The following article will guide you about how to monitor environmental pollution.
Introduction:
The use of instrumentation is an exciting and fascinating part of chemical analysis that interacts with all the areas of chemistry and with many other fields of pure and applied sciences. Analytical instrumentation plays an important role in the production and evaluation of new products and in the protection of consumers and environment.
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Scientists, engineers and technicians in a variety of fields often need information from chemical analysis in their work. Chemists themselves often require information provided by instrumental techniques outside their area of specialisation. Thus, instrumentation provides the lower detection limits required to assure safe foods, drugs, water and air.
For proper understanding of our environment, we must have a clear idea of the identities and quantities of pollutants and other chemical species in air, water, soil and biological samples, etc. These may be either in concentrated point source such as from factory smoke, stacks and sewage discharge or in diffused forms, such as from automobile’s exhausts and runoff from agricultural land. These pollutants eventually endanger the life of both humans and animals. These pollutants can range from parts per billion (ppb) to hundreds of parts per million (ppm).
In order to control the level of these pollutants in the environment, it is necessary to know their chemical or bio-chemical route of formation and degradation, the extent of their occurrence in the environment and their ecotoxicity. Analytical chemistry plays the most vital role in determining the extent of their occurrence in the environment while the degree of their ecotoxicity determines the priority of pollutants and the overall sensitivity required for analytical methods to be employed for their measurement.
To effectively monitor the environment as well as devise pollution abatement methods, it is necessary to know the quality of the environment-quantitatively. It is essential to be aware of the components responsible for environmental pollution and their concentration in waste-water and solid waste. In addition, the environment required for the growth of eco-friendly micro-organisms, which can be utilised to reduce the amount of pollutants in waste, is also required to be evaluated.
The air and water in our environment contain a wide assortment of toxic organic and inorganic pollutants. They enter the environment as emissions into the atmosphere or as discharges to water bodies. These may be either in concentrated point sources, such as from factory smoke, stacks and sewage discharges, or in diffuse forms, such as from automobiles’ exhausts and run-off from agricultural land. These pollutants eventually endanger the life of both humans and animals. These pollutants can range from parts per billion (ppb) or below in rural areas to hundreds of parts per million (ppm) or higher in large industrial and urbanised areas.
In recent years, many chemicals previously considered only moderately toxic have been identified very toxic, e.g. potential carcinogens, and thus have been assigned lower threshold limit values (TLVs). In addition, the number of newly identified toxic substances are increasing everyday.
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Hence, there is an increasing need for rapid screening and monitoring of toxic substances in air and water to meet the requirements of pollution control authorities.
In order to control the levels of these pollutants in the environment, it is necessary to know their chemical or biochemical route of formation and degradation, the extent of their occurrence in the environment and their ecotoxicity.
In the recent past, a great deal of advances have been made in analytical methodology and instrumentation. With the ever-increasing advancement of microprocessor technology, these analytical instruments have become more versatile. These are computer-aided instruments (so-called ‘intelligent instruments) that offer highly improved detection limits (down to parts per billion (ppb) to parts per trillion (ppt) level), better precision, accuracy and increased specificity.
Thus environmental monitoring must be able, in many cases, to detect with accuracy and consistency contaminants present at very low levels. In the determination of the pollutants present, their fate, and their effect on the environment, biotechnology can be of considerable value, especially as molecular biology techniques are increasingly employed.
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Using this information, the level of residual pollutants can be evaluated vis-a-vis the prescribed minimum. The results of such analysis are further used for mass and energy balance, design kinetics of biodegradation processes, design of bioreactors and fermenters, etc.
Chemical Analysis:
For proper understanding of our environment, we must have a clear idea of the identities and quantities of pollutants and other chemical species in air, water, soil and biological samples, etc. These may be either in concentrated point source such as from factory smoke, stacks and sewage discharge or in diffused forms, such as from automobile’s exhausts and runoff from agricultural land. These pollutants eventually endanger the life of both humans and animals. These pollutants can range from parts per billion (ppb) to hundreds of parts per million (ppm).
In order to control the level of these pollutants in the environment, it is necessary to know their chemical or biochemical route of formation and degradation, the extent of their occurrence in the environment and their ecotoxicity. Analytical chemistry plays the most vital role in determining the extent of their occurrence in the environment while the degree of their ecotoxicity determines the priority of pollutants and the overall sensitivity required for analytical methods to be employed for their measurement.
