This article throws light upon the top twenty-three instruments used for monitoring pollutants. Some of the instruments are: 1. Computerized Gas Chromatography or Digital Chromatography 2. Computerized Mass Spectrometry 3. Computerized N.M.R. Spectroscopy 4. Digital pH Meter and Digital Conductivity Meter 5. The Universal Polar Graphic Analyzer 6. Digital Dissolved Oxygen Analyser and Others.
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Instrument # 1. Computerized Gas Chromatography or Digital Chromatography:
In this the peaks are converted into digital form analog-to-digital (A/D) converter before being fed into the computer.
Instrument # 2. Computerized Mass Spectrometry:
Now-a-days we have digital computers in N.M.R. spectroscopy to generate theoretical spectra from given chemical shift and spin-spin coupling constant. With the help of direct reading of shift and coupling constant, the identification of unknown compounds is done very easily.
Instrument # 3. Computerized N.M.R. Spectroscopy:
Now-a-days we have digital computer in N.M.R. spectroscopy to generate theoretical spectra from given chemical shift and spin-spin coupling constants. With the help of direct reading of shift and coupling constant, the identification of unknown compounds is done very easily.
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Instrument # 4. Digital pH Meter and Digital Conductivity Meter:
With the help of these two instruments, we can get the direct readings of pH and conductivity of the solution.
Instrument # 5. The Universal Polar Graphic Analyzer:
With the help of this instrument, we can determine the concentration within 50 second. The model No. 1 4 A is already in the market. Once you insert the sample at a proper place, everything is automatic. The model No. 174 A, EG and G Princeton Applied Research, P.O. Box 2565, Princeton, N.Jo. 8540, U.S.A. is in the market.
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Instrument # 6. Digital Dissolved Oxygen Analyser:
This instrument gives direct reading of the dissolved oxygen present in the sample.
Instrument # 7. Digital Temperature Probe:
The instrument gives reading of the temperature dipped in the solution to be analysed.
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Instrument # 8. Digital Current Metre for Measuring Velocity of Water:
The instrument is used to measure directly the velocity of water when dipped in the river etc.
Instrument # 9. Digital ion Analyzer:
This instrument records the direct concentration and identification of ions present in the sample. The model 945, Tracor Instrument Inc. Texas, U.S.A. is in the market. The company SHIMADZU, Scientific instruments Inc., U.S.A. makes the model No. IP-2A.
Instrument # 10. A Portable Photo Ionization GC for Direct air Analysis:
This is an instrument which permits rapid analysis in the field of multi-component air samples to ppb (parts per billion) level.
Instrument # 11. Video Display Atomic Absorption Spectrophotometers:
The Pye Unicam, a scientific instrument company of Philips makes the said automatic instrument through which we can determine the concentration of 60-70 elements in no time.
The observed increase in the conductivity is proportional to the SO2 concentration in the air if there are no interferences. So with the help of digital instrument, we can read directly the SO2 concentration.
Instrument # 12. Flame Photometric Analyser:
Gas chromatographers will be familiar with the flame photometric detector (FPD) which is used for the analysis of sulphur compounds. In this detector, samples eluting from a GC column are passed through a hydrogen-rich flame.
If any sulphur-containing compounds are present the sulphur is reduced to a diatomic molecule, S2, which is initially in an excited state. On decay to the ground state light is emitted over a wavelength range of 300 to 425 nm, centered at about 394 nm, i.e.
S2* → S2 + hv
The light emitted from the chamber is viewed by a photo multiplier filled with a narrow-band-pass filter.
A schematic diagram of the FPD is shown in Figure 8. This has been incorporated into air monitoring instrumentation for the detection of sulphur.
The first way in which it has been used involves passing sample air directly into the detector as part of the air/fuel mixture. The system then becomes a continuous “total” sulphur monitor. Some degree of specificity can be achieved by the use of appropriate pre-filter (e.g. to remove H2S in the presence of SO2, and vice versa).
Commercial sulphur gas monitors are also available that are essentially custom-built gas chromatographs. Sample injection is by mans of automatic gas sampling valves which are operated at regular intervals. The components of each discrete sample are then separated on an appropriate GC column, to give a continual series of individual chromatograms.
Thus the flame photometric detector can provide a sensitive and selective monitoring system for the sulphur-gases. Its major limitation is the dependence on hydrogen (which fuels the flame), as this introduces strict safety requirements in both the instrument and its installation.
