Sample Preparation
Sample preparation can vary widely and be very complex. Gas samples need special handling and special sampling procedures. Liquid samples may need extraction (e.g. pesticides from water) and extracts evaporated (or concentrated). A preliminary separation may be necessary which can include filtration, centrifugation or distillation. Volatile samples can be separated by GC and, if not volatile, may require derivatization. Trace analysis, may require certain substances to be selectively detected (e.g. by fluorescence detection), thus, fluorescent derivatives may need to be prepared. Sample preparation is an essential part of all chromatographic analyses and will require skills other than those directly associated with the operation of a chromatograph.
Analytical Procedures
Many samples can be analyzed, quite satisfactorily by more than one chromatographic technique. The choice will be discussed later but some indication of the basis of choice will be given. Gas solid chromatography (GSC) is useful for the separation of permanent gases, e.g. nitrogen, oxygen, hydrogen, the inert gases, and the low molecular weight hydrocarbon gases etc. Corrosive gases, e.g. the halogens, hydrogen chloride, sulfur dioxide etc. can be separated by GSC but the chromatographic conduit system must be constructed of inert materials. The conduit system will include the detector sensor that will also come in contact with the corrosive sample.
Essential oils, petroleum fractions, solvents etc., are separated by (GLC) in preference to LC, as higher efficiencies and faster separations are available. Some samples are amenable to both GLC and LC and the choice will depend on the sample. If the sample is relatively involatile and thermally labile, LC might be choice. However, if trace materials are important or there is very limited sample, then the high sensitivities of the GC detectors may be essential. Such samples may need to be derivatized to increase the volatility of the constituents. LC, however, will handle completely involatile samples, thermally labile samples and, due to the versatility of the phase system, will offer far greater selectivity.
