Analytical Chem istry - DePauw University

42 Analytical Chemistry 2.0Fecal coliform counts provide a generalmeasure of the presence of pathogenic organismsin a water supply. For drinkingwater, the current maximum contaminantlevel (MCL) for total coliforms, includingfecal coliforms is less than 1 colony/100mL. Municipal water departments mustregularly test the water supply and musttake action if more than 5% of the samplesin any month test positive for coliformbacteria.3AAnalysis, Determination and MeasurementThe first important distinction we will make is among the terms analysis,determination, and measurement. An analysis provides chemical or physicalinformation about a sample. The component of interest in the sampleis called the analyte, and the remainder of the sample is the matrix. In ananalysis we determine the identity, concentration, or properties of an analyte.To make this determination we measure one or more of the analyte’schemical or physical properties.An example helps clarify the difference between an analysis, a determinationand a measurement. In 1974 the federal government enactedthe Safe Drinking Water Act to ensure the safety of public drinking watersupplies. To comply with this act, municipalities regularly monitor theirdrinking water supply for potentially harmful substances. One such substanceis fecal coliform bacteria. Municipal water departments collect andanalyze samples from their water supply. They determine the concentrationof fecal coliform bacteria by passing a portion of water through a membranefilter, placing the filter in a dish containing a nutrient broth, and incubatingfor 22–24 hr at 44.5 o C ± 0.2 o C. At the end of the incubation period theycount the number of bacterial colonies in the dish and report the result asthe number of colonies per 100 mL (Figure 3.1). Thus, municipal waterdepartments analyze samples of water to determine the concentration offecal coliform bacteria by measuring the number of bacterial colonies thatform during a carefully defined incubation period.Figure 3.1 Colonies of fecal coliform bacteria from a water supply. Source: SusanBoyer (www.ars.usda.gov).