15 Automation in environmental pollution monitoring

15

in envirrmmental pollutiol7 m l l n r i n g

A U -

15.1 INTRODUCTION

The environmental deterioration caused by human action in the last few decades i s alarming as demonstrated by the large number of laws aimed at environmental protection passed i n recent years,

so much so that legal historians

have come to call the 1970s "the great era of environmental

law". Among

others, these laws establish the need for extensive measurements intended t o detect the presence and concentration of a large variety of compounds, both organic and inorganic, in a pleiad of different samples. The major thrust behind the passage of new legislation has been the correlation of adverse health or ecological effects with measurements of the presence and concentration of a variety of pollutants. Such measurements are posslbla today thanks t o significant advances in analytical instrumentation and chemistry, electronics and computer science i n the last two decades. However, there is still a wide gap between the development of sophisticated automatic methods of analysis and their application t o routine measurements of environmental pollutants [l]. This is

no doubt the result of the high cost of automation and the lack of awareness of the real potential and limitatlons of these sophisticated methods. The large variety of pollutants that may in principle occur together in a given matrlx, whether gas, liquid o r solld, poses problems requiring speciflc solutions based on particular methods. The selection of a method for the solution of a given problem usually has two goals. The so-called 'target compounds' (TCs), whose concentrations are desired are usually contalned on a list submitted to the analytical laboratory. On the basls of this list, the laboratory optimizes sampling and the measurement method to be used. Detailed chemlcal instrumental procedures are designed t o isolate the target compounds from the sample matrix and known interferences, concentrate and measure them with various all-purpose or selective detectors. Laboratory control standards are usually employed t o establish that isolation does occur and t o optlmlze the procedure f o r maximum recovery of the desired components and minimum interference from other components. The gas chromatographic electron-capture

detector

for

chlorinated

pesticides are

procedures with examples of

the

Automatic methods o f a n a l y s i s

468

optimized

TC

approach.

However,

the

justification

for

the selection

of

a

specific method requires more information than the decision t o measure target compounds. The broad spectrum (BS) approach i s In sharp contrast to the T C approach. The idea behind the former is t o seek a broad spectrum picture of whatever i s present i n a sample as a major o r minor component. This k i n d o f analysis i s not guided by a predetermined list of compounds t o be measured. The BS approach has existed ever since the beginning of chemical analysis. However, the practical and economic p u r s u i t of t h i s goal was often not possible I n the past. The development of computerized gas chromatography/mass spectrometry systems and other similar technologies

has made t h i s goal a feasible and

desirable alternative f o r many types of samples. With the BS approach, sample preparation i s designed t o be as simple as possible t o preclude losses of significant sample components and t o minimize the possibility of sample contamination. The idea is t o divide the sample Into broad classes of compounds and t o apply all-purpose

chromatographic methods f o r the separation of the

compounds i n each group. Literally hundreds of thousands of compounds can be potentially included i n a broad class, b u t it i s usually safe t o assume t h a t only a f e w compounds are present i n each class i n most samples. The most beneficial result of the BS approach is the frequent discovery of significant b u t previous1 y unrecognized pol I utants. I f the decision is made t o use the BS approach, method selection is re-

stricted. Gas chromatography/mass spectrometry may be a viable choice. On the other hand, if the decision is made t o use the T C approach, careful consideration of the sample type is needed before a method is selected. Environmental systems can be divided i n t o three broad categories: Type 1 systems are relatively closed, i.e.

there Is some control of the

e n t r y of components into the system and all components are well deflned. A n example i s the output from a chemical plant t h a t uses raw materials of know composition, processes them according t o a particular procedure and generates we1I-defi ned products and by-products. Type 2 systems are somewhat open i n that e n t r y of new components i s pos-

sible b u t not frequent o r likely, and components are tentatively defined. An example is the output from a d r i n k i n g water treatment plant t h a uses an uncontaminated ground water source and Chlorinates it t o produce a more o r less constant variety of halogenated methanes. Type 3 systems are wide open t o e n t r y of almost anything a t any time and

components are poorly

defined.

An example Is the Mississippi River at St

Louis, MO. Table 15.1 lists the princlpal specles and parameters controlled In the en-

A u t oma t i on i n envi ronment a l pol 1u t i on moni t o r i n g

469

vironmental field, as well as the techniques used f o r t h e i r detection and quantitation, described In detail in the different sections of t h i s chapter,

TABLE 15.1 Species and parameters most frequently determined and techniques most commonly used i n environmental analysis Species/parameter

Technique

Metals: Hg, Pb, Cd, Ca, Hg

Flame and cold-vapour atomic absorption. I C P

Fe, Mn, A 1

atomic emission spectrometry. Graphite furnace. Zeeman e f f e c t . Anodic s t r i p p i n g voltamnetry. X-ray fluorescence.

ISEs. Colorimetry

Colorimetry. ISEs. Laser Raman spectroscopy.

Oxides/Acids :

S o n , NOx, HzS

Fluorimetry. Coulometry. Colorimetry. LIDAR.

Other compounds: NH3,

NHnNHn, O n , C l n

Colorimetry. Chemiluminescence. ISEs.

Oxygen demand:

BOD, TOD, COD

ICP-AES.

Colorimetry.

Organic cwnpounds:

Pesticides, PCBs'.

Gas chromatography. HPLC.

PAHs2, detergents,

sludge digesters, CO2,

CH4

Other parameters: pH, temperature,

Potentiometry. D i g i t a l thermometer. Thermistor

t u r b i d i t y , conduc-

Thermocouple. Turbidimetry. Nephelometry. Con-

tivity

ductimetry

1

Polychlorinated biphenyls

2

.

Polyaromatic hydrocarbons

Automatic methods o f a n a l y s i s

4 70

The availability and preparation of standards is a major problem in the environmental field. There are standards for many relatively stable constituents such as metals, nutrients or parameters such as turbidity or alkalinity t o name but a few. However, there is a demand for standards for dissolved oxygen, suspended solids. chlorlne, biochemical oxygen demand, oils and fats. It would also be a great aid t o have pH standards containing

some of the

organic substances commonly present In water and introducing errors in the determi nations. The analysis of samples at the trace or ultratrace level poses special problems. As a rule, dilute standard solutions are unstable and readily contaminated. I t is therefore essential t o have a variety of independent, accurate standards over a wide range of trace concentrations. A t present, the National Bureau of Standards has only a few Standard Reference Materials (SRMs) suitable for the analysis of plant, animal and soil materials, none of which is usable in the trace or ultratrace range. The analysis for metal ultratraces requires extremely clean laboratories [2].

This, i n turn, requires the use of a carefully filtered air supply. A l l

reagents used must be certifledly ultrapure and their purity be preserved. Deionized water should be carefully controlled to ascertain the absence of contaminating traces -this

requires all technicians t o exercise care to avoid

contamination by their hair, skin, cosmetics, excretions or even the exhaled air. A strict control of the analytical blank [31 Is obviously another must i n this type of analysis t o avold spurious results i n the determination of some trace or ultratrace analytes.

15.2 SAMPLING 15.2.1 General considerations

Sample collection -and

transport and storage-

represents one of the

chlef problems with which the envlronmental analyst Is confronted. The inherent difficulties posed by the analysis of some types of materlals are augmented by the specific problems presented by the environmental field. The sampling operatlon may disturb the target system t o the point of obtaining a flnal sample irrepresentative of the unaltered system. This may not be too much of a problem with atmospheric measurements, but I t can pose serlous difficultles with biospheric samples. Thus, collection of an air sample f o r analy-

sls i s not bound t o cause much dlsturbance t o the surrounding alr; however, the measurement of the glucogen level In a rat’s liver w l l l lnevltably result In a great disturbance t o the animal’s organlsm. These examples are lllustratlve of one of the major features of environmental samples: their heterogene-

Automation i n environmental p o l l u t i o n monitoring

471

ity. The glucogen measurement is t r u l y representative insofar as the entire organ is exposed. Conversely, atmospheric measurements are hardly representative of even a limited volume Inasmuch as the homogeneity of the sampling field

depends on the stratification in the vertical plane and, to a lesser

extent, on the possible variations in the composition i n the horizontal plane. This entails carrying out multiple samplings t o ensure reliable and representative measurements. Time is another key variable in environmental sampling, not only as regards the sampling frequency, which should be high enough to show any changes in the sampled material (e.g. tidal variations in estuary water), but also as regards the interval elapsed between sample collection and analysis, which should be short enough t o ensure that no essential information is lost through the s a m l e deterioration. Time is also a crucial factor when the results of the analysis are the basis for decisions on the envlronment -particularly

the

working environment. The accessibility of the sampling spot i s also highly influencial in the procedure to be employed in collecting samples [4,5]. The pleiad of problems associated with sampllng have led t o the development of speciflc instrumentation for environmental analysis avoiding this preliminary step (e.g.

chemical sensors capable of performing i n s i t u measure-

ments). The most significant developments in this respect have been aimed at the area poslng the greatest problems in sampllng, namely the biosphere.

In

fact, i n v i v o measurements of some parameters such as alkali-metal and alkaline-earth activities, pH, dissolved oxygen, etc. are by now affordable [6-81. 15.2.2 Sample storage

Unfortunately, not many substances remain unaltered after sampllng. The chief causes of deterioration of collected samples are losses of trace elements by adsorption, particle segregation In heterogeneous powders, dehydration, bacterlal growth i n blological samples, decomposition of the sample mat r i x and formation of volatile compounds from trace elements. Most of these problems can be addressed with speclflc techniques which, however, may introduce some contamination. The potential Occurrence of changes In the sample during storage is closely related to the storage temperature and time. Plastlcs -particularly

PTFE and polyethylene [9,10]-

are the best material for stor-

age of the collected samples, which can also be preserved by cryostorage. This reduces the sample activity t o such an extent that deterioration and interaction tact with the container are kept to a minimum. Acidification is a common resource for

sample stabilization;

however,

it may be contra-Indicated

In

speciation studies [ill. The absorption tubes used i n air sampling are sultable for storage of gas samples as described In Section 15.2.5.

Automatic methods o f analysis

4 72

15.2.3 Sample clean-up The nature of environmental samples often requires a clean-up step between sampling and the appllcation of the analytical method. There are a variety of clean-up procedures available, most of which can be implemented i n an automated fashion (see Chapters 3 and 41, in both segmented and unsegmented flowmethods, robotic methods and HPLC. Liquid-liquid [12-141,

filtration

114,151,

dialysis [12,15],

and solid-liquid extraction

evaporation [12,13],

ature precipitation of lipids and column-switching

low-temper-

methods [16] are all clean-

up methodologies of proven efficiency with environmental samples. 15.2.4 Liquid sampling Water and liquid effluent samples possess the major disadvantage of appearIng to be very easy t o obtain compared wlth solid or gaseous materials: a flask tied t o the end of a string and a bottle to collect the sample seem t o be all the equipment required. For comparison, a factory processing 100 t of coal a day w i l l have to devote much endeavour t o organize its appropriate sampling; a faecal water-treatment

plant dealing with 20 OOO t of material daily

would require an individual inspecting the input and output of materlal at certain intervals by using a small flask to collect samples from each stream and pour them into a larger container from which samples would be anaiysed at the end of the day. Alternatlvely, a river inspector would make a monthly inspection along the river, collecting samples that would also be analysed at the end of the day. The results obtained would provide a pollution contour of the r i v e r in question. Taking into account, f o r instance, that the flow-rate of the Thames at Teddington weir is stated t o be 277 200 t / h and that of the Trent at Colwlck is 2 160 OOO t / h [17], ing sampling t o a r i v e r flow-rate.

there is a great difficulty i n relat-

From these examples it follows that (a) the

significance of the analysis Is limited by the suitability of the sampling program and (b) the chief requirement f o r a satisfactory sampling is the representativeness of the collected samples. The automatic sampling of liquids can be carried out w l t h off-line and online samplers. Off-line

assemblies consist essentially of an suction pump, a

serles of flasks and microprocessor allowlng programming of the intervals between successive samplings. An example of this type of set-up i s the porta-

bl e automatic sampler S-4OOO of the Philips Environmental Protection Series, a scheme of which is depicted In Fig. 15.la. The operatlonal sequence involved the following events: (a) The sampling cycle control ( 1 ) Initiates the sequence by means of an internal quartz-crystal-controller

timer with accuracy of better than 0.03% or

an external slgnal. Cycle Intervals can be set between 3.75 min and 24 h. The

Automation in environmental pollution monitoring

4 73

purges the Intake

P

Fig. 15.1 Portable Automatic Sampler S-4000. ( b ) Control system. (Courtesy o f P h i l i p s ) .

( a ) Operational diagram.

