Gas and atmosphere analysis
Qualitative identifications and quantitative determinations of substances essential for the evaluation of the air quality in the ambient air and in the industrial workplace.
The terms air pollution and gas analysis are used here in the broad sense. Air pollution refers to unwanted air contaminants in both the ambient air and the work environment, since many of the same pollutants are found at both places and are assessed using similar procedures. These pollutants may exist in the gaseous state or as aerosols. Aerosols are liquid droplets or solid particles small enough (0.01 to 100 micrometers) to remain dispersed in the air for extensive periods of time. Gas analysis refers to the analysis of both gases and vapors. The term vapor is used for the gaseous phase of substances that are liquids or solids at standard temperature and pressure. Thus the gaseous states of gasoline, mercury, and water are examples of vapors, whereas methane, hydrogen, and ozone are gases. Many important pollutants are vapors that have arisen from the volatilization or decomposition of organic materials. See also: Aerosol; Air pollution; Industrial health and safety
The qualitative identification of air pollutants can be extremely complex, and may require the use of several instruments which provide complementary information about composition and structure. Since the entire sample is often limited to milligram or microgram quantities, the classical identification methods, such as boiling point and refractive index determinations, functional group tests, combustion analyses, and derivative preparations, have been largely replaced by instrumental methods. Information for identification purposes is now generally obtained from instruments such as mass, nuclearmagnetic resonance, infrared, and ultraviolet spectrometers that rely upon the response of a molecule to an energy probe.
These are identified by means of mass spectroscopy and gas chromatography.
This is probably the single most powerful technique for the qualitative identification of volatile organic compounds, and has been particularly useful in the identification of many environmental contaminants. When a sample is introduced into the mass spectrometer, electron bombardment causes the parent molecule to lose an electron and form a positive ion. Some of the parent ions are also fragmented into characteristic daughter ions, while other ions remain intact. All of the ions are accelerated, separated, and focused on an ion detector by means of either a magnetic field or a quadrupole mass analyzer. Using microgram quantities of pure materials, the mass spectrometer yields information about the molecular weight and the presence of other atoms, such as nitrogen, oxygen, and halogens, within the molecule. In addition, the fragmentation pattern often provides a unique so-called fingerprint of a molecule, allowing positive identification. If the gas is a mixture, interpretation of the mass spectral data is difficult since the fragmentation patterns are superimposed. However, interfacing the mass spectrometer to a gas chromatograph provides an elegant solution to this problem. See also: Mass spectrometry
A gas chromatograph is essentially a highly efficient apparatus for separating a complex mixture into individual components. When a mixture of components is injected into a gas chromatograph equipped with an appropriate column, the components travel down the column at different rates and therefore reach the end of the column at different times. The mass spectrometer located at the end of the column can then analyze each component separately as it leaves the column. In essence, the gas chromatograph allows the mass spectrometer to analyze a complex mixture as a series of pure components. More than 100 compounds have been identified and quantified in automobile exhaust by using a gas chromatograph–mass spectrometer combination. See also: Gas chromatography
A powerful technique for identifying metals is inductively coupled plasma spectrometry. An inductively coupled plasma spectrometer is an emission spectrometer capable of operating at 6000–8000 K (10,000–14,000°F). Emission spectrometers raise the temperature of the sample to a sufficiently high temperature that chemical bonds are broken and the electrons are raised from their stable energy levels to higher energy levels. The excited atoms lose a part of the excess energy by emitting a characteristic wavelength of light. Sensors placed around the plasma can identify and quantify 35 metals in a mixture. See also: Emission spectrochemical analysis
Once a qualitative identification of an important pollutant has been established, further interest often centers on quantifying the levels of the pollutant as a function of time at various sites.
