Detection selectivity

Figure I represents a two-dimensional damage distribution of an impact in a 0/90° CFRP laminate of 3 mm thickness. Unlike in ultrasonic testing, which is usually the standard method for this problem, there is no shadowing effect on the successive layers by delamination echos. With the method of X-ray refraction the exact concentration of debonded fibers can be calculated for each position averaged over the wall thickness. Additionally the refraction allows the selection of the fiber orientation. The presented X-ray refraction topograph detects selectively debonded fibers of the 90° direction.

As an example, we mention the detection of iodine atoms in their P3/2 ground state with a 3 + 2 multiphoton ionization process at a laser wavelength of 474.3 run. Excited iodine atoms ( Pi/2) can also be detected selectively as the resonance condition is reached at a different laser wavelength of 477.7 run. As an example, figure B2.5.17 hows REMPI iodine atom detection after IR laser photolysis of CF I. This pump-probe experiment involves two, delayed, laser pulses, with a 200 ns IR photolysis pulse and a 10 ns probe pulse, which detects iodine atoms at different times during and after the photolysis pulse. This experiment illustrates a frindamental problem of product detection by multiphoton ionization with its high intensity, the short-wavelength probe laser radiation alone can photolyse the... 

While metastable decompositions have been extensively studied for many decades, nowadays the use of tandem MS is almost exclusively limited to the study of decompositions caused by collision. Different kinds of experiments can be carried out by tandem MS the most common is the product ion scan. This is carried out by selecting ions with a given m/z value among all those formed in the source and detecting selectively their product ions formed due to CIDs. It follows that at least two analyzers (the tandem) are required. In... 

D. Following signal detection, select the optimal set of column wells (this will be dependent on the goal of the assay if, for instance, a screening assay is required with a certain cutoff concentration, e.g., 0.1 ng/mL for pesticides in water, then the labeled antigen concentration that yields a B/BO of 50% at 0.1 ng/mL will be ideal). 

R.R. Kunz, W.F. Dinatale and P. Becotte-Haigh, Comparison of detection selectivity in ion mobility spectrometry proton-attachment versus electron exchange ionization, Int. J. Mass Spectrom., 226 (2003) 379-395. A.M. DeTulleo, P.B. Galat and M.E. Gay, Detecting heroin in the presence of cocaine using ion mobility spectrometry, Int. J. Ion Mobility Spectrom., 1 (2000) 38-42. 

Nitrogen compounds in middle distillates can be detected selectively by chemiluminescence. Individual nitrogen compounds can be detected down to 100 ppb nitrogen. Gas chromatography with either sulfur chemiluminescence detection or atomic emission detection has been used for sulfur-selective detection. 

The range of applicability of equation 11.122 depends on the limits of detection of in the sample. The current maximum age attained by direct radioactivity counting is about 4 X 10" a. To measure residual radioactivity, the total carbon in the sample is usually converted to CO2 and counted in the gas phase, either as purified CO2 or after further conversion to C2H2 or CH4. To enhance the amount of counted carbon, with the same detection limit (about 0.1 dpm/g), counters attain volumes of several liters and operate at several bars. More recent methods of direct detection (selective laser excitation Van de Graaif or cyclotron acceleration) has practically doubled the range of determinable ages (Muller, 1979). 

Halogen substituents are of course easy to introduce to heteroaromatic rings, and they also enhance the acidity of the ring protons. n-BuLi will, for example, lithiate the tetrafluoropyridine 179 at —60°C in ether ° but with pyridine itself it leads to addition/reoxidation products . Addition to the ring is the major product with 2-fluoropyridine 180, though some metaUation can be detected selectivity in favour of metaUation is complete with LDA in THF at —75 °C or with phenyUithium and catalytic -Pr2NH at —50°C (Scheme 90) . Similar results are obtained with quinolines . 