Automatic methods o f analysis

4 74

(b)The solenoid valve (3) Inverts the compressor lines t o evacuate the metering chamber (4). The sample is aspirated from (4) into the chamber. (c) Once the sample has been collected, pinch valve ( 8 ) compels the sample t o circulate through the rotary union (9) and pours I t into the sample bottle (10). (d)Pinch valve (2) is closed and the purge of the intake hose is continued. (e) The compressor shuts off, steeping motor (11) advances t o spout t o the next bottle and the system is made ready f o r the next cycle. The control unit (Fig. 15.lb) Is located at the top of the system. The mode switch (1) selects the type of control of the cycle. I n the 'flow'

posi-

tion, the sampling cycle and the timer are controlled by the quartz-crystal clock, intervals being selected through the sample interval switch (6). The sample volume is selected by adjusting the height of the sample used t o introduce the sample into the metering chamber, scaled i n volume units. The bottle advance push-button (3) moves the bottle forward and the manual cycle (4) pushbutton (4) initiates one complete cycle to test the unit. The purge push-button starts the purge cycle and stops it when released. The optional controls for multiple samples (7) and multiple bottles ( 8 ) allow 1-5 placed in a single bottle and up t o four

samples to be

consecutive bottles t o be filled

with the same sample, respectively. On-line sampling systems are simpler than their off-line counterparts and normally consist of a floating piston housed i n a cylinder, generally plastlc, which provldes a flow of adjustable rate. Such a rate should not be too low in order to avoid sedimentation. The plastic cylinder Is used In variable lengths and diameters and is pierced and surrounded by a mesh preventing the entry of large particles that might obstruct the pump. The cylinder is thermally isolated to avoid solar radiation or temperature variations that might result i n erroneous measurements. Whitby e t a l . and off-line

[la]

have designed a sampler that can be used with on-line

systems. It was conceived for the sampling of effluents whose

composition varies with time or which contain solids heavier or lighter than the fluid itself, volatlle solutes or other immiscible liquids. The design is based on the use of a loop of fairly narrow-bore pipework, which is fed from the body of water to be sampled. A mono pump Is used to force the sample continuously

through the

loop (Fig.

15.2a)

at a rate sufficiently

hlgh (500

gal/h) t o promote turbulent flow and also t o prevent the settlement of highdensity sollds by maintaining a high linear flow

velocity.

This stream of

llquid then passes downward through a small closed interceptor chamber which has a limlted retention volume (2-3 L ) and the liquor then flows under gravity

Automation i n environmental p o l l u t i o n monitoring

4 75

a)

L? b)

Inlet

C)

Sample

ocqulslt ion point

I Bi

Return t o loop

PLAN

(a) General layout o f the loop-interceptor system f o r studi e s on a simulated e f f l u e n t . ( 1 ) Thirty-gallon drum holding the eff l u e n t ; ( 2 ) l i g n i n mixer; ( 3 ) mono pump; ( 4 ) outward and return l i n e s o f loop; ( 5 ) interceptor; ( 6 ) sample point. I n the r e a l situations, points A and C are located i n the flow o f l i q u i d e f f l u e n t . (b) I n t e r ceptor vessel (early design). Shaded areas indicate the area i n which suspended solids b u i l d up. ( c ) F i n a l design o f the interceptor vessel. ( 1 ) Swivelling sample-acquisition arm; ( 2 ) K i n e t r o l actuator; ( 3 ) sample delivery tube -represents sample point B. (Reproduced from [18] w i t h permission o f Pergamon Press Ltd).

Fig. 15.2

and i s discharged near the point from which the orlginai sample was drawn. Thls assembly can be used t o promote vigorous agitatlon at the initial sample point, so that the llquld drawn b y the pump is representative o f the -possibly heterogeneous-

effluent stream. The interceptor vessel can be placed a t

a posltion that Is convenient for the coliectlon o f sub-samples and may also be used t o house pH o r ion-selective electrodes t o give a continuous record of the chemical characteristics o f the effluent. The interceptor provides a means

Automatic methods o f a n a l y s i s

4 76

for diverting a sample from the loop system into a suitable sample container. However, b y suitable electronic or pneumatic timlng arrangements, the pneumatically

operated

sampling arm may

be arranged to automatlcally

collect

samples of the following types: (a) snap or grab samples at chosen time intervals;

(b) composite samples;

(c) flow-proportional

samples;

(d) continuous

samples. The arrangement of the loop system is shown i n Fig. 15.2a. The outward loop feeds the interceptor vessel from which samples may be withdrawn (Fig. 15.2b) or which, in improved form (Fig. 15.2~1, permits removal of sample and also insertion of monitoring probes into the continuously

updated

sample. The sample Imp is completed via a pipe which returns the sample close to the pump intake point, thus providing turbulence under real operating conditions. The instruments with on-line sampling units used i n surveillance stations can also include a programmed systert: for emergency samplings, allowing samples to be collected as Soon as sny of the controlled parameters exceeds a preset limit. This aI!ows samples to be obtained in cases of transient or unexpected pol I ution. 15.2.5 A i r sampling

Most of the sampling carrled out in the atmosphere is intended f o r the determination of

pollutants or their

effects i n localized

industrial areas or

areas devoted t o global environmental studies. Only massionally (e.9. measurements of boron [19] or nitrogen oxides in air [20]) are these studles aimed at the Identification of the geochemical cycle of some species i n the atmosphere. Systems for sampling of airborne pollutants usually conslsts of three parts: (a) a means of collecting the air sample; (b) a devlce to trap the pollutant; and (c) a means of measuring the amount of air sampled. The sampling methods frequently used for this purpose are sedimentation [21,22], tion

[23],

[24,25],

impaction,

filtration

the commonest of

and

thermal

or

which are flltration,

electrostatic

centrifugaprecipitation

impaction and electrostatic

precipitation. The size of the collected particles depends on the partlcular method used. Filtration Is the sampling method most frequently used in industrial hygiene work (personal monitors) on account of i t s operational simplicity. A filt e r assembly typically conslsts of a sampllng head and a f l l t e r (passive atmospheric samplers) or a sampling head, f i l t e r and pump (actlve samplers), ali of which are constructed In a materlal introducing no contaminatlon i n the samples. The different types of filters (depth, membrane) and materials (glass fibre, quartz, cellulose esters, PFTE, sllver) used can be readlly adapted t o the unknown analyte.

A utoma ti o n i n en v i rontnent a l pol 1uti o n moni t o r i n g

477

The control of working atmospheres requlres a special type of sampling. I n fact, the level of pollutants t o whlch the individual workers are exposed is more representative than that found in the working place. Obviously, in a petrol station, the individual actually dispensing the fuel w i l l be more exposed t o organic vapours than the cashier, even if the latter is only a few metres away from the former. A common method for sampling i n these 'microenvironments" requires the workers t o carry a badge wlth adsorptive material -typically a charcoal disc pressed into a f i l t e r backlng-

during the working hours

(passive sampling), at the end of which the badge is sent t o the laboratory, where carbon is extracted with an organic solvent (generally CSz) and an aliquot i s injected into a gas chromatograph f o r analysis. This simple method has some disadvantages: (a) the uptake rate of the disc when worn w i l l be affected by the air flow across it and it w i l l perform p w r l y i n still air; (b) the most effective solvents are themselves a health and safety hazard; and (c) the many handling and extraction steps Involved do not lend themselves to a fully automatic system of analysis. Perkin-Elmer have developed a sampler that can be used both for the adsorption of samples by diffusion -as

a badge sampler-

and f o r the introduction

of a controlled alr stream by means of a portable battery-operated suction pump, which allows for spot sampling or the measurement of very low pollutant concentrations.

The sampler is subsequently heated i n a flow of carrier gas

and the vapours are fed directly into a gas chromatographlc column. The monit o r consists of

stainless steel tubes (90 mm x 5.0 mm I.D.)

(Fig. 15.3) that

can be readily removed with a clip. The tubes are supplied with press-on storage caps and gauzes which retain the sample inside f o r months if necessary. I n the sampling mode, the tube Is fitted with a diffusion cap constructed so that the diffusion path length is set precisely t o 15 mm. Diffusion caps are available with semipermeable membranes t o minimize water uptake during sampling, without affectlng component diffusivity. These caps have a gold-anodized

finish for easy ldentlflcation and are also available

membranes f o r non-moisture sensltlve materials -these analysis Is carried out on the Perkln-Elmer

without

have a silver cap. The

Automatic Thermal Desorption

System (ATD 50) and then analytlcal end caps are fltted. These malntaln the sample sealed In the ATD unit and enable the sample t o be dispensed t o the chromatograph without loss o r contamlnatlon. The ATD 50 automatlcally processes up t o 50 sample tubes wlthout attention. Complete control o f all desorptlon parameters coupled with an extensive systems check before each tube Is desorbed ensures that each sample Is subjected t o an optlrnum desorptlon cycle and cannot be lost due t o system o r tube fallure. The desorbed sample passes along a temperature-controlled transfer line which may be attached t o any gas

Automatic methods o f analysis

4 78

chromatograph

or vapour-phase analytical system. An interesting feature of

these tubes i s their reusability.

'0' R\ING

'0'R I N G M E M B l? A N E

STAINLESS STEEL GAUZE

/

ADSORBENT

CAP

N

S T A I N L E S S S T E E L TUBE

\

CAP

GAUZE

Fig. 15.3 Re-usable sampling tube (active or passive). Analyses are carried out on an Automatic Thermal Desorption System f i t t e d t o a gas chromatograph. (Courtesy o f Perkin-Elmer Ltd.).

A pump i s usually employed t o transport the air sample along the sampling

device, where the air stream Impinges on a solid surface (Impactors or adsorption tubes) or a solution (impingers) where the particles or gases settle. The solid use for impaction or adsorptlon and the absorbing solution are suited t o the type of compound t o be determined and t o the nature of the sampled atmosphere. Electrostatic precipitation is specific f o r suspended partlcles and is thus widely used in this type of sampllng 126,271. The collection of gas samples, both organic and Inorganic, f o r analytical purposes has been approached by various authors. Thus, Smith and Murdcck [30] have developed a devlce capable of simultaneously collecting eight equivalent particulate diesel exhaust samples t o study the effect of the type o f filter, extraction solvent and storage conditions used on the measurement of the concentration of five polyhalogenated hydrocarbons. The devlce consists o f a conlcal sampling chamber (18.5 cm diameter, 15 cm height) on whlch ere placed eight cassettes of two 37-mm pieces each, a vacuum manifold containing the critical orifices which control the flow through the 37-mm cassettes, and a sample line f o r attaching the conical sampling chamber t o the diesel exhaust dilution chamber. A 16.5-mm

diameter by 26.5-cm

long Plexiglas6 tube i s used

t o support the sampling chamber and t o separate It from the manifold. A threaded rod wlth wlng nuts Is used t o hold the sampllng chamber, vacuum manlfold and Piexiglass tube rigidly together. The sampling line, a 2.54-cm

Automation i n environment a1 pol 1u t i o n moni t o r i n g

a)

4MPLE LINE

CASSETTES

\

\

SIDE VIEW

37rnm

CASSETTES

(8)

LEFT

SIDE

T O P V I E W ( w i t h top o f f ) Fig. 15.4 Multi-sampler f o r collection o f polyhalogenated hydrocarbons i n diesel exhaust. (a) Side view. (b) Top view. (Reproduced from [30] with permission o f Marcel Dekker, I n c . ) .

4 79

Automatic methods o f analysis

480 aluminium tube

is inserted into the diesel exhaust dilution

chamber.

The

37-mm cassettes are arranged i n the sampling chamber as shown in Fig. 15.4a. The pattern is divided into a r i g h t and a l e f t section, and each position is numbered individually (Fig.

15.4b).

