M. B. Denton

Charge Transfer Device Detectors for Analytical Optical Spectroscopy—Operation and Characteristics

This article is the first in a two-part series describing the operation, characteristics, and application of a new class of solid-state multichannel UV-visible detectors. In this paper, charge transfer devices (CTDs) are described. Detector characteristics pertinent to spectroscopic application—including quantum efficiency, read noise, dark count rate, and available formats—are emphasized. Unique capabilities, such as the ability to nondestructively read out the detector array and the ability to alter the effective detector element size by a process called binning, are described. CTDs with peak quantum efficiencies over 80% and significant responsivity over the wavelength range of 0.1 nm to 1100 nm are discussed. Exceptionally low dark count rates, which allow integration times of up to many hours and read noises more than two orders of magnitude lower than those read by commercially available PDA detectors, contribute to the outstanding performance offered by these detectors.

Spectrochemical Measurements with Multichannel Integrating Detectors

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 spectrophotometry are also discussed.

Qualitative Aspects of an Inductively Coupled Plasma in the Spectral Region between 120 and 185 nm

Investigations of the atomic emission lines produced by a variety of non-metals in the vacuum ultraviolet spectral region are reported. A number of promising analytical lines for oxygen, nitrogen, carbon, bromine, sulfur, and chlorine was observed between 120 and 185 nm using both photographic and electronic detection. A unique experimental configuration employing a side-arm torch which directly couples to the vacuum spectrometer/spectrograph is described.

Characteristics of a “High Solids” Nebulizer for Flame Atomic Absorption Spectrometry

Studies are presented to characterize a new nebulizer developed for direct atomic absorption analysis of extremely complex clinical and environmental materials. Important parameters are described concerning the design and performance of “high solids” spectrochemical nebulizers based on the Babington principle. A new simplified design is presented and mechanisms of undesirable sample wastage are considered. Data are presented concerning the effect of impaction on conventional and “high solids” aerosol generation.

Elemental Analysis with a Plasma Emission Echelle Spectrometer Employing a Charge Injection Device (CID) Detector

An atomic emission spectrometer which allows simultaneous high-precision digital recording of the ultraviolet spectrum has been developed. The instrument employs both a custom-built echelle spectrometer and a custom-built slow-scan charge injection device (CID) based detector system. The system is capable of measuring the wide dynamic range of signal intensities associated with plasma emission sources, and sensitivity is comparable to that of photomultiplier-tube-equipped instruments. Unprecedented speed and flexibility for elemental analysis are provided by the ability to display background-subtracted emission spectra and to have the computer assist in spectral line identification. The design and performance of the optical system, methodologies for CID detector utilization in analytical spectroscopy, and techniques for wavelength calibration are presented. Examples for qualitative analysis and for the qualitative comparison of similar samples demonstrate the sensitivity, flexibility, and speed of the system.

HYDRIDE PRECONCENTRATION FOR INDUCTIVELY COUPLED PLASMA OPTICAL EMISSION SPECTROMETRY.

Studies are presented describing an improved application of the NaBH4 reduction of soluble arsenite to form arsine as a preconcentration approach for ultra-trace level arsenic determination by inductively coupled plasma optical emission spectrometry. Specialized analyte introduction techniques are described for elimination of reaction by-products that would normally extinguish a medium power plasma discharge. An approach is presented to minimize the need for background correction and facilitate a superior arsenic detection limit (≤0.03 ng/ml) in a relatively inexpensive 1.2 kW inductively coupled plasma system.

Information-Based Expert Systems for Atomic Emission Spectroscopy

The development of the direct-current plasma echelle/CID spectroscopic system for atomic emission spectroscopy (AES) provides new alternatives for automated system control and data analysis. With this system, the concept of the “intelligent” spectrometer becomes tangible. The echelle/CID system simultaneously gathers a wealth of spectral information over a wide spectral region. The mechanical stability of the system and the absence of moving parts give rise to reproducible wavelength assignment. The large amount of spectral information acquired has led to the development of information-based expert systems for AES: automated qualitative analysis, semi-quantitative analysis, and an “on the fly” matrix-dependent line selection. These algorithms are effective in situations where there is a large variability among samples. The analytical power of these routines is heavily dependent on their utilization of the large database and the use of fundamental spectroscopic principles. Examples of the use of these algorithms in environmental monitoring, in the identification of chemical waste, in the analysis of geologic materials and steels, and in HPLC-AES are presented.

On the Use of the Inverted Abel Integral for Evaluating Spectroscopic Sources

A numerical method for evaluating the inverted Abel integral employing cubic spline approximations is described along with a modification of the procedure of Cremers and Birkebak, and an extension of the Barr method. The accuracy of the computations is evaluated at several noise levels and with varying resolution of the input data. The cubic spline method is found to be useful only at very low noise levels, but capable of providing good results with small data sets. The Barr method is computationally the simplest, and is adequate when large data sets are available. For noisy data, the method of Cremers and Birkebak gave the best results.

Spark Spectroscopy Using Charge Transfer Devices: Analysis, Automated Systems, and Imaging

An atomic emission spectroscopic system utilizing a spark source for excitation has been developed. The instrument employs a custom echelle spectrometer and a charge injection device (CID) array detector system. This system simultaneously covers wavelengths from 200 to 450 nm with a resolution of 0.02 nm at 300 nm. Solids sample analyses of steels and aluminums were used to demonstrate this systems speed, sensitivity, and flexibility. Automated systems for rapid qualitative and semi-quantitative screening of these materials will also be discussed. Another spectroscopic system based on a commercial imaging spectrograph and a charge-coupled device (CCD) array detector has been used to obtain temporally resolved spectral images of single sparks discharges.

Radiation Exposure Considerations When Employing Microwave-Excited Spectroscopic Sources

Studies are described which demonstrate the presence of microwave radiation levels exceeding the present national safety standards in the vicinity of several common microwave-excited source configurations. Since the various pathological effects of microwave energy are not fully understood, shielding considerations and on-site radiation surveys are recommended.