This is the second article in a two-part series describing the operation, performance characteristics, and spectroscopic application of charge transfer devices (CTDs) in analytical chemistry. The first article in the series describes the new generation of integrating multichannel detectors, the charge injection device (CID), and the charge-coupled device (CCD). The first article also discusses the spectroscopically pertinent characteristics of these detectors and presents performance data for representative devices. This article covers three major topics related to the optimum use of integrating detectors in analytical spectroscopy. The advantages of employing integrating multichannel detectors in analytical spectroscopy, rather than a single detector in a wavelength scanning system or an interferometer, are discussed. Included are detector read noise considerations which have not been considered in previous performance comparisons. When one is employing an integrating detector in luminescence, absorption, and emission applications, achievable sensitivity is dependent on differing detector parameters. In the first case, quantum efficiency and read noise are of the greatest importance, whereas in the later two cases, dynamic range is most significant. The calculation of minimum detectable analyte signal for these three techniques illustrates the differences between integrating detectors and detectors which produce a photocurrent. This discussion also illustrates the great sensitivity that can be achieved with a modern CTD detector. Factors pertaining to the optical design of spectrometers which efficiently use CTDs are presented, along with examples of linear and two-dimensional dispersive polychromators employing CTDs. Low-light-level imaging and a nonconventional method of using a CCD for rapid scanning spectro-photometry are also discussed.

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Spectrochemical Measurements with Multichannel Integrating Detectors

A state-of-the-art self-scanned photodiode array, utilized as a spectrometric multichannel detector, was studied for its applicability to molecular absorption and molecular fluorescence spectrometry. Molecular absorption spectrophotometry requires detectors with high UV to near IR response, high dynamic range, linear response, low noise, and temporal as well as thermal stability. Molecular fluorescence spectrometry requires, in addition, sensitivity to low-light level signals. These requirements were satisfied by the systems under study. Other parameters that govern the overall performance of diode array-based spectrophotometers and spectrofluorometers include: photometric range, stray energy radiation and its effect on photometric accuracy, photometric precision, spectral resolution sensitivity, and signal-to-noise considerations. Various readout methods for the array detectors were examined including: real time, in-memory signal integration, on-target signal integration, variable integration time (VIT), diode grouping, and diode fast access (a pseudorandom access readout). Performance demonstrations of these multichannel spectrometers were made with a few typical chemical systems. The spectrophotometric performance of the diode array system was excellent, practically equal to that of conventional single-channel systems. Exceptions were higher amenability to the adverse effects of stray radiation energy and a dependency of the geometric (wavelength) accuracy on spatial fluctuations (wander) in the light source. Potential long-term UV degradation of diode arrays may present an inherent problem, but was not discussed here because data accumulated up-to-date was considered inconsequential. As a spectrofluorometer multichannel detector, the diode array showed an "unexpected" signal-to-noise (detectability) performance, closely matching that of conventional high gain detectors. However, this performance was achieved only if sufficiently long signal integration times (20 to 160 s) were feasible. Overall performance of the diode array has demonstrated that theoretical multiplex advantages are achievable, resulting in either a considerable improvement in signal-to-noise or a corresponding shortening in observation time.

Spectrophotometry and Spectrofluorometry with the Self-Scanned Photodiode Array:

Surface-enhanced Raman spectrometry (SERS) on a silver-coated filter paper substrate of nitrogen-containing organic molecules is reported. A correction procedure for standardization of measurements is proposed and evaluated to solve the difficult problem of quantitation of adsorbate in SERS. The relative standard deviation obtained through this procedure is around 15%. Linearity (r = 0.999) was achieved up to 50 micrograms/mL aminoacridine. A limited dynamic range is observed, however, due to the limited number of SERS active sites in the substrate. Spectral fingerprinting of three-component mixtures by concentration-dependent selective molecular adsorption on the substrate is also reported.

Mixture analysis and quantitative determination of nitrogen-containing organic molecules by surface-enhanced Raman spectrometry

The application of a gas chromatograph/flame photometric technique to the separation and measurement of four compounds, H/sub 2/S, COS, CS/sub 2/, and SO/sub 2/ in inert gases and hydrocarbon streams, is described. Three columns were necessary to achieve separation of the sulfur compounds from each other and from interferences. These procedures are applicable up to the 50-ppM level. Higher levels are measured after dilution. The lower detection limits in mole ppM are: COS and CS/sub 2/, 0.2; SO/sub 2/, 0.4; and H/sub 2/S, 0.8. The standard deviation for H/sub 2/S at 6.70 mole ppM level is 0.23 and for COS at 3.97 mole ppM is 0.06. Although not measured, CS/sub 2/ is expected to behave like COS and SO/sub 2/ like H/sub 2/S. The accuracy of the blends prepared by means of permeation tubes is estimated to be 3% of the amount present.

Determination of hydrogen sulfide, carbonyl sulfide, carbon disulfide, and sulfur dioxide in gases and hydrocarbon streams by gas chromatography/flame photometric detection

A novel hyperspectral imaging system has been developed that takes advantage of the tunable path delay between orthogonal polarization states of a liquid crystal variable retarder. The liquid crystal is placed in the optical path of an imaging system and the path delay between the polarization states is varied, causing an interferogram to be generated simultaneously at each pixel. A data set consisting of a series of images is recorded while varying the path delay; Fourier transforming the data set with respect to the path delay yields the hyperspectral data-cube. The concept is demonstrated with a prototype imager consisting of a liquid crystal variable retarder integrated into a commercial 640x480 pixel CMOS camera. The prototype can acquire a full hyperspectral data-cube in 0.4 s, and is sensitive to light over a 400 nm to 1100 nm range with a dispersion-dependent spectral resolution of 450 cm(-1) to 660 cm(-1). Similar to Fourier transform spectroscopy, the imager is spatially and spectrally multiplexed and therefore achieves high optical throughput. Additionally, the common-path nature of the polarization interferometer yields a vibration-insensitive device. Our concept allows for the spectral resolution, imaging speed, and spatial resolution to be traded off in software to optimally address a given application. The simplicity, compactness, potential low cost, and software adaptability of the device may enable a disruptive class of hyperspectral imaging systems with a broad range of applications.

Hyperspectral imaging with a liquid crystal polarization interferometer

A comparison of signal-to-noise ratios for single channel methods (sequential and multiplex) vs multichannel methods in optical spectroscopy☆

Theoretical comparison of the signal-to-noise ratios of Fourier transform spectrometry with single slit linear and slewed scan spectrometric methods for the photon noise limited situation

J. J. Fitzgerald

Papers

A comparison of signal-to-noise ratios for single channel methods (sequential and multiplex) vs multichannel methods in optical spectroscopy☆

Theoretical comparison of the signal-to-noise ratios of Fourier transform spectrometry with single slit linear and slewed scan spectrometric methods for the photon noise limited situation

Correction. Selectively-Modulated Interferometric Dispersive Spectrometer for Ultraviolet-Visible Atomic and Molecular Spectrometry