专业英语week 14概要1.ppt

Analytical Chemistry and Problems in Society Analytical chemistry, because it permeates almost every aspect of society, has profound implications for human welfare. An example is the analysis of materials for chemicals of possible toxicity to man. When foods were tested in past decades for toxic materials, they were presumed safe if such materials were not detected. Today, all foods can be shown to contain detectable amounts of a host of toxic materials. Nevertheless, food now is probably generally safer and of better quality than ever before, what has happened? To clarify this seeming contradiction, two terms relating to toxicity and one concerning the limit of analytical measurement should be described. The fatality limit is the level of chemical exposure that when maintained for any length of time results in death. This limit depends on the nature of the chemical, the type of exposure, and on individual human factors such as size, age, general health, and smoking habits. The toxicity limit is the limit where well-being is first noticeably affected. This limit also depends on the chemical, the type of exposure, and individual human factors. The detection limit is the minimum amount of a substance that can be determined by a laboratory test procedure. This limit depends on the state of the art in analytical laboratories. The detection limit is completely independent of the other two and is continually being lowered as knowledge of chemistry improves and more sensitive instrumentation is developed. The difference between detection limit and fatality or toxicity limit is not significant as long as the detection limit is below the toxicity limit. Two or three decades ago, detection limits and toxicity limits for many materials were in the same range. But now detection limits have dropped far below toxicity limits. Figure 1 depicts schematically the relation between the fatality limit, toxicity limit, and changing detection limits for mercury. As indicated in Figu