"Which begs several questions,  first can we really detect at that level with any confidence, "

As an under grad I worked in the lab that held the record for lowest level of quantification for a chemical analysis.  That was in 1960 and the level was a few ppb for iron.  The hard part of that analysis was getting the glass ware clean and keeping it clean.  For instance you could never let the glass dry as the surface chem of glass changes when it drys allowing iron in the glass to migrate to the surface which would contaminate your sample.

Within a decade ppm level quantifications were common for organics althou false positives were pretty common due to both low numbers of theoretical plates in chromotography columns and lack of detectors that provided a good level of selectivity.  Both those factors made many of the ppm level results suspect.  For example you can not trust any of the DDT numbers reported back then for environmental samples.  Those numbers turned out to be pure fiction.  By the early 1980s it was possible to push levels of quantification down to parts per quadrillion for some organics.  We did it in our lab.  Going to gas chromotography columns that had tens or hundreds of thousands of theoretical plates instead of only a few hundred plates and adding a mass spectrometer as a detector totally changed what was possible.

I have no idea what the lower limit is today.  Way below a ppb for sure for most any organic molecule these days.  The biggest problem you have is contamination of equipment resulting in bad data.  When you are working at those levels you better not have macro amounts of the substance of interest anyplace in the lab where you are doing the analysis or you are going to have contamination problems.

Dick

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