[Babcock_Hall]

Hello Everyone,

In isotope dilution mass spectrometry, a multiply deuterated internal standard is typically added at the start of a purification protocol in clinical or forensic toxicology, for example.  How does the sensitivity of the MS experiment change from protium to deuterium, or are they equally sensitive to the detector?  I imagine that use of a standard curve would correct for differences, but I am wondering whether or not there should be differences.  Put another way, should a standard curve have a slope of unity?

[Babcock_Hall]

Quantitative Applications of Mass Spectrometry, Lavagnini I, et al., Wiley (2006) ISBN: 0-470-02516-6
I found a chapter covering quantitative mass spectrometry that had a standard curve in it.  The authors made a series of five mixtures, keeping the concentration of d₃-testosterone constant while varying the concentration of testosterone.  Both compounds were converted into di-heptafluorobutyryl derivatives.  The standard curve placed R, which is the area at m/z 680 over the area at m/z 683 on the y-axis, and concentration of testosterone on the x-axis.  By eyeball, the slope looks to be close to one.  I will keep reading...

[kriggy]

Hi there,
sorry for my late reply to your PM but Im not as active here as I once was.

I cant give you good answer since while I work with isotopes, im synthetic chemist. This means sensitivity of detector is not as important to us. Anyway, I think it mostly depends on the type of MS you are using and position of the labels.
In general, with ESI-MS I dont think there is much difference. With more energetic methods such as EI, I can see different fragmentation patterns due to presence of heavy atoms because of the strenght of the CD bond compared to CH.

What you can observe is LC or GC separation of the cold and D-labeled standard, especialy if its heavily deuterated (D5+). The separation is poor (in our experience) but in several cases we observed peak splitting and/or varisous ion intensities at the start and the end of the peak.

[Babcock_Hall]

I had not thought about the kinetic isotope effect on fragmentation, but that makes sense.  The difference in retention times leads me to wonder more about how to measure and compare the peak areas.

[kriggy]

I think its not that common, it was one case IIRC.
Also, with the addition of the isotope standard, you compare peak areas/intensities in the mass spectrum (ideally in SIM mode) not UV because UV might overlap with other stuff.

[Babcock_Hall]

I think its not that common, it was one case IIRC.
Also, with the addition of the isotope standard, you compare peak areas/intensities in the mass spectrum (ideally in SIM mode) not UV because UV might overlap with other stuff.

Yes, I meant peak areas or peak intensities in mass spectrometry.  My point is that if you took any single scan, the ratio of undeuterated to deuterated compound would not be representative of the true isotope ratio, owing to the difference in retention times.