Chromatography Forum

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?

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

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

Stella,

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

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.

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

what was the retention time for methane?

best wishes,

Rod

Hi Stella

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

Peter

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

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

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

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!

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.