Slmilar studies of collection

efficiency,

blank values and elutlon efficiency of lead sprays collected over four d i f f e r ent solid sorbent packing sampling tubes (silica gel, alumina, Chromosorb and Tenax-GC) in various pore sizes, which showed the suitability of Tenax-GC, have been recently reported [311. Tenax-GC was also used by Lewis et al. [321 to develop a passive sampler for short-term, flow-level air monitoring applications. The small, stainless steel device is straightforward and inexpensive and has a high equivalent sampllng rate, is reusable and rechargeable and is conceived for thermal desorptlon. I t s performance was scrutinized under controlled test chamber atmospheres and in actual outdoor and indoor situations. Sampling rates were calculated f o r several volatile organic chemicals. This design

excels

in

performance

over

other

commercially

available

passive

monitors. Sample collection as a means of avoiding a given interference and determining other analytes has materialized in the development of a K I ring denuder for the collection of N02 [33] as a means of separating this oxide prior to the sampling of airborne particulate matter on filter media such as polycyclic aromatic hydrocarbons liable to undergo oxidation or nitration in the filtration step [34,351. A representative example on account of i t s capacity and performance is the

commercial on-line

gas sampler DS 210 dry-alr-operated

sample-condltioning

probe manufactured by Columbia Scientific Industries. This sampler effects i n a precise manner all the functions involved i n the preparation of the in-stack sample f o r transport and measurement. The sample i s f i r s t filtered and metered to an exact volume by a critical orifice. The measured sample is then diluted with dry eduction air, reducing the relative humldity t o a dew point below that of the ambient operating temperature. A vacuum is drawn on the stack continuously through a f i l t e r and a critical orifice by a newly developed small ejector pump which Is mounted inslde a stack probe. The main air stream (pressurized air) wlth an adjustable flow creates a vacuum I n the space between the primary and secondary nozzle of the ejector pump (Fig. 15.5). The vacuum is used to transport the stack gas through a critical orifice mounted inslde the stack

probe.

The crltlcal orifice determines the stack sample flow

at all

pressures below the critical value. Figure 15.5 shows a detail of the construction of the ejector pump, which Is mounted inside the stack probe inside a steel tube of highly corrosion-resistant steel which forms part of the stack probe. Between the ejector and this outer steel tube a heat exchanger serves to preheat the dilution alr before entering the pump so as to compensate for

Automat i o n i n environmental pol 1u t i o n moni t o r i n g

48 1

changes i n the dilution ratio at v a r y i n g temperatures of the dilution air. The second p a r t of the probe consists of a corrosion-resistant steel mantle which i s screwed on t o the ejector pump end. On the f r o n t end of t h i s mantle is mounted a coarse filter. Inside t h i s ’ p a r t of the probe is mounted the critical orifice with a fine quartz filter.

Both can be exchanged f o r other sample

flows. The CL/PA (calibration/purge a i r ) connection o f the probe consists of a 1/8-in steel tube which ends i n the f r o n t compartment. This tube is used t o supply the critical orifice inlet with calibration gas supplied via the umbilical cord o r t o apply p u r g e a i r at a high flow-rate t o clean the coarse filter. The air stream blowing i n t o the stack removes the dust particles remaining a t the outside of the coarse filter. The diluted sample volume i s approxlmately 5 L o r more -sufficient

t o meet the sample needs (simultaneously) f o r several

analytes, such as total sulphur, SOz, NOx, C02 and hydrocarbons.

,BORE

DILUTION A I R 01 TO VACUUM GAUGE

LITRESiMIY

SE CQNDARY

/ / S T A C K GAS

CRISICAL ORIFICE

C A L t B R A T I 9 N LINE

C A L LINE

Fig. 15.5 Cross-sections of the DS 210 dry-air-operated sample-condit i o n i n g probe. (Courtesy of Columbia S c i e n t i f i c I n d u s t r i e s ) .

15.3 WATER ANALYSERS

Water, the universal solvent, i s everywhere around us. Whlle additives can actually improve water quality f o r some uses, water impurities can cause corrosion and fouling of equipment and be a source of disease and pollution. Automated analysis i s the key t o solving critical water quality problems. Fast detection and correction of abnormalities are also important steps towards c u t t i n g treatment costs and keeping a plant i n compliance w l t h regulatory stat Utes. Table 15.2 lists the different types of Water most CmmOnlY used by humans and the processes typically applied t o t h e i r purification. The parameters t o

TABLE 15.2

Types o f water, f e a t u r e s , processes t o which they a r e subjected and parameters determined

Type o f water Untreated

Features

Process

Parameters

Ground: h i g h 02 and sediment contents Deep: h i g h COZ and s o l i d contents; h i g h hardness Varying t u r b i d i t y which determines treatment

Coagulation, c l a r i f i c a t i o n (chlorination)

pH, t u r b i d i t y ,

Chlorination (fluorination)

Process

One-quarter o f t o t a l i n d u s t r i a l water

Chlorination

Feed

Higher b o i l e r pressure. Higher puri t y required

Elimination o f corrosivity and contaminants pH/phosphate program, chel a t i n g agents, 0 2 t r a p p e r s

Transparency, t a s t e , odour, hardness, b a c t e r i o l o g i c a l act i v i t y (Fe, Mn) pH, hardness, c o r r o s i v i t y , l i t m u s f o r m a t i o n (Fe, Mn) Variable

Condensation

Drainage balance: s e c u r i t y s o l i d l e v e l s / h e a t and chemical l o s s e s Dependent on t h e c o n t r o l r e s u l t s

Drainage-recovery o f chemical levels Variable

Cooling

Recirculated

Waste

Non-recirculated Two c o n t r o l l i n e s : i n f l u e n t and effluent

Acidification, addition of phosphates, chromates and molybdates Chlorination Variable

Drinking

Boiler Purge

chlorine

pH, Ca, Mg, Si032-, (S032-, N 2 H 4 , c h e l a t i n g agents, phosphate, e t c . ) S i O z , N 2H 4, phosphates, pH, conductivity hardness, c o n d u c t i v i t y , t u r b i d i t y , Na+, Si032Hardness, pH, a d d i t i v e s

pH, c o n d u c t i v i t y , phosphates, c h l o r i n e , d i s s o l v e d 0 2 , alkalinity

Automation in environmental pollution monitoring

483

be controlled in these waters (shown i n parentheses) are determined partly by

the nature of the water and partly by the treatment to whlch it has been subjected. There are other types of water and determinations not included in the table than require special solutions and are beyond the scope of this chapter. Nevertheless, some standard practices such as the determination of oxygen in its different possible forms and the speciation of elements are worth considering here. The large-scale surveiliance of waterways as the starting point for the accomplishment of satisfactory levels o f quality i n drinking and industrial waters and the preservation of aquatic fauna Is of utmost importance. Automated water surveillance stations are no doubt the best alternative for the realization of this task. 15.3.1 Off-line water analysers

The range of off-line instruments available for water analysis is wide. I n fact, any analyser with optical o r electrochemical detection can be adapted for this purpose. The use of llquid chromatography for the detectlon and quantitation of detergents or non-volatile organic compounds, of atomic absorptlon spectrometry f o r the analysis for heavy metal traces and of UV spectrophotometry for the determination of phosphates, nitrates and nitrites are representative examples of

the

potential

utilization

of

conventional

analysers for

water analysis. A common practice i n water analysis is the indirect measurement of pollu-

tion from organic matter carried out by means of autoanalysers based on different principles. The PW 9525 total oxygen demand (TOD) meter manufactured by Philips allows the measurement of TOD i n about 2.5 min by burning all the oxidlzable matter and expressing the amount of oxygen consumed i n mg Oz/L. As can be seen from Fig. 15.6, the instrument consists of three essentlal parts: (a) the oxygen dosing and measurlng system (two ZrCh cells); ( b ) the combustlon system (an oven heated at 900'C); and (c) the injection system. A carrler gas (N2) flows through the f l r s t Zr02 cell, which adds oxygen to the nitrogen stream at a constant rate, thereby providing a fixed 02 concentration between 35 and 4OOO ppm. The gas mixture is passed through an oven heated at 9oo'C and contalning a special combustion tube furnished wlth a platinum catalyst. The gas Is then passed through the second ZrOz cell, whlch has a measuring and dosing function. When a known volume of sample Is injected into the oven, the water evaporates and the oxldlzable matter is burnt by the oxygen In the &/Nz

mixture.

The decrease in the 02 concentration In the gas stream resulting from the combustion In the oven Is detected and compensated for by the second cell. The

Automatic methods o f a n a l y s i s

484

amount of oxygen required t o restore the original 02 level i s the sought TOD of the injected sample. The two ZrOn cells are no doubt the key p a r t s of the instrument. They are fitted with two internal porous P t electrodes and two external ring-shaped

electrodes of the same material. The cells are heated at

about 600"C, at which ZrOn i s a solid electrolyte. As an electrical c u r r e n t passes from the inner t o the outer electrodes, the oxygen is transported t h r o u g h the wall of the zirconium tube. The amount of oxygen transported i s proportional t o the magnitude of the current. The oxygen concentration can be measured

by

using

another

property

of

electrolytlc

zirconium,

v i z . the

difference between the On concentration inside and outside the zirconium tube results

in

a

voltage

across

the

electrodes

dependent

on

the

partial

On

pressure inside and outside the tube. I n the f i r s t zirconium tube, the On is dosed i n the carrier gas, while the second takes advantage of the two abovementioned properties of Zr@. The change in the p02 is measured as the voltage drop across the measuring electrodes.

Deviations from the preset p02 are

compensated for by a feedback system that doses o r withdraws the carrier gas in the second cell until the original concentration i s restored. required f o r the dosing

Electronic measuring a n d feedback

i s directly Constant

The c u r r e n t

proportional t o the amount of oxygen

Platinum catalyst Combustion oven

Fig. 15.6 Scheme o f the PW (Courtesy o f P h i l i p s ) .

9525

total

oxygen demand

(TOD) meter.

Automation i n environmental p o l l u t i o n m o n i t o r i n g

485

consumed i n the combustion process. The instrument normally operates with manual injection of discrete samples, although it can be automated by means of a pump and an automatic injection device. Another way of expressing the amount of chemically oxidizable matter is the so-called ’chemical oxygen demand’ (COD), measurement of which can be carried out in an automatic fashion with the Aqua rator marketed by Precision Scientific. This instrument measures COD by causing organic materials to be oxidized on a platinum catalyst at 9OO’C by reduction of COz to CO, yielding a mixture of both gases equalling the number of oxygen atoms required f o r the chemical ox y dat ion:

Hence, the CO readouts of the instrument are proportional to the COD and are safer when the number of oxygen atoms in the organic compound is equal to onehalf of the number of hydrogen atoms. The samples (20 pL) are injected on t o the catalyst and the oxidation products are carried along the Instrument by means of a purified COz stream. The Metrohm 676 COD sample changer is based on the standard procedure for measurement of the amount of KzCr207 required t o oxidize the oxidizable matter present i n 1 L of water, so that the water sample is treated with excess Cr(V1) i n a concentrated sulphurlc acid medium containing AgzSO4 as catalyst and heated at 175’C. The excess of Cr(V1) is determined by back-titration with Fe(I1). The end-point i s detected potentiometrically by means of a metal -ideally gold-

electrode. The tltrated solution Is removed by suction and the ti-

tration vessel is washed with distilled water t o make lt ready for a new sample from the sampler, capable of holding up t o sixteen pretreated samples. Cnobloch e t a l . [36] have designed a relatively simple instrument f o r the analysls of groups of metal ions in natural and waste water based on electrolytic preconcentratlon and subsequent couiometric determination. The scheme of the instrument Is shown i n Fig. 15.7. The measuring cell contains two platinum electrodes and a s t i r r e r whlch creates a thin, well-defined

diffusion layer

around the electrode, resulting in large response signals and hence in high sensitlvity

and

reproducibility.

The

operational

sequence

Is

as

follows.

First, the measuring cell is filled with the waste water sample (pH 3.5); the metals are deposited on the cathode on application of a constant d.c. Voltage and the cell Is emptied. The cell i s then filled with pure electrolyte (buffer solution of pH 3.5) and the metals are passed on t o the solution by applying an Inverse d.c.

voltage. The electric charge consumed is a measure of the

metal concentration In the waste water. The oxidation process Is carried out

Automatic methods o f a n a l y s i s

486

in pure water t o avoid false response signals yielded by other oxidizable substances -particularly

organic matter-

potentially present i n the sample.

The polarity of the electrodes i s reversed after each measurement, thereby ensuring their thorough washing. The number of metal ions that can be detected can be varied by selecting the appropriate deposition voltage, and so can the detection l i m i t by changing the deposition time.

The method developed by

Cnobloch e t a l . is not intended for the accurate determination

of

heavy

metals, but for that of groups. The apparatus acts chiefly as a control activating an alarm when a preset limit is exceeded. The plant operator is then in a position t o take immediate counter-steps to avoid greater perturbations. The method does not replace other accurate methods for the determination of heavy metals in water; however, it limits the number of analyses to be carried out to those instances where critical values are reached or surDassed.

sample collector

Fig. 15.7 Schematic diagram o f a heavy m e t a l ion duced from [361 with permission o f Elsevier).