Techniques for Analysis:
1. Absorption Spectrophotometry:
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Absorption spectrophotometry of light-absorbing species in solution, historically called colorimetry when visible light is absorbed, is still used for the analysis of many water and some air pollutants. Basically, absorption spectrophotometry consists of measuring the percentage transmittance (%T) of monochromatic light passing through a light-absorbing solution as compared to the amount passing through a blank solution containing everything in the medium but the sought for constituent (100 per cent).
2. Atomic Absorption and Emission Analyses:
Atomic absorption analysis has become the method of choice for most metals analysed in environmental samples. This technique is based upon the absorption of monochromatic light by a cloud of atoms of the analyte metal. The monochromatic light is produced by a source composed of the same atoms as those being analysed. The source produces intense electromagnetic radiation, with a wavelength exactly the same as that absorbed by the atoms, resulting in extremely high selectivity. The basic components of an atomic absorption instrument are shown in Fig. 30.1.
The key element is the hollow cathode lamp, in which atoms of the analyte metal are energised so that they become electronically excited and emit radiation, with a very narrow wavelength band characteristic of the metal. This radiation is guided by the appropriate optics through a flame into which the sample is aspirated. In the flame, most metallic compounds are decomposed and the metal is reduced to the elemental state, forming a cloud of atoms.
These atoms absorb a fraction of radiation in the flame. The fraction of radiation absorbed increases with the concentration of the sought-for element in the sample according to the Beer’s law relationship. The attenuated light beam next goes to a monochromator to eliminate extraneous light resulting from the flame and then to a detector.
Atomisers other than a flame can be used. The most common of these is the graphite furnace, which consists of a hollow graphite cylinder placed so that the light beam passes through it. A small sample of up to 100 – µl is inserted in the tube through a hole in the top. An electric current is passed through the tube to heat it-gently at first to dry the sample, then rapidly to vapourise and excite the metal analyte.
The absorption of metal atoms in the hollow portion of the tube is measured and recorded as a spike-shaped signal. A diagram of a simple graphite furnace with a typical output signal is shown in Fig. 30.2. The major advantage of the graphite furnace is that it gives detection limits up to 1000 times lower than those of conventional flame devices.
A special technique for the flameless atomic absorption analysis of mercury involves its room- temperature reduction to the elemental state by tin (II) chloride in solution, followed by sweeping it into an absorption cell with air. Nanogram (10–9g) quantities of mercury can be determined by measuring mercury absorption at 253.7 nm.
3. Atomic Emission Techniques:
Metals may be determined in water, atmospheric particulate matter, and biological samples very well by observing the spectral lines emitted when they are heated to a very high temperature. An especially useful atomic emission technique is inductively coupled plasma atomic emission spectroscopy. The ‘flame’ in which analyte atoms are excited in plasma emission consists of an incandescent plasma (ionised gas) of argon heated inductively by radiofrequency energy at 4-50 MHz and 2-5 kW (Fig. 30.3).
The energy is transferred to a stream of argon through an induction coil, producing temperatures up to 10,000 K. The sample atoms are subjected to temperatures around 7000 K, twice those of the hottest conventional flames (for example, nitrous oxide-acetylene at 3200 K). Since emission of light increases exponentially with temperature, lower detection limits are obtained. Furthermore, the technique enables emission analysis of some of the environmentally important metalloids such as arsenic, boron and selenium.
Interfering chemical reactions and interactions in the plasma are minimised as compared to flames. Of greatest significance, however, is the capability of analysing as many as 30 elements simultaneously, enabling a true multi-element analysis technique. Thus, plasma atomisation combined with mass spectrometric measurement of analyte elements is a relatively new technique that is an especially powerful means for multi-element analysis.
X-ray fluorescence is another multi-element analysis technique that can be applied to a wide variety of environmental samples. It is especially useful for the characterisation of atmospheric particulate matter, but it can be applied to some water and soil samples as well. This technique is based upon measurement of X-rays emitted when electrons fall back into inner shell vacancies created by bombardment with energetic X-rays, gamma radiation or protons.
The emitted X-rays have an energy characteristic of the particular atom. The wavelength (energy) of the emitted radiation yields a qualitative analysis of the elements and the intensity of radiation from a particular element provides a quantitative analysis. A schematic diagram of a wavelength-dispersive X-ray fluorescence spectrophotometer is shown in Fig. 30.4.
