Hi,
ATR-FT-IR spectroscopy is being used for the quantification of a particular compound. Is it not possible to run a known quantity of a sample in a regular FT-IR and then compare the peaks (Absorbance or transmittance) with the unknown quantity of the sample (test sample)? I am sorry if the question sounds silly; But, can someone advise me?
Let me kick some sand around! While Beer's law states;
Absorbance = extinction coefficient x pathlength x concentration (mass/volume)
The problem with IR is:
1. The detector must see a stable area of the ATR cell. You do not need to know it surface area but it must be the same every time.
2. Particle size is very important since the surface area of the compound and thus the detectable mass is critical. I ground the sample (animal feed) with a Flex Mill before analysis.
Yes and no. I quantitated Silicone Oil (Si-O stretch) by measuring its absorbance at 1620 um-1. But I used a liquid cell and not ATR. You will have to ensure the ATR sample has the same mass each time. Remember, with weak radiation (IR) you get a weak response.
Dear Prof,
I understand from your explanation that the mass needs to be the same for the test and the standard. Apart from that, as you said I also understand that the IR intensity varies during a measurement. Is my understanding correct?
Yes. IR complies with Beer's law just like UV and Visible spectroscopy (A=abc).
Let me kick some sand around! While Beer's law states;
Absorbance = extinction coefficient x pathlength x concentration (mass/volume)
The problem with IR is:
1. The detector must see a stable area of the ATR cell. You do not need to know it surface area but it must be the same every time.
2. Particle size is very important since the surface area of the compound and thus the detectable mass is critical. I ground the sample (animal feed) with a Flex Mill before analysis.
Dear Prof. Bruce,
Thank you for your detailed explanation. I feel I've understood it better now. :)
Do not put too much trust in absorbance and Beer's law...
Article Deviations from Beer's law on the microscale - Nonadditivity...
Article Beer's law derived from electromagnetic theory
Article Beer's Law – Why Absorbance Depends (Almost) Linearly on Concentration
Article Employing Theories Far beyond Their Limits-The Case of the (...
if you want to have reliable results, you have to perform good old dispersion analysis and derive the concentration from the oscillator strength.
In Figure 2 of the attached, you an see that a linear relationship of ATR absorbance with concentration of solution of a droplet can be observed. However, one the droplet dries, the range of "linearity" with concentration is greatly reduced, as the deposit rapidly becomes thicker than the physical extent of the evanescent field.
One particularly important aspect, also taken into account in Hugh's paper, is to use Beer's law not in its original form, as it is not generally valid at particular spectral points, but for the integrated absorbance: Article Beer’s law – why integrated absorbance depends linearly on c...
Hello
Kindly look at link here
http://authenticationinart.org/pdf/literature/infrared_spectroscopy.pdf
Hope help you
Good Luck
Saleh Alkarim Nice book, but concerning the Beer-Lambert law, as so many, no longer up to date...
Make a referral peak by adding known compound with known mass which doesn't interfere the interested peak of your unknown sample. Then you can normalize whole spectra for the referral peak position. Then only you can quantify.
Dear dr. Chanaka Sandaruwan
Best contributions
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Dear dr. Thomas Mayerhöfer
Very useful contribution
Best regards
Dear dr. Hugh J Byrne
Very good contribution
Best regards
Article Simple, fast, and accurate methodology for quantitative anal...
http://manu60.magtech.com.cn/rlhxxb/fileup/HTML/20150201.html
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