Chapter 1: The Nature of Analytical Chemistry

Introduction Analytical chemistry: A measurement science that has applications in all fields of science, engineering, and medicine. It consists of: Qualitative analysis: It reveals the chemical identity of the elements and compounds in a sample. Quantitative analysis: It indicates the amount of each substance or analytes (components) in a sample. Ex. quantitative determination of nitrogen in foods establishes their protein content and thus their nutritional value. Analytes: The components of a sample that are determined.

Figure 1-1 Relationship between analytical chemistry, other branches of chemistry, and the other sciences. Agronomy 農藝學 Archeology 考古學 Anthropology 人類學 Metallurgy 冶金學 Meteorology 氣象學 Paleontology 古生物學 Paleobiology 古生物學 Horticulture 農藝學

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1B Quantitative analytical methods Classical methods: Uses mass or volume, along with the use of atomic or molecular masses and well-defined chemical reactions for detecting the presence of an analyte or determining the amount of an analyte in a sample. Gravimetric methods: the mass of the analyte or some compound chemically related to it is determined. Volumetric (titrimetric) methods: the volume of a solution containing sufficient reagent to react completely with the analyte is measured.

Figure 13-1 The titration process

Inorganic Precipitating Agents Gravimetric methods Inorganic Precipitating Agents

Figure 13-2 The two types of titration curves

Continuous Instrumental methods: uses an instrument-generated signal for detecting the presence of an analyte or determining the amount of an analyte in a sample. Electroanalytical methods: electrical properties such as potential, current, resistance, and quantity of electrical charge are measured. Spectroscopic methods: the interaction between electromagnetic radiation and analyte atoms or molecules or the emission of radiation by analytes is explored. Mass Spectrometry:

Figure 21-1 A cell for potentiometric determinations.

29 B Mass spectrometers The mass spectrometer is an instrument that produces ions, separates them according to their m/z values, detects them, and plots the mass spectrum.

IC A typical quantitative analysis Figure 1-2 Flow diagram showing the steps in a quantitative analysis. * The lecture focused on * * *

Sample: The portion of material taken for analysis. Choosing a Method High reliability requires a large investment of time and money. The selected method usually represents a compromise between accuracy required and the time and money available for the analysis. Acquiring the Sample Sample: The portion of material taken for analysis. Sampling: The process of collecting a small mass of a material whose composition accurately represents the bulk of the material being sampled. (When the bulk is large and heterogeneous, great effort is required to get a representative sample.) Accuracy: 準確度 Bulk: 主體、全體 Sampling: 採樣 Assay: 試料、試樣、化驗、鑑定 Representative sample: 代表性樣品

Preparing a Laboratory Sample EX. A solid sample is ground and mixed Processing the Sample Preparing a Laboratory Sample EX. A solid sample is ground and mixed Defining Replicate Samples Replicate samples: Portions of a material of approximately the same size that are carried through an analytical procedure at the same time and in the same way. Quantitative measurements on replicates are usually averaged and statistical tests are performed to ensure reliability. Replicate samples: 重複樣品

Preparing Solutions: Physical and Chemical Changes Most analyses are performed on solutions of the sample made with a suitable solvent. The solvent should completely dissolve the entire sample. The sample may require heating with strong acids, strong bases, oxidizing agents, reducing agents, or some combination of such reagents. Once the analyte is made soluble, the sample must have a property that is proportional to analyte concentration and that can be measured. Replicate samples: 重複樣品

Eliminating Interferences Martrix: The entire group of substances that makes up a sample. Specific techniques or reactions: work for only one analyte Selective techniques or reactions: apply to only a few analytes. Interferents: Species other than the analyte that affect the final measurement by enhancing or attenuating (making smaller) the quantity being measured are called. (Interferences in the sample must be eliminated, EX. Separation, before making measurements.) Separation: The approach used to remove one type of chemical from another.

Calibrating and Measuring Concentration Ideally, the measurement X of a physical or chemical property is directly proportional to the concentration cA of the analyte. cA = kX k is a proportionality constant Calibration: The process of determining the proportionality k between analyte concentration and a measured quantity. Calibration curve: 校正曲線/檢量線

6. Calculating Results Computing analyte concentrations from experimental data is based on the raw experimental data collected in the measurement step and the stoichiometry of the analytical reaction. Evaluating Results by Estimating Reliability The experimenter must provide the uncertainties associated with computed results if the data are to have any value.

