Get the principles and applications of ATR

Get the principles and applications of ATR

Introduction:  The full English name of ATR is Attenuated Total Reflectance. It can be used for the analysis and identification of liquids, pastes, powders, films and solids. This technology was put into use in the 1960s, but it was not widely used . It was not until its application in Fourier transform infrared spectroscopy (FT-IR) that it gained widespread application and rapid development.

Attenuated total reflection spectroscopy technology does not require any processing of the sample during the test process and will not cause damage to the sample. It overcomes the shortcomings of traditional transmission method testing, simplifies the sample production and processing process, and expands the application range of infrared spectroscopy. At present, this technology is widely used in various fields.

1. Basic principles of ATR

When a beam of monochromatic light is incident from one optical medium 1 into another optical medium 2 at an incident angle θ, the light will be reflected and refracted at the interface of the two optical media. When the refractive index n1 of medium 1 is greater than the refractive index n2 of medium 2, and the incident angle θ is greater than the critical angle θc (sinθc= n2/n1), total reflection will occur. When total reflection occurs, an attenuated reflected wave is generated at the inflection point of light, as shown in Figure 1. The distance when the amplitude of the attenuated reflected wave attenuates to 1/e of the original amplitude is called penetration depth (Dp). The depth of the penetrating wave is determined by the wavelength λ of the incident wave, the refractive index n1 of the crystal, the refractive index n2 of the sample, and the incident angle θ of light in the crystal. Dp is calculated as formula (1):

Dp = λ / 2πn1[sin2θ-(n2/n1)2]1/2   (1)

     

Attenuated total reflection ATR uses this technology to obtain the infrared spectrum of the sample. As shown in Figure 1, light undergoes total reflection on the ATR surface, and the reflected wave is attenuated according to the refractive index of the sample to a certain penetration depth. If the sample absorbs in the frequency region of the incident light, the intensity of the reflected light will be weakened at the frequency where the sample absorbs. It can produce spectra similar to ordinary transmission and absorption, so it can be used for qualitative and quantitative analysis of chemical composition.

2. Relevant properties of ATR crystals

In the experiment, we need to select the ATR crystal according to actual needs. The main factors to consider are refractive index, critical angle, incident angle, penetration depth, applicable spectral range, number of reflections, effective optical path, crystal acid and alkali resistance, crystal hardness, etc.

The refractive index of common organic compounds is around 1.5, and the refractive index of the ATR crystal is required to be greater than 1.5. Common ATR crystal materials include diamond (2.4), ZnSe (2.4), Ge (4.0), Si (3.4), KRS-5 (2.37), and AMTIR (2.5).

According to the critical angle calculation formula sinθc= n2/n1, we can get the critical angles of various crystals. Assuming the refractive index of the sample is 1.5, the critical angles of ATR crystals are as follows: diamond (38o), ZnSe (38o), Ge (22o), Si (26o), KRS-5 (39o), AMTIR (37o).

According to the penetration depth Dp = λ / 2πn1[sin2θ-(n2/n1)2]1/2, it can be seen that the greater the refractive index of the crystal, the shallower the penetration depth. The higher the refractive index of the sample, the deeper the penetration depth; the larger the incident angle θ, the shallower the penetration depth; the shorter the wavelength of light, the shallower the penetration depth (Figure 2). It can be seen that the infrared spectrum measured using the ATR accessory has different penetration depths at high wave numbers and low wave numbers. The peak intensity of the low wave number absorption peak is stronger than the peak intensity of the high wave number absorption peak. In order to compare with the ordinary transmission infrared spectrum, the spectrum measured by the ATR accessory needs to be corrected. Taking 1000 cm-1 as the benchmark, the correction formula is:

ATR = AB * ν[cm-1] / 1000 [cm-1] (2)

The number of reflections is related to the length l and thickness t of the ATR crystal and the incident angle θ (as shown in Figure 3). The calculation formula for the number of reflections is: N = l / 2t*tanθ. Then the effective optical path de = N* Dp.

Diamond has the highest hardness, is acid and alkali resistant and chemical corrosion resistant, and is widely used. However, it has strong absorption in the 1800-2300 cm-1 band, and samples with absorption in this area need to be avoided. The applicable spectral range of ZnSe crystal is 20000-650 cm-1, which is consistent with the testing of most samples. However, it is not resistant to acid and alkali, has low hardness, and is prone to scratches. The measurement spectrum range of Ge crystal is narrow, but its refractive index is high, and it can be used to measure high refractive index samples; KRS-5 has the widest spectrum measurement range, but it has the lowest hardness and is not corrosion-resistant, so it is rarely used.

