b5 hazardous substances -monitoring and maintenance of control measures
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Table Of Contents Hazardoussubstances -monitoringandmaintenance of controlmeasures
6T
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alMonitoring Techniques6T
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Element B5: Hazardous substances -monitoring and
maintenance of control measures
Learningoutcomes
On completion of this element, candidates should be able to:
Describe the strategies, methods, and equipment for the sampling and
measurement of airborne contaminants
Outline the principles of biological monitoring
Outline the statutory and other requirements for the monitoring and
maintenance of control measures for hazardous substances
Relevant Standards
International Labour Office, Safety in the Use of Chemicals at Work, an ILO Code
of Practice, ILO, 1993. ISBN: 9221080064
Section 6: Operational control measures (see controls in S.6.5 S6.9)
International Labour Office, Ambient Factors in the Workplace, an ILO Code of
Practice, ILO, 2001. ISBN 922111628
Minimum hours of tuition 6 hours.
1.0 Measurement of airborne contaminants
So far we have examined the way that chemical agents can cause occupational ill-
health, the factors that influence the risk of harm to the individual and some examples
of substances and occupations that present a risk of harmful exposure. This enables us
to recognise when and where there is a risk of exposure to chemical agents. In this and
the following study unit we shall consider the next stage of the occupational health and
hygiene programme, which is to quantify the extent of the problem through
measurement.
Environmental monitoring, the work of the occupational hygienist, is a specialist
function that enables us to assess the risk of harm from exposure to chemical agents
by identifying and quantifying the level of exposure. To understand this important topic
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we must understand the principles of environmental monitoring, and then the actual
techniques used to sample, measure and analyse hazardous substances.
The subject of monitoring techniques and strategies for airborne contaminants is a
substantial topic in its own right and this unit is exclusively devoted to this component
of workplace monitoring for substances hazardous to health.
1.1 Principles of Environmental Monitoring
In our quest to prevent exposure to substances hazardous to health it is essential that
we are able, firstly, to recognise or identify hazardous agents, and then evaluate the
extent to which they represent a risk to health. Environmental monitoring techniques
are designed to enable chemical health hazards to be identified through qualitative
analytical techniques, then measured using quantitative techniques.
The health effects of exposure to chemical agents can be acute or chronic.
Consequently there are different types of measurement to account for this:
Long-term measurements to assess average exposure over a given time
period
Continuous measurements that can detect short-term acute exposure to high
concentrations of contaminants Spot readings to measure acute exposure if the exact point in time exposure is
known
We will be examining the different types of sampling procedures to enable these types
of measurement to be made later, but we begin by considering the range of analytical
techniques that are available to enable us to identify and quantify chemical agents.
An a l y t i ca l T e ch n i q u e s
In simple terms, the analytical techniques that we will be studying in this section, with
the exception of gas chromatography, all generally involve subjecting the substance in
question to a burst of energy (heat, X-ray, infra red, light) and examining the way the
substance responds. The response is characteristic of the substance being examined
and therefore can be used as a fingerprint for the particular agent. This usually
involves comparison of the response with a database of known chemical agents to aid
identification. In addition, the magnitude of the response can be used to estimate how
much of the agent is present. We will see how this operates in practice as we examine
the following specific techniques.
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GasChromatography
Gaschromatography is a valuable technique for the separation, identification and
measurement of organic contaminants. It involves a moving stream of the contaminant
under study mixed with a carrier gas (an inert gas such as helium) which is passed
over a solid, or a liquid adhering to a solid, packed in a column. The technique relies on
the components of the gas mixture being attracted to different extents by the material
in the column. As the gas mixture passes through the column, substances in the
mixture are attracted differently to the stationary column packing and are therefore
separated. The time taken for the substance to pass through the column, the retention
time, is fixed and depends on the particular substance and can therefore be used to
identify the substance. In this way a mixture of substances can be separated, or a
single substance identified from its retention time.
At the end of the separation process the gas mixture passes over a detector which
registers the retention time and also measures how much of the component is present.
If the signal intensity and retention time are plotted on a chart recorder a
chromatogram, such as that shown in Figure 1.2, is produced.
This example is of a hexane mixture and shows clearly the four components of the
mixture and their relative concentrations.
Figure 1.1GasChromatography
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Figure 1.2 - a chromatogram
If you examine the methods for the determination of hazardous substances listed in
Table 1.1 you can see the wide range of substances listed against techniques 1-3 for
which gas chromatography is used as an analytical method.
