Chromatography Forum

Hi everybody !

I have some trouble with my GC Varian 3800. When injected the same standard multiple time, I have a RSD around 10%. If I choose an internal standard, it decreases to 4-5%, but it's still too high. What can I do to improve my RSD ?
I already tested a new syringe, searched for leaks with an electronic leak detector, and verified that my column is correctly installed.


Configuration :
3800 Varian GC
Injector 1177 SSL
Detector PFPD
Autosampler CP-8400
Injection splitless, temperature 200°C, 1µL injected with a 10µL syringe
60°C initial temp, to 300°C (rate : 20°C/min) for 2 minutes
Detector temperature 300°C

What solvent are the standards in ?

What are the compounds of interest ?

What inlet liner are you using ?

Is the detector in square root mode ?

Peter

Hello Peter!

I test the PFPD with OP pesticide Mix from Sigma Aldrich (ref. 861237). It contains carbophenothion, ethion, malathion and parathion at 100ug/ml in hexane.
I tested the non dilute solution with split (split ratio : 200 ) and 1/200 dilution in splitless mode. In the two cases, the RSD was approxymatively 10%.

Regarding the liners, I tried 2 different types :
-Restek 21046-214.5 : spilt liner w/Glass frit 4mm x 6.3 x 78.5
-Agilent 5183-4711 : Liner, split, single taper, glass wool, deactivated

The detector is not in square root mode

Thank you for your help. Every idea can help me resolve the issue.

The first thing to try is to put the detector in square root mode; the "sulfur-squared" response of the PFPD exaggerates differences between samples. You can of course just take the square roots of the areas you already have.

Edit - I am assuming (because you do not provide the information) that you are in sulfur mode, not phosphorus).

What is the solvent in the standard ?, and what are you diluting it with for the splitless injections ?.

Why are you using an inlet temperature of 200C ?

Peter

Thank you Peter for your help!

I will try the square root mode on thursday; and I will keep you updated.
And yes I am in sulfur mode to test the performances of my GC (sensitivity, repeatability...)
When my instruments performance tests will be OK, I will switch in "Sn" mode because I need to analyse Butyltin trichloride. Do you have some tips for the "Sn" mode like gate width, gate delay...?

Regarding my sulfur test method : I work in hexan for my standards, and I diluted my sample (in hexan) because I would like to test the repeatability in splitless mode (because the repeatability in split mode was too high : 10%)
For the inlet temperature, I chose 200°C because I am used to work at this temperature. Do you have some advice regarding this method ? Do you recommend to increase or decrease this temperature ?

Thank you again for your time !

I have never worked with tin compounds, so my advice is to follow the settings in the manual.

Try increasing the inlet temperature to 250C


Peter

Hi,

Have you tried just re-installing your column into the injector? If the column isn't in the correct position (height) in the injector, your RSDs will be affected significantly. I believe the 1177 has a 3.7cm insertion depth from the bottom of the nut.

Hope this helps.

Increase injector temperature to 230-240 at least. Measure and reconnect the column. Dont use glass frit liner for splitless injection but other is ok. There is a tick-tock alignment on varian pfpd detector. Align detector regarding to the pfpd manual. Its easy... The trick for varian gc-3800 is the autosampler parameters. Choose user defined mode. put air plug after sample 1ul. no solvent plug. viscosity delay 2 seconds. inj speed 20. pre-inj delay 1, post inj-delay 2. fill stroke 3