Precision in quantitative analysis

Whenever possible, samples should be dissolved in the mobile phase to provide longest column life and maximum precision in quantitative analysis. The sample solvent must not have stronger eluting properties than the mobile phase, since this will result in wider peaks, and possibly in peak distortion. 

There are two basic methods used in quantitative analysis one uses a reference standard with which the peak areas (peak heights) of the other solutes in the sample are compared the other is a normalization procedure where the area (height) of any one peak is expressed as a percentage of the total area (heights) of all the peaks. There are certain circumstances where each method is advantageous, and providing they are used carefully and appropriately all give approximately the same accuracy and precision. 

The accuracy and precision of carotenoid quantification by HPLC depend on the standard purity and measurement of the peak areas thus quantification of overlapping peaks can cause high variation of peak areas. In addition, preparation and dilution of standard and sample solutions are among the main causes of error in quantitative analysis. For example, the absorbance levels at of lutein in concentrations up to 10 mM have a linear relationship between concentration and absorbance in hexane and MeOH on the other hand, the absorbance of P-carotene in hexane increased linearly with increasing concentration, whereas in MeOH, its absorbance increased linearly up to 5 mM but non-linearly at increasingly higher concentrations. In other words, when a stock solution of carotenoids is prepared, care should be taken to ensure that the compounds are fuUy soluble at the desired concentrations in a particular solvent. 

HPLC has more or less supplanted GC as a method for quantifying drugs in pharmaceutical preparations. Many of the literature references to quantitative GC assays are thus old and the precision which is reported in these papers is difficult to evaluate based on the measurement of peak heights or manual integration. It is more difficult to achieve good precision in GC analysis than in HPLC analysis and the main sources of imprecision are the mode of sample introduction, which is best controlled by an autosampler, and the small volume of sample injected. However, it is possible to achieve levels of precision similar to those achieved using HPLC methods. For certain compounds that lack chromophores, which are required for detection in commonly used HPLC methods, quantitative GC may be the method of choice, for analysis of many amino acids, fatty acids, and sugars. There are a number... 

Because analytical chromatography is used inherently in quantitative analysis, it becomes crucial to precisely measure the areas of the peak. Therefore, the substances to be determined must be well separated. In order to achieve this, the analysis has to be optimised using all the resources of the instrumentation and, when possible, software that can simulate the results of temperature modifications, phases and other physical parameters. This optimisation process requires that the chromatographic process is well understood. 

Anyone who has taken a course in quantitative analysis will tell you about repeating an experiment over and over again to get a large sample of data. It s not uncommon for experiments requiring a titration to be repeated anywhere from 7 to 10 times Once a number of experiments have been completed, you then want to compare the results to see if they are both accurate and precise. Accuracy is the term used to define how close the data have come to the accepted value. Precision is the term used to define how closely the data agree with data obtained from other performances of the same experiment. Hopefully your data will be both accurate and precise. Look at the following data a student obtained from a titration experiment involving an acid and base ... 

There are two approaches to deal with matrix effects in quantitative analysis. One may eliminate the sample constituents responsible for the matrix effects by improving sample pretreatment and/or chromatography. Alternatively, one may reduce or eliminate the influence that matrix effects have on the accuracy and/or precision of the method. 

The standard addition method is commonly used in quantitative analysis with ion-sensitive electrodes and in atomic absorption spectroscopy. In TLC this method was used by Klaus 92). Linear calibration with R(m=o)=o must also apply for this method. However, there is no advantage compared with the external standard method even worse there is a loss in precision by error propagation. The attainable precision is not satisfactory and only in the order of 3-5 %, compared to 0.3-0.5 % using the internal standard method 93). 

These advantages are a) faster analysis time, b) increased accuracy and smaller standard deviation in quantitative analysis, c) precise determination of retention data with Kovdts indices in qualitative analysis, d) increased utility of instruments, e) improvement and application of new techniques which cannot be used without a computer, and f) more information overall. Each of these improvements is a step forward in the progress of analytical science and technology, some of which, however, will earn their keep only in the future. 