1. Charcoal pumped adsorption tubes and gas
chromatography
acrylonitrile,
carbon disulphide,
benzene, styrene
glycol ether,
glycol ether acetate
vinyl chloride
ethlene oxide
chlorinated hydrocarbons
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toluene
mixed hydrocarbons
2. Porous polymer adsorption tubes and gas
chromatography
acrylonitrile, benzene
glycol ether,
glycol ether acetate
styrene
dioctylphthalate
toluene
mixed hydrocarbons
3. Molecular sieve sorbent tubes and gas
chromatography
1,3-butadiene
4. Flame atomic absorption spectroscopy cadmium, lead
tetralkyl lead
5. X-ray fluorescence spectroscopy cadmium, chromium
6. Syringe injection technique organic vapours
7. Permeation tube method organic vapours
8. Colorimetric field method lead, formaldehyde
chromium,
9. Personal monitoring/filter method lead tetraethyl, beryllium
10. Gravimetricfiltration respirable/inhalable dust
coal tar pitch volatiles
11. Adsorbent tube/cold vapour atomic absorption
spectroscopymercury vapour
12. High performance liquid chromatography isocyanates
13. Diffusive sampler
14. Ionselective electrode fluorides hydrogen fluoride
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hydrogen cyanide
15. Infra red spectroscopy quartz
16. X-ray diffraction quartz
17. Phase contrast microscopy asbestos,
man-made mineral fibres
Table 1.1: Methods for the Determination of Hazardous Substances
1.1 Principles of Environmental Monitoring (Cont.)
Flame AtomicAbsorptionSpectroscopy
Flame atomic absorption spectroscopy is a useful technique for the identification and
measurement of metallic substances. The principle of operation is that if certain metals
are heated to high temperatures in a flame, electronic changes in the metal atom
cause a change in colour to the flame. A flame test is a simple way to identify an
element and a basic demonstration of this is the way that common salt (sodium
chloride) sprinkled into a flame will cause the flame to turn yellow. In contrast,
potassium gives a violet flame and lithium and strontium a red flame. Although the red
flames from lithium and strontium appear similar, the light from each can be resolved
by passing it through a prism into distinctly different colours. If the light resolved bythe prism is examined closely it can be seen to consist of a cluster of distinctive lines at
different parts of the spectrum. Each element has a characteristic line spectrum. It is
this particular fingerprint associated with the distinctive electronic changes that occur
when the metal atoms are subjected to high temperatures that is the basis of the
technique.
In practice an atomic absorption spectrometer is used for the analysis and the sample
in question is injected into an air-acetylene flame (to give a suitably high temperature)
and the resultant spectrum is analysed by the spectrometer. Since the resulting
spectrum is characteristic of the particular metal sample, both the identity and the
quantity of substance can be determined.
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Figure 1.3 - Flame AtomicAbsorptionSpectroscopy
Figure 1.4 - A diagram of a flame atomic absorption spectrometer
X-ray Fluorescence Spectroscopy
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X-ray fluorescence spectroscopy is another technique that will directly determine an
element from its characteristic spectrum. The basis of the technique is that if a beam of
X-rays impinges on a sample it will excite some of the atoms. The excited atoms are
unstable and undergo electronic rearrangement which causes emission of energy in the
form of X-rays whose frequencies are characteristic of the particular atom. Thus a well-
defined X-ray spectrum is emitted from the sample which can be used both to identify
the element and also estimate the quantity present.
Figure 1.5 - X-ray Fluorescence Spectroscopy
Infra Red Spectroscopy
Infra red spectroscopy is a widely used general chemical analytical technique. It is
based on the principle that the chemical bonds that connect atoms into molecules are
in a continuous state of vibration and the energy of this vibration falls within the infra
red wavelength range (2.5-15 m). If infra red radiation is passed through a sample,
absorption of energy will take place at the characteristic wavelengths of the chemical
bonds in the molecule. Different substances will contain different bonds and therefore
the absorption spectrum gives a characteristic fingerprint of the substance. You can
see an example of an infra red spectrum in Figure 1.7. Again the infra red spectrum
provides both a means of identifying the substance and also quantifying how much is
present.
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Figure 1.6 - Infra Red Spectroscopy
Figure 1.7 - The infra-red spectrum for ethanoic acid
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X-ray Diffraction
Infra red spectroscopy can sometimes be used to analyse a sample directly on a filter
as a solid and X-ray diffraction is another non-destruvtive analytical technique that can
be used for solids. It is based on the principle that a beam of X-rays passed through a
solid crystal will be deflected and scattered (diffracted) in a characteristic fashion,
which depends on the crystal structure and the spacing between the atoms. A
spectrum of diffracted wavelengths provides a characteristic fingerprint for the
substance.
Automated X-ray diffractometers generate an X-ray beam which is diffracted by a
crystal of the substance being analysed. Both the crystal and an X-ray detector rotate
under computer control to record the angles and intensities of thousands of X-rayreflection spots. After computer analysis of the data a molecular structure can be
determined to aid identification of the sample.
Figure 1.8 - X-ray Diffraction
Further information regarding X-ray Diffraction can be found at
http://www.utc.edu/Faculty/Jonathan-Mies/xrd/xrd.htmlthis also includes movies of
the X-ray Diffraction unit in use.
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Optical Microscopy
The most widely used analytical technique for samples containing fibrous dust, such as
asbestos, is optical microscopy.
To determine the concentration of asbestos fibres, dust sampling is carried out and the
dust is collected on a membrane filter then counted under an optical microscope.
Before counting the membrane filter is rendered transparent by treating it with a
suitable liquid. Since the membrane filter is already marked with a grid pattern, the
number of fibres within any grid square can be counted. A minimum of 20 squares
chosen at random is generally used, or a sufficient number of squares to count at least
100 fibres.
Phase contrast microscopy is used for this purpose, to enhance the contrast between
the fibre on the filter and the background. From the sample of fibres counted, the total
number of fibres collected can be estimated. The volume of air sampled is known from
the sampling time and the flow rate, so the concentration of fibres per unit volume can
be calculated.
Where it is necessary to determine the type of asbestos present, polarised light
microscopy is used. With this technique different types of asbestos fibres show
characteristic colours under various conditions of polarised light, and can thus bedistinguished and identified.
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Figure 1.9 - Optical Microscopy
Information regarding Optical Microscopy and Specimen Preparation can be found at
http://www.doitpoms.ac.uk/tlplib/optical-microscopy/tmicroscope.php?printable=1
1.2 MDHS Guidance on Analysis
In Table 1.1, Methods for the Determination of Hazardous Substances, we noted
the range of substances that can be analysed using gas liquid chromatography. You will
also have noticed that some of the other techniques we have described are also listed
in Table 1.1. This list of techniques is a summary of MDHS (Methods for the
Determination of Hazardous Substances) Guidance on Analysiswhich is a series of
detailed descriptions of analytical methods which have been approved by theHealth
and Safety Executive.
