Help with clarification on gases in GC 5890

[firkinslab]

We have a HP 5890 with an FID and a capillary column (100m x 0.25 mm ID x 0.2 um film thickness).

We want to check if we understand it correctly based on what we have read so far:

1. Air and hydrogen gas do not go through the column, but go to detector to ignite the FID.
Helium, as the carrier gas, is the only gas going through the column.
Is that correct?

2. Is yes, when we have helium gas on, and air and hydrogen off, we should be able to measure the helium flow at the detector exhaust vent. But we can not measure any. Does it mean there might be leak in the GC system?

3. We are trying to measure the carrier flow rate using retention time of methane. If we have all gas flows correctly set according to the manual, why is there hexane detected but not methane? What could be the potential cause?

We have manuals and old notes from former students. It is very confusing because the notes from different sources are not consistent, either do the manuals.

Thank you!

[Consumer Products Guy]

We have a HP 5890 with an FID and a capillary column (100m x0.25 um).

We want to check if we understand it correctly based on what we have read so far:

1. Air and hydrogen gas do not go through the column, but go to detector to ignite the FID.
Helium, as the carrier gas, is the only gas going through the column.
Is that correct? YES. HELIUM OR NITROGEN IS TYPICALLY USED AS FID MAKE-UP GAS AT ABOUT 30-40 ml/min

2. Is yes, when we have helium gas on, and air and hydrogen off, we should be able to measure the helium flow at the detector exhaust vent. But we can not measure any. Does it mean there might be leak in the GC system? YES, IF YOU HAVE THE RIGHT ADAPTER TO YOUR FLOW METER

3. We are trying to measure the carrier flow rate using retention time of methane. If we have all gas flows correctly set according to the manual, why is there hexane detected but not methane? What could be the potential cause? MAYBE METHANE BEING A GAS IS NOT STILL IN YOUR SYRINGE. HEXANE WON"T BE RETAINED MUCH

We have manuals and old notes from former students. It is very confusing because the notes from different sources are not consistent, either do the manuals.

Thank you!

[Peter Apps]

We have a HP 5890 with an FID and a capillary column (100m x0.25 um).

We want to check if we understand it correctly based on what we have read so far:

1. Air and hydrogen gas do not go through the column, but go to detector to ignite the FID.
Helium, as the carrier gas, is the only gas going through the column.
Is that correct? Yes

2. Is yes, when we have helium gas on, and air and hydrogen off, we should be able to measure the helium flow at the detector exhaust vent. But we can not measure any. Does it mean there might be leak in the GC system? Possibly, but the 1 or 2 ml/min flow through the column can be difficult to measure with ordinary flow meters. To check that you have any flow at all remove the column from the detectoo and dip the end into clean solvent - if you see bubbles you have flow.

3. We are trying to measure the carrier flow rate using retention time of methane. If we have all gas flows correctly set according to the manual, why is there hexane detected but not methane? What could be the potential cause? How do you know that it is hexane ? If you are basing the identification on retention time, and given that you do not know the actual flow through the column then it is very likely that methane is eluting at the retention time that you expect for hexane

We have manuals and old notes from former students. It is very confusing because the notes from different sources are not consistent, either do the manuals. Follow the manuals rather than the notes

Thank you!

[daniel_aut]

if you shutdown the fuel and the oxidant the flame is out and I believe that the carrier gas will be automatically cutoff too.

[rb6banjo]

5890's have manual pressure regulators. The flow to the inlet (and column) will be on as long as there is pressure in the source tank.

No apparent flow at the detector? Peter's suggestion is the one to try (bubble into water). No flow but positive inlet pressure = column is plugged or broken. If you get no bubbles, be sure to crank up the inlet pressure a fair amount to make sure that it's no flow.

What's the diameter of your column? This is important to know as you try to set your flows. It looks to me like you've listed the length and the film thickness.

Do you really need 100 m? Those really long columns only make your runs longer. You don't get much improvement in separation. It's not been a worthwhile trade for me over the years.

