Chromatography Forum

Hi All,
I want to develop an LCMSMS method for pesticide of Naled analysis.
The settings of precursor and product ions are 398>127 and 398>109.
I don't see the peak even at 200 ppb.
The mobile phase is 0.1% formic acid, 5mM ammonium acetate in water and pure methanol.
Could anyone help me figure out what caused the problem?

If this is the pesticide you're referring to, it has a molecular mass of 380.8. You're probably looking at a water adduct?

Note that I don't have experience with this particular pesticide. I'd start by infusing it to determine the parent/products (i assume you got them off the internet?).

Definitely so a compound optimization using infusion of the compound in the mobile phase, usually need a concentration of about 1ppm. I like to use a mobile phase composition near what will be coming out of the column at the elution time of the compound, but if that is not known then use 50% of each mobile phase as the diluent for the target compound during optimization, this will allow you to see the mass that will be presented to the mass spec with any adducts that might be occurring.

Lijun-Zhang wrote:
Hi All,
I want to develop an LCMSMS method for pesticide of Naled analysis.
The settings of precursor and product ions are 398>127 and 398>109.
I don't see the peak even at 200 ppb.
The mobile phase is 0.1% formic acid, 5mM ammonium acetate in water and pure methanol.
Could anyone help me figure out what caused the problem?

You are looking at the wrong mass. I developed and validated a method for Naled about 5 yrs ago for a chronic tox study. Was then analyzing samples over the course of about 6 months twice a week. I was going to scribble down some of the parameters today because I read this post this morning and glanced at some of the data today while at work. I do remember the mobile phases were 0.1% formic in water and 0.1% formic in CH3CN. Gradient started at 10% CH3CN and ran to 90% over the course of 4 min with a 4 min re-equib at 10%. Retention was about 4 min. Column was a Thermo Betasil c18, 50x2.1, 5um. I would suggest you infuse on your own inst to get the rest of the optimized parameters, I would bet I infused in 50:50:0.1 MeOH:H2O:formic. When you start to infuse set the DP to about 60 because that is where it will end up when you optimize. The parent mass is like 381 and for some reason I remember there was some trouble initially seeing it. Give it some time when infusing, it may have been one of those that for some reason doesn't jump out at you the instant you hit start. If you don't see it, force it, tell the inst the mass is 380 w/1 charge and optimize, I bet it finds it even if you don't don't see it when scanning. Positive mode.

You had to be looking at an adduct because you have the right product (127).

This is what it should look like...



Standards ranged from 0.0500 to 1.00 µg/L and you are looking at the mid 0.500 µg/L. Flow was 0.5 mL/min

James_Ball wrote:
Definitely so a compound optimization using infusion of the compound in the mobile phase, usually need a concentration of about 1ppm. I like to use a mobile phase composition near what will be coming out of the column at the elution time of the compound, but if that is not known then use 50% of each mobile phase as the diluent for the target compound during optimization, this will allow you to see the mass that will be presented to the mass spec with any adducts that might be occurring.

Since the majority of the compounds I'm asked to make methods for we are given a non-descrip number and mass (sometimes not even mass) and I don't have a clue what the eventual mobile phase will be, I infuse just about everything in 50:50:0.1 MeOH:H2O:formic. 1.00 ppm is where I start too although that will usually be too much for higher end instruments. I also on many occasions settle on a final solvent composition of MeOH:H2O for samples when the mobile phase is CH3CN. Being a little weaker than CH3CN it tends to yield sharper peaks and there is no MeOH around by the time that peak comes off. The validation here was of a formulation of 2 analytes performed at the same time so if I had shown a TIC there would have been another peak about a half minute before Naled. Many times it is a compromise of what must be done for one analyte and what is acceptable for the other.

Hello,

Both m/z 398 [M+NH4]+ (for ammonium adduct, ammonium acetate in mobile phase) and m/z 381 [M+H]+ (protonated molecule) are correct. You should check which option will give better response.

The reason you don't see your compound can be from... lack of analyte in sample to bad ion source settings... you didnt provide much information

Which vendor LC/MS/MS you are using?

Regards
Peter

Organophosphates are really sensitive to ESI conditions. You may have to play a while to get the ionization conditions correct, plus you should monitor for both the H+ and NH4+ adducts. In my experience you'll pick up the H+ adducts for organophosphates.

When dealing with these types of compounds less is more; lower desolvation temps, lower capillary voltages, etc.