HSElink -Methods for the Determination of Hazardous Substances (MDHS) guidance
The MDHS series of guidance sets out approved analytical methods for most chemical
agents that are likely to be encountered in the workplace. They provide reliable and
consistent methods to ensure that accurate measurements of workplace chemical
agents can be made. The use of these standardised methods, in conjunction with the
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hygiene standards that we will consider, enables you, as a health andsafety
practitioner, to demonstrate that adequate controls for chemical agents are in place.
The analytical methods we have considered give an indication of the sorts of
techniques that are available for the identification and analysis of workplace
contaminants. However, before we can carry out any measurements on chemical
agents we must first obtain a representative sample of the contaminant that we are
concerned about. Obtaining a relevant and accurate sample is as important as the
analysis itself and much of the MDHS Guidance on Analysis is concerned with
specifying methods of sampling.
1.2.1 Guidance on Analysis United States of America
The United States has five types of written methods:
1. Methods in the National Institute for OccupationalSafetyand Health (NIOSH)
Manual of Analytical Methods (NMAM). These methods are available in
downloadable files from the Internet at
http://www.cdc.gov/niosh/nmampub.htmlwhich also gives information on
obtaining the full printed version. The NIOSH site also links to MSHA,EPA,
ASTM, and ISO.
2.
Methods developed by the OccupationalSafetyand Health Administration(OSHA) Analytical Methods Manual. OSHA also has a list of partially validated
methods, in the IMIS series, which is available in paper form or CD-ROM. Both
sets of methods can be accessed on the Internet athttp://www.osha-
slc.gov/SLTC/index.html.From this site, OSHA Technical Manual selects OSHA
Samplingand Analytical Methods and ChemicalSamplingInformation selects
the IMIS methods.
3. Methods developed by the Intersociety Committee (IC).
4.
Methods developed by the U.S. Environmental Protection Agency (EPA). Thesemethods are written for ambient air applications, but many are applicable also to
the workplace. The methods are available on the Internet at
http://www.epa.gov/standards.html.
5. Methods developed by the American Society for the Testing of Materials (ASTM).
These are indexed underhttp://www.astm.organd selecting ASTM store.
1.2.2 Guidance on Analysis International Standards
Organisation (ISO)
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Internationally agreed standards (which do not necessarily conform to theCEN/TC137
performance requirements, but generally include precision data according to ISO 5725)
and published by the International StandardsOrganisation,Casa postale 56, CH-1211
Genve, Suisse. Many of these methods are translated into National Standards. The
web site ishttp://www.iso.ch.Selecting ISO catalogue international standards
(HTML) ICS field 13 ICS field 13.040.30 leads to workplace air quality standards.
1.2.3 Comparison of air-sampling methods for nickel in a
refinery
Air sampling methods
United States
Air sampling for substances with time weighted average exposure limits, should be
conducted in terms of the correct sampling technique referred to in the National
Institute for OccupationalSafetyand Health (NIOSH) Manual of Analytical Methods
(MAM) (Plog 2002:505).
NIOSH method 7300, is commonly used for the detection of elements which includes
sampling for the total fraction of nickel dust (National Institute for OccupationalSafety
and Health, Manual of Analytical Methods, Method 7300, 1997:1). Since 1998 theOELfor nickel and nickel species were set for inhalable dust and NIOSH method 7300,
although still widely used, is not a suitable sampling method (American Conference of
Governmental Industrial Hygienists, 2003:43)
United Kingdom
EH 40 (1999:14-15) states that sampling methods that should be used in the United
Kingdom, can be found in theHSEs sampling series The MDHS. The followingrelevant sampling methods for nickel and nickel species are listed namely:
MDHS 14/3 and,
MDHS 42/2.
MDHS 14/3 (2000:1) describes the general methods for sampling and
gravimetrical analysis of respirable and inhalable dust fractions.
MDHS 14/3, measures particulate matter in accordance with the ISOs, as well
astheCENs, respirable dust convention.
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MDHS 42/2 (1996:1) describes the measurement of nickel and inorganic
compounds of nickel in air.
South Africa
The Department of Minerals and Energy (DME) in South Africa, has guidelines for the
gravimetric sampling of airborne particulate matter. The guidelines provide for the
sampling of the total dust fraction and for the respirable dust fraction (Department of
Minerals and Energy, 1994:1). No other reference is made to sampling methods for the
measurement of specific hazardous chemical substances in the Hazardous Chemical
Substances Regulations (South Africa, 1995:5-6).
Discussion
The American, United Kingdom and the South African exposure limits, are set, based
upon obtaining, personal samples, which represents inhalable dust exposure
concentrations of the measured workers (American Conference of Governmental
Industrial Hygienists, 2003:8.; EnvironmentalHygiene40, 1999:14.; South Africa,
1995:26).
It would appear that NIOSH method 7300 which has been set for the measurement of
total dust, using a Casette sampler is the least desirable sampling method to use todetermine compliance to the exposure limits as:
occupational exposure limits are set for the inhalable dust fraction,
the cassette sampler under estimates exposure concentrations,
the cassette sampler collects the total dust fraction which is difficult to define
correctly
1.3 Sampling For Gases and Vapours
Sam p l i n g M e t h o d s
The two basic methods of collecting gaseous samples are:
Grab sampling: An actual sample is taken in aflask, bottle bag or other
suitable container Samples are collected over a period of around a minute
Useful for a peak concentration or when concentrations are relatively constant
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Continuous or integrated sampling: Gases or vapours are removed from the air
over a measured time period and concentrated by passage through a solid or
liquid sorbent
The sample is collected by:
(i) Dissolving in a liquid
(ii) Reaction with a solution
(iii) Collection onto a solid sorbent
Samples are collected over aperiod of up to several hours
Useful if:
(i) The contaminant concentration varies with time
(ii) The contaminant concentration is low
(iii) A time weighted average exposure is required
Samplingmay be achieved:
(i) Actively (using a pump)
(ii) Diffusively (natural diffusion)
Gr a b S am p l e r s
Evacuated Flasks
A flask fitted with a valve at each end (Figure 1.10) is evacuated through one valve
whilst the other valve is kept closed. The open valve is then closed to seal the vacuum.
