AIR SAMPLING INSTRUCTIONS
The basic objective of air sampling is to capture a sample of the contaminants present within the
air in the workplace. There are three principle types of air sampling methods in use. These
methods are, active, grab and passive sampling. Active sampling involves the use of
mechanical pumps and other air collection devices. Grab sampling is an active method which is
used most often to determine worst case conditions and/or identify emission source locations or
“hot spots.” Passive sampling is a method based on the natural tendency of gas or vapor
molecules to move from an area of high concentration to one of lower concentration. Active,
Passive and Grab sampling methods will be addressed in detail in these instructions.

I. ACTIVE SAMPLING:

Active sampling consists of the collection of a known volume of air and depositing of the

contaminant being investigated upon the appropriate collection medium. The Travelers Air

Sampling Guidelines have been published for your use in determining the appropriate air

sampling method for specific contaminants.

To determine full-shift employee

exposure levels to chemicals in

the workplace, it is necessary to

evaluate the Time-Weighted

Average (TWA) contaminant

concentration. Integrated

sampling methods using

rechargeable, battery-powered,

personal sampling pumps are

used for TWA sampling.

Travelers supplies rechargeable,

battery powered, personal

sampling pumps. The pumps

operate over a wide range of

flow rates and are suitable for

evaluating most airborne chemical exposures encountered in the workplace. The pumps

provide a measured air flow for time periods of up to a full work shift, and are of a size and

weight to be considered portable.

Attaching the Pump to the Employee:

Integrated sampling involves the attachment of a sampling train to the employee. The sampling

train normally consists of the pump, appropriate flexible tubing, the media (tube/filter), and any

necessary media holder.

Gilian pumps are equipped with a wide metal clip intended to hold the unit securely on the

employee’s belt or waist band. The media should be placed on the employee’s shoulder or

lapel within several inches of his or her chin in an area called the “breathing zone.” The

breathing zone is defined as an imaginary 9 inch sphere around a person’s head. These

breathing zone samples are considered to represent the employee’s actual exposures. Since

placement of the sampling media can be critical, it should be located as close as possible to the

employee’s face.

Care should be taken to minimize the possibility of the tubing catching on workplace objects or

interfering with the employee’s work. In general, placing the pump on the employee’s left or

right hip routing the tubing diagonally across the back (either under the right or left arm or over

the right or left shoulder) to the breathing zone seems to be the best arrangement.

air sampling of gases

AIR POLLUTION MONITORING

 

Introduction

The Air (prevention and control of pollution) act, 1981 has in its preamble, the objective to take “appropriate steps for the preservation of the natural resources of the earth, which among other things, include the preservation of quality of air.”  The preservation of quality of air means the fixing up of certain minimum standards of ambient air quality in respect of the common and uncommon ingredients of air and the prevention of increase of the concentration of such ingredients in comparison to the fixed minimum quality.

These are many ingredients which may be termed as “primary pollutants.”  These are five primary pollutants which together contribute more than 90% of global air pollution.  These are :

A.          Carbon monoxide, CO

B.          Nitrogen oxides, NO₂

C.          Hydrocarbons, HC

D.          Sulphur oxides, SO₂

E.          Particulates

After being released directly into the atmosphere, these primary pollutants react with atmospheric constituents and with themselves and thus produce secondary pollutants.  In many situations these secondary pollutants are much more dangerous and injurious to the environmental quality.  One of the most common secondary pollutants is ‘Photochemical Smog.’

The preservation of air quality requires constant and continuous monitoring of the ambient air and effective control measures to reduce the emissions from anthropogenic sources.  This also requires efficient methods to forecast the future ambient air quality and implementation of such schemes which may monitor and respond quickly and effectively to control episodal and emergency emissions.  Thus, we may agree that the data collection and their interpretation play very important role in the ambient air quality preservation.  Here, we shall be mainly concentrating on the sampling procedure for various air pollutants.  We shall also discuss difficulties encountered in collecting representative samples and sources of error.  The methods used for measuring gaseous emissions from a stack or a vent depend on the nature of the compound and the purpose for making the measurement.  In addition, the composition and the temperature of the carrier gas stream affect the selection of a sampling technique, analytical method and sampling plan.

Classification of Sampling Methods

The sampling methods used for the study of air pollution can be classified under three different headings.

1.      Sampling of impurities of various nature (ranging from particulate matter to gases).

2.      Sampling under various environmental conditions (ranging from samples taken from chimneys to samples taken in the open air).

3.      Sampling methods varying according to the time factor (ranging from intermittent to continuous sampling).

Difficulties Encountered in Sampling

1.      Collecting samples of true representative character.

2.      Errors arising from methods used for the collection and separation of the various components of pollution.

3.      Difficulty in preventing any change in the concentration of particulate matter in suspension, as a result of sampling operations.

