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Sample Losses During Preparation
Materials may be lost from a sample during laboratory preparation through the following ways
- Losses as Dust or Particulates- small suspended particles in the residue may be readily by any air flow over the sample by air generated by changes in temperature. This can be prevented by storing the samples in covered containers
- Losses Through Volatilization- The loss of volatile elements during heating is minimized by heating without exceeding the boiling point of the volatile compound and by also carrying out reactions in a properly constructed sealed vessel
- Losses due to reactions between sample and container or reactions with other reagents initially used in the container, similarly, the internal surface area of a container, whether used for sample preparation or storage, may cause loss of analyte. Scratches and abrasions increase the surface area, and their geometry make loss of analyte likely.
Sample Contamination In The Laboratory
Contamination leads to biased data that misrepresent the concentration or presence of a specific sample. Therefore, laboratory personnel should take appropriate measures to prevent the contamination of samples. Such precautions are most important when multiple samples are processed together. Possible sources of contamination include:
- Moisture and airborne contamination
- Reagents contaminations
- Glassware/equipment contamination
- Facilities
- Cross-contamination between high- and low-activity samples.
Contamination of samples can be controlled by adhering to established procedures for equipment preparation and decontamination before and after each sample is prepared. Additionally, the results of blank samples (e.g., sand), which are run as part of the internal quality assurance program, should be closely monitored, particularly following the processing of samples with elevated activity.
Cross-contamination is the contamination of one sample by another sample that is being processed concurrently or that was processed prior to the current sample leaving a residue on the equipment being used. Simply keeping samples covered whenever practical is one technique to minimize cross-contamination. Another technique is to order the processing of samples beginning with the lowest contamination samples first.
Laboratory personnel should be wary of using the same equipment (gloves, tweezers for filters, contamination control mats, etc.) for multiple samples. Countertops and other preparation areas should be routinely monitored for contamination.
Preventing Changes In Sample
Once a sample is selected, you must ensure that it does not undergo any significant changes in its properties from the moment of sampling to the time when the actual analysis is carried out, e.g., enzymatic, chemical, microbial or physical changes. There are a number of ways these changes can be prevented.
- Enzymatic Inactivation. Many foods contain active enzymes they can cause changes in the properties of the food prior to analysis, e.g., proteases, cellulases, lipases, etc. If the action of one of these enzymes alters the characteristics of the compound being analyzed then it will lead to erroneous data and it should therefore be inactivated or eliminated. Freezing, drying, heat treatment and chemical preservatives (or a combination) are often used to control enzyme activity, with the method used depending on the type of food being analyzed and the purpose of the analysis.
- Lipid Protection. Unsaturated lipids may be altered by various oxidation reactions. Exposure to light, elevated temperatures, oxygen or pro-oxidants can increase the rate at which these reactions proceed. Consequently, it is usually necessary to store samples that have high unsaturated lipid contents under nitrogen or some other inert gas, in dark rooms or covered bottles and in refrigerated temperatures. Providing that they do not interfere with the analysis antioxidants may be added to retard oxidation.
- Microbial Growth and Contamination. Microorganisms are present naturally in many foods and if they are not controlled, they can alter the composition of the sample to be analyzed. Freezing, drying, heat treatment and chemical preservatives (or a combination) are often used to control the growth of microbes in foods.
- Physical Changes. A number of physical changes may occur in a sample, e.g., water may be lost due to evaporation or gained due to condensation; fat or ice may melt or crystallize; structural properties may be disturbed. Physical changes can be minimized by controlling the temperature of the sample, and the forces that it experiences.