Handbook on "old school" gas chromatography

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

18 posts Page 1 of 2
Hi! I'm messing around with gas chromatograph design, and have just finished my second packed column GC-TCD.

My problem is that all my method optimization, and operation GC knowledge and experience relates to capillary GC instruments with electronic flow control, autosampler and the works.

I'm looking for a handbook covering packed column GC's, head pressure flow control and optimization etc.

My main problem right now is that I get a huge negative peak right upon injection of a gasseous sample. I think it may diffuse to the reference side of my TCD bridge, since my injector volume is rather small.

Any ideas?

Thanks in advance

--Chris
Are you splitting the carrier just before the injection port and running through another column or empty tubing to the reference side?

If so, then possibly use a delay coil of tubing (1-3 feet) that would be long enough to prevent any backflash of the injection solvent and pressure spikes from interfering with the reference flow. In a sense you would be placing the Tee fitting for the two flow paths farther back up the line from the injection port.
The past is there to guide us into the future, not to dwell in.
Thanks for your reply, that makes sense. What I'm actually doing in the current configuration is just running the carrier to the reference side of the TCD, then to the injector.

I was considering a larger injector volume (right now it's 5cm of 1/8" tubing) too.

Could i still put a delay coil between the reference out and the inlet in, or is it just a poor design?

Thanks,

Christoffer
ChristofferBR wrote:
Thanks for your reply, that makes sense. What I'm actually doing in the current configuration is just running the carrier to the reference side of the TCD, then to the injector.

I was considering a larger injector volume (right now it's 5cm of 1/8" tubing) too.

Could i still put a delay coil between the reference out and the inlet in, or is it just a poor design?

Thanks,

Christoffer


Probably want two different lines. As it is setup now you will have the backpressure of the analytical column on the Reference side while the Working side of the TCD is venting to atmospheric, this will result in a different internal pressure in the two sides and may cause a difference in the cooling effect of the carrier on the filaments which would give noise in the signal.
The past is there to guide us into the future, not to dwell in.
That may be right, although my topology is exactly the one used by SRI for their tcds (according to their manuals).

If the inlet and outlet of each side of the tcd is the same dimension tube, I'd think the flow would be the same, wouldn't it? If you have a large pipe with a constricton and then same large pipe afterwards again the flow in the two larger pipe sections would be the same.

I mostly solved the issue too. Smaller injection volume was helping.
My largest problem right now is low flow. At 40 psi I'm only getting 13 ml/min flow through. The column is an 1/8" 50 cm column packed with alumina soaked in paraffin.

Too little particle size?


Thanks again
ChristofferBR wrote:
That may be right, although my topology is exactly the one used by SRI for their tcds (according to their manuals).

If the inlet and outlet of each side of the tcd is the same dimension tube, I'd think the flow would be the same, wouldn't it? If you have a large pipe with a constricton and then same large pipe afterwards again the flow in the two larger pipe sections would be the same.

I mostly solved the issue too. Smaller injection volume was helping.
My largest problem right now is low flow. At 40 psi I'm only getting 13 ml/min flow through. The column is an 1/8" 50 cm column packed with alumina soaked in paraffin.

Too little particle size?


Thanks again


A lot of variables on flow restriction in the packing. How tightly you pack the material, particle size and size distribution, coating thickness, ect.

If the coating is thick and you compress it tightly it could become a plug.

I packed 1/8" copper columns in college but we used commercially prepared packing material usually coated with OV-101 using vacuum to pull the packing into the column so that we had consistency between columns.

Total flow will be whatever the entire system is, but if you have 40psi at the head of the column, you probably have 40psi in the reference side of the detector, but that is before the restriction of the analytical column. If the system is less restricted after the analytical column you can have the same flow rate after, but less back pressure. It may not make that much difference if the flow remains the same. I am just thinking that TCD works on the thermal conductivity of the gas flowing through it, and wonder if the same gas with same flow but higher pressure (more molecules per volume) will have different thermal characteristics, but probably not enough that you can't zero it out. If the injection port is too close to the outlet of the reference cell however, a backflash of solvent into the reference cell would cause signal spikes at the beginning of the run, or just the sudden expansion of the solvent can cause a reverse pressure pulse that would slow the flow in the reference cell momentarily which would do the same thing.
The past is there to guide us into the future, not to dwell in.
Thanks for the advice. I use vacuum (diaphragm pump) and tapping for around half an hour on the straight column, then I coil it. Is it better to coil it? Coiling might compress the packing, now that I think about it.
We would just coil them around a coffee can. Anything round and about 4" diameter will work so that you keep the bends smooth and avoid kinks in the tubing.
The past is there to guide us into the future, not to dwell in.
Thanks for the advice!

I've made a new 3' 1/8" column packed with mineral oil on alumina, and it has dreadful flow - 7 ml/min at 40 psi head pressure.

