Signal Enhancement Technique for Internal Reflection Spectroscopy

S T E P H E N R. L O W R Y , T I M O T H Y E. MAY,* and A H A R O N B O R N S T E I N Spectroscopy Research Center, Nicolet Instrument Corporation, 5225 Verona Road, Madison, Wisconsin 53711

The use of chalcogenide glass for internal reflection spectroscopy (IRS) is described. Chalcogenide glass is a material normally used in infrared fiber-optic systems. A key feature of this concept is the ability to create a tapered section in the IRS crystal in order to increase the interaction with the sample. A liquid cell with integral fiber is described, and its performance as an IRS element is discussed. Several features of the fiber and cell bolder are shown to increase sensitivity and allow for an in- expensive sampling accessory.

Index Headings: Infrared; Internal reflection spectroscopy; ATR spec- troscopy, fiber-optic sensor.

glass allows normal fiber pulling technology to be utilized in making the micro-IRS "crystals."

In this paper we will describe a new infrared internal reflection accessory and sampling method that provides good spectral data for sample amounts of less than 100 #L. The key design goals of the device were (1) small sample volume, (2) chemical resistance, (3) good sensi- tivity, and (4) low cost. We will discuss the advantages of the design and report on several experiments that were performed to evaluate the accessory's performance.

INTRODUCTION

Internal reflection spectroscopy (IRS) has been an ac- cepted infrared spectral analysis technique for over twenty years. Many of the initial applications involved the analysis of polymer films and other solids that pro- vided good surface contact with the IRS crystal. The high chemical resistance of many IRS crystal materials allowed aqueous and other difficult solutions to be easily analyzed. In recent years a number of accessories have been developed for the analysis of liquids by internal reflection spectroscopy. 1 While IRS does not provide the accurate pathlength information that is available with normal transmission spectroscopy, it does provide a sig- nificant improvement in sample handling. In many cases, where the sample is strongly absorbing, the pathlength in a liquid transmittance cell must be extremely short. This can make sample loading and cleaning of the cell very difficult, especially if the sample has a high viscosity or contains particulates.

Internal reflection spectroscopy has been proposed as a good technique for identifying unknown materials that are frequently found during the remedial cleanup of waste dump sites. 2 In this application very little is known about the samples, but they often consist of complex organic mixtures that contain water and corrosive materials. Be- cause of the potentially toxic nature of this application, the sampling techniques should require minimal amounts of sample and the system should be easily cleaned, or better yet disposable.

Recent advances in mid-IR fiber-optic materials have demonstrated that chalcogenide glasses can be used to transmit a broad spectrum of infrared radiation. 3-6 For normal fiber-optic applications, the high refractive index of the chalcogenide glass creates a problem of surface reflection losses in the mid-IR. However, this same high refractive index provides for excellent internal reflection properties. The fact that the chalcogenide material is a

Received 27 August 1992. * Author to whom correspondence should be sent.

EXPERIMENTAL

Initial experiments using small pieces of chalcogenide (2.5 cm long and 400 #m in diameter) for internal re- flection spectroscopy were promising, but the sensitivity was not adequate for measuring mixtures. In these ex- periments the ends of the fiber were polished flat. Most commercially available cylindrical IRS crystals use a con- ical point at the ends of the crystal to increase the angle of incidence of the incoming light and thereby increase sensitivity. 7 A significant cost reduction can be obtained by eliminating the need for the end cones on the crystal. Our experiments indicated that some method of increas- ing the angle of incidence of the light was required. Be- cause the chalcogenide material is a glass, it is possible to create a taper in these fibers by simply heating the fiber and pulling it a very precise distance. This taper creates an area where the infrared radiation is concen- trated at a higher angle of incidence, and therefore the evanescent wave interacts much more strongly with any material surrounding the tapered portion of the fiber. The taper effectively creates a longer pathlength, which increases sensitivity. The smaller surface area of the ta- per also reduces the amount of sample necessary to ob- tain a spectrum. An optical diagram of the cell is shown in Fig. 1.

