Stability analyte

Most often studies will be accepted by regulatory authorities even if they do not contain all information. For example, a summary, the scope, a separate notice regarding the residue definition or a schematic diagram of the analytical procedure are helpful and may avoid additional questions, but they are not essential. Also, detailed specification of standard glassware or chemicals commonly used in residue analysis is less important. Finally, data about extraction efficiency or analyte stability can be offered in separate studies or statements, which are also valid for other methods. However, each method must precisely describe at the minimum ... 

In summary, the CSL guidelines can be simply applied in each laboratory and contain very clear instructions. The validated procedures do not focus on the central analytical part only. Important secondary aspects of the whole procedure (sample processing, analyte stability, extraction efficiency) are also considered. For each parameter which is determined, different criteria for the evaluation of quantitative, semi-quantitative and screening methods are given. Here, it should be noted that compared with other guidelines the requirement for the precision of quantitative methods is very stringent (RSD < 10%). 

The analyte stability during storage and processing of samples or in standard solutions and extracts is not part of method validation in Germany. Therefore, insufficient stability will not be routinely detected and even then more or less only by chance. Also, separate tests for analyte homogeneity and extraction efficiency were not performed. 

In summary, official German analytical methods for pesticide residues are always validated in several laboratories. These inter-laboratory studies avoid the acceptance of methods which cannot readily be reproduced in further laboratories and they do improve the ruggedness of analytical procedures applied. The recently introduced calibration with standards in matrix improves the trueness of the reported recovery data. Other aspects of validation (sample processing, analyte stability, extraction efficiency) are not considered. 

In situations involving acidic/basic analytes, pH is often the most critical property in the extraction, and buffered aqueous solvents are often necessary. Another important consideration is the stability of the analytes in the extraction medium, and method development should entail analyte stability experiments to demonstrate how long solutions and/or extracts can be stored. 

Analyte stability is another source of concern the chemical identity should be preserved all through the analytical process. This is not always guaranteed. Analytes may be oxidised or hydrolysed during extraction. Compound integrity can also suffer in the chromatographic column, through heat-induced decay. Extraction... 

Many of the coagulation factors measured by global coagulation tests have limited stability, and the time and temperature of storage of sample will affect their measurements. Concepts of analyte stability and half-life in plasma extend to markers measured by immunoassay. Markers of platelet activation are affected by artifactual activation in vitro upon collection of the blood specimen. This section will highlight some of the nonanalytical variables that, if uncontrolled, can lead to spurious results and thus affect the interpretation of laboratory data. 

Munch DJ, Frebis CP. 1992. Analyte stability studies conducted during the national pesticide survey. Environ Sci Technol 29 921-925. 

Difficulties of work at high temperatures include stationary phase stability and analyte stability. However, it has been shown that the relatively short residence times in the column at high temperatures tend to minimize the effects of analyte instability... 

The sorbent materials that performed best in the capacity and desorption efficiency tests were investigated further with respect to the stability of the sorbed analyte. Preliminary tests of analyte stability were conducted by a procedure similar to that in the desorption efficiency tests the procedure differed in that samples were stored 7 d prior to analysis rather than Id. To be acceptable, a sorbent material had to exhibit no statistically significant loss of analyte at the 0.05 significance level by a two-tailed t test. 

Freeze and Thaw Stability. Analyte stability should be determined after three freeze-thaw cycles. At least three aliquots at each of the low and high concentrations should be stored at the intended storage temperature for 24 h and thawed unassisted at room temperature. When thawed completely, the samples should be refrozen for 12 to 24 h under the same conditions. The freeze-thaw cycle should be repeated twice more, then analyzed on the third cycle. If an analyte is unstable at the intended storage temperature, the stability sample should be frozen at —70°C during the three freeze-thaw cycles. 

During method development it is important to start by investigating analyte stability. Analyte instability can arise for many reasons ... 

Analyte Stability Analyte stability experiments are carried out mimicking the sample collection, storage, and processing conditions as closely as possible. Stability experiments are conducted for the assay duration in the same matrix... 

Technique Typical Vendor(s) Handles Whole Tissue Chunks Analyte Stability Issues Automation Inexpensive High Sample Throughput References... 

Reference to uncertainty budgets developed according to ISO/GUM principles [8, 9] which are included in each test method. These budgets are used to estimate the uncertainty associated with measurements made using the method. The effects of sample homogeneity and analyte stability are included in the overall estimate of measurement uncertainty. 

STDs in matrix may be prepared ahead of the time and stored as long as analytes stability in matrix has been demonstrated. STD and QC pools are prepared according to the specific validated analytical methods. Spiked solutions cannot be used beyond the established stability. After qualification, the standard and QC pools should be prealiquoted out and stored under designated condition and temperature. 

In the original heroin assay by Goldberger et al., heroin, 6-AM, morphine, and codeine were extracted by LLE and assayed simultaneously by GC/MS. The performance of the assay including analyte recovery and stability was not optimized, and the assay was limited by interference. Selective ion chromatograms of hair and standard extracts are illustrated in Figure 4. In an improved procedure by Goldberger et al., two sets of SPE extracts were assayed independently by GC/MS one set for heroin, and other set for the trifluoroacetyl derivatives of 6-AM and morphine. The extraction process, in combination with the use of aprotic solvents, mild elution solvents, and an enzyme inhibitor, provided maximum analyte stability for the recovery of heroin from hair with minimal (<5%) hydrolysis. The procedure was modified further by Cone et al. for the analysis of cocaine, heroin, and metabolites in hair. 

J. D. Thompson and P. W. Carr, A study of the critical criteria for analyte stability in high-temperature liquid chromatography, ArzaZ. Chem. 74 (2002), 1017-1023. 

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