Detector array

The microchannel plate is a spatially resolved array detector formed of 10 -10 continuous-dynode EMs, each only 10-100 pm in diameter. This detector is used in focal plane mass spectrometers as a replacement for photograph plate detectors and is used in some TOFMS instruments. 

The focal plane camera (FPC), still in initial development, consists of an array of 31 Faraday cups, each 145 pm wide. Up to 15 m/z values can be measured simultaneously. This detector shows improved precision compared with single channel detectors and has the ability to measure fast transient signals such as those from laser ablation. The detector design is described in the references by Barnes et al. and Knight et al. cited in the bibliography. 

Dual electrodes in series are used relatively frequently. Matson et al. extended this approach to an n-electrode, three-dimensional HPLC-ED system. Using coulometric flow-through electrodes, up to 15 such electrodes were incorporated into one housing. Each electrode was held at incremental working voltages from 0 to 650 mV and a series of chromatograms were obtained simultaneously. Each electrode was monitored every 50 ms and the output from each electrode was 

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Spectroscopy Partl, AnaZ. Chem. 1985, 57, 1057A-1073A. Jones, D. G. Photodiode Array Detectors in UV-Vis... 

Colorplate 12 shows a photo of an HPLC equipped with a diode array detector. 

Remcho, V. T. McNair, H. M. Rasmussen, H. T. HPLC Method Development with the Photodiode Array Detector, /. Chem. Educ. 1992, 69, A117-A119. 

A mixture of methyl paraben, ethyl paraben, propyl paraben, diethyl phthalate, and butyl paraben is separated by HPLC. This experiment emphasizes the development of a mobile-phase composition capable of separating the mixture. A photodiode array detector demonstrates the coelution of the two compounds. 

In one instrument, ions produced from an atmospheric-pressure ion source can be measured. If these are molecular ions, their relative molecular mass is obtained and often their elemental compositions. Fragment ions can be produced by suitable operation of an APCI inlet to obtain a full mass spectrum for each eluting substrate. The system can be used with the effluent from an LC column or with a solution from a static solution supply. When used with an LC column, any detectors generally used with the LC instrument itself can still be included, as with a UV/visible diode array detector sited in front of the mass spectrometer inlet. 

The array detector (collector) consists of a number of ion-collection elements arranged in a line each element of the array is an electron multiplier. Another type of array detector, the time-to-digital converter, is discussed in Chapter 31. 

The major advantage of array detectors over point ion detectors lies in their ability to measure a range of m/z values and the corresponding ion abundances all at one time, rather than sequentially. For example, suppose it takes 10 msec to measure one m/z value and the associated number of ions (abundance). To measure 100 such ions sequentially with a point ion detector would necessitate 1000 msec (1 sec) for the array detector, the time is still 10 msec because all ions arrive at the same time. Therefore, when it is important to be able to measure a range of ion m/z values in a short space of time, the array detector is advantageous. 

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. 

Each element of an array detector is essentially a small electron multiplier, as with the point ion collector, but much smaller and often shaped either as a narrow linear tube or as somewhat like a snail shell. 

Array detectors are particularly useful for detecting ions from either a very small amount of a substance or when ionization is not continuous but intermittent. 

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. 

A series of consecutive time bins covers a length of time of a few milliseconds, with each bin representing a time of only a fraction of a nanosecond. When an ion arrives at the microchannel array detector, one time bin notes the resulting electronic pulse. 

One drawback of time bins relates to the events they record. If two or more ions arrive at the array detector at the same instant, the resulting electrical pulse is the same as if only one ion had arrived. The bins are blind to multiple concurrent events. 

To give increased sensitivity when the analysis is not limited by chemical noise, eg, in the ms/ms mode, array detectors (15) have been developed. 

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). 

Hplc techniques are used to routinely separate and quantify less volatile compounds. The hplc columns used to affect this separation are selected based on the constituents of interest. They are typically reverse phase or anion exchange in nature. The constituents routinely assayed in this type of analysis are those high in molecular weight or low in volatility. Specific compounds of interest include wood sugars, vanillin, and tannin complexes. The most common types of hplc detectors employed in the analysis of distilled spirits are the refractive index detector and the ultraviolet detector. Additionally, the recent introduction of the photodiode array detector is making a significant impact in the analysis of distilled spirits. 

There has been a tremendous change in the last two decades as computers have taken over the tedious calculations involved in color measurement. Indeed, microprocessors either are built into or are connected to all modem instmments, so that the operator may merely need to specify, for example, x,j, Y or T, i , b or T, (A, b, either for the 2° or the 10° observer, and for a specific standard illiiminant, to obtain the desired color coordinates or color differences, all of which can be stored for later reference or computation. The use of high intensity filtered Xenon flash lamps and array detectors combined with computers has resulted in almost instantaneous measurement in many instances. 

A capillary electrophoresis systems Agilent CE 1100 (HP, USA) equipped with a diode array detector was used to separate and quantify... 

Separation of C oand C70 can be achieved by HPLC on a dinitroanilinopropyl (DNAP) silica (5pm pore size, 3(X)A pore diameter) column with a gradient from H-hexane to 50% CH2CI2 using a diode array detector at wavelengths 330nm (for C q) and 384nm (for C70). [J Am Chem Soc 113, 2940, 1991.]... 

The particle size analyzer, based on laser light diffraction, consists of a laser source, beam expander, collector lens, and detector (Fig. ] 3.45). The detector contains light diodes arranged to form a radial diode-array detector. The particle sample to be measured can be blown across the laser beam (dry sample), or it can be circulated via a measurement cell in a liquid suspension. In the latter case, the beam is direaed through the transparent cell. 

Photodiodes produce an electric field as a result of pn transitions. On illumination a photocurrent flows that is strictly proportional to the radiation intensity. Photodiodes are sensitive and free from inertia. They are, thus, suitable for rapid measurement [1, 59] they have, therefore, been employed for the construction of diode array detectors. 

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