29 Sampling of Aerosol, Air and Water

Prof. K. Maharaj Kumari and Dr. Anita Lakhani

Contents

1. Introduction
2. Aerosol

                       A. Measurement of Aerosol Physical Properties

                     B.Measurement of Aerosol Chemical Composition

3. Gas Sampling
4. Sampling of Water And Wastewater

Introduction

Rapid industrialization and massive increase in population has led to deterioration of environment. Sampling of environmental components is an important aspect of pollution control activities. Sampling of air and water is a systematic long-term measurement program of atmospheric pollutants and water contaminants, generally in specific locations. The primary reasons are:

(1) to determine the concentration of pollutants

(2) to determine the effects of pollutants on quality of life

(3) assess long-term trends of pollutants.

Sampling is the process of making individual measurements of pollutant concentrations. In passive sampling, a pollutant or pollutants is collected by devices that have no moving parts. In dynamic procedures (i.e., grab, intermittent, and continuous sampling), air is drawn through a device at a known rate using an air-moving system. A grab sample is a small volume of polluted air collected in a period of seconds to minutes. In intermittent sampling, air is collected for a distinct interval over which the concentration is integrated. In continuous sampling, real-time data is collected continuously 24 hours/day etc.

Two kinds of sampling instruments are available:

  (1)   methods which collect samples and are analyzed in laboratory, and

(2)   direct reading instruments capable of sampling a volume of air, immediate analysis and display reading. The sample collection principles are:

(a)   Absorption- A pollutant is collected by reaction with an absorbing reagent

(b) Adsorption- A pollutant is collected on a solid surface.

(c) Impaction-Particles are collected on a surface by the use inertial principle.

(d) Filtration- Particles are collected on a fibrous medium as air passes through it.

During environmental sampling, it is also necessary to collect information on qualitative and quantitative data on the local sources of air pollution, topography, population distribution, land use pattern, climatology, etc, depending upon the objectives of the survey or measurement campaign. The location of the sampling site is dependent on the type of monitoring.

Aerosol

The environmental impact, chemical composition, concentration trends, and health effects of airborne particulate matter have been extensively studied and described in the literature. Current sampling methods involve the use of gravimetric filters or impactor devices and a wide variety of light and laser scattering devices. Many of the analytical methods for the determination of chemical composition of airborne particulate matter require sophisticated equipment and/or the use of strict sample preparation techniques. The tasks of sampling and analysis of airborne particulate matter are often complicated by the complexity of particle size, particle interactions, chemical partitioning between gaseous and particulate phase, and interactions with the sampling medium. The health effects of inhaled particulate matter are associated with both the size and shape of the particulate as well as its chemical toxicity.

There are two types of sampling – continuous and time averaged in–situ sampling. Continuous sampling is carried out by automatic sensors, optical or electrochemical, and spectroscopic methods which produce continuous records of concentration values. The specific time-averaged concentration data can then be obtained from continuous records. Time-averaged data can also be obtained by sampling for a short time – i.e. by sampling a known volume of air for the required averaging time. Samples are then analyzed by established physical, chemical, and biological methods for the concentration values which are the effective average over the period of sampling.

Measurement of Aerosol Physical Properties

Integral measurements

Instruments that provide integrals of specified variables over a given size range are often used for aerosol measurement. For example, condensation nucleus counters provide the total number ofparticles larger than a minimum size, and cloud condensation nuclei counters measure the subset of particles that can form cloud droplets when exposed to water vapor at a specified super saturation. Filter samplers are often used to measure total mass concentrations, integrated with respect to both size and time.

Number concentration

Condensation nucleus counters (also referred to as condensation particle counters or Aitken Nuclei Counters: CNCs, CPCs, ANCs) measure the total aerosol number concentration larger than some minimum detectable size. Health effect studies have suggested that particulate health effects may be more sensitive to number concentration than to mass.

Cloud condensation nuclei concentrations Cloud condensation nuclei (CCN) counters measure the concentration of particles that are converted to cloud droplets by condensation of water (i.e., “activated”) at a specified super saturation. CCN concentrations depend on both the aerosol size distribution and the aerosol composition.

Measurement of Aerosol Chemical Composition

Measurements of particulate mass concentrations are important for regulatory and scientific reasons. There are two basic approaches to the measurement of aerosol particles.

Aerosol measurements may be performed by collecting the particles onto a substrate followed by subsequent analysis. The collection surface may be a porous medium, e.g., a filter that passes the air or gas flow but retains all or a fraction of the particles suspended in the incident aerosol flow. Passage through such a porous medium generally has to be at a rather low flow rate and all of the aerosol flow must pass through extract this medium.

