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
Qualitative
Identification
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.
Organic
compounds
These are identified by means of mass
spectroscopy and gas chromatography.
Mass
spectroscopy
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
Gas
chromatography
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
Metals
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
Quantitative
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.
Methods
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.
Direct
methods
These consist of methods utilizing
colorimetric indicating devices and instrumental methods.
Colorimetric
indicators
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.
Direct-reading
instruments
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
Indirect
methods
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.
Applications
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.
Industrial
hygiene
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.
Indoor air
quality
Complaints about the indoor air quality
became prevalent in the 1970s. NIOSH conducted over 500 investigations and
reported the following causes: building materials contamination (4%),
microbiological contamination (5%), outside contamination (10%), inside
contamination (15%), ventilation inadequate (53%), and unknown (13%).
Ventilation accounted for over 50% of the complaints. In addition to checking
the ventilation system, NIOSH found that the concentration of carbon dioxide
could be used as an indicator of the adequacy of the ventilation. Carbon dioxide
is exhaled by humans; although the level of carbon dioxide is not the cause of
the complaints, it serves as an indicator of the buildup of all air pollutants
emitted by people, such as perfumes and shaving lotions. When the levels of
carbon dioxide increase from the outside levels (250–300 ppm) to above 800 ppm,
complaints about the indoor air quality begin. At 1200 ppm, many people will
complain. See also: Air pollution
Ambient air and stationary source
sampling
In
many sampling situations, regulatory requirements dictate the type of sampling
and analytical methodologies required during ambient air and stationary source
sampling programs (Table 2). However, even in those situations, variables such
as sampling location, sampling time, sampling procedures, gas stream
characteristics, chemical properties, analytical detection limits, analytical
interferences, and test program accuracy and precision requirements must be
evaluated to ensure that applicable quality assurance objectives are satisfied.
Sampling techniques must provide for representative samples with minimal
deterioration and contamination. In addition, the collected sample must be of
sufficient quantity to ensure that the concentration is above the detection
limit of the analytical method.
The procedures employed for ambient air and
source sampling are essentially the same procedures as described for industrial
hygiene. The main differences are that larger volumes of the air are sampled and
higher flow rates are used. One other major difference that is not encountered
in industrial hygiene sampling is isokinetic sampling, which is required for
particulate matter in stacks.
Gas sampling may be accomplished with
evacuated flasks (Fig. 2), bags, canisters, an impinger sampling system, and
sorbent tube sampling techniques. This method is limited by the size of sample
collected and possible nonintegration of sample.
William R. Burg
Michael Glowatz, Jr.
Laurence S. Bernson
Fig. 2 Evacuated-flask sampling
system.
Bibliography
American Conference of
Governmental Industrial Hygienists, Air Sampling Instruments for Evaluation of
Atmospheric Contaminants, 9th ed., 2001
M. J. Boss and D. W. Day,
Air Sampling and Industrial Hygiene Engineering,
2000
National Institute for
Occupational Safety and Health, Manual of Analytical Methods, vols. 1 and 2,
DHHS Pub. 84–100, 1993
G. D. Wight, Fundamentals
of Air Sampling, 1994
E. D. Winegar, Sampling
and Analysis of Airborne Pollutants, 1993
Ali
fazeli=egeology.blogfa.com
| Table
1: Examples of NIOSH analytical methods |
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| Ketones (6, for example,
acetone) | |
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| Gas chromatography-flame ionization
detector | |
| Coconut shell charcoal
tube | |
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| Inorganic acids (6, for example,
hydrofluoric) | |
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| Asbestos and other
fibers | |
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| Mixed cellulose ester
fiber | |
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| Metals (26, for example,
cadmium) | |
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| Inductively coupled plasma-atomic emission
spectroscopy | |
| Mixed cellulose ester
fiber | |
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| Gas chromatography-thermal conductivity
detector | |
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| Gas chromatography-flame ionization
detector | |
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| Ion chromatography-conductivity
detector | |
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| High-performance liquid
chromatography-ultraviolet | |
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| Visible absorption
spectrophotometry | |
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| Gas chromatography-electron capture
detector | |
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| Visible absorption
spectrophotometry | |
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| Organophosphorous
pesticides | |
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| Gas chromatography-flame photometric
detector | |
| Treated solid sorbent for
pesticides | |
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| Particulates, nuisance
dust | |
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| Polyvinyl chloride
filter | |
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| Silica, crystalline,
respirable | |
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| Cyclone and polyvinyl chloride
filter | |
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| Gas chromatography-flame ionization
detector | |
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