The relevant parameters of ATR crystals in the literature are shown in the following :

Crystal	Refractive index	Critical angle	Spectral range(cm1)	Penetration depth(gm) 1000cm-1 ,45°	PH            scope	Hardness    Kg/mm
AMTIR	2.5	37°	11000-840	1.70	1-9	170
Diamond	2.4	38°	4500-2300 1800-400	2.01	1-14	5700
Ge	4.0	22°	5500-675	0.66	1-14	550
KRS-5	2.37	39°	20000-400	2.13	5-8	40
Si	3.4	26°	8300-1500	0.85	1-12	1150
Zn	2.4	38°	20000-650	2.01	5-9	120

3. Issues that need to be paid attention to when using ATR

(1)ATR cleaning

Before infrared spectrum sampling, the background spectrum needs to be collected first. To ensure that the ATR crystal surface is clean and not contaminated, the ATR crystal needs to be cleaned. Often use a soft cloth or cotton ball using solvents such as water, ethanol, acetone, etc. to wipe the surface of the ATR crystal.

(2)ATR sample preparation

ATR is suitable for testing solid and liquid samples. When measuring a solid sample, the solid sample needs to be in close contact with the ATR crystal. Therefore, the indenter of the ATR accessory needs to be used to press the sample against the surface of the ATR crystal so that the attenuated reflected wave can penetrate into the sample. When testing liquid samples, for samples that can be coated on the ATR crystal to form a liquid layer, they can be tested directly after being applied. For volatile samples with low boiling points, an ATR accessory with a liquid cell is required.

(3) ATR crystal maintenance

Different ATR crystals have different degrees of hardness, acid resistance, alkali resistance and chemical corrosion resistance. When using them, you should first understand their properties to prevent scratches, corrosion or even cracks, which will affect the quality of the spectrum or fail to obtain effective spectra.

4. Introduction to ATR attachment types

There are many types of ATR accessories according to user needs. The most common ATR accessories are single total reflection and multiple total reflection.

The principle of single reflection is shown in Figure above. The accessories of single total reflection ATR can be selected from pure diamond, Ge, and ZnSe. As shown in the figure, the upper surface of the ATR crystal is parallel to the stainless steel sample plate, and the bottom end is physically supported to make the sample easy to clean. Figure 5 is equipped with pure diamond ATR accessories, which can meet most usage scenarios. This accessory is also available as a heated ATR model, which can control the temperature range from room temperature to above 120°C.

              

In addition, we have designed an ATR accessory suitable for laboratory desktop Fourier transform infrared spectrometers, as shown in the figure. It can not only match the FOLI10-R and FOLI20 Fourier transform infrared spectrometers, and the FOLI30V vacuum Fourier transform infrared spectrometer, but also match the spectrometers produced by other manufacturers.

                  

The principle of multiple total reflection ATR is as shown above. FOLI10 can provide horizontal 3-time or 5-time reflection ZnSe crystal ATR accessories. The picture below shows FOLI10 using horizontal 5-time reflection ZnSe crystal ATR accessories, which is suitable for liquid testing and film testing.

              

In addition to the commonly used ATR accessories mentioned above, there are also some special ATR accessories. Such as high-pressure diamond ATR for high-pressure testing (left in the picture), single or multiple reflection ATR accessories with variable incident angles (middle and right in the picture), high-temperature and high-pressure ATR accessories, and low-temperature ATR accessories.

The fiber-coupled ATR probe accessory is an accessory often used in recent years to study chemical reaction processes and reaction mechanisms. This accessory inserts the probe into a container such as a reactor and can monitor the concentration changes of reactants and products in real time. Diamond crystals can withstand high temperatures and high pressures and are suitable for studying reaction processes under extreme conditions.

                   

5. ATR related applications

1. Detection and characterization of unknown compounds. The ATR test method is simple and the test results are fast. It is widely used in laboratory organic compound characterization and detection, drug identification, mixture detection, etc.
2. Study on reaction kinetics. The advantage of ATR is that it can be measured in situ and tracked in real time, which is beneficial to the study of reaction kinetics and reaction mechanisms of some compounds, such as the study of 2-Methylpropene polymerization reaction kinetics in polymerization reactions, online monitoring of fermentation processes, and light-induced heterogeneous oxidation .
3. Interface research. ATR mainly obtains the spectral information of the surface of the sample, so it is very conducive to the study of interfaces. Such as surfactant adsorption, organic coating, metal surface reaction, etc.
4. Research on polymer materials. For traditional transmission methods, polymer materials are difficult to prepare and have poor transmittance. ATR does not have such problems, so it has been widely used in the structural analysis of polymer materials such as plastics, fibers, rubber, coatings, and adhesives.
5. Quality inspection and control. ATR sample preparation is simple, easy to operate, and fast to test, making it an important tool and method for analysis and detection in the field of industrial production. Such as SBS quantification in modified asphalt, urea quantification in diesel engines, fatty acid methyl ester (FAME) quantification in diesel, non-destructive testing of textile fibers, leather material analysis, tar and nicotine content determination in cigarettes, and ethylene content determination in polypropylene. There are also quantitative analysis of pharmaceutical ingredients, wine quality control, essential oil classification, etc.