Chromatography Forum

Hello!

I am trying to develop a method for the determination of vinyl chloride and other volatile compounds (Freon 113, MTBE, cis-DCE, TCE and PCE) by using the in-tube extraction with GC-MS.

My system is a Thermo Scientific DSC II, Trace GC, split/splitless injector.
My column is an Agilent DB-VRX 60m x 0.32mm x 1.80um.

The optimal temperature program that I am using for the determination of VOCs except VC is: 40oC for 2min, 15oC/min to 180oC(2min), column flow rate:1ml/min. Temperature of the injector 250oC, transfer line 235oC, direct coupling to the MS, EI mode, ion source at 200oC, scan/s:1, scan range 35-190.
I am using a splitless mode with a liner of 1mm, to have the greatest sensitivity.

the parameters of the PAL are:
Syringe volume: 2.5ml/ TENAX GR
PAL conditions: Incubation temperature: 70oC
Incubation time: 5min
Agitation Speed: 500rpm
Syringe temperature: 60oC
Extraction volume: 1000uL
Extraction strokes: 50
Extraction speed: 200ul/s
Desorption temperature: 250oC
Desorption speed: 100ul/s
Flush time: 2min
Pullup delay: 5000ms
Trap cleaning temp: 250oC

This program works great for the 5 of the 6 compounds, when I am adding the vinly chloride, the peak is really broadening, I know that it happens because of the lack of cryofocusing, and the backflush is huge. So, I tried to put a retention gap to concentrate the VC, but I always have a leak in my system, even if that I tried different connections and specific pressfit. Also, I tried to make a flow program where, I increased the flow rate to 2ml/min for 0.5min and then with the highest ramp to go back to 1 ml/min, but the VC still has the same broadening, and my other peaks had a little bit changed.
I played too, with temperature column programs, but it didn't work.
Next step,I think it is to use split mode, but I would like to have the greatest sensitivity.

Is there anyone that has any other suggestions, except to put on a cryofocusing?

I would be thankful with any answer.

Regards,

bsxenia

What is the temperature of the column when the vinyl chloride leutes ?

Why are you using such a long column, and then a programme rate which is far above the optimum for this length ?

Is the vinyl chloride peak symetrically broadened, or tailing ? It would help if you could post a chromatogram, instructions are in a sticky at the top of the LC page.

What gas volume do you use for desorption from the tenax to the column ?

You are desorbing at 6 ml/min, this will increase pressure in the inlet and cause sample to go in undesirbale directions - e.g. out through the septum purge, back up the carrier gas line. Paradoxically you might get sharper peaks if you desorb more slowly.

Peter

Hello and thank you for your answer,

Well, i am using this column, because the lab where I am working is using it for the determination of VOCs, I cannot buy another one.

I am posting a chromatogram for make it easier, the VC elutes at 5,27min, at a temperature of 95oC.

[img]http://tinypic.com/r/213gioi/6[/img]

The gas volume for the desorption to the column is 500ul.

I tried to decrease the desorption flow rate at 2.4ml/min, but the peak is really squared.

Thanks a lot for your help.

bsxenia

bsxenia,

What are your standards prepared in, meoh? My first suspicion would be that your VC is sitting under a significant methanol loading especially using Tenax. You would not necessarily see it since it if you start scanning above 35 for example. Do you have access to some other adsorbents? Do you have access to some carbowax column that you might be able to use to shift the meoh peak elsewhere.....?

Best regards.

There is a mismatch between flow rates and pressures in the desorption and the column, you are desorbing from the tenax to the column at a flow rate of 100 ul/s, to a volume of 500 ul. This takes 5s, which should give nice sharp peaks. However, at 1 ml/min column flow, the 500 ul gives a plug 30 s wide (which is in the ballpark of you peak widths as far as I can see). During desorption you must be getting some pressure rise in the inlet, that then feeds into the column.

From the chromatogram you post you have both sensitivity and resolution to spare. A split injection is the most straightforward next step.

Given that with the long column your inlet pressure is pretty high, and the gas in the ITEX syringe barrel is at atmospheric pressure you must have gas flow from inlet to syringe as the needle penetrates the septum. If the Tenax is hot at thsi stage the analytes will desorb up into the syringe, which may casue some problems.

Peter

Hello,

Thank you very much for your answers.


AICMM, yes my stds are in MeOH, but unfortunately I don't have any other adosrbant neither another column, but I will start scanning at m/z 49, I hope that I will see a difference.

Well, there is a mismatch of pressures and flow rates, what about to shut down the carrier gas temporarily to avoid peak broadening, and if I can do that, how long could it be shut down, that the MS won't be damaged?

Thanks a lot.

bsxenia

For me, temporarily reducing inlet pressure would be a measure of last resort, and it could never make the inlet band narrower than 30 s anyway.

Try splitting at about 10:1.

Peter

hi
I think tenax are very weak adsorbent for your purpose... i would try another stronger adsorbent like carboxen, chromosorb 106 for lighter analytes...

Peter is right, try the split. If you split, you will automatically reduce the amount of methanol introduced into the column and that could improve your VC peak. Raising the scan amu won't do anything since it is not changing the amount of methanol on the column or in the MS.

Best regards.