Limit of detection determination

The luminescent ATP assay is the most sensitive HTS method available for measuring the viability of cell populations in microwell plates. The limits of detection determined in samples of eukaryotic cells serially diluted from a known concentration may fall below 10 cells per well (Figure 6.7). This enables miniaturization to a 1536-well format. 

The method limit of quantitation and limit of detection must be determined as well as the limit of linearity. The limit of quantitation is defined as the level at which the measurement is quantitatively meaningful the limit of detection is the level at which the measurement is larger than the uncertainty and the limit of linearity is the upper level of the measurement rehabihty (39). These limits are determined by plotting concentration vs response. 

Although the most sensitive line for cadmium in the arc or spark spectmm is at 228.8 nm, the line at 326.1 nm is more convenient to use for spectroscopic detection. The limit of detection at this wavelength amounts to 0.001% cadmium with ordinary techniques and 0.00001% using specialized methods. Determination in concentrations up to 10% is accompHshed by solubilization of the sample followed by atomic absorption measurement. The range can be extended to still higher cadmium levels provided that a relative error of 0.5% is acceptable. Another quantitative analysis method is by titration at pH 10 with a standard solution of ethylenediarninetetraacetic acid (EDTA) and Eriochrome Black T indicator. Zinc interferes and therefore must first be removed. 

It is set that the ion of Pd(II) forms polai ogphic active complex compound with o-hydroxysubstitution azodyes - tropeolin 0 (acetate buffer solution E= -0,58V). The limit of detection Pd(II) ions is 2x10 mol/1. Instead complexforming between the ions Co(II), Ni(II) and tropeolin 0 in the optimum terms of the Pd(II) determination in general is absent. It enables to conduct the Pd(II) determination in presence the 200-multiple surpluses Co(II) and 80-multiple surpluses Ni(II). 

Detection limit of periodate determination is 1,35 pg per sample. The color scale for visual test-determination has been constmcted in working range 5-50 pg periodate-ion per sample. 

The liquid chromatography - tandem mass spectrometry (LC/MS/MS) technique was proposed for the determination of corticosteroids in plasma and cerebrospinal fluid (CSF, liquor) of children with leucosis. Preliminai y sample prepai ation included the sedimentation of proteins, spinning and solid-phase extraction. MS detection was performed by scanning selected ions, with three chai acteristic ions for every corticosteroids. The limit of detection was found 80 pg/ml of plasma. 

The absorption scans were made at a wavelength of 2 = 505 nm (Fig. 1). The limit of detection was 100 ng gentamycin C complex. The best conditions for fluorimetric determination (Fig. 2) were excitation at 2exc = 313nm and detection at 2fi > 390 nm. 

When AMPA or gluphosinate are determined alone, the sensitivity is higher because a higher dilution is not required. For glyphosate, when the transfer volume is precisely adjusted to 280 p.1 for the FMOC-glyphosate-containing fraction, a limit of detection of about 0.2 p.g 1 can be reached (28). 

For example. Figure 13.13 shows the chromatogram obtained when 200 ml of tap water was spiked at levels of 1 p.g 1 of such pesticides. The limits of detection achieved by using this method were between 0.05 and 0.5 p.g 1 although more polar compounds Such as vamidothion or 4-nitrophenol could not be determined (61). 

This set-up, or a very similar one, has been used to determine different group of pollutants in environmental waters (45, 83, 93). For example, with 10 ml of sample the limits of detection of a group of pesticides were between 2 and 20 ng 1 (92) in tap and river water, with this system being fully automated. Figure 13.19 shows the chromatograms obtained by on-line SPE-GC-MS under selected ion-monitoring conditions of 10 ml of tap water spiked with pesticides at levels of 0.1 pig 1 (92). 

A number of ion-selective electrodes are available from laboratory supply houses whilst not intended to be an exhaustive list, Table 15.3 serves to indicate the variety of determinations for which electrodes are available. An indication is also given of the lower limit of detection of the electrodes this figure may vary somewhat according to the source of the electrode but full details are furnished by the manufacturer of the effective range of use of each electrode and of likely interferences. 

In the fifth subdivision, which becomes important when trace determinations are pushed to the limit of detection, Nb is comparable with Nt. Statistical fluctuations in both counts may therefore lead to (1) the... 

The limit of detection is the smallest amount of an analyte that is required for reliable determination, identification or quantitation. More mathematically, it may be defined as that amount of analyte which produces a signal greater than the standard deviation of the background noise by a defined factor. Strictly for quantitative purposes, this should be referred to as the limit of determination . The factor used depends upon the task being carried out and for quantitative purposes a higher value is used than for identification. Typical values are 3 for identification and 5 or 10 for quantitation. 

There are a number of properties of a detector that determine whether they may be used for a particular analysis, with the most important being (a) the noise obtained during the analysis, (b) its limit of detection, (c) its linear range, and (d) its dynamic range. The last three are directly associated with the analyte being determined. 

The limit of detection (LOD) (see Figure 2.6) is defined as the smallest quantity of an analyte that can be reliably detected. This is a subjective definition and to introduce some objectivity it is considered to be that amount of analyte which produces a signal that exceeds the noise by a certain factor. The factor used, usually between 2 and 10 [11], depends upon the analysis being carried out. Higher values are used for quantitative measurements in which the analyst is concerned with the ability to determine the analyte accurately and precisely. 

The attributes required of a method usually include good sensitivity, low hmits of detection, and selectivity. It must be recognized that while low limits of detection will usually require good sensitivity, the latter, in itself, does not guarantee low limits of detection since these are often determined by the levels of interfering materials present and not the absolnte sensitivity of the technique. Low limits of detection allow the analyte to be determined at levels at or below those considered to be harmful or prescribed by legislation or at which it is found in a particular... 

This method was applied to the determination of these oxidized nucleosides in salmon testes using [ Cio, N5]-8-hydroxy-2 -deoxyguanosine (L-8-OH-dG) as the internal standard (Figure 5.65). Four of the oxidized products were below the limits of detection of the method, while the concentration of 8-OH-dG was determined to be 0.93 ppb. 

Signal-to-noise ratio The ratio of the intensity of the analytical signal to that of the noise. This is used in determining the limits of detection and quantitation. 

Sensitivity, Linearity, and Stability of Instruments. Quinine sulfate has been suggested as a standard for determining the limit of detection and linearity of Instruments (44). Some organic materials dissolved In plastics have been suggested as standards because of their stability (22), and many Inorganic Ions dissolved In glasses are quite stable. 

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