5890 FID - Measuring Column Flow

[mlumanog]

I am working with a 5890 II GC with FID and I am trying to adjust the split ratio, however I am having some issues obtaining the column flow out of the FID. I am using a 1-10-100ml bubble flow meter with a FID flow measuring adapter with an O-ring on the end that plugs straight inside the detector.

I have the H2, air, and He turned off with the black knobs on the front panel of the GC. When I plug in the the adapter into the detector, I don't see any movement in my flowmeter. My target is 2-3ml/min. I know it won't be much but I didn't see any movement of the meniscus even from 0-1 ml after some time.

I opened the aux gas and attached the flowmeter to the front of the GC /at the split vent and found it reading 70 ml/min. I lowered this to 30 ml/min by turning the total flow down. I also attached the flowmeter to the septum purge vent on the front and lowered from 7.4ml/min to 2.8ml/min. The gas pressures at the gas cylinders are 60 for Air and He, 40 for H2. The pressure controls inside the GC read 45 for air and He, and 15 for H2.

Any help is appreciated. Thanks.

[chromatographer1]

Most chromatographers do not use a flow meter to measure column flow on capillary columns using FID.

A methane injection is performed and the retention time tells you the linear flow rate through the column and thus the volumetric flow. For 30 meter columns of 0.25mm ID a retention time of 150 seconds will give you a flow rate of 20cm per sec. This equals about 0.6mL per min. (If my memory is correct)

The optimum flow rate for He is about 21cm per sec. Try to stay below 30cm per sec and above 20cm per sec. For hydrogen carrier twice this value is commonly used.

best wishes,

Rod

[Bigbear]

You also should be able to measure column flow the way you did it the first time though I would suggest using a 10 ml or so bubble meter.
The second adjustment you did sounds like make up gas adjustment.

[mlumanog]

Thanks for the tip. Ill look into getting a 10 ml flow meter.

My split ratio needs to be 50:1

[skunked_once]

My split ratio needs to be 50:1

Before you can set your split ratio, you need to know your column flow. Once your determine that, multiply the column flow by 50 to obtain the 50:1 split flow ratio. You adjust this by the "total flow" control knob and measure at the "split vent" outlet. Happy tweaking

[AICMM]

mlumanog,

Several comments. First, try removing your column from the detector and see if you get bubbles in a vial of methanol or isopropanol. You may not have any flow. If you do have flow, you can try squeezing the tubing from the flow meter around the end of the column to measure flow. Second, you don't say what your column head pressure is on the front gauge. That would be good to know. Third, you should tell us what column you are using since the id of the column will dictate the desired flow rate to get the optimum linear velocity (and the proper pressure regime.) Fourth, if you set your column flow to 2-3 mL/min (not bad for a 0.32) then you will need 100 - 150 out the split vent to make your desired 50:1. That's a lot helium.....

The way the 5890 works, you set the total flow high so that you can get a desired back pressure on the gauge, measure the column flow and adjust with column head pressure knob, then when you are happy with that you wind down the total flow to get the desired split flow. Use the two set flows to get ratio at this point.

Now the puzzler. You opened the aux gas and found the split vent at 70 ml/min. They should have nothing to do with each other so if you close the aux gas you should still have flow out the split vent. Is that the case?

Best regards,

AICMM

[Consumer Products Guy]

The others are correct about typicaly using the inlet pressure and the column dimensions to calculate the capillary column flow (Agilent software).

We've also turned off the hydrogen, air, and make up gas and used that Agilent adapter to measure capillary flow. Another way i've done this is to stick the outlet end of the capillary column through one of those cylindrical septa, break off 1/2 inch (in case inserting it plugged the end), then inserting the septum into the tube for the measuring device (bubble meter or electronic).

[mlumanog]

mlumanog,

Several comments. First, try removing your column from the detector and see if you get bubbles in a vial of methanol or isopropanol. You may not have any flow. If you do have flow, you can try squeezing the tubing from the flow meter around the end of the column to measure flow. Second, you don't say what your column head pressure is on the front gauge. That would be good to know. Third, you should tell us what column you are using since the id of the column will dictate the desired flow rate to get the optimum linear velocity (and the proper pressure regime.) Fourth, if you set your column flow to 2-3 mL/min (not bad for a 0.32) then you will need 100 - 150 out the split vent to make your desired 50:1. That's a lot helium.....

The way the 5890 works, you set the total flow high so that you can get a desired back pressure on the gauge, measure the column flow and adjust with column head pressure knob, then when you are happy with that you wind down the total flow to get the desired split flow. Use the two set flows to get ratio at this point.

Now the puzzler. You opened the aux gas and found the split vent at 70 ml/min. They should have nothing to do with each other so if you close the aux gas you should still have flow out the split vent. Is that the case?

Best regards,

AICMM

Sorry. My column head pressure is at 12. The column is a restek capillary column, 30m, 0.32 mb.

When I closed the aux valve and waited for a while, there was no flow out of the split inlet vent. At least none I could tell with my bubble meter.

Thanks for the tips. I'll try checking the detector end with isopropanol to check for any flow at all.

[mlumanog]

The column head pressure at 12 - raises up to 12 when I open the aux valve on the front and a secondary regulator for the aux gas mounted on the top of the GC

[Yama001]

This looks like the GC may have been replumbed (pretty common with these pre GPC 5890s). It sounds like the main flow controller has been replaced by branching off of the detector auxilary flow controller.