Instruments for Sampling Waste Gases and the Atmospheric Sampling

Following are the sampling devices in use:

Devices for General Use

Meters

They are used to determine accurately the volume of the gas collected.  They are fitted with manometers and thermometers to indicate the pressure and temperature of the gas stream sampled.

Probes

These are tubes suitable for penetrating into the gas stream and should be constructed of materials which are non-corrosive and which can withstand special temperature conditions.  Also, they should be constructed of materials which do not react with the substances to be sampled.  Therefore, they should be made of stainless steel or preferably of glass or quartz.  A probe should have suitable length and diameter.  To ensure isokinetic sampling conditions, the opening of the probe should face the gas stream to be sampled.

Suction Devices

Any suction device which has the required volumetric capacity can be used.  Vacuum pumps driven by electric motors are very commonly used.

Devices for Sampling Gases and Vapours

Absorbers

In this process, effluent gases are passed through absorbers (scrubbers) which contain liquid absorbents that remove one or more of the pollutants in the gas stream.  The efficiency of this process depends on –

(1)     amount of surface contact between gas and liquid

(2)     contact time

(3)     concentration of absorbing medium

(4)     speed of reaction between the absorbent and gas

Absorbents are being used to remove sulphur dioxide, hydrogen sulphide, sulphur trioxide and fluorides and oxides of nitrogen.

The equipment using the principle of absorption for the removal of gaseous pollutants includes :

(1)     packed tower,

(2)     plate tower,

(3)     bubble cap plate tower

(4)     spray tower,        and

(5)     liquid jet scrubber absorbers.

Selective chromatographic absorption of gases on small pellets may offer much higher rates that those achieved in packed towers.

A gas can be sampled by means of suitable absorption reagent.  For this purpose, U-shaped absorbers are used.  These absorbers are filled with a certain measured amount of reagent and fitted with a porous glass partition, so that the air or gas led into them is passed through the reagent solution in the form of fine bubbles thus ensuring intimate contact.  Sampling by means of such absorbers is usually carried out at an average rate of about 100-150 litres per hour of gas stream.  The absorbers may be arranged in series of two or more elements containing two or more different reagent solutions so as to absorb different pollutants successively from the same volume of gas or air sampled.  A typical sampling train is shown in Figure – 1 comprising of an impinger, trap, manometer, flow meter, valve and pump.

By selecting the most suitable absorbent solutions, the gaseous components listed below can be determined in concentrations as low as 0.1 ppm by volume.

1.      Oxides of sulphur

2.      Oxides of nitrogen

3.      Ammonia

4.      Hydrogen sulphide

5.      Hydrochloric acid

6.      Hydrofluoric acid

7.      Hydrocyanic acid

This method can also be used for the determination of ozone, hydrocarbons and organic solvents.

Adsorbers

Adsorption is brought about by aspiring the air or gas to be sampled through adsorption columns containing silica gel, activated charcoal or any other suitable agent.  After adsorption , the different pollutants can be extracted from the column in various ways.  For example, by raising the temperature.

The main difficulty in this method is in selecting a suitable adsorbing medium.  This type of sampling is used especially for ozone and light hydrocarbons.

Condensers

Here the gas stream sampled is cooled in suitable containers, thus bringing about the condensation of the volatile substances present.  As in the case of adsorption devices, here also the condensation traps can be arranged either in series or parallels, at decreasing temperatures.  By using various coolants, e.g., ice, liquid air, or liquid nitrogen the components can be separated by fractional condensation.

This method is used in particular for the sampling of odoriferous substances.

Collectors under Reduced Pressure

For some substances like nitric acid and aldehydes having a high molecular weight, absorption in aqueous solutions is sometimes incomplete.  In such cases, it is preferable to use bottles of known volume for collecting under a pressure reduced to 200 mm Hg or even less.

To do this, the absorbent solution chosen is first introduced into the bottle and the pressure is then reduced.  Then the sample is admitted until the internal and external pressures are equal and the container is shaken continuously so as to ensure maximum absorption.

This method is suitable, for sampling the oxides of N₂.

Plastic Containers

Special polythene bags are commonly used for collecting and transporting large volumes of air.  These bags have the advantage that they can be used for successive analysis of small fractions of the sample taken.  Moreover, polythene is inert with respect to many substances including SO₂ and formaldehyde.  On the other hand, plastic bags are not suitable for collecting and storing aerosol suspensions, because of the possible generation of electrostatic charges, as a result of which the aerosols tend to move towards the walls and condense on them.  Plastic bags have been widely used for grab sampling and sample storage before analysis.

Samplers for Mass-Spectrometric Analysis

Sampling or mass spectrometric analysis can be carried out in various ways.  For example, by compressing the gas sample in a pressure flask so as to concentrate a large quantity of gas in a small volume, or by filling evacuated containers.

calculations

CALCULATIONS

·        If the initial and final calibration flow rates are different, a volume calculated using the highest flow rate should be reported to the laboratory. If compliance is not established using the lowest flow rate, further sampling should be considered.

