Optimum make up flow?

[karthic]

Hello

I recently got a SUPELCO CARBOXEN 1010 PLOT capillary column (0.32 mm ID) for separating the syngas (whose composition is similar to transformer gas). We have been optimizing the parameters since a couple of days.
I would like to know if there is any optimum make-up-flow for any kind of system?
I have Argon as my carrier gas and a TCD detector attached and my carrier flow rate is 0.4 ml/min.
Does the increase or decrease in make up flow increase/ decrease my selectivity/ seperation/ resolution?

I would appreciate if I you people can help me out as I am relatively very new to this field of chromatography.

Thanks
Karthic

[Don_Hilton]

I would check the operating manual for the GC to see if the manufacturer has a recommendation. (Specifically the portion on the detector that you are using.)

The TCD works by measuring the thermal conductivity of what ever gas is going past. The greater the difference in thermal conductivity between carrier gas and carrier gas with a peak in it gives rise to the chromatographic peak on the plot of the chromatogram. If you add makeup gas to the flow of gas through the detector, you dilute the analytes - and reduce sensitivity. But, if your detector has any significant volume relative to the column flow, you need to add makeup gas so that what has just been measured will be swept out of the detector so that it does not mix with the next stuff being measured.

If you can’t find a manual for your instrument, my approach would be: Start with a low makeup flow rate (or makeup off) and increase it slowly over several runs of the same mixture. You should see the peaks become narrower as you eliminate mixing in the detector. The peaks will also become smaller as you add makeup gas. When you have sufficient makeup gas to maintain your chromatographic resolution, that’s the optimum. Let me know if this works for you.

[AICMM]

Most TCD's (and most FID's for that matter) were designed around packed column flow rates. Make-up gas is added to capillary flows in order to get capillary flows up to packed flows right before the detector. Furthermore, TCD's balance a reference flow against a column flow and, without some restriction, your reference will have a hard time maintaining that low a flow consistently. So, yes, you probably need a make-up flow. However, TCD's are also concentration dependent so the more make up you have, the less sensitive you are. But, as Don points out, if peaks are swimming around in the detector that's not good and a sharp peak is more sensitive than a broad peak. So, it's all a trade off. I would start with about 20 mL's of make up (on the lower end) and evaluate a couple of other flows against your application. You want the lowest flow that will give you the sensitivity and resolution you need.

By the way, I think your column flow rate is too low. For a 0.32 I would recommend at least a mL/min and probably more.

Best regards.

[larkl]

We just started using the Carboxen 1010 to replace a dual PoraplotQ and 5A mol sieve. It's nice to get everything (perm gases) you need off one column without the valving to worry about. We're starting at 2ml/min (It's the 0.32mm) and ramping up to 5 to speed up the analysis since the CO2 elutes fairly late.