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Air Sampling for
Gases and Vapors
Patrick N. Breysse, PhD, CIH
Peter S.J. Lees, PhD, CIH
Johns Hopkins University
Section A
Introduction
Exposure Assessment Methods
General air-sampling methodology
Air
Pump
Flow
Adjust
Collection
Media
Air + Contaminant
4
Concentration
Mass conta min ant
Concentrat ion =
Volume air
Contaminant mass from laboratory
analysis
Air volume from product of calibrated
air flow rate and sampling time
5
Source: U. S. Government
6
Charcoal Tube Calibration
Calibrator
Tubing
Source: Patrick Breysse
Charcoal tube
Holder
Sampling
Pump
7
Personal Sampling Pumps
Source: Patrick Breysse
8
Section B
Sample Collection Methods
Sampling for Gases and Vapors
Absorptive methods
– Absorption is a molecular process
by which molecules are
homogeneously dispersed in
another substance
– Forming a solution (i.e., requires a
liquid sampling medium)
Methods now mostly obsolete
Continued
10
Sampling for Gases and Vapors
Adsorptive methods
– Adsorption is a molecular process
by which molecules are reversibly
bound to a surface
– Bound by intermolecular or van der
Waals forces
– Uses solids as sampling medium
Most current gas and vapor methods
are adsorptive
11
Absorptive Methods: Impingers
Contaminant bubbled through
absorbent solution
Solution changes according to analyte
Analyzed by wet chemical methods
Messy and unstable; not suited to
personal sampling
Used for gases (NO2)
12
Impingers
Source: U. S. Government
Continued
13
Adsorptive Methods
Gas and vapor molecules “stick” to
solid surface
Highly efficient reversible process
Contaminant stable for transport
Many different adsorbents available
Can be affected by high humidity
Subject to overload
Continued
14
Adsorptive Methods
Some common adsorbent media
– Activated charcoal: Polar solvents
(many hydrocarbons)
– Silica gel: Non-polar solvents
(alcohols and aldehydes)
– Treated diatomaceous earth
– Gas chromatographic supports
15
Adsorbent Tubes
Source: Patrick Breysse
16
Adsorptive Methods: Charcoal
Sampling
– 20–200 ml/min drawn through tube
Analysis
– Analyte(s) desorbed (CS2, or
thermally)
– Typically analyzed using gas
chromatography
Continued
17
Adsorptive Methods: Charcoal
Overload
– >25% on “B” section indicates
breakthrough (loss)
– Discard sample or report as
underestimate
18
Standard Sampling Tube
Direction of flow
Source: U.S. Government
19
Section C
Passive Sampling Methods
Passive (Diffusive) Sampling
Passive sampling relies on the natural
diffusive energy of molecules to move
them to sampling media
Contaminant flux (mass/sampler
area/unit time) is governed by Fick’s Law
Effective flow rates generally 5–30 ml/min
A variety of designs
21
Example: Passive Sampler
Source: Peter Lees
22
Section D
Sample Analysis
Gas and Vapor Analysis
Numerous methods
– Spectrophotometric
– Chromatographic
• Gas chromatography (GC)
• High-performance liquid
chromatography (HPLC)
• GC-Mass spectrometry (GC-MS)
– Wet chemical
24
Analytical Laboratory
Choose analytical laboratory before
sampling
Choose standard method
Know limits of detection
– Calculate minimum sample volume
to be minimally detectable
– Evaluate breakthrough potential
Know interferences
25
Quality Control
Blanks
– At least 10% of samples should be
blanks
Spiked samples
– Whenever possible, submit a
“known” or spiked sample
Continued
26
Quality Control
Split samples
– If possible, submit split samples to
either the same laboratory or two
different laboratories
27
Section E
Direct-Reading Instruments
Direct-Reading Instruments
“Real time” measurement of contaminant
Information saved to data logger
Subsequent data analysis (mean, min.,
max., etc.)
Principle of operation varies
– Infrared absorption
– Ultraviolet absorption
– Electro-chemical method
Continued
29
Direct-Reading Instruments
Numerous designs and manufacturers
Possible uses
– Walk-through survey (preliminary
hypothesis testing)
– Leak detection
– Over-exposure alarm
– Personal monitoring
– Emergency response
Continued
30
Direct-Reading Instruments
Limitations
– Specific versus non-specific
response
– Must know principal of operation
– Must know positive and negative
interference
– Must be calibrated often
– Subject to damage
31
