Stability in solution

It is necessary to study stability in solution in the solvent used to prepare sample solutions for injection in order to establish that the sample solution composition, especially the analyte concentration, does not change in the time elapsed between the preparation of the solution and its analysis by HPLC. This is a problem for only a few types of compound (e.g. penicillins in aqueous solution) when the sample solution is analysed immediately after the preparation of the sample solution to be injected. The determination of stability in solution is more of an issue when sample solutions are prepared and then analysed during the course of a long autosampler run. While the acceptance criteria for stabUity in solution may be expressed in rather bland terms by making a statement such as, e.g. the analyte was sufficiently stable in solution in the solvent used for preparing sample solutions for reliable analysis to be carried out , in practice it has to be shown that within the limits of experimental error, the result of the sample solution analysis by the HPLC method is the same for injections at the time for which stability is being validated as for injections immediately subsequent to the sample solution preparation. While this may be done by a subjective assessment of results with confidence limits, strictly speaking a statistical method known as the Student s t-test should be used. 

The chemical stability of an analyte in a given solution that is stored under specific conditions for given time intervals is an important validation parameter that was discussed in general terms in Section 9.4.4f. An extreme form of instability is the propensity of the analyte to explode or be set afire, and such properties should be available in the MSDS information or, if the analyte is a new compound, the chemists associated with its synthesis or isolation will have discovered such properties before the analyst A crucial special case is the stability of the analyte (and thus of the analytical standard) in solution, as in stock, sub-stock and spiking solutions. The stabihty of analyte or of an analog internal standard in solution should be determined by comparing stored stock solution(s) to freshly made-up stock solution(s). For a stable isotope-labeled internal standard, stability data for the corresponding analyte are often used to estabhsh stahUity for such an SIS. 

Fresh stock solutions should be used immediately after preparation if possible and certainly within 24 hours (one day) of preparation when used for comparison to stored stock solutions that are kept at the anticipated storage conditions. The number of rephcate injections of the freshly prepared and stored stock solutions (or dilutions thereof) should he sufficient to achieve the precision required to make such comparisons statistically meaningful (see Section 8.2). The solutions should be analyzed at relevant intervals spanning the maximum anticipated storage duration in order to expose any trends in the stabihty with time. The storage stahUity of an analyte stock solution is defined hy the period over which the mean peak area or peak area ratio of the stored solution deviates from the mean peak area or peak area ratio of the fresh solution by no more than the acceptable limit established in advance (10 % is a value that is often chosen). If a stock solution falls outside of acceptance criteria, an additional comparison can be made to verify instabihty. If the verification indicates that the analyte in solution was in fact stable (e.g. indicating a prior preparation error rather than instability), then at 

Stability should be clearly established for the anticipated duration of storage and use by comparing peak areas for stored solutions against those for solutions made from fresh weighings of analytical standard at selected time intervals such as two weeks, one month, three months etc. As a practical consideration, it is often desirable to establish a full year of stock stability under appropriate storage conditions. If sub-stocks or spiking solutions are used and made in different solvents, then similar stability studies would be required for these but, for reasons of practicality, probably for shorter periods. 

Additional discussion of the various types of stabdity data that are typically generated in support of validation smdies is given here, and also for the special case of bioanalytical applications in Section 10.4.1. 

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A major goal was to investigate the solid state structures of such compounds by single crystal X-ray diffraction. It was found that Lewis acid-base adducts R3M—ER3 show general structural trends, which allow estimations on the relative stability of the adducts. The experimental results were confirmed by computational calculations, giving even deeper insights into the structural parameters and the thermodynamic stability of simple Lewis acid-base adducts. In addition, their thermodynamic stability in solution was investigated by temperature-dependent NMR spectroscopy. 

Reliable information on the thermodynamic stability of group 13/15 adducts is usually obtained by gas phase measurements. However, due to the lability of stibine and bismuthine adducts in the gas phase toward dissociation, temperature-dependent H-NMR studies are also useful for the determination of their dissociation enthalpies in solution [41b], We focussed on analogously substituted adducts t-BusAl—E(f-Pr)3 (E = P 9, As 10, Sb 11, Bi 12) since they have been fully characterized by single crystal X-ray diffraction, allowing comparisons of their thermodynamic stability in solution with structural trends as found in their solid state structures. 

N-Acetylneuraminic acid aldolase (or sialic acid aldolase, NeuA EC 4.1.3.3) catalyzes the reversible addition of pyruvate (2) to N-acetyl-D-mannosamine (ManNAc (1)) in the degradation of the parent sialic acid (3) (Figure 10.4). The NeuA lyases found in both bacteria and animals are type I enzymes that form a Schiff base/enamine intermediate with pyruvate and promote a si-face attack to the aldehyde carbonyl group with formation of a (4S) configured stereocenter. The enzyme is commercially available and it has a broad pH optimum around 7.5 and useful stability in solution at ambient temperature [36]. 

