Near infrared (NIR) fiber optic spectroscopic analyzers are being used widely in the process industry. One of the long- standing issues with such analyzers is the ability to transfer calibrations from one analyzer to another and from an analyzer to itself after repair. The most frequently services parts of a spectrometer are the lamp and the probe. This paper will examine calibration issues related to probe design, probe to probe uniformity, and the interaction of the probe, spectrometer and fiber combination. Specifically, what impact does transmission, spectral features, vibration sensitivity, pathlength, window wedging, and other probe characteristics have on the spectroscopic system performance and transferability.

A new flow-through high-pressure optical cell, with unique design features, allows spectroscopic monitoring of process streams. The novel design features no windows, has no dead volume and minimizes sample perturbations. A monolithic sapphire block serves as both pressure containment and optical material. The sample chamber is an integral part of the sapphire; thus creating the small, no dead volume sample chamber, while maintaining high optical throughput. Using an auxiliary optical interface adapter, this flow probe can be easily coupled to a fiber optic based spectrometer. The probe is ideally suited for low volume applications, such as pilot plants, GCs, etc. Being of sapphire construction, the probe functions from UV through the near-infrared and has an equivalent pathlength of 2.1 mm. The application to spectral measurements in the near-infrared and visible regions will be illustrated. Direct spectral comparisons to samples run in the probe and in a 2 mm cuvette will be made.

Optimization of a reaction solvent is typically performed when a chemistry if progressed from discovery to scale up. Typically, a large number of solvents are screened to determine which solvent gives the highest rate and yield. Samples are drawn out during the reaction and are analyzed by HPLC. This screening method suffers from a long idle time as the HPLC methods are long and is limited to a small number of samples due to the large number of HPLC samples generated. What is described in this work is an in-situ UV/vis method to perform an on-line analysis of multiple reactions to quickly determine which solvents give the fastest rate. A fiber optic probe is placed directly into the reaction vessel and UV/vis spectra are collected simultaneously from each reaction. Composition profiles and pure component spectra of reactants, intermediates, and products are estimated, using iterative target transformation factor analysis (ITTFA), a type of self- modeling curve resolution (SMCR), without the aid of referee measurements or standards. The results indicate that the method can successfully predict which solvent and can be used as a broad screening tool for the optimization of the reaction. Pairwise analysis of consecutive batches can be used to perform standardless comparisons between the two batches to determine if the reaction proceeded faster or slower, and made more or less product.

Some aspects concerning the calibration procedure are shown for a fiber optic sensor to be used for on-line chromium (VI) concentration measurements. The sensor is based on the measurement of light absorption of a sampling volume between transmitting and receiving optics, made by means of GRIN lenses for sensor size miniaturizing purposes. In particular, the geometrical design of the probe has been discussed, together with the effects of optical and mechanical solutions. These have been set in the probe, referring to the measuring performances to be achieved and, in particular measuring range, sensitivity, accuracy and time stability. The results of preliminary experimental analysis are also described, which allowed validation of the design indication and getting information for probe development.

Fourier transform infrared (FTIR) instruments started out as laboratory devices, which would perform well if they were kept in a temperature-controlled, vibration-free laboratory environment. Today, FTIRs are being used to monitor from the sides of stacks, on production floors, and in the hearts of industrial plants, next t large pumps and motors. In addition, it is becoming increasingly important to individual users to be able to adapt a measurement system to totally different modes of operation at different times. For example, one measurement program may require open-path monitoring, while the next may need extractive process monitoring. To address both of the challenging measurement environments and the adaptability issues, Unisearch has developed the IMx, a modular FTIR industrial monitor. This system has features, which allow it to perform well in the most challenging environmental conditions and its modular design allows for rapid reconfiguration by the user. In addition to adding versatility, the modular approach also has the pleasant effect of reducing system cost. In what follows, we describe the Imx system and show examples of both open-path and extractive data gathered with it in harsh industrial environments.

