vapor pressure measurement

[zimanli]

We are measuring vapor pressure of some semi-volatile chemcals. We can successfully measure it down to 1e-5 Torr. But if we tried to lower the temperature and attempted vapor pressure lower than 1e-5, we always got values way too high than literature values. Is there anyone who has experience in this? I think there is residue chemical somewhere: in gas tight syringe, the needle, the front inlet, or maybe we did not wait long enough for our sample to reach equilibrium.
The experimental conditions are as follow:
Hp-5ms 30m column, 50 uL vapor injection by 250 uL gas tight syringe, splitless mode, 1min purge time, 1ml/min column flow, equilibrate for 2hrs before injection.

Thanks a lot!
ziman

[zimanli]

To make myself clearer, I should put it this way. I injected some sample in GC/MS in splitless mode. Then after a couple of injection, it seems it condenses somewhere (syringe plunger, barrel, needle, septum, liner, column, EI, split/splitless injection port) and it is really hard to come out.
I am looking for expertise in solving this problem.

Thanks everyone.
ziman

[Bruce Hamilton]

First, let me tell you I'm totally clueless, and that also covers using GC to measure vapour pressure .

If you are using a standard method, you should check that all of your equipment is operating correctly and leakfree, and perhaps contact your instrument supplier or method authors for comment about your problems.

If I understand your methodolgy, I'd be quite concerned about your use of a syringe, at the least I would be using a 6 port gas sampling valve, possibly in heated valve compartment set to the desired conditions to ensure the equilibrium was maintained during injection.

I can't see how the syringe would be accurate, especially if at a low pressure, as the moment you inject it, it would be at a different pressure, and also would not be swept empty. You possibly could use a headspace sampler, if available.

Bruce Hamilton

[zimanli]

We assume PV=m/MRT, and measure m by using GC. Yes, you suggestion is helpful, but we need more money for those headspace sampler. People used syringe for vapor pressure measurement down to 10-7Torr. I just would like to know if any one happen to do this and give me some clue about getting rid of condensation.

[zimanli]

I think I would like to know for trace amount analysis, what we shall do to make sure we can minimize error from condensation.

Thanks a lot!

[Peter Apps]

Hi Ziman

What are the temperatures of your sample, syringe, inlet and column ?, and how are you controlling them ?

In general if the syringe is hotter than the sample, and the inlet is hotter than the syringe you will not get condensation. If you get "condensation" on the column it will not be a problem.

How are you controlling the pressure above your sample and in the syringe ?

Peter

[Peter Apps]

Hi Ziman

What are the temperatures of your sample, syringe, inlet and column ?, and how are you controlling them ?

In general if the syringe is hotter than the sample, and the inlet is hotter than the syringe you will not get condensation. If you get "condensation" on the column it will not be a problem.

How are you controlling the pressure above your sample and in the syringe ?

Peter

[Peter Apps]

Hi Ziman

What are the temperatures of your sample, syringe, inlet and column ?, and how are you controlling them ?

In general if the syringe is hotter than the sample, and the inlet is hotter than the syringe you will not get condensation. If you get "condensation" on the column it will not be a problem.

How are you controlling the pressure above your sample and in the syringe ?

Peter

[zimanli]

Hi peter, thanks for your reply.

sample: 3 C, boiling point: 300 C, syringe: ~40 C, inlet: 200 C, column: 46-230 C. I use a temperature controlling bath for the sample vial and a heating tape to wrap the syringe during experiments.

We put our sample into a 20 mL vial with septum cap and let in stay at a certain temperature for ~2 hrs to equilibrate. for the syringe, we just fill sample in the syringe with the speed of 1-20s/50uL. The big change from 1 to 20s is because I thought for high temperature vapor, we should pull the plunger slower for the vapor to equilibrate between the vial and the syringe. For low temperature vapor, since there is not a lot of sample in the vapor, maybe the faster, the less condensation. It is just my thought, I actually doubt it.

My problem is for lower vapor pressure, I always got way too high signal than I expected based on literature values. COuld it be due to the vaporization of my liquid standard is not complete? I am using external calibration by using liquid solution and measuring vapor sample.

Thanks again, peter.

[Peter Apps]

Hi Ziman

With the temperatures that you give I do not think that you can get condensation. Condensation would require either a decrease in temperature as you take the sample, or a dramatic change in pressure.

I presume that you have gas-tight caps on your sample vials, and that you cap them at room temperature. When you subsequently cool them to 3 C the pressure in the vial drops. It drops still further when you take the sample. At the lower pressure the sample vaporizes faster, which MIGHT be the cause of the larger than expected peaks.

Try venting the vials by equilibriating them with a narrow hypodermic needle through the septum. You will not lose significant amounts of sample becuase you are working far below the boiling point. As long as the tip of the syringe needle is far away from the tip of the venting needle you will not get air mixed with the sample.

Peter

[zimanli]

Peter, thanks for your help.
Is vapor pressure dependent on the pressure above the sample, atmospheric pressure or lower pressure?

ziman

[Bruce Hamilton]

How long a time is your syringe in the vial, and what happens if you leave it in the vial for 2 - 4 times as long?. Are you sure it's reached equilibrium, and I'm assuming your sample water bath and syringe are at the same temperature?. If they aren't at the same temperature, how do you compensate?.

Have you tried filling and emptying the syringe back into the same vial, and injecting the empty syringe to see how much sample remains?.

How are you agitating the vial to ensure the vapour space is homogeneous, and have you confirmed that by taking an upper and lower vapour sample for low vapour pressure?.

I'm still rather bewildered by your methodology, but I'm sure you know what you are doing, so please keep having fun.

Bruce Hamilton

[Peter Apps]

Hi Ziman

No. the vapour pressure of your sample depends only on the temperature, but the quantity of the analyte that is held in the syringe and injected in the GC depends on the total pressures in the vial and syringe (mainly because air can move in or out through the needle when you withdraw the needle from the vial).

You could try a gas tight syringe with a valve on it.

Peter

[zimanli]

Thanks a lot, peter, that is really helpful.

Bruce, vapor pressure measurements using GC/MS is not that accurate at all. We use this method to have a good estimate for low vapor pressure chemicals. For vapor pressure measurement, usually we need to use more than one appratus to confirm the number. But we need to try the best we can. The advantage of using GC/MS is that it is fast and could be made portable to field.

ziman