An excitation source, normally an X-ray tube emitting ‘white’ X-rays (a continuum), produces a primary beam of energetic radiation which excites fluorescent X-rays in the sample. A radioactive source emitting gamma rays or protons from an accelerator may also be used for excitation. For best results, the sample should be mounted as a thin layer, which means that segments of air filters containing fine particulate matter make ideal samples.
The fluorescent X-rays are passed through a collimator to select a parallel secondary beam, which is dispersed according to wavelength by diffraction with a crystal monochromator. The monochromatic X-rays in the secondary beam are counted by a detector which rotates at a degree twice that of the crystal to scan the spectrum of emitted radiation.
Energy-selective detectors of the Si(Li) semi-conductor type enable measurement of fluorescent X-rays of different energies without the need for wavelength dispersion. Instead, the energies of a number of lines falling on a detector simultaneously are distinguished electronically.
An energy-dispersive X-ray fluorescence spectrum from an atmospheric particulate sample is shown in Fig. 30.5. A significant advantage of X-ray fluorescence multi-element analysis is that sensitivities and detection limits do not vary greatly across the periodic table as they do with methods such as neutron activation analysis or atomic absorption. Proton-excited X-ray emission is particularly sensitive.
5. Gas Chromatography:
Gas chromatography has played an important role in the analysis of organic materials. Gas chromatography is both a qualitative and quantitative technique; for some analytical applications of environmental importance, it is remarkably sensitive and selective. Gas chromatography is based upon the principle that when a mixture of volatile materials transported by a carrier gas is passed through a column containing an adsorbent solid phase, or, more commonly, an absorbing liquid phase coated on a solid material, each volatile component will be partitioned between the carrier gas and solid or liquid.
The length of time required for the volatile component to traverse the column is proportional to the degree to which it is retained by the non-gaseous phase. Since different components may be retained to different degrees, they will emerge from the end of the column at different times. If a suitable detector is available, the time at which the component emerges from the column and the quantity of the component are both measured. A recorder trace of the detector response appears as peaks of different sizes, depending upon the quantity of material producing the detector response. Both quantitative and (within limits) qualitative analyses of the sought-for substances are obtained.
The essential features of gas chromatography are shown schematically in Fig. 30.6. The carrier gas generally is argon, helium, hydrogen, or nitrogen. The sample is injected as a single compact plug into the carrier gas stream immediately ahead of the column entrance. If the sample is liquid, the injection chamber is heated to vapourise the liquid rapidly.
The separation column may consist of a metal or glass tube packed with an inert solid of high surface area covered with a liquid phase, or it may consist of an active solid, which enables the separation to occur. More commonly now, capillary columns are employed which consist of very small diameter, long tubes in which the liquid phase is coated on the inside of the column.
The component that primarily determines the sensitivity of gas chromatographic analysis, and for some classes of compounds the selectivity as well, is the detector. One such device is the thermal conductivity detector, which responds to changes in the thermal conductivity of gases passing over it. The electron-capture detector, which is especially useful for halogenated hydrocarbons and phosphorus compounds, operates through the capture of electrons emitted by a beta-particle source.
The flame-ionisation gas chromatographic detector is very sensitive for the detection of organic compounds. It is based upon the phenomenon by which organic compounds form highly conducting fragments, such as C+, in a flame. Application of a potential gradient across the flame results in a small current that may be readily measured.
The mass spectrometer may be used as a detector for a gas chromatograph. A combined gas chromatograph/mass spectrometer (GC/MS) instrument is an especially powerful analytical tool for organic compounds. Gas chromatographic analysis requires that a compound exhibit at least a few mm of vapour pressure at the highest temperature at which it is stable.
In many cases, organic compounds that cannot be chromatographed directly may be converted into derivatives that are amenable to gas chromatographic analysis. It is seldom possible to analyse organic compounds in water by direct injection of the water into the gas chromatograph; higher concentration is usually required.
Two techniques commonly employed to remove volatile compounds from water and concentrate them are extraction with solvents and purging volatile compounds with a gas, such as helium, concentrating the purged gases on a short column and driving them off by heat into the chromatograph.