Designs & Prototypes has developed a small, very fast, rugged rotary Fourier transform (FT) spectrometer, which can be used in conjunction with fiber optics to monitor industrial processes. When used in an imaging configuration with a mosaic detector array, multiple processes (or multiple locations in a process) can be monitored simultaneously. The rotary scan allows operation without the laser reference of conventional FT spectrometers, and can easily yield scan rates from 30 to more than 300 scans per second. Single pixel operation at 360 scans per second and 1 cm ^-1 resolution, with excellent lineshape, has been demonstrated. Multiple pixel operation has also been demonstrated using a 3X3 HgCdTe PC mosaic detector. Field testing of a hand portable version with 8 cm^-1 resolution will be performed in late summer 1998. The unit can measure spectra in the NIR, SWIR, and Thermal IR, with the appropriate optics and detector sets. The optical bench for 8 cm^-1 resolution is 2.5X3.5 inches, and 2.5 inches high. It weighs 11.5 oz., and is totally sealed from the environment. Electrons for servo and detector channel functions can be packaged into an enclosure 6X4X2 inches. For very fast real time processing, single or multiple DSPs can be used. The development work has been supported by contracts from the U.S. Army, a large U.S. aerospace company, and a major Australian mining company.

A system has been developed using extractive FTIR techniques to characterize the gaseous emissions from semiconductor wafer process tools. The system provides real-time data collection, processing and display for multiple compounds simultaneously. Tool effluent emission profiles, which track concentrations are produced with time resolutions on the order of seconds. Along with a description of the hardware, sampling and analysis methods, the results of some field testing and system validations are also presented.

A gas cell is used to characterize the analytical performance of an FTIR sensor collecting air monitoring data in an active sampling configuration. Hardware and data collection techniques are described and example data are presented. Instrument performance is characterized and discussed in terms of accuracy, precision, linearity, drift, and reproducibility.

The use of optical spectroscopic methods for quantitative composition measurements in the field of process control is increasing rapidly. Various optical configurations are already in use or are being developed, with the aim of accomplishing the wavelength selectivity needed in spectroscopic measurement. The development of compact and rugged spectrometers for process monitoring applications, has been one of the major tasks for the optical measurements research team at VTT Electronics. A new PbS detector array- based spectrometer unit has now been developed for use in process analyzers, providing 24-wavelengths ranging from 1350 to 2400 nm. Extensive testing has been carried out to examine the performance of the developed units, concerning performance in normal operating conditions, characteristics vs. temperature, unit-to-unit variation and preliminary environmental testing. The main performance characteristics of the developed spectrometer unit include stable output, a band center wavelength (CW) unit-to-unit tracking better than -+ 1 nm, a band CW draft vs. operating temperature less than 1.8 nm in the temperature range +10 degree(s)C...+50 degree(s)C, and optical stray light below 0.1 percent. The combination of technical performance, small size, rugged construction, and potential for medium manufacturing cost (4000-5000 dollars in quantities) make the developed unit a promising alternative in developing competitive high-performance analyzers for various NIR applications.

This paper discusses the possibilities of the light-emitting diode technology in view of application as an instrument light source, especially for on-line process analyzers and hand-held optical sensors. The results of comparative measurements carried out on the spectral power output of selected NIR LEDs and incandescent lamps are summarized. Some aspects concerning the technical optimization of LED- based instruments are dealt with, and the availability of commercial LED devices with a wavelength range from visible to mid-IR wavelengths, are briefly covered. Finally, some examples of LED-based multiwavelength light source modules developed at VT T Electronics over the past few years are briefly reviewed. These comprise a multiwavelength LED source for pulp and paper applications, a prototype 7- wavelength LED spectrometer for the 3.4 micrometers region, and a second generation 32-wavelength SWNIR spectrometer unit for high performance analyzer applications.

Pure gain-coupled distributed feedback (DFB) lasers and complex-coupled DFB lasers, based on a current modulation concept is investigated. An n-doped current blocking layer is formed, either directly on top of the active region or on top of the InGaAsP index-modulation grating. The blocking layer causes a spatial modulation of the carrier concentration at the active region, forming a gain-grating. The blocking layer causes a spatial modulation of the carrier concentration at the active region, forming a gain- grating. The addition of the n-doped current blocking layer did not cause any deterioration in either the I-L or I-V curves. The stop band, which can be clearly observed in the index-coupled and the complex-coupled DFB lasers, cannot be distinguished in the pure gain-coupled DFB lasers, suggesting that the current blocking layer is sufficient to incorporate gain-coupling mechanism to the DFB lasers. The gain- and complex-coupled DFB lasers exhibit significant improvement in single model yield and susceptibility to optical feedback, compared to index-coupled DFB lasers. The concept is applied to the fabrication of DFB lasers at various wavelength from 1.3 to 1.7 micrometers .