[KM-USA]

We have a 5890 with EPC just for the inlet and a 5890 with manual pressure regulation.

if you shutdown the fuel and the oxidant the flame is out and I believe that the carrier gas will be automatically cutoff too.

Not with manual FID valves on 5890. So that means for either of our units here.

[firkinslab]

We have a HP 5890 with an FID and a capillary column (100m x0.25 um).

We want to check if we understand it correctly based on what we have read so far:

1. Air and hydrogen gas do not go through the column, but go to detector to ignite the FID.
Helium, as the carrier gas, is the only gas going through the column.
Is that correct? YES. HELIUM OR NITROGEN IS TYPICALLY USED AS FID MAKE-UP GAS AT ABOUT 30-40 ml/min

2. Is yes, when we have helium gas on, and air and hydrogen off, we should be able to measure the helium flow at the detector exhaust vent. But we can not measure any. Does it mean there might be leak in the GC system? YES, IF YOU HAVE THE RIGHT ADAPTER TO YOUR FLOW METER Yes, we do have a right adapter to the flow meter for the detector exhaust vent.

3. We are trying to measure the carrier flow rate using retention time of methane. If we have all gas flows correctly set according to the manual, why is there hexane detected but not methane? What could be the potential cause? MAYBE METHANE BEING A GAS IS NOT STILL IN YOUR SYRINGE. HEXANE WON"T BE RETAINED MUCH
We set the machine on manual mode and by hand injected methane about 8 ul. For the recent try later yesterday, methane peak did not come out consistently. But we did see sometimes methane peak show up.
It came out at about 16 min.
I then increased column head pressure from 20 psi to 25 psi. Then methane came out at about 14 min.
I increased the column head pressure to 30 psi, which is the maximum, methane peak showed up at 12 min.

We have our helium tank at 50 psi, which is the maximum. The split vent flow was 100 ml/min and the purge vent flow was 1 ml/min to reach a split ratio of 100:1, based on the manual. What could we do to shorten the retention time of methane, to reach the target of 8.4 min for a linear velocity of 20 cm/sec, as described in manual?

We have manuals and old notes from former students. It is very confusing because the notes from different sources are not consistent, either do the manuals.

Thank you!

[firkinslab]

We have a HP 5890 with an FID and a capillary column (100m x0.25 um).

We want to check if we understand it correctly based on what we have read so far:

1. Air and hydrogen gas do not go through the column, but go to detector to ignite the FID.
Helium, as the carrier gas, is the only gas going through the column.
Is that correct? Yes

2. Is yes, when we have helium gas on, and air and hydrogen off, we should be able to measure the helium flow at the detector exhaust vent. But we can not measure any. Does it mean there might be leak in the GC system? Possibly, but the 1 or 2 ml/min flow through the column can be difficult to measure with ordinary flow meters. To check that you have any flow at all remove the column from the detectoo and dip the end into clean solvent - if you see bubbles you have flow. We injected hexane for several times, just to check. And hexane peak always came out. Does it mean the helium is flowing through the column?

3. We are trying to measure the carrier flow rate using retention time of methane. If we have all gas flows correctly set according to the manual, why is there hexane detected but not methane? What could be the potential cause? How do you know that it is hexane ? If you are basing the identification on retention time, and given that you do not know the actual flow through the column then it is very likely that methane is eluting at the retention time that you expect for hexane Sorry I didn't make it clear. So we injected hexane for several times, which is after we failed to see peak of methane when only injecting methane. And hexane peak always showed up. After I posted this topic, we found methane showed up though, but at undesired retention time. Please see the my reply to Consumer Products Guy.

We have manuals and old notes from former students. It is very confusing because the notes from different sources are not consistent, either do the manuals. Follow the manuals rather than the notes

Thank you!

[firkinslab]

5890's have manual pressure regulators. The flow to the inlet (and column) will be on as long as there is pressure in the source tank.

No apparent flow at the detector? Peter's suggestion is the one to try (bubble into water). No flow but positive inlet pressure = column is plugged or broken. If you get no bubbles, be sure to crank up the inlet pressure a fair amount to make sure that it's no flow.

