Optimizing Spectral Data Using a Variable Angle ATR

Here we explore the benefits of using a variable angle attenuated total reflection (ATR) accessory to generate the most informative spectral data. Example spectra of toluene are shown.

The appearance of an ATR spectrum is influenced by several variables including the refractive index of the ATR crystal (n₁) and sample (n₂), the number of internal reflections, the quality of contact between the sample and the angle of incidence (AOI) of the infrared (IR) beam. Having the flexibility to change some parameters, specifically the AOI and ATR crystal type, allows optimization of spectral data to obtain the most useful information for the application.

Materials and Methods

Mid-infrared spectra of toluene were collected using the variable angle, multiple reflection ARTMax II accessory (PIKE Technologies) configured with an AMTIR trough plate. With the ATRMax II, the AOI may be adjusted by turning the center mirror assembly position dial, where the resulting effective AOI is a function of the face angle of the AMTIR crystal and the accessory position dial setting. For data presented, the face angle of the AMTIR crystal was 45^o. Spectra were collected at several AOI values. 30 s of data were co-added for each measurement at a resolution of 4cm^-1.

Results

Most single reflection and multiple reflection ATR accessories have a fixed nominal AOI of 45^o. When using these accessories to measure samples with a refractive index of roughly 1.5, the depth of penetration (d_p) is about 2 microns at 1000 cm^-1 for a diamond or ZnSe ATR element and 0.6 microns for a Ge ATR element. d_p decreases at shorter wavelengths and increases at longer wavelengths.

For total internal reflection to occur, the AOI must be greater than the critical angle (θ), which is defined as θ =sin^-1(n₂/n₁). d_p decreases with increasing AOI while d_p increases exponentially as the angle of incidence nears the θ_c. By probing deeper into the sample—making measurements closer θ_c—the magnitude of the absorbance bands increases significantly. By using a variable angle ATR accessory such as the PIKE Technologies’ VeeMAX III single reflection or the multiple reflection ATRMax II, the absorbance band strength may be optimized to provide maximum chemical information by easily changing the AOI, whether the application requires probing more deeply into the sample by making measurements closer to θ_c or probing closer to the surface at grazing AOI.

Unlike a variable angle, single reflection ATR where only the d_p is changing with varying the AOI, the number of reflections in addition to the d_p is changing when using a variable angle, multiple reflection accessory. Figure 1 shows toluene spectra collected at 43^o and 50^o effective AOI where the number of reflections on the sample is 7 and 5, respectively. The spectrum collected at 43^o shows absorbance bands over 50% greater than the spectrum collected at 50^o. Therefore, the increase in absorbance band strength is because of two contributing factors, increased d_p and the number of reflections. In addition, changing the ATR crystal material provides added flexibility to accommodate various ATR probing depths into the sample.

Figure 1: Toluene spectra collected at an effective angle of incidence of 43^o (blue) and 50^o (red) using the multiple reflection ATRMax II.

Conclusion

Variable angle ATR accessories can optimize spectral data for a given application. By simply changing the angle of incidence, it is possible to probe more shallow or deeper into the sample, as needed.

PIKE Technologies, Inc.

6125 Cottonwood Drive, Madison, WI 53719

Tel. (608) 274-2721, Fax (608) 274-0103

Website: www.piketech.com