I am trying to determine the concentration of a volatile organic compound after it has been passed through a bubbler using nitrogen gas as a carrier. Theoretically I can calculate concentration using Flow Rate and Pressure as mentioned in "Sensors 2015, 15, 18153-18166; doi:10.3390/s150818153". However, to validate the calculation I wanted to use an MQ3 series gas sensor. Calibration has been an issue as I have no known ppm value to initially design my calibration curve. Thus I cannot use the recommended 0.4mg/L of alcohol to determine the Ro value according to the datasheet. Is there an alternative method to experimentally calculate this or another approach altogether to determine the ppm concentration of VOC's?
mq3 datasheet: https://www.sparkfun.com/datasheets/Sensors/MQ-3.pdf
Also I would like to know if there is any other alternative for making custom made/ home made VOC generation set up.
Thank you
Dear Anwesha,
As far I know you don´t need fluxes or pressure to calculate the VOC´s concentrations. If you are using a purging system with nitrogen, then I expect that you will trap the VOCs that have been purged onto a system (e.g. SPME, or others). If that is the case then you will need to get a internal standard and spike it into your samples, so this chemical will be purged together with your other VOCs. Of course this internal standard will have similar properties to your VOCs and of course with 0% chances to be present in your samples. There are a lot of examples in the literature.
Previous to that, you will need to calibrate your system with known concentrations of the VOCs that you are looking for + you IS. You can use the RRF (relative response factor) for that.
I hope it helps you.
Claudio
Dear Claudio,
Thank you for your answer. The purpose of the above mentioned set up is to generate known quantity of vapour for passing it into a gas chamber for performing vapour sensing. I am not using SPME etc. I want to design a simple set up of vapour generation using a bubbler. However in that case I am not sure how to validate the concentration of VOC coming out of the bubbler with nitrogen as carrier gas though theoritically it can be calculated as mentioned in the paper stated above. Please let me know if you need any other information.
Thank you
Anwesha
Please see an example of procedure in attached paper.
Best regards.
AGCh
Thank you Professor A.G.Chmielewski. I have gone through the paper. However it seems here for analysis of VOC GC has been used . Is this is the only way to validate the ppm of VOC generated in my set up or Any other method can be adopted if GC column is not available.
Regards
Anwesha
Sorry, for low concentrations of VOC we are using GC-MS techniques with pre-concentration on sorbents. Best regards. Andrzej G. Chmielewski
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Dear Anwesha,
you could use a PID (photo-ionisation detector) which can measure VOC in a nitrogen flow. the PID is functioning with a lamp to ionise the VOC. a great number of VOCs can be measured like that. The PID is calibrated for a wide range of VOC. I am using the ppbRAE and am quite satisfied with that device. The cost is around 7600 euros.
i hope that will help you
regards,
thu-hoa
Thu-hoa Tran-thi
Hello Thu-hoa, I wanted to ask you if it is possible with PID ppbRAE to measure concentration of different VOCs separately within a mixture of VOCs. For example I wish to use a mixture of acetic acid, formic acid and formaledhyde in a single desiccator. Can i use the PID to measure the separate concentration of each of these VOCs? If yes then how? Do I need to know the vapour phase concentration for each of them before measuring?
Thanks in advance for your answer.
Dear Anwesha and Deepskiha,
The measurement of gas concentration with a PID is based on the ionisation potential of the considered gas. Since the apparatus is equipped with a lamp of a certain Energy (10.6 eV or 11.7 eV), this means that you can measured acetic acid with the lamp of 10.6 eV with no interference of formic acid and formaldehde. in the other hand, it will not allow to discriminate formic acid and formaldehyde because the lamp which has to be used with Energy of 11.7 eV will ionise both compounds.
the only way to discriminate and measure the 3 compounds is to do a colorimetric method and establish a calibration curve for each compound. For formaldehyde, the probe molecule to be used id Fluoral-P which forms with formaldehyde a compound absorbing in the near-UV-visible with an absorption band peaking at 410 nm. For the two acids which have different pKa, you will have to use pH-metry with different dyes which changes color over the pKa zone.
i hope that i answered to your question.
sincerely
thu-hoa
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