1D An integral role for chemical analysis: feedback control systems Feedback system: The process of continuous measurement and control. Feedback loop: The cycle of measurement, comparison, and control Figure 1-3 Feedback system flow diagram.

補充材料

1. Example of Analytical Chemist’s Job Question i) How much caffeine in a chocolate bar? ii) How does that amount compare with quantity in coffee or soft drink? Caffeine Theobromine Theobromine: 可可鹼 咖啡因 咖啡鹼 2017/4/28

Heterogeneous (sample) 2) Sampling Homogeneous (sample) Heterogeneous (sample) 2017/4/28

3) Sample preparation Freezing grinding 2017/4/28

ii) Defat 2017/4/28

Quantitative transfer iii) Extraction Slurry Quantitative transfer 2017/4/28

4) Analysis Chromatography 2017/4/28

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Chromatogram Sample Standard 2017/4/28

Calibration (standard) curve 2017/4/28

5) Reporting and interpretation 2017/4/28

6) Drawing conclusion 2017/4/28

2. Steps in a chemical analysis: Formulate the question: Convert a general question into a specific one that can be answered by a chemical measurement. 2. Select the appropriate method/technology/procedure 3. Obtain a representative sample 4. Sample preparation: Convert the sample into a suitable form for analysis, e.g., concentrate the sample, remove or mask interfering species 2017/4/28

Steps in a chemical analysis: (Cont’s) Analysis: Measure the unknown concentration in replicate analyses Produce a clear report of results, including estimates of uncertainty 7. Draw conclusion: based on the analytical results, decide what actions to take. 2017/4/28

3. Constructing Representative Sample 1) For random heterogeneous material 2017/4/28

2) For segregated heterogeneous material 2017/4/28

4. Criteria in Choosing a Method: 1) Accuracy 2) Precision 3) Sensitivity 4) Detection limit 5) Selectivity 6) Robustness 7) Ruggedness 8) Scale of operation 9) Analysis time 10) Availability of equipment Cost 2017/4/28

Precision: The reproducibility of a measurement. Accuracy: A measure of the agreement between an experimental result and its expected. Precision: The reproducibility of a measurement. Sensitivity: The change in signal per unit change in the amount of analyte. Detection limit: The smallest amount of analyte that can be determined with confidence. Selectivity: A measure of a method’s freedom from interferences. 2017/4/28

Robust (穩健): A method that can be applied to analytes in a wide variety of matrices. Rugged (耐用): A method that is insensitive to changes in experimental conditions. Scale of Operation: The amount of sample and concentration of analyte in the sample for the analysis. 2017/4/28

5. Analytical method vs legislation Technique: A chemical or physical principle that can be used to analyze a sample. Method: A means for analyzing a sample for a specific analyte in a specific matrix. Procedure: Written directions outlining how to analyze a sample. Protocol: A set of written guidelines for analyzing a sample specified by an agency, which has been previously validated. 2017/4/28

APHA = American Public Health Association ASTM = American Society for Testing Materials EPA = Environmental Protection Agency 2017/4/28

Those solutions where water is the solvent 6. Keywords Aqueous solution: Those solutions where water is the solvent Analyte: The component(s) of a sample which is interested Species: The chemical forms of interest, e.g., both Fe2+ and Fe3+ are possible ionic forms of the iron ions in aqueous Example: Determine Fe in ore, Fe is the analyte. Determine Fe2+ and Fe3+ in seawater, Fe2+ and Fe3+ are different iron species. Species 物種 2017/4/28

Representative sample: Sampling: Representative sample: Homogeneous: Heterogeneous: Random heterogeneous material Segregated heterogeneous material Random sample: Composite sample: Segregated 區隔的 Composite 混合的 2017/4/28

A known portion of liquid that represents a part of some larger volume Aliquot: A known portion of liquid that represents a part of some larger volume Quantitative transfer: Slurry: Supernatant: Residue: Decant: Aliquot 次等分 2017/4/28

Interference Masking 2017/4/28

Standard solution: Calibration curve: The result of a standardization showing graphically how a method’s signal changes with respect to the amount of analyte. 2017/4/28

Homework (Due 2015/9/24) End of Chapter 01 Define qualitative analysis and quantitative analysis. Define analyte, matrix, and Interferents. Define sample, sampling, and representative sample. End of Chapter 01