Hollow Waveguides: The Next Generation of Mid-IR Remote Sampling Accessories

Fourier transform–infrared (FT-IR) remote sampling offers a viable solution for the analysis of samples that are not conducive to the use of traditional FT-IR sampling accessories due to constraints imposed by the size of a benchtop instrument's sample compartment or the nature of the sample. Taking an IR probe to a sample is desirable in numerous applications and a necessity in others. Examples of remote IR sampling can be found across many fields. In the fine arts, priceless paintings and artifacts can be analyzed with limited sample handling. Mid-IR analysis of intractable samples such as large painted panels can be conducted. Biomedical applications encompass soft tissue and skin analysis. In chemical production, remote sampling allows for reaction monitoring and the analysis of samples restricted to glove boxes and fume hoods.

The implementation of hollow waveguides in mid-IR remote sampling accessories provides an alternative to traditional mid-IR optical fibers. The most popular type of hollow waveguides consists of reflective-coated silica tubing (Figure 1). The inner portion of silica tubing is coated with Ag, followed by converting of some of the Ag to AgI to form a dielectric layer, which exhibits a highly reflective and a very smooth surface. The exterior of the silica tubing is coated with acrylate polymer to provide additional strength. Until now, hollow waveguides have found limited use in commercial spectroscopy. A hollow waveguide has been used as an alternative to a mirrored gas cell by introducing both the analysis gas and the IR beam into the hollow waveguide (1). Also, hollow waveguides have been used as CO₂ and Er:YAG laser delivery vehicles and as a bidirectional transmission device for the delivery and collection of Raman scattering (2).

Figure 1: Schematic of a hollow waveguide.

In one sampling accessory (Mid-IR FlexIR, Pike Technologies, Madison, Wisconsin), hollow waveguides are used to overcome many of the limitations found with the use of traditional chalcogenide glass and silver halide polycrystalline fibers, which were developed in the 1960s and mid-1970s, respectively. Minimal performance improvements of these fibers have been made over the past 30 years. Chalcogenide fibers exhibit a strong absorption band located near 2170 cm^-1 due to S-H or Se-H bonds. As a consequence, the signal-to-noise ratio (SNR) in this spectral region is significantly decreased (3). To address this issue, two fiber types, chalcogenide and silver halide, often are employed to generate a full spectral range. The chalcogenide fiber generally used in mid-IR spectroscopy covers approximately 6500–2250 cm^-1 and 2050–1000 cm^-1 while the silver halide fiber covers 2100–600 cm^-1. In contrast, hollow waveguides are capable of spanning a spectral range from 11,000 to 400 cm^-1, eliminating the need for a complementary fiber set.

The durability of traditional fibers has been a concern and a hindrance in past mid-IR remote sampling accessories. Additionally, intrinsic flaws originating during the manufacture of glass fibers significantly increase fiber fragility, and often can result in catastrophic failure under routine application use (4). Furthermore, the bend radius of traditional fibers is limited. Contrary to these properties, hollow waveguides offer a robust means of delivering and collecting IR radiation and offer a smaller bend radius.

To illustrate the diverse capabilities of the newest technology in mid-IR remote sampling, three application examples will be presented highlighting the using attenuated total reflectance (ATR), diffuse reflectance, and specular reflectance sampling probes.

Instrumentation

The Mid-IR FlexIR hollow waveguide accessory (Pike Technologies) was equipped with an integrated mercury-cadmium-telluride (MCT) detector for highest sensitivity and fast data collection rates. The spectral range for the narrow-band MCT detector used in the collection of the spectra presented is 5000–700 cm^-1. To expand the spectral range, the system also can be equipped with a deuterated triglycine sulfate (DTGS) or broadband MCT detector. The optical base has been optimized for energy throughput by incorporating diamond-turned optics into an efficient design. Available probes cover several different mid-IR sampling techniques: ATR, diffuse reflectance, and specular reflectance. The sample probe is permanently aligned to the hollow waveguide for consistent analysis results, and the fiber length is 1 m. Three different ATR crystal types are available, including zinc selenide (ZnSe), germanium, and diamond/zinc selenide composite.

All spectral data presented were collected at a resolution of 4 cm^-1 and using a 20-s collection time for both the background and sample spectrum.

