packed column length

[stella]

i'm trying to study with igc a natural mineral, a material with high surface energy, using a packed column. My problem is i get unbelievably
short retention times.
i packed a ss column 1/4 inch o.d 20 cm long, i have a normal flow(20 ml/min), the column temprature is 100C and i get retention times about 20 sec for n-hexane and 30 sec for n-decane!
i tried a ss column 1/8 inch o.d 50 cm long with the above conditions and the results wee almost the same.
the last column i made was 1/4 inch o.d 70 cm long and i saw no progress.
i lowered the flow, in 15 ml/min, in all cases but it made no difference.

any ideas?

[Consumer Products Guy]

Those are pretty short packed columns. When we used packed columns, 6 feet was a common length, or longer. Need more information.

[stella]

Those are pretty short packed columns. When we used packed columns, 6 feet was a common length, or longer. Need more information.

i'll try one about 2m long and we will see. i tried to avoid it because the packing will be difficult and the voids inside the column might be more.

thank you

[MikeD]

Stella,

Are you measuring physicochemical properties of your mineral packing ? Can you explain why short retention times are a problem for you.

[stella]

Stella,

Are you measuring physicochemical properties of your mineral packing ? Can you explain why short retention times are a problem for you.

i'm trying to determine the dispersive component of the surface energy using apolar probes. The sort retention times are a problem to me for two reasons:
1. a high surface energy material normally gives high retention times
2. as i mentioned the retention times range is very narrow, about 10 seconds from hexane to decane.

[chromatographer1]

Just curious..........

what was the retention time for methane?

best wishes,

Rod

[Peter Apps]

Hi Stella

How big are the particles that you pack into the column ?

Peter

[stella]

Hi Stella

How big are the particles that you pack into the column ?

Peter

Hi Peter

i thought this parameter might affect the results, so i packed particles with sizes between 100-250 μm, then 100-500 μm and finally 250-500μm (this size is suggested from bibliography, also). None of this three columns worked, the retention times remained short.

Stella

[Peter Apps]

Hi Stella

How are you measuring the gas flow rate ?

Short retention times could be because the mineral has a low specific surface area - how do the peak shapes look ?

Peter

[stella]

Hi Stella

How are you measuring the gas flow rate ?

Short retention times could be because the mineral has a low specific surface area - how do the peak shapes look ?

Peter

Dear Peter

i measure the helium flow rate with a soap bubble flowmeter.
the peaks are symmetrical in general with a little tail in their end.
indeed minerals have a low specific surface area, but they have been studied with igc.

Stella

[DR]

1) Much longer column - wider bore=easier to pack w/o voids

2) Much narrower particle size distribution - go for as small a particle as you can reproducibly screen without having to screen a dump truck load of it (100-120µ?)

3) Answer Rod's question (Methane RT?) - he has forgotten more about GC than I ever learned I think he wants to help you establish K. It appears you have had good success separating C6 from C10, so count that as some progress!

[MikeD]

But if you are measuring a thermodynamic property in inverse GC long packed columns are a confounding factor. The gas compressibility factor can mess up the calculation.