The world of ppm and lower detection thresholds is a confusing labyrinth of assumptions, equipment quirks, and a place where you definitely can’t confuse accuracy with precision. All of those lovely 9’s queued to the right of the decimal place. I do so want to believe what I see. But so often they are from a tight cluster of bullets away from the bullseye.
For those who must tread in this arcane world, I can only recommend that you find a good analytical lab and get to know the analysts well. They can fill you in on the sorry truth of sub-ppm detection and quantitation.
The trinity of ICP , quadrapole, and the Blessed Dynode allow access to the innermost ring of analytical hell. At the sub-ppm level, most of the periodic table begins to stand out of the background. Once apparently pristine material, like a trailer park divorcee, suddenly reveals a sordid history. Pick your method and stick to it. If you go nosing around with other methodologies, you may be in for a disheartening picture.
Lamentations on Chemistry 
I’ve had several negative experiences with analytical labs. They seem to be run ‘to the breaking point’ with odd shift-work and 60-70 hour work weeks.
The poor people are worked to the bone. Instrument calibration is eveything here. And I’m not sure how people who are in a constant state of exhaustion will go the extra mile to ensure quality calibrations. I liked the people I met, but they were a sad lot. Mostly on temp visas and treated like slaves.
I’ll always remember visiting on a Sat and finding the labs full. Despite being essentially technicians these people were forced to work at least twenty hours more a week than what they were paid.
What a wonderful country we live in! Living to work not working to live!
That’s an interesting observation- analytical sweatshops! Each instrument is capable of just so many runs per week. Idle time is opportunity cost.
“All of those lovely 9’s queued to the right of the decimal place. I do so want to believe what I see.” It seems like chasing purity is akin to chasing “safety”. People want total purity and safety, not realizing the Quixotic nature of the quest.
Yeah, it is a bit weird. Some customers specify 4N or 5N purity entirely on the basis of precaution rather than data. A lot of money gets spent on excess purity out there.