Accuracy related substances

It is often possible to predict with accuracy many properties of ideal solutions, such as dilute gas mixtures, as well as liquid mixtures of closely related substances such as pentane and hexane. On the other hand, liquid mixtures of substances with different... 

The quantitation Limit (QL) is a characteristic of quantitative assays for low levels of compounds in sample matrices, such as impurities in bulk drug substances and degradation products in finished pharmaceuticals. QL is defined as the concentration of related substance in the sample that will give a signal-to-noise ratio of 10 1. The QL of a method is affected by both the detector sensitivity and the accuracy of sample preparation at the low concentration of the impurities. In practice, QL should be lower than the corresponding ICH report limit. 

Experimental Requirements. Solutions of known concentrations are used to determine the linearity. A plot of peak area versus concentration (in percent related substance) is used to demonstrate the linearity. Authentic samples of related substances with known purity are used to prepare these solutions. In most cases, for the linearity of a drug product, spiking the related substance authentic sample into excipients is not necessary, as the matrix effect should be investigated in method accuracy. 

Intrinsic Accuracy. Intrinsic accuracy indicates the bias caused by sample matrix and sample preparation. In this approach, a stock solution is prepared by using known quantities of related substance and drug substance. The stock solution is further diluted to obtained solutions of lower concentrations. These solutions are used to generate linearity results. In addition, these linearity solutions of different concentrations are spiked into placebo. The spiked solutions are prepared according to the procedure for sample analysis. The resulting solutions, prepared from the spiked solution, are then analyzed. If the same stock solution is used for both linearity and accuracy and all of these solutions are analyzed on the same HPLC run, the response of linearity (without spike into matrix) and accuracy (with spike into matrix) can be compared directly. Any differences in response indicate the bias caused by matrix interference or sample preparation. To determine the intrinsic accuracy at each concentration level, one can compare the peak area of accuracy (with matrix) with that of linearity (without matrix) at the same concentration (Figure 3.11). This is the simplest approach, and one would expect close to 100% accuracy at all concentration levels. 

Typically, linearity and accuracy determination covers a wide concentration range (e.g., 50% of the ICH reporting limit to 150% of specification). However, the concentration range for precision will be limited by the availability of sample of different related substance levels. Therefore, to ensure an appropriate method validation range with respect to precision, it is critical to use samples of low and high levels of related substance in precision experiments (e.g., fresh and stressed samples). 

As a final check on the accuracy of the statistical model, the observed results for total related substances and peak B obtained during stability testing of 18 different manufacturing and lab scale batches of drug substance were compared to predicted increases (Figs. 11 and 12). The stability studies represented amorphous and crystalline material with water content ranging from 3-8%. Temperatures used for stability studies were 5°C, 25°C, and 35°C. The duration of the studies was from 11 weeks to 10 months. The... 

Probably no technique in analytical chemistry has been more quickly and widely adopted than gas chromatography. It is ideally suited to the separation and analysis of nonpolar volatile materials such as hydrocarbons. Factors that have contributed to this rapid development are (/) the success of the method in performing quantitative separations of closely related substances, (2) the ease with which the operation can be made automatic, (5) the speed, predsion, and accuracy with which quantitative determinations can be made, (4) the small quantities of simple required, and (5) the rapid development of sensitive detection devices. The prindpal limitation is that the method is restricted to materials that exert vapor pressures of at least 10 mm Hg at the temperature of the column (ranging up to 300°C or somewhat higher). 

A few examples of where it is possible are the following Use of a refractive index detector, trace analysis of related substances (impurities) in pharmaceutical products, with UV detection at very low wavelength, when the accuracy of the result is of little interest [1] (e.g., when chromatography is used to monitor a chemical reaction), and when a relative value is enough. In this situation, the simplified relationship (3) is sufficient, but the analyst must not forget that the proportions found in this way are not the true proportions. 

HPLC provides reliable quantitative precision and accuracy, along with a linear dynamic range (LDR) sufficient to allow for the determination of the API and related substances in the same run using a variety of detectors, and can be performed on fully automated instrumentation. HPLC provides excellent reproducibility and is applicable to a wide array of compound types by judicious choice of HPLC column chemistry. Major modes of HPLC include reversed phase and normal phase for the analysis of small (<2000 Da) organic molecules, ion chromatography for the analysis of ions, size exclusion chromatography for the separation of polymers, and chiral HPLC for the determination of enantiomeric purity. Numerous chemically different columns are available within each broad classification, to further aid method development. 

