Chromatography Forum

Hello,

I use an HP 5890 series II as online-GC. I opened and cleaned the multi-port- valve, injector and detector.
After cleaning, the sig.1 shows the value from 15360 to 28160 when the temperature increase from 40 ° to 240°C. This value depends linearly on the temperature, it is stable when the temperature is constant.
I set the baseline at 15360 and increased the temperature, the sig.1 increase and when it reach to the value of about 17000 the signal in screen (chromatogram) reach to 1 V (the upper limit) and was cut.
Normally, after cleaning some dust can come in to the column and the sigal is high but it is not so high as this time and it goes down gradually. Another thing: when I opened the detector to clean I saw a lot of water in it.
I think about the leak, but I saw the buble at the end of the column. Today I want to check the detector again but it can only be cooled down it to 160°C while yesterday it could go down to 60°C

Please show me the reason and the solution.

Thanks a lot

BuiThuy

BuiThuy,
Please tell the forum what GC detector you are using.
Please also confirm that you say the detector is full of water(?)
Have you positively identified it as water?
Am I right in saying that you have an in-line GC for process monitoring? If so what is the product that is being diverted into the GC by the valve?
WK

Dear WK,

Thank you for your answer

I am using FID detector. My product is a complex mixture of hydrocarbon from C4 to C9+ (C4 is about 80%), that is why I need on-line GC. Last time I opened to much the output valve of the reactor, so that a lot of sample came on to the sample loop (in the liquid form while it should be in gas form) and then on to the column. Therefore I had to clean whole the system.
When I opened the detector I saw a colorless liquid (it is difficult to say a lot or less, it is about some small drops). I did not check if it is water, I just simply thought that because water is one of the products of oxidation of HCs which occures in the FID detector. But I can check it.
I do not have an in-line GC for process monitoring. I have to start, stop, and set up the temperature and other parameters by hand from the keyboard of GC.

Hope this information can make the problem more clearly

BuiThuy

Dear WK,

Thank you for your answer

I am using FID detector. My product is a complex mixture of hydrocarbon from C4 to C9+ (C4 is about 80%), that is why I need on-line GC. Last time I opened to much the output valve of the reactor, so that a lot of sample came on to the sample loop (in the liquid form while it should be in gas form) and then on to the column. Therefore I had to clean whole the system.
When I opened the detector I saw a colorless liquid (it is difficult to say a lot or less, it is about some small drops). I did not check if it is water, I just simply thought that because water is one of the products of oxidation of HCs which occures in the FID detector. But I can check it.
I do not have an in-line GC for process monitoring. I have to start, stop, and set up the temperature and other parameters by hand from the keyboard of GC.

Hope this information can make the problem more clearly

BuiThuy

I have seen water form on FIDs before, but I can't exactly remember the circumstances.
You say it can not cool down lower than 160, is the oven still on?
Turn off the detector temperature and set oven to 20, that will keep the fan blowing and will cool down faster.
You say that dust got into the column during cleaning, are you not removing the column while you perform maintenance?
I suggest cutting the column and re-inserting
As for leaks, you can still see some flow from the end of your column while having enormous leaks. Check all fittings and connections with a leak detector.
Try to flush out your loop also.
After all maintenance is complete, and everything is connected, set your temperatures to the upper limit of your temp program, ignite the FID, and bake the system out for several hours, while moitoring the signal output.
Then get back to us with how you are doing.
Hope this helps.

Now I can decrease the temperature of the detector, last time I forgot to cool down the column so the heat was transfered from column to detector. I am sorry

1. I have done these things:
- cleaned the multi port valve
- replaced new injector
- cut the column at both sides (about 10cm for each side), the column is 50m length
- cleaned the dectector but not opened completly
- disconnected the multi port valve, so now it is not an online GC but a nomal GC
- heated the the column at 230 (upper limit), injector at 200 and detector at 300 (like normal) for several days.
Result: the signal still not drop, it depends linearly on the column temperature, and is stable when the temperature is constant

2. I have thought about these things:
- detector can only be suitable for carbon containing compounds, it can not dectect water, H2, air, N2, He, Therefore if there is a leak somewhere, air can come in to the system. It does not increase the signal. Is it correct?
- If the dust is coming from the detector, the signal will be not depended on the column temperature. Because the detector temperature is constant.
Is it right?
- if the dust is coming from the column, would it be reduced when have been heating for several days?
- If the signal is stil high (several hundred mV, normally, it is 10mV)but stable with column temperature. Should I set the baseline at this value? Does it affect on the results?

I hope to receive more advices.
Thanks

If the rise in signal is really linear with column temperature (in other words you get a straight line rise on the chromatogram) then we can rule out column bleed.

Gas flow through the column changes with temperature, if you are using a megabore column the flow chanegs can be large enough to affect the flame. What are your column diameter and gas flow rates, what gas are you using as carrier gas ?

What data system do you have ?, are you sure that you are monitoring the detector signal and not the oven temperature ? If you inject some gas do you see any peaks ?

Peter

Water in a FID especially a 5890 is caused when the detector heater is switched off or the instrument is switched off with the detector gasses on and hence the flame still on. I have seen this many times with other peoples instruments (never my own of course lol). Usually the signal sits at 830000 as a short develops across the detector. Rust can appear if this has been left for a long time and this can mess up the ptfe insulators.

GCguy

Hi,
It seems to me that your gas line is contaminated, especially the air. The last time the gas trap being replaced was...?
Good luck,

It might be worthwhile determining if it is a detector or column problem. Try using a blanking nut on the detector inlet. This will allow you to light the flame and play about with temperatures. Look for changes as you play with the conditions.

I have seen a similar set of conditions occur when the column was inserted right to the top of the jet rather than being 1~2 mm down from it.

GCguy