Automatic instruments are used to measure analyte concentrations in situ. Sampling, sample preparation, separation, and detection steps are performed onfield. Online gas chromatograph instruments are often used to measure BTEX in ambient air.44'51'61'67 Table 14.10 shows some commercially available online instruments for BTEX analysis:
Online automatic instruments for measuring BTEX use active sorbent sampling coupled with thermal desorption previous to the separation and the detection steps. The aim of collecting such a low volume of air is to achieve low sampling times with total analysis times of 15 to 30 min. Therefore automatic instruments can be applied to control emissions in order to record the variation in concentration of pollutants during a period.
Good agreement between the results of an online instrument and a pumped-thermal desorption analysis are demonstrated.44'51
However, standard deviations and detection limits are better for automatic than for manual analysis; especially if the latter involves a passive sampling-solvent desorption method. This is a consequence of the many sources of errors possible in a manual method which are not present in automatic instruments.
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