Cell Design. Because of the goal to make a low-cost sample cell, polypropylene was molded in two parts, with the tapered piece of the chalcogenide fiber mounted in the middle. The cell is sealed with epoxy, and the ends of the fiber are polished to improve energy throughput. The present cell is a "boat" design with an internal vol- ume of 80-100 #L. A small, sealable, snap top has been designed for the cell to allow the analysis of volatile materials. The cell, called a FiberCell@ (Isorad Ltd, Yavne, Israel) is shown in Fig. 2. The present cell was designed for easy handling and flexibility. Preliminary studies with small drops of material indicate that most of the signal is derived from the sample in the tapered region. Modifications to the plastic cell could reduce the internal volume significantly, but the same sample-han- dling problems that occur with the transmittance cells

30 Volume 47, Number 1, 1993 o0o3-7o28/93/47Ol-OO3052.oo/o APPLIED SPECTROSCOPY © 1993 Society for Applied Spectroscopy

SLOT OPENING

FIBERCELL CONCENTRATOR CONCENTRATOR

FIBER

SAMPLE CAVITY

J

CELL BODY

PATH OF INFRARED RAY THROUGH CELL

Fio. 1. A light ray path through the tapered fiber. Successive internal reflections increase the angle and number of bounces at the surface of the fiber. Concentrator mirrors are mounted in the holder and aligned to cell ends.

are encountered. A flow-through design based on liquid chromatography fittings which should overcome some of the problems of very small cell volumes has been pro- posed.

For the best results, a background spectrum is taken with the empty cell before the sample is inserted. The sample is then introduced into the cell, and a second single-beam spectrum is acquired. This spectrum is ra- tioed to the background spectrum to produce a relative transmittance spectrum which can then be converted to absorbance. This process eliminates the small spectral features caused by the optical fiber and the cell which could cause confusion when one is analyzing trace com- ponents in mixtures. While the overall spectral features are similar to those observed with transmittance spec- troscopy, the actual peak heights and shapes may vary. In IRS types of measurements, the long-wavelength end of the spectrum has an enhanced response due to the deeper penetration depth. The long-wavelength absorp- tion peaks will have higher values relative to the short- wavelength peaks when compared to transmission-type measurements, and band shapes can be affected on the long-wavelength side. Infrared fibers have been used for IRS measurements in a number of situations, and several techniques, such as coating of the fiber, were used to enhance sensitivity, s-t4

The peak heights in this spectrum are directly related to the amount of taper in the chalcogenide fiber. Since this taper can be controlled by the pulling process, it is possible to produce a cell with larger absorbance peaks (greater signal). Narrowing the taper brings the light closer to the critical angle and increases sensitivity. How- ever, as the diameter of the taper is decreased, the total energy through the cell decreases and the relative noise level increases. This pattern indicates that there is a trade-off between increasing the signal by a sharper taper and increasing the noise. Another design constraint is the breakage problems that occur if the tapered section is too thin.

In this work, the cell was designed to provide a peak height of about one "absorbance unit" for the C-O stretching band in pure isopropyl alcohol (see Fig. 3). This arrangement should provide good qualitative in- formation on most organic materials. The critical step in producing the cell is minimizing the variability in the taper dimensions. A special apparatus was designed to provide a mechanism for producing tapered fiber sections

CELL DIAGRAM (not to scale)

FIG. 2. Diagram of the FiberCell@. The 80-~L sample volume sur I rounds the fiber. A cap (not shown) can seal volatile liquids for longer measurement times.

with consistent performance. This device accurately con- trols the heating and the force applied to the fiber during the stretching process.

