Notes of “Introduction of Analytical chemistry” By Skoog, west and Hooler
The Nature of Analytical Chemistry
Analytical Chemistry deals with methods for determining the chemical composition of samples.
Qualitative Analysis (identification) provides information about the identity of species or functional groups in the sample ( an analyte can be identified).
Quantitative Analysis provides numerical : information of analyte ( quantitate the exact amount or concentration).
Analytical Methods
Classical Methods: Wet chemical methods such as determination of boiling point, melting point, density, color or solubility. In addition to gravimetric and volumetric measurements.
Instrumental Methods: Analytical measurements ( conductivity, electrode potential, light absorption or emission, mass-to-charge ratio, fluorescence etc.) are made using instrumentation.
The Role of Analytical Chemistry
- – Analytical chemistry plays a vital role in the development of science.
- – Plays a vital role in many research areas in chemistry, biochemistry, biology, geology, physics and the other sciences.
- – All branches of chemistry draw on the ideas and techniques of analytical chemistry.
- – The interdisciplinary nature of chemical analysis makes it a vital tool in medical, industrial, government and academic laboratories.
Classifying Quantitative Analytical Methods
Gravimetric Methods: Determine the mass of the analyte or some compound chemically related to it.
Volumetric Methods: The volume of a solution containing sufficient reagent to react completely with the analyte is measured.
Spectroscopic Methods: Based on measurement of the interaction between electromagnetic radiation and analyte atoms or molecules.
Chromatographic Methods: Separation techniques and quantitation is based on calibration curve.
Electroanalytical Methods: Involve the measurement of electrical properties such as voltage, current, resistance and quantity of electrical charge.
Stepping Through a Typical Quantitative Analysis
1 – Picking a Method: The first step is the selection of a method. Factors need to be considered in the selection process are:
- Accuracy required
- Cost of analysis (total investment)
- Number of sample to be analyzed
- Time required for analysis
- Skill required
- Complexity of the sample
- Number of components in the sample
2 – Acquiring the Sample: Sampling is the process of collecting a small mass of a material whose composition accurately represents the bulk of the material from which it was taken. To produce meaningful information, an analysis must be performed on a representative sample. For large heterogeneous sample, great effort is required to get a representative sample. Sometime proper sampling is the most difficult step in an analysis and the source of greatest error. The final result of an analysis will never be any more reliable.
3 – Processing the sample: The third step in an analysis is to process the sample. Sometimes no sample processing is required prior to the measurement step such as pH of water sample can be measured directly. Under most circumstances, sample need to be processed in a variety of different ways.
1 – Preparing a Laboratory Sample
2 – Defining Replicate Samples
3 – Preparing Solutions: Physical and Chemical changes.
4 – Eliminating Interferences: Need to eliminate substances from the sample that may interfere with the measurement step. Few chemical or physical properties of importance in chemical analysis are unique to a single chemical species. Species other than the analyte that affect the final measurement are called interferences. An interference causes an error in an analysis by enhancing or attenuating the quantity being measured. A scheme must be devised to isolate the analytes from interferences before the final measurement is made.
5 – Calibration and Measurement: An analytical results depend on a final measurement of a physical or chemical property (X). Ideally, the measurement of the property is directly proportional to the concentration (CA).
CA =kX
where, k is a proportionality constant. The process of determining k is an important step in most analyses. This step is called a calibration.
6 – Calculating Results: Analyte concentrations from experimental data need to be calculated. These computation are based on the raw experimental data collected in the measurement steps, the characteristics of the measurement instruments and the stoichiometry of the analytical reaction.
7- Evaluating Results by Estimating Their Reliability: Analytical results are incomplete without an estimate of their reliability. Some measure of the uncertainties associated with computed results need to be provided.
Analyte: The chemical species to be identified or quantitated. Can be a pure substance or one constituent in a multi-component sample.
Analysis ( of a sample): Investigation of a sample to identify and/or determine (an) analyte(s) or assay a material.
qualitative analysis: Making measurements to determine the identity, structure, or physical properties of a substance.
quantitative analysis: Making measurements to determine the amount of an analyte in a sample.
Analyst: the person who perform the analysis
Analytical method: Logical sequence of operations, described generally, used in the performance of measurements”, e.g., the links of a given analytical technique with particular excitation and detection.
Assay: The process of determining how much of a given sample is the material indicated by its name.
Standard solution: A solution in which the concentration of a solute is known with high reliability.
Standard reference materials (SRMs): Samples of various materials in which the concentration of one or more species is known with very high certainty.
Sample: A portion of material selected from a larger quantity of material.
Blank: A standard that contains no analyte, i.e., a concentration of 0.0. The composition, solvent, electrolyte, etc, should otherwise match the sample test portion.
Blank determination The process of performing all steps of an analysis in the absence of sample. used to detect and compensate for systematic errors in an analysis.
Contaminant or Impurity: A substance, that is introduced unintentionally into a sample or test portion during collection, processing, or measurement.
Interference: A component that is in or is introduced into a sample or test portion that causes a measurement to be higher or lower than the true value.
Matrix: All of the constituents of a sample except the analyte. The matrix is the carrier of the analyte.
Matrix effect: Influence of one or more matrix constituent( s) on the analyte under study. Matrix influences may affect the analyte signal directly by interferences or indirectly by signal depression or amplification.
Validation ( of an analytical method): Process by which it is established, by laboratory studies, that the performance characteristics of the method meet the requirements for the intended analytical applications”.
Sampling: The process of collecting a small portion of a material whose composition is representative of the bulk of the material from which it was taken.
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