Chromatography Forum

Hi,

I am using Restek liners because our local Thermo representative will not work with me because I have an old Thermo device bought second-hand. The only ones to help me install the device and sell me consumables were the Agilent representatives.

So, this one I am using, is a Restek Open Top Uniliner, 4mm x 5.4 x 79.5 for Thermo GCs.

Yes, i believe my column is too low in the injector, since the tech guy that helped me to install the device and connected my column was an Agilent employee and set it up for 2mm in injector. I think i should stick it into the liner's neck. Now is exactly how I draw it in the first picture.

I am using exactly 3 seconds pre-injection and 2 seconds post injection dwell time.

This column setup might be a part of the problem. Other part might be due to "no pulse mode" I employ. It seems everybody use this pulse mode, but I can't. What I can do is loose the constant velocity of 40 cm/s I am using now, and make a pressure program. I can try to initially set up a bigger pressure, to force more vapors into the column, then make a ramp calculated to match the old 40 cm/s program based on column dimensions and temperature.

Monday I will start again the device and give a try with all these. Till then I am gathering ideas and tips

Thank you,
Vlad

CE has probably spotted a major source of the problem - the 2 mm insertion depth is specific for Agilent injectors (and even there I used to increase it). Seat the column into the taper of the uniliner with very gentle pressure - there is no pressure gradient across the seal. I still think that you need to increase the inlet temperature - at least the Thermo Injector has a uniform temperature profile.

Peter

Have you checked the injector menu ? I seem to remember one of the options was splitless with pulse.

Have you downloaded the manuals ?
http://www.gc-gcms-customersupport.com/ ... fault.aspx

Hello wraymogg and all
I am amazed by the fact that the focus of discussion is the inlet.
It is my opinion that most likely the tailing emerges from the ion source
If I am correct, raising the ion source temperature will reduce the tailing
Normally such increased ion source temperature leads to largely reduced
abundance of the molecular ions but not in PAHs in which it is dominant
Thus, my suggestion is to increase the ion source temperature
Amirav (amirav@tau.ac.il)

Hi,

@CE Instruments
Thank you for the link, interesting stuff there.
I don't have a pulse option 'per se', in my imagination 'pulsed' implies some repetition, an oscillation of pressure. But I have other options to try. This is how my screen looks:



For now I am using the constant velocity with 40 cm/s. I will test if 'Surge pressure' works with constant velocity. If it woks, I will program a single pulse at start. Any idea what On time and OFF time would be appropiate ?
If is not working, I can try to make a pressure program, and program a pressure spike, then simulate the constant velocity with a temperature/pressure ramp.

@Amirav
Thank you for your idea. I excluded the ion source's temperature as a problem after I read a couple of papers having great results with the last PAHs with the source temperature at 200 degrees. I can raise it max till 240 degrees, and I actually did. The result was the same. Following are two papers both with same device as mine's, same column type, and they have great results with ion source at 200 degrees.

http://www.biosol.ro/Poze/PAH.pdf
http://www.biosol.ro/Poze/PAH2.pdf

Best regards,
Vlad

P.S. I just discovered that Xcalibur can convert the .ms file into .raw or .cdf... Pfew...

OK so you have a really old system It is fitted with the Finnegan GC that came from Thermo rather than Fisons.
That GC had a very short life as it was replaced with the Trace once it was launched. I guess the surge pressure is what you need and should work with splitless.

Hi,

everything is OK now, I finally got very nice peaks and good separation for the last PAHs. It seems all the good ideas in this forum worked. Thanks to all that contributed.

The things that made it work:

1. Raised injector temperature to 350 ºC
2. Sticked the column into the liner's taper
3. Surge pressure at 33 psi for 0.4 min
4. Rapid injection - 0 sec pre time, 2 sec post time
5. Analyte focusing - started the run at 20 ºC over solvent vaporization temperature, in my case acetonitril, started at 100 ºC
6. Modified the program to elute last PAHs on temperature ramp, not on isothermal part (100 ºC to 320 ºC, 5 ºC per min)

In SIM mode the chromatogram looks great for 10 mg/l.



Now I will try to see how low can I get. By the way, for those wondering about the other temperatures, transfer line at 280 ºC and ionization chamber at 220 ºC.

Best regards,
Vlad

Great feedback - thanks Vlad.

Peter