Measurement of particle composition typically involves the chemical analysis of deposited particles in a laboratory some time after sample collection. Filters are the most commonly used collection substrates, but a variety of films and foils have been used with impactors to collect size-resolved samples. Sampling times vary with ambient loadings, sampling rates, substrate blanks, and analytical sensitivities but typically vary from several hours in urban areas to a day or more under clean background conditions.

Aerosols are determined by three parameters:

Chemical Composition– What is the chemical makeup of the particulate matter?

Mass Concentration– How much particulate matter is being inhaled by exposed persons?

Size Characteristics– How big are the individual particles?

Off-line measurements (Manual Method)

There are three types of particulate samplers: Filters, Impactors and Direct reading samplers

a)  Filter sampling

The capture of aerosol particles by filters is the most common method of aerosol sampling. Air sample is aspirated through one or more openings in the solid casing of the filter holder. This method is a simple, versatile, and economical means for collecting samples of aerosol particles. A wide spectrum of filters, with markedly different physical and chemical properties, are available commercially for aerosol measurement applications in a variety of filter materials, pore sizes, and collection characteristics, as well as a variety of shapes such as discs and sheets sized to fit commonly available filter holders. Filters widely used for aerosol sampling may be classified according to their structure as fibrous filters, porous-membrane filters, Nuclepore filters, and granular-bed filters. Three types of Membrane filters are – Mixed cellulose ester (MCE),Teflon and Polyvinyl chloride.MCE filters are used for microscopy, particle counting and sizing and Fiber counting (asbestos). PVC filters have the advantage of being stable for gravimetric analysis. They do not absorb water vapor, on the other hand, teflon filters have low tare weight and low mass for gravimetric analysis. Filter cassettes of different diameter are available to place filters of 37mm, 47mm etc.

High Volume Sampler

Particle collection onto a filter is due to the simultaneous action of several collection mechanisms (e.g., diffusion, interception, inertial impaction, gravitational settling, and electrostatic attraction). The relative importance of these different mechanisms depends on the particle size, density, shape, and electrical charge, the filtration flow velocity and the mechanical and electrical properties of the filter.

b) Inertial and gravitational collectors

Each stage of a cascade impactor can be analyzed to determine aerosol mass distributions or to assess chemical composition as a function of particle size. Cascade cyclones are useful under elevated temperature conditions such as those found in stack gas sampling. Aerosol centrifuges and some horizontal elutriators size-fractionate over a continuous size spectrum and are used to determine aerodynamic shape factors for irregularly shaped particles. The choice of sampler depends upon the application and the analyses to be performed. Because traditional impactors do not provide much size resolution for sub micrometer particles, micro-orifice and low-pressure impactors have been developed to obtain smaller cut-point diameters as small as 0.05 μm. Low-pressure impactors resemble ordinary impactors but operate at reduced pressures of 0.05 to 0.4 atm, while micro-orifice impactors operate closer to atmospheric pressure 0.8 to 0.9 atm but employ very small orifices (40 to 200 μm in diameter) and the number of nozzles is large to obtain an adequate flow rate.

Impactors

Impactors are used to classify particles according to aerodynamic diameter. The aerodynamic diameter is defined as the diameter of the unit density sphere having the same settling speed as the particle. In Cascade impactor air passes from one stage to the next to remove particles in discrete size ranges, and is exhausted through an after-filter. Each impactor stage consists of one or more jets followed by a collection plate and successive stages are designed to collect smaller particles. Those particles, which penetrate all the impaction stages are collected by a final absolute filter. The size collected by any individual stage depends upon the jet diameter and the air stream velocity in the jet as well as distance to the impaction surface; therefore, the collection of smaller particles in subsequent stages is achieved by using a combination of: smaller diameter jets, higher jet air velocities, or decreased distance to the impaction surface.

                                                  Cascade Impactor

The major limitation of impactors is particle bounce from the collection surface and re-entrainment because of the continuous blowing of the jet on the particle deposit; however, this is minimized by using a sticky surface on the impaction plate, e.g. numerous types of greases and oils. Impactors may be multistage or single stage and multiple fractions of varying size ranges may be collected. They can be used to determine complete size distribution. Impactors are flow rate dependent and are used for bioaerosol assessment. Mostly area-type measurements are done because high flow rate is required.

Cyclones

Cyclones are commonly used size-selective samplers. They are used to define respirable particle sampling criteria. Cyclone removal mimics respiratory system when operated. Flow rate is critical. Cyclones use centrifugal forces for particle collection and utilize a vertical flow inside a cylindrical or conical chamber. Particles with sufficient inertia are unable to follow the air streamlines and impact onto the cyclone walls. The particles are either retained on the cyclone walls or migrate to the bottom of the cyclone cone, while finer particles remain entrained in the air during the upwards spiral.

