Compared with the traditional transmission potassium bromide tablet pressing method, attenuated total reflection (ATR) accessories have the advantages of no complicated sample preparation process, simple operation, and short time consumption, making them increasingly popular among users. InInfrared Attachment | Introduction to the Structure of ATR AttachmentIn the article, we talked about how to choose the appropriate crystal for ATR accessories, as there are different crystals available? Today we will talk about the selection of ATR crystals in practical applications.

Most chemistry students have used oil bath heating pots, and the oil in them is usually silicone oil. The infrared absorption of Si-H bond stretching vibration in silicone oil is generally between 2000-2300cm^–1If we use commonly used diamond crystal ATR accessories for testing, we will obtain the red spectrum in Figure 1, which is displayed between 2000-2300cm^–1No infrared absorption peak of Si-H bond stretching vibration was observed. This is because diamond has strong absorption in this range, making it unusable. Generally, data processing is chosen to make this area consistent with the baseline. If you want to observe the absorption peak in this region, you should use other crystals. The blue spectrum in Figure 1 (with the baseline shifted up for comparison) was obtained by testing the ATR attachment of zinc selenide crystals, and the infrared absorption peak of Si-H bond stretching vibration can be observed. When modifying silicone oil to become hydrogen free silicone oil, we can judge the degree of conversion by this absorption peak.

Figure 1 ATR spectrum of silicone oil

Red: Diamond crystal; Blue: Zinc Selenide Crystal

In addition to the absorption peak of Si-H bond stretching vibration in this region, there are also absorption peaks of isocyanato-NCO groups and cyano-CN groups in this region. Testing these using ATR attachments at 2000-2300cm^–1Diamond crystal ATR attachments cannot be used for samples with absorption peaks.

In the previous article introducing ATR attachments, we also mentioned other crystals, so why choose zinc selenide crystals? Have you ever thought about this question.

The answer is also mentioned in our article introduction. For example, the infrared absorption peak of Si-H bond stretching vibration can be observed in the ATR of germanium and silicon crystals. However, the refractive index of these two crystals is high, resulting in shallow incident depth, and the absorption peak is weaker at this time. In Figure 1, we can observe that the infrared absorption intensity of Si-H bond is average, and the ATR absorption peak of germanium and silicon crystals is weaker. When modifying silicone oil to become hydrogen free silicone oil, it becomes even more difficult to quantitatively analyze when the content decreases.