An integrating sphere is a cavity with high-reflectivity walls, used primarily in precision photometric and reflectance measurements. In addition, it is used to make accurate optical power measurements and to smooth spatial variations in optical beams, by performing the integration of power over the cross-section of the beam. This paper considers the integrating sphere as the containment cavity, in which absorption spectroscopy of uniformly distributed samples is performed. A theoretical presentation of the optical properties of the integrating sphere is made, which leads to an expression for optical transmittance of the sample. This expression is different from the traditional exponential expression of transmittance of Beer-Lambert. The features of this new expression are explored for exploitation in sensitive absorption measurements.

The study of dimethylamineborane, as the reducing agent in electroless deposition of gold, was conducted using electrochemical, elemental and spectroscopic methods. The validity of the mixed potential theory of electroless deposition of metals was used to explain the influence of fluctuating chemical concentrations in the bath. Experimental measurements showed that plating rate depends on the fluctuating concentrations of gold, hydroxide, cyanide and the reducing agents among other variables. The fluctuating chemical concentrations can be monitored using the novel automated chemical sensors discussed.

Many applications for spectroscopy in on-line chemical monitoring do not require high resolution. For these cases, optical filters can be useful and cost-effective. To cover a spectral region using filters, the traditional method has been to select a set of non-overlapping filters. With this set, the instruments can monitor the optical power in each small spectral element. The disadvantage to this approach is the poor peak transmittance characteristics of narrow band filters. This paper discusses the use of overlapping broadband filters to accomplish the task of spectral measurement over the same total spectral region. Greater optical throughput and signal-to-noise ratio can be achieved.

The verification of low water vapor impurity levels in semiconductor manufacturing feed gas supplies is becoming critically important for the development of advanced electronic devices. Ammonia is one of the important precursor gases for electronic manufacturing. In this paper, we present data from a water vapor absorption spectroscopy sensor designed to continuously measure ppb water impurities in pure ammonia gas with a 1 Hz bandwidth. The sensor is built using a near-IR diode laser, commercial fiber optic components, room-temperature InGaAs photodiodes, an ultra- sensitive balanced radiometric detection circuit, and a modified commercially available multipass cell. We present water vapor collisional broadening data by ammonia used to determine the optimal operating pressure for maximum system sensitivity. The commercial multipass cell was modified for ease of alignment, a nearly continuously variable pathlength, and to minimize the atmospheric air pathlength outside of the cell. The computer- controlled sensor is applicable to making water impurity measurements in a number of additional commercially important gases, such as hydrogen chloride, hydrogen fluoride, hydrogen bromide, silane, etc. The sensor is also applicable to moisture measurements in natural gas, and manufacturing dryer applications, such as those found in the plastics industry or the pharmaceutical industry, where in-line process control is critical.

The 1+1 and 2+2 resonance-enhanced multiphoton ionization (REMPI) spectra of gas phase indene near the origin of the first electronic transition at 287.9 nm are reported. This work relates to chemically-specific measurements of trace organic constituents in ambiant air. The spectra were acquired to help assess options for on- and off-resonance REMPI, similar in concept to differential absorbance LIDAR (DIAL) and differential optical absorbance spectroscopy (DOAS). The 1+1 REMPI spectrum for indene at 50 ppbv concentration in air compares well with published high resolution absorbance spectra obtained for much longer pathlength and higher sample concentration. Extensive sequence band structure is observed near the origin. The rotational contours of the origin and sequence bands consist of narrow, nearly symmetric features, accompanied by a lower intensity broad sideband shading to the red. The narrow feature is not as narrow as in the highest resolution absorbance spectrum and cannot be explained in terms of the laser linewidth or intensity (saturation) broadening at high laser power; pressure broadening may be the source. We also report styrene 1+1 REMPI spectrum to document instrument improvements made since our previous study.

A monitoring method for solvent exchanges has been developed and demonstrated. The solvent mixture is monitored in real time by a process fiber optic probe, coupled to a near infrared spectrometer. A partial least squares (PLS) calibration model was developed based on typical solvent exchange systems used in a pharmaceutical bulk drug production. The acousto-optic tunable filter (AOTF) NIR is well suited to process monitoring, as it has no moving parts. The AOTF-NIR continuously fits the PLS model to the currently collected spectrum. The returned values can be used to follow the solvent exchange process. The solvent exchange was monitored by placing a dip-probe into the exchange distillation. The results indicate that the percent of lower boiling solvent in the reaction mixture can be monitored in real time and, thus, be used to determine when the exchange distillation is complete.

