So we have one lab we contract with that sends us samples to be tested for chromium on the MS. Time after time after time, they send us samples, we test them, and then they demand a retest because they aren't happy with the results. Every time. For many months. If it were me, I would send them to a different lab if I weren't happy with the results. It's not that I don't trust our methods, but every method has a flaw, and perhaps their specific types of samples just don't work with ours, or else their samples really don't have the proper amount of Cr in them.
My coworker and I both run these, and we both get different results, and both our results are out of spec: too low. We're a bit tired of it all...
But at least every time we run them, the samples have some Cr in them. When I ran a million of them last night while I was at home, I came in this morning to find that they all had 0.000 ppb...so something wasn't right. The very same method had worked fine the day before. It was reading the Cr QC just fine. I figured maybe there had been a clog last night and started it up again this morning, but I had the same results! Good calibration, good QC, BAD results. ZERO results, actually.
So, my boss and I decided to keep trying different things. First up, we decided to test other isotopes of Cr. We had only been looking at mass 53, but we set it up to read 50, 51, and 54 as well. We started it up again, and VOILA, it was reading and calibrating very well on masses 50 and 51 (53 was crappy, and 54 was completely worthless). HOWEVER. The results were still out of spec, but instead of being too low like they always were with mass 53 they were too high! FRUSTRATION!
First I made a new QC, just to make sure. It was a little high, but still fine. Then I made new standards and started the whole thing over again. Still a little high, but now in the acceptable +/- 10% range. I made a new blank as well. Everything was now working AWESOMELY, but we were still getting high results with 50 and 51, and low results with 53, and complete garbage with 54.
I left it running this afternoon while I went to see a doctor. I hope it behaved for my boss. We'll see if they call us, demanding retests...