I am using a GCMS system to analyze ambient air samples looking for different VOCs. For calibration, we used a standard cylinder with the target gases. We found that with the method we were using at the time there was a small quantity of water going to the detector when sampling and analyzing at the same time. This was causing ion source degradation quite fast even in two weeks the signal will decrease by half or more.
My question is if there is a way to correct those calibrations by normalizing the signal somehow? or I just have to include a huge factor of uncertainty regarding the calibration SD?
Attached is a plot showing the changes in the calibration response during the sampling period. It goes higher when we change the ion source.
If I understand your question correctly you are assaying the VOCs against a mixture of external standards. The performance of the MS as you have noted will deteriorate as the source become progressively contaminated. The rate of contamination will depend on the amount of material that is being pushed through for your analytical procedure. The idea of the external standard is that you can measure the amount of analyte in your sample, independently (up to a point) of the performance of the source. The standard needs to be run often enough (beginning, middle and end of each run, maybe more often) to enable you to correct for the progressive loss of sensitivity of the source.
The short answer is that the external standard provides the assay correction.
Jhonathan - is a great question with many practical implications for environmental analytical chemistry. My quick answer is yes, it is reasonable to normalize your raw data per observed changes in detector calibration. I would also recommend reporting the raw (not-normalized) set of data in (e.g.) in the Supplemental Materials to a publication(s) stemming from this work. Then, it will be transparent how the normalization impacts the results and conclusions. Also, it might be useful to check the integrity of your standard gas mixture(s) which can be impacted by temporal changes ('aging') that affaffectects the nominal (initial) certification. Calibrant verification would require another (more stable) detector. I hope it helps,
Jacek
Was an internal standard run? If so, I would trust using a bracketing approach, correcting between standard injections, using the IS area.
- Badges
- Science topic
- Similar topics
- Filing