For the quantification of hexadecanoic and octadecanoic acids

We have recently been working on the quantification of hexadecanoic acid and octadecanoic acid.

We used a Cary 630 FTIR and Dialpath 200 μm photometry.The quantitative peak is 2855nm

The first graph shows the superimposed spectra of 25mg, 4mg, and solvent

The second graph shows the 4mg, 0.4mg, and solvent stacking spectra

Is there anything I can do to enhance the response of 0.4mg hexadecanoic and octadecanoic acids?

  • Hi Duxact_J

    I've couple of comment on it. 

    The first one is about the background, as i see you are getting a negative Abs, you may consider to check what is causing this shift, this is not affecting the results and the shape but maybe the overrange is scaling the whole spectrum. 

    The second point, about you question is, as you surely know the power of the DialPath is to adjust the Path at needs, so to enhance the signal at 0.4 my suggestion is to use a longer path (keep in mind the Lambert-Beer Equation) and create multiple calibration curves for quantification to be more accurate. 

    As example 3 curves with concentrations between

    0.3 to 2 mg with the longest Path 

    2 to 10 mg with 200 um 

    10 to 25 mg with 200 um

    and then load all the three model on the same Microlab Method and choose the proper results on the correspondent calibration curve. 

    Another try might be to use a Ration between the quantitative peak (2855wn) and a isosbestic point (If you have one) at the lowest Abs.

  • Thank you for responding.

    I consulted with an application engineer who recommended that I replace the Dialpath sampling attachment with a larger optical range.

    I then produced a standard curve today using a Dialpath attachment with a 200 μm light path, and I'm not quite sure if the result passes muster.

    I am new to infrared spectroscopy, is there any requirement for peak shape for quantitative infrared spectroscopy?

  • What can I say, Great Job - its textbook!

    From the STD preparation to the R2. 

    The requirements are the same as in every quantitative analysis, you should pay attention of the selectivity and sensitivity of the peak per Analyte.

    For example, in your case you are using the 2850 wn that should be connected to the CH2 vibration (symmetric stretch) and in fat acids the number of CH2 are "pretty" high so it made this peak a good one. 

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