Analytical Chemistry, one of the major branches of modern chemistry. It is subdivided into two main areas, qualitative and quantitative analysis. The former involves the determination of unknown constituents of a substance, and the latter concerns the determination of the relative amounts of such constituents.

Chemical Analysis, identifying and measuring the chemical components of a sample of a substance. A chemist carrying out a qualitative analysis seeks to identify the substances in the sample. A quantitative analysis is an attempt to determine the quantity or concentration of a specific substance in the sample. For example, determining whether a sample of salt contains the element iodine requires a qualitative analysis; measuring the percentage by weight of any iodine in the sample requires a quantitative analysis.

The measurement of chemical composition is necessary throughout commerce, regulatory government, and many fields of science. Chemical analysis thus takes many specialized forms.

Separation, Purification, and Preparation for Analysis

Chemists are asked to analyse such diverse materials as stainless steel, beer, a fingernail, a rose petal, smoke, aspirin, and paper. The determination of the identity or quantity of a constituent of such materials is preceded by a sampling step—the selection of the amount and uniformity of material required for the analysis—and by the separation from the sample of either the desired constituent or the undesired, interfering constituents. The appropriate separation method depends on the nature of the constituent sought and of the overall sample. Separation relies on being able to use differences in physical or chemical properties. For example, given a simple mixture of salt and sand it is easy to remove the salt as it is soluble in water and sand is not. With a mixture of iron filings and sand, both parts of the mixture are insoluble, but iron is magnetic and sand is not.

Chromatography is the most generally applicable of the separation methods and has many variants according to the interaction that is used. Four important types of chromatography are: gel permeation chromatography, in which large molecules separate according to their size; ion exchange chromatography, in which charged, or ionic, constituents are separated; gas chromatography, which separates the volatile constituents of a sample; and liquid/liquid chromatography, which separates small, neutral molecules in solution.

The goal in conducting a separation is to produce a purified or partly purified form of the desired constituent for analytical measurement, or to eliminate other constituents that would interfere with the measurement, or both. Separation is often unnecessary when the method of analysis is highly specific, or selective, and responds to the desired constituent while ignoring others. Measuring the pH of blood (a measure of its acidity) with a glass electrode is an example of a measurement that does not require a separation step.

Another step preparatory to both qualitative and quantitative analyses is standardization, or calibration. The response of the analytical method and the sensitivity of the mechanical and electronic equipment to the desired constituent must be calibrated, or standardized, using a pure constituent or a sample containing a known amount of constituent.

The numerical result of a quantitative analysis may state the absolute quantity of the constituent or some percentage of it in the sample. The latter can be expressed as weight per cent, molar concentration (moles of dissolved constituent per litre of solution), or ppm (parts per million by weight), among others. The accuracy of the analytical result is reflected by how well it agrees with the true quantity of constituent. The precision of the result is reflected by its reproducibility, or repeatability. Results from repeated measurements are called precise if they all lie within a narrow range of values. Such results are termed highly reproducible. Precision does not necessarily mean that the results are accurate, however, because some part of the measuring process may bias the results towards values that are higher or lower than the true value. Standardization of the analysis often uncovers such systematic errors.

Random errors in a measurement tend to cancel one another out. Accuracy is nearly always improved by averaging multiple determinations. Depending on the method used, measurements may need to be repeated only three or four times. For procedures in which computers are connected to the analytical instruments, as many as 100,000 measurements may be made very quickly. This technique is referred to as signal averaging.

An actual analysis of a sample is commonly based on a chemical reaction of the constituent that produces an easily identifiable quality such as colour, heat, or insolubility. Gravimetric analysis, which hinges on measuring the mass of precipitates of the constituent, and titrimetric analysis, which depends on measuring the volumes of solutions that react with the constituent, are referred to as wet methods; these are more labour-intensive and less versatile than newer instrumental methods.

Instrumental methods of analysis, or analyses that rely on electronic instruments, became important in the 1950s, and today most analytical measurements are conducted with the aid of such devices.

A systematic wet method qualitative analysis of inorganic ions proceeds by separating the ions into groups by selective precipitation reactions, isolating individual ions in the groups by an additional precipitation reaction, and confirming the identity of the ion by a reaction test that gives a specific precipitate or colour. Several schemes exist for doing this, with cations (positively charged ions) and with anions (negatively charged ions). The accompanying table is an abbreviated scheme for the analysis of environmentally important cations of metallic elements.

Organic analysis relies on certain chemical reactions to detect particular functional groups, such as alcohol, amine, aldehyde, alkene, ester, carboxylic acid, and ether. The test reactions are usually employed without prior separation. As an example, alkenes (compounds containing carbon-carbon double bonds) can be identified by the decolorizing effect they have on a coloured bromine solution. For both organic and inorganic qualitative analysis, instrumental methods are currently preferred because they are more sensitive and specific.

These are mainly gravimetric and titrimetric procedures used for inorganic substances. An example of a gravimetric analysis is the determination of chloride ion concentration in a solution by causing the precipitation of insoluble silver chloride (AgCl). The precipitate is then collected and weighed. The analysis yields very accurate results.

