Modulation on secondary column- GCxGC-TOFMS

[kishorebits]

We have a LECO GCxGCTOFMS instrument with cryogenic free modulator. By default the setting of the instrument is to perform modulation on the primary column. That means primary column was present where hot and cold jets used to flow. Today, while changing the secondary column, our LECO engineer fixed columns so that secondary column is now in the modulator (instead of primary column) and union connecting primary and secondary column is in the primary oven. Will there any effect of this setting on 2D seperation ?? I didnot use the two dimensional analysis before therefore I am bit worried this kind of setting will have effect the two dimensional seperation. Moreover, I donot have any data to compare where modulation being performed on primary column versus secondary column. I am really looking forward for the thoughts and suggestions of all the experts in this forum.

[Peter Apps]

Insofar as the modulator focusses what comes into it, it makes more sense to have any potential sources of peak distortion (in this case the connection) upstream of the modulator rather than downstream.

Nonethless, cryofocussing generally works better on siloxane phases because they stay liquid at lower temperatures, and if you have a PEG column as primary and a siloxane as second dimension it might be better to have the modulator on the second column.

If one arrangement does not work, try the other.

Peter

[Don_Hilton]

I have worked with the LECO GCxGC systems for several years - and the configuration shown by LECO is with the press fit in the primary oven. And, this is what I have used with good success. You can put the pressfit between columns into the secondary oven. I've known several people who have done this and believe it gives them some advantave. It is a tight fit and you have to be careful of which manufacturer of press-fit you use. Some press-fits are longer than others. And, the longer press-fits become a bit more of a problem to get into the oven without bending the column too much. (The tighter the bend in the secondary oven, the sooner the column will break!)

I have found that some columns do not seal well to the press-fits and have to be ocassionally reconnected (new cuts on the ends of the columns and a new press-fit) ocassionally to keep going. You can use pollyimide to try to seal the columns into the press fit - but I've even that come apart once or twice. It is a lot more convienient to do the reassembly without having to move things in and out of the secondary oven.

As cold as the cold jets get, I would say that you are mostly freezing things out in the modulator. When the hot jet hits the column, it not all that hot - not more than 30 degrees above the secondary column - and heat transfer is driven by the gradient. So, the question is does the stationary phase film "thaw" out quickly. I've been betting on the thinner stationary phase in the second dimension column. And given that each of the two cooled zones covers about half a centimeter of the column, it may not make much of a difference in the end.

See what gives you good chromatography and push it to the limit. GCxGC is still a new enough field that you can try different things and be no more wrong than the rest of us

[Peter Apps]

Don brings up another point with regard to temperature - the hot jet gets hotter than the second oven, which in turn is hotter than the first, so if the oven temperature is getting close to the upper limit for the stationary phase, the hot jet will take it briefly (but repeatedly) past the limit.

In other words, put the modulator on the column with the higher temperature limit.

Peter

[Don_Hilton]

In the LECO instrument the upper temperature of the secondary column is used as the temperture limit for the modulator and secondary oven. The hot jets are heated by the modulator body. Because the secondary oven is not actively cooled and is inside the primary oven, the higher temperature column is usually put in the secondary oven - and through the modulator.

If you put the column with the lower temperature limit in the secondary oven, the temperature limit for the modulator and secondary oven are set for that column and the primary oven is also limited to that upper temperature limit.

The Zoex modulator is a different configuration and, from what I've been told, does permit a hotter hot jet in the modulator. But that is not the instrument that kishorebits is using, so I'll not wander down that trail.

[kishorebits]

Thanks alot for the replies. It is a great learning experience ever since I joined this forum.
I am currently using DB-1 (0.25mm i.d.) as primary column and Rtx-17(0.18mm i.d.). We have a crogenic free modulator (supposed to be newly introduced in market by LECO).
May it will take some time, but I will definitely update you the observations that I find with current configuration.

[Don_Hilton]

I will be interested to see how things work for you. I typically use narrower columns (0.18 mm first and 0.1 mm second) and have considered going to the diameters you are using to get around some column loading issues. I'll be looking for picures either here or in a journal article.