Sampling gross description

The description of an object in the sense of environmental investigation may be the determination of the gross composition of an environmental compartment, for example the mean state of a polluted area or particular location. If this is the purpose, the number of individual samples required and the required mass or size of these increments have to be determined. The relationship between the variance of sampling and that of analysis must be known and both have to be optimized. The origin of the variance of the samples can be investigated by the study of variance contribution of the different steps of the analytical process by means of the law of error propagation (Eq. 4-21) according to Section 4.3.4. 

In terms of gross morphology, characteristically, coca leaf has two lines which run parallel to the mid-rib on the underside of the leaf. Taxonomically, however, it is difficult to identify leaf and plant material to species on the basis of morphology (leaf and fiower structure) alone and indeed hybridization between species is common [5]. In order to establish that the material comes from the genus Erythroxylon and contains controlled substances, it is therefore necessary to demonstrate the presence of cocaine. In bulk (those which can be seen by the naked eye) samples, this is achieved through a good physical description of the materials and packaging, followed by a combination of presumptive tests, TLC and a tandem technique, usually GC-MS. 

Establishing the absence of a radionuclide becomes important when radioan-alytical chemistry is used to demonstrate that a radiological incident, such as a threatened terrorist act, did not occur. Absence of a substance in a sample is a relative conclusion that could be altered by a more sensitive measurement. A radionuclide is reported as measured or as less than the lower limit of detection. A more sensitive detection method may replace the less than description with an actual value, or continue to report less than, but at a lower value. Such less than values are based on net count rates that may be zero or a positive value sufficiently near zero that is too uncertain, as discussed in Section 10.4, to be reported. This net count rate can be the difference between the gross count rate and (1) the detector background count rate or (2) the count rate in the sample attributed to background from various sources, as defined above. 

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