By Anonymous on Tuesday, May 25, 2004 - 06:34 am:

I am trying to analyse a carboxylic acid compound by LCMS with ESI. I am getting poor sensitivity with negative ionisation and I am getting a +18 m/z with positive ionisation, also with poor sensitivity.I have tried changing buffer conditions, acid for +ve and basic for -ve. Does anyone have any suggestions?

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By RSlingsby on Saturday, May 29, 2004 - 04:41 pm:

Carboxyic acids are best analyzed by IC-MS using an IonPac AS11 anion exchange column, a suppressor, and MS detector. The -ESI conditions for this vary with the model of MS but detection limits are generally in the range of low ppb. If you can provide more information about what you need to do I can send more detailed instructions.

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By MG on Tuesday, June 1, 2004 - 06:58 am:

I have found that nearly all carboxylic acids work just fine on a reverse phase column with ~ 0.1% acetic acid in the mobile phase, and that negative electrospray is the preferred mode. Is this single MS or MS/MS?

If MS/MS, many acids give a predominant fragment from loss of 44 (CO2), but this is nonspecific and often noisy in dirty samples. Try another product ion if available.

If this is single MS, many acids readily lose CO2 at low CID energies. Be sure to optimize your lens voltages (fragmentor, DP, cone, whatever) to maximize the ion of interest.

In negative ESI, check to make sure you aren't producing a corona discharge. You will see this either by a high ESI current (if your instrument provides this info), or by turning out the lights in your lab and looking in the source for a little blue glow on the end of your needle. If corona discharge is present, lower your needle voltage until it goes away. This will give a less noisy signal.