Search

In 1938, Walker Bleakney and John A. Hipple first described the cycloidal mass analyzer as the only mass analyzer configuration capable of "perfect" ion focusing. Why has their geometry been largely neglected for many years and how might it earn a respectable place in the world of modern chemical analysis? This Perspective explores the properties of the cycloidal mass analyzer and identifies the lack of suitable ion array detectors as a significant reason why cycloidal mass analyzers are not widely used. The recent development of capacitive transimpedance amplifier array detectors can enable several techniques using cycloidal mass analyzers including spatially coded apertures and single particle mass analysis with a "virtual-slit", helping the cycloidal mass analyzer earn a respectable place in chemical analysis.

A novel charge-sensitive detector array, termed the focal plane camera (FPC), has been coupled to a Mattauch-Herzog mass spectrograph (MHMS) with an inductively coupled plasma ionization source. The FPC employs an array of gold Faraday cups, each with its own charge-integrating circuit that allows the simultaneous detection of several m/z ratios. The ion-sampling interface of the MHMS has been redesigned to provide better heat transfer away from the sampler and skimmer cones and to reduce the negative effects of turbulent gas flows around the plasma. The instrument has produced limits of detection in the tens to hundreds of parts per quadrillion regime and isotope ratio accuracy and precision of 5% error and 0.007% RSD, respectively. Limits of detection with the FPC are comparable to those obtained with a single-channel secondary electron multiplier (SEM). However, the isotope ratio accuracy and precision are better with the FPC than when the SEM is employed. The dynamic range has been shown to be linear over 7 orders of magnitude.

A study focusing on the ability to utilize isotopic analysis of Pb used in small arms projectiles from varying geographic origin was undertaken using the GV Instruments Isoprobe multicollector inductively coupled plasma mass spectrometer. With the exception of two classes of ammunition, these samples were able to be distinguished from one another. Also, following rigorous normalization and mass bias correction, these data were compared to data collected by geochemists in localities corresponding to projectile manufacture. This comparison was successful when projectiles in certain economically isolated countries were analyzed. Pb isotopic analysis with MC−ICPMS is shown to be useful as a tool for distinguishing between ammunition of various manufacturing origins.

CTDs are an attractive alternative to film, vidicons, photodiode arrays, and photomultiplier tubes

The use of a scientifically operated charge-coupled device (CCD) for the detection and quantitation of aflatoxins on a high-performance thin-layer chromatographic (HPTLC) plate was investigated. A nebulizer-based sample application system was utilized to quantitatively transfer the sample onto the HPTLC plate without disturbing the sorbent layer. Fluorescence excitation of the aflatoxins was accomplished with an ultraviolet transilluminator, which caused the analytes of interest to emit in the blue-green portion of the visible spectrum. The dynamic range, sensitivity, accuracy, and precision of the system were evaluated. Detection limits of the aflatoxins were determined to be in the low picogram range. The existence of aflatoxins in peanut butter samples was analyzed both quantitatively and qualitatively by using multidimensional TLC. The capabilities of the system for performing real-time monitoring of the separation process were also investigated.

A 512-channel Faraday-strip array detector has been developed and fitted to a Mattauch-Herzog geometry mass spectrograph for the simultaneous acquisition of multiple mass-to-charge values. Several advantages are realized by using simultaneous detection methods, including higher duty cycles, removal of correlated noise, and multianalyte transient analyses independent of spectral skew. The new 512-channel version offers narrower, more closely spaced pixels, providing improved spectral peak sampling and resolution. In addition, the electronics in the amplification stage of the new detector array incorporate a sample-and-hold feature that enables truly simultaneous interrogation of all 512 channels. While sensitivity and linear dynamic range remain impressive for this Faraday-based detector system, limits of detection and isotope ratio data have suffered slightly from leaky p-n junctions produced during the manufacture of the semiconductor-based amplification and readout stages. This paper describes the new 512-channel detector array, the current dominant noise sources, and the figures of merit for the device as pertaining to inductively coupled plasma ionization.

