Air Sampling

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The basic need of air sampling and work environment monitoring and analysis is to find the level of pollution and to work out a strategy to reduce it. The air around us is filled with numerous hazardous materials and bacteria that can cause several health problems. Such workplace monitoring is essential for good health and good housekeeping. Air sampling is a method used to find out what type of hazardous materials and bacteria are present in your environment. 

Purpose Of Air Sampling

  1. To determine type and concentration of exposure due to health hazards to workers
  2. To determine the types and effectiveness of the control measures provided, any change if necessary in them and new control measures to be provided
  3. To investigate complaints and
  4. For research purposes.

Types Of Air Sampling

1.Personal Sampling

In personal sampling, the sampling device is worn by the worker near his breathing zone to evaluate personal or individual exposure to him.

2.Area Sampling

In area sampling, the air samples are taken at fixed places in a workroom or confined spaces to evaluate general concentrations of flammable, explosive, or toxic material in the air for the purpose of isolation or restriction to work or to design the control measures. It includes continuous monitors for leak detection, ventilation failure, equipment malfunction, etc.

3.Grab Sampling

Short period (instantaneous) sampling is called grab sampling and long-period sampling is called integrated sampling. Grab sampling is used to measure concentration at a particular time (at least two samples within 5 minutes) e.g. peak value of NH3 or Cl2 at a particular time.

The sample is collected in an evacuated flask or plastic bag, sealed and sent to a laboratory where trace analysis is carried out by gas chromatography, IR spectrophotometry, etc. Direct reading instruments can also be used for grab sampling. Temperature and pressure should be recorded during sampling. It should not be used for reactive gases.

4.Integrated Sampling

Integrated sampling is carried out by direct-reading instruments (e.g. gas detector tubes or digital meters) to measure STEL value for 15 minutes and TLV for 8 hr TWA limits. An air-sampling train consisting of the air-inlet orifice, collection media (solid or liquid sorbent, filters, and passive monitors), air-flow meter, flow-rate control valve, and suction pump, is used by qualified and trained personnel.

Types of sampling are also classified as

(1) Passive or diffusive’ air sampling

Passive or diffusive’ air sampling involves the collection of airborne gases/vapors through a diffusion barrier onto an absorbing medium without the use of an air sampling pump and

(2) Active air sampling

Active air sampling involves the collection of airborne contaminants by means of a forced movement of air by a sampling pump and through an appropriate collection medium i.e. filter.

(3)Selection of equipment

Selection of equipment for air-sampling is important and depends on many factors such as the purpose of sampling, type of sampling, type of equipment available, nature of toxicant, environmental conditions, required accuracy and sensitivity, reliability, the property of air-contaminant, presence of other chemicals which may mix or interfere, duration of sampling, cost, etc.

Direct-reading gas and vapour monitors

  • Colorimetric devices – stain tubes and hand or battery-operated pump
  • Colorimetric paper tape samplers
  • Electrical instruments
  • O2 monitors
  • CO monitors and
  • IR analyzers.

Sampling Calculations

Calculations for gas and vapor concentrations depend on gas laws that

  • At constant temperature, volume decreases as pressure increases and vice-versa i.e. P1V1 = P2V2
  • At constant pressure, the volume is directly ‘proportional to the temperature and
  • At constant volume, the pressure is directly proportional to the temperature.

Standard temperature and pressure (STP) condition is 0 oC and 760 mm of Hg atmospheric

pressure, and at this condition, 1 gm-mol of an ideal gas occupies 22.4 liters volume. If the temperature is increased to 25 °C (with pressure constant), I gm-mol occupies 24.45 liters. Concentration is normally expressed in ppm or mg/m3. Their relationship is given by-

Concentration=mass/volume

Ppm = 24.45 x mg/m3 or mg/m3. = molecular wt x ppm

Volume Vm (ml) of material (solvent) to be used to generate concentration C (ml) : the following equation is used –

VM=C*MW*298*P*Vc /d*24.45*T760*106

where MW = molecular weight of the substance (gm/mol), P = pressure mmmHg, Vc = chamber

volume in litres, d = density (gm/ml), T = absolute temperature of apparatus oK = oC+ 273.

Samples are collected in the areas of

  • Breathing zone of the worker
  • The general atmosphere of the room
  • Operation itself.

The factors determining the duration of sampling or the volume of the air to be sampled are Sensitivity of the analytical procedure

  • TLV, STEL, etc.
  • The expected air concentrations.
  • The number of samples to be collected depends on
  • The purpose of sampling
  • The concentration of the contaminant.

