17 Nov 2021

Choosing the right ATR crystal

In the third and final part in our series by our Application Scientist, Andrew Davies, he explains the factors behind choosing the right ATR crystal and sample plate options. Although it was written with our high performance Quest™ ATR in mind, many of the observations here are true for ATR in general. Be sure to read parts one and two here and here.

Crystal Choice Considerations

When choosing a crystal, it is important to consider chemical compatibility, the required spectral range, and the refractive index of your crystal (nc) and sample (ns). Total internal reflection only occurs when the refractive index of the crystal is significantly larger than the medium it is in contact with. If ns is high then a crystal element with a higher refractive index is required. Many organic compounds have a refractive index ≈1.6 so any crystal with a refractive index greater than or equal to 2.4 is suitable. The Carbon Black case study, below, shows the effect of neglecting to consider ns when choosing your crystal.

Another factor to consider is the penetration depth of the evanescent wave into your sample. Penetration decreases with increasing nc [2]. To maximize signal intensity a crystal with a small nc should be selected (whilst ensuring that nc>>ns).

Diamond (C)

Diamond is an extremely durable material, with excellent mechanical and chemical resistance. It is now the most common choice for routine FTIR analysis for these reasons. Specac uses monolithic diamond rather than diamond wafer applied to a substrate (such as ZnSe) to prevent delamination issues and make available the full spectral range of diamond.

The refractive index of diamond makes it suitable for most samples and gives good signal. Samples with a higher ns will produce strong anomalous dispersion effects, resulting in the appearance of asymmetric peaks with a higher wavenumber dip below the baseline (in Absorbance mode – for transmission mode the feature will appear above the baseline). This effect is examined in more depth in the carbon black case study, below.
Diamond has a strong phonon band absorption between 2600-1900 cm-1 which increases noise in this region and reduces quantification accuracy. This is partially offset by using a small diamond to minimize the pathlength through the material.

Two different diamond options are available: standard diamond and the extended range diamond. The AR coating gives the standard diamond crystal a green-gold appearance when viewed from below, while the uncoated extended range diamond appears a transparent grey. This can be used for identification if there is any doubt. Alternatively, the far IR cut off can also be used.

Standard Diamond Puck

Our standard diamond puck has an anti-reflective coating maximizing the spectral throughput between 7800-400 cm-1. This gives it a superb signal-to-noise ratio (SNR). The standard diamond covers the range of most commercial FTIR spectrometers (typically 4000-400 cm-1 with a KBr beamsplitter).

Extended Range Puck

For applications requiring access to the far IR, an uncoated extended range puck is also available. The lack of AR coating comes as a cost to spectral throughput in the mid IR range. This crystal is suitable for use between 10000-10 cm-1.
 



Zinc Selenide (ZnSe)

ZnSe has a similar refractive index to diamond. The larger crystal size increases spectral throughput relative to the diamond puck and, as a result of these two factors, ZnSe has the highest SNR of any crystal material for the Quest. Additionally, ZnSe has no significant mid-IR peaks giving the best possible data throughout your spectrum with a usable range from 7,800 – 550 cm-1. ZnSe is sensitive to point loads so should only be used with softer materials and should be considered as semi-consumable. The sample pH should be kept within the range 5 to 9. Avoid acidic or strongly basic environments as ZnSe reacts to form toxic fumes under these conditions.



Our ZnSe crystals have an AR coating. Due to the cut-off of ZnSe at 550 cm-1 there is usually no advantage to not applying this coating. However, for the heated puck option this coating is not stable at elevated temperatures and therefore an uncoated ZnSe crystal is used instead.

Germanium (Ge)

Ge has no significant peaks in the mid-IR and has a window of 5500-500 cm-1. It has the highest refractive index out of all the commonly used ATR crystals making it ideally suited to studying materials with a high ns. For the same reason it is also suited to surface studies due to the lower effective penetration depth of the evanescent wave. The cost of this is reduced signal (and hence lower signal to noise) which is partially mitigated by a larger crystal size and an anti-reflective coating. At elevated temperatures it becomes optically opaque and so is unsuitable for use in heated applications. All Ge options in the Quest are AR coated.

Case Study: Carbon Black;

When choosing your ATR crystal, it is important to ensure the refractive index of your crystal (nc) is larger than the refractive index of the sample (ns). Most materials studied by ATR have an ns of ~1.6, suitable for study with any crystal. To maximise SNR choose a diamond or ZnSe puck (lower nc = greater penetration depth). For materials with a high nc or surface studies choose Ge or Si.

Why is nc important?

With increasing ns, the effective pathlength through the sample (and thus signal intensity) also increases. Around an adsorption band, ns increases, with a greater effect for more intense bands. In the most extreme cases, this can prevent internal reflection of light altogether (anomalous dispersion) causing severe band distortion. An example of this is given in Figure 1 showing the spectrum in the CH region of carbon black. With a diamond puck, severe distortions are observed whereas with Ge penetration depth is reduced and distortion mitigated.



Figure 1: Comparison of spectra of a sample with high ns (Carbon Black) recorded on a Diamond and Ge puck. Anomalous dispersion caused by nc≈ns causes severe band distortion when diamond is used (black line), whilst Ge (with nc >>ns) produces the expected spectrum (red line).
 

Silicon (Si)

Silicon offers an intermediate refractive index between diamond/ZnSe and Ge. This allows the user to fine tune the requirements to have nc >> ns whilst minimizing the decrease in signal from reduced penetration depth. It has two spectral windows from 8000-1350 cm-1 and 500-33 cm-1. There are strong phonon bands between 1350-500 cm-1 which obscure the fingerprint region.

Arrow™ (Consumable ATR slides)

Arrow™ is the world’s first consumable ATR option. The ATR crystal is made from a thin wafer of Si and has similar properties to the Si crystal puck. However, the thin wafer reduces the intensity of the phonon bands, which enables Arrow to be used for fingerprint studies. The cost of this is that the crystal cannot withstand a load so can only be used to investigate liquids, or solids that have been dried onto the crystal from a liquid solution or dispersion. Arrow is well suited to batch investigations. Hundreds of samples can be prepared away from the spectrometer and then spectra collected in rapid succession. Arrow is also perfect for studying aggressive chemicals which could cause damage to the crystal or puck metal work. 


Crystal Properties Summary [3]

  Diamond (Standard) Diamond (Extended Range) ZnSe Ge Si Arrow (Si)
Range (cm-1) 7800 to 400 10000 to 10 7800 to 500 5500 to 480 8000 to 1350
& 500 to 33
8000 To 33
Appearance (for identification) Transparent with green-gold hue Transparent grey Transparent green-gold Metallic grey Dark metallic grey Dark grey
nc @ 1000 cm-1 2.4 2.40 2.41 4.00 3.41 3.41
Depth of Penetration 2 μm 2 μm 2 μm 0.7 μm 0.9 μm 0.9 μm
Effective Pathlength†‡ 4.36 μm 4.36 μm 2.83 μm 0.52 μm 0.98 μm 0.98 μm
At 1000 cm-1, 45° angle of incidence and a ns =1.5. See [2] for formulae.
For a single reflection. For multiple reflections multiply by the number of bounces.

For more information on this topic please contact sales@specac.co.uk, who will forward your enquiry to our technical team.
 
References
[1] TN21-01 Basics of ATR Spectroscopy
[2] TN21-02 ATR Penetration Depth
[3] Handbook of Optical Constants of Solids, ed. Palik ISBN: 9780080523750