Phthalates with Varian ion trap

[rp18]

Hi,

we've got a new Varian ion trap GC-MS (431-220) and try to apply a method to determine phthalates in the ppb to ppm range (more exactly down to 0,05 mg/l). I've lots of experience with quads, and on my HP5971 in SIM mode it's no problem to go even lower with the concentrations, but in my new Varian I got a bad signal even at 1 mg/l.
Peaks are tailing (new DB-5 column, fine peaks on HP) and the signal is very low. Phthalates are solved in toluene, injection is 1,5 min splitless, all in all a very "standard" chromatography. Temps are already set quite high. The method is already optimized and used by several institutes, but all use quads.

I've played with several settings, both scan and SIS, but no luck. Got someone from Varian here, but he detected no major problem, tests were OK, it should work.

A technician will check it again later, but maybe someone else encountered the same problems here? Any ideas?

[larkl]

I'm no ones GC/MS expert, but having run both the quads and Varian's ion trap, I'm not surprised. I just don't see that great a sensitivity. We easily get better sensitivity with FID. I have confirmed this with a peer lab in our same company.

[AICMM]

rp18,

What you are saying does not match what I have heard from the ion trap users I have worked with. In general, some other issues but sensitivity is not one of them.

Some quick comments that might start some troubleshooting. First, why the long split and what starting temp for the GC system (not sensitivity but peak shape question?) Second, in scan, what do the phthalates look like in split versus splitless mode? Can you post? Third, ion traps typically like higher source pressures than quads so what is your flow rate and source pressure if you can measure. Fourth, what does the scan of the phthalate look like (ie di-n-ocytyl?)

Best regards.

[rp18]

Thanks AICMM,

the splitless time is standard for high boiling compounds like DEHP, we've tried 1 - 2 minutes. Split injection in this concentration range does not give a peak at all.

We've tested two gradients, one starting from 30 °C, the other from 90 °C, no significant difference in peak shape.

Flow rate is 1 or 1,5 ml/min (both tested), again no real difference.

All diagnostics say everything is OK (pressure ok, no big leakage, air/water check excellent etc.).

The spectra are poor, low sensitivity at higher masses.

[gpronger]

We have historically had Agilent (HP) equipment and added a couple of Thermo GCMS systems. The one aspect that was significantly different between the two was optimizing the injection parameters.

From what you've posted my first guess is you may have a problem in the inlet. I would try a very short (0.5 min or less) splitless time, with the lower GC temp. Have the inlet fairly hot (250C) and see what you get.

The other possibility is the ion trap itself. Unlike a quad, the ion trap is much more "controllable" regarding the characteristics of the mass-spectrum generated. Varian should have application notes on analyses similar to what you're performing. If they can't suggest one, I would set up following their 525 protocol simply because it handles phthalates.

Sincerely,
Greg Pronger

[rp18]

Thanks - the injector part was the one we also thought of causing the problems. But the settings we tried were already taken from Varian apps handling phthalates, although in different matrices/solvents.

Anyway, I think we'd wait for the technician to check if the instrument is OK.

[rp18]

Update:
the service changed some filters, cut off the column (I already did this before), optimized some minor things and although the peak shape got better the sensitivity is still bad. It's not 10 or 50% less, it's magnitudes.

We also tried 16 PAK (EPA), just a rather high - 1 mg/l - standard, but even in scan mode my 20 year old trap EDIT: quad! has a much better sensitivity, with default settings (no tricks, no multiplier offset etc.).

Is this normal? Anyone doing PAK with a Varian trap?

[ChromatographyDude]

The Saturn 2000 ion trap system I work with never has any sensitivity issues for phthalates... The one time in the recent past when I did observe poor sensitivity, I fixed the problem simply by switching the filament. All diagnostics indicated that the filament was fine, but for whatever reason it simply didn't work well. Either try changing from filament 1 to 2, or replace the filament assembly and see if this resolves the problem -- just a suggestion. Good luck! You might also consider raising the filament emission current if you're only using the default value of 10uAmps, I generally run mine at 25uAmps.

[rp18]

Thanks, however, we tried this already. A tec was here yesterday, changed several things, today if air/water check is OK I'll do some more tests.

So at least it should work...

[WK]

Hi.
I have had one filament go on my system recently so had to change to
the other (which had probably been exhausted as well - I am not able to order a spare in - company orders for year end). The upshot is that sensitivity is down to 20% of what it should be so I have reduced the split flow for my SPME work - no great shakes - but interestingly the peaks easily overload. Also I notice in air/water checks the sensitivity is good to start and then after a second or two it goes down again to 20%. Also when aquiring data the RIC decreases over the first second or two.
WK

[rp18]

I'm in no way a novice in GC-MS, but this thing is driving me nuts. I mean, it must be possible to measure 1 mg/l PAH in scan mode, my old HP does this without any problems. This trap is really disappointing. Today autotune had several problems, so I couldn't start a GC run.

The tec dis- and reassembled the trap, adjusted the multiplier, changed gas filters, liner, septum, cut off a piece of column, did several other things. There is a difference in both filaments of the trap, too, but I'm not talking about percents, it's magnitudes.
Again the cal vent was adjusted, RF adjusted, everything, but now the mass signals above m/z 400 are way to high.

Argh.

[WK]

Hi RP18,
I wonder if the trap is not clean if you are getting HMN above m/z 400 ?
You probably suspect this. I have got silchrom coated electrodes. Have you got those installed? Mine usually clean up nicely sonicated in dichloromethane. Otherwise I believe the non-treated electrodes might be more difficult to clean?
WK

[rp18]

The story continues...
The next technician cleaned the trap, background noise is OK now.
OFN Test is OK.
But any splitless application simply does not work. Neither Phthalates nor PAH's. We still get a very high solvent background over the entire run. I suggested injector problems long ago...

So, a simple question: is anyone using the 430/220 GC-MS with 1177 injector type running splitless?

[Ron]

Do you have a chemical trap in the split vent out line? If there is a trap and it gets contaminated this adds restriction, decreases the flow out the split vent, and increases the flow into the mass spec.

Another possibiliy is the valve switching from splitless to split is not functioning properly, although that would be a little odd since the instrument seems to operate properly in split mode. Try measuring the flow out the split vent and see if it matches what is set in the software, especially during the switch from splitless to split operation.

I had a blocked split vent line on a HP 5890 years ago, that caused all kinds of problems and took a while to figure out.

[rp18]

The trap was already replaced and the overall flow is just fine, but the 1177 with EFC doesn't let me measure the split flow alone. But the injector is where I suppose the fault.