I'm a technician in an analytical soil chemistry lab and I've been trying to think of a good reason for these confusing results; any help would be welcome. The instrument is an Elementar TOC/TN analyser, analyzing liquid samples extracted from soil.
Each run includes a standard curve made from a stock prepared in lab, a mid-run and end-of-run 'check' (just one of the points from the curve), a spike, and a QC - ordered from sigma. It analyses for carbon and nitrogen simultaneously with two sensors. Carbon is on an IR sensor, and nitrogen is assessed with an EC cell. Carbon isn't causing any issues, it's accurate and consistent run to run. The nitrogen, however, is giving me some grief.
Run to run, my standard curve has a very consistent slope. The spike recovery is pretty consistent and within +/- 10%. The in-run 'checks' are consistent and pretty accurate, so drift isn't a huge issue. But over the last couple months the QC has been slowly and consistently trending downward, returning a lower and lower value on each run.
I've gone through a couple of bottles of the QC solution and it seems to be decreasing the reported concentration independent of which bottle I use. I'll also note that I've used two batches of standard stock solution, and I make a new curve from the stock solution for every second run.
Ideas as to what might explain these strange results? I'm at a loss. I've thought about the possibility that the EC cell needs to be replaced, but then I would expect the slope of my standards to change or to become less linear. Same with the standards just being contaminated but, again, I'd expect a change in slope if that were the case.
Thanks in advance!