Array detector systems

Figure 15.8—Coupling of a gas chromatograph with an atomic emission spectrophotometer. Effluents from the capillary column are injected into the plasma and decomposed into their elements. Each chromatogram corresponds to the compound containing the element of interest. For a given retention time, indication as to the elements included in a compound can be obtained. The plasma in this example is generated by heating the carrier gas (He) with a microwave generator confined in a cavity at the exit of the column. A diode array detector system can be used for simultaneous detection of many elements (chromatograms courtesy of a Hewlett Packard document).

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. 

Figure 2. Scanning time sequence for multiple integration time array detector system.

The limited availability of affordable commercial RSSF instruments has been an important factor that has prevented the widespread application of RSSF spectroscopy to the study of biological systems. However, in the past year, a significant change in the availability of commercial instrumentation hats come about. There currently are at least five manufacturers of computerized rapid-scanning detector systems. The choices in commercial instrumentation range from a mechanically scanned system with a single photomultiplier detector to photodiode array detector systems. This review includes descriptions of the currently available commercial systems. Because the authors experience in the field of RSSF spectroscopy is limited to the use of diode array detector systems and because most of the commercial instruments have appeared on the market just within the past 12 months, it has not been possible to make detailed performance evaluations and comparisons of the new commercial systems. 

Complete general-purpose array-deleclor-based specirophoiomciers are available commercially for 50(X) to 10,000 and up. Several instrument companies combine array-detector systems with fiber-optic prt)bcs that transport the light to and from the sample. 

Recently, an indirect method to measure the free radical scavenging activity of extracts and, thus, their antioxidant activity has been developed using HPLC coupled to a coulometric array detector system [61]. This was developed to characterise the overall antioxidant activity status of fruit and vegetables. A significant positive linear correlation was demonstrated between the total antioxidant activity determined by using the oxygen radical absorbance capacity assay and that measured using the electrochemical data obtained from the coulometric array detectors. 

Fluorescence spectra were obtained with an 0-SMA diode-array detector system (Spectroscopy Instruments GmbH). 

Other aspects of Raman instrumentation are reviewed elsewhere in this volume. For the purposes of this discussion, we assume a compact spectrograph and array detector system providing a spectral resolution of 20 cm" or better and simultaneous acquisition of a spectral window 1000 cm or more in width. We wish to direct our attention to elements employed in calibrating the system a white-light source for ordinate calibration and an atomic line source and Raman shift standard for abscissa calibration. 

Therefore it is reasonable to prepare already the data acquisition for a three dimensional evaluation in cone-beam-technique by means of two-dimensional detectors. The system is already prepared to integrate a second detector- system for this purpose. An array of up to four flat panel detectors is foreseen. The detector- elements are based on amorphous silicon. Because of the high photon energy and the high dose rates special attention was necessary to protect the read-out electronics. Details of the detector arrangement and the software for reconstruction, visualisation and comparison between the CT results and CAD data are part of a separate paper during this conference [2]. 

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. 

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

Diode Array and Charge-Coupled Detector Systems... 

A method which uses supercritical fluid/solid phase extraction/supercritical fluid chromatography (SE/SPE/SEC) has been developed for the analysis of trace constituents in complex matrices (67). By using this technique, extraction and clean-up are accomplished in one step using unmodified SC CO2. This step is monitored by a photodiode-array detector which allows fractionation. Eigure 10.14 shows a schematic representation of the SE/SPE/SEC set-up. This system allowed selective retention of the sample matrices while eluting and depositing the analytes of interest in the cryogenic trap. Application to the analysis of pesticides from lipid sample matrices have been reported. In this case, the lipids were completely separated from the pesticides. 

The availability of HPLC systems coupled to photodiode array detectors allows for online spectral characterization of anthocyanins. 

An HPLC system equipped with an ultraviolet/diode-array detector (UV/DAD) and automated column switching system is used. 

FIG. 1 Schematic drawing of the high-speed stirring (HSS) apparatus. An organic phase separated from the dispersed system by the teflon phase separator is continuously circulated through the photodiode array detector. 

UV detection, diode-array detector (DAD) and fluorescence have been the detection techniques used, coupled to HPLC for the analysis of OTC. UV detection is set at 355 nm [49-51], 350 nm [40], or at 353 nm [52], Using the diode array detector [49] offers advantages that the target peak can be identified by its retention time and absorption spectrum. Compared to UV detection, fluorescence detection is generally more specific and is less interfered by other compounds in the sample matrix [51]. A HPLC method with electrochemical detection has also been suggested recently. Zhao et al. [53] described HPLC with a coulometric electrode array system for the analysis of OTC, TC, CTC, DC, and methacycline (MC) in ovine milk. An amper-ometric detection coupled with HPLC was developed by Kazemifard and Moore [54] for the determination of tetracyclines in pharmaceutical formulations. 

Procedure Phenolic acids were detected between 210 and 360 nm using a Hewlett Packard diode array detector (HP 1100 HPLC system). The separation was achieved with a Nucleosil 100-5 C18 column 5 pm 4.0x250 mm (Agilent Technologies, USA) at a flow rate of 1.0 ml/min and injection volume of 5 mL. For the elution, a discontinuous acetonitrile-water gradient was used 15% acetonitrile (5 min), 30% acetonitrile (20 min), 40% acetonitrile (25 min), 60% acetonitrile (30 min), 60% acetonitrile (35 min) and... 

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