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 3. SAMPLING CONSIDERATIONS

a. Safety

The main safety consideration is the use of an FID in areas where a potential explosion hazard may exist. Do not use a detector of this kind in any area where a flame would not be permitted unless the instrument has been rated intrinsically safe. If an electron capture detector is used, the radioactive effluent must be safety vented.

b. Type of sample

In contrast to laboratory-type GCs, portable GCs do not have heated injection ports. Therefore, the sample must be a gas or vapor at room temperature. In general, whether the sample is introduced by means of a loop or a gas-tight syringe, most measurements give essentially instantaneous concentrations. Integrated samples may be taken, if desired, by using gas bags, evacuated gas bottles and the like; in that case, the sample concentration is averaged over the filling time of the sample container. It is important to use only those sample containers in which the analyte is stable.

c. Column Selection

Once the decision to use a portable GC has been made, in most cases, the critical question becomes the selection of the column. The following are some hints to aid in the selection of the column best suited to the analysis to be performed [3]:

(1) Retention time is proportional to column length.

(2) Retention time is proportional to the percent loading of the stationary phase.

(3) Column resolution is proportional to both loading and column length.

(4) Non-polar vapors (e.g., hydrocarbons, halogenated hydrocarbons) are separated most efficiently by non-polar stationary phases; polar vapors (e.g., alcohols, ketones, esters) by polar phases. Information on the polarity and selectivity of stationary phases is available in the form of McReynolds constants which are tabulated in the catalogs of various vendors of GC supplies. A useful discussion of the use of McReynolds constants for column selection appears in the catalog of Applied Science, Inc., 2051 Waukegan Road, Deerfield, IL 60015.

(5) Special columns have been developed for various types of separations. In the catalogs of most suppliers of GC columns there is a section on applications and many of the suppliers will be happy to discuss problems and suggest solutions.

4. DATA ACQUISITION AND TREATMENT

a. Types

Data acquisition may be done automatically or manually depending on the particular design of the portable GC. If the instrument does not have provision for automatic data collection, 1/15/98