Air Sampling Methods

Two basic methods employed to collect the gaseous contaminants are :

1. Use of a gas collector, such as an evacuated flask. The collector is resealed immediately to prevent loss before the sample is analyzed and

2. Passing a known volume of gas or air through an absorbing medium to remove the desired contaminants from the sampled atmosphere. The absorbing medium is chosen according to its efficiency for particular contaminants.

Field methods for air sampling

  • Survey of the work environment to collect basic data
  • Sampling principles or strategies to decide the location of measurement (nose level of the worker, at the source of emission, and in the general atmosphere of the workroom)
  • Types of samples
  • The minimum and optimum volume of the sample
  • Duration and time of sampling and
  • The number of samples.

Then air sample is collected and the contaminant is removed for analysis. Gas detection tables, papers, and liquids are used and finally, the results are interpreted.

Air Sampling Strategies

Collection techniques

In Collection techniques, the sampling device is attached to the worker who wears it during his presence in the workplace. It can be held at his breathing zone (nose level). For environmental monitoring, it is placed in a fixed location in the work area. For designing engineering control, it should be placed near the source of emission.

Place of Sampling

In Place of Sampling Purpose of sampling should be decided and accordingly the place, e.g. breathing zone, source of emission, work area, confined space, place of highest concentration, garage, tunnel, etc. should be decided.

Selection of highly exposed worker

A worker who is closest to the source of toxic emissions should be selected. Individual differences in work habits can show different levels of exposure at the same place – the same job, or the same material. Their work methods should be noticed. Air movement patterns should be studied. The ventilation booths, air supply inlets, open doors, windows, combustion, or heating processes are some factors that can produce higher concentrations away from the source.

Time of Sampling

When there are wide temperature differences during different seasons (e.g.summer & winter), samples should be taken during all such seasons. When there is more than one shift, it should be taken in all shifts. For the A.C. area, normally the contaminants remain the same throughout the year. The time of highest degree of hazard should be selected.

Duration of Sampling

The volume of air and duration of the sample depends on the type of measurement i.e. 8-hour TWA TLV or 15 minutes STEL value and also on the sensitivity of the analytical procedure or direct-reading instrument.

Types of Samples

They may be instantaneous or spot samples collected within a short period of 2 to 10 minutes and continuous samples collected over a long period in different shifts or on different days for the same spot or the same worker.

Minimum Required Volume (MRV)

If the volume is insufficient, a false result is possible. For detection of lower concentrations, larger air samples are required. The minimum required volume is given by –

MRV=S*22400*760*273+t / M*TLV*P*273

where MRV = minimum required volume of sample (liters), S = sensitivity of the analytical method

mg, M = molecular weight of contaminant, TLV in ppm, P = barometric pressure in mm Hg, and t = air

temp °C.

If t = 25 °C (or near about) and P = 760,

MRV = S x 24450/M x TLV

and if TLV is in mg/m’ instead of ppm,

MRV = S x 1000/TLV

Sample Analysis Method

They are classified as under:

(1) Chemical Laboratory Analysis:

Micro methods have been developed and accepted as reference methods. They need sophisticated laboratory apparatus and qualified and trained personnel. Their reliability depends on many factors like specificity, accuracy, precision, reproducibility, sensitivity, practical confirmation, etc. The choice and purity of reagents are important. The analysis is carried out by

  • Approximation
  • Titration and Gravimetric methods
  • Spectro-photometry
  • Visible Spectrophotometry
  • Nephelometric method and
  • Calibration Curves. Laboratory equipment includes glassware, accessories, refrigerator, sand bath, etc.

(2) Instrumental or physical methods

Instrumental or physical methods are advanced methods. They have superseded traditional wet chemical methods of environmental estimations. They require regular calibration, skilled operators, and more expense. The methods include :

  • Gas Chromatography
  • Liquid Chromatography
  • Spectroscopy using visible IR or UV or X-rays
  •  Polarography
  • X-ray diffraction analysis
  • Neutron activation analysis and
  • General methods like combustible gas indicators, spectrophotometers
  • and flame ionization detectors. Sensitivity and specificity are important factors for the choice and application of the method.

Analytical methods of Air Sampling

 These are chemical and physical methods used for the determination of contaminants in samples.

Colorometric :

The principle of this method is the development of the color by a reagent which is indicative of the concentration of the substance to be analyzed. Examples of these methods are the determination of zinc, lead, mercury, etc. by dithizone extraction.