6. High-Performance Liquid Chromatography:
A liquid mobile phase used with very small column-packing particles enables high-resolution chromatographic separation of materials in the liquid phase. Very high pressures up to several thousand psi are required to get a reasonable flow rate in such systems. Analysis using such devices is called high performance liquid chromatography (HPLC) and offers an enormous advantage, in that the materials analysed need not be changed to the vapour phase, a step that often requires preparation of a volatile derivative or results in decomposition of the sample.
The basic features of a high-performance liquid chromatograph are the same as those of a gas chromatograph, shown in Fig. 30.6, except that a solvent reservoir and high-pressure pump are substituted for the carrier gas source and regulator. An HPLC chromatogram of some water pollutants is shown in Fig. 30.7.
Chromatographic Analysis of Water Pollutants:
A number of chromatography-based standard methods have also been developed for determining water pollutants. Some of these methods use the purge-and-trap technique, bubbling gas through a column of water to purge volatile organics from the water followed by trapping the organics on solid sorbents, whereas others use solvent extraction to isolate and concentrate the organics. These methods are summarised in Table 30.1.
Liquid chromatographic determination of ions, particularly anions, has enabled the measurement of species that used to be very troublesome for water chemists. This technique is called ion chromatography and its development has been facilitated by special detection techniques using so-called suppressors to enable detection of analyte ions in the chromatographic effluent.
Ion chromatography has been developed for the determination of most of the common anions, including arsenate, arsenite, borate, carbonate, chlorate, chlorite, cyanide, the halides, hypochlorite, hypophosphite, nitrate, nitrite, phosphate, phosphite, pyrophosphate, selenate, selenite, sulphate, sulphite, sulphide, trimetaphosphate, and tripolyphosphate. Cations, including common metal ions, can also be determined by ion chromatography.
Mass spectrometry is particularly useful for the identification of specific organic pollutants. It depends upon the production of ions by an electrical discharge or chemical process, followed by separation based on the charge-to-mass ratio and measurement of the ions produced. The output of a mass spectrometer is a mass spectrum, such as the one shown in Fig. 30.8. A mass spectrum is characteristic of a compound and serves to identify it.
Computerised data banks for mass spectra have been established and are stored in computers interfaced with mass spectrometers. Identification of a mass spectrum depends upon the purity of the compound from which the spectrum is taken. Prior separation by gas chromatography with continual sampling of the column effluent by a mass spectrometer, commonly called gas chromatography-mass spectrometry (GC/MS), is particularly effective in the analysis of organic pollutants.
Total Organic Carbon in Water:
Dissolved organic carbon exerts an oxygen demand in water; often this is in the form of toxic substances and is a general indicator of water pollution. Therefore, its measurement is quite important. The measurement of total organic carbon, TOC, is now recognised as the best means of assessing the organic content of a water sample. The measurement of this parameter has been facilitated by the development of methods which, for the most part, totally oxidise the dissolved organic material to produce carbon dioxide. The amount of carbon dioxide evolved is taken as a measure of TOC.
TOC can be determined by a technique that uses a dissolved oxidising agent promoted by ultraviolet light. Potassium peroxydisulphate, K2S2O8, is usually chosen as an oxidising agent to be added to the sample. Phosphoric acid is also added to the sample, which is sparged with air or nitrogen to drive off CO2 formed from HCO3– and CO32– in solution. After sparging, the sample is pumped to a chamber containing a lamp emitting ultraviolet radiation of 184.9 nm. This radiation produces reactive free radical species, such as the hydroxyl radical, HO. The active species bring about the rapid oxidation of dissolved organic compounds as shown in the following general reaction –
After oxidation is complete, the CO2 is sparged from the system and measured with a gas chromatographic detector or by absorption in ultrapure water followed by a conductivity measurement. Fig. 30.9 is a schematic of a TOC analyser.
Good analytical methodology, particularly that applicable to automated analysis and continuous monitoring, is essential for the study and alleviation of air pollution. The atmosphere is a particularly difficult analytical system because of the very low levels of substances to be analysed; sharp variations in pollutant level with time and location; differences in temperature and humidity; and difficulties encountered in reaching desired sampling points, particularly those substantially above the Earth’s surface.
Furthermore, although improved techniques for the analysis of air pollutants are continually being developed, a need still exists for new analytical methodology and the improvement of existing methodology. Much of the data on air pollutant levels were unreliable as a result of inadequate analysis and sampling methods.