Cyclones have several advantages in air sampling including their large capacity for loading and insensitivity to orientation. Furthermore, they have relatively low cost of construction, ease of operation, and are easily maintained. Unlike impactors, they are not subject to errors due to particle bounce and re-entrainment and do not require special collection surface coatings. However, the flow pattern inside cyclones is complex and not easily modeled; thus, it is not easy to predict cyclone performance without reference to empirical correlations.

Impingers

Impingers utilize inertial properties of particles to effect collection and operate much like impactors, except that the sampled air stream jet is immersed in water at the bottom of a flask. The sampled air stream is accelerated in the impinger orifice to velocities of 60 m/s or greater. The air stream exits underneath the liquid surface immediately above an impaction plate or at a specified distance above the bottom of the collection flask. Particles impinge on the plate or flask bottom, stop, and are subsequently retained by the liquid. Impingers are effective for the collection of particles in the 1μm to 20μm size range. The lower size collected depends on jet velocity and diameter. The upper size collected by impingers is limited because large particles cannot follow the air stream into the impinger.

Elutriators

Elutriators use gravitational settling in a laminar flow to separate particles by aerodynamic diameter. They provide segregation for particles greater than 3μm. Two types of elutriators, vertical and horizontal, are available. The vertical elutriator consists of a vertical duct through which air flows slowly upward. Particles whose sedimentation velocity is greater than the duct velocity cannot follow the air flow and settle out. The ability of the device to distinguish particle sizes is reduced by the distribution of velocities in the duct; nonetheless, it is an effective method for removing large particles.

Aerosol centrifuges

Aerosol centrifuges refer to a class of aerosol samplers that spin at high velocity in order to subject particles to large centrifugal force which is used to deposit the particles on the outer edge of an aerosol chamber. Although several centrifuges have been developed, spiral centrifuge is the one that has seen the greatest application. Generally, particles are collected on a foil which lines the outer wall of the channel and is removed for particle analysis after collection where the deposition distance along the foil is directly related to the aerodynamic diameter of the particle. The resolution of the centrifuge depends on the ratio of the aerosol sampler flow rate to the total flow rate and the overall particle classification may be achieved within the range 0.01 < d < 5 μm at sampling flow rates up to about 1 l/min.

Real-time measurement methods

In real-time or direct-reading instruments, sampling and analysis are carried out within the instrument and the property of interest can be obtained almost immediately. Different aerosol properties, such as number concentration, mass concentration, opacity, size distribution, etc. can be measured. The inorganic constituents of PM2.5 (Na+, NH4+, K+, Mg2+, Ca2+,Cl-, NO3-, SO42-) and associated precursor gases (HCl, HNO3 and NH3) are measured by the Ambient Ion Monitor – Ion Chromatograph. The AIM-IC system provides a Continuous, near-real-time, hourly online measurement of the water-soluble inorganic constituents of PM2.5 and precursor gases.

Ambient Ion Monitor – Ion Chromatograph

Gas Sampling

The reduction of pollution and human health damages requires an integrated approach to air and water quality management. An important step in developing a management strategy is to be able to monitor and evaluate air and water quality. A good monitoring and modeling system is essential for policy making suited to the primary objective of protecting human health. Choice of samples and sampling methods are important for the monitoring results to be significant.

Sampling of Gases and Vapors

The term gases and vapors are used synonymously and the methods employed for their monitoring are same. Gases are substances which are non-condensable at room temperature and under ordinary conditions exist in the gaseous state even when present at high concentrations. Vapors are derived from volatile liquids and may condense at high concentrations. A substance in aerosol or particulate form may coexist with its vapor. The methods applied for sampling gases and vapors are however similar.

Sampling Procedures

Gaseous analysis can be conducted by collecting the sample at the site and performing the analysis in the laboratory or by employing direct reading instruments which sample a known volume of air, perform the analysis immediately and display the results visually. In methods involving laboratory analysis of the samples, a known volume of air containing the contaminant of interest is collected over a short time period usually a few seconds to few minutes. Such a sample is called as Grab sample and the results of the analysis are representative of the contaminant concentration at that sampling site at that time. Grab sampling method is usually employed to monitor different phases of a cyclic process or to monitor and determine the peak concentrations of a contaminant emitted from an industrial process and whose concentrations vary with time. Grab sampling can be performed by the use of various gas sampling devices like evacuated flasks, bottles, metallic cylinders, syringes, plastic bags and gas-liquid displacement containers. Grab sampling does not require air flow measurements, unlike the conventional methods involving adsorption or absorption of the gas in liquid and solid mediums. The volume of sample collected is measured directly from the volume of the container or a known volume is withdrawn with the help of syringes and directly injected into the injection port of the analytical instrument. It is however necessary to record the temperature and pressure during the sampling and correct them to standard conditions while reporting. The collection efficiency of grab sampling is generally 100%, however sample decay can occur from sorption or sample decay. The availability of modern instruments makes this method highly sensitive. Vinyl chloride, for example, is measurable in grab samples by gas chromatography at levels well below 1.0 ppm. Grab sampling is not preferred to collect reactive gases like H2S, oxides of nitrogen and sulfur oxides, which can react with the dust particles and moisture in the sample or react with the sealants and material of the grab sampler and alter the chemical composition of the sample. In such cases it is advisable to analyze the sample immediately after collection.