Titrimetric procedures are commonly based on acid-base reactions such as the titration of ethanoic acid with a solution of sodium hydroxide. Another common reaction employed is that of a complexing agent, such as EDTA (ethylenediaminetetraacetic acid or 1,2-bis[bis(carboxy methyl)amino]ethane), with solutions of metal ions, such as lead or mercury. Reactions suitable for titrations must proceed rapidly to completion, without side reactions that tend to obscure the results. This requirement is more often satisfied by inorganic reactions than by organic functional group chemistry.

Spectroscopy, or the study of the interactions of electromagnetic radiation (emr) with matter, is the largest and most nearly accurate class of instrumental methods used in chemical analysis and indeed in all of chemistry. The electromagnetic spectrum is divided into the following wavelength regions: gamma-ray, X-ray, ultraviolet, visible, infrared, microwave, and radio. The interactions of emr with matter involve absorption or emission of emr energy by means of transitions between quantized, or discrete, levels of energy for electrons, bond vibrations, molecular rotations, and electron and nuclear spins in atoms and molecules. The matter-emr interactions take place in devices called spectrometers, spectrophotometers, or spectroscopes. The spectra produced in these devices may be recorded graphically or photographically in images called spectrograms, which permit convenient study of the wavelengths and intensities of the emr absorbed or emitted by the sample being analysed. Increasingly, however, spectral data are recorded electronically and analysed by computers.

Absorption spectrophotometry in the visible and ultraviolet portions of the emr-spectrum is a common quantitative spectral method for both organic and inorganic substances. One technique measures the relative transparency of a solution both before and after the solution has been made to react with a colour-forming reagent. The resulting decrease in transparency of the solution is proportional to the concentration of the constituent being analysed. A second technique compares the relative transparency with that of a standard solution.

Infrared absorption spectrophotometry is useful for organic analysis because bonds for alkenes, esters, alcohols, and other functional groups have very different strengths and the molecules therefore have different vibrational and rotational energies. They therefore absorb infrared radiation of very different frequencies, or energies. Such absorption spectra appear as peaks when plotted on a spectrograph.

Nuclear magnetic resonance (nmr) spectroscopy depends on transitions between nuclear-spin energy states by absorption of radio-frequency emr energy. In nmr spectra of hydrogen, for example, hydrogen atoms in chemically different states absorb emr at different energies. For example, the organic groups —CH3 and —CH2Cl give very different, well-resolved peaks. Accordingly, nmr is a powerful qualitative analysis tool to deduce the structure of organic molecules.

Fluorescence spectroscopy is the reverse of absorption spectrophotometry. With this technique, molecules are induced to emit light, which they do at energies characteristic of their structure, and at intensities proportional to the sample concentration. This method yields extremely sensitive quantitative results for certain molecules.

In atomic emission and atomic absorption spectrophotometry the sample is heated to a high temperature and thereby decomposed into atoms and ions that absorb or emit visible or ultraviolet emr at energies characteristic of the elements involved. The flame test is a very simple form of this technique. The yellowing of a flame by the addition of salt, for example, occurs because the sodium in salt emits strongly in the yellow portion of the visible spectrum. These methods are especially useful for low concentrations of metallic elements in both qualitative and quantitative analysis.

X-ray fluorescence spectroscopy is useful for both qualitative and quantitative analyses of metallic elements, which emit X-rays at characteristic energies when bombarded by a high-energy X-ray source.

Mass spectroscopy is so named only by analogy with spectroscopy proper. The sample is placed in a vacuum, vaporized, ionized, and given extra energy, all of which cause the individual molecules to fragment. These molecular fragments are then separated according to their masses by electric and magnetic fields in a mass spectrometer. The spectral pattern, or mass spectrum, produced is a “fingerprint” of the molecule, in that molecules display unique fragmentation patterns. The fragment of highest mass is called the parent ion and gives an indication of the formula of the compound. High-resolution mass spectroscopy can be used to find the actual formulae of compounds.

These methods rely on the detection of radioactivity in the form of alpha and beta particles and gamma rays that result from nuclear disintegrations. Radioactivity can be induced in the sample by bombarding it with neutrons. Such a procedure, called neutron activation analysis, is commonly used in industry to identify certain metals in a sample. Neutron activation analysis has the advantage of being rapid and highly automated, and it does not destroy the sample.

When a positive and a negative electrode are placed in a solution containing ions, and an electric potential is applied to the electrodes, the positively charged ions (cations) move towards the negative electrode, or cathode, and the negatively charged ions (anions) to the positive electrode, or anode. As a result, an electric current flows between the electrodes. The strength of the current depends on the electric potential between the electrodes and the concentration of ions in the solution. Hence, this instrumental quantitative method, called conductometry, is often used to measure the ion concentration in a solution.

In a related technique, electrodes specially constructed to accept only specific ions are used to determine the sodium ion or calcium ion concentration or the pH of the solution being analysed. Such ion-selective electrodes are important in several types of clinical analysis.