Chromatography Forum

GCLeo,

By changing the loop you have reduced the pressure drop on the switch to inject. I would agree with Gasman that the better way is a 1mL loop with a 1:1 or 1:2 split. As an alternative, if you have an aux pressure controller available you can try a high volume injection using the Aux EPC. Rotors are all polymeric, by the way, with the material varying from PTFE to PEEK so you should try to match the rotor to the analytes of interest (and the pressure and the temperature....) Valco makes Agilent's valve bodies but Agilent has their own actuators in order to fit under the hood of the GC.

Now for the interesting question, regarding Peter's note on back pressure regulators. I have heard from good sources that you will have to matrix match standard to sample if you use back pressure regulation. I have not tried this myself so I am interested in comments. I have tried back pressure regulation on the sample line and you do improve the detection limit but you also smear out the peak. Other comments?

Best regards.

AICMM, thanks for all informative posts. Not until now had I known the rotor matching issue. We got the valve and heating box from agilent, never reminded the potential issue.

In fact, I'm now using the 1ml loop with 1:1 or 2:1 split injection. Eventually, I am curious how you guys avoid tailing peaks when you do large volume injection. Is it "starting temperature at lower-than-boiling point to condense the sample, and then a temperature ramp" thing? I didn't do so as I set it to get an inline analysis every two minutes.

You cannot run capillary gas phase injection without splitting - a 1:1 or 1:2 split is simply not sufficient for capillary column work, as you will get severe flow disruption and band broadening.

What is the ID of your column? What temperature are you starting your chromatography? What is your phase? It is quite possible that your ghost peak is actually due to the injection.

No, the ghost peak didn't exist before IPA injection. Column ID 253 μm nominal. Starting>190 degree C =10 degree higher than BP, gas phase. I think there are tricks to do to reduce band broadening.

Yes, if you have a back-pressure regulator on the loop you will need more pressure to load it. You will also have more sample in the loop, which will require a higher split ratio.

Is your target peak flattened on top ? Injecting 1 ml as a plug with a 1:1 split gives a starting band 15s wide, and since you are running isothermal this is the minimum peak width you can get. Since detectability depends on peak height rather than area under these conditions you need to increase your split ratio to say 5 or even 10:1 and see how the peak looks.

Peter

Peter,
I still get apexes. You're quite accurate about the band width. I used to use area rather than height and it has been working good. I'll try increasing the split and see if the height changes.

Thanks,
Leo

Now for the interesting question, regarding Peter's note on back pressure regulators. I have heard from good sources that you will have to matrix match standard to sample if you use back pressure regulation. I have not tried this myself so I am interested in comments. I have tried back pressure regulation on the sample line and you do improve the detection limit but you also smear out the peak. Other comments?

I tried back pressure regulation using 1 ml loops and found, like you, that sensitivity increased, and also that it was better just to increase pressure to about 20% of column back pressure. After that, the sample peak compression benefit decreased and peak shapes turned to custard.

In the end, I stopped using the back pressure regulator because I found it more convenient to fully evacuate the sample loop/pressure sensor, and fill to a measured pressure or use flowing stream with a few inches of water head.

If the loops are heated to high temperature, then the use of sufficient length of heated lead-in lines, along with a suitable back pressure regulator, may be justified to ensure consistent sample temperature.

I was really concerned that sample matrix ( especially water content ) could change phase in the loop and affect volume. The persistent concerns about cross contamination when performing trace analyses were just not worth the hassle.

One of the problems with some sample loops larger than 1 ml is that I tended to see wall effects as the carrier flushes the loop, which results in misshapen peaks for some "sticky" but volatile molecules.

The comment about standards is certainly correct for mixtures containing a wide range of analytes, however most gas standard manufacturers offer formulated standards at higher pressures, and may even offer calibration certificates for different end-use loop pressures - as can occur for some online analysers.

Please keep having fun,

Bruce Hamilton