mobile phase vs sample solvent

[jwesley]

I am attempting to analyze a DDT derivative solution using a C18 µBondapack column. My mobile phase is 55% ACN/45% water. My standards are prepared in acetonitrile and my samples are in a MeOH/water with 0.05 M sodium sulphate. I have no trouble seeing the peaks in my standards, but can no longer see the peaks in the samples. Both are at approximately the same concentration. I was previously able to see peaks in the samples using a spherisorb C18 column but was forced to switch to a µBondapack due to resolution issues. Could the difference in solvents be the cause? The injection volume is 5 µl, so I didn't expect a problem. Any other possible causes?

[Uwe Neue]

I can't see a good reason why this would be the case. Have you checked the sample vial?

[jwesley]

I haven't checked the vial itself but have injected several samples with the same result. It appears as though the compound(s) are being retained on the column, but I don't have an explanation (or solution) as to why this is happening?

[Uwe Neue]

Let me verify if I understood you correctly: you inject the sample onto the uBondapak C18 column in a mix of methanol and dilute sodium sulphate and you do not see any peaks. You inject onto the same column with the same mobile phase a sample of about the same concentration dissolved in acetonitrile and you see peaks.

If this is correct, I suspect that the issue is related one way or another to the injection. If you indeed inject sample from these vials, then maybe the issue is an adsorption of the sample to the vials or to the septa or realted issues.

[Ron]

There are deactivated sample vials available that are designed primarily for pesticide analysis. Your samples may be in a signithe ficantly different matrix than the standards, especially if you are analyzing extracts from biological materials. I would tend to suspect the samples are binding to the walls of sample vials, and that is why you are not getting a good response.