Background, generally noise

Often background shot noise limited Generally detector noise limited... 

Alternatively, the background of the spectrum can be used as reference, since it reflects any variation of electronic noise or change in experimental conditions. The background generally depends on the sample absorption, and therefore on the salt content. This can be nevertheless be accounted for in the concentration calibration. The background intensity Jbg, can be... 

Sensitivity strictly refers to the slope of the calibration curve for the method but is widely used (including in this book) in a more general sense of a low value for the limit of detection (LOD, the lowest concentration of an analyte that the bioanalytical procedure can reliably differentiate from background and noise, see discussion in Section 8.4.1). Sensitivity can also refer to the lower limit of quantitation (LLOQ, the lowest amount of an analyte in a sample that can be quantitatively determined with acceptable precision and accuracy. Section 8.4.2). While it is true that a higher sensitivity in the strict sense can lead to lower values of LOD and LLOQ, the latter parameters also vary with the level of chemical background and statistical noise (see discussion of Figure 8.10 in Section 8.4). While the LLOQ is usually the more important parameter in quantitative analyses, the LOD can be significant for some applications (e.g. dioxin analysis. Section 11.4.1). 

The limit of detection (LOD) or detection limit of a method is the lowest analyte concentration that the detector wiU produce a response detectable above the background, or noise level, of the system. The minimum detectable level (MDL) is the concentration level at the LOD and generally defined as three times the noise level (baseline) of the detector. LOD and MDL are the two quantifiable values that can measure the sensitivity of the method. Sensitivity is the smallest difference in the response of the detector (signal) that can be detected for the method. LOD is the smallest amount that is clearly distinguishable from the background or blank. 

The detection limits in the table correspond generally to the concentration of an element required to give a net signal equal to three times the standard deviation of the noise (background) in accordance with lUPAC recommendations. Detection limits can be confusing when steady-state techniques such as flame atomic emission or absorption, and plasma atomic emission or fluorescence, which... 

Different analytical techniques are used for detection of the elemental composition of the solid samples. The simplest is direct detection of emission from the plasma of the ablated material formed above a sample surface. This technique is generally referred to as LIBS or LIPS (laser induced breakdown/plasma spectroscopy). Strong continuous background radiation from the hot plasma plume does not enable detection of atomic and ionic lines of specific elements during the first few hundred nanoseconds of plasma evolution. One can achieve a reasonable signal-to-noise ra-... 

The accuracy with which a wavefront sensor measures phase errors will be limited by noise in the measurement. The main sources of noise are photon noise, readout noise (see Ch. 11) and background noise. The general form of the phase measurement error (in square radians) on an aperture of size d due to photon noise is... 

In general, the number of components N is selected at the point where the addition of a new component does not give relevant additional information within the context of the studied problem or, in other words, when this component explains experimental noise only. Those components explaining proportions of small variance are not investigated, and they are assumed to be mainly related to small background contributions or to noise and experimental error. The selected number... 

Quantification of the limits of detection (LOD), or minimum detectable levels (MDL statistically defined in Section 13.4), is an important part of any analysis. They are used to describe the smallest concentration of each element which can be determined, and will vary from element to element, from matrix to matrix, and from day to day. Any element in a sample which has a value below, or similar to, the limits of detection should be excluded from subsequent interpretation. A generally accepted definition of detection limit is the concentration equal to a signal of twice (95% confidence level) or three times (99% confidence) the standard deviation of the signal produced by the background noise at the position of the peak. In practice, detection limits in ICP-MS are usually based on ten runs of a matrix matched blank and a standard. In this case ... 

Fig. 12.3. General appearance of a calibration curve. The upper limit of the linear range is defined by saturation, the lower by memory and chemical background or adsorption. In addition, the noise level plays a role for the detection limit.

General CE problems (e.g., wall interactions of proteins) have been discussed elsewhere. All ACE techniques working with a ligand added to CE buffer (classical ACE, Hummel-Dreyer principle, vacancy peak analysis) imply the potential problem that any continuously infused matrix can increase background noise and, even worse, deteriorate the ionization of the analyte due to competition. 

Figure 5.12 shows both the dynamic and the static model deformation densities in the plane of the oxalic acid molecule, based on the data set also used for Fig. 5.2. The increase in peak height, due to higher resolution, and reduction in background noise relative to the earlier maps is evident. The model acts as a noise filter because the noise is generally not fitted by the model functions during the minimalization procedure.

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