Transcript WORKPLACE EXPOSURE ASSESSMENT AND FIELD …

UNIVERSITY OF HOUSTON - CLEAR LAKE
SPRING 2015
Known low concentrations of air contaminants
are required for:
testing and validation of analytical
methods;
calibrating instruments, and
toxicological and scientific studies.
Validated sampling and analytical methods
require generation of known standard
concentrations in air.
Range of one-half to two times the
Permissible Exposure Limit (PEL) against
which to compare test results.
Numerous
electronic
instruments
are
secondary measuring devices that require
calibration with accurate known airborne
concentrations.
1. Batch Mixture – simple and convenient
2. Flow-Dilution System – requires a metered
flow of diluent air and a source for
supplying known flows of gases, vapors, or
aerosols combined in a mixing device.
Advantages: compactness, and large volumes
at known low concentrations.
Methods require relatively simple equipment
and procedures.
Serious disadvantage - limited quantities of the
mixture supplied.
For reactive compounds, erroneously low
concentrations may result from appreciable
adsorption losses (50%) of the test substance
on the walls of the chamber.
Not use as primary standards without
verification by chemical analysis.
Disperse accurately measured quantities of test
compound into sealed chamber containing
clean air with mixing.
Keep concentrations outside of the explosive
limits.
Withdraw from center through tube.
Evaluate potential chamber leakage.
Pipet quantity of volatile liquid into the bottle
on paper for evaporation. May be tumbled
with foil for mixing. Withdraw mixture from
bottom through a glass tube to avoid loss.
Disadvantage is that concentration decreases
during withdrawal as clean air enters [5%
without serious loss]. Errors avoided through
use of other methods (i.e. bags, cylinders,
etc.).
Also known hydrocarbon concentrations
needed for calibrating explosive-gas meters.
Many sizes/materials: Mylar, Teflon, Tedlar.
Alternately clean and fill to evacuate/flush.
Meter clean air into bag; add test substance
to air stream; insure complete transfer of
injected material; then, disconnect bag and
plug/cap and mix. Conditioning process for
bags; permeation.
Advantages: (i.e. no dilution; transportable)
and also losses from bags during storage
(e.g. inert vs. reactive materials).
Good for hydrocarbons or carbon monoxide
(CO) due to limited losses. Other substance
losses by:
polymerization, adsorption, or
reaction with steel cylinder walls may change
concentration.
Pressure changes; avoid condensation; clean
regulators; and/or coated aluminum cylinders
for long-term stability.
Possibility of explosion of combustible
substances; excessive heat causes errors.
Recommended to purchase hazardous mixtures
from compressed gas vendors; NIST Traceable
Reference Material.
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Values in appropriate UNITS; for constants in
equation, numerical value corresponds to the UNITS
required.
Ambient pressures commonly measured
barometer and expressed as mm of mercury;
Air volumes measured in liters; and,
Temperature as degrees Celsius.
with
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SI metric units: meter; kg, degrees C; bar/Pascal, etc.
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1 M3 = 1000 Liters; 1 atmosphere = 760 mm Hg
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Ideal gas law; molar gas volume;
partial vapor pressures
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Concentrations – mg/M3 vs. ppm (ug/L)
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Standard conditions: temperature of 25 degrees C
and 760 mm Hg pressure.
See:
Third Edition: Chapter 16, pages 385 – 386
Second Edition: Chapter 14, pages 248 – 250.
Air and test vapor, gas, or aerosol are
continuously and accurately metered and
combined in mixing device.
Primary advantage of compactness.
Large volumes of gas mixture provided from
continuous
operation;
rapidly
change
concentrations.
Lesser explosive hazard due to small volumes.
Only initial losses due to surfaces being fully
saturated. Popular method.
The accuracy of final concentration depends on
accuracy of measurements of air and test
substance flows within system.
Rotameters are commonly used, and mass flow
meters, orifice meters, and critical orifices
frequently employed.
Consider use of pulsation damper for pumps
and error determined based on flow meter.
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First due to calibration is dependent on the
density, and in some cases, the viscosity of the
gas flow, both of which are affected by ambient
pressure and temperature; use absolute
pressure. Depends on device.
Second applied to convert the actual gas flow
rate under ambient conditions to standard
conditions; basis of ideal gas relationship.
High altitude corrections are related to
atmospheric pressures.
Flow-dilution system is comprised of metered
test substance source, metered clean-air
source (required purification), and mixer to
dilute test substance to low concentration
required.
Total flow of mixture must be equal to or
greater than the flow needed.
