Sensitivity of an analytical method

Another approach is to use the sensitivity of an analytical method for determining the acceptance criteria limits. In this case, either the LOD or LOQ can be used as the acceptable residue limit. If the results reported for the swab and rinse samples are less than the LOD or LOQ, the sensitivity limits can be used to calculate the MAC present. 

Sensitivity. The sensitivity of an analytical method is equal to the slope of the calibration line in a linear system. 

The sensitivity of an analytical method means a minimum concentration, minimum amount, or a minimum difference in concentrations, of an element that can be determined by this method. Later on, use will be made of the first meaning of the term. 

The role of flow systems in increasing the selectivity and sensitivity of an analytical method... 

The sensitivity of an analytical method can be defined as the slope of the calibration curve, that is, as the ratio of change in the instmment response with a change in the analyte concentration. Other definitions are also used. In AAS, sensitivity is defined as the concentration of analyte that produces an absorbance of 0.0044 (an absorption of 1%), for example. When the term sensitivity is used, it should be defined. 

In connection with the calibration function performance, several characteristics are combined, such as the sensitivity of an analytical method and its working range. 

The term sensitivity, which is frequently used to describe the quahty of a residue analysis, or the current state of a measuring instrument, is often used incorrectly as a synonym for the lowest possible LOD or LOQ. A sensitive analysis procedure, however, exhibits a large change in signal with a small change in substance concentration. The sensitivity of a procedure thus describes the slope of a linear cahbration function (see Section 3.3.6). At the same confidence interval of the measured points of a cahbration function (Figure 3.161), sensitive analysis procedures give a narrower confidence interval than less sensitive ones The LOD is independent of the sensitivity of an analytical method. 

The terms defined above are all important in the consideration of the overall performance of an analytical method. The greatest sensitivity (response) does not necessarily imply the lowest limit of detec-tion/determination as a more intense signal may also be observed from... 

Verification implies that the laboratory investigates trueness and precision in particular. Elements which should be included in a full validation of an analytical method are specificity, calibration curve, precision between laboratories and/or precision within laboratories, trueness, measuring range, LOD, LOQ, robustness and sensitivity. The numbers of analyses required by the NMKL standard and the criteria for the adoption of quantitative methods are summarized in Table 10. 

It is recommended that the protocol itself contain language that allows for minor modifications of an analytical method or procedure without necessitating an amendment (or a deviation) for example, "Minor modifications in instrumental parameters and/or adjustments in technique may be made in the method during specimen analysis to enhance overall efficiency or the sensitivity, specificity, or selectivity of analyte response."... 

The calibration sensitivity of the analytical method employed is simply determined as the slope of the calibration curve. For example, in the case of methyl paraben, the value of calibration sensitivity obtained was 1.6 mAl I/min///M (Figure 6.22). Analytical sensitivity is defined as the ratio between calibration sensitivity and the value of the standard deviation obtained at each concentration.10 The value of the standard deviation encountered for a concentration of 0.6 //M was 0.1, resulting in an analytical sensitivity for methyl paraben at 0.6 //M of 16 m. II/min///M. As indicated for LOD and LOQ, the values obtained for linearity and sensitivity depend on the analytes employed and the corresponding method and instrumental parameters. For example, Liu et al.9 evaluated the LOD and LOQ for Drug A (released from OROS) for a particular analytical method employing //Pl.C to be 0.5 //g/ml. and 2.0 //g/mL, respectively. 

The purpose of an analytical method is the deliverance of a qualitative and/or quantitative result with an acceptable uncertainty level. Therefore, theoretically, validation boils down to measuring uncertainty . In practice, method validation is done by evaluating a series of method performance characteristics, such as precision, trueness, selectivity/specificity, linearity, operating range, recovery, LOD, limit of quantification (LOQ), sensitivity, ruggedness/robustness, and applicability. Calibration and traceability have been mentioned also as performance characteristics of a method [2, 4]. To these performance parameters, MU can be added, although MU is a key indicator for both fitness for purpose of a method and constant reliability of analytical results achieved in a laboratory (IQC). MU is a comprehensive parameter covering all sources of error and thus more than method validation alone. 

