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 the time between samples will usually have to be determined in the field.

d. Indications

Portable GCs find their most common use as a tool for the gathering of concentration data during industrial hygiene surveys. They may also be used for real time monitoring of leaks, spills, etc., in order to make a judgment as to whether or not a dangerous concentration may have been reached. Some instruments with microprocessor-controlled acquisition and storage of data may be used to assess trends over extended periods. An important consideration, however, is that portable GCs should not, in general, be used for compliance measurements, nor are they meant as substitutes for the usual analytical methods used for determination of exposure such as those in this Manual. There are basically two reasons for this. First, there is no provision for unequivocally identifying a peak obtained, such as could be done in the laboratory using a GC/mass spectrometer system, and second because the separation step in the portable GC is not, as explained previously, as efficient as that obtainable with a laboratory analytical instrument. It is equally important to point out that the field measurement is inherently much simpler and does not have the problems with incomplete adsorption and desorption, sample storage instability, migration, and breakthrough which some of the compliance methods have. Thus while the field measurement is less exact than would be the case with a laboratory measurement, the overall field sampling and measurement process may be, in fact, more exact. 1/15/98