What's the diameter of your column? This is important to know as you try to set your flows. It looks to me like you've listed the length and the film thickness. The diameter is 0.25 mm and the thickness is 0.2 um. I will change them in my first post.

Do you really need 100 m? Those really long columns only make your runs longer. You don't get much improvement in separation. It's not been a worthwhile trade for me over the years.

We have cut column for several times. And there will be about 95 m left. The time we run for each sample's long chain fatty acid is about 40 min

[rb6banjo]

The head pressure you need for a 100 m x 0.25 mm x 0.25 µm column to get optimum flow (~25 cm/s) for helium is huge. Bigger than the gauge can supply on my 5890A. At 40 °C, to set the flow at 25 cm/s for these dimensions, you need about 42 psig of head pressure. The dead volume for methane is about 6.7 minutes under these conditions.

[firkinslab]

The head pressure you need for a 100 m x 0.25 mm x 0.25 µm column to get optimum flow (~25 cm/s) for helium is huge. Bigger than the gauge can supply on my 5890A. At 40 °C, to set the flow at 25 cm/s for these dimensions, you need about 42 psig of head pressure. The dead volume for methane is about 6.7 minutes under these conditions.

I've seen some notes about the high head pressure for our capillary column. But how can we increase the head pressure to increase the flow of helium since it's already at the maximum, and methane only came out at 12 min? On notes, It says: "To increase flow of carrier gas, we need to increase the head pressure...

[Peter Apps]

The head pressure you need for a 100 m x 0.25 mm x 0.25 µm column to get optimum flow (~25 cm/s) for helium is huge. Bigger than the gauge can supply on my 5890A. At 40 °C, to set the flow at 25 cm/s for these dimensions, you need about 42 psig of head pressure. The dead volume for methane is about 6.7 minutes under these conditions.

I've seen some notes about the high head pressure for our capillary column. But how can we increase the head pressure to increase the flow of helium since it's already at the maximum, and methane only came out at 12 min? On notes, It says: "To increase flow of carrier gas, we need to increase the head pressure...

The only way to increase flow is to increase pressure - that is just the physics of gas flow. If you really need to use such a long narrow column you need to change the 2-stage regulator on your cylinder to one that can deliver at least 50 - 60 psi, and possibly change the carrier gas pressure regulator in the GC - I vaguely recall that these regulators were available as an upgrade at one stage.

Peter

[James_Ball]

We have our helium tank at 50 psi, which is the maximum. The split vent flow was 100 ml/min and the purge vent flow was 1 ml/min to reach a split ratio of 100:1, based on the manual. What could we do to shorten the retention time of methane, to reach the target of 8.4 min for a linear velocity of 20 cm/sec, as described in manual?[/color]



Thank you!

The split ratio is not calculated by split vent flow versus purge vent flow, it is split vent flow versus column flow.

At your column length and 40C temp a head pressure is giving you about 0.86 ml/min so you have 100:0.86 split ratio or 116:1, which is close to the 100:1 you are trying to get. Your average linear velocity will only be 15.4 cm/sec so you may not get very good peak shapes, and the methane peak will be late eluting. The flow calculator I have from Agilent says the holdup time is going to be around 10 minutes at those flows, so methane will not elute before 10 minutes unless you increase your head pressure. You will need about 43psi to hit 2ml/min which gives a holdup time of 6.49 minutes.

If you need a column pressure regulator check with Restek, I believe they sell replacement and probably have one rated to go to 50psi, it should be a direct replacement, I have also gotten a gauge from them before that is the same size and fitting as what is in a 5890.

[osp001]

Another option would be to run the hydrogen as carrier gas instead of helium. There are obvious safety risks, but it offers a higher linear velocity than any other carrier gas.

[Peter Apps]

Another option would be to run the hydrogen as carrier gas instead of helium. There are obvious safety risks, but it offers a higher linear velocity than any other carrier gas.

But to get the optimum velocity for hydrogen you need the same inlet pressure as to get the optimum flow for helium.

Peter