cylinders which must be charged before going to the field. Others may need exterior carrier gas cylinders and regulators. It is important to check the carrier gas requirements in the manual of instructions which goes with the particular portable gas chromatograph. It is also important to consider the appropriate shipping regulations concerning compressed gases.
Columns may be of two types, i.e., packed or capillary. The packed column can accept a larger injection volume, but the capillary column has better separating power. Therefore, the choice of a packed or capillary system depends on the complexity of the atmosphere to be sampled and the desired detection limit. It should also be noted that columns for portable GCs may require special geometries, material of construction and hardware specific to the instrument. The users manual should be consulted for information in this area.
The interaction between the injected sample containing one or more compounds in the vapor phase and a liquid phase present, e.g., on the walls of a capillary column or coated on a diatomaceous earth (solid support) in a packed column results in a separation. The stronger the interaction between the component in the vapor phase and the liquid phase, the more strongly the flow of the component will be retarded by the column, i.e., the longer the retention time, tr [3].
The use of a heated column system greatly increases the flexibility of the analysis. As in any isothermal situation, there will still be a usable window of retention time. However, the retention time of any peak is a function of the temperature. As a rule of thumb, the retention time doubles for every decrease in column temperature of 30 C [4]. It is thus possible in many cases to select a temperature at which the peak of interest will have a usable retention time on a particular column. c. Detectors
c. Detectors
After the sample leaves the column, it goes directly to the detector. Detectors vary according to their selectivity, sensitivity, and linearity. The four most commonly used in portable GCs are as follows:
(1) Flame ionization detector (FID) [5] - large linear range; moderate sensitivity; good general detector for organics; low response to CS2, CO, CO2, CC14; non-selective; requires auxiliary gases (air and H2).
(2) Photoionization detector (PID) [6] - large linear range; high sensitivity; good general detector for organics and some inorganic gases; generally lower response to some small molecules, i.e., CH3CN, CH2C12, CC14, CH3OH and low-M.W. hydrocarbons; selectivity can be introduced in some systems by using different energy lamps, if available.
(3) Electron capture detector (ECD) [5] - small linear range; high sensitivity; very selective - in general, responds to molecules containing halogen (fluorine, chlorine, bromine, iodine) atoms, cyano groups or nitro groups; minimal response to hydrocarbons, alcohols, ketones, etc.
(4) Thermal conductivity detector (TCD) [5] - good linear range; low sensitivity; general non-selective detector for volatile compounds including all organics, as well as inorganics such as ammonia, CO, SF6, NO, NO2, and SO2.