When the valve is opened a sample of the atmosphere under test is drawn into the
flask.
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Figure 1.10:Gasor Liquid DisplacementSamplingBottle
Gasor Liquid Displacement Container
A flask similar to the one in Figure 3.3 can be connected to a pump and the vessel
filled with the test atmosphere through one valve by pumping out the air in the flaskthrough the other valve.
Another method is to fill the flask with water and then let the water drain out slowly
from one valve as the test atmosphere is sucked into the flask through the other valve.
Obviously this procedure cannot be used to collect water-soluble gases.
Flexible Plastic Containers
Plastic bags can also be used as grab samplers. They have the advantage of beinglight, non-breakable and simple to use.
Hypodermic Syringes
Syringes of 10 to 50 ml volume can be used to draw a test atmosphere into the body
of the syringe as the plunger is extended. They are available in glass and disposable
plastic and are cheap, convenient and easy to use.
1.4 Continuous Sampling
Active Samplers
Liquid Sorbents The fourtypes of sampler using liquid sorbents to collect gases
and vapours are:
Gaswashing bottles :
(i) Suction applied to an outlet tube causes sample air to be drawn through an inlettube into the liquid contained in the sampler.
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(ii) Suitable for collecting non-reactive gases and vapours that are highly soluble in the
liquid sorbent, e.g. methanol and butanol in water; esters in alcohol.
(iii) Suitable for collecting gases and vapours that react rapidly with the reagent in the
sampling medium, e.g. ammonia neutralised by dilute sulphuric acid.
(iv) The midget impinger (Figure 1.11) is a commonly-used sampler with an air flow
rate of 1.01/min and 10 ml of liquid sorbent.
The impinger is connected to a pump and can be attached to the workers clothing.
(i)Used for collecting gas samples that are only moderately soluble in, or are slow in
reacting with, the reagents in the collecting vessel.
(ii) The spiral or helical structures in the collection vessel provide a higher collection
efficiency by allowing a longer residence time of the contaminant with the reagent for
slower acting and less soluble substances.
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Figure 1.11: The midget impinger
Fritted bubblers:
(i) Used for collecting gas samples that are less soluble in the collecting medium.
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(ii) Design is similar to the impinger but the collection vessel contains sintered or
fritted glass, or multi-perforated plates at the bottom of the collection tube. Air drawn
into these devices is broken up into very small bubbles and the froth that develops
increases the contact between gas and liquid.
Glass-bead columns:
(i)Used for special situations where a concentrated solution is needed.
(ii) Glass beads wetted with the liquid sorbent provide a large surface area for the
collection of the sample. However, the rate of sampling is necessarily low.
Cold Traps
Cold traps are used where it is difficult to use any other method of collection.Vapouris
separated from air by passing it through a coil immersed in a cooling system such as
dry ice (solid carbon dioxide) and acetone, liquid air or liquid nitrogen. The
disadvantage is that water is condensed along with the organic materials being
sampled.
PlasticSamplingBags
Plastic bags, as used in grab sampling, can be used to collect air samples over periods
of a shift or longer in conjunction with a pump.
Solid Sorbents
Absorbent solids can also be used to collect airborne contaminants. The twoprincipal
materials in use are:
Charcoal
Activated charcoal is an excellent sorbent for most organic vapours. The most common
procedure is to use activated charcoal sampling tubes of the type shown in Figure 1.12.
A glass tube with flame-sealed ends contains two sections of activated charcoal
separated by 2 mm portions of polyurethane foam. Immediately before sampling the
ends of the tube are broken and the tube is connected to a calibrated pump to draw
the atmosphere under test through the tube.
The duration of the sampling may be several minutes up to 7-8 hours, depending onthe tubes capacity. The air flow should be checked with a flow meter from time to time
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while the sampling is in progress. At the end of the sampling period the tube is capped
at each end ready for analysis.
The first step in the analysis procedure is to remove the sample from the charcoal,
usually using solvent desorption with carbon disulphide. Although this does not remove
all the sample it is possible to apply a correction to take account of the efficiency of
desorption.
Once the sample has been desorbed from the charcoal it can be analysed using one of
the techniques described later.
Figure 1.12: Charcoal sampling tube
Silica Gel
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Silica gel is another effective sorbent for collecting gases and vapours. The method of
use is similar to that of charcoal, involving sample tubes and a desorption solvent
which, in the case of silica gel, is usually water or methanol.
The advantagesof silica gel over charcoal include:
(i) Many contaminants can be removed from the sorbent by using common solvents
such as water or methanol.
(ii) Certain substances such as amines, nitro compounds and some inorganic
compounds are unsuitable for absorption on charcoal.
(iii) Avoidance of the use of carbon disulphide (a highly flammable and toxic solvent)
for desorption.
The disadvantageis that silica gel has a high affinity for absorbing water and, if there
is much moisture in the air being sampled, the water will displace any absorbed
organic solvents from the silica gel surface. This limits the quantity of humid air that
can be passed through a silica gel absorption tube.