Laser-induced breakdown spectroscopy is considered as a possible technique for monitoring the components in gas streams. The intense plasma created at the focal point of a high-energy laser pulse emits radiation, indicating neutral atomic and singly ionized atomic participation. The emission peaks can be used for quantitative analysis of the gas components.

The responsivity and self-emission of a double-beam FTIR interferometer are modeled in view to simplify the procedure of radiometric calibration. Measurements have shown that the two responsivities associated with each interferometer channel are different in certain spectral regions. It has been attributed to a dissymmetry between the optical transmissions of the two plates forming the beamsplitter. This dissymmetry is also responsible for the instrument self-emission. A model that gives the instrument self- emission in terms of the beamsplitter temperature is presented and tested. An automatic calibration procedure derived from the modeling study is discussed.

A commercial digital signal processor (DSP) board with custom software monitors the operation of an FTIR spectrometer. The DSP board acquires interferogram data simultaneously from both the infrared and reference laser channels of the FTIR spectrometer. This approach permits post-processing of the signals to obtain conventional spectra, as well as a variety of diagnostics. Such diagnostics include extraction of interferometer mirror velocity and the combined transfer functions of the detectors, amplifiers and filters. The DSP board resides in the accessory bus of a personal computer (PC), allowing use of the PC and its peripherals for data display, storage and post-processing. A method for extracting the transfer functions of the laser and infrared channels by the use of solid state emitters is presented. These are the key elements required to monitor and correct the effects of velocity error. The simultaneous digitization of both interferometer channels may be a trend in FTIR spectrometer design, which shifts signal processing further towards soft implementations.

Diagnostic computer programs permit the evaluation of FTIR spectrometer thermal, spectral, and interferogram stability. Raw interferogram data is required under four conditions: (1) equal time increments, (2) error code detection, (3) interferogram center burst threshold change, and (4) user request. Collection of raw interferogram data furnishes a robust means of detection and identification of various instabilities, which occur in passive FTIR spectrometers.

Widespread acceptance of Raman spectroscopy in chemical process monitoring requires instrument calibration, which is automated, repeatable, reliable, verifiable, and transferable from instrument to instrument. Key elements to be calibrated in a dispersive Raman analyzer are Raman emission wavelengths, the spectral response of the instrument, and the excitation laser wavelength. Modern Raman instruments are capable of simultaneously monitoring multiple sample points in a process pipeline. In a typical industrial installation, multiple remote probe heads are coupled to a central instrument (laser source, spectrograph, CCD detector and control/software) via fiber optic cables up to hundreds of meters in length. Instruments must self- calibrate and validate without direct access to remote probe head installations. The presence of a holographic laser notch filter in the system presents unique calibration challenges. The implications of these issues on instrument configuration and calibration/ validation protocol are discussed. Candidate wavelength and intensity calibration references are compared. Examples of industrial Raman applications and their requirements on calibration accuracy and precision are given.

Prototype surface acoustic wave chemical sensor systems are described, which can detect and identify toxic vapors in real-time at trace concentrations. To operate autonomously for long periods, without failure, requires a thorough understanding of the hardware and software requirements of the sensor system. The SAWCAD and SAWRHINO prototypes, which implement several improvements to the hardware, over previously developed systems, are described. Software for vapor detection and neural network identification are also discussed. Preliminary results from two new software enhancements are described. Improved chemical discrimination occurs when the response slopes are incorporated into the analysis of the SAW ambient data. The generalized rank annihilation method is shown to be a powerful tool for extracting pure component analyte signatures from trap and purge gas solid chromatographic SAW data.

Referencing NIST Standard SRM 1921A, a comprehensive approach, has been developed for use by all practitioners of infrared spectroscopy, to calibrate their instruments. This approach utilizes a thin film of Poly 1-phenylethylene) Polystyrene rotationally cast, commonly on an Alkyl Halide crystal, with a radial molecular alignment. Historically, drawn polystyrene films have provided reasonable accuracy with respect to band locations documented by NIST. The principal difference between the two methods lies in their respective ways of manufacture. The rotational method exhibits radial stress patterns where drawn films exhibit parallel stress lines. Comparative studies show that a radial film is just as accurate, yet with fewer shortcomings. Greatest attention is fixed on band position, however, energy absorption at critical points is generally ignored. A radial film can be case in an array of precise pathlengths to monitor an instruments Y coordinate efficiency. Drawn films have limitations. All test films are mounted perpendicular to the directional path of an incident beam. When a conventional film, with transverse and longitudinal orientations, is rotated along a 'Z' axis, or the axis which an incident beam travels, results may differ. This is a well-documented phenomenon. When the orientation of a vibrator changes with respect to a fixed energy beam, the moment of dipole of that vibrator also changes. Due to the configuration of a radial film, there is no significant change in orientation upon rotation. This would be valuable information for anyone using infrared for quantitative purposes. Significance of the rotational method lies in accurate pathlength reproducibility in a variety of thicknesses, a unique molecular arrangement, limited control in the absorbance range, and maintained precision in band location. Lastly, it is a cost-effective measure.