Instrument # 13. Fluorescence Monitors:
Another form of monitor using the principle of florescence has been developed, and this provides a satisfactory alternative to the FPD sulfur analysers. Fluorescence is a process whereby light energy of a given wavelength is absorbed and then re-emitted at a different wavelength, i.e.
AB + hv → AB* → AB + hv’
The change in wavelength occurs because the molecule that is excited remains in that state for some finite period of time (ca 10-8 – 10-4s). This is sufficient for some of the energy to be dissipated in the form of vibration or rotation within the molecule. This results in the emission of light of a lower energy, and hence a longer wavelength.
This phenomenon has been utilized in the development of a monitor for SO2. As shown in Figure 9 UV lamp provides a source of radiation, either continuous or pulsed, which is filtered to admit a narrow band of light into the cell, centred at about 210 nm. The fluorescent radiation is measured at right angles to the incident beam, using a photo multiplier.
Unlike the FPD analyzer, the fluorescence system is specific for SO2. Sample air must be dry and free of dust to avoid fouling of the cell. There is also the potential for interference from fluorescent organic compounds that may be present in the air, but these can also be removed with an appropriate pre-filter.
Instrument # 14. Chemiluminescence Analyser:
The chemiluminescent reaction of nitrogen oxide and ozone provides the basis for the determination of these compounds.
When ozone is added to a gas stream containing nitrogen oxide, the following reactions take place:
NO + O3 → NO2 + O2
NO2 + O2
NO2* → NO2 + hv (> 6000Å)
NO2* + M → NO2 + M (M = gas solvent)
ARNOLDO LIBERTI:
Light emission results when electronically excited NO2* molecules revert to their ground state. To measure NO concentrations, the gas sample is blended with O3 in a flow reactor. The resulting chemiluminescence is monitored through an optical filter by a high-sensitivity photo multiplier positioned at one end of the reactor. The apparatus is shown schematically in Figure 12.
Chemiluminescence’s (CL) emission is a continuum from 0.6 to 3.0 µ.m. In the presence of excess O3, CL intensity is proportional to NO concentration. Greatest sensitivity is obtained at reduced pressure because of quenching effect at higher pressures.
Nevertheless ambient NO concentrations can be measured also at atmospheric pressure. At reduced pressure with a cooled photo multiplier tube this method can detect 0.001 p.p.m. NO.
Response is linear upto 10000 p.p.m. with a linear dynamic range of 107. Because ozone reacts with other atmospheric contaminants to generate CL, a cut-off filter absorbing wavelengths shorter than 600 nm is included in NO monitors. Total oxides of nitrogen (NO + NO2) can also be determined by reducing NO2 to NO with carbon before reacting with ozone.
A real time measurement of NO2 can be realized by means of molecular fluorescence by using laser excitation at 441.6 (He—Cd laser) or at 488 nm (argon ion laser) and photon counting.
The interesting features of this method are the high sensitivity, which is about one part per billion, and the rapidity of the response time, which conduces to ‘instantaneous’ NO2 concentration. By bubbling the incoming air through an oxidizing solution NO is converted to NO2 and determined.
Instrument # 15. Hydrogen Flame Ionization Detector:
The determination of hydrocarbons is carried out by delivering semi- continuously air to a hydrogen flame ionization detector; its response is proportional to the total hydrocarbon content. As methane is a natural air component which is present in fairly high concentration (about 1 p. p.m.), its concentration has to be determined and the value of total hydrocarbon corrected for the methane content.
This is achieved by introducing in air sample into a stripper column through which there is a continuous flow of hydrogen carrier gas. Hydrocarbons heavier than methane are retained and further back flushed, whereas methane and carbon monoxide are passed to a gas chromatographic, column where they are separated, the methane being eluted first.
Alternately, the determination can be carried out by means of a dual detector system. One flame ionization detector, directly fed with the ambient air, measures the total amount of hydrocarbons, and the other, set at the end of a column packed with Porapak Q, measures only methane.
Besides the continuous measurement of total hydrocarbons, it is of major importance to obtain information about various classes of hydrocarbons and also organic contaminants, which have significantly different reactivates and may have a definite impact upon the air quality.
