4 Minimum detectable level, definition

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 ... 

The minimum detectable level, or detection limit, is defined as that concentration of a particular element which produces an analytical signal equal to twice the square root of the background above the background. It is a statistically defined term, and is a measure of the lower limit of detection for any element in the analytical process. (This definition corresponds to the 95% confidence interval, which is adequate for most purposes, but higher levels, such as 99% can be defined by using a multiplier of three rather than two.) It will vary from element to element, from machine to machine, and from day to day. It should be calculated explicitly for every element each time an analysis is performed. 

It is important to specify detectors independent of column parameters and of sample size. One parameter that does this is minimum detectable level, MDL. It is the "level" of sample in the detector at the maximum of the peak, when the signal-to-noise ratio is two. The term detectability is sometimes used for MDL. Variations of this definition are sometimes given which require the signal-to-noise ratio to be either one, three, or five. The parameter is also defined sometimes in terms of root-mean square (rms) noise. Peak-to-peak noise can be taken as six times rms noise. 

If the first definition of sensitivity is used, then "high sensi-vity" is better if detectable level is meant, then "low sensitivity" is better. To avoid confusion, it is better to refer to minimum detectable level or to response factor. 

What is the smallest "linear level Is it the minimum detectable level, or is it a level sufficiently higher than the noise to allow a measurement to be made with a precision equivalent to the allowable deviation from linearity Understandably, instrument manufacturers prefer the former definition, chromatographers with the responsibility of specifying the accuracy of their analyses prefer the latter. 

The sensitivity of taste or odor panels can be measured by the ability of individuals to detect sensory characteristics. Threshold values are measures of the least concentrations of volatile compounds detected in a food matrix (oil or water) or minimum detectable level by at least 50% of the panelists. This definition is, however, now commonly used to refer to detection by 100% of the panelists. There is a considerable difference in the flavor significance of volatile decomposition products formed in oxidized or rancid lipids. Hydrocarbons have the highest threshold values ranging from 90 to 2150 ppm, and the least impact on flavor. Substituted furans with threshold values of 2-27 ppm, vinyl alcohols with threshold values of 0.5-0.3 ppm, and 1-alkenes... 

A detector s limit of detection (LOD) is the lowest concentration level that it can identify with a certain degree of confidence. There are many definitions of LOD, such as the concentration at which the response signal generated is three times the instrument noise level. Here LOD is referred to as the minimum detection level (MDL) of concentration that will consistently cause the detector to alarm. It is affected by background noise and blank signals. The LOD of a detector may vary widely for different chemicals. Environmental and operational conditions could drastically affect the LOD. Manufacturers normally provide LOD information obtained under optimum conditions. 

We have spoken frequently in this chapter about sensitivity and detection limit in reference to advantages and disadvantages of the various techniques. Sensitivity and detection limit have specific definitions in atomic absorption. Sensitivity is defined as the concentration of an element that will produce an absorption of 1% (absorptivity percent transmittance of 99%). It is the smallest concentration that can be determined with a reasonable degree of precision. Detection limit is the concentration that gives a readout level that is double the electrical noise level inherent in the baseline. It is a qualitative parameter in the sense that it is the minimum concentration that can be detected, but not precisely determined, like a blip that is barely seen compared to the electrical noise on the baseline. It would tell the analyst that the element is present, but not necessarily at a precisely determinable concentration level. A comparison of detection limits for several elements for the more popular techniques is given in Table 9.2. 

The limit of determination (LOD) is the minimum level at which a sensor array can detect an analyte in a mixture of other analytes which respond to the array. This differs from the limit of detection which is characterized for a single sensor for a pure analyte in a sample. The definition usually states that the minimum signal that can be accurately detected is three times the noise of the sensor. Therefore, for the single sensor, the limit of detection is three times the noise divided by the slope of the calibration line. In the array case with multiple analytes, the limit of determination is... 

There are several definitions of threshold. The detection threshold is defined as the lowest stimulus eliciting a sensorial response, even though no identification can be reached. The recognition threshold or identification threshold is the level of stimulus at which the specific stimulus can be identified. The differential threshold or just noticeable difference, is the minimum increase in a stimulus required to detect a difference in the perception. 

The limit of detection (LoD) represents the minimum concentration at which the presence of the analyte can be distinguished from its absence with a high statistical probability. There are different possible ways to define the LoD pivoting about the interpretation of significantly different from the blank. The choice of definition will influence which type of error is more likely Type I, false positives or Type II, false negatives. In addition to these considerations, the suitability of a specific LoD is of course affected by the chosen instrumentation and analytical method. Conversely, at higher levels the concentration of an analyte can be quantified. The lowest concentration that can be reliably quantified is known as the limit of quantification (LoQ). The LoQ is often set at a fixed multiple (usually between 2 and 3.3) of the LoD. 

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