Both the detection limit and the limit of quantification, as defined, are often not very stable characteristics of an analytical method, because the blank signal and the signal generated by the very low concentrations of the analyte are frequently dependent on certain analytical parameters, including the purity of reagents, sample matrices, environmental conditions, instrumentation, and the analysts themselves. Sensitivity is a measure of the ability of an analytical method to discriminate between small differences in analyte concentration. It is defined as the analyte signal per unit concentration of the analyte. Despite the apparent simplicity of the sensitivity concept, a degree of confusion surrounds its use. This confusion stems from the perception that the sensitivity of a method is the same as the limit of detection. 

The sensitivity and simplicity of an analytical method always improves when selective detection can be used. Because of detection selectivity, a sample in a complex matrix can be analysed with minimum sample preparation. Wouldn t it be ideal to have an ultra-specific reagent that would allow the determination of a compound in an environment containing thousands of substances ... 

Analytical methods validation—As mentioned earlier, a good CVMP should allow the analytical method to develop concurrently with the product formulation, thus in the early stages of development, an analytical method may not be fully validated but may still be used for cleaning tests as the best available method. At the time of the PAI, however, and definitely by the time the formal cleaning validation occurs, a fully validated analytical method should be developed. This methods validation package should include all the standard parameters, with special attention to the sensitivity of the analytical method as expressed by the limit of detection (LOD) and... 

In a probabilistic risk assessment, both variability and uncertainty in input variables can be taken into consideration. Variability represents the true heterogeneity in time, space, and of different members of a population. Examples of variability are interindividual variability in consumption and in sensitivity to, for instance, an allergen. Uncertainty is a lack of knowledge about the true value of the quantity. An example of uncertainty is associated with the limit of detection of an analytical method and the exploration of the threshold value outside the range of measurements. In contrast to the variability, uncertainty can be decreased, for example, by increasing the number of data points or using a more accurate method of analysis. 

Ultrasonic nebulizers have also been employed in continuous flow systems as interfaces between sample preparation steps in the analytical process and detection by virtue of their suitability for operating in a continuous mode. Thus, preconcentration devices have commonly been coupled to atomic spectrometers in order to increase the sensitivity of some analytical methods. An enhancement factor of 100 (10 due to USNn and 10 due to preconcentration) was obtained in the determination of platinum in water using a column packed with polyurethane foam loaded with thiocyanate to form a platinum-thiocyanate complex [51]. An enhancement factor of 216 (12 with USNn and 18 with preconcentration) was obtained in the determination of low cadmium concentrations in wine by sorption of metallic complexes with pyridylazo reagents on the inner walls of a PTFE knotted reactor [52]. One special example is the sequential determination of As(lll) and As(V) in water by coupling a preconcentration system to an ICP-AES instrument equipped with a USN. For this purpose, two columns packed with two different resins selective for each arsenic species were connected via a 16-port valve in order to concentrate them for their subsequent sequential elution to the spectrometer [53]. 

The thiobarbituric acid (TBA) test was proposed over 40 years ago and is now one of the most extensively used methods to detect oxidative deterioration of fat-containing foods (41). During lipid oxidation, malonaldehyde (MA), a minor component of fatty acids with 3 or more double bonds, is formed as a result of the degradation of polyunsaturated fatty acids. It is usually used as an indicator of the lipid oxidation process, both for the early appearance as oxidation occurs and for the sensitivity of the analytical method (42). In this assay, the MA is reacted with thiobarbituric acid (TBA) to form a pink MA-TBA complex that is measured spectrophotometrically at its absorption maximum at 530-535 nm (Figure 2) (9,43,44). The extent of oxidation is reported as the TBA value and is expressed as milligrams... 

Sensitivity Ability of an analytical method to detect a response variation due... 

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. 