Advances made over the past decade in multispectral and hyperspectral imaging systems, have led to a wide range of new remove sensing capabilities, including the ability to detect and image chemical vapors in the atmosphere. This sensor has application in the detection and monitoring of chemical weapons, as well as environmental pollution monitoring. Key to the continuing development of this technology is accurate and temporally stable radiometric calibration. This paper presents an overview of the system level radiometric calibration approach used for the SAFEGUARD multispectral infrared line scanner. This approach includes radiometric calibration of the sensor at the aperture, corrections for atmospheric effects and group truth validation.

Surface-enhanced Raman spectroscopy (SERS) promises to be one of the most sensitive methods for chemical detection. Unfortunately, the inability of SERS to perform quantitative chemical analysis has slowed its general use in laboratories. This is largely due to the difficulty of manufacturing either active surfaces that yield reproducible enhancements, or surfaces that are capable of reversible chemical adsorption, or both. In an effort to meet this need, we have developed metal-doped sol-gels that provide surface-enhancement of Raman scattering. The porous silica network offers a unique environment for stabilizing SER active metal particles and the high surface area increases the interaction between the analyte and metal particles. This eliminates the need to concentrate the analyte on the surface by evaporating the solvent. The sol-gel is easily coated on a variety of surfaces, such as fiber optics, glass slides, or glass tubing, and can be designed into sample flow systems. Here we present the development of both gold- and silver-doped sol-gels, which have been used to coat the inside walls of glass sample vials for SERS applications. The performance of the metal-doped sol-gels was evaluated using p-aminobenzoic acid, to establish enhancement factors, detection limits, dynamic response range, reversibility, reproducibility, and suitability to commercial spectrometers. Measurements of trace chemicals, such as adenine and cocaine, are also presented.

Series selective and sensitive films were designed and fabricated for sensing volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs). The molecular recognition host reagents were designed and synthesized for optimum binding or organic vapors. Sensing films were fabricated onto a surface acoustic wave (SAW) transducer surface through molecular self-assembly and covalent immobilization. The selectivity and sensitivity of the film are based on host-guest interaction, between the host reagent (cyclodextrins) and gas phase species. The self- assembled film containing cyclodextrins can detect certain organic vapors at parts per billion concentrations.

The detailed kinetics and associated optical waveforms of the spectroelectrochemical sensor in competitive binary mixtures of analytes are presented. The extension of the sensor concept to planar waveguide structures is described. Two different planar waveguide designs have been made and evaluated. One design was an asymmetric slab waveguide with the waveguide layer sandwiched between a silica substrate and a chemically-selective film. The other design was a channel waveguide, which consisted of an ion-exchanged channel between two gold electrodes in a 'bus bar' configuration. These new designs are described in detail and evaluation of them with respect to operation of the sensor in the near ultraviolet region is discussed. Results with the devices in demonstration of the novel spectroelectrochemical concept with three modes of selectivity are given. Instrumentation developed specifically for operation of the sensors and acquisition of sensor data is described.

Highly sensitive, specific and reagent-free optical signal transduction methods for detection of polyvalent proteins have been developed by directly coupling distance-dependent fluorescence self-quenching and/or resonant energy transfer to the protein receptor binding events. The ganglioside GM1 as recognition unit for cholera toxin (CT) was covalently labeled with fluorophores, and then incorporated into a biomimetic membrane surface. In the case using fluorescence self-quenching as a signal transduction mechanism, the fluorescence intensity drops significantly due to aggregation of the fluorophore-labeled GM1 on a biomimetic surface. By labeling GM1 with a fluorescence energy transfer pair, aggregation of the labeled-GM1 results in a decrease in donor and an increase in acceptor fluorescence, providing a unique signature for specific protein-receptor binding. The detection systems can reliably detect less than 0.05 nM CT with fast response (less than five minutes). This approach can easily be adapted to any biosensor scheme that relies on multiple receptors or coreceptors. The methods can also be applied to investigate the kinetics and thermodynamics of the multivalent interactions.

