Analytical sensitivity

The detection limit of a method should not be confused with the so-called analytical sensitivity. Analytical sensitivity is the ability of an analytical method to assess small variations of the concentration of analyte.This is often expressed as the slope of the calibration curve. However, in addition to the slope of the calibration function, the random variation of the calibration function should also be taken into account. In point of fact, the analytical sensitivity depends on the ratio between the SD of the calibration function and the slope. As mentioned previously, the smaller the random variation of the instrument response and the steeper the slope, the higher is the ability to distinguish small differences of analyte concentrations. In reahty, analytical sensitivity depends on the precision of the method. Historically the meaning of the term analytical sensitivity has been the subject of much discussion. 

The relative uncertainty of measurements at or just exceeding the LoD may be large, and often a quantitative result is not reported. The lower limit for reporting quantitative results, the limit of quantitation (LoQ), relates to the total error being considered acceptable for an assay. From a precision profile for the assay and an evaluation of the bias in the low range, LoQ may be determined in relation to specifications of the method. For example, a laboratory may specify that the total error (e.g., expressed here as Bias -i- 2 SD) of an assay is lower than 45% (corresponding to a bias of 15% and a CV of 15%) of the measurement concentration. In this case, the LoQ is the lowest assay value at which this specification is fulfilled. LoQ constitutes the lowest limit of the reportable range for quantitative results of an assay. 

When XRF analysis of thin film samples is performed (i. e., in samples where the product pd of sample thickness d and sample density p is so small that absorption of the incoming exciting and of the outgoing fluorescent radiation in the material can be neglected, see Section 11.4.), there is a linear relation between the collected net X-ray intensity N of a given characteristic line of element i and the irradiated mass ttij, which usually is also proportional to the concentration q of that element in the sample  

The proportionality constants Sj for the various elements are called the sensitivity coefficients of the XRF spectrometer for determination of these elements (expressed in counts s (g/cm ) ) and are important figures-of-merit of the instrument. In Fig. 11.9, the variation with atomic number of the sensitivity of a 

WDXRF spectrometer is plotted, for the case where either the (10 Zj 60) or L (40 Z 80) peak intensities are used as analytical signals. By selection of the excitation conditions (tube anode material, excitation voltage), the shape and location of the maximum in the sensitivity curve can be influenced to suit the needs of the appUcation at hand. 

Instead of using the X-ray intensity collected during a speciflc time t, it is often more convenient to use the net X-ray count rate R  

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For example, when the activity is determined by counting 10,000 radioactive particles, the relative standard deviation is 1%. The analytical sensitivity of a radiochemical method is inversely proportional to the standard deviation of the measured ac-... 

When we carry out conventional studies of solution kinetics, we initiate reactions by mixing solutions. The time required to achieve complete mixing places a limit on the fastest reaction that can be studied in this way. It is not difficult to reduce the mixing time to about 10 s, so a reaction having a half-life of, say, 10 s is about the fastest reaction we can study by conventional techniques. (See Section 4.4 for further discussion of this limit.) The slowest reaction accessible to study depends upon analytical sensitivity and patience let us say that the half-life of a graduate student, 2-2 years, sets an approximate limit. This corresponds to roughly 7 x 10 s. Thus, a range of half-lives of about 10 can be studied by conventional techniques. 

The electrospray process is susceptible to competition/suppression effects. All polar/ionic species in the solution being sprayed, whether derived from the analyte or not, e.g. buffer, additives, etc., are potentially capable of being ionized. The best analytical sensitivity will therefore be obtained from a solution containing a single analyte, when competition is not possible, at the lowest flow rate (see Section 4.7.1 above) and with the narrowest diameter electrospray capillary. 

For low-use rate compounds applied on a grams per hectare basis, it has sometimes been necessary to apply the cumulative seasonal rate in a single application in order to improve analytical detection. Advances in analytical chemistry have greatly improved the trace-level detection of agrochemicals in soil but it is still prudent to verify that sufficient analytical sensitivity exists to detect agrochemicals at their anticipated soil... 

HS-GC methods have equally been used for chromatographic analysis of residual volatile substances in PS [219]. In particular, various methods have been described for the determination of styrene monomer in PS by solution headspace analysis [204,220]. Residual styrene monomer in PS granules can be determined in about 100 min in DMF solution using n-butylbenzene as an internal standard for this monomer solid headspace sampling is considerably less suitable as over 20 h are required to reach equilibrium [204]. Shanks [221] has determined residual styrene and butadiene in polymers with an analytical sensitivity of 0.05 to 5 ppm by SHS analysis of polymer solutions. The method development for determination of residual styrene monomer in PS samples and of residual solvent (toluene) in a printed laminated plastic film by HS-GC was illustrated [207], Less volatile monomers such as styrene (b.p. 145 °C) and 2-ethylhexyl acrylate (b.p. 214 °C) may not be determined using headspace techniques with the same sensitivities realised for more volatile monomers. Steichen [216] has reported a 600-fold increase in headspace sensitivity for the analysis of residual 2-ethylhexyl acrylate by adding water to the solution in dimethylacetamide. 

