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- Posts: 4
- Joined: Mon Feb 20, 2023 7:42 am
- Location: Australia
I have a question regarding a drop in response of my continuous calibration standards over time during a multi-day run.
Context: Analysis of THM's through GCMS headspace analysis using an Agilent 5975C. We run both THM and TPH analysis on this instrument, starting the week with a full calibration set, followed by continuous calibration injections every 10 samples. All samples are liquid water.
Problem: My initial full calibration will be completely fine for all levels and analytes. Recently, after the first day of running I will notice that 2 of my 4 analytes will start to drop in response. For example, when I inject my 50 ppb level, Dibromochloromethane and Bromoform will give results around 35 ppb. While Chloroform and Bromodichloromethane will be fine around 50 ppb.
When I overlay the chromatograms of the initial calibration injection for 50 ppb with the "dropping" continuous calibration 50 ppb injection there will be no significant difference in peak shape, height or width.
The only thing that has been changed with the instrument is we replaced the ion source with a cleaned one, otherwise no other maintenance recently.
Any help would be greatly appreciated.
Kind regards,