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Author: Subject: HELP! Tiny differences in MW??? 253.33 vs 253.44
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[*] posted on 9-2-2022 at 18:19
HELP! Tiny differences in MW??? 253.33 vs 253.44


So I am doing two different procedures to get to target compound 253.33. The first procedure uses a few reagents and according to LCMS has mostly 253.33 but a bunch of other smaller junk peaks. The second procedure uses 1 reagent and is very clean showing just a peak of 253.44 with another minor peak. I have run both procedures many times over the last 8 months and always sent them to a professional lab for LCMS and every time the dirtier procedure comes back as 253.33 and the other cleaner procedure comes back as 253.44. What accounts for this consistent tiny difference? Am I getting two different compounds?

At first I attributed it just to the settings of the LCMS and I assumed both were producing the same compound since both procedures should produce the same compound. But after month after month of getting the same results of 253.33 vs 253.44 there must be a real difference. But what in the world could it be? 0.11 is way too light for even a simple proton and way too heavy for an electron. So what in the world explains it?

Any thoughts would be greatly appreciated. Thank you.
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Sulaiman
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[*] posted on 9-2-2022 at 19:01


Try having your starting chemicals analysed,
maybe natually occuring variations in isotopes?




CAUTION : Hobby Chemist, not Professional or even Amateur
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AvBaeyer
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[*] posted on 9-2-2022 at 19:39


Your question is unanswerable in any helpful manner without a complete description of your chemistry. What are the reaction details for both procedures?

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[*] posted on 9-2-2022 at 19:47


I was with you until I thought about it.

The cleaner procedure that gives me the slightly off and heavier 253.44 I use just stannous chloride and methanol. But the dirtier procedure that gives me the correct 253.33 I use the exact same stannous chloride and methanol but I also throw in catalytic amounts of ammonium chloride. So if the procedure with the added ammonium chloride gave me the slightly heavier weight then based on your reasoning I could maybe say, "ok the hydrogens in the ammonium chloride I have that are donated to the reaction are heavy hydrogens or some other isotope which is producing the slightly heavier weight". But that is not the case. The procedure with the added ammonium chloride is giving me the correct lighter MW whereas the procedure giving the heavier slightly off MW are the same exact reagents used in the other procedure (just missing the ammonium chloride). So the one procedure with the one added reagent is actually giving the correct MW not the slightly heavier MW.
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clearly_not_atara
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[*] posted on 9-2-2022 at 20:13


Isotopic variations can happen in any element. You are talking about a 0.043% difference in molecular weight. The best available techniques have only been able to constrain the atomic weight of sulfur within a range of 0.005%. So you are potentially within a factor of 10 of the best possible.

Meanwhile, the COTS mass spectrometer you are using is probably pretty accurate, but it can still be affected by things like field inhomogeneities, induced currents, multipole effects etc leading to distorted m/z values. Unless we have a team of qualified physicists inspect your equipment, I don't think you're going to find a good answer.

If you're observing identical physical and chemical properties and the LCMS MW difference is 0.04%, you're looking at two samples of the same compound. As I tell my students, four significant figures is a lot...

[Edited on 10-2-2022 by clearly_not_atara]




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 9-2-2022 at 21:15


^^^^ Well that answers that. Thank you.
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