Elemental detection limits monitored

Table 8.25 Principal spectral emission lines, minimum detectable limits, and linear dynamic range of some commonly monitored elements in GC-AED...

XOD is one of the most complex flavoproteins and is composed of two identical and catalytically independent subunits each subunit contains one molybdenium center, two iron sulfur centers, and flavine adenine dinucleotide. The enzyme activity is due to a complicated interaction of FAD, molybdenium, iron, and labile sulfur moieties at or near the active site [260], It can be used to detect xanthine and hypoxanthine by immobilizing xanthine oxidase on a glassy carbon paste electrode [261], The elements are based on the chronoamperometric monitoring of the current that occurs due to the oxidation of the hydrogen peroxide which liberates during the enzymatic reaction. The biosensor showed linear dependence in the concentration range between 5.0 X 10 7 and 4.0 X 10-5M for xanthine and 2.0 X 10 5 and 8.0 X 10 5M for hypoxanthine, respectively. The detection limit values were estimated as 1.0 X 10 7 M for xanthine and 5.3 X 10-6M for hypoxanthine, respectively. Li used DNA to embed xanthine oxidase and obtained the electrochemical response of FAD and molybdenum center of xanthine oxidase [262], Moreover, the enzyme keeps its native catalytic activity to hypoxanthine in the DNA film. So the biosensor for hypoxanthine can be based on... 

The development of these sampling techniques will thus solve the inherent problems encountered in the analysis of trace elements in seawater. The additional pre-concentration which occurs during sampling allows for the simple analysis of samples by existing techniques with even moderate detection limits. In this way, we will be better equipped to monitor the concentration of trace elements in seawater and determine their effect on the environment. 

The trace elemental composition for carbonate was determined by WDS on a JEOL 733 electron microprobe. Accelerating voltage was 15 kV sample current was stabilized at 12 nA on brass spot size was lOjim count time for all elements was 20 s, except for Sr, analysed for 60 s standards were cal-cite (Ca), dolomite (Ca, Mg), coral (Sr) and siderite (Fe, Mn) beam placement was in every case guided by the back-scattered electron image Si was routinely counted by WDS in order to monitor possible contamination from adjacent silicate grains not revealed by the back-scattered image totals between 97 and 103 wt% were accepted. Detection limits are approximately 340 ppm for Mg, 450 ppm for Fe, 310 ppm for Mn and 185 ppm for Sr. 

Complete MCP s can be stacked to provide even higher gains. For response in the vacuum ultra-violet spectral region (50-200 nm) a SSANACON, self-scanned anode array with microchannel plate electron multiplier, has been used (36). This involves photoelectron multiplication through two MOP S, collection of the electrons directly on aluminum anodes and readout with standard diode array circuitry. In cases where analyte concentrations are well above conventional detection limits, multi-element analysis with multi-channel detectors by atomic emission has been demonstrated to be quite feasible (37). Spectral source profiling has also been done with photodiode arrays (27.29.31). In molecular spectrometry, imaging type detectors have been used in spectrophotometry, spectrofluometry and chemiluminescence (23.24.26.33). These detectors are often employed to monitor the output from an HPLC or GC (13.38.39.40). 

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