Chromatography Forum

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

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.

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..

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.

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.

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