Unretained peak shape problems with PTV-GC-MS

[mah]

Hi all,
Just a quick question.
I'm having problems with the shape of butane for testing dead volume of the system/correct column installation/injector problems. The compound has a huge tail in splitless (1 µL) and just a bit smaller tail when splitting a 10 µL injection 1:500. (30 m x 0.25 mm, 0.25 µm, 5 % Phenyl 95 % Dimethylpolysiloxane, 1 mL/min Helium, at 120 oC).
This compound should have a perfect gaussian shape -even on the PTV large volume injector (up to 10 µL here) right?

All the best
/Martin

[Don_Hilton]

Did the retention time match up with the calculated retention time for the column and conditions? If so:

With a 500:1 split ratio, the inlet volume is cleared quite rapidly, so this suggests a dead volume or other mixing issue. My first thought would be to pull open the inlet and look to be sure the liner is intact (and the correct dimension for the instrument), the o-ring (or ferrule) is in good shape, and that the liner is installed to the correct depth.

[chromatographer1]

Make certain there are no particles in the column and the column is installed at the proper location in the inlet, that the column did not 'slip' down before you tightened the ferrule.

tailing is a characteristic of dead volume not being flushed properly, inlet or ouitlet.

best wishes,

Rod

[mah]

Thanks for your suggestions. I did the column reinstallation and there were no visible particles (is there another way?). I still have tailing on butane I havn't run butane on this system before so I dont have reference chromatogram, but to my knowledge this compound should have a perfect peak shape

I injected 1 µL grob mix (in DCM) at 1:100 split (from 0 to 2 minutes) and with a temperature ramp. Most of the peaks are fronting (FAMEs, alkanes, amines) and the DCM peak is split into two peaks; first a small one at 1.4 min and then a larger one at 1.5 min...

[Don_Hilton]

Something seems to be not quite right here. A one minute retention time seems to be rather short for even an unretained compund on a 30 m column. If your flow rate is 1 ml/min and the column diameter is 0.25 mm, off the top of my head, I would expect the retention time to be two minutes or more.

Be sure of your coulm dimensions and be sure all dimensions are correctly entered into your GC method. And be sure that the detector is correctly identified. If you have an EPC that is set to compute flow and the detector is set as an FID and you have an MSD on the end of the column, the flow will be too high. This would not likely be the cause of the split DCM peak, but it will cause problems down the road.

On the split DCM peaks. What was the temperature of the inlet and of the oven at injection?

[chromatographer1]

150 sec (2.5 min) is unretained peak retention time at 0.6mL/min flow rate.

ie, 75 sec at 1.2mL flow rate.

Your fronting peaks are overloaded peaks for the column, too much sample is being injected onto the column.

I am guessing that your installation of the column in the injection liner is faulty, too deep or too shallow.

Remove the column from the inlet and look at the ferrule and see how much of the column extends past it. The column can 'slide' and its depth into the injector can change as you install it.

best wishes,

Rod

[mah]

The grob mix injection program I used was;
isothermal at the PTV inlet 260oC with a glassfrit inside the liner.
split 1:100 from 0 to 2 min, flow 1 mL/min.
Column oven; initial 40oC (hold 2min), 180oC (10oC/min), 300oC (60oC/min, hold 2 min), total 20 min.

I looked at the split DCM peak once again with the MS spectras; (sorry my bad) - the first tailing peak at 1.4 min is butane (which I injected not ages ago, but it should have been swept away...??). The second tailing peak at 1.5 min looks like some kind of halogenated degradates (maybe from DCM combined with other residues)...

In the system there is an EFC, however, this device was (maybe faulty) installed from the vendor as outlet to 'atmospheric' conditions and not 'vacuum'. I have tried to alter the setting, however, then the EFC cannot calibrate the flow Why I have turned it back to 'atmospheric'. But I guess, I agree with Rod, the unretained peak time should be something like 1.4-1.5 min in this system.

I'm running a condensation test this evening to check if there is a contamination from gas lines and injector.

hope you have some suggestions, thanks
Martin

[Peter Apps]

Please tell us the maker of the hardware. I am fairly sure that there must be a setting for column outlet pressure that you can change on the instrument, this should not be a one-time installation setting.

