two columns one detector

[cmoral]

Hi all,

Has anybody put two columns from two injectors into one detector?

I'd like to use a mole sieve column to separate out N2 and O2 and a Q-plot for detection of N2O, both going into a TCD. Each column works well separately into the TCD but the Q-plot won't separate out the N2 and the O2 and the mole sieve won't separate out the N2O.

I have optimized the column and reference flows to the TCD for a single column but not sure how to do it if I try dual columns, except trial and error.

A second option is to put two columns into one injector going to two separate detectors, i.e. FID and ECD, to use the Q-plot column into the ECD. We already would have an RTX volatiles column into the FID.

Any suggestions out there?

thanks.

cmoral

[chromatographer1]

If you put two columns into the one TCD you will double the flow through the detector and reduce the sensitivity of its response.

You could put a valve at the tail of the columns and switch columns to the single TCD but you would perform the analyses sequentially. Would that be a problem?

Good luck.

Rod

[cmoral]

Hi Rod,

Thanks for your response. I was wondering if I could half the flow rate through each column so the total flow into the TCD is the same as if I only put one. It would increase the retention time, which may not be desirable, but it might be acceptable if it worked and minimized the sensitivity loss.

I am not familiar with the valve at the tail of the columns. I had been thinking about that but wasn't sure one actually existed. Could you give me a reference to this valve? It might not be a problem doing the analyses sequentially.

cmoral

[chromatographer1]

A four or six port valve, commonly provided by valco, or one provided by Agilent, or another vendor will suffice.

Halving the flow rate might be a possibility and would be the first I would try.

Good luck,

Rod

[dpr]

We have a molli sieve and ppq on a single injector/detector.

After a single injection the non retained components are passed through and first column which is then switch out of the system.
After the gasses have eluted from the first duty column the valve swithchs the second back in.

In effect, one injector, one detector with two columns in parallel rather than series.

[cmoral]

Thanks to you both for your replies. I will try halving the flows first and then look into the valve.

cmoral

[chromatographer1]

The two column in series using timed events is always a perk, but to keep it simple having a valve merely to change from one analysis to another is sometimes a blessing.

best wishes,

Rod

[cmoral]

An update:

We were able to connect two columns, a molecular sieve in one split/splitless injector and a Q-Plot in the second injector, into one TCD detector using a y-connector - no valve. Column head pressures were 2.5 and 5 PSI, respectively, gases running through both columns at the same time. We were able to separate out N2 and O2 with the mol sieve and N2O with the Q plot with pretty good results by injecting sample into the mol sieve 1 min after injecting into the Qplot.

So that worked.