Method for CO/H2/HCs analysis

Discussions about GC and other "gas phase" separation techniques.

12 posts Page 1 of 1
Hello,

I am looking to analyze CO/H2/CH4/C2H4 (in decreasing order of importance and concentration) in a CO2 stream using a 6890N with a sampling valve. Now, there are two things in which I am a bit clueless and I would greatly appreciate any tips and hints: 1) deciding whether the column/detector/carrier combo would work, and 2) calibration.

Regarding (1), I am planning to use helium as carrier with a TCD. I am aware that analyzing hydrogen in He with a TCD is far from optimal, but can it be done? What should one look out for? Would it be better to use a different carrier gas altogether? (however, I am more interested in accurately determining CO than H2).

There is an FID installed in the GC which I could use (either in series after the TCD, or use a splitter from the column). Would this be a good idea, or is it overkill?

As for the column, I am planning on using a Carboxen-1010 capillary column (http://bit.ly/18ZfdIg), which seems suitable. It is preferable to go with the 0.32 mm column instead of 0.50 mm, correct?

As for the calibration, I have been looking into using gas mixtures of each component (e.g. CO in He at different concentrations) to make a calibration curve for each. However, buying (for example) 3 cylinders of each component seems a bit... wasteful (and costly!). Is there a better way to go about this?

Thanks for your help!

p.s. it might be useful to add that a likely "lower boundary" for the concentration of the analytes (in molar terms) in the carbon dioxide flow would be:
- Carbon monoxide: 1200 ppm
- Hydrogen: 60 ppm
- Methane: 35 ppm
- Ethane: 35 ppm
With the concentrations that you have, this is a two channel system. You will not see hydrogen at 60 ppm with helium gas in a reliable manner. I would suggest the TCD running on nitrogen or Argon carrier gas and using a 1ml sample loop. You would use this for hydrogen and CO. Use the FID for methane and ethylene.

Gasman
GasMan wrote:
With the concentrations that you have, this is a two channel system. You will not see hydrogen at 60 ppm with helium gas in a reliable manner. I would suggest the TCD running on nitrogen or Argon carrier gas and using a 1ml sample loop. You would use this for hydrogen and CO. Use the FID for methane and ethylene.

Gasman


Thanks a lot for your comment. In that case, is there any particular for either argon or nitrogen? Do you suggest connect the FID and the TCD in series, or to split the column outflow between the two detectors?
As long as you do not need to measure nitrogen, then use nitrogen as the carrier gas. Running the TCD and FID in series is also a good idea.

Gasman
I contacted Agilent about the accessory for connecting the TCD and the FID in series, and got the following:

You need a 10 port valve with backflush to vent of the pre column. but with this configuration you won’t see ethylene, since it elutes after CO2. Otherwise you can use a 2 valve system, with one gas sampling valve and a column isolation valve; in this way you’ll detect everything. Obviously you’ll have TCD and FID in series


Um... I got a bit lost there. Why wouldn't the regular setup with the gas sampling valve work?
Guys, regarding the calibration, seeing that the concentrations are rather low, can I assume a linear calibration curve and perform a single-point calibration for each component? That way, I would only have to get one cylinder per component.
It's expensive to have multiple-concentration standards. We generally have a vendor mix all of our analytes in one cylinder and we assume we're operating in the linear range. We have the vendor supply a certification of analysis on the mix and for our mix, they guarantee it for 1 year. In our case, the concentrations are quite low (in CO2) so in my estimation, the linearity thing is really not a bad assumption.
rb6banjo wrote:
It's expensive to have multiple-concentration standards. We generally have a vendor mix all of our analytes in one cylinder and we assume we're operating in the linear range. We have the vendor supply a certification of analysis on the mix and for our mix, they guarantee it for 1 year. In our case, the concentrations are quite low (in CO2) so in my estimation, the linearity thing is really not a bad assumption.


Thanks a lot for your comment, rb6banjo.

Well, an update and some more questions: I contacted Agilent about the possibility of connecting the TCD and the FID in series for my application. They basically said that, for what I want to do, I'd need major modifications to the GC hardware (including additional valves and a second TCD) which would run at more than 25k euros.

Needless to say, we won't be doing that.

This annoys me. Sure, I understand that this is not charity, but still - if I am told that we need to spend 25k euros, I would at least appreciate an explanation of why the suggesting of connecting the TCD and the FID in series would be unsuitable. I still don't see why it wouldn't work. Hell, I don't even see why just splitting the column flow with a tee and send it to both detectors wouldn't work.

Anyways... I think that, at least for the moment, I will be focusing on the CO/H2 analysis only, and leave the hydrocarbons for later. I do need to start getting some useful data, after all, and those two are my most interesting components.