An atmospheric pollutant analysis method does not have to give the actual value to be useful. One which gives a relative value may still be helpful in establishing trends in pollutant levels, determining pollutant effects and locating pollution sources. Such methods may continue to be used while others are being developed.
Air pollutants generally measured may be placed in several different categories. One such category contains materials for which ambient (surrounding atmosphere) standards have been set by the environmental protection agency. These are sulphur dioxide, carbon monoxide, nitrogen dioxide, non-methane hydrocarbons and particulate matter. The standards are categorised as primary and secondary. Primary standards are those defining the level of air quality necessary to protect public health.
Secondary standards are designed to provide protection against known or expected adverse effects of air pollutants, particularly upon materials, vegetation and animals. Another group of air pollutants to be measured consists of those known to be specifically hazardous to human health, such as asbestos, beryllium and mercury. A third category of air pollutants contains those regulated in new installations of selected stationary sources, such as coal-cleaning plants, cotton gins, lime plants and paper mills.
Some pollutants in this category are visible emissions, acid (H2SO4) mist, particulate matter, nitrogen oxides and sulphur oxides. These substances often must be monitored in the stack to ensure that emission standards are being met. A fourth category consists of the emissions of mobile sources (motor vehicles)—hydrocarbons, CO, and NOx. A fifth group consists of miscellaneous elements and compounds, such as certain heavy metals, fluoride, chlorine, phosphorus, polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls, odorous compounds, reactive organic compounds and radio-nuclides.
For some species, an analytical method is well developed and reasonably satisfactory. For others, no really satisfactory method exists. The development of analytical techniques for air pollutants remains a fertile and challenging area for research.
Levels of air pollutants and other air-quality parameters are expressed in several different kinds of units. These are, for gases and vapours, µg/m3 (alternatively, ppm by volume); for weight of particulate matter, µg/m3; for particulate matter count, number per cubic metre; for visibility, kilometres; for instantaneous light transmission, percentage of light transmitted; for emission and sampling rates, m3/min; for pressure, mm Hg; and for temperature, degrees Celsius. Air volumes should be converted to conditions of 10°C and 760 mm Hg (1 atm), assuming ideal gas behaviour.
Analysis of Particulate Matter:
Particles are almost always removed from air or gas (such as exhaust flue gas) prior to analysis. The two main approaches to particle isolation are filtration and removal by methods that cause the gas stream to undergo a sharp bend, so that particles are collected on a surface.
The method commonly used for determining the quantity of total suspended particulate matter in the atmosphere draws air over filters that remove the particles. This device, called a Hi-Vol sampler, is essentially a glorified vacuum cleaner that draws air through a filter. The samplers are usually placed under a shelter, which excludes precipitation and particles larger than about 0.1 mm in diameter. These devices efficiently collect particles from a large volume of air, typically 2000 m3.
The filters used in a Hi-Vol sampler are usually composed of glass fibres and have a collection efficiency of at least 99 per cent for particles with 0.3 pm diameter. Particles with diameters exceeding 100 pm remain on the filter surface, whereas particles with diameters down to approximately 0.1 pm are collected on the glass fibres filters. Efficient collection is achieved by using very small diameter fibres (less than 1 pm) for the filter material.
The technique described here is most useful for determining total levels of particulate matter. Prior to taking the sample, the filter is maintained at 15°-35°C at 50 per cent relative humidity for 24 hours, then weighed. After sampling for 24 hours, the filter is removed and equilibrated for 24 hours under the same conditions used prior to its installation on the sampler. The filter is then weighed and the quantity of particulate matter per unit volume of air is calculated.
The range over which particulate matter can be measured is approximately 2-750 pg/m3, where volume is expressed at 25°C and 1 atm (760 mm Hg, 101 kPa) pressure. The lower limit is determined by limitations in measuring mass and the upper limit by limited flow rate when the filter becomes clogged.
Size separation of particles can be achieved by filtration through successively smaller filters in a stacked filter unit. Another approach uses the virtual impactor, a combination of an air filter and an impactor. In the virtual impactor, the gas stream being sampled is forced to make a sharp bend. Particles larger than about 2.5 µm do not make the bend and are collected on a filter. The remaining gas stream is then filtered to remove smaller particles. Results obtained by the analysis of particulate matter collected by the filters should be treated with some caution. A number of reactions may occur on the filter and during the process of removing the sample from the filter. This can cause serious misinterpretation of data.