Detector specifications have been discussed in chapter 5. They reveal the accuracy and precision attainable in quantitative analysis and also the lower concentration levels that are possible in trace analysis. As in GC, the five specifications of prime importance are detector response, detector noise level, detector sensitivity, or minimum detectable concentration, detector linearity and linear dynamic range [1], The detector response, detector woAe level and the detector are relevant to trace... 

To increase the precision of quantitative analysis, the plasticizer sample is diluted with carbon disulfide, its infrared absorption measured, and compared with absorptions standard of standard samples prepared also in CS2 to cover the range of concentrations from 0.5 to 3 mg/ml. For each suspected (identified) plasticizer, a series of standards has to be prepared and measured. It is also important to select a suitable wavelength for quantitative analysis. For dioctyl phthalate bands at 1725 and 1121 cm" are usually used. For tricresyl phosphate band at 1191 cm is used. Similar to gravimetric method, the results are subject to various interferences when mixtirres of plasticizers or mixture of plasticizers with other additives ate used. 

Types of Internal Standards Three types of internal standards have found a niche in quantitative analysis. The first includes those compounds that have chemical and physical properties similar to those of the analyte. The only requirements are that it should not be isobaric to the analyte and it must elute from a chromatographic column at a different time. This type of internal standard is easy to find, but it does not provide the level of precision and accuracy that is... 

Reliability It is rather difficult to use the accuracy and precision concepts as these capital and basic properties are closely related in qualitative analysis. Their combination has produced a new property called reliability, which is defined as the proportion (percentage) of right yes or no answers provided by individual tests carried out on n aliquots of the same sample to identify an analyte or a family of the analytes. This definition represents the positive side of the errors in qualitative analysis false positives and false negatives. The reliability of the binary response is not an independent property as it strongly depends on the basic properties of sensitivity and selectivity. Moreover, it is in contradiction with productivity-related properties. Reliability is equivalent to certainty and, in quantitative analysis, the uncertainty of a result is a parameter associated with reliability. Indeed, the term is included in the definition of traceability as every experimental datum is affected by specific variations or doubts. As it directly affects the quality of an analytical result, it is necessary to find out an equivalent method to express vmcertainty in qualitative analysis. The term unreliabifity can be... 

Deng, R.-C., WilUams, P. (1989) Factors affecting precision and accuracy in quantitative analysis by secondary ion mass spectrometry. Ancd. Chem., 61,1946-1946. 

The main problems encountered in quantitative analysis are associated with sampling and sample introduction, and with measurement of the peak area and the calibration factors [58]. Chromatographic measurements are usually carried out with a precision of a few percent, but can be much more precise when the sources of error are properly recognized and controlled [65]. 

Whereas IR, NMR. and mass spectroscopy are used mainly for the elucidation of structure and the identification of substances, UV-VIS spectroscopy enables quantitative determinations to be carried out much more precisely and reproducibly. Therefore, its primary areas of application are in quantitative analysis and clinical medicine, in the determination of drug concentrations, in the quantification of pharmaceuticals and as detectors in chromatographic processes (HPLC, TLC) (- Liquid Chromatography) [4], [51, Furthermore, mixtures as well as pure substances can be studied and the components determined quantitatively by methods of multicomponent analysis [6]. Since modem spectrometers operate very rapidly and can be constructed in the form of photodiode arrays, they have the advantage over other analytical methods of being usable not only for observing... 

The method described here makes use of stable isotope dilution mass spectrometry (IDMS) for quantitative analysis of niacin. Isotopically labelled versions of both nicotinic acid and niacinamide are commercially available at a reasonable cost. The use of an isotopically labelled internal standard has distinct advantages in quantitative analysis, as it can correct for analyte losses and makes possible high levels of accuracy and precision (Fassett and Paulson 1989). Sample digestion and clean-up is based on a previously published LC-UV method (LaCroix et al. 1999, 2002 LaCroix and Wolf 2001). This chapter expands on a previous report of LC-IDMS analysis of niacin (Goldschmidt and Wolf 2007), with material from that report used with permission of the publisher. 

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