The second method involves collection of an Integrated sample (average or long term) in which a known volume of air is passed through an absorbing or adsorbing media to remove the contaminant/ gas from the air stream. The sampling period may vary from duration of less than an hour to eight hours. Analysis of these samples yield integrated, average or long term exposure levels. Integrated sampling is employed when the concentrations of gaseous contaminant varies over a short duration, or when the concentrations of the contaminant are low and extended sampling durations are required to meet the sensitivity requirements of the analytical method or to assess the exposure levels of a contaminant in work places in order to establish compliance non-compliance with eight hour or time-weighted exposure health standards.

Absorption

In this method the gaseous pollutant is extracted from the air stream by passing it through a liquid collecting phase, whereby the gas molecules either preferentially dissolve in the liquid phase or may chemically react with it. The extent of absorption of a given gaseous contaminant in a particular solvent depends on its solubility and is limited by the equilibrium partial pressure of the vapor over its solution. The efficiency of absorption of a gas into the liquid phase also depends on the contact surface area hence the absorbing devices are designed to offer maximum contact between the gas bubbles and the absorbing media. This is achieved by transforming the air stream to small finely dispersed bubbles with a relatively long travel time through the absorbent. Solubility of a gas in a particular solvent also depends on temperature; lower temperatures favour the absorption of the more volatile constituents.

Four types of absorption devices have been devised to provide efficient absorption: simple gas wash bottles, spiral or helical absorbers, fritted bubblers and glass beaded columns. The selection of the absorber depends upon the solubility and reactivity of the gaseous contaminant to be collected.

Simple gas wash bottles and midget impinger: These are used for collection of non-reactive gases and vapors that are highly soluble in absorbing liquids. They have the advantage of high flow rate capacity upto 30 litres min-1. The collection efficiency can also be improved by using absorbing reagents which can react with the gaseous contaminant and forming a more stable compound. Examples of these are 2,4-toluenediisocyanante (TDI) hydrolyzed by an absorbing reagent solution to a toluenediamine derivative and p,p-diphenylmethane diisocyanate (MDI) hydrolyzed to methylene dianiline. Midget impingers are commonly employed in sampling of gases and vapors in occupational environments. However they can be used to collect general area air samples from a stationary position; they can be hand –held to collect breathing zone samples or they can be attached to a workers clothing for a personal sample. The efficiency of collection can be enhanced by entraining two or more impingers in series. These impingers have the disadvantage of spillage often encountered during field sampling. However, spill proof impingers have been developed and are commercially available.

Spiral Absorbers: These absorbers are used to collect gaseous contaminants that are moderately soluble in the absorbing reagent or react with it slowly. The spiral/helical structure increases the collection efficiency as the air has to travel a spiral or helical path through the liquid which allows a longer residence time within the tube and results in longer contact between the sampled air and the absorbing medium.

Fritted bubblers: These are more efficient absorbing devices commonly used for sampling of gaseous contaminants that are only slightly soluble or reactive with the absorbing medium. These are usually employed for sampling gaseous contaminants in ambient work atmospheres. The air sample is drawn through a sintered or fritted glass bubbler which is submerged in an absorbing solution or reagent. As the sampled air is drawn through the bubbler small bubbles and a heavy froth develops, increasing the surface area and contact time between the gaseous contaminant and the absorbing solution. The bubble size depends upon the nature of the absorbing liquid and diameter of the orifices from which the bubbles emerge. Frits can be fine, coarse and extra coarse depending on the number of orifices per unit area. Coarse frits are used to sample a gaseous contaminant that is appreciably soluble or reactive with the absorbing medium. Frits of medium porosity are used to sample gaseous contaminants that are difficult to collect while fine porosity frits are used to collect highly volatile gaseous substances.

If concentrated solutions of a gaseous contaminant are required then sample collection is performed over columns packed with glass beads. The beads provide a large surface area for collection. This technique has been employed for collection of benzene and other aromatic hydrocarbons.

