problem with detector sensivity

[o.plaskon]

Hello:)
I have a problem with detector sensivity. Every time when I make calibration curve, the peak area (Ai) for particular (concentration) standard increases. So e.g. for concentration 10 ug/l the peak area is e.g. 100, the next time for the same concentration 10ug/l the peak area is larger, the next time more larger and after some time the peak area is too large and I can't determine this concentration (standard) on the calibration curve. Limit of detection is better with time, but the highest concentration, which is on calibration curve decreases. I can't determine all standards, which I need to have. I can work on one calibration curve only for very short period of time. I inject the sample and standards with SPME technique. Maybe someone had a similar problem? Thank you for all answers:)

[iainconlin]

Hi,

Can you give more details? What type of detector, inlet settings, column etc.?

Have you tried any injections not using the SPME fibre , do you get the same effect?

[o.plaskon]

Thank you very much for answer:) This SPME method doesn't influence on problem. The same situation is when I work with another method. It's the determination of THMs (sample-water)

determination of THMs:
Column-Rtx-5 (or ZB-5 - I tried also on this column) , detector ECD, direct injection
(more details I can write tomorrow)

But when I determine pesticides is the same problem:( So it isn't problem with column

Initially differences between peak areas for the same standard were lower, now peak areas vary very much e.g. by order of magnitude.

[ngocduyfc]

Thank you very much for answer:) This SPME method doesn't influence on problem. The same situation is when I work with another method. It's the determination of THMs (sample-water)

determination of THMs:
Column-Rtx-5 (or ZB-5 - I tried also on this column) , detector ECD, direct injection
(more details I can write tomorrow)

But when I determine pesticides is the same problem:( So it isn't problem with column

Initially differences between peak areas for the same standard were lower, now peak areas vary very much e.g. by order of magnitude.

Which types of standard curve did you use/wanted to get; might the Relationship between injection amount and the peak areas is not really linear.