Laboratory analysis is authentic only when there are standards for comparison of results obtained with the results obtained with standards under an identical set of experimental conditions. However, practically it is not possible to make use of original reference standard every time analysis is carried out as the quantity available is small due to its very high price. It is for this reason that working standards are prepared from high purity materials and their purity is expressed in relation to the purity of standard reference materials. Working standards are assigned an expiry date and must be used only within the validity period.
Proper handling and storage of working standards under prescribed conditions are of utmost importance so as to preserve their integrity over the usage life and these aspects are briefly discussed in the article.
A validated and approved method should always be adopted for the purpose. In case there is no requirement for the prior drying first step is to determine its moisture content and assay by the prescribed method. Such assay is referred to ‘as is’ assay. The assay is also considered as the claim on the label and the value is used in the following relation to arriving at the potency value.
Potency = % Assay / 100- %moisture
A working standard should be stored under same prescribed conditions as the standard reference material. Light sensitive standards need to be stored in amber colored vials or bottles. In all cases, however, adequate precautions should be taken to prevent exposure to the humid environment, light and changing temperatures. Standards marked as a store in’ cool and dry place’ should be stored in a laboratory refrigerator maintained at 2-80 C.
Handle standard vials and bottles carefully to avoid breakages. Always check the validity of standard before using. Container taken out for use should be returned to its assigned storage so as to minimize exposure to outside environment. After removal from refrigerator allow it to come to laboratory temperature for about 10-15 minutes before weighing or volumetric transfer. Any leftover standard after weighing should be discarded and not returned to the original container. Always make use of clean spatulas or fresh disposable tips when making use of micropipettes.
It is advisable to store standards in a desiccator when room temperature storage is required. If drying before use becomes necessary to take required quantity in a clean weighing bottle for drying. After drying do not return the excess to the original container as it can contaminate the original stock. Place the dried standard in a desiccator for equilibration before use.
It is necessary to display the following details on the label.
Name of standard
Batch No/Lot No
Assay(% as is)
Date of Preparation
It is absolutely necessary to maintain proper records relating to standards. Such records should include tabulated details on each standard covering details such as name, analysis parameters, date of validation, validity period and next date of validation. Maintain a separate log book on the usage of standards mentioning name of the user, current stock, quantity used, balance stock available and purpose of use.
Hard copy records are useful for routine reference and for revalidation purpose. Soft records also prove to be useful. Pop-up messages are also helpful to revalidate the standards before the date of expiry.
It can be seen that mere procurement of high-quality standards is not sufficient. Proper handling, storage, and timely validation are even more important and it is only then that their utilization can be justified.