The methods employed chiefly for quantification can be classified for convenience into direct and indirect procedures. Direct-reading instruments are generally portable and may analyze and display their results in a few seconds or minutes, and can operate in a continuous or semicontinuous mode. Indirect methods are those involving collection and storage of a sample for subsequent analysis. Both direct and indirect methods have inherent advantages and disadvantages. By using indirect methods, samples with several pollutants can be simultaneously collected from a number of different sites with relatively inexpensive collection devices and later analyzed at a central laboratory. On the other hand, direct methods may require one instrument for each pollutant at each sampling site, and thus may become prohibitively expensive. However, reduction of the delay before the results are available may be the basis for selecting a direct over an indirect method, or the pollutant in question may not be stable under the conditions of storage. For example, if a worker needs to enter an enclosure with an atmosphere potentially hazardous to life because of an oxygen deficiency, the presence of an explosive mixture, or the presence of a high concentration of a toxic gas, it is essential to have the analytical results available in a few minutes.
These consist of methods utilizing colorimetric indicating devices and instrumental methods.
Three types of direct-reading colorimetric indicators have been utilized: liquid re-agents, chemically treated papers, and glass tubes containing solid chemicals (detector tubes). The simplest of these methods is the detector tube. Detector tubes are constructed by filling a glass tube with silica gel coated with color-forming chemicals. For use, the ends of the sealed tube are broken and a specific volume of air, typically 6 in.3 (100 cm3), is drawn through the tube at a controlled rate. Detector tubes often utilize the same color-forming chemical to detect several different gases, and therefore may be nonspecific for mixtures of these gases. Temperature, humidity, age, and uniformity of packing also influence the performance. Detector tubes for analyzing approximately 400 different gases are commercially available. Accuracy is sometimes low, and detector tubes for only 25 gases meet the National Institute for Occupational Safety and Health (NIOSH) accuracy requirement of ±25%. For some gases, semicontinuous analyzers have been developed that operate by pulling a fixed volume of air through a paper tape impregnated with a color-forming reagent. The intensity of the color is then measured for quantification. Phosgene, arsine, hydrogen sulfide, nitric oxide, chlorine, and toluene diisocyanate have been analyzed by indicating tapes.
With the availability of stable and sensitive electronics, direct-reading instruments capable of measuring gases directly at the parts-per-billion range were developed. Most direct-reading instruments contain a sampling system, electronics for processing signals, a portable power supply, a display system, and a detector. The detector or sensor is a component that is capable of converting some characteristic property of the gas into an electrical signal. While there are dozens of properties for the bases of operation of these detectors, the most sensitive and popular detectors are based on electrical or thermal conductivity, ultraviolet or infrared absorption, mass spectrometry, electron capture, flame ionization, flame photometry, heat of combustion, and chemiluminescence. Many of these detectors respond to the presence of 10−9 g quantities, and even to 10−12 g levels. In addition to improved accuracy, precision, and analysis time, another advantage is that most instruments produce an electrical signal which can be fed into a computer for process control, averaging, and record keeping. Rapid fluctuations and hourly, daily, and yearly averages are readily obtained. Several important instruments are based on the following principles.
1. Heat of combustion. Many portable direct-reading meters for explosive atmospheres are based on the principle of catalytic or controlled combustion on a heated filament. The filament is usually one arm of a Wheatstone bridge circuit. The resulting heat of combustion changes the resistance of the filament (usually platinum), and the resulting imbalance is related to the concentration of the gas. A meter displays the results. This method is nonspecific, and gives an indication of all combustible gases present in the range from about 100 parts per million (ppm) to a few percent.
2. Chemiluminescence. The phenomenon of chemiluminescence is employed for the determination of levels of ozone, oxides of nitrogen, and sulfur compounds. Chemiluminescence is an emissive process that occurs when all or part of the energy of a chemical reaction is released as light rather than heat. A familiar example is the “cold light” of fireflies. Ozone levels in the range from 1 to 100 ppb can be determined by measuring the emission at 585 nanometers, which occurs when ozone is mixed with excess ethylene. Similarly, nitric oxide (NO) levels from 10 ppb to 5000 ppm can be measured by a chemiluminescence method. The analysis of nitrogen dioxide (NO2) is performed by reducing NO2 to NO in a catalytic converter and measuring the NO emission. Mixtures of NO and NO2 can be analyzed by measuring the NO level by chemiluminescence, reducing the NO2 to NO, and again measuring the chemiluminescence level. The NO2 level is obtained by difference. Measurement of the chemiluminescence produced by thermal means in a hydrogen-rich flame allows the detection of sulfur compounds from 1 to 1000 parts per billion (ppb). With the use of appropriate light filters, interferences by other gases can be reduced.