Accessory Design. The FiberCell@ was designed to be used as a normal infrared sampling device that can be mounted in the sample compartment of a laboratory in- frared spectrometer. Because of the small diameter of the fiber, it was necessary to design a beam condenser to increase energy throughput. The FiberCell@ is mount- ed in an accessory interface which is made up of two beam condensers which direct the light from the spec- trometer into the FiberCell@ and then out to the instru- ment's detector. The initial accessory used two elliptical mirrors and four flat mirrors to focus the light on the FiberCell@ and to then re-focus the light for the detector. While the accessory was effective, it was very difficult to align, and the large number of mirrors reduced the ef- ficiency and increased the cost. A second accessory was designed that improved the efficiency and reduced the cost. The beam condensing elements in this accessory consist of parabolic nonimaging concentrators (NICs). The NIC takes the light coming into its entrance aperture and relays it to the exit aperture. The NIC is a special reflecting cone in which light rays within a certain angle will miss the walls going to the exit aperture, and light rays at larger angles, up to the acceptance angle, will make one or more wall reflections. The optical charac- teristics of the concentrators in this accessory have been

FIBER CELL r,J

m

D

~ooo agoo 3~oo

ISPROFANOL

z6oo a~oo z 6 o o l @ O O l~OO NAVENUMBER

6oo

FIG. 3. Spectrum of isopropanol obtained by the FiberCell@. The fiber is adjusted for a response of 1 a.u. at the 953-cm -1 peak.

APPLIED SPECTROSCOPY 31

O

FIBER CELL MCTR DETECTOR

4 0 0 0 3 6 0 0 3 2 0 0 2 8 0 0 2 ~ 0 0 2 0 0 0 1600 1200 8 0 0 ~ 0 0 NRVENUMBER

Fro. 5. Single-beam energy curve of an empty FiberCell@. These 4-cm -1 resolution data were obtained with an MCT-A detector.

Fro. 4. Diagram of the FiberCell@ in the holder. The entire unit fits into the sample compartment of the FT-IR instrument. The kinematic design and clamp hold the cell in alignment.

optimized for the maximum amount of energy getting into and out of the fiber. Coming into the sample com- partment, the light makes an angle to the optical axis that is used as a design limit for the cone acceptance. The input angle and the exit aperture determine the overall size and shape of the concentrator, t5 The NIC trades image fidelity for efficiency in transferring energy to the exit. The light exits the cone at an angle that is larger than that of the incoming light. The product of the entrance aperture area and solid angle is kept con- stant and is equal to the product of the exit area and solid angle. This product, multiplied by the index of refraction of the medium (1 for air), is called the "eten- due," and is a constant for optical systems. The cone's exit light angle is quite large. This configuration gives a better match to the large acceptance angle of the fiber than the unconcentrated incoming light. The exit ap- erture is somewhat larger than the fiber to allow tolerance in positioning the fibercell in the mount. The NIC on the output side of the fiber effectively expands the image of the fiber to better fill the field of view of the detector optics. This feature allows use of the main bench detector without realignment. The concentrators fill the accep- tance angle of the fiber with a homogeneous light field, removing possible hot spots the image of the source could have that affect reproducible energy levels. The initial design used a pair of elliptical mirrors that required a difficult alignment process and placed the cell at an awk- ward position in the sample compartment. The improved design using NICs allows use of standard covers and is in-line with the light path, making alignment simple. Six mirrors have been replaced by two, and four mirror ad- justments have been replaced by one. This "in-line" de- sign also makes it easier to get the cell in and out of the sample compartment with minimal loss of purge. The cell is placed in a slot between the two concentrators and is held in place by a clamp. The polypropylene cell has a special "key" to ensure that the cell can be mounted reproducibly with the same orientation. The accessory,

32 Volume 47, Number 1, 1993

designed and manufactured by Isorad Ltd., is shown in Fig. 4.

Experimental Conditions. All spectra were acquired on a Nicolet 740 FT-IR spectrometer with a high-sensitivity MCT A type of detector, a water-cooled Glowbar@ source, and a mid-IR beamsplitter. All spectra were acquired at four wavenumber resolution with a gain of 4. The FiberCells@ were cleaned between experiments with an ultrasonic cleaner and a two-solvent wash. As with all internal reflection accessories, it is critical that the "crys- tal" surface be clean.

