Stability of analytes

LOD for parent and degradate(s) Sample size requirements Stability of analyte(s)... 

For non-compendial procedures, the performance parameters that should be determined in validation studies include specificity/selectivity, linearity, accuracy, precision (repeatability and intermediate precision), detection limit (DL), quantitation limit (QL), range, ruggedness, and robustness [6]. Other method validation information, such as the stability of analytical sample preparations, degradation/ stress studies, legible reproductions of representative instrumental output, identification and characterization of possible impurities, should be included [7], The parameters that are required to be validated depend on the type of analyses, so therefore different test methods require different validation schemes. 

Some important parameters for testing the robustness of TLC methods include the stability of analyte in the solution being analyzed and on the plate before and... 

Unsuitable for volatile analytes. b Stability of analyte must be established. cCheck specific interference effects. 

The stability of analytical solutions is an important factor to consider dnring method validation. This information can be invaluable to... 

To determine if a solid-phase reaction is complete, on-resin analytical methods are preferable [6]. First, the quantitative or qualitative information obtained from on-resin analysis is more relevant to solid-phase reactions. Secondly, this kind of analysis is fast, direct, and without any alteration to the sample. On the other hand, analysis after cleavage would depend on the cleavage efficiency (to be discussed later) and the stability of analyte to cleavage conditions such as TFA. The... 

The robustness of an analytical procedure is a measure of its capacity to remain unaffected by small but deliberate variations in the analytical procedure parameters. The robustness of the analytical procedure provides an indication of its reliability during normal use. The evaluation of robustness should be considered during development of the analytical procedure. If measurements are susceptible to variations in analytical conditions, the analytical conditions should be suitably controlled or a precautionary statement should be included in the procedure. For example, if the resolution of a critical pair of peaks was very sensitive to the percentage of organic composition in the mobile phase, that observation would have been observed during method development and should be stressed in the procedure. Common variations that are investigated for robustness include filter effect, stability of analytical solutions, extraction time during sample preparation, pH variations in the mobile-phase composition, variations in mobile-phase composition, columns, temperature effect, and flow rate. 

This consideration of the principles of diffusive sampling identifies a range of factors which may influence the performance of a diffusive sampler for monitoring VOC concentrations in indoor air. These factors will potentially be a source of error in such measurements and add to the overall uncertainty of the result given by the measurement procedure. In addition the amount of uncertainty will be influenced by other factors including amount and consistency of background contamination of sorbents, repeatability of analytical determination, formation of artifacts, stability of analyte on the sorbent, recovery of analyte during analyses and presence of interferents. 

The robustness of an analytical procedure is a measure of its capacity to remain unaffected by small, but deliberate variations in method parameters and provides an indication of its reliability during normal usage. During the development phase of the analytical procedure, susceptible parameters should be identified, for example, stability of analytical solutions, extraction time, pH and composition of mobile phase, column lots and suppliers, temperature, flow rate, etc. A factorial design is encouraged. 

As samples are often frozen and thawed, e.g. for re-anal-ysis, the stability of analyte during several freeze/thaw cycles should also be evaluated. The Conference Reports require a minimum of three cycles at two concentrations in triplicate, which has also been accepted by other authors [2, 4, 6, 9,10]. 

The stability of analyte under the conditions of sample preparation (e.g. ambient temperature over time needed for sample preparation) is evaluated here. There is general agreement, that this type of stability should be evaluated to find out, if preservatives have to be added to prevent degradation of analyte during sample preparation [4,9,10]. 

For determining the purity and thermal stability of analytical reagents, including primary and secondary standards. 

When it is possible, assays should be performed on the same day as collection inevitably, however, some samples have to be stored for analyses at a later date, and their stabilities vary. Storage should be at 4, -20, or -40°C, and the laboratory should evaluate the stabilities of analytes under their current operating conditions (Hirata, Nomura, and Tanimoto 1979 Falk 1981 Ito et al. 1998). Samples for ionized calcium and blood gases require additional collection and storage procedures to prevent change of pH and oxygen content (Pickrell, Light, et al. 1973 Pickrell, Mauderley, et al. 1973 Szenci, Brydl, and Bajcsy 1991). 

When information on the stability of analyte in matrix during typical conditions of storage is unavailable from the literature or from proficiency testing (PT) providers, such as the food analysis proficiency assessment scheme (PAPAS), this information shouid be developed in the laboratory as part of the method development and validation. However, a study to obtain information on stability of the analyte first requires the availability of a validated method if results are to be considered reliable. When a method is being developed to introduce a test capability for a new analyte into the laboratory, the new... 

Stability of analytes (and standard) over the analysis period - self-evidently the system under test must not change during the test if the results from the test are going to be meaningfully related to the original sample. 

Temperature - It is important to know the stability of samples awaiting analysis. This can take many forms depending on the type of sample. For instance, biological samples containing analytes must be investigated to find out the stability of analytes in... 

Of course, thermal stability of analytes becomes a concern for all extraction methods (Sections 4.3.2a-d) that involve extended exposure to boiling solvents. A somewhat different approach is discussed below. 

Long term storage stability of analytical samples can be a major issue in several applications, e.g. analysis of pesticide residues in environmental samples and bioanalytical samples. For some validation purposes (e.g. bioanalytical... 

ED is sometimes particularly susceptible to increases in background noise caused by metal ions present in the eluent, introduced from the samples, eluent or from components of the HPLC system. Some analysts add EDTA to their eluents, typically at concentrations of 30-150 mg L (0.1-0.5 mmol L ), with the aim of chelating iron and possibly other metal ions, EDTA may also enhance the stability of analytes, such as catecholamines, during sample storage and analysis. 

The choice of a fluorescent tag depends on the type of derivatization (e.g., precolumn or postcolunui) because the stability of analyte-tag chemical bonds varies greatly. Therefore, for precolunm derivatization it is important to determine the time delay between derivatization and analysis as well as the stability of the reaction product in the mobile phase prior to HPLC method development. For postcolunm derivatization, compatibility of the reaction solvent with the chromatographic system as well as the rate of reaction are variables that must be considered. 

On the other hand, an obvious advantage of offline TLC-MS is the broad list of chemicals, which can be used in the course of the analysis. For example, there are practically no limitations to the solvents that are used for reextraction of analytes from the chromatographic adsorbent. Many MS noncompatible solvents and buffers (phosphate buffer and triethylammonium acetate, for instance) can readily be used for optimal extraction and stability of analytes, as long as they are removed prior to MS analysis. This way, and if care is exercised, labile compounds can be transferred from the TLC plate to the MS with negligible analyte loss, although the overall TLC-MS analysis can still be time consuming. Additionally, analytes extracted from the plates can be redissolved in a minimal amount of solvent used... 

Stability testing is performed to ensure that the analyte concentration does not change during sample storage, sample preparation, and sample analysis. The freeze and thaw stability of analyte in matrix as well as the short-term stability of analyte in matrix at room temperature should be examined. The long-term stability of analyte in matrix stored in freezer should also be tested, as should the stability of stock solution and working solutions ofthe analyte and internal standard. The stability of the analyte (and IS) is satisfactory when the determined concentration is within the limits of accuracy. 

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