You could determine the LOD from linear regression analysis of the linear portion of the calibration curve. See the ICH Q2B guidance for how to perform the calculation using the expression DL = 3.3 F/S where F = the standard deviation of the response, and S = the slope of the calibration curve. Assuming that the sample was introduced into the GC using a gas-tight syringe, did you compressed the gas by pushing the plunger down the barrel before release the sample into the GC. The sample introduction technique often affects the reproducibilty of the peak response for this type of gases.

HI what we are currently doing is choosing a concentration in which we can see a signal and repeat the injection atleast 10 times. Calculate the mean and the standard deviation for that area/conc. the formula used is LOD=mean + 3*standard deviation and LOQ = mean + 10* standard deviation. If u use area for the above calculation, then use ur regression curve equation to calculate the corresponding concentration for that.

I suggest you to perform different calibrations on different measurement ranges. For example do a calibration for low-level CO2 samples (let's say 0-0,5% vol) and another for higher level CO2 samples. Detector linearity cannot be very good for a very wide range like you said (from 0,1%to several % vol!). If your samples differ too much in CO2 concentration you have yo consider also that the same injection apparatus cannot have the same reproducibility on all the injection range...

Hi Christián,

that sounds somewhat strange to me.

Me and my team are doing gas analysis of O2, N2, CO, CO2, CH4 and C2H4 since 30 years.

We never observed a non linear calibration for these compounds!

I expect, there is a wrong part within your GC equipment. Using a GC-TDC without malfunctions, you will always get linear responses for these gases.

If your calibration is nonlinear, something is wrong and it makes no sense to include special calibration functions.

We frequently measure CO2 in the Range between 0,01 % up to nearly 90 % in different gas phases with ease.

I think you are using an wrong column or there is an overload of the TCD.

Working for industrial and academic problem solving as well, I may give you further hints, if you tell me some analytical details (column, TCD design, gas settings etc.).

Hi Peter, thanks for your comments.

The method in GC has following set-up in a Shimadzu GC-2014:

Furnace: 80ºC (isothermal)

Packed Injector, type DIN: 100ºC; TCD:100ºC (65 ml/min,N2 carrier), current: 100 mA.

Injection volume: 500 uL by gas-tight syringe

Column: Supelco Carboxen 1000 (15 ft x 1/8 in )

CO2 signal appears at 12,5 min.

Hi Christián,

thanks for the details.

First of all, nitrogen as the carrier gas is a really bad choice and Supelco Carboxen 1000 as well.

Please, take helium as the carrier gas and switch to a approx. 70-80 cm packed column (1/8"ss) with 60/80 mesh molecular sieve 5A.

Running a temperature programme, it is easy to seoarate O2, N2, CH4, CO, C2H4 and CO2 within 20 min.

Interested in CO2 only? Please select the oven temp. at 180 to 200 °C and you will be satisfied to analyze CO2 within 10 min. or so.

There are a lot of paper claiming CO2 cannot be be eluted using molecular sieve 5A.

That's a very old myth.

It takes you one night at 250 °C to completely condition the Column and at least fur my application you can operate the machine for months without trouble.

IT WORKS !

Who for the hell suggested Carboxen 1000 to use.....

Thanks a lot Peter for your help!

Hi Cristian!

I wouldn´t use a Molsieve colomn if you try to analyse CO2 in air.

If it is not necessary to separate N2 and O2/Ar (what I assume as you use N2 as carrier) I would definitely go for a colomn using Chromosorb or Haysep A or D.

Molsieve is acutally the worst choice for CO2 because you need very high temperatures to get it over the colomn.

I agree with Peter that you better use He as carrier if available, because it gives you a better sensitivity. I also strongly suggest, that you make different calibrations for measuring ranges. From my experience you will never get a linear calibration curve of the range 0.01% (which is actually 1/4 of CO2 concentration in air) to more than 90% as you mentioned in your question. No way a Colomn/TCD is linear over that range and you are definitely close to the detection limit of a TCD (especially if you us N2 as carrier).

if you need further details about operating conditions the colomn I suggested you can check for example the Scanview software of Agilent (free for use) or any other colomn supplier.

If you need precise results for very low CO2 concentrations (below ambient) I would definitely think about a methanizer and FID for your CO2 analysis.

Hope this helps

Stefan

I do not really agree with Stefan's hint.

First of all, Helium does not only increase the sensitivity of a TCD but also extends the linear range.

I assume, a non linear response is a result of a TCD overload....

Molsieve 5A is NOT the worst choice for analyzing CO2. Taking a short (about 70-80 cm * 1/8" or even 1/4" diameter) will elute CO2 isothermally at 180 °C or so.

Analyzing CO2 at 0,03% in the atmosphere is not a problem at all.

Chromosorb may be overloaded at high CO2 concentrations.

By the way, not every suggestion from the Scanview software is the best selection.

That's what I think is the difference relying on some software against more than 30 years of experience and daily use in industrial research and university.

There are some software solutions on the market which do not offer best results ore even make some good analyses even worse (e.g. MS).

