When we go about the task of determining the accuracy of a method, we are focusing upon the identification and elimination of systematic errors. Don't be misled by the statement that 'good precision is an indication of good accuracy. The VIM uses the terms 'repeatability' and 'reproducibility' instead of the more general term 'precision. Additional Notes: 1. These conditions are called repeatability conditions.
Repeatability conditions include the same measurement procedure, the same observer, the same measuring instrument, used under the same conditions, the same location, and repetition over a short period of time. A valid statement of reproducibility requires specification of the conditions changed. The changed conditions may include principle of measurement, method of measurement, observer, measuring instrument, reference standard, location, conditions of use, and time. When discussing the precision of measurement data, it is helpful for the analyst to define how the data are collected and to use the term 'repeatability' when applicable.
The frequency distribution of the measurements approximates a bell-shaped curve that is symmetrical around the mean. The arithmetic mean is calculated using the following equation:. Typically, insufficient data are collected to determine if the data are evenly distributed. Most analysts rely upon quality control data obtained along with the sample data to indicate the accuracy of the procedural execution, i. The analysis of at least one QC sample with the unknown sample s is strongly recommended.
Acid-Base Titrations The techniques of analytical chemistry are relied on heavily to maintain a benign environment. The text is written clearly in understandable prose. When these terms are mentioned again, the text links the reader back to the original definition in case more information is needed. Give yourself an additional incentive to complete the course. Examples of rapidly expanding fields in this area are genomics , DNA sequencing and related research in genetic fingerprinting and DNA microarray ; proteomics , the analysis of protein concentrations and modifications, especially in response to various stressors, at various developmental stages, or in various parts of the body, metabolomics , which deals with metabolites; transcriptomics , including mRNA and associated fields; lipidomics - lipids and its associated fields; peptidomics - peptides and its associated fields; and metalomics , dealing with metal concentrations and especially with their binding to proteins and other molecules. Branches of chemistry.
Even when the QC sample is in control it is still important to inspect the data for outliers. There is a third type of error typically referred to as a 'blunder'. This is an error that is made unintentionally. A blunder does not fall in the systematic or random error categories. It is a mistake that went unnoticed, such as a transcription error or a spilled solution. In the situation where a limited data set has a suspicious outlier and the QC sample is in control, the analyst should calculate the range of the data and determine if it is significantly larger than would be expected based upon the QC data.
If an explanation cannot be found for an outlier other than it appears too high or low , there is a convenient test that can be used for the rejection of possible outliers from limited data sets.
The range is always calculated by including the outlier, which is automatically the largest or smallest value in the data set. The standard deviation of a population is symbolized as s and is calculated using n. Iron can be detected by chemical means if it is present in a sample to an amount of 1 part per million or greater. Some very simple qualitative tests reveal the presence of specific chemical elements in even smaller amounts.
The yellow colour imparted to a flame by sodium is visible if the sample being ignited has as little as one-billionth of a gram of sodium. Such analytic tests have allowed chemists to identify the types and amounts of impurities in various substances and to determine the properties of very pure materials.
Substances used in common laboratory experiments generally have impurity levels of less than 0. For special applications, one can purchase chemicals that have impurities totaling less than 0. The identification of pure substances and the analysis of chemical mixtures enable all other chemical disciplines to flourish. The importance of analytical chemistry has never been greater than it is today. The demand in modern societies for a variety of safe foods, affordable consumer goods, abundant energy , and labour-saving technologies places a great burden on the environment. All chemical manufacturing produces waste products in addition to the desired substances, and waste disposal has not always been carried out carefully.
Disruption of the environment has occurred since the dawn of civilization, and pollution problems have increased with the growth of global population. The techniques of analytical chemistry are relied on heavily to maintain a benign environment.
The undesirable substances in water, air, soil, and food must be identified, their point of origin fixed, and safe, economical methods for their removal or neutralization developed. Once the amount of a pollutant deemed to be hazardous has been assessed, it becomes important to detect harmful substances at concentrations well below the danger level.
Analytical chemists seek to develop increasingly accurate and sensitive techniques and instruments. Sophisticated analytic instruments, often coupled with computers, have improved the accuracy with which chemists can identify substances and have lowered detection limits.
An analytic technique in general use is gas chromatography , which separates the different components of a gaseous mixture by passing the mixture through a long, narrow column of absorbent but porous material. The different gases interact differently with this absorbent material and pass through the column at different rates. As the separate gases flow out of the column, they can be passed into another analytic instrument called a mass spectrometer , which separates substances according to the mass of their constituent ions. A combined gas chromatograph—mass spectrometer can rapidly identify the individual components of a chemical mixture whose concentrations may be no greater than a few parts per billion.
Similar or even greater sensitivities can be obtained under favourable conditions using techniques such as atomic absorption, polarography, and neutron activation. The rate of instrumental innovation is such that analytic instruments often become obsolete within 10 years of their introduction. Newer instruments are more accurate and faster and are employed widely in the areas of environmental and medicinal chemistry.
Modern chemistry, which dates more or less from the acceptance of the law of conservation of mass in the late 18th century, focused initially on those substances that were not associated with living organisms. Study of such substances, which normally have little or no carbon, constitutes the discipline of inorganic chemistry. Early work sought to identify the simple substances—namely, the elements—that are the constituents of all more complex substances.
Some elements, such as gold and carbon, have been known since antiquity, and many others were discovered and studied throughout the 19th and early 20th centuries. Today, more than are known. The study of such simple inorganic compounds as sodium chloride common salt has led to some of the fundamental concepts of modern chemistry, the law of definite proportions providing one notable example. This law states that for most pure chemical substances the constituent elements are always present in fixed proportions by mass e.
The crystalline form of salt, known as halite , consists of intermingled sodium and chlorine atoms, one sodium atom for each one of chlorine.
Such a compound, formed solely by the combination of two elements, is known as a binary compound. Binary compounds are very common in inorganic chemistry, and they exhibit little structural variety. The subject uses portfolio evaluation. In case of a re-sit examination all the parts of the portfolio must be repeated.
Exam registration requires that class registration is approved in the same semester. Compulsory activities from previous semester may be approved by the department. Fundamentals of Analytical Chemistry, 9th Ed. Douglas A. Skoog, Donald M.
West, F. James Holler, Stanley R.
Crouch Thomson Learning B. Additional course material given at the lectures and the exercises.