In mass spectrometry, several advantages can be derived when multiple mass-to-charge values are detected simultaneously and continuously. One such advantage is an improved duty cycle, which leads to superior limits of detection, better precision, shorter analysis times, and reduced sample sizes. A second advantage is the ability to reduce correlated noise by taking the ratio of two or more simultaneously collected signals, enabling greatly enhanced isotope ratio data. A final advantage is the elimination of spectral skew, leading to more accurate transient signal analysis. Here, these advantages are demonstrated by means of a novel Faraday-strip array detector coupled to a Mattauch−Herzog mass spectrograph. The same system is used to monitor elemental fractionation phenomena in laser ablation inductively coupled plasma mass spectrometry.

A second-generation Faraday-strip array detector has been coupled to an inductively coupled plasma Mattauch−Herzog geometry mass spectrograph, thereby offering simultaneous acquisition of a range of mass-to-charge ratios. The second-generation device incorporates narrower, more closely spaced collectors than the earlier system. Furthermore, the new camera can acquire signal on all collectors at a frequency greater than 2 kHz and has the ability to independently adjust the gain level of each collector. Each collector can also be reset independently. With these improvements, limits of detection in the hundreds of picograms per liter for metals in solution have been obtained. Some additional features, such as a broader linear dynamic range (over 7 orders of magnitude), greater resolving power (up to 600), and improved isotope ratio accuracy were attained. In addition, isotope ratio precision as low as 0.018% RSD was achieved.

A Mattauch-Herzog geometry mass spectrograph (MHMS) has been equipped with a novel array detector, the focal plane camera (FPC). The FPC consists of an array of gold Faraday cups, each coupled to its own integrator, with interrogation of the integrators performed by a multiplexer. The initial coupling of this instrument with a pin-type glow discharge source has provided limits of detection in the single to hundreds of nanograms per gram regime; isotope ratio accuracy and precision better than 5% error and 0.2% RSD, respectively; and a linear dynamic range of at least 6 orders of magnitude. A current weakness of the FPC is its pixel size, which limits both sensitivity and baseline resolution (to R = 130). The minimum data acquisition time for multiple images at present is 1 ms/image, with a dead time of 3.2 ms between images, which will limit the ability of the FPC to monitor extremely short transient signals.

The ability of the inductively-coupled plasma (ICP) gas chromatographic (GC) detector to determine empirical formulas is evaluated. Elemental compositions and empirical formulas of hydrocarbons and halogens are determined as the compounds are eluted from a chromatograph. The observed accuracy and precision are found to be of sufficient quality to indicate that the technique holds promise for the determination of empirical formulas. (Author)

A novel fluorescence spectrometer is described using a two-dimensional charge-coupled device as the detector and a mercury pen lamp as the excitation source. The fluorescence spectra and intensity of anthracene in ethanol standards at concentrations ranging from .0001 to 10 to the -11th power M are shown. The limit of detection measured is 10 to the -12th power M, or 227 parts-per-quadrillion. The linear dynamic range obtained with this instrument is 6 orders of magnitude with excellent linearity using 10 second signal integration time. Keywords: Charge coupled device; Molecular fluorescence; Multichannel detectors; Charge transfer device; Anthracene; Rayleigh spectra; Raman spectra.

This article is the first in a two-part series describing charge transfer devices (CTDs) -- a relatively new class of multichannel detectors for the ultraviolet to near-infrared spectral regions. An overview of the operation and the characteristics of CTDs relevant to analytical spectroscopy is presented. The sensitivity and dynamic range obtainable from CTD detectors are discussed and compared to the sensitivity and dynamic range obtainable from other spectroscopic detectors. Unique capabilities such as the ability to nondestructively readout the detector array and the ability to alter the effective detector element size using the process of binning are described. Detector array formats ranging from single elements to extremely large arrays, large photoactive areas, high quantum efficiencies, dark count rates allowing long integration times and low read noises all contribute to the outstanding performance and great flexibility offered by CTDs.