Co-eluting peaks in GC/MS

[alexandros]

Hi everyone,
I am trying to detect bisphenol A with a 5890 II GC/MS with bisphenol A-d16 as an I.S. The two peaks are almost eluting at the same time, wich means a cannot measure the areas and quantifing the samples.
Any ideas how i can achieve better resolution between the two peaks?

Thanks,
Alex

[LiVD]

Given the fact you're using an MS, that shouldn't be a problem... Just do your quantification on their mass traces instead of on the TIC.

[James_Ball]

Given the fact you're using an MS, that shouldn't be a problem... Just do your quantification on their mass traces instead of on the TIC.

Yes, with GCMS you do not have to completely resolve the peaks if you have different masses present in each spectra. With those two compounds there should be a peak in the d16 version that is 16amu greater than the main peak in the undeuterated version which would make them easy to quantitate even if they completely co-eluted.

[alexandros]

I performed the analysis and the mass spectra provided all the data i needed. I will try to change the oven programm to achieve better resolution, only because i enjoy develop GC methods.
Thank you very much anyway!!
Alex

[rb6banjo]

It is quite likely that you will never resolve those 2 chromatographically.

The reason you use deuterated materials as standards is because they are easier to sort out with a mass spectrometer. Benzene is molecular weight 78 (with 6 protons). Benzene-d6 is molecular weight 84 (each deuteron is 2, protons are 1). After you collect your chromatogram with the mass spectrometric detector, you sort the mass spec. data by the characteristic masses for each material. Thus, if (more likely when) they coelute, it doesn't matter.

If I were working on a method for trace benzene, at a minimum I would monitor m/z = 78 and m/z = 84 (benzene gives a pretty good molecular ion). You need to think along this line in developing your BPA method using deuterated BPA as your internal standard.

[Peter Apps]

16 deuteriums is a mass increment of 16, which is a bit more than a methyl group and so it is slightly surprising that you get a complete coelution. What column are you using, and what temperature programme ?

Peter

[alexandros]

Peter, i do use an Rtx-5MS GC column and the oven temperature program is: 80 oC, 20 oC/min to 150 oC (hold 2 min), 10 oC to 250 oC and 10 oC to 280 oC (hold 2min). The total time is 20.50 min.
The method was previously developed in the laboratory where i work. Fορ anyone interested, there is the corresponding paper published.

[Peter Apps]

Even with those very steep temperature ramps I am surprised that the labelled and native compounds co-elute completely.

Peter

[Bigbear]

As others have said I doubt you can totally resolve the two peaks. I wouls suggest you inject each component singly and look at the spectra. You will see the patterns are similar but the duterated will be about 16 daltons more. Choose the ions that describe each of the compounds and go from there.