Chromatography Forum

Dear All,

I am working with a thermo LCQ Decq Xp plus, off site for reparations but will be back next week.

I am planning the first method to develop on the instrument, pesticides.

I have read few papers on direct injection of samples (no concentration, no clean-up, sometimes the sample is only filtered) but this was done on drinking water samples, quite clean. The injection volume is usually equal or superior to 100uL.

I am quite interest in this as the method can be really cheaper for my company but I have few questions and remarks.

1-I am planning to do methods with a high number of compounds (20-100), what do you think of interferences?
2-My HPLC injection loop is 20uL, I can easily change it to 100-200uL. how does that affect the system? Do I need to change something else in that case.
3-Can the MS survive a routine running of dirty samples (without clean up), I guess a more thourough cleaning of the source will be required.

Thank you for any point of view.

If you're running drinking water, I wouldn't imagine they would be very dirty. I would use a diverter valve to keep any unretained junk from going into your MS, and to keep any late-eluting junk out as well. I would use a 0.2-micron syringe filter to filter all samples prior to injection, and run an inline filter as well. Changing out the loop might require you to adjust expected retention times of your target compounds, if you've already developed the chromatography for the samples. Other than that, the only problem I can envision is suppression in the samples, which you would only be able to remedy by changing the injection volume or the chromatography.

Thank you for the advices.

The method is a new in-house method, I will start working on it next week. I will definitely think about using the divert valve on the MS. The samples are for environmental studies, I suspect some of them will be quite dirty. We indeed have 0.2um filters on site, I will use them.

I checked the HPLC injection syringue which is 25ul, it can be easily change with a higher volume syringe which can allows to increase the injection volume.

Many thanks

I'd be very surprised if a DecaXPplus is anything like sensitive enough for trace pesticide analysis in water without concentration, but I'd love to be proved wrong! Also remember that this ion trap isn't particularly good at filling its trap selectively for MRMs, so its sensitivity is limited by the capacity of the trap. If an ion is only 1% of the total ions entering the trap, a DecaXP will have only 1% of the sensitivity you'd see if you injected a pure sample (100% of ions entering the trap). And electrospray gives a signal dependent on concentration, not total amount injected. Using a 100uL injection won't improve the sensitivity situation, it just gives the instrument longer to carry out a lot of MRMs, if that's what you're doing (this being a slow instrument).

lmh,

Thank you for pointing out the sensitivity problem. I plan to do some SPE methods with the instrument and will try DAI methods (Glyphosate, to avoid a long derivatisation step) to see if I have some reliable results or not. I don't have much hope about that but then I have to try...