INDUSTRIAL HYGIENE SAMPLING OF GASES AND
VAPORS
UNIVERSITY OF HOUSTON - CLEAR LAKE
SPRING 2015
PURPOSE
Introduce techniques to evaluate EXPOSURES to
gases and vapors arising in or from the workplace.
Additionally:
- be aware of technology for assessment of
environments;
- indoor and ambient air; and,
- capabilities and limitations of various
referenced methods.
SAMPLING GASES AND VAPORS
When developing a sampling strategy, review
available sampling and analytical methods for
contaminants of interest.
Select most suitable!
e.g. OSHA, NIOSH
i.e. published and validated methods.
EPA methods used for lower level indoor
air pollutants and toxics in ambient air.
SAMPLING METHOD
Select a method that meets sampling and analytical
ACCURACY and PRECISION requirements of the
referenced standard in unique field conditions.
Usually stipulate measurement at the PEL within +/25% of the “true” value at a 95% confidence level.
EPA – related to ambient air.
ANALYTICAL LABORATORY
Select and consult with qualified analytical laboratory,
e.g. AIHA for participation in appropriate Laboratory
Accreditation Programs.
Labs assist with method choice that meet sensitivity
and specificity criteria for environment under
evaluation. Choose sampling media and strategy
compatible with method and review special handling.
Two key factors:
knowledge of occupational
environment AND overall perspective of the limitation
of the chemistry of sampling/analysis parameters.
SENSITIVITY
Exercise caution when using traditional workplace
sampling method for measuring contaminants in
indoor
or
ambient
air
because
expected
concentrations may be below the working range.
To obtain sensitivity, NIOSH recommends exceeding
recommended air volume while observing the
suggested maximum flow rate.
The conservative value protects against break-through
(e.g. primary vs. back-up sections of sampler) under
“worst-case” conditions of high %RH and/or
concentrations.
GAS/VAPOR
For IH purposes, a substance is a GAS if this is
normal physical state at room temperature (25
degrees C) and one-atmosphere pressure.
Examples: CO, Cl, Oxygen, and Nitrogen.
If substance is normally a liquid at normal
temperature and pressure, then the gaseous
component in equilibrium with liquid state is a
VAPOR.
Examples: CCl4, HCOH, and Benzene.
SAMPLING PLAN
Designing a sampling plan involves consideration of
the following:
location of samples,
number of workers, and
duration of sampling.
Also other factors:
noise,
equipment,
size,
flow rate, and
security of equipment/sample integrity.
SAMPLE TYPES
Two basic types of samples
used to assess employee exposure:
-
Integrated
-
Grab.
INTEGRATED SAMPLING
For gases and vapors, passage of a known volume
of air through an ABsorbing or ADsorbing medium
to remove the desired air contaminants during
defined time period.
Absorb – penetration into material; physically
dissolve or chemically react with sample medium.
Adsorb – traps airborne chemicals onto surface.
Contaminants of interest are collected and
concentrated over time to obtain the average
exposure levels during the entire sampling period.
GRAB SAMPLING
Direct collection of an air-contaminant mixture into
a device (i.e. sampling bag, syringe, or evacuated
flask) over a short interval of a few seconds or
minutes.
Represents the atmospheric concentrations at the
sampling site at a given point in time.
WHOLE AIR SAMPLING
Collection of air into a sealable container (e.g.
stainless steel canister or sampling bag) for
subsequent analysis.
Example: Figure 11.10 [insert – follows]
Obtained over a short period of time as grab
samples
OR
integrated over a longer period of time to obtain
Time-Weighted Average (TWA).
INTEGRATED SAMPLING
Cover the entire period of exposure required
because airborne contaminant concentrations during
a typical work shift vary with time and activity!
Grab samples do not reflect average exposures.
Most integrated sampling is done to determine the
8-hour TWA and/or STELs for comparison with
OSHA PELs, ACGIH TLVs and NIOSH RELs.
Personal Pocket Sampling Pump
INTEGRATED SAMPLING
CONSIDERATIONS
Appropriate sample duration and flow rate chosen
relative to the purpose of sampling, sensitivity of
analytical method, and expected concentration of
contaminant.
Accurate measurement of flow rate and time which
depends on precise determination of the mass of
contaminant collected and volume of air sampled.
ACTIVE SAMPLING
Means of collecting an airborne substance with a
mechanical device such as air sampling pump to
draw the air/contaminant mixture into or through
the sampling device.
Examples: sorbent tube, treated filter, or impinger
containing a liquid media.
A key element is calibration that reliably measures
the pump flow rate, thus allowing for an accurate
determination of air volume.
ACTIVE SAMPLING
Uses mechanical device to collect airborne
substance such as air sampling pump to draw the
air/contaminant mixture into or through the
sampling device.
AIR SAMPLING PUMPS