A measurement system optimized for process control in the industrial environment has been developed and successfully commercialized. The system comprises a central unit, which contains all sensitive electronic and electro-optic parts. Fiber optics is used to transport the probing laser light to the measuring points in the process. Extremely rugged sensor heads are used to interface to the harsh industrial environment. Adaptation to the different applications is solely made up by changing the type of sensor head used. Six different process control applications will be presented. Ammonia slip monitoring in the NO(subscript x4/ reduction process in power stations, waste incinerators and heavy-duty diesel engines. Measurement of water vapor and oxygen in municipal waste to energy plants. Monitoring of oxygen and the thermodynamic gas temperature in steel pellets manufacturing. Monitoring HF reduction in a dry scrubber and HF emission from a pot room. Experiences of CO emission peak monitoring to protect electro filter in a chemical waste incinerator. Finally, we will describe measurements of HCI in the raw gas to access the calorific value of waste and to optimize bag-house filter operation.

A chemical sensor scheme, based on selective sensing surfaces and highly sensitive integrated optical transduction is presented. Self-assembly techniques are used to covalently attach species selective films onto the surface of silicon nitride waveguides. Exposure to targeted analytes results in selective absorption of these molecules onto the waveguide surface, causing a change in the effective refractive index of the guided modes. These relative changes in effective refractive indices of TM and TE modes are measured using Zeeman interferometry. The measurements demonstrate reversible, real time sensing of volatile organic compounds at ppm level. Improvements in the waveguide design are proposed to further increase the sensor performance.

The Raman spectra of fluoro-organic compounds show specific emission bands for carbon-fluorine bonds in the range 500- 800 wave numbers (cm^-1)). With very limited exceptions, biological materials do not contain carbon- fluorine bonds. Fluoro-organic compounds introduced into biological samples can be detected by a Raman emission signal. Normal mode C-F bond bands are observed: (1) at 710- 785 cm ^-1 for trifluoromethyl groups; (2) at 530-610 cm ^-1 for aromatic organofluorine bonds; (3) a range centered at 690 cm ^-1 for difluoromethylene groups. Specific examples of normal mode C-F bond emissions for organofluorine compounds containing trifluoromethyl groups are: 1-bromoperfluorooctane, 726 cm ^-1; perfluorodecanoic acid, 730 cm ^-1; triperfluoropropylamine, 750 cm ^-1; 1,3,5-tris- (trifluoromethyl)-benzene, 730 cm ^-1; Fluoxetine (Prozac) commercial powdered pill at 782 cm ^-1. Compounds containing aromatic C-F bonds are: hexafluorobenzene, 569 cm ^MIN1; pentafluoropyridine, 589 cm ^-1. Difluoromethylene groups: perfluorodecalin, 692 cm^-1; perfluorocyclohexane, 691 cm ^-1. Raman spectra were observed with a standard single monochromator. The 510.8 nm light source was a copper-vapor laser operated at 3-10 watts with 10-12 nanosecond pulses at 10 kHz repetition rate. Detection was made with a time-gated photomultiplier tube. Resonance Raman spectra were also observed at 255.4 nm, using a frequency doubling crystal. Observed spectra were free of fluorescence with very sharp strong C-F lines.

Planar waveguides have evanescent fields sensitive to index of refraction changes in the volume immediately above the waveguide surface. Optically combining one guided sensing beam with a reference beam in an interferometric configuration generates measurable signals. Applying a chemically selective film over the sensing arm of the interferometer provides the basis for a chemical sensor. Acid-base chemistry either on the waveguide surface or incorporated into a polymer film, deposited on the waveguide, allows for reversibly sensing pH in solution or ammonia in air. The pKas of the sensing groups can be spaced across the pH range to permit continuous change over the entire range. Alternatively, sensing and reference arms can have different sensing groups with pKas, which bracket the acidity or basicity of the target analyte. A polymer film thicker than the evanescent field shields the optical beam from environmental changes, but also permits facile transfer of either protons or ammonia. Multiple interferometers can be fabricated on a single integrated optic chip. Channels not used for acid-base sensing can be used to cancel out interferants, or to measure other analytes. Sensitivities achieved to date are in the ppbv range for ammonia and <0.01 pH in the pH sensor.

In this paper, we present a chemical sensor based on the modification of an optical resonator: the optical path length of the resonant cavity is changed by the chemical in question, thus shifting its resonant frequency.