Thermal Desorption
Another method of desorbing the collected sample is to heat the sample tube and drive
off the substance that has been absorbed. This avoids the use of hazardous solvents
such as carbon disulphide and provides a less laborious method. In general this is not a
practical method with charcoal sorbents because the high temperature needed to drive
off the sample would result in its decomposition. Consequently this method uses
carbon molecular sieves or porous polymer sorbents.
The thermal desorption procedure uses larger tubes than previously described,
desorption can be made fully automatic and analysis can be carried out using gaschromatography.
1.5 Sampling Equipment
We have seen from the descriptions given above that the continuous sampling
procedure involves a sampling device (either liquid or solid sorbent) connected to a
sampling pump and an air metering device. This enables the contaminated air to be
pulled through the sampling device at a known flow rate. From this both the amount of
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contaminant and the total volume of air can be determined to enable the average
concentration of airborne contaminant to be calculated.
Pump
The pump should have an adjustable flow rate and be able to operate continuously for
a period of up to 8 hours. For personal sampling the pump should be able to be worn
by an operator whilst carrying out their normal duties.
Flow Measurement
Flow measurement is important in enabling an accurate estimation of the total volume
of air that has been sampled. An external flow meter with a known level of accuracy
should be used rather than relying on any flow meter built into the pump. These are
useful as a guide to the operating flow rate and indicate that the pump is working, but
are not accurate enough unless calibrated in some way during the sampling process.
One method of measuring flow is to use a bubble flow meter. This consists of a
calibrated tube with a soap film that is drawn along the tube by the pump under test.
The passage of the film is timed between two marks on the tube which represents a
known volume. From these measurements the flow rate for the pump in terms of
volume per unit time can be calculated.
Analysis of Gases and Vapours
The description of the various methods for continuous sampling given above shows the
range of sample collection methods available. Table 1.2 lists a range of gases or
vapours that can be sampled by absorption on charcoal. Table 1.3 gives examples of
types of sampler used for the collection of airborne contaminants, the sorbent used and
the analytical method used to determine the quantity of substance collected.
GasorVapour Desorption
Acrylonitrile Carbon disulphide
Benzene Carbon disulphide
Carbon tetrachloride Carbon disulphide
Chlorobenzene Carbon disulphide
Chloroform Carbon disulphide
1,2-Dichlorobenzene Carbon disulphide
Dichloromethane Carbon disulphide1,2-Dichloropropane 15% acetone in cyclohexane
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2-Ethoxyethanol 5% methanol in dichloromethane
Ethylene oxide Carbon disulphide
2-Methoxyethanol 5% methanol in dichloromethane
2-Methoxyethyl acetate 5% methanol in dichloromethaneStyrene Carbon disulphide
Tetrachloroethylene Carbon disulphide
1,1,1-Trichloroethane Carbon disulphide
Trichloroethylene Carbon disulphide
Vinyl chloride Carbon disulphide
Table 1.2: Examples of Gases and Vapours that can be Sampled byAbsorption
on to Charcoal and the Desorption Medium
Gasor
VapourSampler Sorbent Analysis
Acetaldehyde Bubbler Water Iodoform reaction
Acetic acid Wash
bottle
Glycerol/water pH change
Acetonitrile Syringe Permanganate Colour change
Amines Bubbler HCl in isopropanol Ninhydrin/spectrophotometry
Ammonia Bubbler Dil H2SO4 Phenol/hypochlorite/
spectrophotometry
Aniline Bubbler Dil H2SO4 Spectrophotometry
Benzene U-tube Silica gel Spectrophotometry
Butanol Bubbler Water Chromate oxidation
Carbon
disulphide
Glass
beadsCopper/diethylamine Colour reaction
Chlorine Bubbler Methyl orange Spectrophotometry
Ethanol Impinger Water Chromate oxidation
Formaldehyde Impinger Bisulphite Iodine titration
Hydrogen
sulphide
Bubbler Iodine soln Iodine oxidation
Methanol Impinger Water Fuchsin/formaldehydeNitrobenzene Bubbler Ethanol Spectrophotometry
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Nitrogen
dioxide
Bubbler Naphthylethylenediamine Colour reaction
Ozone Impinger KI Titration
Phenol Impinger Ethanol SpectrophotometrySulphur
dioxide
Impinger Tetrachloromercurate Spectrophotometry
Toluene U-tube Silica gel Spectrophotometry
Toluene Impinger Acid Diazotation/coupling/
diisocyanate spectrophotometry
Table 1.3: Examples of Samplers, Sorbents and Analytical Methods for
CommonGasandVapourContaminants
Calculation of Result
As indicated above the collected sample is analysed either directly if a gas sample or
liquid sorbent, or after desorption if collected on a solid sorbent.Gassamples will be
expressed directly as a concentration in ppm. Samples absorbed in another medium
will initially be expressed as a concentration which can be converted to a mass by
multiplying by the sample volume. In calculating the actual average concentration of
airborne contaminant, factors such as the sampling efficiency of the collector (i.e. what
percentage of sample dissolves in the collecting medium) and the desorption efficiency
(i.e. how much of the sample is recovered from the sorbent after desorption) need to
be included in the calculation. These factors are usually determined by using samples
of known concentration as a control.
1.6 Diffusive Samplers
We have seen how continuous sampling can be carried out by pumping contaminated
air through a collection device to trap and measure the quantity of airborne
contaminant. This is termed active sampling since the process involves the active
movement of air through the sampler.
Another important method of sampling involves the use of a passive sampler or
diffusive sampler. This is a device which takes samples of gas or vapour from the
atmosphere under test by a physical process such as diffusion, but does not involve the
forced movement of air through the sampler.