Classical polarography is not optimised for analytical sensitivity. This inefficiency has been remedied in pulse polarography. Instead of applying a steadily increasing voltage on the mercury droplet, in pulse... 

E + AE at the instant just before the drop fall this has such an overwhelming effect that, although basic acquires values with a considerable pre-electrolysis component, Ai remains sufficiently large to reach high analytical sensitivity limits of detection of 10 8 M35 by DPP compared with 10 6-10 7Mby NPPhave been obtained. Further, where by DPP in fact AijAE per drop is determined, AlijAE2 can be established for consecutive drops. 

The PIXE microbeam technique has a spot size in the range 1-10 pm, and this enables a study of the spatial distribution of elemental concentrations. The advantage of p-PIXE over EPMA is a very much increased analytical sensitivity due to the much lower Bremsstrahlung background generated by the proton beam. The detection limits are of the order 0.1% for EPMA and 0.001% using the p-PIXE technique. 

The analytical sensitivities of the different quantitation methods have been compared using serial dilutions of patients specimens (Butterworth et al., 1996) and the Eurohep HBV DNA standards (Zaaijer et al., 1994). In both cases, bDNA was shown to be about 10-fold more sensitive than the liquid hybridization (Abbott) and the hybrid capture (Digene) assays. Using the Eurohep HB V standards, the detection limits were 2.5 X 106 genomes/ml for bDNA and 2.5 X 107 genomes/ml for both liquid hybridization (LH) and hybrid capture (HC) assays. 

A prototype bDNA assay was developed for quantification of HGV/GBV-C RNA in serum (Pessoa et al, 1997). The assay employed target probes based on the relatively conserved sequence in the 5 untranslated region of the HGV/GB V-C genome. Preamplifier molecules and incorporation of isoC and isoG into the sequences common to bDNA assays were used to enhance the analytical sensitivity. The provisional limit of detection was 32,500 genome equivalents/ml based on dilutions of a 700-nucleotide synthetic HGV/GBV-C RNA transcript. The run-to-run variance of the assay was <15%. 

The impact of an ion beam on the electrode surface can result in the transfer of the kinetic energy of the ions to the surface atoms and their release into the vacuum as a wide range of species—atoms, molecules, ions, atomic aggregates (clusters), and molecular fragments. This is the effect of ion sputtering. The SIMS secondary ion mass spectrometry) method deals with the mass spectrometry of sputtered ions. The SIMS method has high analytical sensitivity and, in contrast to other methods of surface analysis, permits a study of isotopes. In materials science, the SIMS method is the third most often used method of surface analysis (after AES and XPS) it has so far been used only rarely in electrochemistry. 

With all the features that increase analytical sensitivity, it is possible to obtain very good quality pyrolysis Ar MAB mass spectra from 50,000 cells... 

The low concentrations of lead in plasma, relative to red blood cells, has made it extremely difficult to accurately measure plasma lead concentrations in humans, particularly at low PbB concentrations (i.e., less than 20 pg/dL). However, more recent measurements have been achieved with inductively coupled mass spectrometry (ICP-MS), which has a higher analytical sensitivity than earlier atomic absorption spectrometry methods. Using this analytical technique, recent studies have shown that plasma lead concentrations may correlate more strongly with bone lead levels than do PbB concentrations (Cake et al. 1996 Hemandez-Avila et al. 1998). The above studies were conducted in adults, similar studies of children have not been reported. 

The robustness of an analytical procedure for the determination of the analyte A in presence of some accompanying species i = B,...,N under influence of various factors fj(j = l,...,m) according to Eq. (4.30) is in reciprocal proportion to the sum of all their cross sensitivities, SA multiplied by the actual amounts, x and the specific influencing strengths, hjy of the factors multiplied by their actual values (in relation to xA) see Danzer (2004). Because of the way measurements are obtained, the range of their values is range = (0... oo), so it makes sense to calculate the relative robustness which includes the analyte sensitivity and amount itself, SAA xA, as follows ... 

Fluorescence resonance energy transfer (FRET) has also been used very often to design optical sensors. In this case, the sensitive layer contains the fluorophore and an analyte-sensitive dye, the absorption band of which overlaps significantly with the emission of the former. Reversible interaction of the absorber with the analyte species (e.g. the sample acidity, chloride, cations, anions,...) leads to a variation of the absorption band so that the efficiency of energy transfer from the fluorophore changes36 In this way, both emission intensity- and lifetime-based sensors may be fabricated. 

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