The automatic off-line

detector. (Repro-

speciation of elements i n water has been imple-

mented in a straightforward manner by flow-injection analysis with the aid of configurations adapted t o the characteristics of the systems under study. Chromium has been speciated upon reaction between Cr(V1) and 1,5-dlphenylcarbazide (DPC), which yields a coloured compound absorbing at 540 nm and f r e e from interference from Cr(II1). The manlfolds used for this purpose allow the simultaneous o r sequential determination of Cr(V1) and total chromlum after oxidation of Cr(II1) with Ce(1V) by use of a single- or dual-beam spectrophotometer. The configuration i n Fig. 1 5 . a uses a single sample injection that is split Into two channels reaching the sample and reference flow-cell,

respec-

Automation i n environmental p o l l u t i o n monitoring

tively, of a dual-beam

spectrophotometer

(Fig.

48 7

15.8al).

The upper channel

merges with a DPC c u r r e n t and yields a signal proportional t o the Cr(V1) concentration i n the sample upon passage t h r o u g h the flow-cell. The lower channel also merges with a Ce(1V) solution

which oxidizes Cr(II1) t o Cr(V1) after

merging with an indicator reagent; hence the signal obtained from the cell corresponds t o the contribution of the two oxidation states Initially present i n the sample. A similar configuration with two flow-cells aligned with the l i g h t path of a single-beam

spectrophotometer i s shown In Fig. 15.8a2,

while Fig.

15.8a3 depicts a configuratlon with merging p r i o r t o a single flow-cell,

The

sequential speciation of chromium can be carried o u t with the aid of a selecti n g valve (V2) which allows o r prevents the passage of a Ce(1V) stream, thereby facilitating the determination o f Cr(V1) o r total chromium by using two sample injections (Fig.

15.8b).

All

these configurations o f f e r

good mixture

resolution with excellent reproducibility and determination ranges, which show the effectiveness of the methods used [37]. A configuration similar t o t h a t i n

a) DPC WATER SAMPLE

Hz

so4

ox

DPC

TZ 1.I5 1.33

0.59

q ( m L . min-1)

b) DPC ox HzS04 WAT E R SAMPL

lw

F I A configurations f o r the o f f - l i n e speciation o f chromium. (a) With a s p l i t t i n g p o i n t ( A ) and a dual-beam detector (a.1): w i t h two c e l l s aligned w i t h the l i g h t path (a.2); w i t h a merging p o i n t p r i o r t o a s i n g l e f l o w - c e l l (a.3). (b) Sequential determination w i t h the a i d o f a selecting valve. DPC: diphenylcarbazide; Ox: oxidant [Ce(IV)]; q: flow-rate; V i : i n j e c t i o n valve: L and 0: reactor length and inner diameter; V 2 : s e l e c t i n g valve; W: waste. (Reproduced from [37] w i t h permission o f Elsevier).

Fig. 15.8

Automatic methods of a n a l y s i s

488

Fig. 15.8b has been used for the speciation or arsenlc (as arsenite and arsenate) by use of the Molybdenum Blue reaction after complexation of A s ( V ) with Mo(V1). A KIOJ solution is used to oxidize As@- in this case. The calibration garphs are linear in the range 106-10-4M for both ions and mixtures in ratios up to 20:l can be readily resolved [381. 15.3.2 On-llne water analysers

This type of system features a number of advantages over off-line analysers, namely sampling Is more representative, the risk of contamination by the containers or changes in the sample composition by storage is minimal and the evolution of the system under study can be continuously monltored, which results in more useful information.

15.3.2.1 Single-parameter analysers There are a host of commercially instruments available for the monitoring of a single parameter in both industrial and domestic water. All these instruments are very similar and only differ signiflcantly

in the sensing system

used, which is suited to the analyte (parameter) to be determined. Colorimetric analysers. The generic scheme of this type of instrument

IS

depicted i n Fig. 15.9a. The water sample i s introduced into the analyser b y means of the sample pump and mixed with a measured amount of reagent which

IS

introduced by its corresponding pump. The mixture then flows along the system to the cell, where the colour development Is monitored. The cell is the cylinder of the photometer pump and from i t the sample-reagent mixture Is pumped through some delay loops around the mixing spool and out of the drain. As the volumes delivered by the sample and reagent pumps are smaller than the capacity of the photometer pump, some of the solution expelled by the photometer piston’s previous stroke Is drawn back into the sample cell. A pinch valve assembly functions to control the direction of the flow through the analyser. This is accomplished by opening and closing tubes at the appropriate times i n relation to movements of the pump pistons. A flush system prevents the buildup of reagent crystals In the upper part of the photometer block cylinder and piston. Demineralized water, placed i n the reagent compartment, is pumped through the area of the sample cell that is above the photometer piston seal, rinsing the area with each pump cycle (Fig. 15.9b). Also rinsed are the front sectlons of the other pumps where the flush system water carries away any salt that would Increase the wear of the seal. Two flush pumps are mounted on the pump panel t o provide the energy t o move the demineralized water through the system. The inlet flush pump on the left side of the panel moves the rinse water to the point where It enters the Photometer block and the outlet pump of the r i g h t draws It from the photometer block and pumps It out of the draln.

489

Automation in environmental pollution monitoring

a)

DRAIN

STANDARD SAMPLE

SAMPLE

FLUSH OUTLET PUMP

REAGENT PUMPISI

FLUSH !NLET PUMP

Fig. 15.9 Sample and reagent flow diagrams ( a ) and f l u s h ( b ) o f a single-parameter water analyser w i t h c o l o r i m e t r i c detection. (Courtesy o f Hach Co.).

Hach manufacture a serles of single-parameter analysers with the above-mentioned features

f o r the determination

of copper (Model 61 7001, hexavalent

chromium (Model 31700), permanganate (Model 314001, high-range silica (Model

61500), phosphate (Model 31500), ozone (Model 32000), f r e e chlorine (Model 61 loo), chlorine dioxide (Model 316001, hydrazine (Model 321001, phenolphthalein alkalinity

(Model 614001, total alkalinlty (Model 612001, chelant (Model

61300), hardness (Models 31000 and 610001, etc. On the other hand, the trace analysers marketed by Hach are gravity-based and control the sample-to-reagent ratio by means of precisely selected capillaries located i n the sample and reagent flow paths (Fig. 15.10). The temperature of sample and reagents i s controlled d u r i n g the mixing and reaction phase i n o r d e r t o ensure reproducibllity. The temperature control Is maintained because the head regulator and delay blocks are secured t o a mounting plate t h a t Is an integral p a r t of the sample heater. I t i s important,

however, t h a t the sample flow be relatively

constant because wide fluctuations w l l l affect temperature control. The water sample entering the analyser flows f i r s t t h r o u g h the sample heater, where It

Automatic methods o f analysis

4 90

is heated at 50"C, and then enters the head regulator, constant head above the water capillary a t 5 mL/min.

which

provides a

As it leaves, it mixes

with reagent 1 and drops i n t o t h e f l r s t delay block, where a delay allows the f i r s t step of the reaction t o take place. This mixture then drops Into the second delay

block, where it Is mixed with reagent 2, which produces the

monitored product. This final mixture is detained i n the second delay block f o r several minutes t o ensure t h a t the colour can develop f u l l y

before it

flows on the sample cell f o r measurement. This i s the operational

basis f o r

the trace silica (Models 651C and 1234D) and trace phosphate (Model 2359C) anal ysers.

ig-

Ti;

Healed

Drain Block

F h . 15.10 Scheme o f c o l o r i m e t r i c gravity-flow tesy o f Hach Co.).

autoanalyser.

(Cour-

A colorimetric analyser f o r t h e determination of phenol in waste water in-

volving p r i o r distillation of the sample b y means of a home-made device has been reported by Goodwing and Marton [39]. lation unit and Fig.

15.11b

Flgure 15.11a shows the distil-

i t s connection t o the analyser.

The distillation

assembly consists of a distlllatlon coil, a condenser and a heat exchanger.

Automation i n environmental p o l l u t i o n m o n i t o r i n g

The

distillation

coil

has an expansion chamber

49 f partially

filled

with

1-mm

glass beads, a bottoms draw t o remove hot acid and an overhead draw t o remove vapours. The cycle time includes 1 min f o r sampling and 3 min f o r washing.

E

5

Fig. 15.11 Colorimetric phenol analyser. (a) D e t a i l s o f the d i s t i l l a t i o n u n i t , condenser and heat exchanger. ( b ) General scheme o f the i n strument. A, sampler; B, heating bath w i t h heating rod; C, d i s t i l l a t i o n head; D, condenser; E, heat exchanger; F, s t r i p - c h a r t recorder; G, colorimeter; H, c o i l ; J, s i n g l e bead-string reactor; K, p e r i s t a l t i c pump; ( 1 ) wash water; ( 2 ) sample; ( 3 1 , a i r ; ( 4 ) phosphoric acid; (5) overhead condensate; ( 6 ) 4-AAP reagent; ( 7 ) F e ( I I 1 ) reagent; ( 8 ) a i r ; ( 9 ) l e v e l control; (10) bottoms draw; (11) debubbler draw. (Reproduced from [391 w i t h permission o f Elsevier).

Turbidimetric There are two basic types of on-llne water turbldimeters: surface scatter and flow-through. I n turbidimeters of the former t y p e the sample flows i n t o a constant-level

well. The l i g h t beam i s dlrected t o the sur-

Automatic methods o f a n a l y s i s

492

face of the cup and the radiation scattered by the particles i n the liquid surface is measured at a specific incidence angle. This t y p e of instrument has the advantage that the optical elements are never in contact with the sample. I n the latter type of instrument, the sample flows through a tube and the l i g h t impinges on it through an optical window located i n a lake o r at the end of the tube, after which the scattered l i g h t i s measured. The Model 5 surface

scatter

turbidimeter and the Model 17208 flow-range

process turbidimeter

marketed by Hach are representative examples of each type of turbidimetric on-line water analyser. Dissolved oxygen detectors. These consist essentially o f an electrochemi-

cal cell with a gold cathode and a silver anode immersed in an electrolyte solution separated from that of the sample by a chemically resistant polymer membrane permeable t o the gas, so t h a t p a r t of the dissolved oxygen i n the sample proportional t o i t s partial pressure diffuses across t h e membrane and i s reduced i n the cathode, giving rise t o a c u r r e n t proportional t o the oxygen concentration i n the sample. The PW 9600 dissolved oxygen transmitter and PW 9610 dissolved oxygen sensor manufactured by Philips include a transmitter

unit which accommodates two printed circuit boards --one of which provides the measuring circuitry-

with solid-state components. Both instruments have alter-

native housings f o r the sensing element. The immersion probe o r flow assembly used depends on the particular requirements of each Installation. The two instruments provide continuous automatic measurements of dissolved oxygen i n water, f u l l y compensated f o r the sample temperature. Organic carbon a n a l y s e r s A variety of procedures have been reported f o r

the automated determination of carbon-containing compounds in water b y oxidation with peroxydisulphate and the aid o f UV radiation o r Ag as catalyst. I n 1969, Erhardt E401 proposed an automated procedure f o r the analysis of sea

water by oxidation with peroxydisulphate under UV light. The COz produced was absorbed by an alkaline solution and the resulting change I n conductivity was a measure of the concentration of dissolved COz. Another method, developed by Goulden and Brooksbank 11411, used UV irradlation o r an. A g catalyst t o effect the oxidation of the samples and measured the Co2 generated b y means of an I R analyser. Princz e t a l . [42] also developed a less common method f o r the determination of COD based on potentiometric measurements. The instrument, depicted i n Fig. 15.12, has an autosampler with alternate samples and blanks, a UV tube reactor, a peristaltic pump and a flow-cell with a CO., gas electrode. The practical procedure involves the prior separation of inorganic carbon b y addition of HzS04 t o the sample u p t o pH 2-3, followed by s t r i p p i n g of the evolved COz with

COz-free

air.

The

pretreated

solutlon

is reacted

with

the

oxidant

(KzSZQ) and introduced I n t o the sampler, from which It i s passed t h r o u g h the

A utoma t ion

in en v i ronmen tal pol 1u tion moni tor i n g

493

reactor tube, where organic carbon is oxidized and measured i n the controlledThe generated emf is recorded analogically by the log-

temperature flow-cell. graph device.

co2 ELECTRODE

uv

AUTO

Fig. 15.12 Potentiometric COD permission o f Elsevier).

analyser. (Reproduced

from

[ 4 2 ] with

The Technicon Monitor I V i s a segmented-flow analyser for the continuous on-line monitoring of COD, TOD and TC. For the determination of TOD and COD, the sample Is acidlfed and sparged t o remove organic carbon, and later mixed with HzS04 and KzS208 and subjected to UV light in a UV digester. The COz generated is isolated from the matrix by means of a dlalyser with a gas-permeable silicone rubber membrane. The acceptor stream contains phenolphthalein dissolved i n a carbonate-hydrogen carbonate buffer, the change of colour resultIng from the absorption of COZ being monitored by a colorimeter. The reaction sequence f o r 15.13a.

the

determination

of

each

parameter

is

illustrated i n

Fig.