Biomedical Application — ATR Probe: The simplicity of ATR sampling has led to its use in numerous biomedical applications. Confining the ATR crystal sampling surface to an FT-IR sample compartment limits in vivo studies. Remote ATR sampling, however, expands the flexibility of FT-IR studies and applications in this field. For example, remote ATR sampling makes it possible to investigate chemical diffusion through the skin, residual chemicals retained on the skin from body lotions and washes, and skin aberrations.

The objective of this biomedical application was to investigate residual chemicals found on human skin after the application of a commercially available sunscreen spray. A spectrum was collected before and after the application of the skin care product using an ATR probe (FlexIR ZnSe, Pike Technologies).

Spectral data of untreated skin clearly shows the IR chemical signature of skin including the amide I and amide II bands at 1650 and 1550 cm^-1, respectively. The result from spectral subtraction allows for the investigation of the sunscreen chemicals remaining on the skin (Figure 2). The ability to collect in vivo data allows for the optimization of formulations and the study of time-based efficacy of existing and new products.

Figure 2: Untreated and sun-screen treated skin spectra collected using the Mid-IR FlexIR accessory configured with a ZnSe probe.

Intractable Panels — Diffuse Reflectance and Specular Reflectance: One method of mid-IR analysis of intractable objects is with an in-compartment accessory via abrasion testing, where sample particles are collected by rubbing an area of the original sample with an abrasion disk, silicon carbide, or diamond. Then the abrasion disk with embedded sample scrapings is analyzed using a diffuse reflectance accessory (5). The abrasion disk method has drawbacks as it scratches the surface of the primary sample and can alter the chemical nature of the native sample through the abrasion process. An alternative nondestructive sampling technique such as remote sampling for intractable samples is highly desirable.

Reflective type measurements can be classified as either diffuse or specular. Smooth coatings and thin films on reflective substrates are candidates for specular reflectance measurements. Using this sampling technique, the reflected beam from the sample is collected at an angle of incidence equal to that of the incoming beam as it is delivered to the sample. Diffuse samples scatter the reflected beam across a wide range of angles and in IR sampling must be gathered using a collection optic.

To illustrate the nondestructive mid-IR testing using remote sampling, two intractable samples were analyzed. One sample consisted of a coating on a smooth reflective surface, conducive to specular reflectance measurements. The other sample type had a painted diffuse surface.

Figure 3: Spectrum of a coating on aluminum collected using the Mid-IR FlexIR accessory configured with a specular reflectance probe.

Figure 3 shows the spectrum of a coating on a smooth reflective surface obtained by using the system configured with a specular reflectance probe, and Figure 4 shows spectra of painted diffuse panels collected with the diffuse reflectance probe. The two diffuse painted panels clearly show differing chemical properties. In each sample spectrum, the high SNR results in quality spectra. The spot size of both probes is 2.5 mm in diameter. The test area for these probes allows for measurements of small defects. Remote sampling provides a convenient method of nondestructive analysis.

Figure 4: Spectra of intractable panels with a diffuse finish collected using the Mid-IR FlexIR accessory configured with a diffuse reflectance probe.

Conclusions

Hollow waveguides enhance mid-IR remote-sampling accessories. The diversity of sampling probes covering ATR, diffuse reflectance, and specular reflectance used in conjunction with hollow waveguides and high precision optics offers the ability to collect quality spectra of a wide range of samples, which can be prohibited with traditional in-compartment FT-IR sampling accessories.

Jenni L. Briggs is with PIKE Technologies, Madison, Wisconsin.

References

(1) R.H. Kozodoy et al., Appl. Spectrosc. 50(3), 415–417 (1996).

(2) Y. Komachi et al., Opt. Lett. 30(21), 2942–2944 (2005).

(3) J.A. Harrington, Infrared Fiber Optics. Retrieved from http://irfibers.rutgers.edu/pdf_files/ir_fiber_review.pdf

(4) C.D. Rabii and J.A. Harrington, Opt. Eng. 38(9), 1490–1498 (1999).

(5) PIKE Technologies. n.d. Diffuse Reflectance – Theory and Applications. Retrieved from http://www.piketech.com/technical/application-pdfs/Diffuse_Theory&Appl.pdf