Column length

Basic questions from students; resources for projects and reports.

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I am relatively new to the gc world. I utilize two shimadzu 8a tcds. One he carrier. One switches between ar and n2. I also have a shimadzu fid. just switched to these from the ancient gow macs that finally gave out. My question is pertaining to column lengths. Can anyone offer some in depth insight on the pros and cons of the column length for both ms and pq from an expert point of view. I find myself lowering temps and turning current up to help separate peaks when I believe adjusting column size could benefit me.

One mix that gives me big issues is a neon lung diffusion. .3 co / .5 ne / 21 o2 bal n2 on the ms.

If it help I use ez chrom.

Like I said any knowledge in the pros and cons in column sizing from someone who has "been there done that" would be greatly appreciated.


Thanks.
Column dimensions are key in getting good separations. More narrow-bore columns give you better efficiency and ultimately better separation. By increasing the length you get better separation at the cost of longer analysis time. Your resolution will only improve by about 40% if you double the column length.

The best way to get better separations is to change the selectivity of the stationary phase. For instance, m- and p-xylenes are not separable on DB-1 type phase but if you switch to a wax, you can separate all 3 isomers - in the same length of column.

You don't have a wide variety of stationary phases that are options for you with those analytes. Subambient oven temperatures might be your only option.
The best way to get better separations is to change the selectivity of the stationary phase.
True in general, but for fixed gases (as in this case) there are only a limited number of options.

To address the original question: to a good first approximation, if you double the column length you will improve resolution by 40% (doubling the length doubles the plate number, and resolution is a function of the square root of the plate number). However, the retention times will approximately double, and the pressure required to maintain your flow rate will also increase. Assuming you are using a molecular sieve column, the selectivity (relative spacing of the peaks) should remain about the same.
-- Tom Jupille
LC Resources / Separation Science Associates
tjupille@lcresources.com
+ 1 (925) 297-5374
We would typically do that separation with 1/8" packed column, 2m long, with helium. You're looking for small concs of neon and CO though so you might need to up the injection volume to 1ml which will then ruin the peak shape of the balance gases.
Where can I buy the kit they use in CSI?
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