RESULTS AND DISCUSSION

A number of experiments were performed to evaluate the performance of the FiberCell@. The experiments were designed to define the basic performance specifications for the accessory.

Energy Throughput. One of the most important per- formance features of a micro-accessory is the amount of energy that gets through the system. While this is not totally the case with an internal reflection experiment, because the energy must also interact with the sample on the surface of the crystal, it is still a very important first criterion for performance. Figure 5 shows the single- beam spectrum through the FiberCell@. While there are no major features in the spectrum, a number of small peaks can be observed. These peaks are due to absorp- tions in the chalcogenide fiber (2200, 1260, 1050 cm -1) and from the adhesive (2900, 1730, 1600 cm -1) used to seal the fiber into the cell. All of these features are con- stant and do not affect the final spectrum. This curve shows that spectra can be measured from above 4000 cm -1 to about 750 cm -~ with good sensitivity. The 750- cm -1 cutoff is due to the lattice vibrations in the glass. Measuring the energy throughput of the FiberCell@ is actually difficult because the beam condenser is designed to focus the energy on one end of the FiberCell@ and to refocus from the other end. This means that very little energy actually gets through the accessory when the FiberCell@ is not present. The relative throughput of the accessory and cell can be determined by dividing the single-beam spectrum of the accessory by a spectrum from the spectrometer when the accessory is removed.

FIBER CELL MCT£ r]ETBCTOR

~ Be s~eo s~oo sloc ~7BO aSoo 1~oo 1sob i4oo fOB NAVENUHBEB

Fro. 6. No ise ]eve] p e r f o r m a n c e over t he 4 0 0 0 - 7 5 0 cm -1 reg ion. T h e m e a s u r e m e n t t ime was 100 s with 8192 da ta points . 10 20 30 40 50 60 70 80 90 100

12

11

10

9

8

7

6

5

4

3

2

1

0

'E

The accessory transmits about 2 % of the total open beam energy. While spectra can be obtained with a TGS de- tector in less than five minutes, the performance is sig- nificantly better with a cooled MCT detector.

Signal-to-Noise Ratio. A good estimate of system sen- sitivity can be obtained from ratioing two single-beam spectra that were acquired with no sample in the cell. The noise level in this spectrum can be used in the cal- culation of the signal-to-noise ratio. Unlike transmit- tance spectroscopy, where this is the only critical factor, the actual signal must also be determined in IRS. Figure 6 shows the 100% line through the FiberCell@. This spectrum was acquired with a 100-second measurement time. This spectrum shows low noise level throughout the spectrum with no significant attenuation at either the high or low frequencies. A peak-to-peak noise level of about 0.05 % T was measured at 2000 cm -1. On the basis of an isopropyl peak absorbance of 1.0, this results in an absorbance signal-to-noise ratio of about 460.

Average Pathlength. Because of the optical design of the accessory and the taper in the fiber, the spectral absorbance is affected not only by the normal IRS vari- ables, such as refractive index of the sample, wavelength dependence, and temperature, but also by the effective pathlength. It is extremely difficult to calculate the op- tical path through the FiberCell@. The probe pathlength is a complex function of the incident angle at which the light enters the fiber and the exact shape of the taper. The increased sensitivity due to the change in diameter in the fiber comes from two causes. ~6 The first involves the depth of penetration of the light into the sample. In IRS there is an effective depth of penetration that is a function of the angle the light makes with the surface of the fiber. The depth varies exponentially with the angle the light makes to the surface, down to the critical angle where the depth becomes infinite and the light is lost. The critical angle is related to the difference in index of refraction between the fiber and its external covering. As the light nears the critical angle, it extends deeper into the sample. A large critical angle is generally preferred; thus the large index of the chalcogenide is desirable (n = 2.77). The internal reflections of the ray along the narrowing taper bring the light closer to the critical angle and thus increase the penetration depth. As depth in-

