller volume, e.g., less than 1 ml, a Kuderna-Danish concentrator is used. In this case, the solution is slowly heated in a warm water bath until the necessary volume is obtained. Also, an air-cooled condenser provides the solvent reflux [2].

Sample cleanup is especially important for analytical separations such as GC, HPLC, and electrophoresis. Often, many solid matrices, as soil, biological materials, and natural products, contain hundreds of interferences at higher concentrations than those of the analytes. So, a cleanup step is vital to separate the trace amount of analyte from interferences. On the other hand, some high-boiling materials can cause a variety of problems such as the adsorption of analyte in the injection port or in front of a GC or HPLC column. Therefore, some positive and negative errors can

Some other cleanup techniques include gel permeation chromatography (GPC), acid-base partition cleanup, solid-phase extraction (SPE), and column chromatog-

Gel permeation chromatography (GPC) is a size-exclusion method which contained organic solvents (or buffers) and porous gels to separate macromolecules larger than analytes of interest. GPC is used to eliminate lipids, proteins, polymers, copolymers, natural resins, cellular components, viruses, steroids, and dispersed high-molecular-weight compounds from the sample. This method is suitable for both polar and nonpolar analytes [2]. On the other hand, GPC is usually used to remove high-boiling materials which condense in the injection port of a GC or the

Acid-base partition cleanup is a liquid-liquid extraction procedure to separate

acids such as organic acids and phenols from the base or neutral analytes like amines, aromatic hydrocarbons, and halogenated organic compounds, by adjusting pH. Also, this cleanup method is applied for petroleum waste prior to analysis [2]. Solid-phase extraction cartridge is a traditional column chromatography which is applied to clean up the biological, clinical, and environmental samples. Some of

**Figure 3.** *The schematic diagram of membrane pervaporation.*

*Gas Chromatography - Derivatization, Sample Preparation, Application*

schematic diagram is shown in **Figure 3**.

*2.11.2 Optimization of membrane extraction*

*The schematic diagram of membrane pervaporation.*

*2.11.1 Membrane modules*

sample solution into the membrane phase and dissolve into it. Then, the analytes diffuse across the membrane phase and permeate into the acceptor or permeate phase and evaporate into the gas phase, as well. An additional step is occurred in headspace sampling mode, and the analytes transport into the headspace phase from the bulk aqueous phase. In both cases, the concentration gradient across the membrane is the driving force for the analyte transport across the membrane. Its

Membranes can be categorized both based on its structure in two kinds, porous and nonporous, and based on its geometry in two types, flat sheet and hollow fiber. Membranes which are applied in pervaporation and gas permeation are especially hydrophobic and nonporous silicone (polydimethylsiloxane or PDMS) membranes. Aqueous organics dissolve into the membrane phase and are extracted, while the aqueous contaminants are unextracted into the membrane. The microporous membranes in pervaporation are usually made of polypropylene, cellulose, or Teflon. The disadvantage of this membrane is to permit the passage of large quantities of water. Usually, water must be removed before it enters the analysis instrument.

As understood the name, flat-sheet membranes are flat, like a sheet of paper, and can be made as thick as less than 1 mm. However, the typical holders are necessary to hold them in place. In-tube hollow fiber membranes are 200–500 mm in diameter and also allow fluids to flow both inside and outside. Hollow fibers are selfsupported and offer the advantage of larger surface area per unit volume and high packing density. A large number of parallel fibers can be packed into a small volume.

Several factors, which affect the extraction efficiency and sensitivity by the membrane, such as temperature, membrane surface area, membrane thickness, geometry, sample volume, and sample flow rate, should be optimized for specific applications. Higher temperature has two opposite effects on the extraction efficiency. On the other hand, it facilitates mass transfer by increasing diffusion coefficient and, on the other hand, decreases analyte partition coefficient in the membrane. So, the temperature of the membrane module should be controlled to avoid fluctuation extraction efficiency and sensitivity. Another effective parameter

**64**

**Figure 3.**

is the membrane thickness. Faster mass transfer is achieved by using thinner membranes, and in the case of hollow fibers, using longer membranes and multiple fibers is better. Also by using the larger volume of the sample, higher sensitivity can be obtained. However, larger volumes take longer to extract, but this lower sample flow rate makes the extraction efficiency increase.
