Chromatography Forum

Is this a correct statement; If the boiling point of Benzyl alcohol is 205°C then the vapor pressure of Benzyl Alcohol is 760mmHG at 205°C?

I hope this is correct because I'm going to go on.

Currently, I am trying to analyze Benzyl alcohol Organic Impurities via the USP method. The injection port is 200°C. My display of head pressure is 4.0psi. Is the actual meaning of head pressure 4.0psi above the atomospheric pressure of 14.7psi which 1atm (760mmHg) equals?

If so does that mean the total pressure in the injector is 18.7psi or 967mmHg?

Presuming all of the above is correct, could this explain the large carry over peaks that I am seeing when I run the temperature program of the GC without actually making an injection?

Mr. Burger,

I would advise turning up the injection port temperature. That could certainly be a source of your carryover issues.

Best regards,

AICMM

Also, how much are you injecting and what liner are you using? Splitelss injection? Unless you are concerned about degrading compounds, I would suggest an inlet temperature of 250 degrees.

You do not have to be above the boiling point of your compund to move it from the inlet to the GC column - but it helps.

Be sure your inlet liner is properly installed and has a good o-ring seal (use a fresh o-ring each time the liner is changed - and it should be changed often - for some that is weekly, others it is daily)

The reason for asking about liner and injection volume is that if you inject too much material into the liner, the expanding sample vapors push out of the liner, out of the inlet, and back into the gas supply lines. Cleaning up after this is a bit time consuming. Improper installation of a liner (or a broken liner) will lead to funny gas flows. The problems are typically other than what you describe, but with a relatively cool inlet - it would allow for vapors to be trapped on cold spots.

Charles - I'll look up the USP assay of benzyl alcohol once I get to work. If the USP assay states 4 psi, that's what it means as your total inlet head pressure on the 30-m × 0.32-mm, 0.5-μm film thickness, G16 column detailed.

We all must remember that even the "beloved" USP allows considerable flexibility in allowable changes/modifications to even its own methods which do not require any validation work. These are listed at the end of the recent <621> chapter, and in some other USP publications.

Yes, there are many USP assays that cause us to raise our eyebrows....

OK, I looked up the NF Monograph.

Don - the injection volume in the monograph is 0.1 ul.

Charles - where do you get that 4.0 psi inlet pressure from? The monograph states 25cm/sec linear velocity, and Agilent flow calculator with your conditions calculates as 14.1cm/sec and a flow of 0.717 ml/min, considerably less than the NF monograph.

If you're getting peaks and stuff without an injection, take that inlet temperature up and bake your column at 220C (the maximum in the NF) to try to reduce that.

Funny you mentioned my pressure, I went and checked my column dimensions plugged into the system and I found out I was showing a 15m column installed instead of the 30m that I am using. Magically the pressure went to approximately 8 PSI after that correction. Thanks for that heads up CPG.

After getting everything squared away, I decided to start from scratch, new liner, septum, column trimming, and etc. After conditioning the system, I proceded with a blank collection. Only the temperature programs were activated. The septum was not pierced in anyway. Anybody care to guess what happened next?

Yes, a large peak detected at Benzyl alcohol. Ugh! SO last night I baked out my injector. I hope this is going to take care of that problem, but since I want to get a head start on troubleshooting, is it possble that I could still have a Benzyl alcohol (or any peak for that matter) show up after baking out the injector (at 400°C) for 12+ hours? How and what could it be?

is it possble that I could still have a Benzyl alcohol (or any peak for that matter) show up after baking out the injector (at 400°C) for 12+ hours? How and what could it be?

I'm assuming that you baked this out with carrier flowing, and with the column hooked up and it at a high temperature as well.

I'm assuming that you baked this out with carrier flowing, and with the column hooked up and it at a high temperature as well.

Yes
No
Yes

put a ferrle plug in, capped the injector head and ran a 100 ml/min total flow through the injection system. Half out of the spilt vent, the other half out the septum purge.

Result was a very clean baseline during a no injection blank run.