In laboratory parlance, its the difference between sensitivity and specificity.
Screening tests, ie . field tests, are by design very sensitive -- they have a very low false negative rate, which is what you want if you're looking through a broad range of materials for a weapon. However, the down side of any extremely sensitive test is a lack of specificity ( ie, a high false positive rate ). In this case pesticide residues cause a false positive test -- pesticides are incidentally very closely related to nerve agents chemically.
A highly specific test is the second tier test for all the samples that test positive on the screen. It will be designed to have a very low false positive rate, and very specifically identify chemical weapons, and weed out the 'pesticide' residue samples that are flagged as positive in the first screen.
Its a scientific method with broad usefulness.
I agree but I don't actually believe that all of these "false positives" are false positives. I believe they are true positives. High sensitivity in the interest of caution in the field is great and all but an ROC curve with a spike at specificity=1 would be totally worthless. ;-)
What seems to be going on is like you say, that they have some field tests for "pesticide-like" substances. The field test comes up positive but some later screening test fails to identify the substance.
But that doesn't mean the substance was REALLY a "pesticide"! Pesticides stored in camoflaged drums near military installations? Why?? Are we required to turn off our brains when thinking about these matters??
The more likely explanation is that designing a substance which would be explainable as merely a "pesticide" (or explainable in some other way) to inspectors was a primary design criterion for all Iraqi CW scientists. Think of it this way: suppose they did have ongoing CW programs. If so, it would be almost certain that they would "camoflage" it so as to be invisible to (perhaps not-very-motivated) inspectors. Indeed, it stretches credulity to suggest that this would not be a feature of a hypothetical Iraqi CW program. The idea that Iraqi CW programs (assuming they had them) would take place in buildings labelled "Iraq Chemical Weapons Research Facility", and would labor long and hard, investing many man-hours and petrodollars, to create substances which all known tests would identify as "chemical weapon and nothing else" and thus would be immediately confiscated, is just absurd. How stupid are we supposed to believe these hypothetical Iraqi CW scientists were, anyway?
Its a scientific method with broad usefulness.
Of course, but that usefulness does not extend to ferreting out prohibited activity in the arena of chemical weapons, which is by its very nature likely to be designed to come up "false positive" on whatever test is in use.