Chromatography Forum

we have two methods using phosphate buffer and H3PO4 respectively as mobile phase(MP). Now we want to identify some impurities using LC-MS in those two compounds. I know both phosphate buffer and H3PO4 are not MS compatible. In this case, should I change the MP to MS compabitle acids? If so, how should I know if the elution order will be the same? How to line up the peaks? Is there any rule of thumb? Any references?

Please help! Thanks in advance!

Apple

Sometimes the easiest thing to do (before overanalyzing) is to make a few simple runs with common MS friendly mobile phases.

You could start with acetic acid or formic acid at 0.1% or adjust the pH with their ammonium salts at about 10 mM.

Try out your new buffer system with UV or PDA before gearing up the MS. It is not always necessary, but depending on your MS source, you may want to change the column diameter to optimize the flow rate.

About the elution order, unless your molecules are isomers/isobars you should know which one is eluted when as you have molecular mass information. Also most of the times, you will not notice a lot of selectivity differences changing your phosphate buffer to a ammonium acetate/formate buffer (same pH, in a reverse phase column). Depending on the application, you might (or not) notice some resolution loss...

If you are not sure about the elution order, you can collect the peaks on your phosphate system and (co)-inject them onto the MS-compatible system .
Unless you are hunting for peaks around the LOD, two or three injections usually are enough to obtain detectable amounts of collected peaks.