Chromatography Forum

Hi.. Need a help regarding calibration.
Currently I am using Agilent GC with FID. I have to analyse the benzyl alcohol and benzaldehyde. For this I am using Elite-1 capillary column. I did the calibration (internal standard). But every time the response factor is changing for both the analytes. Also I am wondering why the peak area is changing every time with injection (here % area is same). Also I am supposed to get for e.g. for 10 ppm peak area= 10000 and if I will change concentration to 20 ppm I am supposed to get peak area more than 10 ppm peak area (here I think approx. twice I should get). But I am not getting peak area double but sometimes it will be less. Please provide me your valuable suggestions.
Thanks in advance.

Welcome to the forum.

Most likely you have poor injection repeatability, or a problem with inlet conditions. Possibly you are using the wrong solvent.

Please post all the details of your method and set-up.

Peter

Thanks for your suggestions.
I am giving method details below:
Injection temperature =200 ,
Oven tempt. program- 60 for 1 min and ramp of 40 to 230 and hold of 1 in.
Injection volume- 1 ul
For solvent I used toluene and cyclohexane both but faced same difficulties again.

Hello

I'd recommend:

1.Check syringe - check if plunger is moving freely and there is no contamination. Also check if it is tight (leaking?)
2.Check inlet - leak/split vent trap problem
3.Check liner - position of wool inside (perhaps evaporation problem)

Regards

Tomasz Kubowicz

How much variation are you seeing?

Does the internal standard response co-vary with the analyte response? e.g. if the analyte response half what it should be is the internal standard response also half?

If the variation is not too drastic and the internal standard co-varies then it may not be too much of a problem.

Are you manually injecting or using an autosampler.

Gasman

Yeah I am manually injecting samples.

As Peter earlier mentioned, you probably have poor injection reproducibilty. Manual injection is not easy, and you should look at the results of your calculations using the ISTD method. The purpose of ISTD is take out the problems with manual injections. Look at the ratio of the area of the ISTD peak and your peaks of interest. The ratio should be close for multiple injections. If the ratio is changing then you have serious problems. You will need to make sure that you push the needle into the injection port the same depth and the time that the needle is in the injection port is always the same. If the needle is in the injection port for any length of time (seconds), it will heat up and more sample will evapourate from the needle. If you look at any of the modern autosamplers, the needle is in the injection port for a fraction of a second.

Gasman