Chromatography Forum

Hello,

I'm not very experienced in GC. My normal field of work is HPLC.

So I would like to ask if anyone could tell me, how to scale a GC method for another (capillary)-column?
I'm especially interessted about the temp-gradient slope scaling if length is changed.

Is it the same way as in HPLC (scaling according to column volume)? This would mean: double the length -> halve the slope?

Does the film thickness also have some scaling-influences to think on or "just" loadability?

Thank you.

Double the length - halve the slope is as good a rule as any other. For film thickness - usually you would use thinner films for heavy compounds that elute at higher tempratures, and thick films for more volatile compounds.

Peter

The type of the column stationary phase should be considered also, when trying to transfer method from one to another column

check out Agilent website for method translation software.

http://www.chem.agilent.com/cag/servsup ... /main.html

generally speaking, when you translate a method, it's assumed that same stationary phase chemistry is used(just like in HPLC). Usually, a smaller id column is used to replace a bigger id column. A few things to keep in mind for column transfer:

1) maintain phase ratio (beta): If you have a 30 m x 0.53 mm X 3.0 um column, and you want to change to 30m x 0.45 mm column, film thickness needs to be 2.55 um to maintain the phase ratio of 44.

2) smaller id columns usually have better column effiency, which means shorter column length for smaller id column to achieve same theoretical plate number.

3) smaller id column can have faster carrier gas linear velocity without losing efficiency. It translates to shorter analysis time.

4) oven temperature program (analysis time) is shorter for smaller id columns.

Thanks for your replies

JI2002,
about the phase ratio:
what's its influence to the chromatography? any rule of thumb if I change the ratio?
Is there a comparable HPLC term (for linking the theory)?

K=k*phase ratio

By maintaining phase ratio, theoretically k is kept the same. This applies to both GC and HPLC. In GC, phase ratio is determined by two parameters: column id and film thickness.

Beta = r/2df

in which r is column radius and df is film thickness.

I think it's a little complicated in HPLC.

The mobile phase volume in HPLC column can be calculated as

Vm = pi * (d/2)^2 * L * porosity d is column id, L is column length and porosity is the total porosity(both internal and external).

The amount of stationary in umol can be calculated using the following formula:

Stationaty phase in umol = A * B * pi * (d/2)^2 * L * (1- porosity) * D

A is specific surface area of the stationary in m^2/g
B is the surface coverage in umol/m^2
D is the skeleton density of the stationary phase in g/cm^3

The ratio of the two (the two are not in same unit) will be:

Vm/stationary phase in umol = porosity/(A * B * (1- porosity) * D)

So phase ratio in HPLC is determined by total porosity, the stationary phase specific surface area, surface coverage and its skeleton density.