LC-MS/MS for sugars

[mariia.maak]

Hello! I'm trying to measure glucose and ribose using MRM. However, I can't find the precursor ion for either of them. On the attached picture is glucose in 10%MeOH matrix. The instrument is set to a negative mode. The neutral ion mass is 180.06 Da. Can anyone explain what is it at the mass of ~162 Da and later at ~197 Da?https://imgur.com/a/jbrADbQ#YqT9171

[Gaetan Glauser]

In principle you should get an (M-H)- at m/z 179 in negative mode. However, the declustering potential you applied is very high and in-source fragmentation may have occured, leading to m/z 161 (loss of H2O). Your instrument might be a bit detuned if you see a peak at m/z 162. Try to reduce the declustering potential and look if m/z 179 appears.

[mariia.maak]

In principle you should get an (M-H)- at m/z 179 in negative mode. However, the declustering potential you applied is very high and in-source fragmentation may have occured, leading to m/z 161 (loss of H2O). Your instrument might be a bit detuned if you see a peak at m/z 162. Try to reduce the declustering potential and look if m/z 179 appears.

Thank you for the suggestion. I started at the DP of 0 and was seeing high-intensity peaks at 197 Da. Increasing the DP helped in reducing those peaks. But changing DP didn't help in identifying glucose at 179..

[lmh]

You've got isotope peaks (presumably isotope peaks) at intervals of +2, which suggests to me that it's not organic. It might be something salty.

Sugars ionize really badly except at high concentration; if you've used the sort of concentration that you'd normally pick for developing an MRM method, you might not have a visible signal from the sugars at all.

[mariia.maak]

You've got isotope peaks (presumably isotope peaks) at intervals of +2, which suggests to me that it's not organic. It might be something salty.

Sugars ionize really badly except at high concentration; if you've used the sort of concentration that you'd normally pick for developing an MRM method, you might not have a visible signal from the sugars at all.

Thanks! I might try increasing the concentrations when developing an MRM method, though I don't expect my sample to have high concentrations of sugars... I am also using ESI if that matters, maybe other sources are more efficient in this case.

[JMB]

I agree with LMH that you have isotopic clusters of 2 amu within the cluster.
The cluster at m/z 160.7 to 166.6 looks like a Cl3 pattern of 27:27:9:1 theoretical relative intensity for 35Cl ratio to 37Cl of 3:1.
The cluster at m/z 195.6, 197.6, 199.6 is then very likely a Cl4 cluster with m/z 201.5 , 203.6 not shown; relative intensity theoretical of 81:108:54:12:1.
Do you have a scan that includes up to m/z 250???

If m/z 160.7 is a 35Cl3 ion, then [M + Cl3]- = 160.7, and M = 55.7 as a neutral.
Is the mass calibration good?? I wonder if that should be 56.

You might test your solution with acidified AgNO3 solution for chloride.

Regards,
JMB