Integrated methods require relatively constant source
of suction calibrated to recommended flow rate
(within +/- 5% with collection media in-line).
AIR SAMPLING WITH PUMPS




Personal sampling (i.e. breathing zone) OR
area sampling for potential source(s).
Features – constant flow capabilities/back pressure;
intrinsically safe; etc.
Maintain desired flow rate over the entire sampling
period with in-line sample collection device.
Pressure drop; constant flow vs. constant pressure
CALIBRATION
Pump flow must be calibrated with the entire
sampling train assembled as used.
Good IH practice requires both pre- and post-pump
calibration on the same day under pressure and
temperature conditions similar to those at site.
Not addressed with built-in rotameters (not
precision devices and do not provide a quantitative
measure of the airflow).
CALIBRATION STANDARDS
Two Terms:
 Primary – direct and measurable linear dimensions
(length and diameter of cylinder).
 Examples:

spirometers and bubble meters
Secondary – flowmeters that trace calibration to
primary standards and maintain accuracy with
reasonable care and handling in operation.
 Examples:
precision rotameters, wet test meters, and
dry gas meters
Refer to instructions from manufacturers.
SAMPLE COLLECTION MEDIA
Consult published air sampling methods to
determine the appropriate collection media for a
specific chemical contaminant.
Review methods to determine applicability relative
to field conditions.
Such as: VP, BP, reactivity; interferences, also
humidity/temperature effects, proper measuring
range; physical state of the contaminant sampled;
multiple phases (i.e. particulate and vapor, etc.).
ACTIVE SAMPLING – ADVANTAGES
-
-
Select method used by compliance personnel
during OSHA inspection.
Offers calibrated, measured airflow for
assurance in accuracy of sample volume.
Sorbent tube samples have a secondary layer
for back-up indicating break-through.
Multiple phases can be assessed by a series of
samplers.
ACTIVE SAMPLING – DISADVANTAGES
-
-
Cumbersome equipment and may interfere
with job tasks during shift for personnel.
Pump calibration is time consuming and
requires technical training.
Pump may become somewhat less reliable at
maintaining constant flow over the entire
sampling period, and more frequent calibration
may be necessary.
PASSIVE SAMPLERS
Passive sampling is the collection of airborne gases
and vapors at a rate controlled by a physical
process such as diffusion through a static air layer
or permeation through a membrane without the
active movement of air through sampler.
Operate on principle of diffusion.
Organic Vapor Monitoring Badges
PASSIVE SAMPLERS
Performance characteristics evaluated in the NIOSH
Validation Protocol include:
- Analytical recovery
- Sampling rate and capacity
- Reverse diffusion
- Storage stability
- Analyte concentration
- Exposure time
- Face velocity
PASSIVE SAMPLERS
Performance characteristics evaluated in the NIOSH
Validation Protocol include:
- Relative humidity
- Interferences
- Monitor orientation
- Temperature
- Accuracy and precision
- Shelf life
- Behavior in field
DIFFUSIVE SAMPLERS
Rely on movement of contaminant molecules across a
concentration gradient, which for steady-state
conditions can be defined by:
Fick’s first law of diffusion.
Consist of diffusion gap between external air and a
sorbing medium to collect chemicals of interest, but
also to maintain the concentration as close to zero as
possible at the end of the diffusion path.
DIFFUSIVE SAMPLERS
Each gas/vapor sampled has a specified diffusion
coefficient (D). Uptake rates can vary under various field
conditions. Validation!
See Equations and Units on Page 274 (Third Edition).
OSHA ISSUES
Report (1998) attempted to determine sampling rate
variation of specific passive sampler designs.
Concept of passive sampling equated to active
sampling with pump error of +/- 5%.
Significant - use of sampling rate variation for
passive sampler along with the analytical error
component allowed calculation of the overall
Sampling and Analytical Error (SAE).
SAE must be used by OSHA inspectors along with
sample results to determine if PEL exceeded.
Therefore, passive sampling methods can be used by
OSHA.
PASSIVE SAMPLERS
Various types are commercially available.
Some samplers designed to collect broad range of
compounds, whereas others due to collection media
preferentially obtain a single chemical or family of
chemicals.
Examples: activated charcoal sorbent – organic
vapors and GC analysis; chemical treated sorbents or
filter paper for HPLC analysis.
Direct-reading passive samplers based on colorimetric
techniques; may not be as accurate as lab analytical
methods.
PASSIVE SAMPLING –
ADVANTAGES
-
Easy to use, samples can be collected by
personnel with less technical training.
Less expensive.
Less obtrusive for wearer.
For most applications, the mass of contaminant
collected by passive samplers is not
significantly affected by temperature or
pressure.
PASSIVE SAMPLING – DISADVANTAGES
-
May not be OSHA/NIOSH methods referenced
in order to insure reliability of data.
Sampling rate, if theoretically calculated, may
not prove valid under field conditions.
Reverse diffusion may be a factor.
Environmental parameters may influence the
collection efficiency of passive samplers.
Examples: stagnant air; high face velocities.
Low uptake rates may not provide sensitivity
required for low-level determinations, and
