Webinar Recap: Transmission is still a proven technique for FT-IR

 About the presenter

Jeff boasts 35 years of expertise in the FT-IR spectrometer accessory field. He has delivered presentations at numerous OEM workshops and trade exhibitions and holds memberships in esteemed organizations such as Coblentz and the American Chemical Society.

Synopsis of the talk

While ATR is the “go-to” for most analyses nowadays, the method of transmission FTIR still has a role to play. In this webinar, Jeff D’Agostino draws on years of experience to review the main liquid, solid, and gas transmission techniques, along with some rationale for why transmission is sometimes the best option for a particular analysis.

• • What can an IR do?
  • Get to know liquid transmission
  • Get to know Solid transmission
  • Get to know gas analysis
  • Transmission vs ATR

During the presentation, we explored multiple illustrative examples to help you understand more about transmission.

View the webinar’s recording:

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Questions & Answers:

KBR questions

Q: How important is it to use a vacuum when pressing KBr pellets?

Specac’s evacuable pellet dies have a fitting for a vacuum pump to be attached. The basic premise is that removing excess air from the die during compression helps to bind the particles together better, while also removing any excess moisture from the material.

You don’t need to use a vacuum – it is possible to produce adequate pellets without this and you will still get great results. But you may find that your KBr pellets have a cloudier appearance due to microscopic inclusions of air or moisture. Any moisture will also be visible as a broad peak in the region of 3400 cm^-1.

In fact, when using the Mini-Pellet™ press pellet you don’t have the option for vacuum.

Q: What’s the best sample to KBr ratio for a pellet.

This ultimately depends on how highly absorbing the sample is. Samples that absorb more light will need more dilution in the KBr, whereas less absorbing samples will require less.

A good rule of thumb is to start at a ratio of about 1% sample by weight. Run a test spectrum of the 1% pellet and then adjust up or down as required.

Bear in mind that any peaks that will be used for quantitation need to be under around 1.0 absorbance units in intensity. The reason is that the detector’s response becomes nonlinear at higher intensities and is therefore less reliable for quantification.

Q: If I don’t have dry KBr available, is it possible to do a quick grinding and drying procedure?

Yes. You can try using a heat lamp to dry the mixed KBr sample in the mortar and pestle for 10-15 minutes. It won’t perfectly dry the mixture, but it will reduce any resulting cloudiness a little bit.

Q: What’s better for analysing micro- and nano-plastics – FTIR or Raman spectroscopy?

The two techniques are complementary. It depends on what you’re looking at and how quickly you need to collect data.

Most Raman instruments are microscopes and are therefore able to pick out small particles easily, but larger particles (down to a few hundred microns) are possible to analyse directly on an ATR instrument. With an IR microscope, such as the in-compartment SurveyIR accessory, you can analyse samples down to tens of microns in size.

It generally takes a little bit longer to collect a Raman spectrum of a target particle since the amount of signal returning to the detector is very small and needs some time to integrate. FTIR is faster and that can be an advantage.

Q: What’s happening to germanium material when you heat it, so that its transparency to IR light decreases?

Germanium is a very versatile optical material with good mid-IR transmission properties, but its transparency reduces at higher temperatures. We showed a graph of this effect in the webinar.

The reasons are a little beyond the scope of what we normally consider while doing FTIR spectroscopy, but we dug into it a little bit further. Thermal energy is absorbed by free electrons in the material whose energies fall within the bandgap between the valence and conduction bands [1]. This increases the refractive index of the material, which inhibits the transmission of light through the material and increases its reflectivity [2]. We’ll be over stretching ourselves if we go into any more detail than that, but you can check out the references below for more information!

An interesting result in our poll!

During the session we polled our audience on the “mull” technique.

For those who don’t know, a mull is created by grinding and mixing a powder sample with a petroleum-based oil such as Nujol®. The resulting liquid—or mull—can be used in a liquid transmission cell for analysis.

Since the oil itself has an IR spectrum, only regions of the spectrum where there are no peaks due to the oil may be interpreted as coming wholly from the sample powders.

As a technique it has long since been surpassed by ATR in most cases, so it was surprising to find that some of our audience are still using it – and recently too!

The webinar is available on-demand HERE for anyone who missed it.

[1]https://www.crystran.co.uk/optical-materials/germanium-ge

[2]https://www.elantechnology.com/wp-content/uploads/2018/03/Elan-Technology-Refractive-Index.pdf