Strange GC issue

[shibboleth]

Over the last couple of months I have started to do quantitation on timber samples and have been having an interesting time with some uncertainties with one of the analytes.

For the most part I have two actives in methanol to which a DBP internal standard is added. The issue I am having is one of the analytes is giving consistently varied results even from the same sample. The other analyte is fine however. Also RRFs are reasonable..

I have found that some of the troubled analyte is being hung up pre-coloumn as a flush after the standard shows. The amount that is being hung up is varied as well. The pre-coloumn is tapered with glass wool packing which, when changed stops this 'sticking' of the problem analyte for a short while before the problem returns and slowly grows over time.

To add to this, when I do this whole procedure with acetone instead of methanol the problem is gone. This is with a split-less injection of which there is far too little analytes in the standard or sample to overload the coloumn.

Any ideas on what is causing this issue?

[Peter Apps]

You have a build up of non-volatile deposits in the inlet liner. The solution is more frequent liner changes or more effective sample clean up before injection. The acetone - methanol difference is probably due to the methanol extracting more crud from the sample than the acetone does.

Peter

[shibboleth]

This issue still occurs with a standard even after rinsing the out of the coloumn/liner so that it is clean.. Surely if it was the 'crud' in the extracted samples this would not occur with a standard?

[Don_Hilton]

What volume are you injecting? And what ID inlet liner are you using (I assume 4 mm)? What kind of instrument and inlet are you using? And what is the head pressure at injection time?

If you are using an injection that is too large for the inlet liner you can be flashing materials back into the gas supply lines for the inlet... Some more details may help.

Also, the best treatment of a used inlet liner is placement in the trash.

[shibboleth]

Instrument is a Shimadzu GC-2010

The injection volume is 1µL
Inlet liner is 3.4mm LNR SH17A (have also used a 2.6mm LNR SH17A without a taper, with glass wool, same thing)

Pressure is 51.6 kPa
He total flow rate 27.2mL/min
Column flow rate is 1.15mL/min
Linear velocity is 32.5 cm/sec

[Peter Apps]

I have never found that just rinsing an inlet liner is an effective way of cleaning it. Unless you are doing an extensive sample cleanup I would expect methanol extracts of wood to very quickly lead to dark coloured deposits in the inlet - does your rinsing procedure remove these ?

How do you rinse the column ?

What are the two "actives" that you are interested in ? - one is affected by the problem and the other not, so there might be some troubleshooting clues in their chemical properties.

Peter

[shibboleth]

The rinsing removes a certain amount but never completely.
For rinsing the coloum just several blank runs with methanol.

The two actives are triadimefon and cyproconazole. Cyproconazole is the one that is sticking/creating issues.

I think I will try a SPE clean up and see how that goes.


Cheers for all the help guys!!

[Peter Apps]

It sounds as what you call "rinsing" is just a blank run - this will have a very limited effect of any dirt in the inlet; if it could be shifted by a blank run it would not have accumulated there in the first place.

What happens if you put in a new inlet liner ?

Peter