Minimum detectable concentrations

The method of stripping voltammetry (SV) is one of the most perspective methods in concentration range of thallium(I) determination of 10 -10 M. Achievement of high sensitivity of thallium(I) determination needs carrying out its additional concentration and sepai ation from other metals which ai e close by electrochemical properties. For these purposes it is offered to use a method of coprecipitation with collector. The combination of SV and a method of coprecipitation on a collector have shown that minimum detectable concentration can be decreased by 2-3 orders of magnitude. 

Path length (m) Absorbence Minimum Detectable Concentration (ppm) " (20 metre cell)... 

To unambiguously identify the presence of a peak and, in addition, be able to give some proximate estimation of its size for quantitative purposes, the peak height needs to be at least 5 times the noise level. The detector sensitivity, or the minimum detectable concentration, (Xd), is defined as that concentration of solute that will give a signal equivalent to twice the noise level and, consequently, the concentration of solute at the limiting (k ) value must be 2.5Xd. 

Equation (33) shows that the maximum capacity ratio of the last eluted solute is inversely proportional to the detector sensitivity or minimum detectable concentration. Consequently, it is the detector sensitivity that determines the maximum peak capacity attainable from the column. Using equation (33), the peak capacity was calculated for three different detector sensitivities for a column having an efficiency of 10,000 theoretical plates, a dead volume of 6.7 ml and a sample concentration of l%v/v. The results are shown in Table 1, and it is seen that the limiting peak capacity is fairly large. 

It is instructive to compare the sensitivity which may be achieved by absorption and fluorescence methods. The overall precision with which absorbance can be measured is certainly not better than 0.001 units using a 1 cm cell. Since for most molecules the value of emax is rarely greater than 105, then on the basis of the Beer-Lambert Law the minimum detectable concentration is given by cmin> 10 3/105= 10 8M. 

Detector Sensitivity, or Minimum Detectable Concentration Pressure Sensitivity Flow Sensitivity Temperature Sensitivity... 

Detector Sensitivity or the Minimum Detectable Concentration has been defined as the minimum concentration of an eluted solute that can be differentiated unambiguously from the noise. The ratio of the signal to the noise for a peak that is considered decisively identifiable has been arbitrarily chosen to be two. This ratio originated from electronic theory and has been transposed to LC. Nevertheless, the ratio is realistic and any peak having a signal to noise ratio of less than two is seriously obscured by the noise. Thus, the minimum detectable concentration is that concentration that provides a signal equivalent to twice the noise level. Unfortunately, the concentration that will provide a signal equivalent to twice the noise level will usually depend on the physical properties of the solute used for measurement. Consequently, the detector sensitivity, or minimum detectable concentration, must be quoted in conjunction with the solute that is used for measurement. 

The flow sensitivity of a detector will also be one of the factors that determines the long term noise and thus will influence the sensitivity or minimum detectable concentration of the detector.lt is usually measured as the change in detector output for unit change in flow rate through the sensor cell. Again, the refractive index detector is the most sensitive to flow rate changes. 

Where Q, is the minimum detectable amount, R the detector noise level and s the detector sensitivity [135,146,151,152]. For a concentration sensitive detector the minimum detectable concentration is the product of Q, and the volumetric gas flow rate through the detector. The minimum detectable amount or concentration is proportional to the retention time, and therefore, directly proportional to the column radius for large values of n. it follows, then, that very small quantities can be detected on narrow-bore columns. 

NRC. 1998b. Minimum detectable concentrations with typical radiation survey instruments for various contaminants and field conditions. Washington, DC Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission. NUREG/CR-1507. 

Figure 4.18. Minimum detectable concentrations as a function of atomic number and proton energy for thin organic specimens in a typical PIXE arrangement. (Reproduced by permission of Johansson...

Fujiwara et al. [94] found that, when present as a heteropolyacid complex, molybdenum(VI), germanium(IV), and silicon(IV) produced CL emission from the oxidation of luminol, and similar CL oxidation of luminol was observed for arsenic(V) and phosphorus(V) but with the addition of the metavanadate ion to the acid solution of molybdate. A hyphenated method was therefore proposed for the sensitive determination of arsenate, germanate, phosphate, and silicate, after separation by ion chromatography. The minimum detectable concentrations of arsenic(V), germanium(IV), phosphate, and silicon(IV) were 10, 50, 1, and 10... 

The observed signal to noise ratio is less than that expected from the minimum detectable concentration of analyte. 

Did this FDA position make any scientific sense It implied that if aflatoxin could be detected by reliable analysis, it was too risky to be consumed by humans, but that if the aflatoxin happened to be present below the minimum detectable concentration it was acceptable. (Analytical chemists can never declare that a chemical is not present. The best that can be done is to show that it is not present above some level - 20 ppb in the case of aflatoxins, and other, widely varying, levels in the case of other chemicals in the environment.) To be fair to the FDA, perhaps the word acceptable should be withdrawn the agency s position was not so much that all concentrations of aflatoxin up to 20 ppb were acceptable, but that nothing much could be done about them, because the chemists could not determine whether they were truly present in a given lot of food until the concentration exceeded 20 ppb. 

Population Mean Limit of Detection Minimum Detectable Concentration Limit of Quantification... 

The Minimum Detectable Concentration (MDC) is the detection limit expressed as a concentration. 

For some laboratory-built systems, it is possible to detect on the order of 10 labeled molecules. In commercial systems, where the optical alignment from run to run has to be more robust, more typical limits for state-of-the-art instruments are a few hundred fluorophores, which for a detection volume of (100 pm) =1 nL translates into a minimum detectable concentration of a few hundred femtomolar. Experimentally, there are several major factors that limit the sensitivity of detection [49]. For maximum sensitivity, the excitation light intensity should be sufficient to photobleach most of the fluorophores by the time they exit the detection volume. The collection optics are extremely important, and should be designed for spatial rejection of light originating from outside the detection volume as well as for efficient collection of as much of the isotropic fluorescence emission as feasible. 

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... 

As a finite mass of solute is placed on the column, any increase in peak volume necessary to compensate for high extra column dispersion will dilute the solute concentration as sensed by the detector. Consequently, as the sensitivity, or minimum detectable concentration of the detector, has a limit, increasing the column diameter will result in a reduced mass sensitivity. 

The sensitivity of the detector (Xd) (or minimum detectable concentration) is defined as that concentration of solute that will provide a signal equivalent to twice the noise level. Now the concentration of solute at the peak maximum is approximately twice the average concentration of the solute in the peak, volume. Thus, the minimum detectable mass will be that mass (m) that, when dissolved in a volume of mobile phase equivalent to the peak volume, will produce a concentration of Xp/2. 

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