GENERAL RULES FOR WEIGHING
Your instructor will explain the specific operation of your specific balance.The main objectives are to protect all pans from dust and corrosion, avoid contamination or change in load (of sample or container), and avoid draft (air convection) errors.
Some general rules for the weighing:
1. Never handle objects with your fingers to weigh. A piece of clean paper or tongs should be used.
2.Weigh at room temperature, thus avoiding currents of air convection.
3. Never place chemicals directly on the pan, but weigh them in a vessel (weighing bottle, weighing dish) or on powder paper. Immediately brush off spilled chemicals with a soft brush.
4. Always close the balance case door before making the weighing. Air currents are going to make the equilibrium unstable.
Although modern digital balances do not have user-manipulable weights, corrosion can still cause problems. Volatile corrosive substances (e.g., iodine or conc. HCI ) should never be weighed in open containers in a balance.
Collecting a representative sample is an aspect of analytical chemistry that the beginning analytical student is often not concerned. Because the samples handed to him or her are assumed to be homogeneous and representative.
Yet this process can be the most critical aspect of an analysis.
The sampling process can limit the significance and accuracy of the measurements. Unless sampling is done properly , it becomes the weak link in the chain of the analysis.
A life could sometimes depend on the proper handling of a blood sample during and after sampling. If the analyst is given a sample and does not actively participate in the sampling process, then the results obtained can only be attributed to the sample “as It has been received. “And as mentioned above, the custody chain must be documented.
Instructions for sampling:
Many professional societies have specified definite instructions for sampling given materials. The Association of Official Analytical Chemists International and the American Public Health Association .
By appropriate application of experience and statistics, these materials can be sampled as accurately as the analysis can be performed. Often , however, the matter is left up to the analyst. The ease or complexity of sampling will, of course, depend on the nature of the sample.
The problem is to get a sample representing the whole. This sample is called the gross sample. Its size may vary from a few grams or less to several pounds , depending on the type of bulk material. Once a representative gross sample is obtained, it may need to be reduced to a small enough size for handling. This is called the sample.
Once the specimen is collected, it will be analyzed as an aliquot, or component. This aliquot is called the analysis sample. It is possible to perform several replicate analyzes on the same sample by taking separate aliquots.
Gross sample in clinical laboratory:
In the clinical laboratory , the gross sample is usually satisfactory for use as the sample because it is not large and it is homogeneous (e.g.. blood and urine samples). The analysis sample will usually be from a few milliliters to a fraction of a drop (a few micro-liters) in quantity.