FTIR Cary 630 - Simethicone Identification

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?

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.

Hi, thanks again for the reply.

The cell I am using is 500 microns, not 0.5 µm, sorry, that was a typo.

My Dialpath apparatus contains the following sets of pathlenghts:

- 100, 500, 1000 microns

I have made some tests with the 100 microns cell, but there is still signal saturating the detector. The following image is the blank (toluene) spectrum using the 100 microns cell.

I have also made some tests with the ATR module (0.02 mm pathlength) but in this case It is not possible to properly identify the signal, because the band intensity is very low compared to the rest of the spectra. The following image shows overlay spectra, red being standard solution and blue being blank (toluene). The circled area shows the band of interest.

I have concluded that to proper identify the sample I would need a dialpath with pathlength 30 or 50 microns. The problem is that I am using an USP method as reference and It states that the cell must have 0.5 mm pathlength. My question is: using the current cell (0.5mm), is there a way of reading the sample without saturating the detector?

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:

Rep1:

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!