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Table Of Contents Hazardoussubstances -monitoringandmaintenance of controlmeasures

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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

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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

Documents

B5 Hazardous Substances -Monitoring and Maintenance of Control Measures