Signal values, trace analysis

In principle, all performance measures of an analytical procedure mentioned in the title of this section can be derived from a certain critical signal value, ycrit. These performance measures are of special interest in trace analysis. The approaches to estimation of these measures may be subdivided into methods of blank statistics , which use only blank measurement statistics, and methods of calibration statistics , which in addition take into account calibration confidence band statistics. 

The limit of detection (LOD) is the smallest amount of a specific analyte that is visible with given significance. It is an important parameter for trace analysis. A distinction between the lowest concentration of the analyte in the sample (relative value) and the smallest absolute amount that can be detected with the respective method is important. This also has to take the type of detector into account that can either be concentration dependent or amount/mass dependent in its signal generation. The latter type of detectors is in general destructive in that it is not possible to recover the sample. With concentration-dependent detectors, such as the common UV absorbance detector, the concentration detection limit can only be improved on the detector itself, while the absolute detection limit depends both on the detector and on the column dimension and separation efficiency. For truly mass proportional detectors, the column dimension does... 

In trace analyses, the slope of the analytical evaluation curve (in logarithmic form) is usually I at higher concentrations ij may be > 1 as a result of self-reversal. In trace analysis by AES, the intensity of the spectral background /u can be used as reference signal and cr should be replaced by Cu the concentration at which the line and background intensities are equal (background equivalent concentration or BEC value) ... 

The electrical signal from a detector is amplified and fed into a recorder or computer for analysis. A typical recorder trace is shown in Figure 3.5. Each peak represents a component in the original mixture. A peak is identified by a retention time, the time lapse between injection of the sample and the maximum signal from the recorder. This number is a constant for a particular compound under specified conditions of the carrier gas flow rate temperature of the injector, column, and detector and type of column. Retention time in GC analysis is analogous to the R value in thin-layer or paper chromatography. 

For trace determinations of cesium cations the procedure is executed in two steps First, 3 /il of a standard solution containing a known amount of cesium chloride is applied to the emitter by means of the modified syringe technique s and a signal at m/z 133 is recorded. S nd, between 0.2 pA and 1 1 of sample are applied to the same emitter and desorbed under identical conditions. From the peak areas of the evaporation profiles obtained in both measurements, the unknown amount of the alkali element present in the sample is calculated. Usually, one analysis (calibration + sample analysis) can be performed within 30 min. Thus, cesium in sample sizes of 0.2 to 1 jul, which contain 0.3 to 1000 pg of the element, can be determined. The accuracy of repeated measurements of a standard solution is 10% and that of the technique for the determination of unknown concentrations 20 %. A sensitivity between 1.4 and 2.5 X 10 C per g is obtained for cesium. Since a good sensitivity value for an organic FD ion, namely the molecular ion of cyclophosphamide, has been reported to be 1 to 2X 10 C per it is clear that FDMS is about a factor of 100 more sensitive for the [Cs] ion. 

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