Chromatography Forum

Dear all,

I have seen some places where they inject a large volume of sample using a split/splitless injector in splitless mode with some silanised glass wool at the bottom of the glass liner.

What do you think of this? Is it possible without a temperature programmable injector?
How could they avoid the backflashing and the overflowing of the solvent?

Regards

I think very poorly of it.

Your concerns are justified.

I have used a special injection liner in certain GC designs that allow direct injection with a large volume for evaporization of the liquid sample when the injection is carrier out slowly so no backflashing could occur.

But your description does not come near to what I have done successfully.

Rod

Thanks Rod,

I see...

And if the sample is injected slowly?

Is manual injection an option?

Please note that I did not use an injector with a split, the split had been by-passed by the extension of the liner which had a 100% flow into the column.

The Varian autosampler has a variable injection rate which is controlled by the computer.

But to simplify and to accelerate the research I did my injections manually. And did them in a controlled manner (VERY SLOWLY) so the vapor cloud never reached a volume so that it would exceed the flash volume of the liner.

My samples were permitted to refocus on the column and the large amount of solvent permitted to pass through the column before the oven temperature was allowed to increase and move the analyte bands down the column.

Unfortunately, a split/splitless injector is not designed in this manner and its flash volume is too small to inject a large sample without having solvent and analytes going everywhere possible as the volume and the pressure increases during the injection.

Success of this technique is dependent upon the creation of the vapor volume from the sample being less than the amount of vapor entering the column per unit amount of time.

The type of solvent and the analytes, the film thickness of the column and the measured stability of the hand of the chemist are all factors which affect the result.

Use the right hardware and the right technique and improvements of two or even three orders of magnitude can be attained. But if everything is not appropriate, then tailing and wailing are usually the result.

Good luck,

Rod

It can be done, but as Rod says, only under special conditions. There is a lot of literature out of Grob's lab and alsewhere on solvent effect focussing and large volume injections - several years old now though.

Peter