Low-Cost Infrared Microspectroscopy
Figure 4: Normalized signal-to-noise Improvement in FTIR technology (Perkin Elmer FTIR spectrometers).
microspectroscopy involve contaminant identification, and in the authors’ 33 years of experience, the vast majority of con- taminants studied have been 50 micrometers in size or larger. Although many advancements have been made in the field
of FTIR spectroscopy in the last 30 years, the basic design and operating principles of the infrared microscope have generally remained unchanged. Te majority of improvements have been made in the interferometers to which microscopes have been coupled. Specifically, developments in audio digital electronics dealing with signal processing have greatly improved the SNRs achieved by commercial instruments [11]. Figure 4 illustrates the normalized improvement in signal-to-noise performance from 1984 to 2014 for selected FTIR instruments. Te outlier in this plot is the Perkin Elmer Spectrum GX, which at the time was a very high-end research-grade FTIR. Along with advances in audio digital electronics came
advances in array detectors (charged couple device [CCD] and now complementary metal-oxide semiconductor [CMOS]) used for image capture and documentation. Today high-qual- ity image capture cameras and soſtware can be obtained for less than a few hundred dollars. Finally, essentially over the same time period, major advances have been made in attenu- ated total internal reflection (ATR) accessories. Tese advances were the result of 1) the ability to produce high-quality, sin- gle-crystal synthetic diamonds for use as internal reflection elements and 2) improvements in optical fabrication using diamond-turning processes to produce precise, yet complex, optical surfaces. Today ATR measurements dominate the field, which includes ATR microscopes. While many improvements have been made, one issue
that is seldom addressed is cost. Most high-end microscope systems of today range in price between $100K to $250K and offer standard transmission, reflection, and ATR sampling modes. Many higher-priced microscope systems also offer
2020 March •
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Figure 5: Photograph of the SurveyIR 5X in the sample compartment of the Frontier FTIR.
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mapping and imaging functions in these modes. With the increased per- formance of modern FTIRs and the need to reduce cost, Czitek developed the SurveyIR 5X microscope, signifi- cantly reducing the end-user cost over other systems. Te people respon- sible for the development of this microscope include Gregg Ressler, the principal engineer for the Spec- tra Tech IR-Plan microscopes, and David Schiering, who was involved in the development of the Perkin Elmer microscopes of the early 1990s. Te SurveyIR 5X can be placed directly into the sample compartment of an FTIR and employs either the standard instrument mounted DTGS detector or a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector. Te sampling modes include infrared transmission, reflection, and ATR, along with transmission, reflection, and oblique white light illumination with image capture. We illustrate the
use of this microscope by evaluating its signal-to-noise perfor- mance relative to a high-end microscope and its ability to char- acterize small particle contaminants.
Materials and Methods All spectra were collected using a SurveyIR 5X microscope
accessory produced by Czitek interfaced to a Perkin Elmer Frontier FTIR. Detectors used in the evaluation included the standard DTGS detector and a liquid-nitrogen-cooled MCT detector. ATR measurements were made using the diamond internal reflection element (IRE) integral to the microscope. A
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