% P-xylene concentration by volume

FIG. 7. P lo t of peak area vs. concent ra t ion for peak a t 795 c m - ' of p-xylene . T h e curve is approx imate ly l inear only over narrow regions of concentra t ion.

creases, the volume in which light interacts with the sample increases. The penet ra t ion depth for the FiberCell@ is not simply defined by a single light angle as in other IRS configurations. The light in the fiber comes to the surface over a range of angles. The effective penetration depth can be found from empirical data for liquids with known indices of refraction. The average pathlength of the taper was estimated from a series of data taken for seven cyclohexane peaks (n = 1.426). By a comparison of the peak areas between a spectrum from a liquid cell of known pathlength and a spectrum ob- tained with the FiberCell@, the pathlength as a function of wavelength was empirically determined. '7 The average pathlength for a FiberCell@ varies from about 7 ~m at 3000 cm -1 to 25 ~m at 750 cm -1. Typical IRS penetration depths are on the order of the wavelength of the light that is used.

The second increase in sensitivity comes from increas- ing the number of interactions between the light ray and the sample. The rays of light traverse across the diameter of the narrow part of the fiber more frequently than the wider part. The number of reflections varies inversely with fiber diameter and the tangent of light angle, thus increasing the number of reflections per unit length in the tapered region. The tapered fiber gives more light interactions with the sample than a nontapered fiber.

Quantitative Analysis. Several papers have discussed the linearity problems with internal reflection spectros- copy. ls,19 These concerns apply equally to this design. In order to bet ter unders tand the l imitat ions of the FiberCell@, a simple quantitative analysis experiment was performed with para-xylene in cyclohexane. Figure 7 shows that the peak area for the 795-cm -1 peak in p-xylene is not linear with concentration over a wide range (1 to 95 % by volume), p-Xylene solutions ranging from 10 to 30 % xylene were analyzed with the FiberCell@, and the spectra were used as a calibration set for a partial least-squares (PLS) analysis. The percent error in pre- dicted concentrations varied from 0.1 to 0.9%. These

A P P L I E D S P E C T R O S C O P Y 33

results suggest that the FiberCell@ can be used for quan- titative analysis with proper calibration.

Disadvantages. The tapered area is extremely delicate, and any contact with a pipette or needle can break the fiber. We have run multiple samples in a FiberCell@ without damage, but we have also destroyed several by careless sample insertion. Viscous liquids are not a prob- lem if reasonable care is used in filling the cell. Cleaning in an ultrasonic bath generally works well, but we have run samples that badly contaminated the cell after dry- ing onto the surface of the crystal. In most cases we managed to clean the surface after many cycles in the cleaning bath with different solvents and cleaning so- lutions.

The body of the cell is made of polypropylene, and it should not be used with any solvent that can dissolve or react with polypropylene. The temperature of the sample must be kept below 50°C, since the thermal expansion of polypropylene is several times that of the fiber and can cause stress that will break the fiber.

CONCLUSIONS

In this paper we have described the design of a new accessory for infrared internal reflection spectroscopy. The results of this work show that the FiberCell~ can be used for routine analysis of liquid materials that would be extremely difficult to analyze by normal transmittance spectroscopy methods. This preliminary work has dem- onstrated that the infrared absorbance spectra obtained with a FiberCell@ can be modified by varying the amount of taper in the sampling crystal. The extremely small size of the active sampling area suggests that this sampling concept can be used for acquiring spectra from less than 10 #L of sample.

While this design does have certain limitations, we believe that it will prove valuable in a number of appli- cation areas.

ACKNOWLEDGMENT

The work on which this publication is based was supported in part by a grant from the Israel-U.S. Binational Industrial Research and

Development Foundation (BIRD). The opinions and information con- tained herein are those of the authors and not necessarily those of the foundation.

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34 Volume 47, Number 1, 1993