Smallest detectable concentration

Although weak fluorescence in the P— P lines was observed, from a practical standpoint only the P— P lines were found to be intense enough for resonance fluorescence work at low atom concentrations. The fluorescence count rates using the whole of the fully allowed P— P transition of F were typically 2 counts s" at [F] = 1 X 10 cm . These data set a lower concentration limit of 1 X 10 cm" for the smallest detectable concentration of F P atoms. Similar lower limits for O, Br, and I atoms are appreciably less, and are continually being improved by better attention to collimation and detection. Because of the low count rates observed in the F-atom resonance fluorescence studies, it is a better approach to use resonance absorption with a non-reversed line source (see above). 

In most biochemical sensors, detection limit is a very important parameter to evaluate sensing capability of a device, which denotes the smallest detectable amount or concentration of analytes. For microring sensors, detection limit is proportional to the variation of waveguide parameters affected by occurrence of analytes the index change of the surrounding medium, 8nc, in homogeneous... 

Smallest detectable partial pressure ratio (concentration)... 

Figure 4.11 shows the screen print-out for the measurement. The peak height for Argg in the illustration is determined to be 1.5 10" 2Aand noise amplitude A to be 4 10 A. The minimum detectable concentration is that concentration at which the height of the peak is equal to the noise amplitude. This results in the smallest measurable peak height being... 

However, a detectivity of 10 9 g/mL for a concentration detector means that this concentration is the smallest one that can be detected at that concentration in the detector cell. Consequently if a sample of this concentration is introduced into a chromatograph and run, its concentration will be somewhat less when it reaches the detector, because we have seen that an analyte is diluted and its zone becomes wider as it passes through the column. Thus, the amount of analyte that can actually be run and detected can differ from the MDQ. This situation has produced other terms like minimum detectable concentration, MDC, which will depend on the peak width (in milliliters). Analogously, for the mass flow rate detectors, the minimum detectable mass, MDM, will depend on the peak width in time units. In both cases, the quantity to be injected depends on... 

LOD) quantitative detection limit limit of determination. The smallest detectable concen- tration an analytical instrument can determine at a given confi- dence level. IUPAC defines the i quantitative detection limit as Cld = ks/m, where k is 10, s is. the standard deviation of in- i strument readings taken on a j blank (a solution with zero concentration of analyte), and 1 m is the slope of a plot of in- strument response vs. concen- oration, as calculated by linear regression. J... 

An extremely high sensitivity is claimed for this system but it is difficult to interpret the data in terms of minimum detectable concentration The smallest cell (1.4 pi) (a cell volume that would be suitable for use with microbore columns) is reported to give a sensitivity of about 2 x 10 RI units at a signal-to-noise ratio of two. Consequently, for benzene (RI = 1.501) sensed as a solute in n-heptane (RI=1.388 ) this sensitivity would represent a minimum detectable concentration of 5.6 x g/ml. The alternative 7 pi cell would decrease the minimum detectable concentration to about 1 x 10 g/ml, similar to that obtained for other refractive index detectors. However, the cell volume is a little large for modem high efficiency columns. 

The detection limit (Cmin p) is the smallest analyte concentration which can be detected by a given method with a reasonable certainity level P. This concentration is given by... 

The laboratory may often be required to make detection decisions about samples, but when the analyte activity is low enough, the relative uncertainty in the result may make it difficult to distinguish between a small positive activity and zero. The performance characteristic of the measurement process that describes its detection capability is called the minimum detectable value, minimum detectable activity, minimum detectable concentration (MDC), or lower limit of detection (LTD). These terms have been used to denote the theoretical concept of the smallest true value of the analyte in a sample that gives a specified high probability of detection. 

Limit of Detection. One must also consider the possibility of errors of the second kind (that is, false negatives or the probability of falsely concluding that the sample does not contain determlnand, when In fact It Is present). For a sample whose true concentration Is equal to the criterion of detection, that probability Is equal to 50%. Wilson chose to reduce that value to 5%, as Illustrated In Figure 2. The limit of detection Is defined as being twice that of the criterion of detection, or 4.65og. Thus, the limit of detection is the smallest sample concentration that can be detected with 95% probability. 

Instrumental detection limit gives the smallest possible concentration which can be achieved by the instrument. The instrumental detection limit is derived by using the optimum instrumental parameters and the pure solvent (water) as a sample. The instrumental detection limit is useful in comparison of the performance of the different spectrometers. 

Voltammetric techniques that can be applied in the stripping step are staircase, pulse, differential pulse, and square-wave voltammetry. Each of them has been described in detail in previous chapters. Their common characteristic is a bell-shaped form of the response caused by the definite amount of accumulated substance. Staircase voltammetry is provided by computer-controlled instruments as a substitution for the classical linear scan voltammetry [102]. Normal pulse stripping voltammetry is sometimes called reverse pulse voltammetry. Its favorable property is the re-plating of the electroactive substance in between the pulses [103]. Differential pulse voltammetry has the most rigorously discriminating capacitive current, whereas square-wave voltammetry is the fastest stripping technique. All four techniques are insensitive to fast and reversible surface reactions in which both the reactant and product are immobilized on the electrode surface [104,105]. In all techniques mentioned above, the maximum response, or the peak current, depends linearly on the surface, or volume, concentration of the accumulated substance. The factor of this linear proportionality is the amperometric constant of the voltammetric technique. It determines the sensitivity of the method. The lowest detectable concentration of the analyte depends on the smallest peak current that can be reliably measured and on the efficacy of accumulation. For instance, in linear scan voltammetry of the reversible surface reaction i ads + ne Pads, the peak current is [52]... 

