Photodiode array spectrometer system

A Dual-Beam Photodiode Array Spectrometer System for Liquid Chromatographic Separation Methods Development... 

The classical silicon photodiode linear array manufactured by Reticon was the first detector marketed successfully. Similar solid state linear array detectors based on charge coupled devices, or charge.injection devices may also be of interest. Typical of the families of detectors, the Reticon detectors are built in a number of elements/array sizes. Commercially available units have anywhere from 128 to 1024 elements/array. Each individual element in the array is 1 x 1 mil to 1 x 100 mil in area, and are spaced on approximately 1 mil centers. The spectrometer system discussed in this article was built using Reticon-type devices. 

Most of the detectors permit peak recognition but provide no structural information, which can be particularly important for identification of unknown compounds. From this point of view, the spectro-metric detectors, specifically mass spectrometer and photodiode array detectors, add a third dimension to the multidimensional system and give additional information useful in components identification. 

The main components of an LC-MS are the HPLC apparatus, an optional UV or photodiode array detector, the interface, the mass spectrometer and a computer system for data management and evaluation. The interface is the key component of the LC-MS system. All other components must be adapted to the particular interface that is used. Most commercially available systems work with thermospray, electrospray, or particle beam interfaces. Each interface has a distinct mode of action and its own operational parameters. 

The commercial softwares, initially developed by instrament manufacturers for open-access operation, were adapted to enable unattended data acquisition and automated data processing for large series of samples from an autosampler supporting the 96-well microtitre plate format, which is the sample format of choice in combinatorial synthesis. Initially, mainly Gilson 215 or 233 XL autosamplers were used, but other systems have become available from other instrument manufacturers. The complete system is under control of the MS data system. It consists of a 96-well-plate autosampler, an LC pumping system, eventually a UV-photodiode-array detector (DAD) in series and/or evaporative hght scattering (ELSD) detector in parallel, and the mass spectrometer eqnipped with ESI, APCI, and/or atmospheric-pressure photoionization (APPI). 

Detailed experimental procedures for obtaining infrared spectra on humic and fulvic acids have been reported previously 9,22,25-26) and will be briefly described here. Infrared spectra were taken on the size-fractionated samples by using a Fourier transform infrared spectrometer (Mattson, Polaris) with a cooled Hg/Cd/Te detector. Dried humic and fulvic materials were studied by diffuse reflectance infrared spectroscopy (Spectra Tech DRIFT accessory) and reported in K-M units, as well as by transmission absorbance in a KBr pellet. Infrared absorption spectra were obtained directly on the aqueous size-fractioned concentrates with CIR (Spectra Tech CIRCLE accessory). Raman spectra were taken by using an argon ion laser (Spectra-Physics Model 2025-05), a triple-grating monochromator (Spex Triplemate Model 1877), and a photodiode array detector system (Princeton Applied Research Model 1420). All Raman and infrared spectra were taken at 2 cm resolution. 

Products/technologies The Waters LC-MS solutions for drug discovery consist of a Waters Alliance HPLC system (with optional 996-photodiode array or 2487 UV/Vis detectors), Waters 2700 Sample Manager (for 96-well microtiter plates), Micromass Platform LC or Platform LCZ benchtop API mass spectrometer, and MassLynx software with special options for combinatorial... 

An HPLC detector measures the concentration (or mass) of eluting analytes4 by monitoring one of their inherent properties, such as UV absorbance. A detector can be universal to all analytes or specific to particular classes of analytes. Common detectors and their attributes are listed in the Table 4.2. Early HPLC detectors are spectrometers equipped with small flow cells however, most modern units are compact and designed solely for HPLC. The ubiquitous UV/visible variable wavelength absorbance and the photodiode array detectors (PDA) are covered in more depth in this section. Note that mass spectrometers (MS) and nuclear magnetic resonance spectrometers (NMR) are discussed in the section on hyphenated systems. 

CCD array at the end of the section. Typical photodiode arrays from Princeton Instruments and other manufacturers have individual diodes with a center-to-center spacing width of 25 tm and a height of 2 mm. This height matches the height of the 1-to-l image of the arc in a Xe lamp. The center-to-center spacing of the photodiodes and the dispersion of the spectrometer determine the spectral resolution of the system. 

The optical detection systems used in MIPs are the same as those used for other atomic spectrometers and can be either single or multichannel. Fourier transform-based spectrometers have also been used. Conventional optical systems are best designed if the plasma is viewed from the exit of the discharge tube, as is possible with the TMqio type cavity, rather than through the walls of the discharge tube, which become etched. The commercially available AED uses a computer-controlled silicon photodiode array detector which has multielement detection capability over segments of spectra. In recent years, MIP sources have also been investigated as ion sources for mass spectrometry. 

The common detector for AAS is the photomultiplier tube (PMT). The construction and operation of a PMT has been described in Chapter 5. While PMTs are the most common detectors, solid-state single and multichannel detectors such as photodiode arrays (PDAs) (discussed in Chapter 5) and charge-coupled devices (CCDs) (discussed in Chapters 5 and 7) are increasingly being nsed in AAS spectrometers. Many small systems, particularly those dedicated to one element snch as a dedicated CVAAS mercury analyzer, use solid-state detectors instead of PMTs. Multielement simnltaneous AAS systems also use multichannel solid-state detectors to measure more than one wavelength at a time. 

RP HPLC has proved to be the method of choice for the separation of a variety of flavonoids in different samples. The phenolic nature of these compounds requires the use of acidic mobile phases for satisfactory separation and peak shapes, whereas the detection is usually carried out with photodiode array detectors which are also very helpful for their identification of the characteristic absorption spectra of the flavonoids. In the last decade, mass spectrometers connected to HPLC systems introduced a greater selectivity and sensitivity in flavonoid analysis. Improving the characteristics of the stationary phases and developing more sophisticated instruments as well as devices for more efficient and faster sample preparation are the challenges for all modem analysts. Discovering... 

FIGURE 24.7 Single [M + H] ions and UV chromatograms at 260- and 330-nm wavelengths of flavonoid compounds present in leaves of narrow-leafed lupine (Lupinus angustifolius) recorded using a hybrid LC/MS/UV system (photodiode array quadru-pole time-of-flight mass spectrometer) (reproduced from Reference [79]). 

The simplest type of experiment with an OTTLE is aimed simply at identifying reaction products. The electroactive species in solution is electrolysed at constant potential until the current decreases to zero (typically 30-60 s), and then the spectrum of the product is recorded. Provided this product contains a strong chromophore, a conventional spectrometer is usually adequate alternatively an RSS or photodiode array system is used. In view of the timescale... 

Milano et al. [153, 154] and Cook [34] introduced an approach to derivative spectra by substituting electronic wavelength modulation for the mechanical systems used in derivative spectrometers. This effect is achieved by superimposing a low-amplitude, periodic wave form on the horizontal sweep signal. In this way spectra were generated. Warner et al. [155] applied a vidicon detector for fast detection of fluorescence spectra and obtained derivatives of the stored data by digital computation. Cook et al. [156] also made use of a silicon vidicon detector for multichannel operations in rapid UV-VIS spectrophotometers with the possibility of first-order differentiation. For the same purpose Milano et al. [93, 157] used a multichannel linear photodiode array for detection of spectra in polychromator optics and stored data manipulations (d ). Technical explanations of the principles of diode array and vidicon devices cem be found in [158-161]. 

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