unknown peak

[paddy1969]

Hi

I am getting a peak at 143 when running MeOH: h20 (50:50) thorough the ESI MS. It has a significant ion count of 1x10^8.

Does anyone have ideas as to what it could be and how i could get rid of it?
thanks
paddy

[BHolmes]

Is it just one peak or is there a series of peaks separated by the same amount (ie. +/- 14m/z, typical of oils in fingerprints)?

If you are truly injecting a clean, solvent only sample then this unknown peak must be a contaminant of some kind. I would first look at the quality of your solvents - make sure you are using highest grade possible.

If you are using highest grade available, then its contamination from your system. I did a quick search and found that 143m/z could be a polymeric plasticizer (http://nvlpubs.nist.gov/nistpubs/jres/0 ... 65_A1b.pdf page 2 and http://www.chem.agilent.com/Library/sel ... 5451EN.pdf page 149), could be from packaging (solvent, vials, etc...), vinyl or plastic gloves, or from o-rings damaged by high temperature.

[bisnettrj2]

ESI positive or negative? What sorts of salts or acids have been used on this system recently? Are you pumping through a column or directly from the pump to the source?

[paddy1969]

Is it just one peak or is there a series of peaks separated by the same amount (ie. +/- 14m/z, typical of oils in fingerprints)?

If you are truly injecting a clean, solvent only sample then this unknown peak must be a contaminant of some kind. I would first look at the quality of your solvents - make sure you are using highest grade possible.

If you are using highest grade available, then its contamination from your system. I did a quick search and found that 143m/z could be a polymeric plasticizer (http://nvlpubs.nist.gov/nistpubs/jres/0 ... 65_A1b.pdf page 2 and http://www.chem.agilent.com/Library/sel ... 5451EN.pdf page 149), could be from packaging (solvent, vials, etc...), vinyl or plastic gloves, or from o-rings damaged by high temperature.

Thanks for your reply. Yes I am using a clean solvent. (i will try another bottle to see if resolves it to confirm it!)

[paddy1969]

ESI positive or negative? What sorts of salts or acids have been used on this system recently? Are you pumping through a column or directly from the pump to the source?

Thanks for your reply.
ESI positive is giving the peak. I am pumping directly from the pump to the source. I have been doing only loop injections for the past couple of months in MeCN: H20 (50:50). No salts or acids have been used recently.

[bisnettrj2]

Can you try a different source of water, or methanol, or both to see if it goes away?

[paddy1969]

Can you try a different source of water, or methanol, or both to see if it goes away?

I tried MeCN from 2 different sources, meoh n water from a diff source too but comb of both meoh: h20 or mecn:h2o (50:50) gave the same peak. However when i ran just mecn 100% i got a peak at 550.

[bisnettrj2]

So, methanol:water from two different suppliers yields a mass at 143 in positive ESI. That would seem to indicate that you're getting a system peak from the MS source or the LC. You can infuse mobile phase with a syringe pump to isolate the MS source. If the peak goes away, that indicates it is coming from the LC. If the peak remains, then that points to the source (or the tubing connected to the source).

I'll ask the bigger question here - does this peak at 143 matter? What are you doing that you can't ignore this peak? If you're not looking for 143, or anything within a few Daltons of 143, then I wouldn't worry about it.

[kerri]

I have seen the 550.6 peak many times in solvents. I have found it to be disteryldimethylammonium chloride (the [M-Cl]+ peak).

I am not familiar with m/z 143.

K

[paddy1969]

So, methanol:water from two different suppliers yields a mass at 143 in positive ESI. That would seem to indicate that you're getting a system peak from the MS source or the LC. You can infuse mobile phase with a syringe pump to isolate the MS source. If the peak goes away, that indicates it is coming from the LC. If the peak remains, then that points to the source (or the tubing connected to the source).

I'll ask the bigger question here - does this peak at 143 matter? What are you doing that you can't ignore this peak? If you're not looking for 143, or anything within a few Daltons of 143, then I wouldn't worry about it.

Thanks ..will try isolating the source and if that doesnt work, I could ignore it.