how to ionize methyl benzoate to detect it by ESI LC MS

[ym3142]

I am using Finnigan Ion Trap LXQ

I tried to use ammonium formate, fomic acid, TFA, or HCl along with MeOH/ACN as mobile phase.

With direct infusion, I could see nice peak of 137. However, with mobile phase coming I almost compeltely lost the 137 peak, under what ever combinatio of mobile phase (above)

A LC MS injection gave no peak for the ester.

I also tried to post column addition of Lithium chloride. At infusion with mobile phase I saw nice 143. But an LC MS injection did not create this peak in MS.

Why? What is the possible reasons? Do you have a suggestions for me to try?

Thanks in advance

[JMB]

1. Methyl benzoate is a prime candidate for GC/MS, NOT LC/MS (as you have discovered).

2. If you must use LC/MS, I would suggest that you run in the negative ion mode, with post-column addition (PCA) of LiCl.

Run (a) & (b) below WITHOUT any additives in mobile phase.

(a) Prepare Me benzoate in MeOH, run by direct infusion in MeOH 100%. If you see the [M+Cl]- signal as the characteristic doublet at m/z 171, 173 then repeat this with an injection onto a column. Do you see the Me benzoate peak by MS at the same RT as given by the UV detector ?

(b) Repeat the column injection with MeOH/H2O (1:1, v/v); track the peak elution with the UV detector; do you see [M+Cl]- ?

If you get a good result with (b), then remove the PCA of LiCl, and run instead in MeOH/aqueous 10 mM NH4Cl (1:1); you should get [M+Cl]- again.

[ym3142]

Thanks JMB, for the detailed instruction.

I am not sure of "a) Prepare Me benzoate in MeOH, run by direct infusion in MeOH 100%. "

I tried me benzoate (H2O:methanol =1:1) : LiCl(0.1M) =1:10 infusion. I did not see 171.

[ym3142]

Hi JMB

I infused 0.4mg/mL methyl benzoate in methanol, . To 10 mL of that solution I added a few particle of LiCL solid particles and a few drops of water.
to dissovle the solid
That I infused gave no 171

[JMB]

While I ponder this (i.e. I don't have a better idea at the moment), do you have an APCI source ?

If yes, run the various expts., again WITHOUT additives, under positive ion mode. You might expect to see [M+H]+ and [MH - MeOH]+ signals.

Start with direct infusion, then column (MeOH 100%), then MeOH/H2O (1:1) as before.

JMB

[ym3142]

Hi JMB

Could explain the 2nd step of "then column (MeOH 100%),"? DO you want 100% MEOH as mobile phase or sample diluent?

Thanks

[JMB]

Yes, MeOH as both diluent 100% and mobile phase 100%

JMB

[lmh]

(1) I would worry about adding a non-volatile salt like LiCl in electrospray.

(2) MeBenzoate is volatile. Its boiling point is 200 degrees. It's probably giving low sensitivity because the electrospray process is drying it off, just like a solvent. You could consider reducing all possible drying temperatures as low as you think you can get away with... the fact that you see it in positive mode direct injection as an M+H ion is promising, but when you do the chromatography you're putting solvent in and drying it off again. It's a bit like tipping acetone into a bit of wet glassware and blowing air over it to dry off the water.