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Sample Identification
Laboratory samples should always be labeled carefully so that if any problem develops its origin can easily be identified. The information used to identify a sample includes:
- a) Sample description,
- b) Time sample was taken,
- c) Location sample was taken from,
- d) Person who took the sample,
- e) Method used to select the sample.
The analyst should always keep a detailed notebook clearly documenting the sample selection and preparation procedures performed and recording the results of any analytical procedures carried out on each sample. Each sample should be marked with a code on its label that can be correlated to the notebook. Thus if any problem arises, it can easily be identified.
Sample Storage Methods
Complete preservation of samples, regardless of source, is a practical impossibility. Regardless of the sample nature, complete stability for every constituent can never be fully attained. At best, sample preservation only slows the biological and chemical changes that inevitably continue after the sample is collected. Following collection and during transportation, samples should be kept at 6oC or on ice. Samples requiring preservation should be preserved as soon as possible after collection to maintain the integrity of the sample.
Methods of preservation are intended to retard biological action, retard hydrolysis of chemical compounds and complexes, and reduce volatility of constituents. Preservation methods are limited to pH control, chemical addition, amber or opaque bottles, filtration, refrigeration, and freezing.
To minimize the potential for volatilization or biodegradation between sampling and analysis, keep the sample as cool as possible without freezing.
Analyze the samples as quickly as possible upon arrival at the laboratory. If immediate analysis is not possible, storage at 2-4o C is recommended for most samples.
Use chemical preservation only when it is shown not to interfere with the method of analysis. No single preservation method is entirely satisfactory; choose the preservation with regard to the analyses being made. Because a preservation method for one analysis may interfere with the preservation for another, samples for multiple determinations may need to be split and preserved separately.
Sample Storage Prior To Analysis
All samples are properly stored from the time they arrive at the laboratory to disposal. Samples are refrigerated at 4oC prior to analysis unless method SOPs indicate other storage conditions.
Nature of Sample Changes: Some analyses are more likely than others to be affected by storage before analysis.
Temperature, pH, and dissolved oxygen are best determined in the field. Temperature changes quickly and pH may change significantly in a matter of minutes. Dissolved gases (oxygen, carbon dioxide) may be lost very quickly. With the changes in the pH-alkalinity-carbon dioxide balance, calcium carbonate may precipitate and cause a decrease in the values for calcium and total hardness.
In general, the shorter the time that elapses between the collection of a sample and its analysis, the more reliable the analytical results. However, it is impossible to state exactly how much time may be allowed between sample collection and analysis. Changes occurring in the sample depend on the character of the sample, the analysis to be made, and the conditions of storage. Changes caused by the growth of microorganisms are greatly retarded by keeping the sample in the dark and at a low temperature.