Dear Mr. Harynuk,

you should use Nitrogen or Helium.

Regards

Amir

Dear Mr. Harynuk,

I recommend helium as carrier gas and nitrogen as make-up gas.

Best regards

Valentina Domingues

Dear James:

You can use Nitrogen (high purity) 99.999%. for avoid noise in baseline...it is less expensive than CH4/Ar mix!. Its necessary a good HC, O2 and moisture trap since can interact with the stationary phase and cause significant problems such as high baseline noise and column bleed in the output gas chromatogram which both reduces the analyser sensitivity and decreases column lifetime. Halocarbons content must be low since ECD is very sensitive to these compounds

Hi Mr. Harynuk,

working as responsible analytical head for 1bout 25 years in the field of analysing chlorinated pesticides, best results have been found using He als the carrier in capillary GC plus Ar/CH4 as the make-up gas due to sensitivity, stability and a fine linearity.

Thing it would take a ot of time do discus the details.

Very often some colleques use N2 in order to avoid an extra expensive gas supply - why not ask the guys dor their reasing to use N2?

Best way is trying it out by switsching the make-up using a three way Whitey valve.

Both gases are good. Nitrogen is much cheaper. I absolutely agree with Cristián Cerda-Barrera.

dear james

it's a long and dated debate on which carrier gas suite better for ECD: somewhere I read that N2 give a larger sensitivity, while P5 a greater dynamic range.

However consider Janos,

the low price is NOT a criterion on trace analysis of chlorine using an ECD. It is still the analytical quality and performance.

Making GC-R&D for 3 decades, I found the gas is still the cheapest part in high qualitiy instrumental Analysis !

Helium is usually recommeded for its high purity however it is costly than nitrogen. However if you can secre high qulaity nitrogen then it is fine.

Thank you Peter and Christian for your suggestions (and for actually answering the question that was asked!).

The reason I ignore price is that for me, P5 and ECD-grade nitrogen are the same price (to within $20) once you account for different tank sizes and fill pressures. If I want TAG-grade nitrogen, it is actually much more expensive than P5 here.

In practice I think there is very little to choose between nitrogen or argon/methane in terms of performance (sensitivity, dynamic range etc.). I did have some data but it was from a couple of decades ago and instrument electronics have moved on a bit since then. In my experience the quality of argon/methane was more consistent than high purity nitrogen (in the UK at least) so I preferred argon/methane. As long as you ensure you have the correct chemicals traps in place you are unlikely to have issues with either though. I don't accept the price argument either, it's not a significant cost element overall.

I would also have a preference for using hydrogen as a carrier from a purist point of view. Although if you plan to move to MS after your ECD work you may find this easier if you use helium on both instruments so porting GC methods is easier.

Colin.

Hi Colin,

from my point of view you are right in most of your arguments !!!!

However, to replace Helium as the carrier gas with Hydrogen will give another problem:

While using H2, you will need appr. 3 ml H2 /min instead of 1 ml He. This may be critical for some older MS Machines like the HP 5971 or 5972 with diffusion pumpn. That won#T work at all.

so I agree with Colin; Ar/Methan ist a much better choice as a make-up for the ECD. However, is is really necessaray to combine a selective ECD with a selective MSD - catching the ration of Cl and its isotopes?.

Hy hint.

Keep the line:

Helium as a carrier for a fine separation and Ar/CH4 for optimum sensitivity, Linearity etc.

Best of regards,

Peter

Yes sorry Peter, I should have made it clearer. I meant hydrogen as a carrier for the ECD not MS which is why I suggested helium if planning to use MS after the ECD working on the assumption most workers use helium for MS.

I have used ECD coupled with MS using a system where the effuent from the analytical column is split to the two detectors. You can also do it the other way and connect two (identical) columns to one injection port one going to the MS and one to the ECD. It's actually a pretty useful technique if looking for unknown halogenated compounds in a complex matrix. The ECD trace will give some clues as to the area to focus on (although of course ECD will respond to a wide variety of compounds not just halogenated species). With a "hideously complex matrix" as James describes this you could spend many unfruitful hours working through the MS data alone so using ECD as a screen makes sense.

Colin.

Manjula, don't think I said anything about using hydrogen with helium make-up gas ?

Anyway relax hydrogen as a carrier with argon/methane as a make-up gas is perfectly safe. The methane in the mix doesn't cause a problem either !!

Colin.

For the curious, the following is largely irrelevant to the conversation, and why all I was wondering about is the m/u gas on the ECD.

We are looking to identify some unknown chlorinated compound in some petroleum sample that distils below 250oC at atmospheric pressure. The ECD is just to give us an idea where to look, the FID (which I did not mention, but which will receive some of the outlet flow) is to give us some idea of how much other junk is in the neighbourhood (this is on a 6890). This way we know where to look and how worried we need to be about our filaments on the MS.

Then we can move to either to MS (7890/5973) which has EI, or a MS/MS (brand new Bruker triple quad) with EI/PCI/NCI capabilities working in MS mode to get mass spectral signatures of the area, which we will then likely export as raw data for analysis with some custom data interpretation tricks we write in MATLAB to obtain a pure spectrum for the unknown.

Then we go back and do targeted MS/MS analysis to confirm the identity of the molecule we're looking for.

We're not setting up to do this in anything that remotely resembles a routine analysis. Just some fun detective work.

Nitrogen gas sufficient for its use as a make up gas

Nitrogen gas is ideal and sufficient to be used as make up gas for ECD

That does indeed sound like fun James. I would be interested to hear how you get on. Good luck on your quest.

Colin.

Your biggest issue isn't the makeup gas; it's the lack of selectivity using the ECD. If it's in a truly complex hydrocarbon matrix with ANY heteroatoms present then you're going to get a big ol' mess. You're way, way better off using a Hall detector if you know the compound is actually chlorinated.

Peter's point about porting the chromatography is moot. MS is vacuum chromatography; ECD is atmospheric. Your chromatography would change no matter what.

Hi Mark,

did you never heard about couppling GC/MS with GC/FID (at atmospheric pressure against vacuum).

This technique is in safer hand for the experienced chromatographeur, not for beginners. Assuming you are somewhat experienced, you may handle thist problem easily unsing external references !