In order to achieve this aim, it is required to trap the organic components of a certain volume of air and to analyse the sample. One of the most efficient systems is the device described by Bruner and co-workers, which consist of a trap, filled with suitable material (graphitized Carbon Black) set in a Dewar flask with liquid nitrogen. After sampling, the trap is connected with a gas chromatographic column and heated up.
The desorbed compounds are directly injected into a chromatographic column where volatile hydrocarbons such as alkanes and olefins with carbon number C2 – C6, usually found in an urban area, are separated. For the determination of heavier compounds a solvent extraction with carbon disulphide is required and with a suitable column a full ‘spectrum’ of the organics present in the air can be obtained.
Instrument # 16. Correlation Spectrometer:
Correlation Spectrometer is one of the spectroscopic techniques that is used with remote and long path sensors where a replica of an absorption or emission spectrum is compared with a dispersed light beam which has passed through the target gas. The basic principle is as follows.
Reflected or scattered radiation from a distant source is collected in a telescope and dispersed through a spectrometer of the grating or prism type. The spectrum of the radiation is projected into an optical mask which carries a photographic replica of the spectrum of the gas being detected.
An oscillating refractor plate or some other suitable means is used to vibrate the spectrum of the gas which has to be analysed across the mask and the output of the photo detector behind the mask is sensed for the presence of a beat signal. If there is a correlation between the incoming radiation and the mask, there will be beat signal as the dispersed radiation vibrates periodically in and out, matching with the mask.
An automatic gain control keeps the average D.C. output of the photo-detector constant so that the amplitude of the beat signal becomes a quantitative measurement of certain pollutants; the lack of beat signal indicates the absence of a certain species (Figure 16).
The principle of the instrument is based on the Lambert—Beer law of absorption (I = I0e-acl). The correlation spectrometer can use as an energy source natural daylight or artificial light; it can be set on aircraft to obtain profiles to pollutants when flights are made across pollution sources such as power stations and large industrial establishments.
Instrument # 17. Differential Infra-Red Spectrometer:
Long-path monitoring can be realized by means of a two-beam differential infra-red spectrometer using a tunable CO2 laser. The use of two beams— one tuned to a prominent line of the absorption spectrum of the desired gas, the other to an adjacent non-absorbing region-eliminates the effect of atmospheric optical turbulence and scintillation, interference from molecules and other scattering.
An urban air pollution monitoring centre may be envisaged as system of lasers emitting radial beams from a central location. Corner reflectors will return the radiation to receivers which determine the differential absorption over two-way path on both wavelength channels.
Another technique frequently proposed for long-path measurements involves Raman scattering of a laser beam. Raman spectra result from inelastic collision of monochromatic photons with molecular species during scattering, resulting in bands or lines of shifted wavelength. This wave-length shift is a function of the molecular vibrational modes and permits unique identification of the molecules.
Instrument # 18. X-Ray Fluorescence Spectroscopy:
When a primary X-ray from an X- ray tube or a radioactive source hits a sample of material, the X-ray can either be absorbed by an atom or scattered through the material. The process in which an X-ray is absorbed by an atom by transferring all its innermost electron is called the “photoelectric effect.”
If the primary X-ray had sufficient energy, electrons are ejected from the inner shells, creating an excited atom with vacancies on inner shells. These vacancies present an unstable condition for the atom.
As the atom returns to its stable condition, electrons from the outer shells are transferred to the inner shells and in the process emit a characteristic X-ray whose energy is the difference between the two binding energies of the corresponding shells.
Because such element has a unique set of energy levels, each element produces X- ray at a unique spectrum of energies, allowing one to identify the element and to measure the elemental composition of a sample . The measuring of compounds in samples by XRF is illustrated in figure 16 and 17.
In XRF spectroscopy, many elements can be measured simultaneously. XRF is non-destructive and requires minimal (or no) sample preparation — the filter is inserted directly into the instrument for anlaysis. This technology is relatively inexpensive. Because quartz filters used in high-volume samplers cause high background XRF analysis. Filters used in the dichotomous samplers are preferable.
X-ray fluorescence spectrometry can be used for all elements with atomic number from 11 (sodium) to 92 (uranium). Typical elemental detection limits for this method range between 2 and 2000 ng m-3. XRF depends on the availability of excellent standard particulate matter standards.
