Chromatography Forum

Dear all,

My colleages ran a mixture of samples today. The column is HP-INNOWAX (PEG). We have not used this column for a long time.

Here are some parameters:
carrier gas: He 1ml/min,
inlet: split 100:1, inj vol 1ul
detector: FID, Air 250ml/min, H2 25ml/min, make-up gas: He 1ml/min

Then we find that some peaks are tailing, some are fronting and some are symmetrical.

Hence, he made a run of dodecene only. The solvent peak is tailing but dodecene is fronting.

Should I consider the column is bad? or is it normal for different peak shapes in the same run (especially in PEG column)?

Did you try to cut a few centimeters of the column. And the liner is it old also? Try to check if it looks dirty, and if you have a new one I would change it.
Regards,

At_om

Anthony_Ng,

I am assuming that you are running a 0.25 column based on the column flow rate. So why are you only running 1 mL/min of make-up? Detector wants to see at least 10 mL total flow and more like 20-25 mL/min. Increase your make-up flow and see what that does. You might try increasing your H2 and air flow as well.

Best regards.

Hi Anthony

Tailing is nearly always due to adsorption or absorption, often onto muck inthe inlet. Solvents often show a tail because there is such a large quantity and becuase they get no focussing when they transfer from inlet to column. Compounds with polar functional groups e.g -OH, -NH2 often tail when other compounds give symmetrical peaks.

Fronting is due to concentration overloading, and is expected when there is a mis-match between the polarity of the stationary phase and the analyte. In this case you have a non-polar analyte (dodecene) on a polar column. Acids and alcohols on non-polar columns give exactly the same problem.

What you see is not necessarily a symptom of system problems, but you do need more make-up as AICMM says, and it would be worth checking the inlet for muck.

Peter

Thanks for your suggestion. Indeed I am rather new in GC so some parameters are wrongly set.

The column dimension is : 60m x 0.25mm (id) x 250um (film thickness)

I was told the proportion of Air:H2 is around 10:1, so I set to 250ml/min:25ml/min

When I set make-up gas flow higher, the flame goes off immediately after ignition. That's why I wrongly set to 1ml/min and it doesn't go off at this setting.

Column head is newly cut for about 10cm, new septum and clean liner replaced.

Beside ananlyte overloading and polarity mismatch, will fronting caused by wrongly/poorly set oven temp programming? (I start at 70oC for 5min, ramp at 15oC/min until 220oC and keep there for 20min). Solvent peak comes out at 5min, Dodecence comes out at about 9min.

Locate the manual, and set up the detector as specified in the manual, taking care to set the column flow first.

Most manufacturers offer guidance on how to set appropriate flowrates for the specific detector design. The "10:1" rule is not a good start - as you have discovered. Also ensure the detector is hot, it makes ignition more consistent.

In my experience, good selection of detector parameters ( gas flowrates and temperature ) will mean that you almost never have to reset the detector gas flowrates for most capillary column flows ( 1 - 5 ml/min.).

Peter and AICMM have described the most common causes for the problem you see.

If you want to know if the column is still OK, duplicate either the separation on the column's CoA, or an earlier analysis that worked.

Please keep having fun,

Bruce Hamilton