Chromatography Forum

I have gas phase standards of 1 mg/L, 0.1 mg/L, 0.01 mg/L and 0.001mg/L of benzene in them. The 0.1 to 0.001 mg/L turned out really linear, but the area counts of 1 mg/L one is always about 25 times of that of 0.1 mg/L instead of 10 times. I have checked everyting and tried at least three times of making new stadards, different split ratios and injection volumes. I also tried SIM and SCAN mode. But it has always been like that. Does anyone have any idea how/why this would happen?

I am using HP5971A GC-MS and a BETX column.

Thanks a lot! Hong

Are you manually injection these samples or using an online switch?

hong2009,

If it were linearity, typically your high standard will report low, not the other way around. Also, I would expect SIM to saturate even quicker which would mean not linear in SIM even below the 1 mg level. If you have good linearity below that then you the other three appear correct.

Are all 4 of the standards made from the same stock? How are you making them, and what steps have you taken to mix them? Since you are doing different injection volumes, I assume you are using a syringe?

Best regards.

Hi guys,

Yes, the lower levels are all made from the 1mg/L one. And I normaly take out 50ml of 1mg/L gas and put that into 450ml of air to make 0.1 mg/L sample in a Tedlar bag; To make 0.01mg/L sample, taking 5ml of 1mg/L and putting into 495ml air in a Tedlar bag; and so on...

I do agree if the 1mg/L is saturating the detector, it should report lower area counts, and that is why I do not understand what my problem is. The weird and consistent thing is that the area counts of 1mg/L is always 25 times of that of 0.1 mg/L instead of 10 time. So I do suspect this is a system problem.

I always manually inject 50 uL intot he GC-MS using a 500uL syringe.

Any idea of why this would happen?

Thanks!

Hong

hong2009,

My first suspicion, without more information, would be to suspect the amount of diluent air you are adding. How do you arrive at your dilution volume of air, ie how is that measured?

Best regards.