The detection limit depends on the precision of the absorbance measurement. the less the noise, the less the quantifiable concentration. However, my example was a special case. There are not much gaseous substances measurable by UV/VIS spectroscopy. Normally, IR absorbance is used for gas analysis, measured by FTIR spectrometers. For measurement of gases in low concentrations, "multi-pass" configurations are used to increase the measurement pathlength and therefore the absorbance intensity.

Perhaps you should clarify the goal of your question... There are many techniques used in gas analysis like GC GCMS, FTIR, Raman and some more... Again, it depends if want to identify the composition of a gas mixture, or the quantitative amounts of gases, or detect traces of gases. It makes a big difference if want to measure from far away (atmospheric gases) or can draw a sample... and so on...

Yes.

Absorption spectra of gases or vapors can be measured in similar fashion as in case of liquid solutions. But the only problem is the required concentration in the sample cell in order to obtain observable spectrum.

Thank you

try the following: give one drop of toluene into a standard absorbance cuvette and close it with the plug. Measure the absorbance spectrum from 200 - 400 nm. Due to high vapour pressure, you will see the characteristic toluene absorbance spectrum...

thanks. as I realized , the vapor concentration must be high. do you know how much is the lowest concentration? what do I do for gases like oxygen or methane?

The detection limit depends on the precision of the absorbance measurement. the less the noise, the less the quantifiable concentration. However, my example was a special case. There are not much gaseous substances measurable by UV/VIS spectroscopy. Normally, IR absorbance is used for gas analysis, measured by FTIR spectrometers. For measurement of gases in low concentrations, "multi-pass" configurations are used to increase the measurement pathlength and therefore the absorbance intensity.

Perhaps you should clarify the goal of your question... There are many techniques used in gas analysis like GC GCMS, FTIR, Raman and some more... Again, it depends if want to identify the composition of a gas mixture, or the quantitative amounts of gases, or detect traces of gases. It makes a big difference if want to measure from far away (atmospheric gases) or can draw a sample... and so on...

The question is much too vague

What do you want to measure

Why do you want to measure in the gas-phase.

Please specify your question. We can not answer such general remarks

thank you every one. Mr. Menges's suggestion solved my problem.

thanks

Hi Sara

If you have access to a cavity ringdown lab. is possible to measure with very high confidence the spectra and concentration of a desired compound in gases. CDR spectroscopy is very sensible as lower than ppb.