The sparging step is omitted i n the determination of total carbon,

which is measured i n a direct manner. The sparging of the samples for removal of inorganic material may cause some organic compounds t o be lost, particularly when volatile or water-insoluble

substances are present. These losses can

be evaluated by comparing the readings obtained with and without sparging i n an acidic medium -the

acldified samples w i l l provlde a lower concentration

f o r the inorganlc carbon than that actually present. As a rule, waste Water rarely contains volatile or very insoluble materials, which are more likely t o be found i n other types of water. The essential components of the instrument, shown i n Flg. 15.13b, are : (1) overflow sampler; (2) solenoid valve,

which

facilitates the introduction of the sample or the reference solution into the manifold; (3) proportioning pump; (4) UV digester; (5) manifold assembly, with

Automatic methods o f analysis

494

W

G

DETECTOR

b) LIQUID STREAM

OVERFLOW

SOLENOID

REFERENCE SOLUTION

+SPARGINGH M A N IFOLD A S S EM BLY

uv

OIGESTiONW

CONTAINER S RECORDER

DETECTOR

Fig. 15.13 Monitor I V system. ( a ) Reaction sequence. (b) Block diagram o f the interchangeable module design. (Courtesy o f Technicon).

A u t oma t i o n i n en v i ronmen tal pol 1u t i o n moni t o r i n g

495

the required glassware, tubing, heating baths, dialysers and fittings -this unit is intended to facilitate manifold changeover for the analysis of different analytes-;

(6) detectors -the

analyser can use an IS€ or colorimeter

depending on the parameter t o be assayed-;

(7) recorder (the Monitor I V

provides d i r e c t analogue data output and strip-chart printouts for permanent record of the analytical values; and (8) sparging system, consisting of an impingement pump, sparging coil and an additional gas-liquid separator. The sparging air is supplied by an air pump and purified from organic contaminants by passage through an active carbon f i l t e r . The sparging coil is by-passed for total carbon measurements. The instrument optionally allows: (a) Dual sample analysis. A timer and a programmable valve permit the aiternate analysis of two streams with auto-correction (standardization) at preset intervals. An auto-correction feature is provided for monitoring of applications involving d r i f t s arising from system changes such as reagent degradation or pump tube delivery changes, and electronic long-term drifts. Depending on the expected nature of the drift, the auto-correction unit may be set t o correct f o r either the baseline d r i f t or sensitivity drift. (b)Use of high-low alarms. This module warns the operator about any abnormal change i n the concentration of the parameter being determined. (c) Use of a continuous water clarifier. A module which provides samples from which particles with diameters greater than 0.5 pm have been f i l t e r e d out for samDle streams. 15.3.2.2 Uulti-parameter

analysers

The legislation on water pollution and quality control necessitates analyses for an increaslngly larger number of parameters. These demands have been

met by commercial firms with the manufacture of multi-parameter analysers. The Phllips Environmental Protection range Is representative of this type of instrument. The PW 9835 Automatic Water Monitor allows the measurement of up t o six water

quality

parameters (pH,

pCI, redox potentlal,

conductivlty,

dissolved

oxygen and temperature) i n ground waters i n a continuous automatlc fashion. The period of unattended, maintenance-free operation may be as long as 1 month or more, depending on local conditions. Two Important features contributing t o accurate, unattended operatlon are automatic sensor cleaning and automatic callbration of the instrument. The sensors can be kept from fouling by algae and other materials normally present i n ground water by an optionally available automatic ultrasonic cleanlng unit, which operates periodically at preset time intervals. Automatic calibration provides proof that the automatic water monitor Is working properly. Reliable reference values are given daily and

Automatic methods o f a n a l y s i s

496

t r u l y correct measuring data over the full unattended period are assured. A turbidimeter can be optionally included. The autoanalyser consists of two distinct parts, an upper electronic section and a lower wet section. The electronic section consists of three potentiometric systems for measurement of pH, pC1 and redox potential, a conductimetric detector, a voltammetric system for measurement of dissolved oxygen, a temperature transmitter and a control unit effecting the following functions: (a) Control of the sequence for the calibration process.

This

includes

timing the motion of the slides in the measuring block, filling the individual compartments

i n the measuring block

with fluid and flushing.

The

calibration cycle takes 41 min. (b)Generation of electrical and visible status signals (0 o r 15 V ) and acceptance of incoming signals for remote control. (c) Control of the sampling pump and hence of the sample stream. through the monitor. (e) Supply of +24 and -24 V d.c. power for the measuring transmitters. ( f ) Thumbwheei adjustment t o allow the user t o select the time at which a calibration cycle should start automatlcaliy.

This i s adjustable from 0 t o 16

h subsequent t o setting. (9) Fault detection i n case the water pressure inside the measuring block becomes too high, i n which event the sample pump is turned off; the air pressure for

pneumatic operation is too low, in

which event the monitor

is

switched off; power consumption by the sampling pump is too high, in which event it is switched off; or the sliding valves i n the measuring block are positioned imprope rly

.

The electronic unit is completed by recordlng equipment which can be from a multi-channel

recorder t o up to four dual-line

recorders. Data transmission

is achieved by mounting the appropriate equipment In the recorder section. It Is also possible to mount a microcomputer-based pollution data reductor.

The wet section includes: ( 1 ) a PW 9825 flow-through measuring block into which the appropriate sensors are mounted; (2) bottles containing electrolyte for

replenishing the contents of

double-junction

reference electrodes;

(3)

pneumatic devices consisting of pressure-reducing valve tubing and pneumatic valves; and (4) a number of 5-L containers for calibration liquids, placed i n the bottom of the cabinet. I n the measuring step, the PW 9825 block functions as an ordinary pipe through which the sample flows at a rate of 1-2 m/s. I f an ultrasonic transducer is needed, it is placed before the electrodes, which are also mounted i n the

block.

During calibration,

the

block

i s flushed

and

drained, and the water supply i s switched off. The sliding valves mounted i n the block are pushed upwards

so that the pipe is divided into measuring cham-

Automation i n environmental p o l l u t i o n m o n i t o r i n g

497

bers. The calibration liquids are then dosed pneumatically into these chambers. Overflow

pipes allow f o r the elimination of a i r and excess calibration

liquid, After 20 min, the sliding valves are pushed downwards, the pump i s switched on and the block i s flushed. A second calibration i s performed i n the same manner, with the second calibration liquid dosed t o the chambers by means of their respective pneumatically operated valves. After the second callbration, t h e water dissolved oxygen

monitor is switched t o the measuring cycle. i s carried

Calibration of

o u t by exposing the sensor t o air.

D i r t and

superfluous water on the membrane are removed b y an a i r stream d u r l n g the f i r s t f e w seconds of both calibration cycles. A l l positions and modes o f the measuring block are indicated by small semiconductor lamps on the control panel. Comber and Nicholson [431 reported a home-made design f o r the continuous monitoring of r i v e r s and estuaries. The instrument allows the detection and electrochemical quantitation of cyanide, sulphide, ammonia and p H (ISEs), as

well as salinity,

dissolved oxygen and temperature (Fig.

15.14).

The sample

$-) Logger

U

Fig. 15.14 Flow diagram o f continuous,water monitor. Electrodes: 1, cyanide; 2 , sulphide; 3, ammonia; 4 , pH; 5 , reference; 6, s a l i n i t y ; 7 , oxygen; 8 , temperature. V : valve; P: pump. (Reproduced from [ 4 3 ] w i t h permission o f the Royal Society o f Chemistry).

Automatic methods o f a n a l y s i s

498

intake can either be mounted on a metal pole fixed t o the bow of a vessel -approximately

1 m below the surface-

f o r use i n horizontal profiling of a

r i v e r course, o r it can be lowered t h r o u g h the water column for depth profiling. The dissolved oxygen and salinity probes are mounted alongside the sample intake and are calibrated i n the laboratory. The water is pumped on the boat at a rate of approximately 500 gal/h by using an on-board pump (Pz). The power f o r t h i s pump and the remainder of the equipment is derived from the vessel's accumulator, either directly o r via an electronic inverter t o provide 240 V and 50 Hz. Later developments extended the scope of t h i s instrument t o analyses f o r metals by anodic stripping voltammetry and ion exchange, the determination of n i t r i t e s and nitrates by

FIA and on-line research on organic compounds

with the aid of a fast-scan monochromator. The principle behind the FIA technique (reversed mode) has been exploited t o develop a completely automated

instrument [44] (Fig. 15.15) i n which the

sample i s continuously pumped along the system and In which different injection valves [45] o r a single injection valve aided by a selecting valve [46] are used t o introduce the reagents required for the determination of each anaiyte i n the appropriate sequence and at the required time intervals. The reacting plug is driven t o the flow-cell

ttt

of a photometric detector,

and the

1

t

Injection and

Reaction Detector

I

Waste Water

a Printer

Fig. 15.15 Completely automated instrument f o r the p o l l u t a n t s , based on the reversed F I A p r i n c i p l e .

determination o f

Waste

499

Automation in environmental pollution monitoring

signal yielded is recorded o r acquired by a microcomputer through a passive interface and later processed. The active interface also used allows actuation of the pump and the injection valve(s), so that, by use of a suitable program, the instrument is started at the required time intervals, injects reagents according t o the preset sequence, collects data provided by the detector and compares them with the calibration graph f o r each analyte stored in the program. A tolerated limit i s also stored f o r each analyte, so t h a t if the concentration of any of them surpasses the limit, an alarm system warns the operator. This analyser allows the quantitation of anions (CN-, NOz-, S2-) and cations

Fe,

(Al,

Cu)

[46].

The

Incorporation

of

a

glass-calomel

electrode

p r i o r t o the injection system along the sample channel allows the determination of pH [47]. The reversed FIA (rFIA) mode also allows f o r the on-line

speciation of

elements. Thus, different rFIA configurations (Fig. 15.16) have been used f o r the speciation of chromium based on the indicator reaction between Cr(V1) and 1,5-diphenylcarbazide

(DPC). Figure 15.16a shows the configuration used f o r

the completely continuous monitoring of Cr(V1) and periodic measurement of Cr(II1). The sample circulates continuously along the system and merges with an acidic DPC stream, i t s evolution i n the water being continuously monitored. A t time intervals dependent on the required frequency, Ce(1V) is injected t o

oxidize Cr(II1) t o Cr(VI), which yields a signal concentration initially

present i n the sample

15.16b consists of two sub-systems total chromium (2).

The selecting

proportional t o the Cr(II1)

[a]. The

configuration in Fig.

f o r the determination of Cr(V1) (1) and valve,

SV, allows the arrival of one o r

another stream t o the detector and hence the obtainment of the Cr(V1) o r total chromium concentration, the Cr(II1) concentration being calculated by difference

[el.

The configuration i n Fig. 15.16~ uses both rFIA and the asynchro-

nous merging zones mode f o r the speciation. The dual injection valve used simultaneously volume

of

inserts i n t o the system a large volume of DPC and a small

Cr(VI),

so t h a t the latter merges at point A with the tailing

portion of the DPC plug, yielding a f l r s t signal corresponding t o the reaction of Cr(V1) alone in the heading portion o f the DPC p l u g and a second, larger signal corresponding t o the tailing portion of the plug, where Cr(II1) has been oxidized and therefore contributes t o the analytical signal [49]. An rFIA-asynchronous merging zones conflguration has be used f o r the speciation of up t o nine different chromlum forms -aquo tetrahydroxylated

Cr(II1) and molecular, anionic and

cludes a glass-calomel

complex, mono-, di- and dlmerlc Cr(V1).

It in-

mlcroelectrode inserted in the sample stream p r i o r t o

the merging with the reagents, and a microcomputer whlch acqulres the measured pH and chromium concentratlons -Cr(VI)

and total Cr. These data are pro-

cesssed by a computation program In which the equillbrium constants of the

Automatic methods o f a n a l y s i s

500

1

CCL'

12

Sample DPC

q

im L rnin-'1

LI = 6 0 0 c m

L2 = 6 0 c m 5 L O nm

-i

tl. Alw t

A I

0

W

22

2 2

n PC

Fig. 15.16

W

rFIA c o n f i g u r a t i o n s f o r t h e s p e c i a t i o n o f Cr(V1) and Cr(II1). ( a ) Completely continuous determination o f Cr(V1) and p e r i o d i c measurement o f C r ( I I 1 ) . ( b ) Sequential method f o r t h i s s p e c i a t i o n by use o f a s e l e c t i n g v a l v e f o r determinatlon o f C r l V I ) (channel 1 ) and t o t a l chromium (channel 2 ) . ( c ) Asynchronous merging zones f o r simultaneous determination o f b o t h species. q: flow-rate; V i , VI and V z , i n j e c t i o n valves; S: s e l e c t i n g valve; W: waste. The recordings obtained a r e shown t o t h e r i g h t . (Reproduced from [481 w i t h permission o f Springer Verlag).

