Gas Chromatography: Evaluation

Evaluation

Scale of Operation 

Analytes present at levels from major to ultratrace compo- nents have been successfully determined by gas chromatography. Depending on the choice of detector, samples with major and minor analytes may need to be di- luted before analysis. The thermal conductivity and flame ionization detectors can handle larger amounts of analyte; other detectors, such as the electron capture de- tector or a mass spectrometer, require substantially smaller amounts of analyte. Although the volume of sample injected is quite small (often less than a micro- liter), the amount of available material from which the injection volume is taken must be sufficient to be a representative sample. For trace analytes, the actual amount of analyte injected is often in the picogram range. Using the tri- halomethane analysis described in Method 12.1 as an example, a 3.0-μL injection of a water sample containing 1 μg/L of CHCl₃ corresponds to 15 pg of CHCl₃ (as- suming a complete extraction of CHCl₃).

Accuracy 

The accuracy of a gas chromatographic method varies substantially from sample to sample. For routine samples, accuracies of 1–5% are common. For analytes present at very low concentration levels, for samples with complex matrices, or for samples requiring significant processing before analysis, accu- racy may be substantially poorer. In the analysis for trihalomethanes described in Method 12.1, for example, determinate errors as large as ±25% are possible.

Precision

The precision of a gas chromatographic analysis includes contribu- tions from sampling, sample preparation, and the instrument. The relative stan- dard deviation due to the gas chromatographic portion of the analysis is typically 1–5%, although it can be significantly higher. The principal limitations to preci- sion are detector noise and the reproducibility of injection volumes. In quantita- tive work, the use of an internal standard compensates for any variability in injec- tion volumes.

Sensitivity 

In a gas chromatographic analysis, sensitivity (the slope of a calibra- tion curve) is determined by the detector’s characteristics. Of greater interest for quantitative work is the detector’s linear range; that is, the range of concentrations over which a calibration curve is linear. Detectors with a wide linear range, such as a thermal conductivity detector and flame ionization detector, can be used to analyze samples of varying concentration without adjusting operating conditions. Other de- tectors, such as the electron capture detector, have a much narrower linear range.

Selectivity 

Because it combines separation with analysis, gas chromatography provides excellent selectivity. By adjusting conditions it is usually possible to design a separation such that the analytes elute by themselves. Additional selectivity can be provided by using a detector, such as the electron capture detector, that does not re- spond to all compounds.

Time, Cost, and Equipment 

Analysis time can vary from several minutes for sam- ples containing only a few constituents to more than an hour for more complex samples. Preliminary sample preparation may substantially increase the analysis time. Instrumentation for gas chromatography ranges in price from inexpensive (a few thousand dollars) to expensive (more than $50,000). The more expensive mod- els are equipped for capillary columns and include a variety of injection options and more sophisticated detectors, such as a mass spectrometer. Packed columns typi- cally cost $50–$200, and the cost of a capillary column is typically $200–$1000.