Chromatography Forum

Hi All,

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,

Welcome to the forum.

If peaks are the same shape, height and width then they should have the same area, and then the change in results can only be due to a change in where the integrator is putting the baseline. I would guess that you have a progressive build up of minor contaminants, bleed and ghost peaks creating beasline drift, and that the integrator's detection of peak onset and end is affected.

Is your IS rising? If the area count on the dropping analytes is the same that's the only thing that makes sense. You're concentrating or losing some compounds in the headspace. Is this soils headspace or something? 524/8260 is what comes to mind when I hear THM.

Normally we would do THM by EPA 524 or 8260 using purge and trap. Never tried it with Headspace analyzer.

When you say same height and width of peaks are you looking at Total Ion Chromatogram peaks or the Extracted Ion Chromatogram peaks normally used in quantification? Sometimes the total peak will have similar sizes but the extracted will be smaller if the tune shifts a little over time.

Do all four analytes use the same Internal standard or different ones? This would be how it changes as mentioned above if one internal changes response and another doesn't.

We probably need a little more information on the setup and analysis to trouble shoot it completely.