Au t m a t i o n i n envi ronmentsl pol 1u t i o n moni t oring

50 7

nine above-mentioned species are stored and which yields the concentration of each [501.

15.4 A I R ANALYSERS

Atmospheric pollution, on account of its special features, is the most important aspect of environmental pollution.

The most recent trends i n this

field point t o atomic absorption and emission techniques (metals) 1511, the use of GC and HPLC for organic compounds [52-551 and the development of new electrochemical sensors [56-601 facllitating off and on-line

measurements. I n

s i t u measurements, carried out by remote sensors [60,611, aiiow the detection and quantitation of air volumes i n the atmosphere at a distance from the instrument without the need for sample collection. Measurements are mostly based on the optical properties of matter (light absorption, emission or scattering) and the instruments used can be located in a fixed position or on a moving platform, either on the ground or i n the atmosphere (planes, balloons, satellites). Two general types of light sources can be distinguished depending on whether they use passive or active sensors. Passive sensors use natural light and depend on the intensity, spectral distribution, etc. of the source -light coming from the sky or from the sun itself after passing through the atmosphere. They are often used in the limb sounding configuration, looking at sunrise or sunset through the whole atmosphere t o increase the integrated signal over very long paths. I n that case, they measure a quantity which Is the product of a concentration and a distance. Passive Sensors can be classif ied

into

radiometers,

ultraviolet

vidicons,

spectrometers,

i nterferometers,

Fourier transform analysers and laser heterodyne amplifiers of spectral lines. Active sensors have built-in light sources -generally

tasers-

and are obvi-

ously more complicated and expensive than passive sensors. The source can be at a long distance from the detector (hundreds or thousands of metres), and the llght collected

by the telescope allows the measurement of the total

amount of a gas along the light path rather than Its concentration.

Most

passive detectors can be used as active detectors by f i t t i n g them with a light source. The commonest are laser absorption spectrometers and LIDARs (monowavelength, differentlal, Raman or fluorescence).

Active and passive sensors are

Intended for different appllcations. Passlve Instruments are better suited t o stratospheric observation In balloons or satellites because of thelr compactness and low energy consumptlon. A t ground they can provide a huge amount of information on the nature of chemical compounds. Active sensors have a much hlgher spatial resolution and should be more useful f o r studying the atmospheric phenomena from the ground t o a distance of about 5-10 km. The chief

Automatic methods o f analysis

502

drawbacks of these instruments arise from the difficulties In interpreting the complete information provided by the data; not only are the spectral data often confusing, especially i n the lower frequency

region (infrared, micro-

wave), but also the atmospheric parameters are changing so much and so quickly i n space and i n time that very great care is necessary not to adopt simplistic assumptions causing large errors [621. The problems associated with the sampling and automation of this stage

were dealt with i n their corresponding sections, so this section is exclusively devoted to the automated instrumentation available for this type of sensor.

As with water analysers, air analysers are classified into off- and on-line anaiysers, and the latter i n t u r n Into single-parameter

and multiparameter.

15.4.1 Off-line alr analysers

Off-line systems are typically used for the detection of the metal analytes occurring in the atmosphere as particulates or fumes which, after collection by an appropriate f i l t e r , are subjected to dry ashing followed by acid decomposition f.631 or, more commonly, ICP-AES

wet ashing [63], and quantltation by

[641, flame AAS [65], normal or furnace A A S [66,67], or X-ray fluor-

escence spectroscopy [68]. Oguma and van Loon [69] reported a method for the

a)

F

P

%T

C

M

A

W Fig. 15.17 Assembly f o r determination o f vepour Sampling apparatus: (b) measuring instruments.

mercury i n a i r . (a)

Automa t i o n i n environmental p o l l u t i o n moni t o r i n g

503

determination of total mercury vapour in air by AAS using the apparatus depicted in Fig. 15.17a for sample collection. Air is pumped through a 0.45-pm Millipore f i l t e r (B), the moisture traps (M) and the collection tube (C), in this order, at a rate of 2 L/min, checked by the passage of the air through a flow-meter

(F) by turning the three-way

stopcock ( K ) at 10-mln intervals.

A f t e r an appropriate sampling time, the collection tube is dislodged from the

system and installed in the measuring system (Fig. 15.17b), where I t is warmed t o room temperature and then heated at 100-115'C for 5 min. During these two

stages, a nitrogen stream is passed through the collection tube, the pyroiyser

(N), the cooling coil

( G ) and the amalgamation tube (HI,

in this order, at a

flow-rate of 0.5 mL/min. Then stopcocks K and L are shut off and the amalgamation tube is heated at 500°C for 30 s. The mercury vapour thus released is swept into an open-ended glass tube (20 cm x 1.5 cm I.D.) with a 5 mm I.D. inlet tube fused in the middle that is placed i n the burner head of a Perkin Elmer 3058 atomic absorption spectrometer and aligned in the usual manner t o

allow maximum light from the hollow-cathode lamp t o reach the detector. The height of the recorded peaks is used t o determine mercury. The analytical system is standardized at a known temperature by means of a septum inserted between the cooling coil (Gi) and the stopcock (K). 15.4.2 On-line alr analysars 15.4.2.1 Single-paramter analysers

The on-line

single-parameter

instrumentation for the determination of

gases employs a variety of detection systems. Good proof of concern about the pollution caused by sulphur dioxlde Is the large number of analysers developed for the continuous monitoring of this parameter. The literature review by Hoilowell et a l . [70] on the instrumentation for continuous monitoring of S0.r deals with the different detection techniques used for this purpose, namely conductimetry, colorimetry, coulometry, electrochemical transducing, flame photometry alone or in conjuction with gas chromatography , non-d Ispersi ve and dispersive absorption and condensation nuclei formation. Recent studles were oriented t o the resolution of the prob-

lem of real-time on-line analysis for S0.r at low concentratlons [71] by means of a dispersive infrared detector or a coated piezoelectric mass transducer [72].

The device used In the latter case, Fig. 15.18, allows the rapid varia-

tion of the crystal environment i n terms of the gas composition, temperature and water composition of the gas stream. The oscillator and frequency meter board of the Interference rack are controlled by a microcomputer. Most commercial instruments available f o r the determination of So;, are based on the UV light excitation of the So;, molecules In gas mlxtures [731 and

Automatic methods o f a n a l y s i s

504

the measurement of the resultant fluorescence (e.9. Model SA700 Fluorescence SO;! analyser from Columbia Scientific Industries Corporation) or of the light

emitted b y the sulphur species in passing through a hydrogen-rich flame (Sul-

phur Dioxide analyser from the same company).

Water b o t h

C r y s tal driving circuit

F M board interface

Water- bath

Microcomputer

plotter

Fig. 15.18 Scheme o f experimental instrument f o r on-line measurement o f SO2 with detection by a piezoelectric c r y s t a l sensor. (Reproduced from [72] with permission o f the Royal Society o f Chemistry).

On the other hand, the single-parameter

gas analysers manufactured by

Philips (PW 9755 So2 monitor, PW 9780 HIS monitor, PW 9775 CO monitor, PW 9760

NO monitor and PW 9765 N02 monitor) are based on continuous coulometric titrations (Fig. 15.19). The air Is aspirated from the atmosphere via an air sampler and dust f i l t e r and i s then pumped at a constant flow-rate through the monitor where the analyte concentration is measured. On entering the monitor, the air is f i r s t passed through a chemical scrubber t o f i l t e r out unwanted components that might affect the analyte reading.

The filtered air is then

passed at a constant rate through an aqueous solution of KBr, Br2 and HzSO4 in the thermostated cell. By use of the two electrodes immersed in the electrolyte, the free bromlne concentration Is converted t o a redox potential which Is compared with a known reference potential. A control amplifier senses

Automation i n environmental p o l l u t i o n monitoring

the difference

505

between these two voltages and sets u p a flow of electric

c u r r e n t i n the electrolyte via two more electrodes.

This electric c u r r e n t

converts bromide ions t o free bromine, whose concentration i n the electrolyte is therefore restored. As the extent of reduction depends on the amount of analyte (SOz o r HzS) passing t h r o u g h the electrolyte, the electric c u r r e n t used w i l l be directly proportional t o the amount of analyte present in the sample. By keeping the air flow-rate constant with the aid of a vacuum pump and a thermostatically controlled

critical

i n the air can be readily calculated. i s passed t h r o u g h a convection-cooled

orifice,

After

the analyte concentration

leaving the electrolyte, the a i r

Peltier cooler.

The water evaporated

from the electrolyte solution i s carried by the a i r stream and i s condensed i n the cooler, falling on the electrolyte solution, the level of which is therefore kept constant. The measurement, zero-checking and span-checking functions can be automatically selected by means o f a three-way operated

under

remote control.

motor-driven

valve

I n the measurement position, a i r from the

sampling system (and dust f i l t e r ) passes only t h r o u g h the chemical scrubber on i t s way to the measuring cell. When the valve i s set t o the zero-checking position, the a i r i s sucked t h r o u g h an active charcoal f i l t e r t o remove the analyte before reaching the chemical scrubber. The purified a i r then enters

Fig. 15.19 Block diagram o f gas analyser w i t h coulometric detection. (Courtesy o f P h i l i p s ) .

Automatic methods o f a n a l y s i s

506

the measuring cell and gives rise t o an output signal which defines the zero level. With the valve in the span position, the above-described

procedure is

the same as that between the zero-filter and the chemical scrubber, the air passing through a standard source where it i s dosed with a known amount of analyte. The standard source consists of a permeation tube with liquid SO2 and HzS thermostatically maintained at a constant temperature.

I n the CO monitor, the air, after passing the scrubber, enters a column where i t reacts with 120s at 160°C to yield f r e e iodine. This reaction is necessary on account of the electrochemical inertia of CO. I n this manner, as the reaction efficiency is constant, the amount of 12 formed will be directly proportional to the CO concentration i n the air. The I z vapour is passed through a measuring ceil with two electrodes which contains the electrolyte and is kept at 37'C.

On entering the cell, the vapour is passed through a

graphite cathode, where iodine is reduced to iodide and an electric current proportional to the amount of iodine and hence t o that of CO in the air is generated. The current flow causes a potential difference between the two electrodes which is measured by a potential amplifier. A l l other stages of the process are the same as those taking place i n the So;! and HzS analysers. The Philips NO and NOz monitors

also correspond t o the scheme in Fig. 15.19.

A f t e r isolation from potential interferents, NO must be oxidized t o NOz. The

air stream is passed through a K I solution which reacts with the No;! to yield iodine, which gives the measured current upon reduction. A home-made analyser with a seif-scanned photodiode array as a multiplex

sensor has been used f o r laboratory detection and measurement by dispersive spectroscopy of trace amounts of polluting NOz. The on-line

data acquisition

and numerical analysis system allows the elimination of some systematic errors and d r i f t s (Taylor filtering) and the noise associated with high spatial f r e quencies (low-pass filtering). The experimental set-up 15.20.

The

white

light source,

S,

illuminates slit

used is shown i n Fig. Fi

homogeneously. Two

spherical mirrors Mi and Mz with a focal distance of 15 cm for the image of F i on the slit (Fz) giving access to the dispersion system (an array of 1200 lines/mm). Tube T, f i l l e d wlth No;!

at different partial pressures, intercepts

the beam betwen Mi and Mz. Lens Li, diaphragmed by D, forms the image of Fz onto the microphotodiodes (pPh). A cylindrical lens, Lz, allows the light to

be concentrated by a factor of 10 in the direction perpendicular t o the line. The 1728 photodiodes (15 pm spatial period and 16 pm length) are cooled by a Dewar vessel

containing a mixture of methanol and d r y ice. The electric sig-

nals from the detection system are acquired by a computer. The gathered data allow the calculation i n real time and for each diode the mean and variance of n steps and eliminates the unevenness of the photodiode responses, defective

A u t oma t i o n i n en v i ronmen t a1 pol 1u t i o n moni t o r i n g

50 7

transmission from the optical system and the unvenness of the spectral source by carrying out a delayed homogenization. The analyser compares favourably with commercially available instruments with a single detector [741.

S

-1

I

DE WAR

Fig. 15.20 Experimental set-up f o r determination o f NO;! by means o f a photodiode system and dispersive spectroscopy with computerized data acquisition and treatment. (Reproduced from [741 wih permission o f the N a t u r a l Research Council o f Canada).

