Positive systematic bias

[wan]

I observed positive systematic bias between quantitation values obtained from 2 different ion pairs in my GCMS and LCMSMS analyses.

Given that I am using a 13C and 15N labelled internal standard and had ensured there is no matrix suppression, I was wondering what other parameters should I be looking at to track the source of this bias?

[Loekie]

None pure internal standards? maybe the labeling was not 100% and some native non-labeled is still present. a blank including only the internal standard could give an answer to this. Depending on how much native compound is in the internal standard lowering the concentration of the internal standard to minimize it's contribution could be one solution or correcting for it during the data processing another.

[Peter Apps]

How are you making up samles and standards ? - in detail please. Solvent evaporation is a possible cause of bias.

Peter

[wan]

Hi Loekie, we did run a sample containing only the labelled compound. There is very little non-labelled as far as we can see.

Hi Peter, I wish I can provide more information, but I realised I can't after talking to my supervisor. I guess I will have to look for the answers myself.

Thank you for your replies. I appreciate it.