Following the establishment of specificity, the method(s) should be validated to allow for use in release and stability testing. Such validation is typically less stringent than for final methods (sec Chapter 12), but should demonstrate specificity, linearity, range, accuracy, and analysis repeatability for the API. For related substances, specificity should be demonstrated and the limit of detection (LOD) and limit of quantitation (LOQ) should be established for the API to serve as surrogate values for the LOD and LOQ of impurities for which authentic substances are not available. To achieve a sufficient LOD and simultaneously keep the API in the linear dynamic range of the detector, it may be necessary to use different sample concentrations for the analyses of the API and related substances. It is additionally beneficial to repeat the separation on new columns from different batches to ascertain that the separation obtained can be maintained column to column. 

Another recent innovation deals with the chemistry of the capillary wall. For example, capillaries have been improved by coating their inner surfaces with novel chemistries, or by adding a replaceable dynamic coating solution, allowing better resolution of analytes (109-114). Carrier solutions or background electrolytes can be enriched with a variety of additives, permitting the separation of closely related substances at baseline resolution (see Refs. 115-118 and Sec. II). These improvements have lead to better precision, selectivity, and accuracy of separated analytes. Precision and accuracy is improved for both peak migration and peak area counts for the simple and complex analytes. 

A thorough review of the chemistry of many of the naturally occurring bile acids and related substances may be found in the monograph by Fieser and Fieser (1). A tabulation of known acids and alcohols, their properties, and a brief description of their origins is presented in the next section of this chapter. Identifications in some cases have not been confirmed but the burden of accuracy has been left with the original authors. 

With chlorobenzene and bromobenzene, which show no change of temperature or of volume when mixed together, and have the same critical pressure, and with methyl and ethyl alcohol, which have widely different critical pressures, the differences D -D, are certainly within the limits of experimental error in the other cases the differences are very small, and it may probably be concluded that the actual boiling points of mixtures of closely related substances may be calculated with very considerable accuracy from the... 

The method development by MEEKC is useful for the determination of active and excipient compounds as well as related substances of drugs, with very high precision and accuracy. The application of MEEKC in pharmaceutical analysis offers a powerful tool in quality control of pharmaceutical laboratories. 

Another related issue is the computation of the intensities of the peaks in the spectrum. Peak intensities depend on the probability that a particular wavelength photon will be absorbed or Raman-scattered. These probabilities can be computed from the wave function by computing the transition dipole moments. This gives relative peak intensities since the calculation does not include the density of the substance. Some types of transitions turn out to have a zero probability due to the molecules symmetry or the spin of the electrons. This is where spectroscopic selection rules come from. Ah initio methods are the preferred way of computing intensities. Although intensities can be computed using semiempirical methods, they tend to give rather poor accuracy results for many chemical systems. 

The ideal gas law is an example of an equation of state, an expression showing how the pressure of a substance—in this case, a gas—is related to its temperature, volume, and amount of substance. A hypothetical gas that obeys the ideal gas law under all conditions is called an ideal gas. All real gases are found to obey F.q. 6 with increasing accuracy as the pressure is reduced toward zero (which we write P — 0). Therefore, the ideal gas law is an example of a limiting law, a law that is strictly valid only in some limit—in this case, as P—> 0. Although the ideal gas law is a limiting law, it is in fact reasonably reliable at normal pressures, and so we can use it to describe the behavior of most gases under normal conditions. 

The heating curve of a substance, like that in Fig. 6.26, shows how its temperature changes as heat is supplied at a constant rate, usually at constant pressure. How are these curves produced Simple laboratory heaters can be used to obtain a crude estimate of a heating curve. However, for accuracy, one of two related techniques is normally used. 

Rodebush has also implied that the accuracy with which very low temperatures can be measured is restricted by the uncertainty principle and by the nature of the substance under investigation. However, the accuracy of a temperature measurement is not limited in a serious way by the uncertainty principle for energy, inasmuch as the relation between the uncertainty in temperature and the length of time involved in the measurement depends on the size of the thermometer, and the uncertainty in temperature can be made arbitrarily small by sufficiently increasing the size of the thermometer we assume as the temperature of the substance the temperature of the surrounding thermostat with which it is in either stable or metastable equilibrium, provided that thermal equilibrium effective for the time of the investigation is reached. 

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