How to do >1ml gas injection

[GCleo]

We have a 6890 equipped w/ FID and capillary column. Used to do liquid injection. Now need to inject gas samples greater than 1ml. Do we need to replace the liner in order to do the large volume injection?

[AICMM]

GCleo,

Do yourself a big favor and buy a manual gas injection valve, 6-port X 1/16 inch fittings. Install this upstream of your current injection port and use the carrier gas line from the EPC to provide gas into the valve. Valco has a drawing of how you would plumb this up but if you still don't understand then re-post and a number of people can help you out.

I have found that trying manual injections of larger volume results in pressure pulsing which leads to poor reproducibility. Valve is not that expensive and much more reproducible.

Best regards.

[Peter Apps]

The six-port injector valve is most likely the best option, but you may be able to manage without it.

What liner do you have in at the moment ?, why do you need to inject such large volumes ?, are you going to be doing split injections ?, if so what split ratio do you plant to use ? What is the diameter of the column ?

Peter

[GCleo]

Thanks, AICMM and Peter. I've got a 6-port valve and 10cc sampling loop and setup a continuous sampling method. However, I keep getting a background peak which I consulted an Agilent fellow who advised it's arisen from flow upset due to the large volume and the pressure upset during injection, which I agree with. Have you fellows seen something similar?

I've got another issue here. I injected some isopropanol samples last week with the valve, however for some reason, thereafter the GC keeps generating a ghost peak that appears in tandem with the flow upset peak; in other words, now we get two background peaks, which is really annoying. I tried taking off the sampling loop and flushing it with compressed air and acetone, the area of the peak has decreased from hundreds to about 20 and then hangs there no matter how many times I flushed it, it really haunts like a ghost. I am running out of idea how to get rid of it . I am very sure it's not from the injection source neither from residues in column. The peak is very consistant, same retention time same height. -- Can someone suggest how to get rid of it? Thanks in advance.

[mbicking]

I'm not the best GC guy around, but having tried some similar experiments with 1 mL loops, I'm not sure you will be able to eliminate this. Remember that the GC is operating under a certain headpressure, required to maintain the desired flow (in constand flow mode). When you switch the valve, you now have 10 cc of gas at atmospheric pressure that need to be pressurized up to the column headpressure. This takes a finite amount of time, and as a result the flow through the column is going to be momentarily disrupted. How this affects the baseline will depend on how flow sensitive your detector is.

Now, the problem of ghost peaks may be a different problem. If you have less volatile materials in your gas sample, and the loop is at a lower temperature than the gas stream, you could see some condensation inside the loop. Can you find a way to heat the loop between runs?

[GasMan]

Did you inject liquid isopropanol with the valve? If yes, it is quite likely that this has been absorbed by the rotor, and you are seeing the isopropanol coming out of the rotor. I have had customers complain about liquid sample valves showing a ghost peak for isopropanol, and it turns out that Valco test their liquid sample valves with this alcohol. The only way would be to replace the rotor.

Would you please post some answers to the previous postings, such as the split ratio you are using. There is no point in taking a 10ml sample loop and then using a large split ratio

Gasman

[GCleo]

Did you inject liquid isopropanol with the valve? If yes, it is quite likely that this has been absorbed by the rotor, and you are seeing the isopropanol coming out of the rotor. I have had customers complain about liquid sample valves showing a ghost peak for isopropanol, and it turns out that Valco test their liquid sample valves with this alcohol. The only way would be to replace the rotor.

Would you please post some answers to the previous postings, such as the split ratio you are using. There is no point in taking a 10ml sample loop and then using a large split ratio

Gasman

Thanks, Gasman. Do you mean the rotor of the valve is contaminated? how much is the rotor? I don't recall I did liquid injection with valve. It was gas sample. FYI, I use a 10:1 split, 0.2min load, 1min injection, 6ml/min flow in column, which really gobbles up a lot helium. I know it's really rare to take a 10ml loop but I tried it works better for us than 1ml. BTW, do you work for Valco? Is there any difference between Valco's valve and Agilent's? I noticed that Valco's 6-port valve is more expensive than Agilent's.

[GCleo]

I'm not the best GC guy around, but having tried some similar experiments with 1 mL loops, I'm not sure you will be able to eliminate this. Remember that the GC is operating under a certain headpressure, required to maintain the desired flow (in constand flow mode). When you switch the valve, you now have 10 cc of gas at atmospheric pressure that need to be pressurized up to the column headpressure. This takes a finite amount of time, and as a result the flow through the column is going to be momentarily disrupted. How this affects the baseline will depend on how flow sensitive your detector is.