extended sampling times (> 24 hours) may
enhance effects of reverse diffusion.
GRAB SAMPLING
Collected sample measures gas and vapor
concentrations AT A POINT IN TIME and used to
evaluate “PEAK” exposures for comparison to
“Ceiling” limits.
Identify
unknown
contaminants,
evaluate
contaminant sources, or measure contaminant levels
from intermittent processes/other sources. Collected
using syringes, canisters, or bags, etc.
Instantaneous (as well as integrated) measurements
of gases/vapors also may be performed using
detector tubes or direct-reading instruments.
Colorimetric Tubes and Sampling Pump
Colorimetric Gas Detection Tubes
GRAB SAMPLING –
ADVANTAGES
-
-
After
collection,
frequently
analyzed
immediately
by
GC
or
direct-reading
instruments.
Therefore, quick decisions made in field/site
about source of leaks;
Confined Space Entry (CSE);
PPE, etc.
GRAB SAMPLING – DISADVANTAGES
-
-
For most applications, contaminants are
collected but not integrated over time. Only
some devices will allow use of a metering device
to collect sample(s) at or near constant flow over
period of time for TWA.
For low contaminant concentrations, analytical
instrument may not be sensitive for detection.
Multiple grab samples to assess full-shift
exposures is time-consuming process and subject
to error.
OPERATIONAL LIMITS OF SAMPLING AND
ANALYSIS
Inherent limitations of method:
- Sampler capacity
- Limit of Detection (LOD)
- Limit of Quantification (LOQ)
- Upper measurement limits define useful range of
referenced method.
These factors determine the minimum, maximum, or
optimum volume of air to be sampled and also
confidence that can be placed in the results.
Discuss with lab before sampling!
SAMPLER CAPACITY
Predetermined conservative estimate of total mass of
contaminant that can be collected on medium without
loss or overloading.
NIOSH definition of 2/3 of experimental breakthrough capacity of the solid sorbent [i.e. 67% of
mass of contaminant on sorbent at the break-through
volume].
Break-through volume is: volume of an atmosphere
containing two times the PEL for the contaminant that
can be sampled at the recommended flow rate before
the efficiency of the sampler degrades to 95%.
LIMIT OF DETECTION (LOD)
Lowest concentration level determined to be
statistically different from a blank sample.
Recommended value of the LOD is the amount of
analyte that will give rise to a signal that is three
times the standard deviation of the signal derived
from the media blank.
LIMIT OF QUANTIFICATION (LOQ)
Concentration level above which quantitative results
may be obtained with a certain confidence.
Recommended value of the LOQ is the amount of
analyte that will give rise to a signal that is ten
times the standard deviation of the signal from a
series of media blanks.
Corresponds to a relative uncertainty in the
measurement of +/- 30% at 99% confidence level.
DETECTION LIMIT (DL)
Described both in terms of:
- DL of the analytical procedure, and
- DL of the overall procedure.
OSHA reports, in general, that detection limits (DL)
are defined as: amount of analyte that gives a
response that is significantly different (three SD)
from the background response.
UPPER MEASUREMENT LIMIT
Useful limit of the analytical instrument
(mg of analyte per sample).
Sample above Upper Measurement Limit, then redilute and re-analyze.
Review and discuss with analytical lab.
TARGET CONCENTRATION
Preliminary estimate of airborne contaminant
concentration relative to the purpose of testing.
Parameter can be used to determine the minimum
and maximum air volumes.
Can be estimated by: using previous sampling data,
use of direct-reading instruments, or by relying on
the professional judgment of the IH.
CALCULATIONS
Sample volumes – minimum and maximum.
Working range is range of contaminant
concentration that may be quantitated at a specified
air volume.
Lower boundary of working range is defined by a
sample that has a mass of contaminant equal to the
LOQ. Upper boundary is defined by sampler
capacity.
Refer to Page 278 (Third Edition) for formulas.
SOLID SORBENT MEDIA
Adsorb onto surface; Effectiveness determined by:
 Trap and retain nearly all contaminants from air
 Amenable to desorption from sorbent
 Sufficient capacity to retain contaminant quantity for
analysis without large media pressure drop
 Not cause chemical change of contaminant
except by analytical method as needed
 Absorb contaminant of interest in presence of other
substances, possibly in higher concentrations than
necessary for compound.
COLLECTION EFFICIENCY OF SOLID
SORBENTS
Various Factors:
 Temperature
 Humidity
 Sampling Rate
 Other Contaminants
 Sample Break-through
- 25%
- Migration
DESORPTION
-
Solvent extraction - extract contaminants of
interest from adsorbent materials.
Examples: carbon disulfide; mixtures.
-
Thermal desorption – drive contaminant off
sorbent by high temperature; entire mass of
contaminant introduced directly into analytical
instrument without dilution; and, measure low
airborne concentrations.
DESORPTION EFFICIENCY
Measure of how much analyte can be recovered
from the sorbent tube; determined typically at 0.1,
0.5, 1, and 2 times target concentration based on
the recommended air volume and expressed as a
percentage of analyte spike on media.
To be determined for each lot number of solid
sorbent used for sampling and also done in the
concentration range of interest.
TYPES OF SORBENT MATERIALS



