Reference standard material purity

The quantitative analysis is only as good as the reference material allows. The reference standard is used as the primary standard against which all determinations are made. Reference standard characteristics should be appropriately determined and documented for each batch. Copies of this documentation must be included with study records and must be available for inspection. The reference standard should be (1) of the highest purity obtainable, (2) independently characterized to establish identity, strength or potency, and purity, (3) in the most stable form, and (4) stored under suitable conditions. 

Before an instrument is used to validate a method, its performance should be verified using generic standards [22,23], Satisfactory results for a method can only be obtained with well-performing equipment. Special attention should be paid to the equipment characteristics that are critical for the method. For example, if detection limit is critical for a specific method, the instrument s specification for baseline noise and for certain detectors also the response to specified compounds should be verified. Any material used to determine critical validation parameters, such as reagents and reference standards, should be checked for accurate composition and purity. 

The material from an appropriate bulk supply of the matrix material is weighed into a suitable container and homogenised. Where appropriate, portions of the matrix material are tested for the analytes of interest and for possible interferences. Pesticide and environmental test samples are usually prepared by spiking known amounts of substances of specified chemical purity into the bulk sample matrix. The spiking process is witnessed and cross-checked by a second scientist and full records of the process and of reference standard solutions used are maintained so that they can be verified if required. 

The primary reference standard is normally prepared on a laboratory scale using pure starting materials, reagents, and solvents and should be of the highest purity that reasonably can be obtained. The synthetic procedure used to make it and the method(s) used for its purification, also should be provided. (If applicable, the method of manufacture section can be referenced.) The purification procedure is normally performed until little or no change is observed through two consecutive cycles in assay purity and levels of impurities. 

Standardization Prepare a Standard Solution containing a total of about 10% solids, using sugars of known purity (e.g., USP Fructose Reference Standard USP Dextrose Reference Standard, or NIST Standard Reference Material maltose, Aid-rich Chemical Company or equivalent) that approximates, on the dry basis, the composition of the sample to be analyzed. Dissolve each standard sugar, accurately weighed, in 20 mL of purified water contained in a 50-mL beaker. Heat on a steam bath until all sugars are dissolved, then cool, and transfer to a 100-mL volumetric flask. Dilute to volume with water and mix. Freeze the solution if it is to be reused. 

Identify the materials used and give information on the degree of and criteria for purity, but do not reference standard laboratory reagents. Give the chemical names of all compounds and the chemical formulas of compounds that are new or uncommon. Use meaningful nomenclature that is, use standard systematic nomenclature where specificity and complexity require, or use trivial nomenclature where it will adequately and unambiguously define a well-established compound. 

The ultimate stereochemical identity test is, of course, the direct resolution of the enantiomers using chiral liquid or gas chromatography (9). When compared to a reference standard of the racemate, and under experimental conditions that will resolve the peaks of both enantiomers, the occurrence of two equal peaks will identify the racemate, and one peak will signify an enantiopure material. A proof of the stereochemical identity of the analyte can be provided, based on a match of retention times with a reference standard of known stereochemistry. Inequality between the peaks is a measure of enantiomeric enrichment. Therefore, it is conceivable that both stereochemical identity and purity can be established from a single experiment. 

Direct quantification is carried out in situ rather than after spot elution. The simplest direct method involves visual comparison of sample zone size and/or intensity (color) variation according to concentration against reference standards developed on the same plate.f This qualitative/semiquantitative approach is specified in various pharmacopeias for the purity analysis of active raw materials and formulated products. These pharmacopeial methods are designed for analyses at several levels 1) simple detection of impurities as additional spots 2) detection and identification of impurities by comparison to the R( values distances of standards or 3) detection, identification, and estimation of amounts of impurities by comparing intensities between samples and standard dilutions of the same compounds. ... 

Authentic Materials Authentic materials (AM) are reference standards, which are qualified for identity and approximate purity. The chromatographic purity of an AM need only be 80% or greater. They are not used in quantitative assays but are generally used to establish chromatographic system suitability and as identity comparators for spectroscopic assays. Authentic materials are usually available in small amounts and are frequently obtained by preparative chromatography. 

The critical parameter in the discussion of reference standards is the process of qualification. FDA guidance on this topic states that A non-compendial standard should be thoroughly characterized to assure its identity, strength, quality and purity. 3 It is therefore left to the pharmaceutical firm to establish thorough characterization for the ARS of each NCE. The overall qualification process for an ARS includes characterization of the material, proper form selection, and assignment of purity. Some general considerations in this process include... 

Certified Reference Standards (Standard Reference Materials, SRMs) for clinical laboratories are available ffom the NIST and the IRMM. Cholesterol, the first SRM developed by the NIST, was issued in 1967. Today, the lists from the NIST and IRMM are extensive (Table 1-10 and Table 1-11, respectively). Not aU standard reference materials have the properties and the degree of purity specified for a primary standard, but each has been well characterized for certain chemical or physical properties and is issued with a certificate that gives the results of the characterization. These may then be used to characterize other materials. 

Purity is the measurement of the quantity of a prevalent component of a drug substance when only that component is present. The purest material is generally regarded as a reference standard and is used to determine the purity of a drug by a comparative UV spectroscopic method. However, the precision of the UV method may not be high in some cases. Furthermore, the absolute purity of a reference material may be impractical to measure thus, UV for purity determination may have its limitations. 

A reference standard, or reference material, is a substance prepared for use as the standard in an assay, identification, or purity test. 

Reference standard, primary A substance that has been shown by an extensive set of analytical tests to be authentic material that should be of high purity. This standard can be (1) obtained from an officially recognized source, (2) prepared by independent synthesis, (3) obtained from existing production material of high purity, or (4) prepared by further purification of existing production material. 

Selection of a reference standard for most analytical methods is a straightforward task, involving acquisition of the relevant analyte in a state of known composition and purity. However, some analytes are not available in pure form, and structurally similar analogs (e.g. free base) may be used instead. Standard materials should be of a known purity, and the source, lot number, expiration date and certification of identity should be recorded (FDA, 2001). 

For all analytical testing, standard substances are required as reference material. These standards exhibit defined quality, and serve for all types of identity, purity, degradation product and potency assays as reference quantity. In QC, reference and working standards are used. Reference standards are highly purified and extensively characterized by all appropriate physico-chemical, biochemical and immunochemical methods. Working standards are used in daily practice, and are calibrated against reference standard for routine use. 

As indicated by the weight losses in hydrogen, the samples were impure nevertheless, the evaluation data were sufficient to indicate that the particle size was small and that the magnetic properties were remarkable in view of the low purity of samples. Extrapolation of the data derived from the impure samples predicts a Q value for the pure material of 110 to 130% of the value shown by the reference standard carbonyl powder. 

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