Chemiluminescence techniques have been used for the determination of a variety or atmospheric pollutants [751, particularly ozone. Most the commercial instruments available for the determination of 03 are based on chemiluminscent reactions and use the Nederbragt e t al. method [76], which utilizes the light emitted upon reaction of ozone with ethylene gas (Meloy OA 325-2R and OA 350-2R ozone anaiysers) or that between 03 and Rhodamine [77,78]

o r a disc with this

reagent and galllc acid

B over silica gel

(Philips PW 9771 03

Monitor). Ray e t al. have developed a chemiluminescence analyser for the measurement of atmospheric ozone using Eosin Y in ethylene glycol (Fig. 15.21). The sample gas is drawn through the detection cell at 1-7 L h i n by a diaphragm pump. Fluid and air are separated at the reservoir, and the dye solution is recircuiated. The sample gas flows across a paper or glass-fibre pad

mat that

is saturated with the organic dye dissolved i n an alcohol solvent. The cell design allows the separate entry of air and dye solution through the upper inlet and their joint evacuation through the lower outlet. The response of the instrument, which compares favourably

with that of commercial instruments

(e.g. TECO Model 48P) i s linear i n the range 0.2-400 ppb [79].

Automatic methods o f analysis

508

,

air

in d e t e c t o r 3

a1 r out

PU J P

I fluid tank

Fig. 15.21 Scheme o f chemiluminscence ozone analyser. PTM: photom u l t i p l i e r tube. (Reproduced from [791 with permission o f the American Chemical Society).

The determination of hydrogen peroxide in the atmosphere has attracted much interest since it was acknowledged to promote the oxidation processes converting So;! t o HzSO4 i n the clouds below pH 4.5. The chief source of H202 dissolved i n the clouds i s that formed photochemlcally

in the atmosphere.

Vapour H202 concentrations i n the range 4 pptv t o 6 ppbv can be determined by an automatic fluorlmetrlc method developed by Lazrus e t a l . [80] and based on the selective peroxidase-catalysed

decomposition of H202 by (p-hydroxyphenyi)

acetic acid (POPHA), which yields a dimer of the latter that absorbs at 320 nm and emlts at 400 nm. Peroxidase also catalyses the formation of fluorescent dimers by hydroperoxides, so that i n order t o be able t o distinguish H202 from organic peroxides a dual-channel

flow system with a dual-cell

fluorimeter is

used. The above-mentioned reaction yields a measure of ail peroxides i n one channel, while the other is treated with the enzyme catalase, which destroys H202 selectively prior t o the cataiysed oxidation, so that the second channel provides an analytical blank for measurement of the H a . An example of an autornatlc analyser f o r the determinatlon of organic pollutants is the Meloy HC 500-2c self-contained

from Columbia Sclentlfic Industries. It is a

system f o r monitoring ambient concentrations of non-methane

hydrocarbons (NMHC), methane and total hydrocarbons. Sample air is f i r s t introduced directly into the flame ionization detector t o yield a total hydrocarbon reading whlch Is stored In an electrlcal circult. The pneumatic system is automatically switched

so that the sample air passes through a catalytic

converter before it i s introduced into the detector, which converts ali the NMHC Into a non-detectable specks. Hence, only the methane i n the sample i s

Automation i n environmental p o l l u t i o n m o n i t o r i n g sensed and

its reading is stored

509

in another electrical

holding circuit.

A

differential amplifier subtracts the methane reading from the total hydrocarbon reading to provide an NMHC reading as the output signal. Automatic dust meters are essentially based on two principles: light scattering and adsorption. Beta absorption is probably the most reliable means of making measurements insofar as these are independent of the phyiscal, chemical and optical properties of the monitored dust. Such is the technique used in the PW 9790 Dust Monitor from the Philips Environmental Protection Series. The instrument consists of three major parts: the measuring and control units and the vacuum pump, The measuring unit, shown i n Fig. 15.22, consists of a filter tape drive system, a beta source and detector and a heated air sampling gate. Heating the air sampling gate eliminates condensation on the f i l t e r tape. A i r is drawn through the section of the filter in the sampling gate by the vacuum pump. As the flow-rate of the pump is kept constant, by operating the pump for a preset time the volume of the air sample is accurately determined. The control

unit regulates the overall timing of the dust monitor. It also contains

the electronics used for the up-down counter, for derivation of output signals and for generation of the required d.c. power supplies. Plug-in wire links on one of the printed circuit boards enable a wide choice of counting (10 s to 4 min) and sampling periods (10 s to 14 h ) to be preset.

UPPER TAPE TENSION CONTROL LOOP

TAPE BETA SOURCE

A I R SAMPLE I N L E T FROM AMBIENT AIR

FILTER TAPE SUPPLY REEL

-t--t

-

I

DETECTOR COUNT. PULSES TAKEN V I P

GEIGER-MULLER DETEC TOR OPERATIVE DURING COUNTING PERIODS

M

I

/ I

AIR OUTLET TO PUMP ICONSTANT FLOW- RATE)

DUST DEPOSITED DURIG SAMPLING PERIOD

LOWER TAPE TENSION CONTROL LOOP

TAPE - FILTER TAKE-UP REEL

Physical arrangement o f the Philips PW 9770 Dust Monitor. (Courtesy o f Philips).

Fig. 15.22

Automatic methods o f analysis

510

Radiation from a low-level beta source is passed through the clean glassfibre f i l t e r and measured with a Geiger-Milller detector. The output pulses of the detector are counted by an electronic up-down counter over a preset counting period. A known volume of air is then passed through the same section of the filter, upon which dust from the air is deposited. The f i l t e r Is exposed to the beta radiation, but this tlme the output pulses of the detector are used to count down from the number attalned with the clean tape towards zero. Because of the dust f i l m on the f i l t e r , however, more of the radiation is absorbed and the count rate is lower than before. The time taken t o count down to zero is therefore longer than the preset counting period. The difference in time between the up and down-counting periods, which is accurately measured, bears a direct relationship to the mass of dust and, as the volume of the air sample is known, the result can be expressed as a concentration (pg/mJ). No repetitive calibration is required because the long-term d r i f t is compensated by setting the zero level for each measurement -the to the beta count achieved through the clean tape-

zero level is equivalent and the short-term d r i f t

occurring durlng the sampling period Is negligible thanks t o the extremely high stability of the instrument. The dust monitor can be operated contlnuously

or started by remote control. The output signal is also suitable for on-

line transmission t o a remote data-handling ments can be logged on-site

centre.

Alternatively,

measure-

by uslng a recorder connected to the recorder

output, or the analogue or digital slgnals can be used when the results are to be transmitted over a distance. The dust monitor can be used f o r more than

measuring dust concentrations. further

As the f i l t e r

collects actual

studies are possible for completion, colour,

shape,

dust samples, etc.

An X-ray

fluorescence spectrometer, f o r instance, can provide an elemental analysis of the sample for lead and iron.

15.4.2.2 nulti-paramter analysers There are many Instances i n which the control of the composition of the atmosphere o r that of gases dumped into it requires the automatic rnonitorlng of more than one parameter. Multi-parameter

air analysers can be classified

according to the characteristics of the analytes t o be determlned into: (a) those in which only one product need be monitored (e.g.

nitrogen oxides

analyser) thanks to the interconvertibility of the sample components; (b) those using a single detection system responding t o two different species (e.g. I R analyser for the determination of CO and COz); (c) those requiring a detector per analyte to be determlned -these

generally consist of a set of

single-parameter analysers or of single- and multi-parameter analysers used in conjunction.

51 1

Automation i n environmental p o l l u t i o n m o n i t o r i n g

The functional principle behind nitrogen oxldes analysers is generally the reaction of nitric oxide with ozone to yield exclted

nitrogen dioxide and

water. The transition of the excited molecule t o the ground state is accompanied by the release of radiant energy, which is suitably measured: NO + &-,NOz* + 02 NOz*+NOz + hv

The radiation intensity is proportional t o the rate of mass flow of NO i n the reaction chamber, where it is mixed with ozone. The radlant energy Is converted to an electric output by means of a photomultiplier tube and i t s associated electronics.

A converter transforms the NOz initlally present i n the

sample t o NO, so that, by means of a solenoid valve, the incoming air sample i s alternately passed through a converter by-pass

t o sense NO alone and

through the converter to detect the sum of NO and N G , defined as NOx. An electric subtraction circuit provides the NOz concentration as the difference between NOx and NO. Figure 15.23 shows a block diagram of an analyser of t h i s type, namely the Model 1600 NO/NOz/NOx Analyser from Columbia Scientific Industries. As can be seen on the left, the Instrument consists of an ozone generator and a reaction chamber where the 03 i s mixed with the sample, which is passed

or not through the oxide converter through the action of the motor-

ized three-way

valve. The right-hand side of Fig. 15.23 depicts the sensing,

discrimlnation and data delivery systems, namely an optical f l l t e r collecting the emission from the reaction chamber, a photomultiplier tube (PT) and an electronic circuit that allows the arrival at each recorder and/or microcomputer of the slgnal corresponding to the sum of the concentrations of the analytes or of each of them separately. The panel meter can display the value of any of the three signals without the need t o interrupt the instrument's operation. The three-way

valve works at a rate of 480 cycles/h,

which sub-

stanclally reduces the errors associated with the rapid change in the NOx levels i n the surrounding environment. Gas flow-rates are kept within 21% of the initial settlng by means of orifices kept at 50'C

and protected by stain-

less 7-pm steel filters. The photomultplier photocathode is kept refrigerated at 1wO.l'C

t o reduce background noise and achieve a detection l i m i t of 0.002

ppm. I n order t o avoid the contribution of ammonia to the NOx measurement, the converter operates below 4OO'C.

The chemlcal/catalyst cartridge Is designed so

that the air sample must pass through a packed column t o ensure intimate contact between the air and the catalyst, and so guarantee 99% conversion. Infrared spectrometers are the best alternative t o the contlnuous monitoring of CO and COz. An autoanalyser of thls type i s depicted schematically i n

HEATED AND

ORIFICE FLclW FLQW METER

u

n

>

v

n

>

v

o

>

;;-O_ 0

F i g . 15.23

0

"

a

>

U

a

>

V

n

>

E ; C "c c

o

n n ,,,

o

z s : 0

0

Block diagram o f nitrogen oxides analyser. (Courtesy o f Columbia S c i e n t i f i c Indust r i e s

Corporation).

0 -+I

a t-.

cn

Automation in environmental pollution monitoring

513

Fig. 15.24a. A portion of the gas from the atmosphere o r an industrial chimney is sucked by a pump and passed through a f i l t e r with a pore size ensuring retention of unwanted particles -this

i s t h e sampling step, which may include

the measurement of the moisture i n the collected gas. I n the measuring stage, the sample i s passed t h r o u g h a solenoid valve, a pressure gauge and the I R analyser, which measures the absorption of both analytes at their characteristic wavelengths. The data generated and the sample pressure are acquired by recorders and/or a computer which allows the determination of the atmospheric concentration of both analytes. The solenoid valve allows the a r r i v a l of CO and COZ standards at the detector t o r u n the pertinent calibrations. The j o i n t use of single- and multi-parameter t o r i n g of a variety

of analytes.

analysers permits the moni-

The instrument depicted i n Fig. 15.24b i s

used f o r monitoring blast furnace processes. D i r t y gas i s extracted from the duct and pulled t h r o u g h the f i l t e r by a pump. A small portion of the clean wet gas after the pump is fed t o the moisture analyser, b u t the major portion is cooled

i n a refrigerator t o remove the bulk of the moisture

pumped t o the analysers.

before it is

The relatively d r y gas flows approximately 150 ft

from the pump shed to the analyser room. By means of a by-pass valve the second pump boosts the gas pressure sufficiently t o force the gas t h r o u g h a final d r i e r and i n t o the analyser. A major portion of the gas extracted from the plant duct is passed t h r o u g h the by-pass valve. Only a small portion of the total volume i s used f o r analysis. A large volume of gas i s pumped t o decrease the residence time i n the cleaning system because the analysers are located at a distance of about 300 ft from the plant duct. The volume of the gas apportioned f o r analysis i s dried t o a moisture concentration of a few ppm. A solenoid

valve

system

automatically

or

manually

allows the

selection

of

blast

furnace gas o r any of three calibrating gases t o flow t h r o u g h t h e pressure controller

and i n t o the analysers.

cell,

CCz cell

the

atmosphere.

and,

finally,

All the gas flows t h r o u g h the hydrogen the

CO cell

before

it i s vented

to

the

Signal lines are connected from each analyser t o the respectlve

recorders, which are equipped with transmitting slide wires t h a t f u r n i s h a signal t o the computer terminal and then t o the main computer. With f e w exceptions, a well-designed

analyser system meets the requirements o f several

processes. However, a sampling system is not predictable. Although the same raw material is used, the particulate matter i n the gas from one furnace may vary significantly enough from the dust i n a second furnace t o require redesign o f the f i l t e r i n g media. The difference is not i n the amount of dust, b u t i n i t s composition and physical characteristlcs. Figure 15.24~ shows the scheme of an analyser f o r basic oxygen processes (BOP).