From my point of view, for the expected working range from 0,1 ... 65% CO2 a methanizer will be not necessary.

Regards,

Peter

Ask two scientists and you get three different opinions :-)

I´ll stick with my opinion not to use MS5A. However, I second Peters opinion that it will work at high temperatures.

Last time I used a packed colomn was on RV Sonne during a cruise off-shore Sumatra. We routinely analysed C1-C5 and CO2 using a Haysep colomn with TCD and FID connected in a row (exhaust TCD to inlet FID).

Re-conditioning of this colomn is easier done because it needs less high temperatures (we routinely re-condition our packed MS - colomn at 300°C)

Currently I am using a packed 1/16" colomn (Porapack) with a liquid nitrogen trap for isotope analyses of C1-C4 on GC-irMS.

Anyhow, I mentioned scanview etc only, if Crisitan wishes to change colomn. I am your opinion, that marketing of producers is biased, but Cristian could find some good starting values for method set-up (carrier gas flow, temperature) in that case.

Peter, you are right about the methanizer. I thought Cristian is measuring down to 100ppm but he wrote 0.1%. Methanizer not necessary :-)

Regards

Stefan

Every GC method that works is fine. :-))

The Molsieve 5A technique worked fine for gas analyse in gastight bags with roasted ground coffee with hundred of organic volatiles with ease and an average column 'life time' of more than 1 year without frequently conditioning before replacing the Molsieve at the cost of one $US or so.

Anothe main application has been the reliable determination of microbiological released CO2 in earth...

Also, I do not expect an oven temperature of around 180 °C is very high compared to PAH analyses, requiring 300 °C in a run.

Anyway, I think, more importing is the linear respone of the TCD.

This mainly depends on a stable carrier gas flow, because the TCD is flow sensitive. I have found some Electronic flow regulators on the market not worth the price compared to some precise proportional flow controllers on the market.

A carrier flow of 65 ml/min for an 1/8" column is by far too high.

For a fine two channel TCD you can get a linear range of 1: 10^5. However this is reduced using "switching" TCDs.

And- keep the injection volume as constant as possibe, becaus the dead volume in a gastight syringe greatly influences linearity.

Regardds,

Peter

@ Robert: No offense taken :-)

Have a nice long free weekend :-)

Stefan

Dear colleagues:

Discussion has been interesting and your comments are valuable.

Respect to concentration of samples, some samples coming from lab-reactor gas exhaust (biomass pyrolysis/combustion) and in some cases (like pyrolysis) CO2 concentration are very low at T

Dear Peter

I have read your comment regarding Co2 analysis by GC. Is it necessary to analyze Co2 gas at higher oven temp. 180 to 200 °C. I analyze Co2 at isocartic oven temperature 45°C and i am getting peak at RT 1.2 minutes.There is tailing in the peak. I am using packed column (1/8"ss) with 60/80 mesh molecular sieve 5A. and carrier gas is helium. I think for trace level (ppm or ppb) of Co2 we need methanator and FID detector is use for analysis.Please give your feed back.

Dear Tiwari,

I think for sure you aure NOT detectin CO2 using an 1/8" packed Molsieve Column.

At 45 °C CO2 will not elute from the column !!!!!

I think you will get the signal from co-eluting O2/N2 , but NOT CO2 .

Below 0,01 % a TCD is more or less without sense - even operating a TCD at highest sensitivity.

As mentioned by Stefan Schloemer, you can take a FID with a methanizer. Probably not in the ppb range.

Another option is a 12 m Molsieve capillary together with MS detection in SIM-mode.

What is also possible : Inject a large volume without filament current at 40 -50 °C and than switch back the filament of the TCD after elution of O2/N2 and start a fast temperatur program up to 230 °C.

For a more detailed hint, I shoud know mor details about your sampling and analysis.

Greetings,

Peter

Hi Cristián,

the higher the carrier flow rate, the lower the TCD sensitivity as the TCD has a concentration depending response.

(close to the response of an HPLC-UV detector. )

Please take that in mind in order to analyze low CO2 concentrations.

thanks dude

is TCD suitable for detection of CO2/CH4 and/or CO2/N2 mixture in an equimolar mixtures?

A TCD is THE detector of choice for the analysis of CH4/CO2 as well as for CO2/N2 mixtures using a packef Moldieve 5A column at about 60 to 80 cm.

As pointed out earlier, O2, N2, CO, CH4, C2H6 and CO2 can be easily analysed in a 15 min single GC run in high ptecision not obtained by methanizing or eben wiht ms detection.

Me and my coworkers did this simple routine task for about 3 decades.

(Long before there were "applications" on CD or in catalogues available !!

Regards, Peter

Dear colleagues:

I have a question, it is possible determine CO2 with a Mol Sieve 5A PLOT Capillary GC Column?

Thanks

Roberto

Co2 gas works specifically with a certain temperature and does require high temperature, check the carrier flow rate, induce the intensity by using helium or nitrogen

CO2 will be permanently retained in a MS5 column. Over time the column will start to behave abnormally.