-
- Posts: 11
- Joined: Wed Aug 25, 2021 3:05 pm
Background: I've very recently taken over running a Metrohm 940 Professional IC Vario that is set up for simultaneous detection of cations and anions. The person who was running the instrument knew less about it than I do (which is saying a lot), so there are no established protocols or procedures for anything regarding it. My training consisted of "put the vial in the rack and press the play button".
Purpose: The lab is looking for the concentrations of standard inorganic cations and anions in samples we are sent from the field (we do a water extraction on the samples). The testing we do is not governed by any regulating body, although we are currently trying for ISO 17025. The method that has been cited on past reports is ASTM D4327, which is similar to EPA 9056A (part of SW-846).
Problem: In my opinion, in order to have any integrity as a lab and any confidence in the reports we send, we need to establish some kind of QA/QC for what we're doing. The first thing I'm attempting to tackle is defining a reportable range, and in order to do that I need to determine an LLOQ. However, given that I have no defined or regulated method, I'm struggling with how to do this. I also don't have a good grasp of statistics, despite my best efforts, so I get lost just trying to Google things. For example, I don't want to report a value of 0.005 ppm because I don't trust it, but I need to be able to provide a reason as to why I don't trust it.
Questions:
1. To determine the LLOQ, can I just pick a really low concentration, make a standard and run it several times, and then calculate something like %RSD and say, if %RSD is less than X we're good to go?
2. I've read in several places that the LLOQ can be calculated as 10 time the std dev of the blank. But I don't understand how that's possible for IC, because the blank run is a graph, not a single data point. How do you calculate std dev of multiple graphs?
3. Each anion/cation of interest will have its own LLOQ, correct? And it is standard practice to set this value as the lowest point of a calibration curve?
4. What other questions should I be asking?