Method of Operation
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Pollutants are removed from the atmosphere at a rate controlled by diffusion through a
static layer or permeation through a membrane. The mass uptake by the diffusive
sampler depends on the concentration gradient (i.e. the concentration of contaminant
in the atmosphere compared to the concentration of contaminant in the sampler), the
time of exposure, and the area of sampler exposed to the atmosphere. Complications
to the process include fluctuating concentrations, sorbent saturation, wind velocity and
turbulence at the sampler surface, temperature and pressure.
The two principal types of design are shown in Figure 1.12. In Figure 1.12 (a) you can
see a badge-type sampler which has a flat permeable membrane supported over a
shallow layer of sorbent. Figure 1.12 (b) shows the tube-type sampler which has a
smaller permeable membrane supported over a deep metal tube filled with sorbent.
There are diffusive equivalents of most of the active systems, such as a liquid-filledbadge equivalent to the impinger, a charcoal badge equivalent of the charcoal tube and
also a thermal desorption badge. It is accepted that active and diffusive sampling are
complementary approaches with each having useful areas of applicability, and that
there seems to be no significant difference between accuracy and precision of diffusive
sampling and active pumped sampling.
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Figure 1.12 (a) Badge Sampler & Figure 1.12 (b) Tube Sampler
Factors Affecting Performance
Temperature and pressure
Mass uptake is independent of pressure but depends on the square root of absolute
temperature. In practice this means that temperature dependence at ambient
temperatures can generally be ignored but increased temperatures may adversely
affect the capacity of the sorbent.
Humidity
High humidity can adversely affect the absorption by charcoal badges.
Concentration variations
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It is possible that a sudden rapid fluctuation in contaminant concentration may be
missed before it has a chance to diffuse into the sampler. Since the time taken for
diffusion into the sampler varies between 1 and 10 seconds the sampling time will
usually be well in excess of this and therefore this effect will not present a significant
problem.
Sorbent efficiency
Diffusive samplers rely on the sorbent having a high affinity for the contaminant being
sampled and therefore a suitable sorbent being selected for the contaminant in
question.
Face velocity
This is an important parameter: if there is insufficient air movement over the face of
the sampler, transport of pollutant to the membrane will be limited and the effective
sampling rate will be reduced.
If there are high air velocities inducing turbulence in the sampler body the diffusion
path length will be reduced and the sampling rate increased. The geometry and design
of samplers should be such that sampling rates should be constant within the range of
air velocities likely to be encountered in the workplace, but badge-type samplers usedin static positions may experience air flows below the critical value for this type of
sampler.
Calculation of Result
The method of calculation of the result is similar to that for active samplers in that the
collected sample is analysed and the total mass of the sample determined; the total
sample volume is calculated from the effective sampling rate (which depends on the
diffusion coefficient of the substance and the length and area of the diffusion path [thegeometry of the sampler]) and the time of exposure (sampling time). This gives an
average concentration in mg/m3.
1.7 Sampling Procedures
In the previous two sections we examined the range of techniques available to analyse
and measure workplace contaminants and also the different methods available to
sample gases and vapours.
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We are now ready to move on to consider how these occupational hygiene techniques
are used in practice. We shall now examine the different types of sampling procedure
that are available for use in the workplace, and look at some examples of actual
measuring instruments that may be used for environmental monitoring.
Before we look at sampling procedures we must remind ourselves what the purpose of
environmental measurements are:
To give a qualitativeanalysis of an atmosphere, i.e. to indicate the presence of,
and identify, contaminants.
To provide a quantitativeanalysis, i.e. to determine exact concentrations and
assess compliance with hygiene standards, or to assess exposure.
To indicate the developmentof a potentially hazardous concentration, i.e. toact as an alarm system.
Before any analysis of a workplace atmosphere is carried out, it is vitally important that
a representative sample of the environment under test is obtained. Any analysis,
however sophisticated, is useless in terms of the data produced unless the sample
analysed is representative of the particularhazardbeing monitored.
T y p e s o f Sam p l e
You will remember that there are in general threeforms of sample:
The spot or grabsample, taken at one point (or in a limited area); it is
representative of the sample area at that point in time.
The time averagedsample, taken over a period of time and after analysis the
results will give the total contamination. A time average can be deduced by
dividing total concentration by the time. This is sometimes termed continuous
sampling.
The continuous monitoredsample, continually taken and analysed during the
monitoring. At the end of any period of time a record of the variation inhazard
level in the vicinity of the sample point is obtained. This system is used onvinyl
chloride(chloroethene) plants. Continuous sampling systems can be used in
conjunction with alarm systems; when a set level is exceeded the alarm is
activated.
Sam p l e P o s it i o n i n g
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The equipment used to carry out an analysis may vary, depending upon the type of
sample taken and the sampling location. There are threegeneral positions for samples
to be taken:
In the general working atmosphere, e.g. ozone monitoring in a welding shop or
oxygen deficiency in a closed vessel (grab sample).
In the operators breathing zone, e.g. dust collectors (time averaged sample).
At a position close to the contaminant generation, e.g. where beryllium metal is
being machined (continuous monitored sample).
Sam p l i n g F r e q u e n c y
Some sampling procedures are laid down in guidance notes in conjunction with
Statutory Regulations. For asbestos fibres, sampling should take place for 4 hours to
conform with the requirement of the control limit. This may be altered to take into
consideration factors that might upset the taking of a viable sample, e.g. if the
collected fibre density was low, then extra time would be used to provide the required
conditions. In this case the fibre concentration would be adjusted to give the corrected
time requirement.
Samplingfrequency will depend to some extent upon the risk level of the contaminant
being analysed. When entering a confined space for inspection purposes, an initial
sampling of the atmosphere would be satisfactory, provided the environment was safe.