For example, volatile particulate matter may be lost from the filter. Furthermore, because of chemical reactions on the filter, the material analysed may not be the material that was collected. Artifact particulate matter forms from the oxidation of acid gases on alkaline glass fibres. This phenomenon gives an exaggerated value of particulate matter concentration.
One of the major difficulties in particle analysis is the lack of suitable filter material. Different filter materials serve very well for specific application, but none is satisfactory for all applications. Fibre filters composed of polystyrene are very good for elemental analysis because of the low background levels of inorganic materials.
However, they are not useful for organic analysis. Glass-fibre filters have good weighing qualities and are therefore very useful for determining total particle concentration; however, metals, silicates, sulphates and other species are readily leached from fine glass fibres, introducing error into analysis for inorganic pollutant analysis.
Impactors cause a relatively high velocity gas stream to undergo a sharp bend, so that particles are collected on a surface impacted by the stream. The device may be called a dry or wet impactor, depending upon whether collecting surface is dry or wet; wet surfaces aid particle retention. Size segregation can be achieved with an impactor because larger particles are preferentially impacted and smaller particles continue in the gas stream.
The cascade impactor, illustrated in Fig. 30.14, accomplishes size separation by directing the gas stream onto a series of collection slides through successively smaller orifices, which yield successively higher gas velocities. Particles may break up into smaller pieces from the impact of impingement; therefore, in some cases impingers yield erroneously high values for levels of smaller particles.
A number of chemical analysis techniques can be used to characterise atmospheric pollutants. These include atomic absorption, inductively coupled plasma techniques, X-ray fluorescence, neutron activation analysis and ion-selective electrodes for fluoride analysis. Chemical microscopy is an extremely useful technique for the characterisation of atmospheric particles. Either visible or electron microscopy may be employed. Particle morphology and shape tell an experienced microscopist a great deal about the material being examined.
Reflection, refraction, microchemical tests and other techniques may be employed to further characterise the materials being examined. Microscopy may be used for determining the levels of specific kinds of particles and for determining their size.
Direct Spectrophotometric Analysis of Gaseous Air Pollutants:
It is obvious that measurement techniques that depend upon the use of chemical reagents, particularly liquids, are cumbersome and complicated. It is a tribute to the ingenuity of instrument designers that such techniques are being applied successfully to atmospheric pollutant monitoring. Direct spectrophotometric techniques are more desirable when they are available and when they are capable of accurate analysis at the low levels required.
One such technique, non-dispersive infrared spectrophotometry, has been described for the analysis of carbon monoxide. Three other direct spectrophotometric methods are Fourier transform infrared spectroscopy, tunable diode laser spectroscopy and differential optical absorption spectroscopy. These techniques may be used for point air monitoring, in which a sample is monitored at a given point, generally by measurement in a long absorption cell. In- stack monitoring may be performed to measure effluents.
A final possibility is the collection of long- line data (sometimes using sunlight as a radiation source), an approach which yields concentrations in units of concentration-length (ppm-metres). If the path length is known, the concentration may be calculated. This approach is particularly useful for measuring concentrations in stack plumes.
Dispersive absorption spectrometers are basically standard spectrometers with a monochromator for selection of the wavelength to be measured. They are used to measure air pollutants by determining absorption at a specified part of the spectrum of the sought-for material. Of course, other gases or particulate matter that absorb or scatter light at the chosen wavelength interfere. These instruments are generally applied to in-stack monitoring. Sensitivity is increased by using long path lengths or by pressurising the cell.
Second-derivative spectroscopy is a useful technique for trace gas analysis. Basically, this technique varies the wavelength by a small value around a specified nominal wavelength. The second derivative of light intensity versus wavelength is obtained. In conventional absorption spectrophotometry, a decrease in light intensity as the light passes through a sample indicates the presence of at least one substance— and possibly many absorbing at that wavelength. Second-derivative spectroscopy, however, provides information regarding the change in intensity with wavelength, thereby indicating the presence of specific absorption lines or bands which may be superimposed on a relatively high background of absorption. Much higher specificity is obtained. The spectra obtained by second-derivative spectrometry in the ultraviolet region show a great deal of structure and are quite characteristic of the compounds being observed.