Adsorption

Adsorption is a common air sampling method used to collect trace quantities of insoluble and non-reactive gases and vapors on solid adsorbents. Adsorption is a surface phenomenon whereby gases are concentrated and bound by intermolecular forces to the adsorbent phase. Under equilibrium conditions at constant temperature the volume of the gas adsorbed is proportional to the partial pressure of the gas in compliance with the Freundlich’s adsorption isotherm. Adsorption also depends on the active surface area of the adsorbent material. Various constituents of an air sample are absorbed in amounts inversely proportional to their volatility. Several solid adsorbents are widely used such as activated carbon (charcoal), silica gel, alumina, molecular sieves, porous polymer beads and gas chromatographic supports.

The method involves drawing a known volume of air at a controlled flow rate through a small tube packed with an appropriate sorbent. As the air passes through the tube, the molecules of the contaminant are adsorbed onto the surface of the sorbent and remain chemically and physically unchanged. The contaminant is then desorbed from the sorbent by a suitable solvent or thermally for subsequent analysis. Thermal desorption has the advantage that the entire sample can be removed from the sorbent and analyzed at once, this increases the sensitivity of the analytical method.

Several factors are to be considered in selecting a suitable adsorbent. A high relative surface area is important to maintain a large contact area for adsorption and allowing maximum space between adsorbent granules for maximum air flow. Silica gel and alumina offer surface areas of 200-600 m2 g-1, activated carbon offers 500-2000m2 g-1, while gas chromatographic columns offer considerably large surface areas per g. The adsorbing properties (adsorption affinity) for polar or non-polar compounds are also important for selecting adsorbent to collect a given gaseous contaminant. The polar adsorbents have more affinity for polar molecules. Polar adsorbents such as silica gel, can therefore be used either for short duration sampling in atmospheres that contain relatively high concentrations of contaminants or in atmospheres that are sufficiently low in moisture content so that the adsorbent does not become saturated with water vapor before the sampling is complete. Silica gel is an amorphous form of silica formed by reacting sodium silicate with sulfuric acid. It is electrically polar and attracts polar molecules to the active sites on its surfaces. The use of silica gel offers the advantage that desorption of contaminants can be easily accomplished, it can effectively be used to collect certain inorganic substances for which charcoal is unsuitable. Charcoal is an amorphous form of carbon formed by burning wood, nutshells, animal bones and other carbonaceous materials. It is electrically non-polar and adsorbs organic vapors and gases in preference to atmospheric moisture. Charcoal has an extensive surface area as large as 930 m2 g-1. In industrial settings, organic solvent vapors are found as mixtures and not in a single pure form. Therefore, air sampling and analytical technique which is capable of collecting, separating, identifying and determining the concentrations of each individual constituent of a mixture is required. Gas chromatographic tubes filled with charcoal are preferred for sampling in such conditions. The organic vapors are adsorbed onto charcoal during sampling. The adsorbed material is desorbed by extracting in CS2 and subsequently analysis is performed with flame ionization or electron capture gas chromatography. A disadvantage of the method is that desorption of the contaminant is not always 100%, and dilution of the sample occurs from solvent extraction resulting in lowered sensitivity and the solvent CS2 is also toxic and inflammable. Further compounds such as O3, NO2, Cl2, H2S react with activated charcoal; hence these cannot be collected by this method. Amines and particularly the aromatic amines like methylaniline, N,N-dimethylaniline, p-nitroaniline, o-nitroaniline are not easily desorbed from charcoal and must be collected with other adsorbents like silica gel.

Adsorbent Tubes

Impregnated Solid Adsorbents

The development of Ion chromatography as an analytical tool for analysis of the ionic forms of gaseous agents led to the development of solid sorbent tubes impregnated with absorbing reagents for the collection and analysis of reactive gases. The sampling with impregnated solid adsorbents involves the collection of a gaseous contaminant in standard solid sorbent tubes impregnated with an appropriate absorbing reagent which changes the collected gas to a more stable ionic form. This is achieved by drawing the air through the sorbent tubes at a controlled flow rate for a known period of time. The sample is then desorbed with an appropriate solvent, followed by ion chromatography analysis.

Impregnated solid sorbent tubes containing have been utilized for SO2 and formaldehyde with efficiency between 95 and 100%. Impregnated solid adsorbents, facilitates the collection and analysisof certain gaseous contaminants which cannot be analyzed by the conventional methods. The sorbent tubes can easily be handled in the field without any kind of spillage.