If welding is to be carried out, regular grab samples may be satisfactory. If there is a
likelihood of excess fume generation then continuous monitoring would be more
appropriate.
In processes where lead is ahazard,bi-monthly sampling is recommended, provided
conditions remain satisfactory. More frequent samples are required if stable conditions
cannot be achieved.
Me a su r e m e n t P r o c e d u r e s
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There are basically twogeneral procedures used for making environmental
measurements:
The first procedure involves taking the sample, then carrying out an analysis in
separate equipment, often away from the sampling position, i.e. in a laboratory.
Measurements of dust concentration, e.g. asbestos fibres, are carried out in this
way.
In the second procedure, sampling and analysis takes place in the same
instrument. One of the most commonly used instruments is the stain tube
detector for gaseous contaminants.
Me t h o d s o f Sam p l in g
There are twomain ways that an airborne contaminant can be sampled: diffusion
sampling and mechanical sampling, which we looked at earlier in this unit.
In diffusion samplingthe contaminant passes over the sampling system in natural
air currents and diffuses into a chamber containing an absorbent material. At the end
of a given period of time, usually an 8-hour shift, the sampler is sent off to a laboratorywhere the contaminants can be desorbed and analysed. The system is sometimes
described as passive sampling.
An example of such a system is the Draeger ORSA (ORganic SAmple) personal gas
measuring unit illustrated in Figure 1.13. The small glass tube containing the special
absorbent activated charcoal, is supported in a clip that can be worn in the breathing
zone of the person at risk. The mass of contaminant absorbed on the charcoal depends
upon its concentration in the air, the time of exposure and its diffusion characteristic
(i.e. some materials will diffuse quicker than others and therefore more mass will be
absorbed).
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Figure 1.13
With a knowledge of the diffusion characteristics, time of exposure and mass absorbed
(from analysis), the time averaged concentration can be calculated.
The mechanical samplingsystem uses a pump to provide air flow through the
sampling device or analysing instrument. The use of reciprocating diaphragm pumps or
peristaltic pumps enables volume or air flow measurements to be monitored as each
stroke of the diaphragm or rotation of the compressor delivers a measured quantity.
This is sometimes called active sampling.
1.8 Analytical Mechanisms
There are threebasic analytical mechanisms used in environmental measuring
instruments: chemical, electrical and physical. They can be used separately but are
more generally used in combination, depending upon the particular analysis involved:
Chemicalreactions are usually designed to produce a coloured product which
enables a qualitative analysis to be made, i.e. simple detection of a contaminant.
Quantitative analysis is done by measuring the depth of colour produced; or, in stain
tubes, the amount of reactant used in the detection reaction.
Electricaldetection is usually arranged in conjunction with chemical or
electrochemical processes, e.g. combustion on a resistance wire or current generation
between electrodes.
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Physicalmethods may involve the use of ultraviolet or infra red radiation. The
absorption of the radiation by the gaseous contaminant is proportional to its
concentration, e.g. some mercury vapour analysers use ultraviolet radiation systems.
Other physical processes are visual microscopic analysis, e.g. asbestos fibres,
gravimetric analysis, size classification, using cyclone separators.
2.0 Measuring Instruments
For examination purposes you need to be able to describe the principles of operation
and methods of use of selected types of instruments. The information presented here
will not make you a competent analyst. To use hygiene equipment you will have to
receive practical training and develop a technique.
S t a i n Tu b e D e t e c t o r s - T h e D r a e g e r
Stain tube detectors provide a convenient method of analysing gaseous contamination
of the workplace air.
The principle of operation is very simple: a known volume of air is drawn over a
chemical reagent supported in a glass tube. The contaminant reacts with the reagent
and a coloured product, a stain, is produced.
The technology behind the manufacture of commercially viable stain tubes and their
accurate functioning is extremely complex. It has taken many years to develop since
the idea was first put into operation in about 1920, when carbon monoxide in mines
was detected and analysed by this technique.
Stain tube detectors are now made to allow grab samplingor long-term sampling,
operated by hand bellows, hand pistons or motorised pumps. The ubiquitous
breathalyser is a stain tube detector system, but the contaminated air is blown throughthe tube to provide a volume of sample controlled by the bag.
Draeger Multi-gas Detector
The Draeger as it is more generally known, is a common instrument used for
environmental testing. The unit consists of two main parts, the bellows pump and the
Draeger tube, selected to suit the particular measurement to be carried out:
Bellows Pump
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The bellows hand pump is shown in Figure 2.1. and the basic structure of the bellows
hand pump is shown in Figure 2.2.
The pump is designed to draw in 100 cm3of air with one stroke. To achieve this, the
bellows must be fully compressed before it opens automatically by the spring to its
maximum volume, controlled by the limiting chain. This mode of operation is
comparable to a dosage pump. The time taken for the bellows to open fully from the
closed position gives one pump stroke. The stroke time will depend upon the type of
Draeger tube being used and can vary from three seconds to 40 seconds.
Owing to the time involved and the number of strokes required for a particular
measurement, it is very important to have a stroke counter fitted to the unit.
Never rely on memory!
Figure 2.1: The bellows hand pump
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Figure 2.2: The basic structure of the bellows hand pump
Detector Tubes
The detector tubes contain a reagent which reacts with the contaminant in the air flow
passing through it to cause a coloured reaction.
The method of controlling the colour developed is either by drawing a fixed volume of
air through the tube using a specified number of strokes, or by counting the strokes
required to produce a colour change.
In the first methodthe tubes are marked with a graduated scale; the longer the stain
produced the higher is the concentration of contaminant. This is the most commonly
used system: they are sometimes called scale tubes.