Desirable use of glass or Teflon parts.
Maintain small pressure drops and operate
system at close to atmospheric pressure. Also
minimize dead volume for rapid response
time.
Various devices available to provide high
concentrations of gases/vapors for dilution with
pure air to achieve the concentration desired.
Advantages
Disadvantages
Selection of a source device depends on needs of
application and available equipment.
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Figures:
3rd Edition
2nd Edition
Vapor Pressure
16.6/16.7
14.6/14.7
Motor Driven Syringes
16.8
14.8
Diffusion Source Systems
16.9
14.9
Porous Plug Sources
16.10
14.10
Permeation Tube
16.11/16.12/ 14.11/14.12
Source Devices
16.14
14.14
Miscellaneous Generation Systems e.g. electrolytic generator; aerated chemical
solution mixture
Permeation tubes are useful sources for
liquefiable gases and volatile liquids.
Potential
precision;
NIST
certifications.
Calibrated at 25 C to within 1% of rate with
95% confidence.
When maintained in a dry controlled
environment, can serve as primary standards.
Commercial types span gas standard generator
for instrument calibration; caution due to
pressure!; environment carefully controlled;
gravimetric calibrations.
Also See:
Third Edition – Chapter 16, pages 392 – 396.
Second Edition – Chapter 14, pages 254 - 257
Variables involved include:
 Source strength
 Desired concentration
 Flow rates
Examples of Calculations – refer to
Chapter 16 (2nd Edition – Chapter 14)
and also Appendix.
Preparation of aerosols more complex and
difficult than gas/vapor mixtures.
Major consideration is size distribution.
Commonly - lognormal distribution!
Describes values characterized by a:
Geometric Mean (GM) and a
Geometric Standard Deviation (GSD).
Usual aerosol source devices supply range of sizes.
Special types supply uniform-sized particles.
If the geometric standard deviation is less than 1.1, the
particles are considered homogeneous, or monodisperse.
If material is a mixture, composition may vary with size.
Consider surface electrostatic charges. Determined by
study purpose and/or objectives.
Proper design of exposure chamber needed to sample
aerosol – e.g. aerosol/size distribution.
EXAMPLES:
- Dry Dust Feeders – agglomerate/packing;
fibrous aerosols are difficult.
- Nebulizers – compressed air for liquid use.
- Spinning
Disc
Aerosol
Generators
–
monodisperse aerosols from liquids.
- Miscellaneous Generation Systems – duplicate
natural formation of dusts.
Multipurpose Calibration Components/Systems
Commercially available for testing analytical
methods and calibrating instruments.
Standards Completion Program – 516 methods
evaluated by NIOSH contract labs. Each set up
a multipurpose system for simultaneously
producing several test concentrations in the
desired range.
A range of temperature and pressure changes can be
tolerated before corrections are applied to the volume
or air sampled during an exposure assessment.
All OELs and environmental exposure standards and
limits are expressed at 25 degrees C and 1
atmosphere (760 mm Hg), defined as normal
temperature and pressure (NTP).
Therefore,
corrections
needed
for
meaningful
comparisons related to published exposure limits.
Apply temperature and pressure differences
between NTP and field conditions into program
to monitor exposure to air contaminants.
Air volume changes if different from standard
temperature and pressure (NTP).
To compare air monitoring data with OELs, make
corrections of volume of air sampled in the field
to NTP standard condition.
Gas laws: Boyle, Charles, Gay-Lussac, and
Dalton laws for basis of understanding
effects.
Assume that air behaves as an ideal gas.
Small error (less than 0.5% at concentrations
less than 5000 ppm by volume).
Enables occupational hygienists to calculate
the concentration of air contaminants in the
workplace and the ambient environment.
Temperature correction is adjustment of
volume by difference between the field
sampling temperature and the desired
standard temperature of 25 degrees C (298
degrees K).
Volume at standard condition (NTP) either
increases or decreases by the ratio of the field
and standard temperature (Ts over Tf).
Evaluate to determine need for correction.
Pressure correction is adjustment of volume
by difference between the field sampling
pressure and the desired standard pressure of
1 atmosphere (760 mm Hg).
Volume at standard condition (NTP) either
increases or decreases by the ratio of the field
and standard pressure (Pf over Ps).
Evaluate to determine need for correction.
Understand relationship - calculate the
concentration in parts per million from mass
per volume concentration (mg/M3).
Temperature and pressure correction applied
to the air sample volume in cubic meters and
not to molar gas volume.
Same concentration in ppm is independent of
temperature and pressure since calculation on
volume-per-volume basis.