The sensitivity and simplicity of an analytical method always improves when selective detection can be used. Because of detection selectivity, a sample in a complex matrix can be analysed with minimum sample preparation. Wouldn t it be ideal to have an ultra-specific reagent that would allow the determination of a compound in an environment containing thousands of substances ... 

The precipitation (Section 10.2) and extraction (Section 10.4) of coordination compounds under controlled conditions and their optical characteristics (Section 10.5) have been exploited, together with a miscellany of other procedures, for detecting selectively small traces of elements in very small volumes of a test sample. Such spot reactions have been painstakingly collected, refined and recorded by Fritz Feigl.83... 

Carbamates and substituted ureas are a numerous group of pesticides widely used to control weeds, pests, and diseases in fruit trees, vegetables, and cereals. Carbamate residues in foods are commonly extracted with water-miscible solvents and determined by using a liquid chromatograph equipped with a sensitive detector, frequently a UV detector. In addition, to obtain adequate detection selectivity, the postcolumn fluorimetric labeling technique is used for methyl carbamates. Substituted ureas are normally extracted from foods with organic solvents, and they can be determined directly by HPLC-UV or after postcolumn derivatization by fluorescence determination of their derivatives. 

A number of composition analyzers used for process monitoring and control require chemical conversion of one or more sample components preceding quantitative measurement. These reactions include formation of suspended solids for turbidimetric measurement, formation of colored materials for colorimetric detection, selective oxidation or reduction for electrochemical measurement, and formation of electrolytes for measurement by electrical conductance. Some nonvolatile materials may be separated and measured by gas chromatography after conversion to volatile derivatives. 

In general, standard methods applicable to a vast majority of compounds of interest to ensure throughput capabilities are critical for LC/MS screens. Although not optimized for specificity, standard conditions provide a systemic measure of control. This control results in data that has high quality, reliability, and comparability. With a strategic selection of compounds that have similar molecular weights, structural features, and chromatographic properties, the detection selectivity and precision are satisfactory for this particular type of analysis. 

Perakis, N. Separation et detection selective de composes soufres dans les fractions lourdes des petroles. Geochimie des benzo[b]thiophenes Ph. D. dissertation, Univ. of Strasbourg, 1986. 

The detector converts a change in the column effluent into an electrical signal that is recorded by the data system. Detectors are classified as selective or universal depending on the property measured. Selective (solute property) detectors, such as fluorescence detectors, measure a physical or chemical property that is characteristic of the solute(s) in the mixture only those components which possess that characteristic will be detected. Universal (bulk property) detectors measure a physical property of the eluent. Thus, with refractive index (RI) detectors, for example, all the solutes which possess a refractive index different from that of the eluent will be detected. Selective detectors tend to be more sensitive than universal detectors, and they are much more widely used. Universal detectors are more commonly used in preparative chromatography, where a universal response is desired and sample size is large. 

Hormann et al. [157] monitored various European rivers for levels of Atrazine, Terbumeton and dealkylated metabolites GS26571 (2-amino-4-tertbutylammo-5-methoxy-l,3,5-triazine) and GS30033 (2-amino-4-chloro-5-ethylamino-l,3,5-triazine). The compounds were extracted into dichloromethane and quantitated by gas chromatography with nitrogen-specific detection. Selected results were verified by gas chromatography with mass fragmentographic detection. The limit of detection was usually 0.4pg L 1. 

The mechanistic basis to radical reactivity of organotransition metal compounds is still not very well developed. Mechanisms very often remain speculative, since the information about involved intermediates is scarce. Mechanistic information can be gathered by using physical methods, such as ESR spectroscopy. Changes in the oxidation state of metal complexes indicating SET, paramagnetic metal centers or the radicals themselves, provided their lifetime allows it, can be detected (selected reviews [73-75]). CIDNP measurements can also provide valuable information, but were rarely used in the past [76-78]. 

One of the most attractive detectors for screening of CWC-related chemicals is the atomic emission detector (AED), which is capable of detecting selectively any element below the nanogram level. The... 

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