Two notes deserve mention. First, Dr. Rogan provided evidence that his NOAEL might be overstated by a factor of two, due to the use of an analytical method that is known to overstate PCB concentrations. As a result, a NOAEL of 0.05 pg/kg/day can be reported for the studies reviewed by Tilson et al. (1990) for the most sensitive population (i.e., embryos and children). (Note, some panehsts thought an additional factor of 10 should be apphed to his NOAEL to account for sensitive populations, but other panelists disagreed.) Second, the extrapolated NOAEL for the Tryphonas study was derived as follows The LOAEL in rhesus monkeys of 5 pg/kg/day was divided by a factor of 10 to convert the LOAEL to a NOAEL of 0.5 pg/kg/day in monkeys. This NOAEL was then divided by 3 to account for interspecies variation. Therefore, ATSDR calculated the NOAEL in the non sensitive human population as 0.166 pg/kg/day, rounded up to 0.2 pg/kg/day. To account for the most sensitive population, such as the developing embryo and fetuses, ATSDR divided the 0.2 pg/kg/day by a factor of 10 and resulted in an MRL of 0.02 pg/kg/day. As a result, ATSDR s MRL (i.e., 0.02 pg/kg/day) is lower than the NOAEL in the most sensitive human subpopulation reported in the studies reviewed by Tilson et al. (1990) (i.e., 0.05 pg/kg/day). 

Table 13.1 lists the detection limits (King, 1984) of a number of spectroscopic and chromatographic analytical methods employed in supercritical extraction studies. A gravimetric method requires a few milligrams of material for analysis TLC analysis can detect materials at solubility levels one billion times lower If we are measuring the threshold pressure at which the compound dissolves at a detectable level, an analytical method that can identify 10 g will result in the identification of a lower threshold pressure than can be ascertained in the gravimetric method. The threshold pressure is therefore a function of the sensitivity of the analytical method. By itself, it is certainly not the critical criteria for determining the feasibility of a potential SCF process. Nevertheless, the work of Stahl provides us with a wealth of solubility information that we briefly consider here. 

Considering both the compound and amount related effects compromising the precision of carbon isotope analyses, the sensitivity of the analytical method used can be appointed to an amount down to approx. 5 ng for numerous anthropogenic contaminants. However, it has to be noted that in comparison the precision of the analyses of halogenated and tin containing compounds is generally lower. 

In this chapter, we have covered the way s to understand, estimate, and interpret various criteria required for assessing the validity of an analytical method. Whatever the complexity or simplicity of computations and models needed, the primary obj ective of an analytical method should never be forgotten Can each measurement be trusted or, equivalently, is the measurement error acceptable All the information needed to make a decision is contained in the measurement error profile. The key performance characteristics such as the linearity, accuracy, precision, limits of quantification, and sensitivity are readily obtained from this profile and can easily be understood and interpreted by an analyst. 

It has been proved that the selectivity of an analytical method is directly connected to the complexity of the matrix from which the analyte must be determined. As a result, the same method can be more selective or less selective, depending on the qualitative and quantitative composition of the matrix from which the analyte must be determined. For example, two types of electrochemical sensors are described for the assay of thyroid hormones L-T3 and L-T4. The first is an amperometric biosensor based on L-amino acid oxidase (l-AAOD),270 whereas the other is an amperometric immunosensor based on anti-L-T3 and anti-L-T4.271 If T3 and T4 have to be determined from phramaceutical products, both types of sensors have the necessary sensitivity and selectivity. When it is required to determine both hormones in biological fluids or in thyroid tissue, the proposed biosensors are not selective enough because l-AAOD catalyzes the reactions of both thyroid hormones. The amperometric biosensors can only make the discrimination between the two thyroid hormones, namely, L-T3 and L-T4, since the specific antibody reacts only with the specific antigen. 

A high sensitivity of the method creates low selectivity. The interrelationship between selectivity and sensitivity must be a strong consideration in the validation of an analytical method. As was shown in Chapter 9, the best quality and reliability for analytical information is obtained only when the sensitivity is at a medium level and the selectivity is high. 

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