GC Newbie - please be gentle!

[Don_Hilton]

I would move the column to a split/splitless injector. and get away from the on column inlet. You can then use a liner with glass wool that will catch a lot of the non-volatile junk. I notice that Restek has a uniliner that will take a 0.53 column. So, I expect that the Varian inlets can support a 0.53 column. And, you would have the option of using the other types of GC liners.

I'd rather be discarding old liners than cutting the end off the column, if it is an option.

[Peter Apps]

Hi H-H

What inlet (not autoinjector) do you have on your GC ? - there are various models available. To find out, press the inlet icon on the GC touchpad and look at the top left of the status screen for the model number. My guess would be that it is a 1079.

Before you go any further down a road that is not leading you in the direction that you need to go I think that you need to completely rethink this method. To put it bluntly the advice that you are getting about what columns are compatible with split splitless injections is just plain wrong. You can do any kind of injection you want to onto a megabore column. "megabore" by the way just means an i.d. of 0.53 mm.

As Don says, the GC needs to be set up to do split injections, with a plug of glass wool in a single gooseneck liner. If there is nobody in your lab who can do this, you need to get Varian technical support in, although I have to say that this is not a viable way to generate results routinely. You are probably going to have to change the liner once day. Use a retention gap, you can get these from column suppliers. The retention gap needs a polar deactivation.

The next question is the choice of stationary phase in the column. It has to separate the ethanol from interfering peaks, especially whatever you dilute the samples with. You might even consider filtering the raw samples and injecting as is with a high split ratio. All the column companies have applications notes, look for the one that most closely matches your needs, and then ask their technical support for their opinion.

Always make sure that you are talking to technical support and not sales when you ask for technical advice !

Peter

[HPLC_Help]

Thank you, Don and Peter. I spoke with Restek tech support for about 40 minutes yesterday, and the specialist told me exactly what you are saying (except for the glass wool part - but I'll call her back).

The column stationary phase does seem to be appropriate for our application. I am moving to a 0.32 mm, however, to help improve variability in using a split/splitless injector. I also am ordering polar deactivated guard column to cut into a retention gap. (The inlet on the Varian is a 1041.).

Do I pack my own liner with glass wool, or is the liner sold with the glass wool?

Thank you all, especially for adhering to my initial request - to "be gentle."

H_H

[Don_Hilton]

Purchase the lines already packed with glass wool. The glass wool should be deactivated by the vendor in the liner. I ran into a vendor who did not do this - about 15 years ago - and they have changed since then. If you try inserting glass wool into an inlet, some of the glass fibers will break, giving very active surfaces.

[Peter Apps]

Hi H-H

The inlet that you have is indeed an on-column injector, but it is better suited to packed columns than to megabore capillaries. Is it the only inlet on the GC ?

You may be able to convert the 1041 inlet to take a removable glass liner that can be packed with glass wool. Speak to Varian (their web site is singularly uninformative), Restek, and Supelco.

Peter

[HPLC_Help]

Hi, Peter -

We're moving to the other injector, which is capable of handling a liner, and moving to a smaller-bore column.

Why is the mega-bore not as good on the on-column inlet? Also - what's a packed column?

Thanks!
H_H

[Peter Apps]

Hi H-H

A packed column is the kind that you are used to - a tube with packing in it, and a stationary phase coated onto the packing particles. Because gas is less viscous and has a higher diffusivity than liquid, GC packed columns are nearly always longer than LC columns, and have larger particles. GC packed columns are usually 1/8 or 1/4 inch diameter with lengths in tens of cm, GC capillary columns are 0.53 mm or smaller in diameter, with lengths in tens of m, and the statioanry phase is a coating on the inside wall of the tube.

There is no problem with megabore columns in on-column inlets. In fact it is WAY easier to do an on-column injection into a megabore (0.53 mm)column than into a normal bore (0.32 mm) because you can use an ordinary syringe needle that is stiff enough to penetrate a septum for the megabore, but you need a special thin needle for the narrow bore, so thin that it cannot penetrate a septum without some extra bits to help it. The problem that I alluded to is that the 1041 is a packed column inlet that can be used for megabore capillaries, rather than a specialised capillary on-colum inlet, which allows you to do injections at low temperatures.

Peter

[HPLC_Help]

Thank you all for your help. I've just ordered some new supplies, and here's where we're starting when they arrive:

0.32mm column (same type as before); retention gap (polar deactivated); goose neck liner w/ glass wool; split injection.

inlet temp - 250C
column temp - 40C
ramp temp to 240C @ 25C/min.

Again, our samples are ethanol in methanol.

Thank you all for (basically) overseeing our method development! I'll let you know how it goes.

H_H