Ion Exchange :

By this technique, it is possible to separate elements from one another. Mercury in urine, fluorine in urine, and fluoride sample can be separated for further analysis.

Gravimetric Method

This method depends on the formation of a precipitate or a residue that can be weighed. An example is an analysis of dust samples for free silica.

Volumetric Method

By the use of the standard solution for titration. Examples are acid gases which are titrated with a basic reagent.

Physical Methods

The physical methods widely used for the determination of the various contaminants are emission spectroscopy, infrared and ultraviolet spectroscopy, mass spectroscopy, polarography. X-ray diffraction and gas chromatography.

Air Sampling Instruments

Various sampling and monitoring instruments are used to measure or/and control toxic hazards of air pollution and other non-chemical hazards Sampling devices are of two types.

  • Direct reading devices or instruments (DRI) and
  • Integrated sampling devices.

The basic requirements or main parts of any sampling instrument are :

1. Source of suction, which may be an electrically or hand-operated pump, an aspirator, or squeeze bulb. They are of three types – Low flow. High flow and Dual range.

2. Absorbing medium – It should be able to efficiently retain the contaminants to be sampled by\adsorption, absorption, chemical reaction, or mechanical retention filter or collection media. Sorbent tubes, bags, filters, liquid, and impingers are used to collect airborne contaminants for analysis. Sorbent tubes (activated charcoal. Silica gel) work on the principle of adsorption. They are used for non-reactive insoluble gas/vapor and give an accurate assessment of TWA exposures. Adsorbed material is desorbed (extracted) and analyzed in the laboratory.

A cyclone device is used to collect and separate respirable particulate matter. Rapid circulation of air separates particulates according to size. The grit pot remains in place during sampling. Impinger is a glass bubble tube. It contains liquid mediators complete absorption of reactive chemicals like acids and anhydrides.PVC filters are used for respirable dust’ e.g. silica, crystalline.

Flowmeter (rotameter) to indicate and control the rate of suction of air to calculate the amount of air sampled. The flow meter attached to the instrument must be calibrated with a wet or dry gas meter.

Aircheck Sampler Pump is used to sample gases, dust, and particulate matter. Its features are

  1. Programmable timer operation
  2. Range 1-3500 ml/ min
  3. The fault indicator will light if the flow is restricted or the battery voltage drops to a minimum.

A sampler Pump is useful for specific toxic gases that cannot be monitored through the gas detector. It can be used while manual handling of toxic chemicals, pumping operations and pump gland/seal leakage, etc.

Gas Detector is used to detect all toxic and combustible gases and oxygen content in the workroom, manholes of storage tanks, tank cars, confined space, pumping station, etc. Its range is 0 to 50 ppm for toxic gas, 0 to 100% for LEL, and 0 to 25% for oxygen, Its operating range is 0 to 40° C for toxic gas, 18 to 40°C for combustible gas, and 0 to 40°C for oxygen. Its humidity range is 10 to 90% RG. Long time extreme humidity reduces the sensor life.

Specific Gas Detectors are also available to measure, record (print), and give an audio-visual alarm when a set limit is reached. They give a direct reading by digital display. Such instruments are portable but costly. They are available to detect C12, H2S, SO2, CO, phosgene, etc. An HS meter operates on an electrochemical polarographic cell. They should be properly calibrated and used within their ranges of ppm, LEL, humidity, and temperature. Dragger gas detector tubes for such specific gases and sampler pumps are also utilized.

For Toxic Substances: Midget impinger, low volume, and high volume air sampler, personal air sampler, electrostatic sampler, hexlet, gravimetric dust sampler, gas detector, and doctor tubes. Millipore filter holder and filter papers, phase control microscope, binocular microscope, microscope illuminator, dust counting cells, microscope stage micrometer (2.0 mm x 0.010 mm division), filter/microscope eyepiece, cover glasses for counting chamber, spencer bright line counting chamber, MSA calorimetric carbon monoxide tester, scrubber for use with carbon monoxide tester, MSA combustible gas indicator.

Osha’s some permissible exposure limits in parts per million (ppm)

Substanceppm
Acetaldehyde200
Acetic acid10
Acetic anhydride5
Acetone1000
Acetonitrile40
Acetylene tetrabromide1
Carbon dioxide5000
Carbon monoxide55
Dioxane (Diethylene dioxide)100
Ethyl acetate400
Ethylbenzene100
Ethyl chloride1000
L.P.G. (Liquified petroleum gas)1000
Methyl acetylene (Propyne)1000
Diisobutyl ketone50
Cyclohexane300
Chlorobromomethane200

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