Clarification on OPGV

[Roxanne42]

So I'm definitely a little confused about OPGV.

30-m column, 0.32 mm ID and 0.25 um film, carrier gas is helium

Various sources give different values for the OPGV of helium - some say 25 cm/s, some 30 cm/s and some 35 cm/s. According to the van Deemter plot, the optimal value would be of 27-28 cm/s for helium (although I think the OPGV is usually defined as being 1.5-2 times that value for ultimate compromise between resolution/speed)

In the Agilent manual, it is suggested to use a flow of 2.6 mL/min for a 0.32 mm column (this would lead to a head pressure of 11 psi and an average flow rate of 40 cm/s - NTP). In the same manual, they also suggest a pressure of 7 psi (which would lead to a column flow of 1.5 mL/min and an average flow rate of 25 cm/s!).

Now I'm aware that the OPGV value varies with temperature, along with several other factors/variables and I'm also aware that the relationship is not linear and that slight deviations from the OPGV should not affect the outcome of the chromatography by much). What I am looking for though is just a general good starting point to set in the default method I wish students to use to develop their own methods. What would be a good starting value for the average flow rate/column flow/head pressure...?


Thanks,

Roxanne.

[lmb]

Dear Roxanne,

I would like to start with some shocking news. OPGV (optimal practical gas velocity) of helium at 100 degree Celsius in 10-m long, 0.1-mm id (internal diameter) column is close to 90 cm/sec. For 1-m long column with the same id (these columns are popular in a certain class of real applications), OPGV is about 210 cm/sec.

I know that those are not the columns you are asking about. However, the examples provide a reality check for other widely known recommendations.

OPGV was proposed by Scott and Hazeldean in 1960 [1] as “average linear gas velocityâ€

[Roxanne42]

Hi there,

Thank you so much for the reply. It does clarify things.

I also searched 6890 manual for the recommendation to use 7 psi pressure for your column. The only such recommendation that I found was for column conditioning. This is different from using a column for analyses of real samples. Please let me know if I miss other 7 psi entries.


It's in the 6850 Series Control Module User Information manual actually (p. 40), just below the recommended starting flows/pressures.

Again, thanks!

Roxanne.