Three considerations determine how samples are introduced to the gas chromato- graph. First, all constituents injected into the GC must be volatile. Second, the ana- lytes must be present at an appropriate concentration. Finally, injecting the sample must not degrade the separation.
Gas chromatography can be used to separate analytes in complex matrices. Not every sample that can potentially be analyzed by GC, however, can be injected directly into the instrument. To move through the col- umn, the sample’s constituents must be volatile. Solutes of low volatility may be re- tained by the column and continue to elute during the analysis of subsequent sam- ples. Nonvolatile solutes condense on the column, degrading the column’s performance.
Volatile analytes can be separated from a nonvolatile matrix using any of the extraction techniques. Liquid–liquid extractions, in which analytes are extracted from an aqueous matrix into methylene chloride or other or- ganic solvent, are commonly used. Solid-phase extractions also are used to remove unwanted matrix constituents.
An attractive approach to isolating analytes is a solid-phase microextraction (SPME). In one approach, which is illustrated in Figure 12.19, a fused silica fiber is placed inside a syringe needle. The fiber, which is coated with a thin organic film, such as polydimethyl siloxane, is lowered into the sample by depressing a plunger and is exposed to the sample for a predetermined time. The fiber is then withdrawn into the needle and transferred to the gas chromatograph for analysis.
Volatile analytes also can be separated from a liquid matrix using a purge and trap or by headspace sampling. In a purge and trap, an inert gas, such as He or N2, is bubbled through the sample, purging the volatile compounds. These compounds are swept through a trap packed with an absorbent material, such as Tenax, where they are collected. Heating the trap and back flush- ing with carrier gas transfers the volatile compounds to the gas chromatograph. In headspace sampling the sample is placed in a closed vial with an overlying air space. After allowing time for the volatile analytes to equilibrate between the sample and the overlying air, a portion of the vapor phase is sampled by syringe and in- jected into the gas chromatograph.
Thermal desorption is used to release volatile analytes from solids. A portion of the solid is placed in a glass-lined, stainless steel tube and held in place with plugs of glass wool. After purging with carrier gas to remove O2 (which could lead to oxida- tion reactions when heating the sample), the sample is heated. Volatile analytes are swept from the tube by the carrier gas and carried to the GC. To maintain efficiency the solutes often are concentrated at the top of the column by cooling the column inlet below room temperature, a process known as cryogenic focusing.
Nonvolatile analytes must be chemically converted to a volatile derivative before analysis. For example, amino acids are not sufficiently volatile to analyze directly by gas chromatography. Reacting an amino acid with 1-butanol and acetyl chloride produces an esterfied amino acid. Subsequent treatment with trifluoroacetic acid gives the amino acid’s volatile N-trifluoroacetyl-n-butyl ester derivative.
Analytes present at concentrations too small to give an adequate signal need to be concentrated before analyzing. A side benefit of many of the extraction methods outlined earlier is that they often concen- trate the analytes. Volatile organic materials isolated from aqueous samples by a purge and trap, for example, can be concentrated by as much as 1000-fold.
When an analyte is too concentrated, it is easy to overload the column, thereby seriously degrading the separation. In addition, the analyte may be present at a con- centration level that exceeds the detector’s linear response. Dissolving the sample in a volatile solvent, such as methylene chloride, makes its analysis feasible.
To avoid any precolumn loss in resolution due to band broadening, a sample of sufficient size must be introduced in a small volume of mo- bile phase. An example of a simple injection port for a packed column is shown in Figure 12.20. Injections are made through a rubber septum using a microliter sy- ringe. The injector block is heated to a temperature that is at least 50 °C above the sample component with the highest boiling point. In this way rapid vaporization of the entire sample is ensured.
Capillary columns require the use of a special injector to avoid overloading the column with sample. Several capillary injectors are available, the most common of which is a split/splitless injector.7 When used for a split injection only about 0.1–1% of the sample enters the column, with the remainder carried off as waste. In a splitless injection, which is useful for trace analysis, the column temperature is held 20–25 °C below the solvent’s boiling point. As the solvent enters the column, it condenses, forming a barrier that traps the solutes. After allowing time for the solutes to concentrate, the column’s temperature is increased, and the separation begins. A splitless injection allows a much higher percentage of the solutes to enter the chromatographic column.
For samples that decompose easily, an on-column injection may be necessary. In this method the sample is injected on the column without heating. The column temperature is then increased, volatilizing the sample with as low a temperature as is practical.