The class I FruA isolated from rabbit muscle aldolase (RAMA) is the aldolase employed for preparative synthesis in the widest sense, owing to its commercial availability and useful specific activity of 20 U mg . Its operative stability in solution is limiting, but the more robust homologous enzyme from Staphylococcus carnosus has been cloned for overexpression [87], which offers unusual stability for synthetic purposes. Recently, it was shown that less polar substrates may be converted as highly concentrated water-in-oil emulsions [88]. 

When the concentration of the stabilizer in solution is high, the particles are forced into restricted conformations this decreases the entropy and, in turn, increases the... 

Nebulizer formulation conforms to sterile product preparation, which means that drug stability in solution in the presence of additives must be evaluated. Historically, it was sufficient to use antimicrobial agents in the formulation, notably benzalkonium chloride. Adding antimicrobials is not now considered an acceptable approach to the formulation of nebulizer solutions. The solubility of the drug is important since it may impact upon the performance of the solution in a selected nebulizer. Additives may form complexes with the drug. 

We note that the calculation of At/ will depend primarily on local information about solute-solvent interactions i.c., the magnitude of A U is of molecular order. An accurate determination of this partition function is therefore possible based on the molecular details of the solution in the vicinity of the solute. The success of the test-particle method can be attributed to this property. A second feature of these relations, apparent in Eq. (4), is the evaluation of solute conformational stability in solution by separately calculating the equilibrium distribution of solute conformations for an isolated molecule and the solvent response to this distribution. This evaluation will likewise depend on primarily local interactions between the solute and solvent. For macromolecular solutes, simple physical approximations involving only partially hydrated solutes might be sufficient. 

In general, the polymers incorporating decaborane that have been reported in the literature tend to be soluble in polar solvents, and generally have poor stability in solution with degradation of the decarborane cage.4... 

Rhodococcus equi XL-1 has also been demonstrated to have superior stability in solutions containing up to 20 mM HCN when compared with several Rhodococcus erythropolis... 

Electroless deposition should not be confused with metal displacement reactions, which are often known as cementation or immersion plating processes. In the latter, the less noble metal dissolves and eventually becomes coated with a more noble metal, and the deposition process ceases. Coating thicknesses are usually < 1 pm, and tend to be less continuous than coatings obtained by other methods. A well-known example of an immersion plating process that has technological applications is the deposition of Sn on Cu [17] here a strong complexant for Cu(I), such as thiourea, forces the Cu(I)/Cu couple cathodic with respect to the Sn(II)/Sn couple, thereby increasing the thermodynamic stability in solution of thiourea-complexed Cu(I) relative to Sn(II). 

The heavy metals copper, manganese, cobalt and zinc were omitted individually and in combination from MS and B5 media to determine the effect on antibody stability in solution [63]. When IgG, antibody was added to these modified media in experiments similar to the one represented in Figure 2.2, only the B5 medium without Mn showed a significant improvement in antibody retention relative to normal culture media. Nevertheless, protein losses were considerable as only about 30% of the added antibody could be detected in the Mn-free medium after about 5 h. The beneficial effect of removing Mn was lost when all four heavy metals, Cu, Mn, Co and Zn, were omitted simultaneously. The reason for these results is unclear. Addition of the metal chelating agent ethylenediaminetetraacetate (EDTA) had a negligible effect on antibody retention in both MS and B5 media [63]. 

When a TLC method is validated, two of the tests that are done are (1) stability on plate and (2) stability in solution of the drug substance to determine how quickly a sample must be applied to the HPTLC plate and developed before degradation occurs, if it occurs at all. For example, five vials are prepared by placing 25 mg of drug substance in each vial, and labeling them as time 0,1,2, 3, and 4 h. The experiment begins... 

Furthermore, by in vitro experiments, it has been verified that stereospecific activation of alprenoxime is organ-specific, occurring in the eye and not systemically. When administered locally to rabbits, marked decreases in intra-ocular pressure were observed, whereas oral administration elicited almost no cardiac effects. Such ketoximes represent promising chemical delivery systems in the treatment of glaucoma However, a major limitation is their poor stability in solution, seemingly due to hydrolysis of the oxime function. 

An oximine (i.e., a substituted oxime) analogue of alprenoxime was examined in an attempt to overcome the problem of low stability in aqueous solution. To this end, the methoxime analogue of alprenolol (11.74, R = Me) was prepared and evaluated [109]. Stability in solution was greatly improved at neutral pH. Topical administration to rabbits produced a decrease in intraocular pressure that had the same onset and intensity as that produced by alprenolol, but that lasted longer. Alprenolol was, indeed, formed in eye tissues as a metabolite, with the peak concentration reached 30 min after topical administration of the methoxime. 

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