Chromatography Forum

Dear all,

I have a Perkin Elmer Clarus 500 GC/TCD with packed columns (pora pack q and molsieve 5A) for separation of CO, CO2, N2, O2, CH4, C2H4 and C2H6. However because of the packed columns I can not reduce the analysis time below 15 minutes.

I have read few examples of a micro GC applications, among which I have seen that a combination of 5-10 meters of capilary columns (0.32 mm) poraPlot Q and Molsieve 5A can separate this mixture isothermally in less than 5 minutes. However, I am not sure if my TCD detector can be used in case of a small flows and even smaller amounts injected into the column. I need to determine all the mentioned species quantitatively untill a lower limit of 500 ppm.

I would realy apreciate any comments or advices!

Critical is the amount of dead space in your injector and your TCD. Not being familiar with the Clarus 500 TCD, I don't know what to tell you.

The following micropacked column (not a capillary) using a HP6890 µTCD worked well. 100µL valve loop

2 meter 1 mm ID column packed with MS 5A 80/100, oven 70°C, 11cc/min. TCD 150°C



hydrogen is the first peak: then oxygen 2500 ppm, nitrogen balance, methane 1250 ppm , CO 1900 ppm

The detection limits are well below 500 ppm.

best wishes,

Rod

Thanks Rod,

I checked my TCD, Perkin Elmer is saying that there is no problem when working with a capilary columns 0.32 and 0.53 mm without any problems and without aditional make up gas if the flow is bigger than 5cc/min. These are the specifications of the TCD:

Operating 100 °C to 350 °C in 1 °C increments temperature.

Sensitivity with a detector temperature of 100 °C : 9 μV/ppm nonane at 160 mA at the bridge

Minimum detectable quantity: Typically < 1 ppm nonane

Linearity: > 105

Power supply: Constant current with four selectable settings:
1: ±40 mA
2: ±80 mA
3: ±120 mA
4: ±160 mA

Signal filtration: 50, 200, 800 msec

Filament protection : Self-limiting and resetting after transient overloads in either channel

Makeup gas: Not required for 0.32- to 0.53-mm i.d.
columns with flows ³ 5 mL/min
Required for 0.25-mm or smaller
i.d. columns

So I guess it should not be a problem. However, the problem here arrises when I want to analyse CO2 as well. I have a packed Molsieve 5A column, but I cant let CO2 pass to it. So the current method is letting only the permanet gasses (CO, N2, O2 and methane) to pass through PoraPack Q and to Molsieve. Next eluting component is CO2 and the second column, Molsieve is closed untill all other components (CO2, ethane, ethylene, acetylene) come out from the PoraPack, after which Molsieve is again connected.

What I was reffering was the brochure for the Micro-GC by agilend :

http://www.youngin.com/application/AN-0809-0098EN.pdf

they managed to separate permanent gasses on molsieve capilary column and light hydrocarbons + CO2 on a poraPlot Q in less than 1 minute each. However they have a 250 nanoliter TCD . Therefore I want to use same length of the columns and similar flow, however I do not expect so fast analysis and so low detection limits.

You see the microGC has these four columns but from the chromaotograms they appear to be single dimensional. They also use zero dead-volume valving, connections, and detectors to achieve their chromatography. But look at the column flow rates to the detectors and the amount of sample injected !
Calculate for yourself the column flow rate for an unretained peak at 0.45 min with a 8m 0.32mm ID column, with a sample size volume of 1/5 second at that flow rate. The near zero dead-volume detector is required with these specifications.

Also high temperature conditioning of the MS5A capilaries is routinely required because when they inject hydrocarbons and CO2 on the sieve columns which must be removed periodically or the sieve will become blocked and ineffective, as you are aware.

Look however at a recent posting under "Setting up a GC-TCD" for a chromatogram of CO2 eluting from a MS5A column.

Your application can be done multidimensionally with a single 10 port valve using micropacked or capillary columns, configured in a column step configuration where the sample is injected upon a Q, D, N, or A polymer column (#1) and after the hydrogen - methane plug elutes from column 1 onto a MS5A or MS 13x column (#2). The column #1 is valved switched to a position BEHIND the MS column without reversal of flow to the detector. The fixed gases continue their path through the MS5A column and back into the polymer column onto the detector. The CO2 and hydrocarbons are remaining on column #2 until after the column switching is performed and elute onto the detector while the fixed gases are eluting onto column #1 again for a second time, passing through it after eluting from column #2 with the proper sizing of columns the fixed gases can elute AFTER the CO2, methane, C2s plug or BEFORE, depending upon your preference.

One can also use a three column configuration where there is another polymer column immediately before the detector which receives the flow from column #1 after it is switched behind column #2 to delay the elution of the CO2-C2s.

Of course if you want to use TWO TCDs then you can configure your valve to measure fixed gases from the MS5A on one and the CO2-C2s on the other with a single injection, which is easier, faster, and less complicated. I would recommend this last option if possible.

This is a simple multidimensional application, one of the first I learned at Philips Petroleum back in the 1980s.
I have applications using as many as 7 valves to process refinery gas samples. with only 1 detector. Then things can get complicated !

You may wish to review the VICI web site and look at the applications there to guide you and to be your source of the necessary hardware.

Good luck.

Rod

stan83,

I would be surprised of a 0.32 application without make-up gas on a conventional TCD. I have been surprised before. However, if you are not hurting for sensitivity, make-up should be easy enough to do. If you do add make-up you WILL lose sensitivity since the TCD is a concentration dependent detector. 0.53's often run well at packed (or micro-packed) flow rates.

I would advise taking Rodney's advice on micro-packed columns as a really good place to start. Still use the same rough flow-regime, still able to use pretty big loop volumes and will improve your separation time. Besides, a lot cheaper place to start.

Of course if you have a cool $40 or $50 grand to drop on the micro..... I have a customer who it has done well by as long as you don't have to service it... And it is fast...

Best regards,

AICMM