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Transmission vs ATR spectroscopy | Animated Guides

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What are the differences between using ATR spectroscopy and transmission spectroscopy for analysis and which option is better? Read this article to find out.

Which is better for spectroscopic analysis, transmission or ATR?

ATR | Attenuated Reflectance Spectroscopy

For the ATR technique, IR radiation passes through an Internal Reflection Element (IRE) crystal material. The IRE crystal surface is in contact with a liquid or solid sample species to be measured and as the IR light passes through the IRE crystal it interacts at the crystal/sample interface.  For many ATR measurements the IR light interacts with the sample at a 45 degrees incident angle to the crystal/sample interface.

IRE crystals with a high refractive index (RI) of 2.4 or higher at 1000cm-1 (e.g. zinc selenide, diamond, germanium) are needed for the ATR technique. Typically, at a 45 degrees incident angle, light radiation penetrates the sample to a depth of circa 1 micrometer, where it can be absorbed. 

Transmission Spectroscopy

However, for the transmission technique, the incident radiation does not pass through an IRE crystal to interact with a sample species. IR light passes directly into a solid, liquid or gaseous sample where a specific wavelength or frequency of light will be absorbed by the sample.

IRE crystals with a high refractive index (RI) of 2.4 or higher at 1000cm-1 (e.g. zinc selenide, diamond, germanium) are needed for the ATR technique. Typically, at a 45 degrees incident angle, light radiation penetrates the sample to a depth of circa 1 micrometer, where it can be absorbed. 

Transmission Spectroscopy

However, for the transmission technique, the incident radiation does not pass through an IRE crystal to interact with a sample species. IR light passes directly into a solid, liquid or gaseous sample where a specific wavelength or frequency of light will be absorbed by the sample.

Any frequency of light not absorbed by the sample will be transmitted through the sample to a detector, provided the sample is not too thick for complete absorption of all the frequencies of light in the range. 

Sample Preparation Methods

There are different ways in which sample is prepared for both techniques. 

It may be necessary to carry out some form of prior sample preparation procedure when measuring a sample using a transmission technique. For transmission if the sample is a solid, KBr pellets are usually used. NaCl or CaF2 windows are typically used when liquids are analyzed through transmission.

However, when using the ATR technique, the sample preparation is usually much simpler and quicker. Either a solid or liquid sample can be directly loaded onto the IRE crystal. Liquid samples by their nature invariably make good and consistent contact against an IRE crystal surface, but solid samples need to be brought into better contact for the ATR measurement to work.

A clamping arm with a pressing anvil on an ATR accessory applies pressure to solids for uniform contact with the IRE crystal and the force and subsequent pressure can be controlled.

Spectra Comparison | ATR vs Transmission

When comparing spectra taken using ATR and transmission, there are slight differences between them. Resulting spectral peaks from an ATR spectrum are very similar in intensity and frequency positions to a spectrum taken using the transmittance technique, however there are small variations in peak intensity and peak position.  (e.g. carbonyl band absorptions show ATR peak shift). 

The shift is due to the amount of reflected radiation which is dependent on the differing indexes of refraction of the IRE crystal and sample at different frequencies of interacting light. Anomalous dispersion occurs at specific frequencies of absorption resulting in a change of refractive index of sample and in penetration depth radiation into sample. Thus, peak shifts are optical effects due to changes in refractive index. 

Advantages & Disadvantages of Transmission

Transmission advantage 1: high quality spectra

The transmission technique offers the possibility in sample measurement that any FTIR spectra obtained is likely to be of high quality and is satisfactory for qualitative analysis determination with a wide variety of spectral libraries available to compare results.

The main drawbacks for the results obtainable with use of the transmission technique mainly lie within the sample preparation procedure.

Transmission disadvantage 1: KBr pellets can be tricky

When using KBr pellets for solid sample measurements, spectral quality depends on the pellet thickness and for the sample particles to be of a uniform and consistent size dispersed throughout the KBr matrix for correct interaction and absorption of the light radiation to avoid light scattering. KBr is hygroscopic by nature and the quality of any KBr pellet to be made may be affected by any uptake of moisture. 

KBr pellets are a well-established practice for IR spectral transmission analysis, but the overall results are dependent on the way that a KBr pellet can be made. Experience is key.  

Transmission disadvantage 2: water can dissolve NaCl windows

When using NaCl windows for liquid samples, any water content in the sample will cause the NaCl windows to fog and potentially dissolve. Hence, it may be necessary to use an alternative water tolerant window material such as CaF2 for specific liquid sample types.

Transmission disadvantage 3: air bubbles disrupt liquid analysis

When liquid cells are filled with a liquid sample, the solution must be free of air bubbles to avoid spurious light interaction absorption effects. If a liquid cell is not constructed consistently between samples, then this can lead to the collection of IR spectra that are not particularly reproducible. In addition, it can sometimes be difficult to fully recover a liquid solution from a short pathlength (less than 25 microns), liquid cell assembly.

Advantages & Disadvantages of ATR

When compared to the transmission technique, the ATR technique has several advantages. 

ATR advantage 1: a quick and flexible technique

It enables a quick and easy way to obtain high quality IR spectra for qualitative analysis from (usually), minimal sample preparation. The ATR technique is sample non-destructive with a potential for easier sample recovery than when using a solid or liquid transmission technique. Overall, when compared to a method of sampling using the transmission technique, ATR can be considered as time efficient. 

The ATR technique for the collection of IR spectra is not affected by a sample thickness that is crucial to the transmission technique being employed. The light radiation for interaction with a sample penetrates only a few micrometers into the surface of a sample species.

ATR advantage 2: results are more reproducible

ATR can also provide highly reproducible results for a wide variety of sample types such as threads, yarns, fabrics, fibres, pastes, powders, suspensions, polymers and rubbers. which may otherwise be difficult and/or time consuming to sample using a transmisison technique.

Of course, there are less ATR spectral libraries than existing tranmission spectral libraries to consult and compare for any ATR spectrum that are taken, which may be considered as a limitation of the ATR technique. However, ATR users can create libraries of their own to best compare for their sampling applications should this be necessary.

Conclusion 

The transmission technique for a spectral sample measurement has been used with FTIR spectroscopy from its introduction. The ATR technique has developed over the past 30 years or so for its adoption in use with the development of more energy efficient and powerful FTIR spectrometers.

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