Simultaneous Array Detectors

FIGURE11.7 A DCD used for simultaneous measurement of ions separated by a Mattauch-Herzog double-focusing magnetic sector mass spectrometer. 

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Judging from the degree of apparent interest and the number of papers published in the field of elemental TOF-MS over the last 3-4 years, it appears that this marriage is one full of promise for the future of elemental analysis. Perhaps the primary reason for such a trend is the need for a truly simultaneous mass spectrometer capable of extending capabilities beyond current instrumentation. The fields of ICP and GD atomic emission spectroscopy have been revolutionized by the incorporation of simultaneous array detectors. This revolution is just now beginning in the mass spectrometry field. 

An array ion collector (detector) consists of a large number of miniature electron multiplier elements arranged side by side along a plane. Point ion collectors gather and detect ions sequentially (all ions are focused at one point one after another), but array collectors gather and detect all ions simultaneously (all ions are focused onto the array elements at the same time). Array detectors are particularly useful for situations in which ionization occurs within a very short space of time, as with some ionization sources, or in which only trace quantities of a substance are available. For these very short time scales, only the array collector can measure a whole spectrum or part of a spectrum satisfactorily in the time available. 

Recording of the dispersed ion beams can take place simultaneously across a plane, as in an array detector, or, as described here, by being brought to a focus at one point sequentially. 

The ions in a beam that has been dispersed in space according to their various m/z values can be collected simultaneously by a planar assembly of small electron multipliers. All ions within a specified mass range are detected at the same time, giving the array detector an advantage for analysis of very small quantities of any one substance or where ions are produced intermittently during short time intervals. 

Sohd-state multi-element detector arrays in the focal planes of simple grating monochromators can simultaneously monitor several absorption features. These devices were first used for uv—vis spectroscopy. Infrared coverage is limited (see Table 3), but research continues to extend the response to longer wavelengths. Less expensive nir array detectors have been appHed to on-line process instmmentation (125) (see Photodetectors). 

ICP-MS presents various shortcomings as compared to the requirements of an ideal PS-MS technique (Tables 8.62 and 8.56). Simultaneous detectors, as in ToF-MS or array-detector atomic mass spectra (ADAMS), offer several advantages in terms of sensitivity, precision, LOD (50ppq), resolving power and sample throughput. PS-ToFMS and ICP-ADAMS are still in their infancy. 

There is a recent trend towards simultaneous CE separations of several classes of food additives. This has so far been applied to soft drinks and preserved fruits, but could also be used for other food products. An MEKC method was published (Lin et al., 2000) for simultaneous separation of intense sweeteners (dulcin, aspartame, saccharin and acesulfame K) and some preservatives (sorbic and benzoic acids, sodium dehydroacetate, methyl-, ethyl-, propyl- and isopropyl- p-hydroxybenzoates) in preserved fruits. Ion pair extraction and SPE cleanup were used prior to CE analysis. The average recovery of these various additives was 90% with good within-laboratory reproducibility of results. Another procedure was described by Frazier et al. (2000b) for separation of intense sweeteners, preservatives and colours as well as caffeine and caramel in soft drinks. Using the MEKC mode, separation was obtained in 15 min. The aqueous phase was 20 mM carbonate buffer at pH 9.5 and the micellar phase was 62 mM sodium dodecyl sulphate. A diode array detector was used for quantification in the range 190-600 nm, and limits of quantification of 0.01 mg/1 per analyte were reported. The authors observed that their procedure requires further validation for quantitative analysis. 

Y. Zuo, H. Chen and Y. Deng, Simultaneous determination of catechins, caffeine and gallic acids in green, Oolong, black and pu-erh teas using HPLC with a photodiode array detector. Talanta 57 (2002) 307-316. 

Nowadays, forward geometry instruments are often constructed to be used in combination with an (optional) array detector, e.g., the JEOL HX-110 (EB), the Thermo Finnigan MAT 900 (EB) and the Micromass Autospec (EBE) instruments can be equipped in that way. The array detector is then located at the focus plane of the magnet, but different from the Mattauch-Herzog design it only covers a comparatively small m/z range simultaneously (Chap. 4.8). 

Fig. 4.63. Array detector in the focal plane of a magnetic sector to detect a small mass range simultanously. By courtesy of Thermo Electron (Bremen) GmbH.

Another application utilizing the coulometric array detector is the simultaneous determination of biogenic amines, kynurenine, and indole derivatives of tryptophan. The method employed a CIS column with a phosphate-acetate mobile phase (pH 4.1) containing methanol and sodium octyl sulphonate (Vaarman et al., 2002). 

In step 2, the migration times of the solute and the marker of the electroosmosis, such as mesityl oxide, were measured at each pH and converted to the effective mobility. When the CE instrument is equipped with a photodiode array detector, the spectrophotometric method is available simultaneously. The buffers should be exchanged every five runs, because the pH of the buffer was changed by electrolysis during CE analyses. The details of the experimental conditions are described in Ref. 20. 

Ferreira. Development of an HPLC/ CA090 diode-array detector method for simultaneous determination of seven hydroxyl-cinnamic acids in green coffee. J LiqChromRelTechnol 1997 20(13) CA091... 

CCD detector consists of 224 linear photodetector arrays on a silicon chip with a surface area of 13 x 18 mm (Fig. 4.16). The array segments detect three or four analytical lines of high analytical sensitivity and large dynamic range and which are free from spectral interferences. Each subarray is comprised of pixels. The pixels are photosensitive areas of silicon and are positioned on the detector atx -y locations that correspond to the locations of the desired emission lines generated by an echelle spectrometer. The emission lines are detected by means of their location on the chip and more than one line may be measured simultaneously. The detector can then be electronically wiped clean and the next sample analysed. The advantages of such detectors are that they make available as many as ten lines per element, so lines which suffer from interferences can be identified and eliminated from the analysis. Compared with many PMTs, a CCD detector offers an improvement in quantum efficiency and a lower dark current. 

Figure 3.6 Schematicofa double-focusing sector field mass spectrograph with Mattauch-Herzog geometry with a linear imaging curve (double focusing for ions of all masses simultaneously) modified field combination with 70°. In the mass spectrograph, photographic ion detection or focal plane array detectors are used for quasi-simultaneous detection of separated ion beams. (H. Kienitz (ed.), Massen-spektrometrie (1968), Verlag Chemie, Weinheim. Reproduced by permission of Wiley-VCH)...

Recently, Hieftje et al.15-16 equipped a small double-focusing mass spectrograph built in house with Mattauch-Herzog geometry with several ion sources (such as glow discharge, an inductively coupled plasma ion source or a microwave plasma torch) and a novel array detector for simultaneous ion detection. 

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