FTIR Cary 630 - Simethicone Identification

Hello everybody,

I have a problem in the identification of simethicone capsules. I am currently following the USP method, which indicates that I should use the standard solution, blank and sample solution according to the assay, therefore, the samples are diluted in toluene, circa 2,5 mg/mL.

I have created a new library and added the spectra of standard and blank. The problem is when I compare the spectra from the library with the spectra from a recently prepared standard I get good similarity results in both the comparision standard-standard and standard-blank. This way I can´t properly identify simethicone in a quantitative way, because there is no significant difference between the responses. Also, the scalling is not helping me to do a qualitative comparision, because It is auto-scaled (0 to 1). Is there any way to fix the false positive result and to adjust the graffic scaling?

First one is standard, second one is blank.

  • Hi Scharf,

    The % transmittance is very very low across the entire spectrum (normally the signal varies from 100% down to 0% rather than 1% to 0%), and in some places it seems the signal is saturating the detector (the areas illustrating flat lines at zero). There is also some strange behaviour / "spikes" on either side of most peaks. Without additional information it will be difficult to comment, but I would be interested in knowing:

    - What sampling interface / sample cell is being used to collect the data?

    - What solvent is the background spectrum taken on?

    - Does the Cary 630 FTIR currently pass its performance tests?

  • Hello David, first of all, thank you for your reply.

    The sample cell used is the dialpath 0.5µm. The background solvent is the analysis blank, which is toluene.

    The equipment is new and have passes all the performance tests.

    I have made a few more tests, but I am still having problems with signal saturating the detector. Could it be that the background could not compensate these signals? (because they saturate the detector). For example, after collecting the background I read the same sample (blank) and get the current spectrum:

    In my interpretation, there are a lot of problems. First, the scalling, which includes signal with transmittance higher than 100. Second, the flat lines. Also, normally in literature, the scale is reversed (0 being on top). 

    I have also read the sample the same way as described before but using absorbance. 

    The spectra here is overall better, but i get some signal with 5 Abs, which i think is the limit of my detector (5 Abs equals 10^-5 transmittance).

    The problem is: these signal with higher absorbance do not maintain their wave number, and so I can´t properly use the spectra for identification.

    Do you have any suggestion on what I could do to avoid those signal that saturate the detector?

  • The Dialpath accessory usually comes with one of two sets of pathlengths:

    - 30, 50, 100 um

    - 50, 100, 200 um

    As far as I know, 0.5 um is not a support pathlength (minimum is 30 um). If the signal is saturating the detector (which seems to be the case, from what I can see above), then I recommend using a shorter pathlength than whichever one is being used, especially given that toluene has fairly intense absorbance bands.

  • I *think* this might solve the issue? Please let me know how you get on when you have a moment to try it out.

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