Area Falling

[chromatographer1]

Restek can make these with wire bundle terminations. Not my favorite configuration.

Supelco can make these with integrated stainless steel screen terminations, industry standard configuration. As long as the screens are properly installed (a quick visual check before installation is required) these are much better for long life and consistent performance.

You definitely have a crushed packing or packing void problem with pressures that different between columns.

best wishes,

Rod

[AICMM]

punt,

Can I suggest trying a smaller loop as a diagnostic? Perhaps a 0.5 or 0.25 to see what the peak shape looks like...

Best regards,

AICMM

[punt]

Hi Rod,

After much troubleshooting, i realised it could be the moisture that cause my H2 peak shape to change from fronting to symmertical. When i inject a lower concentration of H2 like 60%, i can see that the peak is tailing. After i condition the Molsieve5A column at 280C for few hours, the peak profile change from symmetrical back to fronting and the area and retention time is now more stable. Thanks for your useful information.

Some question i would like to ask you, for molsieve 5A and 13X column, how many times can we regenerate at high temperature? Usually wat kind of symptons do we see for such column before we decide to change to a new column? Other than moisture, how does CO2 absorption to the molsieve column affect the column performance? Thank you

AICMM,
I did not try to change to a smaller sample loop becoz i dun think the problem lies with the overloading as i have 2 similar GC with the same configuration. If its a overloading issue, the other GC which is alright should be facing the same problem. Moreover when i Inject a 60 percent H2 Std, the peak shape also look fronting.

Thank you

Regards
Punt

[chromatographer1]

When the column is dry and the sample size exceeds the limit the column can separate and keep a proper equilibration the peak shape will be asymmetrical (fronting). This doesn't mean you can't inject and integrate your peaks properly. We are talking about symmetry for a publication quality chromatogram.

As the pores are blocked with CO2 or water the peak will tend to broaden and tail. When an asymmetrical (fronting) peak begins to tail it appears to be symmetrical.

When the pores become badly blocked, the peak will be shorter and wider than historical examples.

Mole sieve packing can be reconditioned many many times if done slowly in temperature and using a large a flow of carrier as possible.

(the idea is to remove the water at temperatures as low and as quickly as possible )

The packing will crumble with time. It is more friable when dry than when 'wet'. When enough 'dust' is present the peaks will tail and broaden. When you are unhappy with peak shape and peak separation, replace the column.

The more often you condition the column, the better. The lower temperature you use to condition the better. (I never exceed 200°C) The slower the carrier flow when you condition the worse. The longer you condition the better.

best wishes,

Rod

[punt]

hi Rod,

When you mention using a large flow of carrier gas to condition molsieve column, what kind of flowrate are u refering or best for conditioning.
Is the flow rate measure going into the column or leaving the column?

Thank you

[chromatographer1]

I would use the maximum pressure the GC was capable of using, usually 80 to 100 psi.

The more quickly you can remove the water at a lower temperature the less damage to the mole sieve you will cause, the longer the column will last and the better the peak shapes will be.

The WORST possible technique would be to take a WET MS column and heat it above 250°C using a minimal flow rate.

Rod