32A-2 Sample Injection system For high column efficiency, a suitably sized sample should be introduced as a “plug” of vapor. Slow injection or oversized samples cause band spreading and poor resolution. Calibrated microsyringes, such as those shown in Fig. 32-3, are used to inject liquid samples through a rubber or silicone diaphragm, or septum, into a heated sample port located at head of the column.

Figure 32-3 A set of microsyringes for sample injection.

The sample port (Fig. 32-4) is usually kept at about 50oC greater than the boiling point of the least volatile component of the sample. For ordinary packed analytical columns, sample sizes range from a few tenths of a microliter to 20 mL. Capillary columns require samples that are smaller by a factor of 100 or more. For these columns, a sample splitter is often needed to deliver a small known fraction (1:100 to 1:500) of the injected sample, with the remainder going to waste.

Figure 32-4 Cross-sectional view of a microflash vaporizer direct injector

Commercial gas chromatographs intended for use with capillary columns incorporate such splitters, and they also allow for splitless injection when packed columns are used.

For the most reproducible sample injection, newer gas chromatographs use autoinjectors and autosamplers, such as the system shown in Fig. 32-5. With such autoinjectors, syringes are filled, and the sample injected into the chromatograph automatically.

Figure 32-5 An autoinjection system with autosampler for gas chromatography.

In the autosampler, samples are contained in vials on a sample turntable. The autoinjector syringe picks up the sample through a septum on the vial and injects the sample through a septum on the chromatograph. With the unit shown, up to 150 sample vials can be placed on the turntable. Injection volumes can vary from 0.1 mL with a 10-mL syringe to 200 mL with a 200-ml syringe.

Standard deviations as low as 0 Standard deviations as low as 0.3% are common with autoinjection systems. For introducing gases, a sample valve, such as that shown in Fig. 32-6, is often used instead of a syringe. With such devices, sample sizes can be reproduced to better than 0.5% relative.

Figure 32-6 A rotary sample valve Figure 32-6 A rotary sample valve. Valve position (a) is for filling the sample loop ACB; position (b) is for introduction of sample into the column.

Liquid samples can also be introduced through a sampling valve. Solid samples are introduced as solutions or alternatively are sealed into thin-walled vials that can be inserted at the head of the column and punctured or crushed from the outside.

32A-3 Column Configurations and Column Ovens The columns in gas chromatography are of two general types: packed columns or capillary columns. In the past, the vast majority of gas chromatographic analyses used packed columns. For most current applications, packed columns have been replaced by more efficient and faster capillary columns.

Chromatographic columns vary in length from less than 2 m to 60 m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into an oven for thermostating, they are usually formed as coils having diameters of 10 to 30 cm (Fig. 32-7).

Figure 32-7 Fused-silica capillary columns.

Column temperature is an important variable that must be controlled to a few tenths of a degree for precise work. Thus, the column is normally housed in a thermostated oven. The optimum column temperature depends on the boiling point of the sample and the degree of separation required. Roughly, temperature equal to or slightly above average boiling point of sample results in reasonable elution time (2 - 30 min).

For samples with a broad boiling range, it is often desirable to use temperature programming whereby the column temperature is increased either continuously or in steps as the separation proceeds. Fig. 32-8 shows the improvement in a chromatogram brought about by temperature programming. In general, optimum resolution is associated with minimal temperature.

Figure 32-8 Effect of temperature on gas chromatograms Figure 32-8 Effect of temperature on gas chromatograms. (a) Isothermal at 458C. (b) Isothermal at 1458C. (c) Programmed at 308 to 1808C

The cost of lowered temperature, however, is an increase in elution time and, therefore, the time required to complete an analysis (Fig. 32-8 a & b) Analytes of limited volatility can sometimes be determined by forming derivatives that are more volatile

Temperature programming in gas chromatography is achieved by increasing the column temperature continuously or in steps during elution.