Inorganic Sorbents – silica gel (polar; %RH); less
reactive than charcoal
Elemental Carbon – charcoal types; organics; high
adsorptive capacity; stable compounds; high
humidity parameters
Carbonized or Graphitized Sorbents – low to
moderate surface area; intermediate to high
volatility; stable compounds for thermal desorption
Sorbent Tubes
Sorbent Tubes
TYPES OF SORBENT MATERIALS


Organic Polymers - selectivity to particular
applications, and stability at high temps enables
thermal desorption (i.e. Tenax – broad range of
organics can be collected).
Other Sorbent Materials –
sorbents with PUF; sorbent
combinations; sorbent/filter
combinations – OVS, etc.
Sorbent Tubes
OTHER MEDIA


Chemically treated filters – derivatize/desorb
Liquid absorbers
Gas washing bottles – e.g. impingers
 Fritted glass bubblers

Filter Cassettes
Filter Cassettes
OTHER MEDIA

Sampling bags/evacuated containers/canisters

-
Situations:
direct-reading instruments;
leaks/emergencies; peaks;
highly volatile compounds
Precautions – storage time; reaction; diffusion.
CALCULATIONS







Total Mass of Contaminant
Airborne Concentration by sample volume (mass
over volume)
Air Volume (flow rate x sample time)
Unit Conversions – mg/M3 to/from ppm
Temperature/Pressure Corrections
Time-Weighted Averages (TWA)
Potential Work Shift Adjustments
See Formulas and other Handouts/PPTs.
CALCULATIONS




Unit Conversions – mg/M3 to/from ppm
Temperature/Pressure Corrections
Time-Weighted Averages (TWA)
Potential Work Shift Adjustments
Formulas on Page 286-287 (Third Edition).
REMEMBER
CALCULATIONS
Range of temperature and pressure changes tolerated
before corrections are applied to the volume of air
sampled.
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 to published OELs.