Dynamic range describes the ratio of the highest to lowest intensity feature in a mass spectrum. The lowest intensity feature needs to provide meaningful information on an ion, such as with the S/N ratio > 3. Dynamic range is an important factor when mass spectrometers are used to analyze samples with large concentration differences for example, when one of the sample molecules has an abundance several thousand times lower than others. Dynamic range is distinct from detection limit of a mass spectrometer because detection limit concerns the smallest detectable sample quantity without considering the interference of other species. 

C d = the smallest reliable detectable concentration an analytical instrument can determine at a given confidence level. 

The term limit of detection XN [37,38] is defined as the smallest detectable amount of a compound. A quantitative analysis with reported concentrations is possible only when the analytical result is equal or larger than the limit of detection, because the required significance level is achieved only for such an analytical result [39] (see Figure 9.9). In contrast to the detection criteria, the limit of detection is a quantitative statement of the amount of substance. 

Figure 3.159 Definition of the limit of detection (LOD) from the decision limit. The limit of detection (LOD) is defined as that quantity of substance, concentration or content which is given using the calibration function from the smallest detectable signal...

Limit of detection, the lowest concentration of a substance that can still be detected unambiguously (assessed in the signal domain). The value is obtained from the decision limit (smallest detectable signal) using the cahbration function or the distribution range of the blank value, typically expressed in a S/N value > 3. 

A method s detection limit is the smallest amount or concentration of analyte that can be detected with statistical confidence. The International Union of Pure and Applied Chemistry (lUPAC) defines the detection limit as the smallest concentration or absolute amount of analyte that has a signal significantly larger than the signal arising from a reagent blank. Mathematically, the analyte s signal at the detection limit, (Sa)dl, is... 

The smallest concentration or absolute amount of analyte that can be reliably detected. 

Typical elemental detection limits are listed in Table 1. The detection limit is the concentration that produces the smallest signal that can be distinguished from background emission fluctuations. The continuum background is produced via radiative recombination of electrons and ions e — M+ hv or M + e + e — ... 

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. 

The half-life of the 2,3,7,8-tetrachlorodibenzo-p-dioxin in isooctane was estimated to be 40 min for the 0.5 meter exposure and 3 hours for the one meter exposure. The half-life in 1-octanol was essentially the same. The 24-hour photolysis products of the 2,3,7,8-tetrachlorodibenzo-p-dioxin were examined by gas chromatography. The smallest concentration of 2,3,7,8-tetrachlorodibenzo-p-dioxin that could be detected by the instrument was 0.5 ppm. When an injection of the 24-hour photolysis product was made, no tetra was detected. An additional confirmation of the disappearance of the 2,3,7,8-tetrachlorodibenzo-p-dioxin in the 24-hour photolysis products was obtained when thej yiaterial was submitted to the Chemical-Biology Research Laboratory for rabbit testing. No chloracnegenic activity was indicated. ... 

Sensitivity is a measure of the smallest concentration that can be either measured [limit of detection (LOD)] or accurately quantitated [limit of quantitation (LOQ)]. In the USA, the method for measuring LOD or LOQ is left up to the method developer. European requirements for determining LOD and LOQ are very specific the LOD is based on the mean plus three standard deviations for 20 control blank samples, and the LOQ is defined as the lowest concentration giving an acceptable CV. 

The limit of detection (LOD) is an important criterion of the efficiency of an analytical method. It is characterized by the smallest value of the concentration of a compound in the analytical sample. The detectable amount of anilide compounds is in the range 0.01-0.5 ng by GC and 0.1 ng by HPLC. The limit of quantitation (LOQ) ranges from 0.005 to 0.01 mg kg for vegetables, fruits and crops. The recoveries from untreated plant matrices with fortification levels between 10 and 50 times the LOD and the LOQ are 70-120%. The relative standard deviation (RSD) at 10-50 times the level of the LOD and LOQ are <10 % and <20%, respectively. 

The LOD is an important criterion of the efficiency of an analytical method. It defines the smallest value of the concentration of a compound in the analytical sample. Detectable amounts of neonicotinoid insecticides range from 0.5 to 1 ng by HPLC. The LOQ ranges from 0.005 to 0.01 mg kg for vegetables, fruits and crops. 

The limit of tolerable error is generally smallest in a minor component assay. It will have been determined that a particular minor component must be at or below a threshold concentration for the product to be usable. Therefore, the decision to accept or reject an entire production batch may depend on the analytical result. Typical batches may contain the contaminant at a concentration very similar to the specification limit. In a minor component assay, the major component may be overloaded and out of the proper range of detection of the assay. Even so, the minor component may be at such low levels that assay noise interferes. 

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