-All the tubing from gas cylinder to inlet is 1/16". could that throttle the flow?

-Could my solid support (Al2O3) be too finely powdered?
ChristofferBR wrote:
Thanks for the advice!

I've made a new 3' 1/8" column packed with mineral oil on alumina, and it has dreadful flow - 7 ml/min at 40 psi head pressure.

-All the tubing from gas cylinder to inlet is 1/16". could that throttle the flow?

-Could my solid support (Al2O3) be too finely powdered?


Either the packing is too fine, or the coating too thick. Difficult to control the amount of mineral oil on the alumina I would imagine. Have you tried using something like sand instead? You could sieve it and eliminate the finer particles and use larger particles and see how that works. The plates will be lower but if the flow works better who knows.
The past is there to guide us into the future, not to dwell in.
Brave soul, packing your own columns.

How did you apply the mineral oil to the alumina, what solvent did you use for the mineral oil? You may be overloaded.

If you can get your hands on Supina's book, it is very helpful about how to make packings.

As an alternative, I would suggest diatomaceous earth instead of alumina. Alumina has its own reactivity which is not as bad as DA.

Best regards,

AICMM
I think definately the packing is too fine. Recommended packed column support mesh size is 80-100, and my alumina has the (very scientific) consistency of potato starch - feels much finer than 100 mesh!

Sand is a possibility. some inorganic salts might do it too (moisture sensitive!)
but they're both straying further from usable and closer towards classroom demonstration. I'd like to be able to make decent columns.

Diatomaceous earth is definitely ideal, used commercially too. It's just a bit harder to find for sale for laypeople, but I have seen health shops selling it online.

Either way I'm gonna buy some 80 and 100 mesh test sieves and make a little shaker so I get good particle size control. It's a fairly easy variable to take out of the picture.

The first support coating i just shook the support with a bit of mineral oil. Now i've adapted a method outlined in ASTM E260: https://archive.org/details/gov.law.astm.e260.1996/page/n1

it's a pretty good reference for packed GC, actually. Basically my method is:
1. 1g mineral oil dissolved in 50 ml heptane(s)
2. addition of 9g support (Al2O3)
3. swirl gently and pull a vacuum on the mixture to draw adsorbed air out.
4. filter suspension on a coarse glass frit funnel under vacuum
5. gently transfer dry-ish phase to a dish and let dry in air at slightly elevated temp.

This should give a max 10% loading, probably much less. It's adviced to extract a known amount of the finished support's phase to determine actual loading but I don't have the capacity for that.

Thank you all for your good advice! I'll look out for Supina's book!

--Chris
Chris,

Since you seem fairly fearless, https://www.diatomaceousearth.com/pages/contact-us

Best regards,

AICMM
That looks good! If it's a health supplement chances are SOME store will sell it!

The only issue is that the food grade product they sell is MUCH too fine for solid support use! the analysis sais smaller than 35 microns, and the optimum for GC is 80-100 mesh (~170) microns. It makes sense for a food grade product not to feel like eating sand I suppose.

The other commercial use for diatomaceous earth is to kill certain bugs, where it's being sprayed as a powder, implying small dust-like particles as well. I don't suppose there's a way to increase particle size of the D.E.?

I've found several archaic GC references (60's and 70's) using powdered brick or fire brick as stationary phase or solid support. It may be worth trying, since it's at least very economical.

Perhaps making a diatomaceous earth clay, firing it and then ball milling it into a coarse powder, then sieving out the 80-100 mesh size fraction would work ok.

Edit:
Here's an abstract of using ground ceramic tiles as solid support. Haven't read the article yet but it looks promising.
https://doi.org/10.1016/S0021-9673(01)97002-5
I remember from my instrumental analysis class talking about using ground brick as the support in the first GC columns.

I still like when GCs had manual flow controller/back pressure regulators. You couldn't get a precise flow control, but I never had to worry about them failing like I do now with all of the electronic flow controllers.
The past is there to guide us into the future, not to dwell in.
18 posts Page 1 of 2

Who is online

In total there are 11 users online :: 0 registered, 0 hidden and 11 guests (based on users active over the past 5 minutes)
Most users ever online was 599 on Tue Sep 18, 2018 9:27 am

Users browsing this forum: No registered users and 11 guests

Latest Blog Posts from Separation Science

Separation Science offers free learning from the experts covering methods, applications, webinars, eSeminars, videos, tutorials for users of liquid chromatography, gas chromatography, mass spectrometry, sample preparation and related analytical techniques.

Subscribe to our eNewsletter with daily, weekly or monthly updates: Food, Environmental, (Bio)Pharmaceutical, Bioclinical, Liquid Chromatography, Gas Chromatography and Mass Spectrometry.

Liquid Chromatography

Gas Chromatography

Mass Spectrometry