Instrument # 19. Gas Chromatograph Mass Spectrometer (GC-MS):
The GC-MS is an analytical instrument combining a gas chromatograph (GC) and a mass spectrometer (MS). Gas chromatographs have excellent separation capabilities and achieve a very high degree of separation when using a capillary column.
Mass spectrum can be used to determine the molecular structure of a substance from the mass spectrum of the substance. FID and ECD, conventional gas chromatograph detectors, were only able to provide qualification information of the retention time.
Mass spectrometers excel at qualifying components, and when used as a detector of gas chromatograph, the mass spectrum data can also be used at times other than during the retention time, making it possible to perform peak identification and qualification.
A mass spectrometer consists of the ion source, quadrupole mass separator, Ion detectors, and a data-processor. Each of the sample molecules separated by the gas chromatograph is introduced into the ion source. In the ion source, thermo electrons emitted by the filament accelerate to 70eV and collide with the sample molecules (electron impact ionization method, EI).
As a result, an electron is knocked out of the sample molecules, creating positive charged ions (molecular ions). At the same time, energy from the electrons is transferred to the sample molecules, so the molecular ions split, creating positive charged ions. This process is called “fragmentation”, and the decomposed ions that result from this process are called “fragment ions.”
The molecular ions and fragment ions formed in this way are polled from the ion source as an ion bundle and introduced into the quadrupole mass separator. The quadrupole mass separator consists of four metal rods (electrodes) that are parallel in respect to each other.
The ion bundle is introduced into the units in the direction of the long axis. The relative electrodes are charged with a direct current with a 180° phase shift, and the electrodes are also in a high-frequency electric field.
The mass to charge number ratio (m/z) of the ions that pass through the path between the electrodes in the direction of the long axis is dependent on the voltage applied to the electrodes. The ions that can pass through the unit are detected and amplified in the detector. This information is then converted into electric signals that are processed by the data-processor.
There are two methods of measurement used by mass spectrometer, the scanning method and the selective ion monitoring method (SIM method). With the scanning method, a specific mass number range is scanned at set intervals to measure the mass spectrum.
This method is used to obtain total ion chromatograms, mass chromatograms and mass spectrums. With the SIM method, the type of ion to be measured is set in advance of measurement, and this limitation result in improved sensitivity. This method is used primarily to quantify trace components.
Instrument # 20. Fourier Transform Infrared Spectroscopy (FTIR):
FTIR is mainly meant for the detection of air pollutants. Fourier Transform Infrared Spectroscopy (FTIR) can detect and measure both -criteria pollutants and toxic pollutants in ambient air. FTIR can measure more than 120 gaseous pollutants in the ambient air, such as carbon monoxide, sulphur dioxide, and ozone.
FTIR technology can also measure toxic pollutants, such as toluene, benzene, and methanol. Principle of FTIR is based on the fact that every gas has its own “fingerprint,” or absorption spectrum. Sensor FTIR monitors the entire infrared spectrum and reads the different fingerprints of the gases present in the ambient air.
Ambient air is drawn into a sample chamber and a beam of infrared light is passed through it. CO (always present in analyzers) absorbs infrared radiation, and any decrease in the intensity of the beam is due to the presence of CO molecules. This decrease is directly related to the concentration of CO in the air.
A special detector measures the difference in the radiation between this beam and a duplicate beam passing through a reference chamber with no CO present. This difference in intensity is electronically translated into a reading of the CO present in the ambient air, measured in parts per million.
Instrument # 21. Gas Chromatography Flame Ionization Detector (GC-FID/PID):
Gas chromatography (GC) coupled with a flame ionization detector (FID) is employed for qualitative and quantitative determination of volatile organic compounds (VOCs) in air pollution monitoring. The gas chromatograph, or GC, consists of a column, oven and detector.
In the gas chromatograph, a sample goes to the column, separates into individual compounds and proceeds through the hydrogen flame ionization detector. The flame in a flame ionization detector is produced by the combustion of hydrogen and air. The signal from the flame ionization detector is then amplified and output to a display or external device.
When a sample is introduced hydrocarbons are combusted and ionized, releasing electrons. A collector with a polarizing voltage located near the flame attracts the free electrons, producing a current that is proportional to the amount of hydrocarbons in the sample.
