Array element selection

Element-selective detectors. Many samples, e.g. those originating from environmental studies, contain so many constituent compounds that the gas chromatogram obtained is a complex array of peaks. For the analytical chemist, who may be interested in only a few of the compounds present, the replacement of the essentially non-selective type of detector (i.e. thermal conductivity, flame ionisation, etc.) by a system which responds selectively to some property of certain of the eluted species may overcome this problem. 

Powerl first reads the numerical values of all cells, and stores them in an array. The mathematical operations are then performed on these stored data, so that there is no ambiguity involved. Consequently, Powerl yields correct results for either column. However, Powerl may fail when the selected block is too large (which, depending on the amount of available memory, and the version of Excel used, typically will happen only when the array has more than a few thousand elements). In that case, use Special Copy Values to duplicate the numerical values of the array elements in another block, then use a macro such as Power that does not suffer from size limitations. 

Methods for improving the time resolution capabilities of the sensor array have been identified, and an improved prototype is nearing completion. The inclusion of additional sensor array elements has been considered as a mean to further improve selectivity. However, this will increase the complexity of the sensor and may not improve selectivity enough to justify the added complexity. An evaluation of the benefits and risks of utilizing more array elements in the sensor is underway. The two-channel array prototype promises significant improvement in the false alarm performance of the sensor, improving on the already excellent selectivity of previous generations of Fido sensors. 

Phased arrays versus steering by element selection... 

FIGURE 25.9 Steering by element selection for a curvilinear array. The beam will shift to a new location as the center of the active aperture is shifted over the entire airay. 

The AED employs a microwave-induced He plasma to dissociate eluted analyte molecules to their component atoms and excite them to emit at characteristic wavelengths. This is very similar to the mechanism in the argon plasma inductively coupled plasma source (cf. Section 7.3.1). A spectrometer with a diode array detector (Figure 7.26b and c) isolates and measures the intensity of sensitive emission lines unique to each element. Depending on the relative sensitivity and proportion of atoms in the molecules, separate element response channels may display peaks in several element-selective chromatograms. These data may be combined with retention... 

The minimum critical mass calculations involved arrays of plates, cubes, or square cross-section rods in water. A minimum critical array mass is found by changing the spacing of elements, d, in a particular array until a maximum value of keff is found when keff is plotted versus d. A second element size is then selected for the array and the maximum keff and the associated array element spacing found. The mass and spacing of the minimum critical mass (keff = 1.0) array are obtained by linear interpolation between the masses and spacings at the maximum keff values. 

The required acoustic verifications depend on what the probe is made for. If the probe is used as an angular scamiing system with a fix set of elements, then we think it is only needed to characterize the array behavior with a few selected time delay laws to isolate the angular steering capability and the foeusing capability as explained before. 

Contrary to the expectation that a sulfur-containing substituent will be a catalyst poison, a phenylthio group serves as an effective selectivity control element in TMM cycloadditions. A single regioisomer (30) was obtained from the carbonate precursor (31) in good yield. The thermodynamically more stable sulfide (32) is readily accessible from (30) via a 1,3-sulfide shift catalyzed by PhSSPh. A wide array of synthetically useful intermediates could be prepared from the sulfides (30) and (32) with simple transformations (Scheme 2.10) [20]. 

Drushel [58] and others [31,59] have described the needs of the chromatographer in the area of detectors. Specific texts concern detection in quantitative GC [54], diode-array detection in HPLC [48], selective detectors [39] and element-specific chromatographic detection by AES [60], electrochemical detectors [61] and laser detectors [62]. 

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