The gas flows from the plant duct t h r o u g h the f i l t e r s t o t h e pump.

Automatic methods o f analysis

514

Solenoid Conlral

a)

."

Almosphcre

TOP

I

Gas Flaw

Solcn o l d

Anolyrrr

c;3 Pump

C)

Anolyr Room

Calibrating Gosrs

Soirnoid Control

S l g n o l s To C o m p u t e r

Fig. 15.24 ( a ) Analyser w i t h I R d e t e c t o r f o r t h e determination o f CO and Con. Analyser w i t h single- and multi-parameter detectors f o r b l a s t furnace ( b ) and basic oxygen process ( c ) . (Reproduced w i t h permission o f Academic Press).

515

Automat i o n i n environmental pol 1u t i o n monitoring

A f t e r the pump, a portion of the clean wet gas is diverted t o the moisture

analyser. The major portion flows through a refrigerator to remove moisture to approximately 0.8% V/V. The gas pressure is boosted by a pump and diverted to the paramagnetic oxygen analyser and the C02 infrared analyser. I f CO i s also present in the process gas, both CO and C02 cells are included in the same analyser cabinet. The recorders are equipped with transmitting wires which furnish signals to the computer located approximately 500 f t from the analysers. A response time of 20 s is critical in the BOP process,

so that the

sampler must be located in the dirty gas stream after the f i r s t thorough mixing. The composition and physical characteristics of the dust particles fer

considerably from those i n the blast furnace.

dif-

The selection of filter

media is based on the dust characteristics. The anaiysers can be calibrated manually or automatically. The CO and C02 analysers are standardized at two points on the concentration curve, one in the neighbourhmd of the low end and another in that of the maximum concentration in the process gas. The 02 analyser is also checked at the high and low ends with calibrating gases. Analyses f o r pollutants i n open atmospheres are more useful on account of their versatility, the use of single-parameter modules or analysers making up multi-component monitoring stations adaptable t o the requirements of the area to be monltored. A typical scheme of one such station, namely a multi-component monitoring system marketed by Philips, is depicted i n Fig. 15.25. The oper-

ational principle behind each of the measuring systems (coulometry for SOZ,

HzS, NO and N02, and chemiluminescence for G) was dealt with i n describing single-parameter

analysers.

A

pollutant analyser performs the functions of

sampling, analysis and calibration. The actual pollutant analysis and calibration take place in the monltor. This contains the essential items such as filters, standard source, valve (which controls the zero-checklng,

span-setting

and measurement sequences), detector and air-flow controller. The supply cabinet houses the air suction pump (suitable for use with up t o three monitors except for the 03 monitor) and a pre-wired, position can accommodate one plug-in,

three-position module rack. Each

electrical supply unit, and the number

of supply units fitted (up t o three) depends on the number of monitors used. The remaining module positions can be fitted

with accessories such as a

transmission frame (two positions) o r a control clock (one position). The a i r sampling, can be carried out i n two ways: (a) by means of a sampler with a bullt-in dust f i l t e r which can serve up t o three monitors depending on the ambient dust concentration In the atmosphere, and ( b ) by use of a multl-component monitoring station, often much better for drawing a large air sample through a large-bore pipe. Both methods ensure the removal of water, insects and large dust particles from the sampled air. Regardlng data transmission, In

I

I

1

I

Sampling

I

I

i=l

2

Ambient

h

1

1

c for m a x . 3 n a i t w s . depending on dust load

I

II

I I I I

I

h

C

H rt

~~

CLOCK

/I '

W975011 Supply cabnet PW 9750100

PW 9750100

-G

S U D D ~ Vcabinet P W 9 7 5 0 / 0 0

a

I I I I I

I

1-

k.

0

x

3 3. 0

9 0 -+I

k

Fig. 15.25

Scheme o f a multi-component a i r pollution monitoring system. (Courtesy o f Philips).

2c1 v,

Automa tion i n

envi runment a l pol 1tit i o n moni t o r i n g

a simple installation where a visual

517

indication of the analysis i s sufficient,

t h e direct o u t p u t signal o f t h e monitor can be logged b y a recorder. When it i s necessary, however, t o transmit the output signal t o a remote data-handling centre, the signal must be converted i n t o a form suitable f o r transmission over switched o r leased telephone wires, o r via radio links -see

the next

section.

15.5 DATA ACQUISITION, TRANSMISSION AND PROCESSING. SURVEY NETWORKS The need f o r the rapid acquisition of the results of an environmental analysis i s of outmost importance insofar as such results may indicate the need t o take drastic and urgent steps in cases where some danger o r damage may be otherwise expected. Hence, the rapid acquisition of data by means of an A/D converter i s virtually indispensable in environmental analyses t o complete the achievements of automatic sampling and analysis [81-841 -the

automatic sam-

p l i n g of pollutants in water and a i r i s beyond the already broad scope of t h i s chapter [47]. An instrument specially designed f o r t h i s purpose is the microprocessor-based

Pol I ution Data-Red uctor (micro-PDR)

from

Phi I i ps, conceived

f o r f u l l y automatic pollution monitoring and p r o v i d i n g a straightforward and

I n t c rconnrctron bu I

f---T-T

4

Random Acccss Memory (RAM) Temporory dola storagc

I

Progrommablc Read only Memory (PROM1 Pcrmancnl program slwagr

I1

3

Punched Paprr Tape Control

Rimate

0UlP"t

Connrctlon

Fig. 15.26 Block diagram o f a microprocessor-based p o l l u t i o n data reductor. (Courtesy o f P h i l i p s ) .

Automatic methods o f a n a l y s i s

518

efficient means of automatically acquiring, processing and recording the data from a series of pollution monitors. I n addition, the system can start calibration cycles and use the results to correct the data from the analysis of the samples. The output data can be recorded in a variety of supports: magnetic tape cassettes, numeric or alphanumeric print-outs and punched paper tape. Remote printing and connection to a larger processing system are also possible. A block diagram of the micro-PDR is shown in Fig. 15.26. The primary functions of the system are data acquisition and processing, output of control signals to monitors, timing and data output. Data c o l l e c t i o n . Analogue input signals (0-5 V or 0-20 mA) are received

either direct from a local monitor or via the supervision multi-tonal (MDF) equipment when coming from remote monitors. The data inputs are switched serially b y the solid-state

multiplexer, under the control of the microprocessor,

to the A/D converter. Here, the analogue signals are converted t o digital form and then taken, together with up t o eight digital signals, via an interconnection bus to the microprocessor and the temporary data store (the RAM). A l l monitors are sampled once per minute. Data processing. The microprocessor functions according to the program

stored in the programmable read-only memory (PROM), a solid-state data-storage device whose contents is virtually indestructible. The main tasks of the 8-bit microprocessor are: (a) control of the input multiplexer; (b) start and control of the calibration cycle; (c) correction of measured values according to the results of the calibration cycle; ( d ) computation of periodic averages; (e) calculation of output data; ( f ) output of data and (9) control of perip herals. Output of c o n t r o l s i g n a l s t o t h e monitor. Up t o three control signals can

be sent to the pollution monitors via the digital output control. One of these signals is the calibration cycle control signal. Timing.

An accurate,

real-time

digital

clock

Is used for

timing all

events. A t the heart of the digital clock is a crystal whose output is used t o derive the minute, hour and day signals. From these, the microprocessor obtains the months and years. Data output. Ouput information is taken via appropriate control units to

the respective output peripherals, as shown i n the block diagram in Fig. 15.26. The modem output can be used either to connect a remote teletypewriter

to the micro-PDR,

or t o connect this to a central station.

connections are achieved

over

either

leased or

(dlalled link). During normal automatic operation,

switched

These remote telephone

lines

the following output data

are registered: (1) station identification, (2) status message, (3) date and time, (4) 1/2-h average, (5) 24-h average, (6) all data or functions manually entered into the system.

Automation i n environmental p o l l u t i o n m o n i t o r i n g Protection against

519

loss of data due t o power f a i l u r e . The microprocessor

and digital clock are provided with reserve power supplies, which ensures that, if the normal mains supply should fail, the timing continues uninterrupted and there is no loss of data from the RAM. A f t e r restoration of the normal supply following a power failure, the time, date and stored data are recorded by the output equipment and normal automatic operation is resumed. Recharging of the reserve power-pack also takes place when the operation from the mains supply is restored. No reserve supply is required by the PROM, whose memory contents are unaffected by disturbances t o the mains. The most complex fashlon of performing environmental sampllng, analysis and control is probably that used by automatic water and atmospheric pollution survey networks. A survey network consists of a series of stations, each of which has a modular multi-parameter anaiyser transmitting data to and receiving commands from a central computer that controls and organizes all the stations making up the network. The scheme of one such network for water control is depicted i n Fig. 15.27.

Fig. 15.27 Philips).

Scheme o f

river

water

survey

station.

(Courtesy

of

The site of a survey station must be in accordance wlth t h e pursued objec-

Automatic methods o f analysis

520

tives: in the case of water survey, the site should be as close as possible to the drainage point, where collected samples will be more representative than those taken at points where the waste waters would have merged throroughly with those of the river and the results obtained would be representative of the water quality of the river itself. Dual sampling can be carried out at the confluence of two water-ways to separately measure, record and transmit the data correspond i ng t o both. The overall functioning of a survey station involves three basic operations: data transmission and communication, data reduction and data recording. Data transmission by means of varlous systems such as the multitonal survey system (MDF) allows slgnals t o be sent i n both directions between the stations and the central unit by means of private or hired telephone lines. A single telephone lines enables transmission of up t o 25 data channels according to the recommendations from CCITT.

Digital transmission

(MDT), based on the

time-division multiplex principle, can prove economical i n sending large volumes of measurement, status or remote control signals, which must be converted to digital form prior t o transmission. Switched telephone lines shared with other users are of use when great distances are to be covered and hired lines are too expensive. I n this case, stations require a self-contained control and memory system t o collect and store information. This "Intelligent station" device sends information to the pollution data reductor i n the network centre when requested by this, so that the telephone line Is not continuously engaged. Normal transmission/reception units (radio links operating i n the VHF or UHF bands) can also be used for this purpose. This method allows up to 16 survey stations to be linked to the data reductor on the same carrier f r e quency, which is accompllshed by using a simplex radlo-link

with selective

calls t o each station in the network. The so-called 'pollution data reductor' (PDR) is the unit where data are processed. Depending on the size and purpose of the network, it can be as simple as an analogue tape recorder or as compllcatad as a programmable computer with various output readouts and the capability t o carry out alarm functions in real time. This information can be used t o warn those industrles causing pollution or being on the verge of suffering i t s consequences. The PDR can be of one of two types: fixed-program (without computer) and stored-program (wlth computer). Fixed-program PDRs are chiefly used in uncomplicated networks whose principal aim is to record data, whether instant concentrations i n analogue form or average concentrations In digltal form. Stored-program

PDRs are em-

ployed when on-line calculation and recording are required. The typical functions performed by this processing system are as follows: (a) estlmatlon of short-term pollution predictions; (b) checking of the functioning of the sta-

Automat ion i n envi ronment a1 pol 1ution moni t o r i n g

52 1

tions, transmission network and PDR; (c) recording of data on punched paper o r magnetic tape for batch processes; ( d ) data concentration and line connection t o a central pollution processor (CPP). The key u n i t in a stored-program PDR is the programmable mini-processor,

whose software consists of (a) a monitor

program r u n n i n g i n real time and governing the remainder of the programs, equipment and peripherals; ( b ) a program f o r handling of messages (errors, warnings, recordings of pollution data); (c) a program controlling data acquisition throughout the network every minute, hour, fraction of hour o r day; ( d ) a calibration program controlling the calibration cycle and the interpretation of the calibration data; (e) a keyboard function program controlling the communication between the user and the machine, and between the control centre and the stations i n the network. As a rule, it i s more convenient t o transmit measurements t o a control

centre, laboratory o r office, where they can be immediately read from the teletypewriter, from punched paper o r magnetic tape. The on-site recording of data i n the station is also of use f o r field research, although more limited as regards long-term information. A data reductor can also be installed in t h e station o r be connected t o it by remote control f o r averaging of some parameters (recording of integrated values).

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522

Automatic methods o f analysis

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Automation i n environmental p o l l u t i o n monitoring

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