Page:NIOSH Manual of Analytical Methods - Chapter I.pdf/3

 The choice of detector, and, therefore, in most cases, the instrument, will depend on the chemical to be monitored, the nature of any other contaminants, and the sensitivity required.

d. Limitations

Most portable GCs have no provision for heating the column, detector, or injection port. This limits analyses to the more volatile compounds, and results in variation of retention times with ambient temperature. Also, late-eluting peaks will be poorly formed and will not be very useful for quantitation. This imposes a severe restriction on the number of usable column packings for any given contaminant. It also affects the ability of the system to work well in complex atmospheres, since all quantifiable peaks must have short retention times, and those with long retention times will appear as poorly-defined peaks which may interfere with quantitation of the well-shaped peaks of later injections. Consequently, if contaminants other than the one of interest are known or suspected to be present in the workplace atmosphere, it is essential that these compounds be considered when the selection of a column is made in the laboratory prior to field work.

There is a similar limit to the usable retention time region for a capillary column operated isothermally. Its advantage is that within this usable region it does more efficient separations. Some portable GCs have provision for backflushing the column, which is useful in removing strongly adsorbed compounds which are trapped on the front of the column.

An important point to remember is that an observed retention time does not constitute unequivocal identification of a contaminant; an independent, specific identification must be made to confirm the identity of any peak. However, the knowledge that the compound in question is in use or is likely to be present plus the presence of a peak with the expected retention time is strong evidence for the identity of the contaminant. Thus, some knowledge of the chemicals and the processes in use in the atmosphere to be examined is essential. Retention time on a second column of different retention characteristics is a valuable aid to identification.

Such important parameters as the detector drift with time, retention time variation with temperature, usable battery life, etc., should be determined in the laboratory prior to use in the field. 1/15/98