5890 II plus flow rate problem

[faris]

Hi guys. When the oven elevates the flow is declining gradually so by the time the oven reaches 300C the flow rate is 0.980 mL from 1 mL. Any help

[larkl]

Wow, I'm amazed that it's that good. 0.98 vs a set point of 1.0? Is that a typo. With a pneumatic instrument that running constant pressure, the flow will drop as the temperature increases. It'll drop a lot more than 0.02. GC's were run this way for years successfully (and many still are) in constant pressure mode. I've had GC's that had to be run in constant pressure mode, even with EPC, as we were using 100M columns and we didn't have enough head pressure to keep the flow constant throughout the temperature program.

[faris]

[quote="larkl"]Wow, I'm amazed that it's that good. 0.98 vs a set point of 1.0? Is that a typo. With a pneumatic instrument that running constant pressure, the flow will drop as the temperature increases. It'll drop a lot more than 0.02. GC's were run this way for years successfully (and many still are) in constant pressure mode. I've had GC's that had to be run in constant pressure mode, even with EPC, as we were using 100M columns and we didn't have enough head pressure to keep the flow constant throughout the temperature program.[Yes it is running it constant pressure mode]

[faris]

Wow, I'm amazed that it's that good. 0.98 vs a set point of 1.0? Is that a typo. With a pneumatic instrument that running constant pressure, the flow will drop as the temperature increases. It'll drop a lot more than 0.02. GC's were run this way for years successfully (and many still are) in constant pressure mode. I've had GC's that had to be run in constant pressure mode, even with EPC, as we were using 100M columns and we didn't have enough head pressure to keep the flow constant throughout the temperature program.[Yes it is running it constant pressure mode]

But previously I didn't have this trouble Also, the base line is elevating at the end of the run

[Peter Apps]

Hi Faris

For the flow (which cannot ne measured directly in any case) to drop by 2% during a run is not a problem - it will have NO discernable effect on the chromatography. The rise in the baseline is due to column bleed, and is also normal. How much bleed you get depends on all sorts of factors - the best way to minimise it is to use very clean carrier gas.

Peter

[lmb]

Hi Faris,

I agree with Peter. A 2% change in flow rate should have a minor effect on chromatography.

However, there are some contradictions in your data. In temperature-programmed analysis at constant pressure, the column flow should be inversely proportional to about 1.7-th power of absolute temperature. Example: if temperature increases from 100 ºC to 300 ºC (i.e. from 373 kelvin to 573 kelvin) then the flow rate at fixed pressure should drop from 1 mL/min at 100 ºC to about 0.48 mL/min 300 ºC. (More accurately, the flow is inversely proportional to 1.685 power of absolute temperature for helium, and to 1.698 power for hydrogen.) The fact that, in your case, the flow drops only by 2% suggests that there might be some problems with your flow measurements, or maybe the pressure is not constant, or something else.

Good luck with solving your problems.

[II5tickmanII]

Have you checked your regulators are set at the correct values and are not causing the flow to decrease over the temp ramp as demand on the system increases due to the greater pressure required to maintain flow?

Failing that if your using cylinders are they giving sufficient supply?