·        Generally, sampling is conducted at approximately the same temperature and pressure as calibration, in which case no correction for temperature and pressure is required and the sample volume reported to the laboratory is the volume actually measured. Where sampling is conducted at a substantially different temperature or pressure than calibration, an adjustment to the measured air volume may be required depending on the sampling pump used, in order to obtain the actual air volume sampled.

·        The actual volume of air sampled at the sampling site is reported, and used in all calculations.

The laboratory normally does not measure concentrations of gases and vapors directly in parts per million (ppm). Rather, most analytical techniques determine the total weight of contaminant in collection medium. The lab calculates concentration in mg/m3 and converts this to ppm at 25 ⁰C and 760 mm Hg using following Equation. This result is to be compared with the PEL without adjustment for temperature and pressure at the sampling site.

TLV in ppm =  (TLV in mg/m³  ) X (24.45) / (Gram Molecular weight of

                                                                             substance)

OR

TLV in mg/m³ = (TLV in ppm) X (Gram Molecular weight of substance)/24.45

Where:

24.45 =molar volume at 25  C(298 K) and 760 mm Hg

Mwt =molecular weight

NTP =Normal Temperature and Pressure at 25 C and 760 mm Hg

NOTE: When a laboratory result is reported as mg/m3 contaminant, concentrations expressed as ppm (PT) cannot be compared directly to the standards table without converting to NTP.

Adjustment for temperature and pressure: Formula is as bellow:

ppm(PT) = mg/m3 X 24.45/ Mwt X 760/P  X  298/T

·        Time-Weighted Average: The average full shift exposure level calculated by weighing the various concentrations throughout the workday with respect to time.

TWA = C₁T₁ + C₂T₂ + CnTn  / 8 hr

            Where, TWA = Time-weighted average concentrations in ppm/ or mg/m³

                                 C        = Concentration of contaminant during an incremental exposure  

                                        time

                          T       = Time : Incremental Exposure Time

·        Threshold Limit Values for Mixtures :

Ø    Most threshold limit values are developed for a single chemical substance.

Ø    However, the work environment is often composed of multiple chemical exposures both simultaneously and sequentially.

Ø    It is recommended that multiple exposures that comprise such work environments be examined to assure that workers do not experience harmful effects.

Ø    There are several possible modes of chemical mixture interaction.

1)    Additivity occurs when the combined biological effect of the component is equal to the sum of each of the agents given alone.

2)    Synergy occurs where the combined effect is greater than the sum of each agent. 

3)    Antagonism (Reveres Effect) occurs when the combined effect is less.

Ø    The general ACGIH mixture formula applies to the additive model.

NOTE: The guidance contained does not apply to substances in mixed phases.

Ø When two or more hazardous substances have a similar toxicological effect on the same target organ or system, their combined effect, rather than that of either individually, should be given primary consideration. In the absence of information to the contrary, different substances should be considered as additive where the health effect and target organ or system is the same.

That is, if the sum of:  C₁/T₁ + C₂/T₂ +…..Cn /Tn = 1

Where, C₁ indicates the observed atmospheric concentration and T₁ is the corresponding threshold limit)

It is essential that the atmosphere is analyzed both qualitatively and quantitatively for each component present in order to evaluate the threshold limit of the mixture.

Example: Air contains 400 ppm of acetone ( TLV,750 ppm),150 ppm of sec-butyl acetate (TLV,200 ppm) and 100 ppm of methyl ethyl ketone (TLV,200 ppm).

400/750 + 150/200 + 100/200 = 0.53 + 0.75 + 0.5 = 1.78

Threshold limit is exceeded.

Ø Special case when the source of contaminant is a liquid mixture and the atmospheric composition is assumed to be similar to that of the original material, e.g. on a time-weighted average exposure basis, all of the liquid (Solvent) mixture eventually evaporates. When the percent composition ( by weight) of the liquid mixture is known, the TLV s of the constituents must be listed mg/m3.

1/ fa/TLVa+fb/TLVb+fc/TLVc +…fn/TLVn

Example: Liquid contains ( By Weight)

          50% heptan e: TLV 400 ppm or 1600 mg/m3

          30% methyl chloroform : TLV 350 ppm or 1900 mg/m3

          20% perchloroethylene : TLV 50 ppm or 335 mg/m3

          TLV of Mixture = 1/0.5/1600+ 0.3/1900 + 0.2/335

                                   =  1/0.00031 + 0.00016 + 0.0006

                                   =  1/0.00107 =  935 mg/m3

of this mixture:

50% heptane or (935)(0.5) = 468 mg/m3 is heptane

30% methyl chloroform (935)(0.3) =  281 mg/m3 is methyl chloroform

20% perchloroethylen (935)(0.2) = 187 mg/m3 is perchloroethylen

These values can be converted to ppm :

Heptane = 117 ppm

methyl chloroform = 51 ppm

perchloroethylen = 29 ppm

TLV of mixture =  117 + 51 + 29 = 197 ppm