Now, the problem of ghost peaks may be a different problem. If you have less volatile materials in your gas sample, and the loop is at a lower temperature than the gas stream, you could see some condensation inside the loop. Can you find a way to heat the loop between runs?

Merlin, sorry didn't see your reply in the first place. Yes, I absolutely agree what you said. In fact, when I asked the question I didn't expect a yes but some smart interpretation, and I got yours. Regarding the ghost peaks, I think GasMan may have given the right answer as I forgot to mention that we have a heating box that encloses the valve and is 200 degree C, so the scenario you mentioned may not exist here. But I don't understand why the rotor would absorb isopropanol especially at such high temperature. Isn't isopropanol a common chemical? the rotor material should've been made of something with low absorption efficiency.

[GCleo]

Did you inject liquid isopropanol with the valve? If yes, it is quite likely that this has been absorbed by the rotor, and you are seeing the isopropanol coming out of the rotor. I have had customers complain about liquid sample valves showing a ghost peak for isopropanol, and it turns out that Valco test their liquid sample valves with this alcohol. The only way would be to replace the rotor.

Would you please post some answers to the previous postings, such as the split ratio you are using. There is no point in taking a 10ml sample loop and then using a large split ratio

Gasman

Gasman, forgot to ask, can I take out the rotor and WASHED it with acetone? is isopropanol gonna be displaced by acetone? I assume the rotor is made of some metal, there should be a way to clean it up rather than replacing it. How do you think?

[GasMan]

If you injected iso propanol in the gas phase, you probably have not contaminated the rotor. You could still have some stuff in the loop. I see more problems with the large loops due to the pressurization problem, and would prefer to see a 1 ml loop with a 1:1 split, the same amount of sample will go into the column and you will use less helium.

Agilent uses Valco valves, and I do not work for Valco.

Gasman

[GCleo]

If you injected iso propanol in the gas phase, you probably have not contaminated the rotor. You could still have some stuff in the loop. I see more problems with the large loops due to the pressurization problem, and would prefer to see a 1 ml loop with a 1:1 split, the same amount of sample will go into the column and you will use less helium.

Agilent uses Valco valves, and I do not work for Valco.

Gasman

Gasman, I just dont' understand isopropanol is volatile and as I mentioned above, we have a heating box covering the valve and loop and the temperature is 200C. I also took off the sampling loop and flushed with compressed air for quite a while. I even tried passing acetone through the loop but the ghost peak is still there refusing to go. It's really annoying. We'll have some visitors tomorrow, I don't want them to see two background peaks. One is definitely enough, just like a wife

[GCleo]

Now I'm sure it's not the rotor. I replaced 10ml loop with 1ml and the ghost peak is gone. It's definitely the loop. It could be a lesson for anyone who's using valve and sampling loop: DON'T MESS with isopropanol!

[Peter Apps]

To avoid pressure / flow disruptions put a back pressure regulator downstream of the loop when the valve is in the load position, and set it to match the head pressure on the column. The sample loop will then fill with pressurized gas before the regulator starts bleeding gas, and with the pressure in the loop being the same as that in the column, there will be no flow pulse. This assumes that you have sufficient sample volume to make the back pressure regulator work.

Peter

[AICMM]

GCLeo,

By changing the loop you have reduced the pressure drop on the switch to inject. I would agree with Gasman that the better way is a 1mL loop with a 1:1 or 1:2 split. As an alternative, if you have an aux pressure controller available you can try a high volume injection using the Aux EPC. Rotors are all polymeric, by the way, with the material varying from PTFE to PEEK so you should try to match the rotor to the analytes of interest (and the pressure and the temperature....) Valco makes Agilent's valve bodies but Agilent has their own actuators in order to fit under the hood of the GC.

Now for the interesting question, regarding Peter's note on back pressure regulators. I have heard from good sources that you will have to matrix match standard to sample if you use back pressure regulation. I have not tried this myself so I am interested in comments. I have tried back pressure regulation on the sample line and you do improve the detection limit but you also smear out the peak. Other comments?

Best regards.

[GCleo]

To avoid pressure / flow disruptions put a back pressure regulator downstream of the loop when the valve is in the load position, and set it to match the head pressure on the column. The sample loop will then fill with pressurized gas before the regulator starts bleeding gas, and with the pressure in the loop being the same as that in the column, there will be no flow pulse. This assumes that you have sufficient sample volume to make the back pressure regulator work.

Peter

Peter, thanks for your suggestion. I had thought about back pressure regulation but didn't do further study or search the product. A quick question: if the pressure builds up in the loop in loading position attributed to the back pressure regulator, does this mean you need higher pressure to load up sample?