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.

  • Through your experiments and performance tests I think you've identified the problem! It seems to be pathlength related. May I ask, do other samples work correctly on the dialpath? It definitely seems like a pathlength issue given that the ATR spectrum looks superb, but I understand ATR cannot be used for this purpose due to low intensity of signal of interest.

    Which USP is being followed, if I may also ask? I also wonder if the 0.5 mm pathlength designated in the USP is specific to a particular instrument, or class of instruments, or accessories.

    Internally we use toluene to double-check the pathlengths are set correctly on the Dialpath and TumblIR modules, so I'm still a bit surprised that toluene would saturate the detector even at 100 um. You can also perform a pathlength check yourself using the "Pathlength_DialPath" method with pure toluene as the sample.

    Please let me know if the Dialpath accessory works on other samples correctly, and if the pathlengths are correctly set for all its intended pathlengths, just so we can rule out any instrument-related issues.  Lastly, you can also perform the Performance Tests on the Dialpath module itself to double-check its performance, rather than using the ATR module for performance tests.

  • Hello David,

    We normally only use the ATR module, this is the first method that uses the dialpath, so I don´t have any other sample or reference to compare.

    The USP Method that I am following is USP for Simethicone Capsules:

    The identification refers to the <197> Infrared Spectroscopy 197S:

    The requirements are:

    The <197> also states that It is possible to use a different method from the table by following General Notices, 6.30 Alternative and Harmonized Methods and Procedures.

    I have performed a pathlength check with toluene using the 0.5mm and the 0.1mm cell:


    Rep 2:

    Rep 3:

    Overlay Spectra:

    All the replicates have passed the method check, but there is a huge difference regarding the value. I have not quite understood how the method works, could you please clarify it for me?

    The results obtained via 0.1mm cell are better. All the value obtained was 0, and the spectra are quite similar.

    Overlay spectra:

    About the performance test, I have tried doing It with the dialpath module. The problem is that i have not found a clear guide on how to perform the test. I have performed It with toluene, but It has not passed the signal to noise test. Do I need to put sample on the dialpath to perform the performance test? or should I perform the same way as the ATR module?

    Could you please inform me on how to perform a performance test with the Dialpath Module? Step-by-step if possible.

    Thanks again for replying, It is helping me quite a lot.

  • Hi Scharf,

    No worries! Thank you for being so thorough!

    Looking at the results from the toluene pathlength test, as well as the USP requirements, I strongly suspect something strange is occurring within the DialPath accessory. The spectra and pathlength results are not at all what I would expect.

    At this point, it may be best to connect with our online tech support team, that is the fastest and most efficient platform to resolve such issues: https://www.agilent.com/en/contact-us/page

    Select your local region, and call the number displayed. You may have to navigate a few prompts, follow "Molecular Spectroscopy", "Service Request", "Diagnose an Issue with a Troubleshooting Expert over the phone" (actual prompts and order of prompts may vary according to language).

    I'm confident that the issue is relegated to the Dialpath module, since the ATR module data is excellent. I'm sorry I couldn't be of greater assistance!

  • Hello David,

    I have contacted the agilent services here in Brazil and It appears that I have made a mistake during the pathlength method. I used toluene as background, so the pathlength resul was not correct. Here are the correct results:

    0.5mm cell: Value = 501,89

    0.1mm cell: Value = 100,37

    Do you have any more ideas on what is happening with the saturation? I think that the 0.5mm cell is too sensible for the identification analysis, but the USP Method states that the 0.5mm cell must be used.

  • Splendid work matey! Those results actually look quite good!

    If the peak of interest for the analyte exists outside areas of toluene peak saturation, then I wouldn't worry about the saturation all that much, since it will not affect other signals. It is likely that the USP has taken this into account, and that the 0.5 mm pathlength distance might be best if the analyte peak of interest is a very low intensity signal.

    If that analyte peak can now be seen correctly, and is not saturating the detector, I believe you can indeed perform the experiment as intended, even if some of the toluene peaks saturate the detector elsewhere. Otherwise, as you had pointed out, the 0.1 mm pathlength cell can be used and a deviation can be created, stating something like "the detector on the Cary 630 is very sensitive and thus a lower pathlength was required to accurately record the results" (please note this is merely a suggestion, as while I am very familiar with compliant environments, I do not currently work in one).

    Good job!

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