Chromatography Forum

Hello,
I am just a novice in the LC-MS field. WE have An LC-MS Zq4000 a single quadrapole analyser but only an ESI probe which is for polar compounds. Can I use non polar compounds for analysis.

2.What are the solvent that I can use if compounds are not soluble in Acetonitrile or methanol or water/acetonitrile.

3. At what concentration do I inject the sample if i use direct injection to the mass

4. What do I need for cleaning samples before injecting into the machine.Can you suggest accessories needed for the LC system and as well for Mass system for solvent purification and sample cleaning.

5.Can i dissolve sample in DMSO if at first they do not dissolve in the above stated solvents.

Thank you,
Johnny

I'm no MS guru, but in my experience I have never generated an ESI signal from a completely nonpolar molecule. There always had to be some polar funtionality for the charge to transfer from the solvent/analyte droplet to the analyte upon desolvation.

This question may be better posed in the Hyphenated techniques forum where someone with extensive LC-MS knowledge could comment.

Polarity is a relative thing. I would consider triglycerides a nonpolar type of molecule since on many reverse phase columns it can be highly retained. Most compounds that contain heteroatoms, even though chromatographically nonpolar in nature, can be persuaded to ionize under certain conditions.

We have used acetone and THF as solvents to get nonpolar compounds off the reverse phase column. Normally add 0.1 ml/min post column of 25 mmolar ammonium acetate in methanol to 1.0 to 1.5 ml/min flow of the solvent. It will enhance the ionization of many molecules. At times, I have added 100 micromolar potassium aceate in methanol post column in methanol under similar conditions if the molecules like cationization better..

Also have used 50/50 isopropanol/acetonitrile with post column addition of the ammonium acetate. Can see some experimental detail on page 20 of setup for postcolumn addition.

http://littledomain.com/james/files/pit ... int.ppt#20

The presentation also shows how to run 5 experiments at once on single MS analyzer type instrument to get MW info in pos and negative and fragmenation data.

We also do some size exclusion with post column addition of the proper reagent to enhance ionization.

Many times the MS total ion response looks very noisey and most people would say thus useless. However, if one uses the UV response to guide the background subtraction of the background from signal, one can dig out a lot of useful qualitative data.

Many people have reported the used of other solvents such as hydrocarbons and methylene chloride. Some have worried about the addition of hyrocarbons for APCI since combustible, but most think not a problem in standard LCMS sources.

Also, THF can swell and somewhat weaken PEEK tubing, and suggest using stainless steel. Normally, post column use of PEEK with THF is OK since at lower pressure. Still might use stainless steel precolumn for safety.