A question on open tublular column

[Sandra7]

How can H be increased through changing column dimensions (Diameter, length, and film thickness)

Here is my note on H:
H = L/N,
L= length of column
Plate Number = N (originally number of theoretical plates)
(H is constant under specific conditions, while N is proportional to L)
N = (tR/σ)2 (= 16(tR/wb)2 )
N is an absolute measure of column efficiency but depends on length
Plate Height = H = length of column needed to get N of 1

I would say, increase L or decrease diameter. As for film thickness, I have to think about it. Any tip?

[krickos]

Hi

I would say you are on the right track for the first two.

Assuming this is an introduction course to GC, I would say for filmthickness, look closer on the C term in the van deemter equation you had in your other thread, you will notice it consists of 2 parts, that should give you a clue.

[Sandra7]

Hi

I would say you are on the right track for the first two.

Assuming this is an introduction course to GC, I would say for filmthickness, look closer on the C term in the van deemter equation you had in your other thread, you will notice it consists of 2 parts, that should give you a clue.

Thanks.
Yes, I see the C term described in the textbook. I am about to read the whole section (lots pf pages).

BTW, I meant to ask "How can H be decreased through changing column dimensions (diameter, length, and film thickness (for open tubular column)?

The new question I'd like to ask is whether there is any disadvantages in making these changes. May be I'll know for sure once I finish reading the whole section. Skimming through the text material, the answer would be "no" (not for these changes). Please correct me if I am wrong.

[Sandra7]

Hi

I would say you are on the right track for the first two.

Assuming this is an introduction course to GC, I would say for filmthickness, look closer on the C term in the van deemter equation you had in your other thread, you will notice it consists of 2 parts, that should give you a clue.

Thanks.
Yes, I see the C term described in the textbook. I am about to read the whole section (lots pf pages).

BTW, I meant to ask "How can H be decreased through changing column dimensions (diameter, length, and film thickness (for open tubular column)?

The new question I'd like to ask is whether there is any disadvantages in making these changes. May be I'll know for sure once I finish reading the whole section. Skimming through the text material, the answer would be "no" (not for these changes). Please correct me if I am wrong.

On a second thought, changing the length of the column might result in poor separation of analytes. Forgot to think about L earlier.

[Sandra7]

On a second thought, changing the length of the column might result in poor separation of analytes. Forgot to think about L earlier.

But, when L is changed, N is chnaged and so no change in H. So, only diameter and file thicknes change can be done. Disadvantage is on the rentention factor (capacity factor).

[thohry]

I think when changing the column dimension, you should also keep the phase ration constant, so you know how to choose the right film thickness.

[krickos]

On a second thought, changing the length of the column might result in poor separation of analytes. Forgot to think about L earlier.

But, when L is changed, N is chnaged and so no change in H. So, only diameter and file thicknes change can be done. Disadvantage is on the rentention factor (capacity factor).

Need to partly correct myself and extend a bit:

No it is not true that H is unchanged if L is for exampled doubled because the doubled L do not give you twice the amount of N. If you read up further you will find and extended resolution formula thats goes like: Rs=(squareN/4)(alpa-1/aplpha)(k´/1+k´). So doubling the L will gain you much less N then you expected.

Your initial question confused me a bit (increasing H?) you typically want to do the oposite ie higher N per L which leads to a lower H.

In GC (WCOT) increasing L alone is usually only timeconsuming and give little payback but sometimes work for your application. If combined with other changes you can gain more.

To lower H its is usually more effective (or in combination with lowering/increasing L) to improve the B and C factors in the van deemter equation. B mean optimising the gas flow (cm/s) for your carrier gas typically a few cm/s above otimum for your gas (provided GC can do constant flow).
C means speeding up equilibrums, like thinner phase (optimized flow also helps here), smaller ID.

[Sandra7]

On a second thought, changing the length of the column might result in poor separation of analytes. Forgot to think about L earlier.

But, when L is changed, N is chnaged and so no change in H. So, only diameter and file thicknes change can be done. Disadvantage is on the rentention factor (capacity factor).

Need to partly correct myself and extend a bit:

No it is not true that H is unchanged if L is for exampled doubled because the doubled L do not give you twice the amount of N. If you read up further you will find and extended resolution formula thats goes like: Rs=(squareN/4)(alpa-1/aplpha)(k´/1+k´). So doubling the L will gain you much less N then you expected.

Thanks for this explanation.

Your initial question confused me a bit (increasing H?) you typically want to do the oposite ie higher N per L which leads to a lower H.

Sorry about that.

In GC (WCOT) increasing L alone is usually only timeconsuming and give little payback but sometimes work for your application. If combined with other changes you can gain more.

Great info.

To lower H its is usually more effective (or in combination with lowering/increasing L) to improve the B and C factors in the van deemter equation. B mean optimising the gas flow (cm/s) for your carrier gas typically a few cm/s above optimum for your gas (provided GC can do constant flow).
C means speeding up equilibrium, like thinner phase (optimized flow also h[[elps here), smaller ID.

Thanks. The newest lecture covered optimization. I'd read up on that soon. Thanks a lot.