3. Infrared absorption. Measurement of carbon monoxide utilizes a variety of other principles, including infrared absorption at a fixed wavelength, nondispersive infrared absorption, measurement of the heat of reaction in oxidizing carbon monoxide to carbon dioxide, and gas chromatography.
4. Gas chromatography. The gas chromatograph is widely used both for the analysis of collected samples and as a semicontinuous direct-reading instrument. The availability of many different detectors capable of measuring the effluent from the gas-chromatograph column and the development of valves for automatic injection of samples and for directing sample flow have further extended the versatility of this instrument. Accessories are available that can collect samples from about 12 different distant sites through sampling lines for sequential injection directly into a gas chromatograph for analysis. Another application involves the use of valves to inject an air sample into a gas chromatograph equipped with a flame ionization detector to measure the methane content. A second air sample is injected directly into the detector to measure the total hydrocarbon content. The methane signal is subtracted from the total hydrocarbon content to provide an indication of the smog potential.
5. Light scattering. Portable dust monitors using light-scattering principles can directly measure concentrations to 0.01 mg/m3. The sampling period can be varied but is typically about 1 min. The results are displayed as the average value for the sampling period. Some instruments can distinguish between dust particles and fibers. These have had some success in measuring airborne asbestos fibers.
6. Electrochemical sensors. Gases such as oxygen, carbon monoxide, carbon dioxide, sulfur dioxide, and hydrogen sulfide can be measured by using electrochemical sensors. Usually a separate sensor is required for each gas, but several can be combined into a single instrument.
7. Ionization detectors. Flame ionization detectors and photoionization detectors are capable of measuring many organic compounds in the parts-per-million range. These detectors produce ions, and a current is produced in proportion to the type of molecule and the number of the molecules. Flame ionization detectors use a hydrogen flame to produce the ions, and the photoionization detectors use ultraviolet light to produce ions. See also: Chemiluminescence; Light-scattering techniques
For indirect methods, the main collection devices are freeze traps, bubblers, evacuated bulbs, plastic bags, and solid sorbents. Because of their convenience, solid sorbents dominate collection procedures. NIOSH developed a versatile method for industrially important vapors, based on the sorption of the vapors on activated charcoal or, to a lesser extent, on other solid sorbents such as silica gel and porous polymers. Typically, in this technique a few liters of air are pulled through a glass tube containing about 0.004 oz (100 mg) of charcoal. The charcoal tube is only 7 cm × 6 mm (3 in. × 0.2 in.), and has the advantage that it can be placed on the worker's lapel. A battery-operated pump small enough to fit into a shirt pocket is connected by a plastic tube to the collecting device, so that the contaminants are continuously collected from the breathing zone of the worker. Many solvent vapors and gases are efficiently trapped and held on the charcoal. The ends of the sample tube are then capped, and the tube is returned to a laboratory for analysis. In the laboratory the tube is broken open, and the charcoal poured into carbon disulfide to desorb the trapped vapors. Following desorption, a sample of the solution is injected into a gas chromatograph for quantification.
This technique has been highly successful for several classes of compounds, such as aromatics, aliphatics, alcohols, esters, aldehydes, and chlorinated compounds. Sulfur- and nitrogen-containing compounds can also be analyzed by using a gas chromatograph which is equipped with a sulfur- or nitrogen-sensitive detector.
The major applications of gas and atmosphere analysis involve industrial hygiene, indoor air quality, hazardous waste sites and spills, and sampling of ambient air and stationary sources.
The Occupational Safety and Health Administration (OSHA) has established exposure standards for about 600 substances. Because a number of factors affect exposure, sampling at the breathing zone of the worker is preferred. Filters, sorbent tubes, impingers, and passive monitors are used to collect the samples (Fig. 1). Both OSHA and NIOSH have developed a number of sampling and analytical methods (Table 1).
Fig. 1 Collection devices for monitoring air in the workplace. (a) Solid sorbent (charcoal) sampling tube. (b) Midget impinger. (c) Cowled cassette and its filter. (d) Passive monitor, used for nitrogen dioxide. (e) Piston-type detector tube pump.