Exactly so Steve,
Andre, even with the best will in the world unless the PID is set up exactly right using the correct bulb, draws from a suitable headspace, has time to equalise and the correct calibration and span gas for the single most meaningful gaseous VOC or SVOC compound you are measuring (lets ignore possible cross contamination, interference and other effects such as moisture and temperature for sake of argument) you will be very lucky to see any correlation between it and any subsequent chemical analysis result of perhaps greater than about 20-30%.
As a screening tool, a PID has some very limited field use but it is not a magic box that gives a definitive answer, despite the fact that even some top consultants seem to think it does. At best it may give a relative one, for a given value of relative. Even then, not only are you removing the very thing you actually want to measure, but also changing the conditions by exposing it and taking a sample of it. In a limited, mixed and relatively open material such as a stockpile, frankly I would be amazed if you get any meaningful correlation at all between the gas/PID result and general soil concentrations of VOC and SVOC by GCFID following extraction for example.
Sad to say this general opinion that a PID gives a definitive and meaningful result is down to ignorance mainly and a general lack of understanding about what a VOC/SVOC actually is, how it reacts in the general or specific environment as a consequence of what it is and what a PID actually genuinely measures and especially how it does. I still chuckle when I remember the consultant asked me if 15ppm was significant compared to 10ppm on their PID results. Probably still gets paid a lot more than I do :)
Regards,
Chris Swainston
Principal Engineer
The Geotechnical Centre
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