GENERAL ASPECTS OF GAS CHROMATOGRAPHY .1 Applications of Gas Chromatography

Gas chromatography is a unique and versatile technique. In its initial stages of development it was applied to the analysis of gases and vapors from very volatile components. The work of Martin and Synge (2) and then James and Martin (3) in gas–liquid chromatography (GLC) opened the door for an analytical technique that has revolutionized chemical separations and analyses. As an analytical tool, GC can be used for the direct separation and analysis of gaseous samples, liquid solutions, and volatile solids.

If the sample to be analyzed is nonvolatile, the techniques of derivatization or pyrolysis GC can be utilized. This latter technique is a modification wherein a nonvolatile sample is pyrolyzed before it enters the column. Decomposition products are separated in the gas chromatographic column, after which they are qualitatively and quantitatively determined. Analytical results are obtained

from the pyrogram (a chromatogram resulting from the detection of pyrolysis products). This technique can be compared to mass spectrometry, a technique in which analysis is based on the nature and distribution of molecular fragments that result from the bombardment of the sample component with high-speed electrons.

In pyrolysis GC the fragments result from chemical decomposition by heat. If the component to be pyrolyzed is very complex, complete identification of all the fragments may not be possible. In a case of this type, the resulting pyrogram may be used as a set of “fingerprints” for subsequent study.

Pyrolysis may be defined as the thermal transformation of a compound (single entity) into another compound or compounds, usually in the absence of oxy- gen. In modern pyrolysis the sample decomposition is rigidly controlled. One should keep in mind that pyrolysis gas chromatography (PGC) is an indirect method of analysis in which heat is used to change a compound into a series of volatile products that should be characteristic of the original compound and the experimental conditions.

Gas chromatography is the analytical technique used for product identifica- tion (under very controlled conditions) and must be directly coupled to a mass spectrometer when information other than a comparative fingerprint (pyrogram) is required, such as positive identification of peaks on the chromatogram.

Ettre and Zlatkis (6) classified pyrolysis types according to extent of degrada- tion of the sample compound:

1. Thermal Degradation. Usually occurs in the temperature range of 100–300^◦C but may occur as high as 500^◦C. This type may be carried out in the injection port of the instrument. Rupture of carbon–carbon bonds is minimal.

2. Mild Pyrolysis. Occurs between 300 and 500^◦C, and carbon–carbon bond breakage occurs to some extent.

3. Normal Pyrolysis. Occurs between 500 and 800^◦C and involves cleavage of carbon–carbon bonds. Very useful for characterizing polymers and co- polymers.

4. Vigorous Pyrolysis. Occurs at temperatures between 800 and 1100^◦C. The end results is the breaking of carbon–carbon bonds and cleaving organic molecules into smaller fragments.

The pyrolysis process may be performed by three different methods:

1. Continuous-Mode Method. May involve tube furnaces or microreactors.

In this mode the heated wall of the reactor is at a higher temperature than the sample and secondary reactions of pyrolysis products will most likely occur.

2. Pulse-Mode Pyrolysis. Sample is in direct contact with a hot wire, thus minimizing secondary reactions. Although the temperature profile is repro- ducible, the exact pyrolysis temperature cannot be measured. Another dis- advantage is that the sample weight cannot be known accurately. This is also known as Curie point pyrolysis.

3. Laser-Mode Pyrolysis. Directs very high energies to the sample, which usually result in ionization and the formation of plasma plumes. Thus, laser pyrolysis results in fewer and sometimes different products than ther- mal pyrolysis.

To a first approximation, good interlaboratory reproducibility of the pyrolysis profile is obtainable; however, intralaboratory matchings have been disappointing.

Several major parameters influence pyrolysis reproducibility:

1. Type of pyrolyzer 2. Temperature

3. Sample size and homogeneity

4. Gas chromatographic conditions and column(s) used 5. Interface between the pyrolyzer and the gas chromatograph.

Therefore, optimization of the pyrolyzer by use of reference standards is impor- tant. Thermal gradients across the sample may be avoided by use of thin samples.

For good results in PGC, one must have rapid transfer of the pyrolysis products to the column, minimization of secondary reaction products, and elimination of poor sample injection profiles.

