Chromatography Forum

Hi all

I understand that both instruments have their advantages or disadvantages. But I would like to be more specific in my question. I am running organophosphate pesticides residue analysis upon Alliance separations module 2695 that attached to Quattro Ultima tandem mass spectrometer (QqQ) ESI+. I do have 6 OPs that are usually detected with GCMS and LCMSMS.

As you now, the only accessible (free) pesticides data spectra of LCMSMS available can be found on http://www.bfr.bund.de/de/bfr_daten_fue ... -5831.html
However, these acquisition parameters are for ESI-MS/MS (API 2000), and it has been argued that may not be the same with other instruments.

The use of LCMSMS in pesticide residue analysis is of increasing these days since it has been validated that LOQ is from 0.1 to 1 ng/ml where the median LOQ for GCMS is 100 ngml.

I am facing trouble of sensitivity on my LCMSMS instrument; there is only one paper in which they use the same (both LC and MS platform) instrument that I am using now. Thus, I followed the exact parameters and 5 out of 6 compounds were detected with poor sensitivity and one could not be detected at all.

Before making the decision of returning to our (old hero!! GCMS), I have couples of inquires:

How can I improve the sensitivity of my current compounds using LCMSMS ?
As far as I understand, the collision energy is of important to achieve good intensity. Thus, optimization of CE is the first step. Do I have to use different CEs randomly with the obtained ions transitions? Or there is a specific protocol.
I am using column that its history is not known (has not been used for 2 years, and no clue which compounds were separated on it (definitely not pesticides).

I would like to be advised on how to improve the sensitivity (which exact parameters that I have to focus on) and what is the sign that can be used to decide that the LCMSMS is not sensitive? And return to GCMS is the solution.

Note: this is my first time standing in front of and operating LCMSMS. So I may skip some basic protocols.

Thanks

To get best sensitivity, you should probably first optimize your source conditions to get good precursor ion generation and transmission, THEN optimize collision conditions.

I have optimized the following source parameters:
1. Capillary voltages 2.0 kv. No significant enhancement in sensitivity when increase or decrease the voltage.
2. Sample cone voltage 35 k. No significant enhancement in sensitivity when increase or decrease the voltage.
3. Source temperature 100 c. No significant enhancement in sensitivity when increase or decrease the temperature.
4. Desolvation gas temperature 350C. No significant enhancement in sensitivity when increase or decrease the temperature.
5. The flow rates for desolvation gas and cone gas (N2) were set at 100 and 600 L h−1, respectively.
The only thing left is the pressure of fragmentation ions, it is setting based on the usual work on instrument.

If you were more specific as to which OP you actually are trying to detect, the answers would also be more detailed...

In general the OPs can be detected with ESI+ with good sensititvity (see e.g. http://www.chem.agilent.com/Library/app ... 3774EN.pdf). If you don't get a good sensitivity, you are either trying to detect the wrong ions (did you run a scan to find the right precursors?), or something is wrong with your MS.

Did you do a mass tuning of the instrument recently?
When did you last clean the source?

In most cases the MRMs from API 2000 are transferable to your MS. However, you need do optimization by direct infusion to fine tune the compound dependent parameters such as collision energy and cone voltage for each analyte. Capillary voltage, source temperature and Desolvation gas temperature are compound independent, you have to find the right balance among the analytes.

Refer to your instrumental manual for step by step instruction.

BTW, for OP, LC-MS/MS is better than GC-MS