Using CO2 to cool a PTV/Oven.

[Mendicant]

Good day, chromatographers.

Our laboratory is undergoing a long-overdue expansion. Many headaches to contend with. But also many possibilities for improvement!

We are running some complex mixtures of large organic molecules that have some very aggressive ramp temperatures. To reduce run time of sequences, we want to be able to cool the oven and inlet to get back down to the injection temperatures faster.

The 7890 manuals only reference using liquid nitrogen and CO2. We've never dabbled with using cylinders in a liquid phase. Is it possible to just use industrial grade compressed gas for this purpose? Would there be anything special we need to set up connection-wise to be able to use liquid?

Thank you.

[James_Ball]

Liquid CO2 is the easiest to use as you can get it in small regular cylinders, you just have to request a siphon tube to draw the liquid from the bottom. Liquid nitrogen is not that complicated either, but you will need a special dolly to move the heavy liquid dewars around the lab. I have moved them by hand but just hope you don't have to go far if you do, and a regular dolly is frightening when trying to control one.

Something else we installed to help cool the oven faster was a powered vent attached to the oven heat outlet. Works like a dryer vent but all of them are connected to a blower on the roof. It removes the heat quickly and also improves cooling by helping the oven fan draw in more cool air.

[Peter Apps]

CO2 is easier to work with than liquid nitrogen, and has the advantage that when you are not using it you can just shut off the cylinder valve and not lose any - which you cannot do with liquid nitrogen.

Plumbing liquid CO2 is as straightforward as it can be - 1/8 inch tubing from the cylinder (no regulator) to the control valve on the GC.

Peter

[GOM]

Hi

As has been pointed out there are advantages and disadvantages to using Liquid N2 and CO2.

From my experience with CO2 with a dip tube - although preferable - the cylinder doesn't last that long.

Some other thoughts that are of no use to you in your current query and for that I apologise

1. The instrument/oven size heating and cooling are essentially based on 50+ year old technology.

2. The size of the instrument is based around this.

3. The size of the instrument does make it a stable platform for adding autosamplers/MS etc

4. All the mods/tweaks/accessories etc are really trying to cool down the instrument oven! The column itself has a very low thermal mass. Perhaps it is time to radically rethink column heating and cooling rather than trying to come up with solutions to work around an old flaw

5. What is your cycle time and what would you like it to be?

6. What is your sample?

It is those last 20-30°C of cooling that takes the longest. Is there another approach that would allow you to get your sample on at a higher oven temperature? - for example - liquid CO2 cooling of the front couple of cm of your column using a tee piece to allow you to focus your sample.?

I have used this one successfully in similar circumstances

It is a more cost effective use of C02 cooling - you are cooling the column and not the whole oven

See " indirect cooling " adaptor from SGE
http://www.sge.com/uploads/5a/8f/5a8fb7 ... 0044-A.pdf

Regards

Ralph

[markf]

GOM: Rethink the issue, sort of like that fancy new Agilent GC? (Have only seen one in an exhibition, have yet to see one working, or anyone that bought one, for the matter.

[GOM]

Hi Mark

Yes, I agree, I need to rethink the issue. Thank you

Which is the main rate determining issue - PTV inlet cooling or oven cooling?

I was only wondering if separate CO2 cooled PTV injection port and CO2 cryofocusing of the sample on the column might be another approach. Both of which I have used successfully in combination for rapid cooling.

Regards

Ralph

[Peter Apps]

Good day, chromatographers.

But also many possibilities for improvement!

We are running some complex mixtures of large organic molecules that have some very aggressive ramp temperatures. To reduce run time of sequences, we want to be able to cool the oven and inlet to get back down to the injection temperatures faster.

Thank you.

Time for some lateral thinking - if your molecules are large you probably do not need to cool the oven to anywhere near room temperature - it is the last few degrees that take the time. Have you verified that the separation suffers if you use higher starting temperatures ?

Peter

[Mendicant]

Hi Mark

Yes, I agree, I need to rethink the issue. Thank you

Which is the main rate determining issue - PTV inlet cooling or oven cooling?

I was only wondering if separate CO2 cooled PTV injection port and CO2 cryofocusing of the sample on the column might be another approach. Both of which I have used successfully in combination for rapid cooling.

Regards

Ralph

That is an interesting question. The temperature range the PTV goes through is higher than what the oven goes through. But the oven is larger than the PTV and thus takes more time to heat and cool over the same temperature range.

It would behoove me to try testing that out and observing which of the two takes longer to cool under the current setup.

[Mendicant]

Good day, chromatographers.

But also many possibilities for improvement!

We are running some complex mixtures of large organic molecules that have some very aggressive ramp temperatures. To reduce run time of sequences, we want to be able to cool the oven and inlet to get back down to the injection temperatures faster.

Thank you.

Time for some lateral thinking - if your molecules are large you probably do not need to cool the oven to anywhere near room temperature - it is the last few degrees that take the time. Have you verified that the separation suffers if you use higher starting temperatures ?

Peter

The methods we are using the PTVs for are mostly for determining things likes waxes, alkyl esters, diglycerides, triglycerides, and the like.

Man, we really have to heat that column past the stationary phase's normal limits to get those triglycerides through! But we treat the columns like they are consumables and it is just a cost of doing business.

We definitely like to have the PTV cooled on that wax method, so then we can do the small methyl and ethyl esters in the same run. If we go too much higher than the solvent's boiling point, we do start to get some measurable discrimination. And I don't like that so much.

[Peter Apps]

So cool the PTV with CO2 and see how high a column starting temperature you can use and still separate the light esters. As Ralph pointed out cooling the whole oven is a waste of time and coolant.

Peter

[James_Ball]

What solvent are you using?

I have found that if you can get the oven as little as 10c below the solvent boiling temperature you can get good results with solvent effect, the rapid heat to about 10c above and begin your normal ramp from there.

The only solvent so far that gives trouble with temperatures is Methylene Chloride since it boils near 40c, but most others are 60c or above so reaching 50c on the oven is pretty quick just using the fan.