Analytical concentration definition

The technical terms homogeneity and inhomogeneity defined in analytical chemistry must be distinguished from the physicochemical concept of homogeneity and heterogeneity (Danzer and Ehrlich [1984]). Whereas the thermodynamical definition refers to morphology and takes one-phase-or multi-phase states of matter as the criterion, the analytical-chemical definition is based on the concentration function... 

The guidelines provide variant descriptions of the meaning of the term linearity . One definition is, ... ability (within a given range) to obtain test results which are directly proportional to the concentration (amount) of analyte in the sample [12], This is an extremely strict definition, one which in practice would be unattainable when noise and error are taken into account. Figure 63-la schematically illustrates the problem. While there is a line that meets the criterion that test results are directly proportional to the concentration of analyte in the sample , none of the data points fall on that line, therefore in the strictest sense of the phrase, none of the data representing the test results can be said to be proportional to the analyte concentration. In the face of nonlinearity of response, there are systematic departures from the line as well as random departures, but in neither case is any data point strictly proportional to the concentration. 

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. 

The linearity definition is on the slide. As long as the measurement result versus the analyte concentration fits a straight line we call this linearity. In an ideal case hnearity could extend over several orders of magnitude. This is, for example, the case for certain elements when determined by ICP-OES. In other cases it can be less than one order of magnitnde. Linearity over a large concentration can be very time saving, becanse fewer dilation steps might be necessary (see also chapter 9). 

Suspensions or slurries pose a problem since by definition they are not homogeneous. The problem is how to obtain reliable quantitative results from suspensions. One method for dealing with suspensions is to prepare individually weighed samples and stress them at concentrations greater than the final analytical concentration. Prior to analysis the samples are then diluted to the final analytical concentration with a solvent that completely dissolves the sample. For example, if the final analytical concentration desired is 0.3mg/mL, suspensions could be prepared at a concentration of approximately 1 mg/mL by adding 3mL of the appropriate solvent to samples of approximately 3 mg in 10-mL volumetric flasks. Prior to assay, the samples can then be diluted to volume with a solvent capable of completely dissolving the sample. 

The MDL is one of the secondary data quality indicators. The EPA provides the definition of the MDL as the minimum concentration that can be measured and reported with 99 percent confidence that the analyte concentration is greater than zero (EPA, 1984a). 

Extrapolation of the calibration curve above or below the calibrated range is not acceptable. The rule of thumb is that samples with analyte concentrations that exceed the upper calibration point by more than 10 percent must be diluted. Sample concentrations that are below the lower calibration point are estimated, not definitive, values. 

Definition The limit of detection is a parameter of limit tests. It is the lowest concentration of analyte in a sample that can be detected, but not necessarily quantitated, under the stated experimental conditions. Thus, limit tests merely substantiate that the analyte concentration is above or below a certain level. The limit of detection is usually expressed as the concentration of analyte (e.g., percentage, milligrams per gram, parts per billion) in the sample. 

Further research is required in this relatively new field to establish definitive data regarding differentiation between internal incorporation (ingestion) and external contamination (environmental), effect of hair type on analyte incorporation, time course of analyte appearance, dose vs. analyte concentration relationships, and the mechanism of drug entry into hair. Already this novel technique has proved useful in a wide variety of applications, and will unquestionably become more popular in future years. 

Some of the concepts used in defining confidence limits are extended to the estimation of uncertainty. The uncertainty of an analytical result is a range within which the true value of the analyte concentration is expected to lie, with a given degree of confidence, often 95%. This definition shows that an uncertainty estimate should include the contributions from all the identifiable sources in the measurement process, i.e. including systematic errors as well as the random errors that are described by confidence limits. In principle, uncertainty estimates can be obtained by a painstaking evaluation of each of the steps in an analysis and a summation, in accord with the principle of the additivity of variances (see above) of all the estimated error contributions any systematic errors identified... 

The dissimilarity score for two identical spectra will never reach zero because of the presence of noise in the detector that cannot be compensated. There will always be some residual dissimilarity, which imposes a practical limit on the definition of a match threshold. The noise level of a given spectrum at a specific analyte concentration depends on the spectrometer, the characteristics of the diode array, and the scan speed of the detector, the lamp, and detector electronics. 

We noted earlier that the detection limit is directly related to SNR and is often defined as an analyte concentration yielding a signal that is some factor, k, larger than the standard deviation of the blank, (Jbk- It is useful to define the detection limit for Raman spectroscopy as the minimum detectable value of the cross section-number density product, or ( SD)min- Of course, the concentration detection limit in terms of D or molarity will depend on the magnitude of p, but (PD)min is a more general definition that directly indicates spectrometer performance. In the vast majority of analytical Raman measurements at low values of PD, the SNR is background noise limited, so abk In... 

The limit of quantitation (LOQ) is defined by the maximum acceptable level of uncertainty in the measured values, and is generally considered to be the analyte concentration that yields a signal equal to the blank signal plus 10 standard deviations, as shown below. These and alternative definitions of LOD and LOQ are presented elsewhere.3... 

The word sensitivity is often used in describing an analytical method. Unfortunately, it is occasionally used indiscriminately and incorrectly. The most often-used definition of sensitivity is the calibration sensitivity, or the change in the response signal per unit change in analyte concentration. The calibration sensitivity is thus the slope of the calibration curve, as shown in Figure 8-14. If the calibration curve is linear, the sensitivity is constant and independent of concentration. If nonlinear,. sensitivity changes with concentration and is not a single value. 

This excitement about second-order sensor calibration has led to a search by chemometric researchers to find equivalent instrumentation that gives rise to second-order data. The definition of second-order instruments is slowly solidifying and currently a foundation has been established to classify which techniques are true second-order devices. This definition of second-order instruments is simply two sensor arrays which are independent of each other. However, in order for the arrays to be independent, one of the arrays must modulate the sample s analyte concentrations. The best known instrument... 

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. 

There are recognized definitions and guidance documents related to the assessment of the analytical response for an analyte in a measurement system. These usually are expressed in terms of the relationship between the concentration of analyte present in the sample material and the associated response from a detector. The relationship between the analytical response and the analyte concentration is plotted and usually expressed as a linear regression equation. lUPAC defines the calibration curve in terms of the calibration function in analysis in the lUPAC Gold Book the calibration function in analysis is defined as follows ... 

Definition The Method Detection Limit (MOL) is defined as the minimum concentration of a substance that can be measured and reported with a 99% confidence that Che analyte concentrations are greater than zero. "... 

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