CHEM LEC 1.pdf
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ANALYTICAL OBJECTIVES CHAPTER 1 Analytical chemistry is concerned with the chemical characterization of matter and the answer to two important questions: what is it (qualitative analysis); and how much is it (quantitative analysis)....
ANALYTICAL OBJECTIVES CHAPTER 1 Analytical chemistry is concerned with the chemical characterization of matter and the answer to two important questions: what is it (qualitative analysis); and how much is it (quantitative analysis). 1.1 Chemicals make up everything we use or consume, and knowledge of the chemical composition of many substances is important in our daily lives. Analytical chemistry plays an important role in nearly all aspects of chemistry, for example: Agriculture - Nitrogen content of a fertilizer determines its value Food and Nutrition - Foods must be analyzed for contaminants and essential nutrients Medicine - Blood glucose must be monitored in diabetics Environmental - The air we breathe must be analyzed for toxic gases Forensic - presence of trace elements from gun powder on a perpetrator’s hand Manufacturing – quality assurance of manufactured products depends on proper chemical proportions Metallurgical - carbon content of steel will influence its quality; and pharmaceutical chemistry - purity of drugs will influence their efficacy What is Analytical Science? The Federation of European Chemical Societies held a contest in 1992 to define analytical chemistry, and the following suggestion by K. Cammann was selected [Fresenius’ J. Anal. Chem., 343 (1992) 812–813]. Analytical Chemistry provides the methods and tools needed for insight into our material world... for answering four basic questions about a material sample: What? Where? How much? What arrangement, structure or form? These cover qualitative, spatial, quantitative, and speciation aspects of analytical science. 1.1 What is Analytical Science? The Division of Analytical Chemistry of the American Chemical Society developed a definition of analytical chemistry: Analytical Chemistry seeks ever improved means of measuring the chemical composition of natural and artificial materials. The techniques of this science are used to identify the substances which may be present in a material and to determine the exact amounts of the identified substance. 1.1 What is Analytical Science? Analytical chemists serve the needs of many fields: In medicine, analytical chemistry is the basis for clinical laboratory tests which help physicians diagnose disease and chart progress in recovery. In industry, analytical chemistry provides the means of testing raw materials and for assuring the quality of finished products whose chemical composition is critical. Many household products, fuels, paints, pharmaceuticals, etc. are analyzed by the procedures developed by analytical chemists before being sold to the consumer. 1.1 What is Analytical Science? Environmental quality is often evaluated by testing for suspected contaminants using the techniques of analytical chemistry. The nutritional value of food is determined by chemical analysis for major components such as protein and carbohydrates and trace components such as vitamins and minerals. Indeed, even the calories in food are often calculated from its chemical analysis. Analytical chemists also make important contributions to fields as diverse as forensics, archaeology, and space science. 1.1 Qualitative vs Quantitative Qualitative deals with the identification of elements, ions, or compounds present in a sample (we may be interested in whether only a given substance is present). Quantitative deals with the determination of how much of one or more constituents is present. 1.2 Qualitative vs Quantitative Qualitative analysis tells us what chemicals are present. Quantitative analysis tells us how much. 1.2 Selective vs Specific Qualitative tests may be performed by selective chemical reactions or with the use of Instrumentation. A clear distinction should be made between the terms selective and specific: A selective reaction or test is one that can occur with other substances but exhibits a degree of preference for the substance of interest. A specific reaction or test is one that occurs only with the substance of interest. 1.2 Selective vs Specific Unfortunately, very few reactions are truly specific but many exhibit selectivity. Selectivity may be also achieved by a number of strategies: Sample preparation (extractions) Instrumentation (selective detectors) Target analyte derivatization (derivatize specific functional groups) Chromatography, which separates the sample constituents. 1.2 The Analytical Process The Analytical Process 1.3 The Analytical Process 1.3 The Analytical Process 1.3 The Analytical Process 1.3 The Analytical Process 1.3 The Analytical Process 1.3 The Analytical Process 1.3 Validation of a Method— You Have to Prove It Works! Two types of error may occur: random and systematic. Systematic error is consistent, repeatable error associated with faulty equipment or a flawed experiment design. These errors are usually caused by measuring instruments that are incorrectly calibrated or are used incorrectly. 1.4 Validation of a Method— You Have to Prove It Works! However, they can creep into your experiment from many sources, including: A worn-out instrument. For example, a plastic tape measure becomes slightly stretched over the years, resulting in measurements that are slightly too high. An incorrectly calibrated or tared instrument, like a scale that doesn’t read zero when nothing is on it. A person consistently takes an incorrect measurement. For example, they might think the 3/4″ mark on a ruler is the 2/3″ mark. 1.4 Validation of a Method— You Have to Prove It Works! Random error (also called unsystematic error, system noise or random variation) has no pattern. One minute your readings might be too small. The next they might be too large. You can’t predict random error and these errors are usually unavoidable. 1.4 Validation of a Method— You Have to Prove It Works! Examples of causes of random errors are: electronic noise in the circuit of an electrical instrument irregular changes in the heat loss rate from a solar collector due to changes in the wind. 1.4 Analyze Versus Determine—They Are Different The terms analyze and determine have two different meanings. We say a sample is analyzed for part or all of its constituents. The substances measured are called the analytes. 1.5 Analyze Versus Determine—They Are Different The process of measuring the analyte is called a determination. The constituents in the sample may be classified as major (>1% of the sample), minor (0.1 to 1%), or trace (
