Decreasing TCD response over time

[karthic]

Hello,
I have a problem which has been bothering me since last week. I am using a SUPELCO PLOT Column for gas analysis (H2, O2, Co, CO2, N2, C2H2, C2H4, C2H6, CH4). I do manual injection (100ul) using Gas tight Hamilton syringes. I have been following this technique since over 3 months and had no problem. Recently, I observed that my responses on TCD went lower than normal (eg if my TCD response for CO was 44.12 for 20% Co my current response was 25.65 for teh same 20% Co). I have seen that responses for all the gas compounds has gone down.
While checking the possible options, I checked my syringe (which was fine), I used new liners etc but I my problem continues.
SO MY QUESTION IS THAT WHETHER THE TCD RESPONSES FALL OVER TIME WHEN THE INSTRUMENT IS USED FOR A LONG TIME. DOES THAT NEED ANY KIND OF MAINTENANCE? OR THERE MAY BE SOME PROBLEM WITH THE COLUMN LIKE CUTTING 10 CM FROM THE FRONT PORTION MAY SOLBE THIS ISSUE?

I would be glad if I recieve an early response from you.

Thanks
Karthikeyan

[AICMM]

karthic,

Have your retention times stayed the same? If so, the first place I would look would be the aux flow rates or the reference flow rates. If the cylinder pressure has changed and you don't have secondary pressure control you could have increased your flows and caused a decrease in sensitivity. This is especially true if responses have dropped across the board. None of the compounds you are shooting should have a strong effect on filament except, maybe, excess oxygen. This can be mitigated somewhat by a lower filament current but you already appear to have poor sensitivity to % levels of CO. If possible, can you post a chromatogram?

By the way, what carrier gas are you using and at what flow.

Best regards.

[karthic]

My retention time is the same for all the peaks. On the pressures, we do have a secondary pressure regulator and looks like we have no leaks as I have used the leak detector for checking that. I will try uploading my chromatogram in the next few hours if possible. Until then please ahve a lok at my parameters.

Here are some parameters which may help you decide what is/ are going wrong.

GC conditions

• SUPELCO PLOT 1010 Column
• 100 ul manual injection using a gastight Hamilton syringe.
• Carrier gas is Argon. Inlet temp 200 C. Split is 30:1.
• Argon gas flow rate is 0.4 ml/min for 12 min and ramp at 0.1 ml/min2 to 0.8 ml/min.
• Initial temp 32 C hold for 12 min and then ramp the temperature at 30 C/min to 236 C. Finally hold for 1.2 min.
• Also, My TCD outlet goes into an FID for detection of gases like CH4, C2H2, C2H4, C2H6 as these gases have a better response for these compounds.

Let me know if you need any otehr information.

Thanks for your help.
Karthic

[AICMM]

karthic,

Some more things to think about. One, as a diagnostic, disconnect your detector from the FID and see what effect that has on your TCD response. Two, what is your make up flow set at? Three, is your split set manually or by EPC and if manually, have you actually measured it recently, although if this were messed up the FID would be lower as well.

Best regards.

[karthic]

Hello

I still have to check out disconnecting the TCD to the FID.

The make up flow for TCD is 4.5 ml/ min and reference flow is 16 ml/min.

As TCD and FID are connected, on the FID my H2 flow is 45 ml/min and Air flow is 450 ml/min. I do not have any make up floe for FID.

I have a couple of more questions:
IS THE TCD DETECTOR AFFECTED BY OXYGEN?
IS THE TCD DETECTOR AFFECTED BY WATER WHICH IS SOMETIMES ACCIDENTLY INTRODUCED AS I AM DEALING WITH FEREMNTATION SAMPLES?
DOES THE TEMPERATURE AFFECT THE RESPONSE OF THE TCD DETECTOR?

I TRIED TO INCREASE THE TEMPERATURE OF TCD DETECTOR TO 325 C TO REMOVE IF THERE WAS ANY CONTAMINATION ON IT BUT IT DID NOT WORK.

Again, thanks for your help.

Karthic

[karthic]

On your question on SPLIT
It is carried automatically. So, I think it should not be of much conern.

Karthic

[Don_Hilton]

Qustion: You tell us that the TCD resonse is down. What about the FID response? Is it down also? If I understand, your flow is from the column, through the TCD and then into the FID. If the TCD response has changed and the FID response has not changed (or they have changed, but differing in change between the two), I would look at the detectors. If both have changed, I would look upstream from them.

[AICMM]

karthic,

Increasing the temperature of a TCD decreases it's sensitivity. You want to run it at the minimum possible temperature to avoid condensation and in your case this sounds like it is around 200 or 220C.

Yes, oxygen will affect the filaments, basically it will oxidize them like a light bulb filament will oxidize. At low concentrations this is not significant but if shooting air it may impact the filament lifespan.

From the flow conditions you describe it sounds like your bridge may be slightly out of balance. Assuming you like your separations, I would increase your make-up flow so that column plus make-up match reference although this will cost you a bit in sensitivity. I would also strongly recommend diagnosing without the FID since it may be putting back pressure on the TCD. You might think about doing this as a split (parallel) rather than as a serial detector configuration. On the other hand, with back pressure you should have seen a slight retention time shift.

Finally, this is a perfect application for the HID. I have been working with a similar project using an HID for pyrolysis gases and get great signal for the components you list.

Best regards.

[karthic]

Hello everybody

I think I have solved the problem with my GC. The solution was much more simple that I had anticipated. I thought of what I had changed before this problem occurred. Except a few septa's the only thing I had changed was the LINER. This LINER was different from the previous ones. My Lab Manager had asked me to use a CYCLOSPLITTER LINER from RESTEK. He said that would work great for me. But little did I know about it till this morning when I was going through its specifications. It said "I CANNOT USE BIG SAMPLE SIZE".

So, I changed the liner to the LOW PRESSURE DROP LINER and I could get back my old peak areas for all my compounds.

I really appreciate everybody who viewed and replied to my post for your time and valuable suggestions. I came to know a lot about effect of oxygen on TCD's, effect of filament temperature on response etc etc (which is truly encouraging as I am not an analytical chemist but still my research group depends on me for these kind of maintenance and operation.

Also, I would like to know more about the HID as I am also working with the gases obtained from gasifier. Besides CO, CO2, H2, N2, O2, C2H2, C2H4, C2H6. It would be ideal to look into SOx, NOx, ammonia, Benzene, Toluene etc. I would like to know if there is any column and/ or detector in the market which can detect all these compounds in a single run and whether it is feasible with the Agilent 6890 series.

Once again. Thank you all.

[Ron]

The HID is a very sensitive detector, in many cases detection limits are in low ppb range. The problem is that the linear range is limited, so if you want to measure low and high concentrations in the same run you may have some problems.

[Don_Hilton]

Thanks for posting the resolution of the problem. It helps all of, including those making suggestions, to learn.

[AICMM]

Karthic,

I can tell you lots more about HID's since I make one. aicmm at flash.net.

Best regards.