Principle of Operation:
It is used for monitoring low-level specific volatile organic compounds such as benzene, toluene, ethylbenzene and xylene. It is particularly well adapted for applications in air quality monitoring. Its metrology is based on the gas chromatographic separation of the compounds of interest combined with a detection achieved by a photo-ionization detector (GC/PID) or FID.
Sampling, analysis and Data Handling are the three main functionalities of PID. The sampling is performed in cyclic mode with two tubes filled with selective sorbents while one tube is collecting sample, the other one is desorbed. This allows the instrument to achieve nearly 100% sampling time coverage.
The analysis is performed, first through a pre-concentration tube interfacing the sampling tubes and the chromatographic column, thus eliminating interferences. The desorbed sample is then injected into a fused silica capillary column for separation.
A controlled temperature gradient oven permits a fast and accurate separation of volatile organic compounds. Compounds are identified by their elution time through the capillary column.
Instrument # 22. The Photometric Sensor:
The photometric sensor is extremely shown in fig. 10. Light from a light emitting diode (:LED) is coupled to a low cost 1 mm diameter optical fibre, which leads the light into the sensor body. An identical fibre occupies the opposite side of the sensor. The interior of the sensor body is an absorbance chamber through which the pore fluid of the soil passes.
A porous plastic membrane prevents soil particles from blocking the light beam whilst allowing pore fluid to pass through. The wavelength of light is chosen to match the maximum absorbance of the pollutant. In this case we have selected an amber LED which emits light at 620 nm and a green dye tracer which absorbs light at this wavelength.
The receiving optical fibre is coupled to a silicon photodiode which is part of a photoelectron system. A logging circuit ensures that the voltage which is output from the detector is a log function of the light intensity detected by the photodiode. By the Beer-Lambert Law [for example Straughan and Walker (1976)] the light absorbed, A, by the dye,
A log I0/I = εCl
where I0 is the pre-sensor light intensity and I is the post-sensor light intensity. This absorbance, A, is a linear function of its concentration, C, and the path length, I, where ϵ is the extinction coefficient which is constant at a given wavelength.
Rearranging the above expression:
log I = – ϵ Cl + log I0. (2)
The electronics are arranged to produce an output voltage which his a log function of the light intensity reaching the detector, so that the output varies linearly with dye concentration. The signal from the detection system is digitized and stored using a Handscope 12 bit data acquisition system (from Tie Pei Engineering, Leeuwarden, The Netherlands) which is resident in a 486 personal computer.
These sensors are calibrated by testing with known concentration of pollutant. The sensor output (volt) versus concentration graphs is linear for both sensors with regression coefficient of around 0.998. The calibration graph for sensor 1 is shown in Fig. 11.
Instrument # 23: Potentiometer Fluoride Anlyser:
Hydrogen fluoride and soluble fluoride particulates can be determined by potentiometer procedure making use of a fluoride electrode as sensing device. Figure 6 shows the scheme of this anslyer. The gas reacts with an adsorbing solution made of a citrate buffer, which ensures constant pH and ionic strength.
From the E.M.F. supplied by a cell consisting of a fluoride-ion electrode and a calomel electrode, the free fluoride concentration of the solution is obtained. The absorbing solution is renewed continuously at a predeterminate rate in order to have the sensor, whose response is a logarithmic function of fluoride concentration, operate in the range where it exhibits a higher sensitivity.
As both the flow of sampled gas and the flow of absorbing solution are kept constant, the measured tension is linearly correlated to the content of fluoride in the sampled gas. The measurement is carried out over a definite period of time selected gas in accordance with the flow of the sampled gas and the absorbing solution, and the measured fluoride is thus the average over the selected sampling time.
The approach of the various instruments described to monitor a pollutant either by laboratory analysis or by anlysers is the same. These instruments can be called ‘point’ sensors as they measure the concentration of the given pollutant a single point. Another approach for monitoring is the ‘remote sensing’.
This term indicates the use of instruments which can provide the average concentration of pollutant in a certain area either by looking at the emissions as they exit at the mouth of a stack or by sampling an optical volume at a point within the plume and conducting a spatially integrated measurement across the diameter of the plume.
Remote sensing can be performed also by means of a ‘long- path sensor’; this term indicates any device, which permits one to measure extended or diffused sources, such as oil refineries and chemical complexes between two points.
Remote and long-path sensors should greatly simplify the monitoring of a certain area and a variety of spectroscopic techniques have been applied for these instruments.