Optimal Sample Storage Conditions

Providing each collection site with standardized collection kits ensures consistency across sites. Kits may contain collection tubes, sample preservatives, sample preservation tubes, and sample labels. 

After samples are processed they are frequently stored temporarily at the collection site until being shipped for analysis or long-term storage. Consideration should be given to what temperature is acceptable for temporary storage. Many sites only have -20°C freezers. The impact of storage at -20 versus -80°C should be assessed in advance to selecting clinical sites. The ability of the site to monitor the freezer temperatures where sample will be stored should also be evaluated. 

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As mentioned before, RM producers go to great lengths, as are required by the ISO Guide 31 (1996) to prove homogeneity and stability, and to establish the best sample size and storage conditions for an optimal shelf life. This information is normally provided in the RM certificate. Nevertheless, users must pay particular attention to a number of procedures in the use of RMs to avoid invalid results. 

All samples must be stored appropriately to minimize the loss of activity due to protein denaturation, lack of stabilizers or presence of inhibitors. Optimal storage conditions will vary for different enzymes and the nature of the sample, blood, tissue, etc. Such information would be sought from specialist textbooks. 

We monitored the residual moisture of a protein product at different temperatures to evaluate its dependence on the temperature of storage. The stoppers were prepared by our normal manufacturing process, which had previously been optimized for occluded water removal. The product was placed at 5°C, the normal storage condition, and 25°C, an accelerated stability condition. Residual moisture in the 5°C samples was monitored for 36 months in the 25°C samples, for 12 months. The data are plotted in Figure 2. Residual moisture increased slowly in the 5°C samples from the initial value of 0.5% to about 1.0% in 36 months. In contrast, residual moisture rose much more rapidly at 25°C, attaining a value of 1.8% in 12 months. The data were fitted to a straight line and the slopes of the curves were calculated. Whereas the slope of moisture uptake at 5°C was only 0.02% per month, that at 25°C was 0.11 % per month. In other experiments (data not shown), we have found that moisture is gained at an even faster rate at 37°C (approximately 0.50% per month). 

Zhang Ling-Su, Wong GTF. Optimal conditions and sample storage for the determination of H2O2 in marine waters by the scopoletin-horseradish peroxidase fluoromeuic method. Talanta 1999 48 1031-1038. 

Smith, S. and Morin, R, Optimal storage conditions for highly dilute DNA samples Arole for trehalose as apreserving agent, J Forensic Sci, 50, 1101, 2005. 

To rationalize the results of a screen and to derive structure activity relationships (SAR) guiding the additional optimization of the compounds, it is prerequisite that the activity of a compound sample results from the structure being attributed to it. This can only be ensured if the compounds included in the screening collection are reasonably pure. Typical purity requirements are in the range of 85% to 95%. Impurities that interfere with the assay technology especially must be avoided. To remain in that state of purity, the compounds must be chemically stable under the conditions of storage, and because the fresh production of screening solutions from powder sample is not feasible for each individual HTS, the compound must be stable in DMSO solution over a period of time if the stock solution is intended to be used. 

Sample stability assessment includes the likely range of conditions to which a sample might reasonably be exposed. This includes process stability, assessing the impact of short-term storage under less than optimal conditions (as can occur on the laboratory benchtop during processing) and at least one cycle of freeze thaw. 

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