Is my pressure too high?

[AZBiodiesel]

Hey guys I am new to setting up GC's so I have a question. I am using an HP5890 with on column injection and FID, no EPC only manual control. I have a 0.53 mm guard column attached to 0.32 analytical column. The method requires 3ml/min flow thru the column. Checking with the bubble meter attached to the FID i am only getting 1.2 ml/min of carrier gas @ 70psi at the gc regulator. The regulator itself only goes up to 100psi. First of all does this sound like too high psi for the flow rate on that column? It is such a small bore so I don't know. Also could this hint to a leak somewhere in the system? I am too broke to buy a leak detector and I have heard to not use leak checking solutions? Any help would be appreciated thanks!

[Consumer Products Guy]

Hey guys I am new to setting up GC's so I have a question. I am using an HP5890 with on column injection and FID, no EPC only manual control. I have a 0.53 mm guard column attached to 0.32 analytical column. The method requires 3ml/min flow thru the column. Checking with the bubble meter attached to the FID i am only getting 1.2 ml/min of carrier gas @ 70psi at the gc regulator. The regulator itself only goes up to 100psi. First of all does this sound like too high psi for the flow rate on that column? It is such a small bore so I don't know. Also could this hint to a leak somewhere in the system? I am too broke to buy a leak detector and I have heard to not use leak checking solutions? Any help would be appreciated thanks!

I used a 5890 with manual pressure controls today, great GC. You need to download Agilent's Flow Calculator utility, several versions are available such as at http://www.chem.agilent.com/en-US/Support/Downloads/Utilities/Pages/GcPressureFlow.aspx
70 psi at the regulator is plenty. We use helium as carrier, and you didn't state column length, but with a 30 m 0.32mm i.d. column that utility tells me that 15.8 psi helium head pressure will deiver 3.00 ml carrier flow at 100C oven temperature. 0.32mm i.d. column is not that small, we've used smaller, but I used 0.32 today, and used about 14 psi head pressure myself. How exactly are you hooking up (connecting) the bubble meter to measure the capillary flow?

[Peter Apps]

70 psi (nearly 5 bar) is way too high for the flow you measure unless the column is exceptionally long.

Either the measurement of the flow is wrong, or you have a plumbing problem. Are you sure that you have the units of pressure correct ?, 70 KPa (0.7 bar) is not so far off. As a double check inject some methane or other unretained gas (e.g. lighter gas) and measure how long it takes for the peak to appear at the FID. This gives the average linear velocity of the gas through the column, multiply that by the cross sectional area and you have the volume flow. It will not be exactly the volume flow measured at the exit because the gas decompresses as it flows down the column but it is close enough for first pass trouble shooting.

A leak is possible. How are you connecting the column to the guard column ? If you have no leak detector you can use isopropanol/ water 1:1 as a bubble solution (do not use anything with soap or detergents in it). You will have to cool the inlet and detector so that it does not boil.

What carrier gas are you using ?, and how long is the column ?

Peter

[AZBiodiesel]

Hi, carrier gas is helium, guard column is 5m, analytical is 15m, so 20m overall. Im using an agilent ultimate union to join the two. So I will go check right now because it seems like that a leak in the union might be the cause. I have the bubble meter hooked up via the adapter piece that sticks in the FID. I am using the calculations given in the hp 6890 operations manual to find mL/min 1/t if measuring the meniscus from 0 to the first mark, 10/t from 0 to second mark etc.

[fsistere]

Hi
Make up of the FID is closed? maybe you aren't measuring only carrier gas

[fsistere]

Sorry
I'm said a stupid thing. I'm thinking in hollidays...

[Don_Hilton]

Excessive pressure to drive a column may mean an obstruction in the column. How have you joined the guard column to the analytical column? (I have had a column which required excessive pressure to drive it - and the problem went from one instrument to another with the column. There were no leaks and I confirmed column flow rates with an unretained analyte.)

[AZBiodiesel]

Hello to all thanks for your input, i was calculating the flow wrong. I wanted mL/min, what i was calculating was mL/s. (1/t)60 is what i should have been doing. OOPS.

Also i was using the stopwatch on my phone not the stopwatch on the gc which calculates it for you! do what the manual says lol