Sensitivity of the method

The specific nature of UV absorption for certain structures when combined with the high sensitivity of the method enables trace quantities (— 1 ppm) of molecules in a matrix transparent to UV beams to be analyzed. Benzene in cyclohexane is an example. 

Of the six parameters shown in Figure 13.18, the most important are peak height and return time. The peak height is related, directly or indirectly, to the analyte s concentration and is used for quantitative work. The sensitivity of the method, therefore, is also determined by the peak height. The return time determines the frequency with which samples maybe injected. Figure 13.19 shows that when a second sample is injected at a time T after injecting the first sample. 

One limitation of this method that should immediately come to mind is the restriction to relatively low molecular weight polymers. This is a corollary of the fact that chain ends are inconsequential for very long chains. Hence, the sensitivity of the method decreases as the molecular weight of the polymer increases. As a general rule, molecular weights in the neighborhood of 25,000 represent the upper limit for applicability of this method. 

Epichlorhydrin (ECH) detection starts with detecting epoxide cycle using hydrochloric acid in combination with sodium chloride the reaction product - 1,3-dichlorhydrin - is extracted in diethyl ether and concentrated by removing the latter. Gas-liquid chromatography with a flame-ionization detector is used to detect glycerin 1,3-dichlorhydrin. The sensitivity of the method is 0.01 mg/dm. ... 

Note Vapor-phase derivatization was appreciably more homogeneous than that produced by dipping and simultaneously increased both the precision and the sensitivity of the method. 

Further techniques which may be applied directly to the solvent extract are flame spectrophotometry and atomic absorption spectrophotometry (AAS).13 The direct use of the solvent extract in AAS may be advantageous since the presence of the organic solvent generally enhances the sensitivity of the method. However, the two main reasons for including a chemical separation in the preparation of a sample for AAS are ... 

Reagents. In view of the sensitivity of the method, the reagents employed for preparing the ground solutions must be very pure, and the water used should be re-distilled in an all-glass, or better, an all-silica apparatus the traces of organic material sometimes encountered in demineralised water (Section 3.17) make such water unsuitable for this technique unless it is distilled. The common supporting electrolytes include potassium chloride, sodium acetate-acetic acid buffer solutions, ammonia-ammonium chloride buffer solutions, hydrochloric acid and potassium nitrate. 

Determination of appropriate measuring and analysis methods. Decisions must be made on the selection of appropriate and available measuring and or analytical equipment and tools. The characteristics of the methods must be discussed in terms of specificity, accuracy, precision, sensitivity of the methods, and locations of measuring and/or sampling (off-line, at-line, on-line, in-line, non-invasive). 

A hanging electrolyte drop has also been applied to determine ionic species in solution using differential-pulse-stripping voltammetry procedures [69]. Particular emphasis was given to assessing the selectivity and sensitivity of the method. The technique of current-scan polarography has also been applied in the study of electron-transfer [70] and coupled electron-transfer-ion-transfer [71,72] reactions at the ITIES in this configuration. 

We will describe a systematic approach to renormalize the intrachain interactions towards a coarser level for three different modifications of polycarbonates. The advantage of examinig three modifications of the same polymer gives a first hint of the sensitivity of the method. The three modifications of the polycarbonate are BPA-PC, BPZ-PC, and TMC-PC. The structures are given in Fig. 6.1. Although the backbone sequence is the same they have re-... 

Recoveries from fortified bananas were 89 and 79% at levels of 0.02 and 0.2 mg/kg, respectively [81]. The sensitivity of the method was such that 0.6 pg of the pure derivative could be detected. 

A different MS-based ee-assay makes use of a proline-derived mass-tagged acylating agent.95 In the course of derivatization it is necessary that some degree of kinetic resolution comes about. The sensitivity of the method was reported to be 10% ee. It can also be applied to the reaction of a prochiral compound lacking enantiotopic groups, as in the transformation of acetophenone to phenylethanol. 

Van den Berg [131] used this technique to determine nanomolar levels of nitrate in seawater. Samples of seawater from the Menai Straits were filtered and nitrite present reacted with sulfanilamide and naphthyl-amine at pH 2.5. The pH was then adjusted to 8.4 with borate buffer, the solution de-aerated, and then subjected to absorptive cathodic stripping voltammetry. The concentration of dye was linearly related to the height of the reduction peak in the range 0.3-200 nM nitrate. The optimal concentrations of sulfanilamide and naphthyl-amine were 2 mM and 0.1 mM, respectively, at pH 2.5. The standard deviation of a determination of 4 nM nitrite was 2%. The detection was 0.3 nM for an adsorption time of 60 sec. The sensitivity of the method in seawater was the same as in fresh water. 

Lee [524] described a method for the determination of nanogram or sub-nan ogram amounts of nickel in seawater. Dissolved nickel is reduced by sodium borohydride to its elemental form, which combines with carbon monoxide to form nickel carbonyl. The nickel carbonyl is stripped from solution by a helium-carbon monoxide mixed gas stream, collected in a liquid nitrogen trap, and atomised in a quartz tube burner of an atomic absorption spectrophotometer. The sensitivity of the method is 0.05 ng of nickel. The precision for 3 ng nickel is about 4%. No interference by other elements is encountered in this technique. 

The older methods for vitamin Bi2 used the optical density of the culture medium as a measure of growth rate of the assay organism [470,471]. The sensitivity of the method could be increased somewhat by the following 14C uptake as a measure of growth [472,473]. These methods are sensitive to 0.1 ng, well below the amounts of the vitamin which could be measured by any available chemical technique. 

---