In my icpms analysis there are 4 measurements taken for each sample. The counts per second for some elements, especially Iron, drifts downward for each of the 4 measurements. For example one sample has 7.9x107, 6.9x107, 6.4x107, and 5.2x107 cps
What might cause this?
The calibration curve is good for Iron, and most other elements do not have this issue in the samples.
Assuming you are using an autosampler with liquid samples and vacuum/peristaltic pumping to load the samples to a pneumatic nebuliser:
Which isotope of iron are you observing?
- could there be an interference adding to the earlier replicates that is decreasing as the sample continues?
Are these the replicate sweeps of a single measurement, or 4 individual samples?
-could the later replicates be diluted by the carrier solution?
For how long is each observation (dwell time)? 10 ms or 100ms or longer?
How are you transporting the sample to the nebuliser? Is there a t-piece for internal standards or is it direct from the sample tube?
Is your sample settling in the sample vial before it gets sampled? e.g. Blood and plasma are notorious for settling out, could your material be doing the same? Sampling from the bottom of the vial could have more, denser material with extra Fe coming to the nebuliser first.
Which other elements are doing the same thing, which aren't? Are they on the same order of magnitude as the iron?
Yes. An SP4 autosampler with liquid samples and peristaltic pumping into a micromist nebulizer.
The rsd is an issue with all Fe isotopes we analyze, 54, 56, and 57. This issue doesn't happen often, but when it does it would be good to understand why.
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