[ussername]

I measured gas composition on GC with TCD and I obtained these data:
 Component Name    Ret.Time    Area               
   ----------------       --------  ----------           
           1                  1.00     102456     = methane CH4       
           2                  1.20     379129     = carbon dioxide CO2
and I know that:
[tex]\frac{x(CO_{2})}{x(CH_{4})}=\frac{Area(CO_{2})}{Area(CH_{4})}=\frac{379129}{102456}[/tex]

What does area number physically mean? (answer 'that is area bellow some peak' won't help me).
How can we exactly (step by step) get to that area number from measured voltage on TCD (do we have to measure sthg else, for example eluate flow rate around TCD filament)?             

[Arkcon]

(do we have to measure sthg else, for example eluate flow rate around TCD filament)?           

No, because that's included in the "area under the peak."  Oops, sorry about that, but that's the way cinematographers talk.

So, you've gotten a response over time, and the software has computed for you when the analytes started and stopped coming off the column, and gave you the area.  Do you need something else, or are you asking how the software computed it, or are you asking why we do this this way?

[ussername]

So, you've gotten a response over time, and the software has computed for you when the analytes started and stopped coming off the column, and gave you the area.  Do you need something else, or are you asking how the software computed it, or are you asking why we do this this way?

I'm asking how the software computed the number (based on what formulas and physical laws - e.g. heat conduction equation) and what does the number mean (has it some unit?).
'Area under the peak' is only geometric interpretation, not physical.

[Arkcon]

Absolutely. The integrator has given you the area under the curve by the geographic method. Your detector can only give a signal it sees, at the time it sees the signal.  The integrator plots it, and sum the area over time.  And that's the area under the curve.  You can do this yourself for any curve you can plot, that's how you determine the integral of a curve.

If you've taken calculus, you know that you can also determine the integral, or area under a curve, by taking the derivative.  And modern, advanced chromatographic software can do that as well.  It does help for merged peaks and noisy baselines. 

How a thermal conductivity detector works is googleable: https://en.wikipedia.org/wiki/Thermal_conductivity_detector

The units of chromatography detectors, say for the TCD detector for a gas chromatograph, or the UV detector for an HPLC or standalone UV detector reading a cuvette are arbitrary.

[DrCMS]

I measured gas composition on GC with TCD and I obtained these data:
 Component Name    Ret.Time    Area               
   ----------------       --------  ----------           
           1                  1.00     102456     = methane CH4       
           2                  1.20     379129     = carbon dioxide CO2
and I know that:
[tex]\frac{x(CO_{2})}{x(CH_{4})}=\frac{Area(CO_{2})}{Area(CH_{4})}=\frac{379129}{102456}[/tex]

I think you are wrong if you think the ratio of the two peak areas is exactly the same as the ratio of the amounts of the two gases.  Different materials will have  slightly different detector response factors that have to be taken into account to convert a peak area to a quantification of that substance.   The thermal conductivity of methane is about twice that of carbon dioxide.

[Arkcon]

and I know that:
[tex]\frac{x(CO_{2})}{x(CH_{4})}=\frac{Area(CO_{2})}{Area(CH_{4})}=\frac{379129}{102456}[/tex]

I'm sorry.  But this isn't appearing for me.  I wouldn't even have noticed it if not for DrCMS:'s quote.  What are you looking to say here?  The parser isn't working for me, and I can't parse Mathjax myself.

[ussername]

Absolutely. The integrator has given you the area under the curve by the geographic method. Your detector can only give a signal it sees, at the time it sees the signal.  The integrator plots it, and sum the area over time.  And that's the area under the curve.  You can do this yourself for any curve you can plot, that's how you determine the integral of a curve.

If you've taken calculus, you know that you can also determine the integral, or area under a curve, by taking the derivative.  And modern, advanced chromatographic software can do that as well.  It does help for merged peaks and noisy baselines. 

How a thermal conductivity detector works is googleable: https://en.wikipedia.org/wiki/Thermal_conductivity_detector

The units of chromatography detectors, say for the TCD detector for a gas chromatograph, or the UV detector for an HPLC or standalone UV detector reading a cuvette are arbitrary.

I don't see here any answer to my questions 
I read TCD on wiki, but there is no explanation about how one can get area from measured voltage and what is the meaning of area number.

I think you are wrong if you think the ratio of the two peak areas is exactly the same as the ratio of the amounts of the two gases.  Different materials will have  slightly different detector response factors that have to be taken into account to convert a peak area to a quantification of that substance.   The thermal conductivity of methane is about twice that of carbon dioxide.

I ready don't understand you, because (again) I don't understand how one gets area from measured voltage and what is the meaning of area number.
But in our lab we use this algorithm to determine selectivities for membranes during gas mixture separation.

TeX code is:

Code: [Select]

\frac{x(CO_{2})}{x(CH_{4})}=\frac{Area(CO_{2})}{Area(CH_{4})}=\frac{379129}{102456}

[Arkcon]

Well, I tried to explain that the response over time gives a collection of points that can describe a curve.  You can integrate that curve, geometrically or by using calculus, you get an area.  We typically describe the area as the response, in chromatographic determinations.  And the Latex is appearing for me now, so I see you're describing the ratios of your two areas.  Of course, like DrCMS: said, your areas are useless for quantitation, unless you have run a standard first.  Or you have been given a calibration factor, like DrCMS: told you, which lets you know the corresponding responses of the two compounds on a TCD.

You keep being very abstract, asking "what does this mean" and "how does this work."  So I'd like to ask you what answer do you need to give, and what pieces are you missing that prevent you from figuring the answer out.  I'm sorry you can't get the "meaning of area number" from how the TCD detector works, but the detectors responce, by the change in thermal conductivity of the flowing carrier gas, over time, is exactly where the area comes from.

[ussername]

You keep being very abstract, asking "what does this mean" and "how does this work."  So I'd like to ask you what answer do you need to give, and what pieces are you missing that prevent you from figuring the answer out.  I'm sorry you can't get the "meaning of area number" from how the TCD detector works, but the detectors responce, by the change in thermal conductivity of the flowing carrier gas, over time, is exactly where the area comes from.

Also from what I introduced, one approach could be:
[tex]\frac{Area(CO2)}{Area(CH4)}=\frac{x(CO2)}{x(CH4)}=\frac{n(CO2)}{n(CH4)}[/tex]
Also area is proportional to total amount of the substance passed thought TC detector.
Area means:
[tex]Area(CO2)=\int_{t_{min}(CO2)}^{t_{max}(CO2)}response\cdot dt[/tex]
Now what is the response exactly (what physical variable is it)?
In my chromatogram in lab there is:
[tex]Voltage [mV] = f (Time)[/tex]
So the area is bellow function:[tex]\Delta U=f(t)[/tex]?