Passive Dosimeters

Passive dosimeter is a simple sampling device for sampling both organic and inorganic vapors, usually employed in industrial settings. This sampling technique does not require any sampling pump, has a low unit and capital cost. The technique is based on Fick’s first law of diffusion or the permeation properties of the gas to be measured. The dosimeters consist of solid sorbent tubes and the samples can be analysed by gas chromatography. Collection of contaminant gases takes place by two different mechanisms. The gas is first adsorbed into a bed of uncoated solid adsorbent like charcoal or silica gel or the contaminant may react with a chemical coating present on the collection surface. Charcoal is generally used as sorbent for collection of non-polar organics while silica gel is used for polar gases and chemical coatings for reactive gases. The method offers almost 100% collection efficiency as the concentration of the contaminant gas at the boundaries (surface) is almost zero and a gradient of concentration exists in the stagnant air layer. Each gas has a unique diffusion coefficient in air, and due to the gradient of concentration, is transported to the surface with a velocity dependent on ambient concentration. Passive sampling has been used for measurement of ambient SO2 with the lead peroxide candle method. In this method lead peroxide is exposed within a louvred box to ambient air over a month. The SO2 reacts with lead peroxide, forming sulphate, which is measured by turbimetric methods using BaSO4. A major limitation of the method arises from the much lower transference rates of pollutants to the absorption medium. Passive sampling can only be used for long-term sampling or for determination for high pollutant levels.

Condensation Methods

The condensation method is used to collect gaseous contaminants in liquid or solid forms primarily for identification purposes or to sample gaseous contaminants which are difficult to collect by other methods such as sulphur trioxide. The condensation of gases from the atmosphere depends upon cooling the gas stream to temperatures below the boiling or freezing point of the gases to be collected. Samples are collected by drawing the air sample through a single or series of cold traps immersed in dry ice and acetone, liquid air or in liquid nitrogen refrigerant bath system. The collection traps are double walled, with the sampled air passing through the space between the walls. The temperatures used in the condensation device are selected to ensure condensation of the gas by lowering its vapor pressure below 1.33mb to prevent significant evaporation during sampling. Condensation methods have the advantage that the contaminant is preserved in its natural state without any chemical reaction. The method is particularly useful if the concentrations of a given contaminant are low and a highly concentrated sample is required for analysis. A major drawback of the method is that condensation units are bulky and is usually not portable to be applied in field measurements; secondly, unwanted gases and water vapor in the air may also condense out. This is prevented by employing sequential traps at progressively lower temperatures. The condensation systems cannot be employed for unattended operation.

Direct Reading Instruments

Direct reading instruments have the capability of accurately and immediately detect potentially hazardous concentrations of airborne contaminants. Direct reading instruments are of two types. The first type consists of those that produce a color change either in solution or detector (indicator) tubes through which the air sample is drawn or on chemically treated papers exposed to contaminated atmospheres. The second type comprises those that have electronic circuitry and are capable of sampling a volume of air, performing both qualitative and quantitative analysis internally and displaying the results on a dial, digital display, or strip chart recorder or printout.

Direct Reading Instruments with Indicators

Three types of direct reading colorimetric indicator systems are used for the determination of concentrations of gaseous contaminants: liquid reagents, chemically treated papers and detector tubes containing solid supports treated with chemical reagents. All the three systems utilize the chemical properties of contaminant gases to produce a reaction with a color-productive reagent. In instruments utilizing liquid reagents, the reagent solution is carried into the field with the air sampling unit such as impinge or bubbler. This method has been widely used to determine the concentrations of nitrogen oxides in air by passing a known volume of air at a controlled rate through Saltzman’s reagent taken in fritted glass bubbler. The air is passed until a perceptible color change occurs. The concentration of nitrogen dioxide in air is inversely proportional to the time required to produce a perceptible color change which can be conveniently measured. A major disadvantage of this method in the field is that they are inconvenient and bulky to transport.

Papers impregnated with chemical reagents were widely employed in the past to detect presence of toxic gases and vapors in work atmospheres; these are rarely used today because of the availability of more sensitive, sophisticated and accurate methods. Papers impregnated with mercuric bromide for the detection of arsine, lead acetate for detection of H2S, a mixture of o-toluidine and cupric acetate for the detection of HCN and detector tabs for CO. The observed time required for a color change after exposure of a specific paper or detector tab to an agent is an indication of the concentration present.

Detector tubes have been widely used as a convenient and economical tool for the detection of potentially toxic agents. They were first developed for monitoring of CO and H2S. A detector tube unit comprises of a pump, a colorimetric indicator tube which is a sealed glass tube containing a granular material such as silica gel, alumina or pumice impregnated with a chemical reagent that reacts with the contaminant in the air stream as it is drawn through the tube. For sampling and measurement, the two sealed ends of the tube are broken and the specified end of the tube is inserted into the rubber septum of the inlet of the pump and affixed volume of air is drawn at a controlled rate. After a short specified time has been allowed for color development, the concentration is determined. The concentration can be determined by either of the following three ways :

1) by comparing an absolute length of stain produced in the column of indicator gel or a ratio of the stain length to the total gel length against a calibration chart;

2) by comparing a progressive change in color intensity with a chart of color tints; or

3) noting the time required to produce an immediate color change in which the air volume sampled is intended to be inversely proportional to the concentration of the atmospheric contaminant. There are certain limitations of this method:

a) results obtained from matching tube color change with charts of tint is highly subjective among readers

b) results could be erroneous at high or low temperatures due to temperature dependence of the rate of chemical reaction;

c) lack of specificity of the indicator tubes, two or more gases can interfere with each other during the color producing reaction.