When employing PGC for qualitative and quantitative analysis of complex unknown samples, it is essential to use pure samples of suspected sample compo- nents as a reference. One should never base identification of unknown pyrolyzate peaks on the retention time of pyrolyzate product peaks obtained from the standard (7). A peak in the chromatogram from the pyrolysis of the unknown may be from the matrix and not the suspected component. The use of selective detectors (i.e., a NPD with a FID or a FID with an ECD) will furnish element information but not molecular or structural information about the component peak. The matrix components (in the absence of the suspected analyte) may yield the same peak at the same retention time.

Another important variable in PGC is temperature control. Small changes in temperature may have pronounced effects on the resulting chromatogram. The effects may be manifested in several ways:

1. Increased number of peaks 2. Decreased number of peaks

3. Partial resolution of overlapping peaks

4. Increase or decrease in the peak areas for same sample size of unknown (indicating different pyrolysis mechanism)

5. Changes in peak shape of pyrolysis products

Thus, caution must be used when identifying a peak on a pyrogram for an unknown. This means that a reliable identification should not be based on reten- tion time data. The two best techniques for identifying unknown peaks are

infrared spectroscopy (IRS) and mass spectrometry (MS). Mass spectrometry is the better of the two techniques because one obtains a mass number that may be matched with a mass number in a library of mass spectra of known compounds.

All the ions from a known compound must be present for positive identifica- tion. Infrared spectroscopy will validate the presence of functional groups in the molecule. If the peak is single entity, one may match the spectrum (IR) obtained with a spectrum of a standard compound.

In addition to analysis, GC may be used to study structure of chemical compounds, determine the mechanisms and kinetics of chemical reactions, and measure isotherms, heats of solution, heats of adsorption, free energy of solution and/or adsorption, activity coefficients, and diffusion constants (see Chapter 12).

Another significant application of GC is in the area of the preparation of pure substances or narrow fractions as standards for further investigations. Gas chro- matography is also utilized on an industrial scale for process monitoring. In adsorption studies it can be used to determine specific surface areas (4,5). A novel use is its utilization for elemental analyses of organic components (8–10).

Distillation curves may also be plotted from gas chromatographic data.

Gas chromatography can be applied to the solution of many problems in various fields. A few examples are enumerated:

1. Drugs and Pharmaceuticals. Gas chromatography is used not only in the quality control of products of this field but also in the analysis of new products and the monitoring of metabolites in biological systems.

2. Environmental Studies. A review of the contemporary field of air pollution analyses by GC was published in the first volume of Contemporary Topics in Ana- lytical and Clinical Chemistry (11). A book by Grob and Kaiser (12) discussed the use of LC and GC for this type of analysis. Many chronic respiratory diseases (asthma, lung cancer, emphysema, and bronchitis) could result from air pollution or be directly influenced by air pollution. Air samples can be very complex mix- tures, and GC is easily adapted to the separation and analysis of such mixtures.

Two publications concerned with the adaptation of cryogenic GC to analyses of air samples are References 13 and 14. Chapter 15 covers the application of GC in the environmental area.

3. Petroleum Industry. The petroleum companies were among the first to make widespread use of GC. The technique was successfully used to separate and determine the many components in petroleum products. One of the earlier publications concerning the response of thermal conductivity detectors to con- centration resulted from research in the petroleum field (15). The application of GC to the petroleum field is discussed in Chapter 13.

4. Clinical Chemistry. Gas chromatography is adaptable to such samples as blood, urine, and other biological fluids (see Chapter 14). Compounds such as proteins, carbohydrates, amino acids, fatty acids, steroids, triglycerides, vitamins, and barbiturates are handled by this technique directly or after preparation of appropriate volatile derivatives (see Chapter 14).

5. Pesticides and Their Residues. Gas chromatography in combination with selective detectors such as electron capture, phosphorus, and electrolytic conduc- tivity detectors (see Chapter 6) have made the detection of such components and their measurement relatively simple. Detailed information in this area may be found in a monograph by Grob (16) and Chapter 15 of this book.

