Sampling for Airborne Contaminants

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Sampling forAirborne Contaminants

• Industrial Hygiene
• IENG 341
• Carter J. Kerk
• Industrial Engineering Program
• SD Tech
• Spring 2006

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Reading Assignment

• Nims, Chapter 5

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Outline

• Introduction
• Why sample the air?
• Sampling particulates
• Sampling gases and vapors
• Standard sampling and analysis methods
• Laboratory analytical techniques
• Direct-reading methods
• Air sampling strategies

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Introduction

• Inhalation in the number one route of entry into
  the body for airborne materials
• We must be able to identify and evaluate air
  contaminants
• Sampling
• Analysis methods
• Interpretation

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Why Sample the Air?

• Knowledge of conditions so you can apply controls
  if necessary
• Engineering, administrative, PPE
• Monitor concentrations
• During normal exposures
• Before and after controls are implemented
• In cleanup operations (asbestos)
• Confined Space Entry
• For regulatory reasons
• To determine required respiratory protection
• 29 CFR 1910.134

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Sampling Approaches

• Direct-Reading or Real-Time Sampling
• Immediate or rapid feedback
• Integrated Sampling
• Draw air across a collector (or sampling medium)
• Sample is analyzed in a laboratory

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Sampling Pumps

• Individual exposures
• Small, light-weight (1-2 lb)
• Rates from 1 cc/min to 4 l/min
• Low end for gases
• High end for particulates
• Worn for an entire shift
• Area Sampling
• Bigger and heavier
• Higher rates (2 15 l/min)
• Fixed location for an entire work area

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Sampling Particulates

• Filters
• Commonly used (25 of sampling methods)
• Type depends on the contaminant and the method
• Thin filters placed in rigid holders (cassettes)
  with a support pad beneath
• Open Face Sampling
• Top portion of cassette is removed
• When distribution across media is important
  (asbestos)
• Closed Face Sampling
• Small plug in top cassette is removed for
  sampling
• As in metal sampling

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Filter Structure

• Not like a regularly-shaped screen or sieve
• Maze of interconnected tunnels and pores
• Passing air turns, branches, changes speed
• Impaction contaminants collide with surface
• Interception contaminants stick to surfaces
• Electrostatic attraction electrically charged
  particles stick to attracted to media
• Factors particle size, electric charge of
  particle/filter, type of filter, flow rate
• The capture efficiency of a filter is not limited
  by the size of the pores

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Mixed Cellulose Ester Filters

• MCEF a commonly used filter
• Manufactured from a polymer that starts as a
  liquid, is spread out in a thin layer
• As it solidifies (or dries) small pores open up
• Average size of openings can be controlled by the
  manufacturing process
• 0.4 0.8 mm pore sizes
• For metal fumes
• Entire filter is dissolved in acid and analyzed
  for metal content
• For asbestos
• A section of the filter is cut, treated with
  acetone vapor to make it clear, then viewed
  through a microscope for fibers

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PVC (Polyvinyl Chloride) Filters

• Have good resistance to acids and bases
• Do not absorb much water vapor (hydrophobic)
• Used to collect dusts (e.g. silica)
• Gravimetric Analysis
• Placed in a dessicator and pre-weighed
• After sampling, back to the dessicator and
  post-weighed
• Mass of collected dust and sampled air volume
  determine the airborne concentration
• Filters can further be analyzed with x-ray
  diffraction to identify forms of crystalline
  structure

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PVC (Polyvinyl Chloride) Filters

• Recall the three fraction ranges of dust
• Inhalable
• Thoracic
• Respirable
• A cyclone filter can be placed on the inlet side
  of the filter cassette to allow only the
  desirable fraction(s) to enter

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Teflon Filters

• Polymer filter
• Teflon (PTFE)
• Polytetraflouroethylene
• Like PVC filters, they are chemical-resistant and
  hydrophobic
• Used for aromatic hydrocarbons
• Benzopyrene (given off from hot tar or asphalt)

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Glass Fiber Filters

• Layers of fibers arranged in a seemingly tangled
  mat
• Used for collecting particulates and some
  droplets of contaminants, such as mercury and
  acid gases
• Sometimes used as an upstream, pre-collection
  device, so that larger particles do not reach a
  second filter of another type
• This allows simultaneous collection of two
  different physical forms

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Potential Problems Associated with Filter
Collection

• Overloading
• Too much particulate collection can overload the
  filter and cause an error
• Static Electricity
• Filters pick up a charge and can cause an error
  in gravimetric analysis
• Moisture or physical damage
• After getting wet, filters can tear or burst
• Contamination with interfering substances
• Contaminant material causes error in
  determination of desired material

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Sampling Gases and Vapors

• Sorbent Tubes
• Passive Samplers
• Impingers
• Grab Samples

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Sorbent Tubes
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Passive Samplers

• Collection accomplished by diffusion into a
  sampling device
• No pump
• Passive samplers (sample badges)
• Small clip-on devices worn on the collar
  throughout the work shift
• Then sealed in a container and sent to a lab for
  analysis
• Some use a color change agent in presence of a
  specific contaminant
• Simple

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Impingers

• For contaminants that are nonreactive and highly
  soluble in a specific solution
• Impinger is a glass container with a measured
  volume of the specific solution
• A sample pump creates a vacuum drawing air
  through the solution
• Sealed and sent to lab for analysis

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Grab Samples

• Integrated sample from a very brief sampling
  period, like a snapshot
• Useful for evaluating
• compliance with a ceiling or peak limit
• Screening or basic identification
• Sample bags
• Teflon or Tedlar bag connected to a pump
• Bag is then sealed and shipped to lab for
  analysis
• Evacuated container
• Cylinder from which air has been removed and
  sealed
• Seal is broken and contaminated air is drawn in
• Container is then sealed and shipped to lab for
  analysis