Electronic Direct Reading Instruments

Direct reading instruments have electronic sensors utilizing infrared and ultraviolet radiation, flame and photoionization and chemiluminescence capable of detecting and measuring airborne concentrations of gases and vapors in a few seconds. Most of these instruments are equipped with automatic continuous recording devices which generate real time data representing peak exposure concentrations at any point in time as well as time weighted data (averaging concentrations over time from a few seconds or minutes to full 8-hr work periods or longer. Most recently these direct reading instruments have been linked to microcomputers which allow immediate treatment and reduction of a mass of exposure data to a readily usable form on termination of sampling, correction of interferences, and automatic analysis for multi-component mixtures. The operating principles of direct reading instruments are based upon the physical and/or chemical properties of the gaseous agents they detect and quantify. Instruments based on physical properties comprise of an electronic detector or sensor that generates electrical signal in response to a physical phenomena. For example the mercury vapor meter which is based on the absorption of UV light by Hg vapor showing a strong absorption line in the 253.7 mμ region of the UV spectrum. The instrument consists of an absorption chamber with a UV light source located at one end of the chamber and a photosensitive detector/sensor element located on the other. Air is drawn through the instrument’s absorption chamber where UV light is absorbed by Hg vapor present in the air stream. The presence of the Hg vapor reduces the UV radiation reaching the photosensitive detector element in proportion to the concentration present. The change in intensity of UV radiation reaching the photosensitive detector element which is connected to one arm of a Wheatstone bridge creates an unbalanced condition that is detected and displayed on a meter. In instruments based on the chemico-physical principle, the gas or vapor undergoes a chemical reaction and a physical method is used to detect the changes caused by this reaction. Either the consumption of one of the reactants or the production of one of the products is measured. Oxidation reduction reactions are examples of chemico-physical detection methods. The chemical part of the method is the oxidation–reduction reaction; the physical part is the measurement of electrons involved in the process. An example of such an instrument is the Mast Ozone meter in which O3 is used to oxidize KI to molecular I2 and KOH. The free I2 produced reacts with a thin layer of H2 that covers a wire electrode. The reaction consumes both H2 and I2 to yield HI. The removal of H2 from the thin layer allows a polarization current to flow through the wire, regenerating the H2 layer. For each O3 molecule in the sample, two electrons flow in the circuit. The micro coloumb sensor counts these electrons and displays the concentration of O3 in ppm.

Sampling of water and wastewater

A pre-requisite in developing a monitoring plan is to clearly identify the objectives of the monitoring. Water and wastewater monitoring may be undertaken to Gaining an understanding of an aquatic ecosystem and the physical, chemical and biological processes that operate within it,

Review of water quality within specified criteria.

It is also important to consider the number of sites, number of replicates and the frequency of sample collection and amount of sample to be collected in planning a sampling design, to provide confidence in interpretation of results. A poor sampling design or too few samples can result in data that is too variable to reveal an impact, disturbance or trend. Samples should be collected from sites and times that provide a representative sample free from contamination and should not decompose, thus providing an accurate description of the overall quality of the water and wastewater. Sampling sites should also be located in safe and accessible areas, be well mixed to ensure a homogenous sample is collected and be easily identifiable for later sampling and comparable over time.

Two types of sampling techniques are generally adopted for sampling of natural and waste water.

These are grab or spot sampling and composite sampling.

Grab or Spot Samples

Grab samples are discrete samples that are taken at a location to provide a record of the water quality characteristics at that location and time of sampling. They do not provide any information about the concentrations outside that point in time. If grab samples are employed, a high number of samples are required to show the nature of change over time. However, taking manual grab samples is labour intensive and often impractical for long, intensive sampling plans.

Composite samples

A composite sample is a sample consisting of two or more sub-samples mixed together in known proportions. Composite samples may be collected manually by combining grab samples, or by an automatic sampler. Compositing samples increase the temporal and spatial extent of sampling, without increasing the number of samples or sampling and analysis costs. These types of samples are used when the average water quality characteristics are of interest over a given period of time or volume of flow. They are more appropriate than grab samples when the distribution of constituents within the waste stream is random or when the variability within that stream is low. Composite samples are also useful when the determination of loads of constituents is required. However, compositing does have its limitations. A prior knowledge of the stream is required to determine if composite samples are appropriate, which is done by a pilot study of discrete grab samples. Additionally, compositing may mask variability within the waste stream by hiding peak and low concentrations. Composite samples are not appropriate for analytes that degrade during sampling or transport (e.g. dissolved oxygen, chlorine) or for easily contaminated samples such as microbiological sampling.

