film thickness calculation in capillary column

[jdezeeuw]

Hi
I am looking for reference for actual calculation of the average filmthickness in WCOT capillary columns used for GC. I know it can be done via the chromatography using the specific retention volume, the column ID, and phase density, but I need a reference for the formula.
Cannot find anything related on the web. Anybody out there that has this? Thanks!
jaap de Zeeuw

[thohry]

Hi
I am looking for reference for actual calculation of the average filmthickness in WCOT capillary columns used for GC. I know it can be done via the chromatography using the specific retention volume, the column ID, and phase density, but I need a reference for the formula.
Cannot find anything related on the web. Anybody out there that has this? Thanks!
jaap de Zeeuw

You can do that with isothermal condition using the equation:
k = K/beta
where
k is the retention factor
K is distribution coefficient,
beta is the phase ratio which is = (gas phase volume)/(stationary phase volume).

[jdezeeuw]

Thank you, this is possible. The challenge is that the K is depends on the liquid phase characteristics which vary a lot. Also there are very few reliable literature values for K. Problem is that the GC phases are not 100% defined. OV-101 has different K then OV-1 although both are 100% polysiloxanes. If you look at bonded phases the story becomes more complex. If we include polar phases it becomes even worse.

I am looking for the formula that has the specific retention volumes.

regards
jaap

[Peter Apps]

Hi Jaap

I suspect that this is buried deep in the packed column literature - the phase volume in the column has the same effect on retention no matter the column geometry - probably in one of the early books.

Happy new year

Peter

[jdezeeuw]

yes, probably: I did a little math and the equation I came up with is this:

df = 0.5 x (r x k) / Vg x SD

where:
df = film thickness in um;
r = column radius in um;
k = retention factor reference component at test temp.;
Vg = specific retention volume for reference component at test temp. in cm3/g on the phase used
SD = specific density of phase in g/cm3;

so it's pretty easy. Now we only need a list of Vg values. I do have a few numbers we used and it fits pretty good

jaap