The thing is that now I'm doubting again my column choice. As I mentioned, I thought about using a Carboxen-1010 capillary column, but I am worried about the sensitivity of the TCD and the low sample volume along with the low concentrations (as well as the price, a bit). Is perhaps a packed column more suitable? I wrote to Restek about the ShinCarbon ST and I was told it should work, and I have also seen the HayeSep D mentioned in the literature.

Maybe I'm just overthinking it and I should buy, install, and test instead :?
I agree with you. If I'm you, I try the tee idea. I have a GC in my lab that has the ability to heartcut from one column to a second one and the effluent from the second column is split to a mass spectrometer to a PID. The output of the PID goes to the sniff port. They add some humidified air to the PID outlet to help the volatiles get to the nose cone for me to sniff. Lots of splitting with tees.

That's an inexpensive option that should/could work for you. FID's are pretty sensitive. You could choose your restrictor tubing such that very little (enough) effluent goes to the FID and most of it goes to the TCD.
Re. TCD and FID in series, look at this, gives part numbers etc.
http://www.chem.agilent.com/Library/Sup ... F03044.pdf
Really it's just a bit of 1/16" pipe with 1/8" Swagelok adapters.

For a range of concentrations we use one of these
http://www.signal-group.com/index.php/a ... as-divider
for validation, in practice we assume (have proved) linearity so just go with single point through zero calibration.

A four-component calibration mix is no trouble to make, and a lot less trouble to use than several single-component cylinders. I'd agree with Gasman, use nitrogen carrier but use a trap in the line as it may have a small trace of CO in it, plus CO2 is in the air so will ingress through any leaks.

The alternative to daisy-chaining the detectors is a two-column system, you'll need either two inlets or one with a ten-port valve to inject simultaneously. The data will look the same as if they were in series (apart from the timing), you'll still have two channels to process.

Am a bit busy to look at column choices but plenty of guys on here know more about that than me anyway!
Where can I buy the kit they use in CSI?
Gaston wrote:
Hello,

I am looking to analyze CO/H2/CH4/C2H4 (in decreasing order of importance and concentration) in a CO2 stream using a 6890N with a sampling valve. Now, there are two things in which I am a bit clueless and I would greatly appreciate any tips and hints: 1) deciding whether the column/detector/carrier combo would work, and 2) calibration.

Regarding (1), I am planning to use helium as carrier with a TCD. I am aware that analyzing hydrogen in He with a TCD is far from optimal, but can it be done? What should one look out for? Would it be better to use a different carrier gas altogether? (however, I am more interested in accurately determining CO than H2).

There is an FID installed in the GC which I could use (either in series after the TCD, or use a splitter from the column). Would this be a good idea, or is it overkill?

As for the column, I am planning on using a Carboxen-1010 capillary column (http://bit.ly/18ZfdIg), which seems suitable. It is preferable to go with the 0.32 mm column instead of 0.50 mm, correct?

As for the calibration, I have been looking into using gas mixtures of each component (e.g. CO in He at different concentrations) to make a calibration curve for each. However, buying (for example) 3 cylinders of each component seems a bit... wasteful (and costly!). Is there a better way to go about this?

Thanks for your help!

p.s. it might be useful to add that a likely "lower boundary" for the concentration of the analytes (in molar terms) in the carbon dioxide flow would be:
- Carbon monoxide: 1200 ppm
- Hydrogen: 60 ppm
- Methane: 35 ppm
- Ethane: 35 ppm



Hi Gaston,

I can see that there has been a while with this post, but I am new to this GC system and I am struggling by myself.

The thing is that I would like to analyze hydrogen in low concentrations, O2 and CO2, if possible. I have Shimadzu 2014 in my lab with capillary column carboxen 1010 and TDC. I use Ar as carrier gas. The system is not configured with any valve, I intended to inject the gas sample with a gas-tight syringe.

Were you be able to analyze H2 with this column? I tried to apply this exact method on the Sigma website https://www.sigmaaldrich.com/technical-documents/articles/analytical-applications/gc/astm-d3612-96-gc-analysis-of-transformer-gas-g002209.html but apparently, I cannot see any good resolution peaks when I inject either H2 and O2.

I injected two different samples of 300microliter H2 and O2 separately just to see retention times and peaks. The retention time for O2 was much more different than the Sigma's chromatogram, H2 was as expected. But both intensities were low.

I really appreciate any single help.
Arca.y,

Couple of suggestions. First, if I were you, I would start a fresh post. Second, I doubt you are going to be able to do low level O2 when using a syringe, back diffusion into the syringe will kill you. Third, low level H2, O2, and CO2 (all together) very hard with TCD - carriers for H2 not good for O2 and CO2 and versa-visa. Fourth, I do not know Shimadzu very well (by choice) but I am wondering if they have a high current and low current switch on their TCD.

Honestly, probably more appropriate to consider an HID if you want low levels of all three.

Best regards,

AICMM
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