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Standard Sampling and Analysis Methods

• Sampling and analysis methods have been developed
  and validated for many airborne contaminates
• OSHA Reference Methods and NIOSH Manual of
  Analytical Methods
• Sampling media to be used
• Sampling flow rate
• Volume of air to be sampled
• Instructions for sample preservation and handling
• Detailed procedures for analysis

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Standard Sampling and Analysis Methods

• Before acceptance a NIOSH method must be shown to
  be able to provide a result that is within 25 of
  the actual concentration, 95 times out of 100
  tries
• Flow rates must be calibrated
• Laboratory techniques have boundaries of
  reliability
• LOD, Limit of Detection
• LOQ, Limit of Quantification
• Smallest amount of contaminant that can be
  reliably detected and quantified, respectively
• Study the NIOSH method for sampling and analyzing
  Ammonia in Table 5-2 of text

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Gas and Vapor Analysis

• Gases and vapors may be collected on sorbent
  tubes, filters, or in solutions
• A titration method adds chemical reagents until
  an endpoint is reached (color change)
• Intensity of the color is proportional to
  concentration and measured with spectrophotometer

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Gas Chromatography (GC)

• Works like a still
• Contains a distillation tower
• Different molecules pass through at different
  speeds
• A graph is produced
• The area under the peak in the graph is
  proportional to the amount of material
• Graph is compared to known concentrations

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Mass Spectrometry

• Mass Spec or MS
• Sample is bombarded with beam of electrons,
  causing ionization (charged particles, or ions)
• Each ion has a specific mass
• The mass to charge ratio is unique, m/e
• The intensity of each m/e value is proportional
  to the amount of the ion produced
• The highest m/e value is assigned 100
• The MS plot is like a fingerprint unique to each
  compound

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Mass Spectrometry Similar to gas
chromotography, the output produced by the mass
spectrometer must be compared against a known or
reference output to determine the compound that
is present. These mass spectra are for n-octane
(A) and 2,2,4-trimethylpentane (B).
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Absorption Spectroscopy

• Involves the amount of energy absorbed by a
  compound
• The wavelength where the energy is absorbed
  indicates the identity of the compound
• See Electromagnetic Spectrum (next slide)
• Ultraviolet and Infrared Spectrometry
• organics
• Atomic Absorption (AA)
• metals

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Absorbance of energy at specific wavelengths is a
characteristic of organic compounds. These
patterns of absorbance are like fingerprints, in
the sense that they allow the specific compound
to be identified. Shown here are the absorbance
spectra of wavelengths in the ultraviolet region
for benzene and toluene.
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Chlorinated hydrocarbons tend to absorb energy in
the infrared region. Shown here are IR spectra
for five common industrial solvents. Wavelengths
are in micrometers.
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Inductively Coupled Plasma (ICP)

• Utilizes ability of electrons to absorb energy
• However this technique measures the energy loss
• Intensity of emissions is proportional to the
  amount present
• Used for metal scans

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Fluorescence Spectrometry

• Similar to ICP, but used for organic compounds,
  especially aromatic rings

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Summary of the common methods used to analyze air
samples in the laboratory.
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Direct-Reading Methods

• Calibration, maintenance, and recordkeeping are
  critical
• Types
• Combustible Gas Meters
• Oxygen Meters
• Photoionization Detectors
• Flame Ionization Detectors
• Length-of-Stain or Detector Tubes

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Gas Meters

• Field use, lightweight
• Uses leak detection, initial evaluation,
  screening for contaminants, oxygen deficient or
  enriched
• Multi-gas or multi-function units
• Sensors
• Wheatstone bridge circuit (see next slide)
• Metal-Oxide semiconductor (MOS)
• Detects changes in electrical conductivity
• Thermal conductivity
• Tests the ability of tested air to conduct heat

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Direct-Reading, Multi-Function Gas Meter
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LEL, LFL, UEL, UFL
Word of Caution The presence of a combustible
gas just above the UEL is potentially worse than
a concentration approaching the LEL. This is
because a little dilution of the mixture will
push the concentration into the combustible
range. Any reading above the UEL should be
considered potentially very hazardous!
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Other Cautions on Combustible Gas Meters

• Sensors have a limited life
• Follow mfg specs and change on schedule
• Sensors can be contaminated by interfering
  compounds
• Heated filament or flame devices can present a
  risk of igniting an explosive or combustible
  atmosphere
• Flashback arrestor
• Electronic components can also present an
  ignition source

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Photoionization Detectors

• PIDs
• Utilizes light energy from a tiny UV lamp
• The energy is absorbed by the contaminant
  molecules
• Detects organic vapors such as alcohols, ketones,
  ethers
• But, detection is non-specific
• Useful for screening purposes

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Flame Ionization Detectors

• FIDs
• Utilizes energy from a heat source hydrogen
  flame
• Energy is absorbed by the contaminant molecules
• Useful in detection of hydrocarbons and most
  organic compounds
• Also non-specific, but useful for screening
  unknown atmospheres

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Detector Tubes

• Also known as Length-of-Stain tubes
• Similar to an oversized sorbent tube
• Sealed at both ends
• Contains a solid sorbent coated with a reagent
  that reacts with a contaminant and causes a color
  change
• Marked with graduations along the length
• Length of stain indicates the concentration
• Snap off both ends and attach to a pump
• Useful for screening and leak detection

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HW5

• Complete Critical Thinking Questions on p 146-7
• Due ?