Before you worry about the Grob mix, you need to sort out the tailing butane peak, since this is probably a symptom of a system problem that might in turn be messing up your Grob mix.

What inlet liner did you use for the butane ? Some PTV liners are tightly stuffed with glass wool, or even adsorbents, which would for sure give you a tailed gas peak. With a packed liner you might not be getting a 500:1 split through the inlet because of the resistance to gas flow.

I presume that you are using an autosampler ?, if so, check the needle insertion depth. If it is too small the sample is injected into a partially dead volume near the septum and thenslowly bleeds down into the carrier flow. Also check the septum purge flow (if you have one, some PTVs do not).

Peter

[mah]

The hardware is from Varian (CP-3800) with a 1079 PTV-injector and there is a septum purge vent. The flows seems to adds up (EFC-outlet, septum purge and column). I have set the septum purge to something like 2 mL/min. The injection needle level should be ok, but I will check...

The liner I use is a gooseneck split liner with a glass frit, deactivated (#392611946, page 199, http://www.varianinc.com/image/vimage/d ... red/gc.pdf). I would not expect any tailing of butane because of this liner.

The condensation test gave some answers. After the butane and grob test injections, I kept the injector (in splitless mode) and the GC oven at 40oC. After 4 hours I started a temperature program on the injector and GC oven (40oC hold 2 min, 350oC at 20oC/min and hold 20 min; total 37.5 min). The first chromatogram looked very bad with many huge humps and the second a bit better, and third almost like a normal baseline. This morning I ran the test again aften 12 hours trapping (and no butane/grob injections before) at 40oC; all the blank chromatograms looks fine now. I think there is some kind of build up of residues in the inlet...

all the best /Martin

[Peter Apps]

Hi Martin

There is nothing in the set up that can obviously cause a butane peak to tail. The contamination that you see on the condensation test is very likely accumulating at the top of the column rather than in the inlet, and it is a separate issue to the butane tailing.

What was the temperature of the inlet when you did the butane injections ? - it is just conceivable, but very unlikely with the film thickness that you have, that if you injected with the inlet cold the little bit of column inside the inlet would retain the butane.

If you have not already done so please check your split and septum purge flows with a separate flow meter - it is not unknown for GC onboard controlers to give inaccurate readings.

I think that if you look in the column setup of the GC you will find a setting for the outlet pressure - is this what you have already tried to change ?, and then the flow control malfunctioned ? You also need to select the carrier gas, and the column dimensions. If all of these are correct but any of the other flows (including the hold up time on the column) are incorrect you may have an EFC fault.

What is the history of the GC ? - has it been used for lots of dirty samples for example ?

Peter

[mah]

Hi Peter,
I did the butane injections at isothermic conditions; 120oC (inlet) and 150oC column oven.
I have measured the split vent and septum purge flow with an external flow meter...and these seems to add up.

Few months back we had a clogged split line maybe due to injection of very dirty samples (extracts of soils and tissues). We managed to clean the lines out and baked the injector (and connected stainless steel lines) at 450oC in an oven, but without gas flowing through. Unfortunately, we observed some kind of a black-coffee looking liquid film in the lines just before the EFC . We could not see any contamination of EFC at this point.

I will try and get hold of a new EFC (can't be too expensive?) within few days.

Thanks
/Martin

[Peter Apps]

Hi Peter,
I did the butane injections at isothermic conditions; 120oC (inlet) and 150oC column oven. this should be OK, but if you have a chance try it with the inlet hotter than the column
I have measured the split vent and septum purge flow with an external flow meter...and these seems to add up. which suggests that the EFC itself is OK, at the moment, but with crud in the lines it is only a matter of time before it dies

Few months back we had a clogged split line maybe due to injection of very dirty samples (extracts of soils and tissues). We managed to clean the lines out and baked the injector (and connected stainless steel lines) at 450oC in an oven, but without gas flowing through. Unfortunately, we observed some kind of a black-coffee looking liquid film in the lines just before the EFC . We could not see any contamination of EFC at this point. I suspect that the split/splitless lines and/or valve are resricted or stuck, so that you are always doing a splitless injection

I will try and get hold of a new EFC (can't be too expensive?)You might be in for a surprise here within few days.

Thanks
/Martin