In the second method, used less frequently, the greater the number of strokes taken,
i.e. the greater the volume of air sampled, the smaller is the concentration of the
contaminant, e.g. for the olefine 0.5% detector tube, five strokes indicate 500 ppm,
while ten strokes indicate 200 ppm.
2.0 Measuring Instruments (Cont.)
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A scale tube is illustrated in Figure 2.3.
Note the colouration on the used tube indicating a concentration of 50 ppm carbon
monoxide, the stroke number n = 10, and the arrow showing the end to be inserted
into the pump.
The formation of the colour shows just how precisely the indicating material has to be
made. The reagent has to be evenly distributed through the carrier material and
accessible to the contaminant so that it reacts quickly enough to give the colouration
within the scale markings.
As the reagent is used up by the contaminant, the contaminant is able to passs further
along the tube to react, and a higher concentration is indicated.
Automatic Multigas Detector
A refinement on the bellows type pump is the automatic multigas detector. This is an
electrically operated bellow pump model which can be set to switch off when the
selected number of strokes for the particular tube is complete. It is useful where an
operator has to be free during testing and where the measurements require a high
number of strokes.
Polytest Tubes
Some tubes, called polytest tubes, are designed to make qualitative measurements to
determine only the presence of potentially harmful substances. Varying colour and
stain length sometimes give an indication of the possible contaminant.
2.1 General Method of Operation of Stain Tube Detectors
Select the appropriate tube for the measurement being made, taking note of anypossible cross-sensitivity.
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Break the end off the tube to be inserted into the pump. Use the tube end-breaker
provided.
Insert the tube into the pump and exhaust the bellows by fully depressing the front
plate.
Allow the system to remain in this state for a few minutes and check for possible
leaks.
If there are no leaks, break off remaining tip in an uncontaminated atmosphere.
Cover end with rubber cap provided.
Select the sampling position, remove rubber cap and proceed to carry out the
sampling procedure, e.g. the given number of strokes for a scale tube and the time
allowed for the colour to develop fully.
Note the reading and record the result and sample position.
Remove the stain tube, cover both ends with a rubber cap and dispose of it
according to the manufacturers instructions.
Problems with Stain Tube Detectors
You should be aware of some of the problems related to the use of stain tube detectors
as their control will help to make fuller and more effective use of the stain tube
system:
The rate of flow of air is important, so the stain tube should have the ends removed
properly.
The accuracy of the sampled volume is critical, therefore the bellows action must be
fully operated for every stroke. The number of strokes must be recorded accurately,
hence the need for an effective counter. Leaks must be eliminated.
There may be the possibility of cross-sensitivity of tube reagents to other substances
than the one being analysed. This will be indicated on the data sheet accompanying the
particular stain tube.
There may be problems caused by variations in temperature and pressure. Stain
tubes are designed to operate at about 20C and one atmosphere pressure. Variationin atmospheric pressure will probably be within the limits of accuracy of the system,
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although changes in altitude could cause problems. Normal variations in temperature
may be problematic; remember, a change of 10C can cause a reaction rate to be
doubled or halved. With ambient temperature ranging between 0C and 30C, the
potential for error is considerable.
Because of the complexity of the indicating reagent, tubes have a shelf life, so care
must be taken to turn over stock and only to use currently operative tubes.
Reagent complexity also causes a variation between each tube; hence, judgments
cannot be made on one grab sample.
Hand-operated stain tube systems are capable only of a point in time or grab
sample. Long-term tubes have now overcome this problem.
2.2 Long-term Stain Tubes - Draeger Polymeter
To overcome the problem of point-in-time analysis, long-term tubes have been
developed. The Draeger Polymeter long-term testing system consists of a battery
powered peristaltic pump, providing 15 cm3/minute air flow rate, with a built-in counter
and a special long-term stain tube. The whole unit is small enough to be carried, with
the stain tube in an extension section, or to be easily positioned for static operation.
The stain tubes are marked in (equivalent to ppm). They are similar to the normal
stain tube except that the time-weighted averageconcentration is not indicated but
has to be worked out.
The average values are calculated using the following equation:
The air volume is calculated by multiplying the number of revolutions of the peristaltic
pump by the amount of air displaced from the pump tube during one revolution of the
tube compressor.
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V a p o u r A b s o r p t i o n T u b e s
As we saw earlier vapour absorption tubes with an identical appearance to stain tubes
have now been developed to collect samples of organic vapour on activated charcoal or
silica gel. They can be attached to a long-term polymeter or an automatic multigas
detector. After an appropriate sampling time, the tubes can be sealed and sent off to a
laboratory for more sophisticated chemical analysis, e.g. gas chromatography.
2.3 Oxygen Monitor
Analysis of a working environment to monitor or determine the concentration of
oxygen is very important. For concentrations below 20% oxygen, the possibility of
death or brain damage from simple anoxia has to be considered. For concentrations
above 20% enhanced fire risk is the problem, with the possibility of horrific burns to
operators and excessive fire damage to property.
In the operating condition the oxygen in the air sample monitored diffuses into the
sensor through a special membrane. It then passes into the electrochemical measuring
system, where the resultant electrochemical process produces electric current directly
proportional to the oxygen concentration.
The principles of operation of the sensor probe are illustrated in Figure 2.4.
The signal produced by the electrochemical reaction is transmitted to a direct readoutgauge giving the oxygen concentration in percentage oxygen. The instrument can be
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pre-set to a given oxygen concentration which activates an alarm system. The
electrochemical sensor is not directly attached to the main instrument but is connected
by a lead, thus enabling more flexibility in use when it is carried by the operator. The
instrument is also suitable for static monitoring in workplaces, especially where
compressed or liquid oxygen is being used.
E l e c t r o n