There are two basic types of composite samples in water sampling:

• time-weighted samples
• flow-weighted samples.

Time-weighted samples are sub-samples of equal volume taken at constant intervals during the sampling period. For example, four samples are taken six hours apart to create a 24-hour composite sample. In flow-weighted sampling, the sub-samples are proportional to the effluent flow or volume during the sampling period. A flow-weighted sample can be created by taking samples at constant intervals but with varying sample volumes that are proportional to the flow at the sampling time; or by taking samples of equal volume that are taken at the time when fixed amounts of effluent have passed the sampling point.

Separate samples must be collected for chemical and biological analysis as the sampling and preservation techniques are different. Moreover for an accurate analysis, it is desirable to allow short-time interval between sampling and analysis. For example temperature, pH and dissolved gases should be determined as quickly as possible preferably in the field. Similarly redox reactions can cause errors in analysis, microbial activity can reduce the phenol and COD values, change the NO3-—NO2-—NH3 or alter the relative proportions of SO42- , cause oxidation of S2-, I- and CN-, reduce Cr(VI) to Cr(III) which precipitates readily, while Na, SiO2 and B may be leached from the glass containers and metals may be adsorbed on the container walls. Colour, odour and turbidity may change with aging of the sample. Hence careful sampling and preservation techniques should be adopted. Table below lists the sampling, preservation and analytical methods along with the permissible values for potable water for some important water quality parameters.

Summary of Sampling, Preservation and Analytical Methods and Permissible Values for Important Water Quality Parameters

Parameter	Sampling Container	Preservation Method	Analytical Method	Permissible value for water potable
pH	Plastic/ Glass	Refrigeration	pH Meter	6.5-8.5
Colour	Plastic/ Glass	Refrigeration	Visual/ Spectrophotometry	25 Hazen unit
Electrical Conductance	Plastic/ Glass	None required	Conductivity Meter	2000 µScm-1
Alkalinity	Plastic/ Glass	Refrigeration	pH  metric Titration	600mg/L
Turbidity	Plastic/ Glass	Refrigeration	Nephelometry	10 NTU
Hardness	Plastic/ Glass	Refrigeration	Titrimetric	600 mg/L
DO BOD	Plastic / Glass	Refrigerate/Store in dark	Winkler’s Method	>6 mg/L 2 mg/L
COD	Plastic / Glass	Acidify and Refrigerate	Titrimetric	10 mg/L
NO3-, NO2-	Plastic / Glass	Filter and Freeze	Ion Selective /Colorimetric (UV)	20, 3 mg/L
NH3	Plastic / Glass	Refrigerate	Ion Selective/ Corimetric.	1.5 mg/L
Organic Nitrogen	Plastic / Glass	Refrigerate	Kjeldahl	100 mg/L as N
F-	Plastic (teflon)	Refrigerate	Ion Selective/ SPADNS	1.5 mg/L
Cl-	Plastic/ Glass	Refrigerate	Argentometric	250mg/L
SO42-	Plastic/Glass	Refrigerate	Turbidimetric	400 mg/L
CN-	Amber glass	Refrigerate	Ion Selective/	0.05 mg/L
Cu	Acid washed Plastic/Glass	Acidify with nitric acid	AAS/ICP	1.5mg/L
Fe	Acid washed Plastic/Glass	Acidify with nitric acid	AAS/ICP	0.3 mg/L
Hg	Glass	Acidify with nitric acid	AAS/ICP (Cold Vapour)	0.001 mg/L
As,  Cr  (as Cr VI)	Amber glass	Acidify with nitric acid	Hydride	0.05 mg/L
Pb	Acid washed Plastic/Glass	Acidify with nitric acid	AAS/ICP	0.1 mg/L
Se, Cd	Acid washed Plastic/Glass	Acidify with nitric acid	Hydride	0.01 mg/L
Anionic  Detergents (as MBAS0	Plastic/Glass	Refrigerate	Colorimetry	0.05 mg/L
Hydrocarbons, Oil and Grease	Glass	Refrigerate	Gas Chromatography	Absent
Pesticides	Glass	Refrigerate	Gas Chromatography	Absent
Phenolic Compounds	Amber glass	Refrigerate	Gas Chromatography	Absent
Total  Coliform (MPN/100mL)	Plastic/Glass	Refrigerate	MPN Method	10

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• Manahan SE: Environmental chemistry, Lewis publishers (2005).
• Roy M. Harrison and Roger Perry, Handbook of air Pollution Analysis, Springer, Netherlands (2011).
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