Mobile Phase Contamination

[Widebore]

I have observed a peculiar mobile phase LC-MS artifact for some time when using gradient elution. The mobile phase contains water, methanol, formic acid and ammonium hydroxide. At high aqueous content, the contaminant can not be seen. However, as the gradient progresses to its maximum at 90% methanol, a large peak elutes with an m/z of 326.4. The TIC baseline becomes raised for a while after the main part of the peak elutes. I use high purity water, LC-MS grade methanol, LC-MS grade formic acid and LC-MS grade ammonium hydroxide. Has anyone seen this before using a similar mobile phase?

[DSP007]

I have observed a peculiar mobile phase LC-MS artifact for some time when using gradient elution. The mobile phase contains water, methanol, formic acid and ammonium hydroxide. At high aqueous content, the contaminant can not be seen. However, as the gradient progresses to its maximum at 90% methanol, a large peak elutes with an m/z of 326.4. The TIC baseline becomes raised for a while after the main part of the peak elutes. I use high purity water, LC-MS grade methanol, LC-MS grade formic acid and LC-MS grade ammonium hydroxide. Has anyone seen this before using a similar mobile phase?

It.s may be water or column contamination .Prior to this were analyzed on a column? 326 m/z is type steroids, retinoids and oher pharmaceutical substances ... .4 - may by chlorine radical in molecula .

[Widebore]

Hi, We typically analyse environmental samples for pesticides, herbicide, insecticides etc. I shall start with the water. I'll have to buy some to see if it that. Further input would be appreciated.

thanks

[Gaetan Glauser]

I know of a peak at 327.008 that contains chlorine atoms (you should see the isotope at 329.006) and is often present as a contaminant in ESI positive. I am not sure where it comes from because it always kept little intense and I didn't bother to find this out. It may also come from your MeOH or additives. The fact that it elutes as a wide peak at 90% MeOH let suggest that it is retained on the column at lower % MeOH during equilibration and beginning of the run, thus accumulating for a while before elution occurs. To test this hypothesis, you can bypass the column and run 100% H2O, then 100% MeOH, and finally 100% MeOH or water + additives (FA and/or NH3). This will also enable you to determine whether the column is responsible or not.

[Gaetan Glauser]

Hi,
the ion at m/z 327.008 I talked about contains a bromine atom, not a chlorine. Sorry for the mistake, I have just realized it.