Chromatography Forum

Dear colleagues,

How do you go about setting the flows for a micropacked column in a 6890 GC? I'm currently still trying to analyze the amount of CO in a CO2 stream using a 0.53 mm ShinCarbon ST micropacked column with a split/splitless inlet and a TCD with helium as carrier gas, but it's not working well.

When I installed the column, I calibrated the column flow vs. the inlet pressure using a flow meter and I got this calibration curve.



The recommended flow rate for this micropacked column is 3 to 5 mL/min, so I chose 5 mL/min (for no particular reason at all). That means I have to set my inlet at 4.06 bar.

Okay, now the question becomes, how do I set the split ratio?

I thought then that given that

Total flow = Septum purge vent + column flow + split vent

and that the purge vent and column flows are fixed automatically by the EPC and by the inlet pressure, respectively, the split ratio is controlled only by the total flow to the inlet. Is this correct?

So, if I set a total flow of 33 mL/min at 4.06 bar, 5 mL/min will go to the column and 3 mL/min will go to the septum purge, leaving a split vent flow of 25 mL/min, that is, a 1:5 split ratio.

Does this make sense?

It only makes sense if you are working with a capillary column. You are in effect working with a 1/16 inch diameter packed column. The only way that I see that you will get this to work is to use the Agilent flow calculator and find an equivalent capillary 'column' to your micro-packed column. In the calculator, you enter a pressure of 4.06 bar, the carrier gas your are using, and vary the column length and/or diameter until you get 5ml/min of flow. You then enter this length and diamter into the 6890 as the column dimensions. It should then work, as the system 'thinks' that you have a capillary column installed.

Gasman

But, in that case, what are "reasonable" operating parameters for a micropacked column? Restek's instructions mention that the column flow should be between 3 and 5 mL/min (and that's why I set the inlet pressure at 4.06 bar and operate at constant pressure mode), but it does not say anything regarding the total flow in split mode. That's why I was trying to do the analogy with the split ratio.

But then, these columns are usually not used in split mode?

Your calculations for split flow are correct - it simply needs to be the split ratio times the column flow.

Peter

Gaston,

Why are you using a micro-packed column with the capillary inlet? The reason for the split mode is to prevent overloading of a capillary column, which results in poor peak shape. A micro-packed column will have a higher capacity than a 'standard' capillary column and will allow higher concentrations before you get overloading. How are you injecting your gas sample into the GC and what concentration range are you expecting for your CO.

I would not recommend using any micro-packed column with a capillary inlet, prefering to inject the sample directly onto the column using a 6 port valve. I would control the amount I inject by choosing a suitable sample loop size on the valve.

Gasman

Gasman,

Truth be told, I don't really know why I am doing it that way I am injecting the gas sample through a six-way valve with a 1 mL sample loop (the purpose of the GC is to do on-line gas analysis), but the six-way valve goes into the S/SL inlet. I have not changed the "plumbing" after inheriting the GC from an unknown previous owner, but initially it had a capillary column installed, which I guess explains why the six-way valve discharges into the split/splitless inlet and not directly into the column.

Is it then possible to "bypass" the S/SL inlet and inject directly into the column?

I am expecting my CO content to be around 5000 ppm (with CO2 as the balance gas), but it is highly variable on the "effectiveness" of the reaction I am measuring.

IF you wish to carry on using your micro-packed column, it will not be easy to use the capillary inlet, especially if you already have the valve plumbed upstream of the inlet. It will be easier and probably cheaper in the long run, to change over to a PLOT column. However, this will depend on what other compounds are likely to be present in your sample. Will oxygen be present in the sample?

Gasman