Definition, detection limit

Detection Limit Criterion. The criterion used to define the detection limit, or perhaps as important, the protocol used to measure it, can be critical in establishing a valid detection limit. Currie [9] has described the wide variation in detection limit definitions for radiochemical measurements reported in the literature. lUPAC [2] recommends the detection limit, c, be defined as the concentration of an analyte equal to a background-corrected signal, xl - X3, three times the estimated standard deviation of a single determination using 20 measurements of the blank. 

Significance tests, however, also are subject to type 2 errors in which the null hypothesis is falsely retained. Consider, for example, the situation shown in Figure 4.12b, where S is exactly equal to (Sa)dl. In this case the probability of a type 2 error is 50% since half of the signals arising from the sample s population fall below the detection limit. Thus, there is only a 50 50 probability that an analyte at the lUPAC detection limit will be detected. As defined, the lUPAC definition for the detection limit only indicates the smallest signal for which we can say, at a significance level of a, that an analyte is present in the sample. Failing to detect the analyte, however, does not imply that it is not present. 

Normal distribution curves showing the definition of detection limit and limit of identification (LOI). The probability of a type 1 error is indicated by the dark shading, and the probability of a type 2 error is indicated by light shading. 

The detection limit is another value which is often quoted, and this may be defined in a variety of ways. The most widely accepted definition is that the detection limit is the smallest concentration of a solution of an element that can be detected with 95 per cent certainty. This is the quantity of the element that gives a reading equal to twice the standard deviation of a series of at least ten determinations taken with solutions of concentrations which are close to the level of the blank. 

Later in 1995, lUPAC came up with additional recommendations for the definition of LQD. Detection limit is defined as, The minimum detectable value of the net signal (or concentration) is that value for which the false negative error is given a. a ... 

Depending on the method used to define the detection limit, the value can vary up to an order of magnitude, thereby rendering these numbers meaningless. This is why it is very important to have uniform definitions for each of these terms, in order to be able to compare values across matrices, methods and laboratories. ... 

Comparing equations (10) and (5), the lUPAC definition for detection limit, the difference is that RMSE is used instead of For dynamic systems, such as chromatography with autointegration systems, RMSE is easier to measure and more reliable than for reasons discussed earlier. Both are measures of variance and, although dissimilar, provide similar information. This is apparent in the equations used to calculate the values of. Tb and RMSE ... 

Environmental Protection Agency Instruction 10 CFR Ch. 1 (7-1-94 Edition) Appendix B to Part 136 - Definition and Procedure for the Determination of the Method Detection Limit - Revision 1.11, US Environmental Protection Agency, Washington, DC (1994). 

As in case (1), the analyte is definitely detected, apart from the remaining risk of error a. Also in this case, an analytical result x, can be calculated formally from y by means of the corresponding calibration function. But it must be noted that the relative uncertainty of results nearby of the detection limit amounts about 50% (see Sect. 7.5, item (2), p 201). 

As there are few data on the ratio of thoron concentration to radon, we have assumed that the value is constant and is equal to 0.14, Error(4) and Error(5) are assumed to be negligible for the error calculation. The error due to ambiguities in classification and discrimination of tracks according to the shape or the size was assessed by experiment and the relative error (1 S.D.) was less than 10%. The relative error due to the variations of F, f, and (V a) is less than 13% as described in the previous section. The combined error due to these factors is estimated to be 16%. The total error can be obtained by combining this error and Error(l). If we use the definition that the lowest detectable limit is the radon concentration at 50 % relative S.D., the lowest detectable... 

Based upon the available data, derivation of AEGL-1 values was considered inappropriate. The continuum of arsine-induced toxicity does not appear to include effects consistent with the AEGL-1 definition. The available human and animal data affirm that there is a very narrow margin between exposures that result in little or no signs or symptoms of toxicity and those that result in lethality. The mechanism of arsine toxicity (hemolysis that results in renal failure and death), and the fact that toxicity in humans and animals has been reported at concentrations at or below odor detection levels (-0.5 parts per million (ppm)) also support such a conclusion. The use of analytical detection limits (0.01 to 0.05 ppm) was considered as a basis for AEGL-1 values but was considered to be inconsistent with the AEGL-1 definition. 

Limit of detection The method you choose must be able to detect the analyte at a concentration relevant to the problem. If the Co level of interest to the Bulging Drums was between 1 and 10 parts per trillion, would flame atomic absorption spectroscopy be the best method to use As you consider methods and published detection limits (LOD), remember that the LOD definition is the analyte concentration producing a signal that is three times the noise level of the blank, i.e., a S/N of 3. For real-world analysis, you will need to be at a level well above the LOD. Keep in mind that the LOD for the overall analytical method is often very different than the LOD for the instrumental analysis. 

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. 

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. 

A number of toxicologically active 2,3,7,8-substituted PCDDs and PCDFs, planar PCBs and PCDD, PCDF, and PCDT homologs were measured in fish, SPMDs, and sediments. Only two target compounds exceeded the detection limits of 0.2-1 pg g in SPMD field blanks (see definition in Chapter 5). These exceptions were octachlorodibenzo-p-dioxin (OCDD) and octachlorodibenzofuran (OCDF) which were present in SPMDs at about 5 pg g However, negative... 

Definition The detection limit defines the smallest flow or the lowest amount of analyte necessary to obtain a signal that can be distinguished from the background noise. The detection limit is valid for one well-specified analyte upon treatment according to a certain analytical protocol. [27-29]... 

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