Hi

I would like to read your comments on this. We analysed an extracted sample by Agilent GC with dual FID/NPD, and Agilent GC-MS, with the same column type HP5-MS. It was found that, on the GC-NPD, a peak (retention 17mins, and containing phosphorus) degraded when the inlet temp was set to 200°C but not at 150°C. Based on this run, the GCMS inlet, transfer line and ion source were subsequently set to 150°C and the extract re-run - but, sadly, the peak seen on the GC-NPD still cannot be observed on the GCMS. Any suggestions on what further approach i should take inorder to observe this peak on GCMS.
Conditions follow:
GC column : 30m x 0.25um x 0.25mm
GC inlet = 150°C, splitness,2uL inject (GC-NPD/FID) or 1uL (GCMS)
Oven program = 40°C for 2mins, ramp = 10°C/min, final temp 260°C for 5mins
NPD temp = 300°C, air = 60mL/min, H2 = 3.1mL/min, makeup (N2) = 10mL/min
FID temp = 250°C, H2=40mL/min, Air = 400mL/min, Makeup = 10mL/min
MS (see above for temp), full scan.

Thanks in advance.

The NPD is generally speaking much more sensitive than a mass spectrometer operating in scan mode. That is why most pesticide screeing is done with a selective detector, and if a possible hit is detected the sample is concentrated down and then shot on a GCMS for confirmantion. There are a few things you can try to improve the sensitivity of the mass spec.

1. Increase the injection volume.
2. Increase the electron multiplier voltage.
3. If you have some idea what the compound is you may be able to decrease the scan range and increase signal to noise.
4. Concentrate the sample.

If you have a tenative identification from the NPD you can look in a MS library and try a SIM analysis, although you wlll lose the confirmation ability of full scan analysis.

Good Luck.

Are you sure this is not a nitrogen-containing peak? Most P compounds are more thermally stable, if you are working with pesticides.

Do some calculations (assume it is a known compound) and concentrate the unknown sample to a point where it should show. For example, if it takes 5 ng of chlorpyriphos (use an analyte with a retention time close to the unknown) to give a decent peak on the MS, make sure you are injecting at least that much of the unknown material.

You have already determined this material is difficult to analyze, and I presume can't find it from retention tables. It is therefore likely your MS system will need quite large amounts of analyte. My system has been cold for two months, and it took 40 ngs of some materials (guthion) to show a peak. This is probably a column/injector absorption problem. Changing instrument conditions may not help.

If you are desperate, install a new column and all injector parts (septum, insert, any disks, etc.) and try again. This will give you the best chance of seeing this stuff. You can also use a shorter column, the highest flow rate the system will handle, and the least heat possible. Try to shorten the retention time of the 'surrogate' peak (similar retention time) as much as practical to maximize the signal to noise and sharpen the peak up.

You have a difficult problem, with no guarantees. Good luck.