6. Foods. The determination of antioxidants and food preservatives is an active part of the gas chromatographic field. Adaptations and sample types are almost limitless, and include analysis of fruit juices, wines, beers, syrups, cheeses, beverages, food aromas, oils, dairy products, decomposition products, contami- nants, and adulterants.

2.2.2 Types of Detection

The various detectors employed in GC are discussed in Chapter 6. Our purpose here is only to categorize the detection system according to whether they are an integral-type system or a differential-type system. This classification is an old one; any detection system can be made integral or differential simply by a modification of the detector electronics. A more modern categorization would be instantaneous (differential) and cumulative (integral). Chromatograms that result from this classification of detectors are shown in Figure 2.18.

2.2.3 Advantages and Limitations

From the limited discussion so far one can visualize the versatility of the gas chromatographic technique. There are so many reasons for this, and we shall

FIGURE 2.18 Types of chromatogram:a, differential chromatogram;b, integral chro- matogram; c, peak resolution; O, injection point; OX, injector volume; OY, detector volume; OA, holdup volume V_M; OB, total retention volume V_R; AB, adjusted reten- tion volumeV_R−V_M; CD, peak base; FG, peak widthw_b; HJ, peak width at half-height w_h; BE, peak height;E, peak maximum; CHEJD, peak area (space incorporated within these letters); KL, step height of integral chromatogram.

enumerate some of the advantages. It should be stressed that what one person considers a disadvantage may be an advantage to someone else. Additionally, a current disadvantage, may be an advantage several years from now.

A few broad comments regarding GC would include the following:

1. An Analytical Technique. This is used not only for the qualitative identifi- cation of components in a sample, but also for quantitative measurements.

2. A Physical Research Technique. This may be used to investigate various parameters of a system, such as determination of partition coefficients, thermodynamic functions, and adsorption isotherms (see Chapter 12).

3. A Preparative Technique. Once the analytical conditions have been deter- mined, the system may be scaled up to separate and collect gram amounts of components.

4. An Online Monitoring Probe. A gas chromatograph can be locked into a process line so that the process stream may be monitored on a 24-h basis.

5. An Automated System. A gas chromatograph may be interfaced to a computer with an automatic sampler so that routine analyses can be run overnight.

Following are some overall advantages of GC that should be stressed:

1. Resolution. The technique is applicable to systems containing components with very similar boiling points. By choosing a selective liquid phase or the proper adsorbent, one can separate molecules that are very similar physically and chemically. Components that form azeotropic mixtures in ordinary distillation techniques may be separated by GC.

2. Sensitivity. This property of the gas chromatographic system largely ac- counts for its extensive use. The simplest thermal conductivity detector cells can detect a few parts per million; with an electron capture detector or phosphorous detector, parts per billion or picograms of solute can easily be measured. This level of sensitivity is more impressive when one considers that the sample size used is of the order of 1 µL or less.

3. Analysis Time. Separation of all the components in a sample may take from several seconds up to 30 min. Analyses that routinely take an hour or more may be reduced to a matter of minutes, because of the high diffusion rate in the gas phase and the rapid equilibrium between the moving and stationary phases(see Chapter 5).

4. Convenience. The operation of GC is a relatively straightforward proce- dure. It is not difficult to train nontechnical personnel to carry out routine separations.

5. Costs. Compared with many analytical instruments available today, gas chromatographs represent an excellent value.

6. Versatility. Gas chromatography is easily adapted for analysis of samples of permanent gases as well as high-boiling liquids or volatile solids.

7. High Separating Power. Since the mobile phase has a low degree of viscos- ity, very long columns with excellent separating power can be employed.

8. Assortment of Sensitive Detecting Systems. Gas chromatographic detectors (see Chapter 6) are relatively simple and highly sensitive, and possess rapid response rates.

9. Ease of Recording Data. Detector output from gas chromatographs can be conveniently interfaced with recording potentiometers, integrating sys- tems, computers, and a wide variety of automatic data storing modules (see Chapter 6).

10. Automation. Gas chromatographs may be used to monitor automatically various chemical processes in which samples may be periodically taken and injected onto a column for separation and detection.