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FTIR and Raman

Spectral Libraries

Frequently Asked Questions

Q: Can I search ATR/FTIR vs transmission FTIR spectra or vice versa?

A: Yes. Search over the fingerprint region. Since the greatest differences between ATR and transmission spectra are in the C-H stretch region, and higher, searching on the fingerprint region is practical. We believe this is due to the relatively small changes in peak positions between ATR and transmission in the fingerprint region and the nature of the correlation coefficient and least squares search algorithms, which are optimal for medium bandwidth spectra such as infrared and Raman spectra. Searching ATR vs ATR will often provide higher hit scores than ATR vs transmission but you can still obtain good and informative hits at or near the top of the hit list.

Q: Can I add to the library?

A: No, but that’s OK. With most programs you can build user libraries and search them at the same time as an FDM library and the hit list will be merged so you get the same effect.

Q: What are the advantages of ATR over transmission sampling?

A: ATR sampling is faster, requires far less skill and is far more reproducible than transmission sampling.  Most labs save time, effort and money with ATR.

Q: Why did FDM rerun several ATR libraries?

A: We reran everything to have superior libraries. The first generation of FDM ATR libraries were run on ATR crystals mounted with a metal foil seal.  At that time, 2004-2008, the crystal choices were Zinc selenide (ZnSe), a diamond/ZnSe composite or Germanium. The lower end of the spectral ranges were 550 cm-1 or 600 cm-1. The most recent generation of ATR accessory base and ATR crystal mount are much more rugged, allowing the use of monolithic diamond ATR crystals brazed into a steel mount. This means superior baselines, largely mitigated phonon bands and a wider spectral range. So, of course, we reran everything. We bought many new compounds as well so the libraries are larger.

Q: Which ATR libraries were rerun?

A: The FDM ATR Polymers, FDM ATR Retail Adhesives & Sealants, FDM ATR Organics, FDM ATR Inorganics, FDM ATR Dyes, and the FDM ATR Essential Oils.  For this reason our libraries stand apart and above all others.

Q: Why does FDM provide ATR libraries with spectra measured on both diamond and Germanium crystals?

A: Diamond and ZnSe both have a refractive index of 2.4. Germanium’s refractive index is 4.0, which is considered much higher. Compounds with functional groups with strong absorbances will also have high refractive index values. Compounds with refractive index values near or above 2.4 will produce spectra that appear distorted from anomolous dispersion in the vicinity on the strong absorbance on a diamond or ZnSe ATR crystal at 43°, and more so at 45°, angles of incidence.  Over the years people have tried to address this with ATR correction algorithms.  We believe the best approach is to record a spectrum on a Germanium ATR crystal.  Spectra recorded on a Germanium ATR crystal are more transmission-like than those recorded on a diamond ATR crytal and are superior to an algorithmic approach because they are authentic. That authenticity means your data is easy to defend as it is not derived from a public or proprietary algorithm or errors in the implementation of an algorithm. Germanium ATR crystals are about the same price as ZnSe crystals. Lastly, Germanium ATR crystals often provide superior baselines relative to diamond or ZnSe.

Q: What is meant by saying spectra run on a Germanium ATR crystal are more transmission-like than those recorded on diamond or Zinc selenide (ZnSe)?

A: Because of Germanium’s high refractive index the effect of ATR peak broadening from anomolous dispersion on the high frequency (high wavenumber) side of a peak is greatly reduced compared to diamond or ZnSe. Thus, the peaks are more transmission-like.