Air Sampling & Instrumentation Flashcards
Absorption
The removal of gaseous constiutent from a gas stream by pentration of the gas molecules into a liquid or solid matrix. Needs bubbling to achieve good mixing.
Properties/Types of Absorption Sampling
Properties
- Low volatility, low viscosity, low foaming, non flammable
Types
- Fritted Glass (MFGB), Impinger (MI)
Adsorption
The removal of gaseous constituents from an airstream by a solid matrix in which the gas molecules are deposited on the surface of the adsorbent
Factors affecting adsorption
- Nature of material (surface area, polarity, reactivity, etc.)
- Temperature
- Velocity of air stream
- Concentration of gases
Active adsorption
air pulled thru tube
Break-thru criteria for chemical sampling media
NIOSH > 10% on back up
OSHA > 25% on back up section
―Migration – diffusion of sample from front to rear over storage time
5% on pre/post calibration of pumps = bad sample
Field Blanks: 10% of sample, Minimum 2
Passive adsorption
relies on molecular diffusion (ficks first law)
Critical Air Velocity 0.2 m/sec
Performance affected by: Face velocity, temperature, humidity, pressure, and reverse
diffusion
Common adsorbents
- activated charcoal
- silica gel
- porous polymers
Conductivity:
Gases form electrolytes in solution and change conductivity of solution (NH3, SO2)
Potentiometry:
Gases react with reagents to change pH of solution (O2, CO, H2S)
Coulometry:
measurement of the number of electrons transferred across a solution/electrode interface
Flame ionization:
measurement of conductivity of gas as a result of ionization of gas by flame (total Hydrocarbons)
Thermal conductivity:
resistance of heated filament changes with temperature changes of filament caused by thermal conductance of gas flowing over filament (wheatstone bridge)
Fume
Solid particles formed by condensation after heating (0.001-0.2 m)
Dust
Solid particles formed by mechanical processes (0.5-50 m)
Mist
A dispersion of liquid particles (40 - 400 m)
Smoke
Solids from incomplete combustion
Fog
Liquid formed by condensation
Fiber
aspect ration of 3-1
Aerosol behavior effected by
1) size
2) shape
3) density
4) Hygroscopic
Stokes Law (particle settling rate)
Vs = 0.003SGd2(cm) Or Vs = 0.006 x SG x d2 ( m)
Terminal settling velocity is proportional to d2
Filter medium - Forces Acting on an Aerosol
- Sedimentation – gravitational effects (based on Stokes Law)
- Impaction – Rapid changes in Air direction impacts particle on filter
- Diffusion – Random displacement due to air‘s Brownian Motion
- Interception – Airflow of particle past a collecting medium 1/2 particle size to be captured by ―touching” electromagnetic attraction (oil wipes this out)
Filter medium - Forces promoting
retention
- Van der Waals
- Electrostatic charge
- Capillary forces due to liquids
Three major types:
Filtering, Impaction, and impingement
Impingement:
not commonly used as a technique anymore
- Greenberg-Smith: Flow rate 1 cfm, pressure drop 3”Hg
- Midget impinger: 0.1 cfm,
- Particle counting - (see Ocular microcopy for more information) Mppcf – million particles/cubic ft (6.4 mppcf = 1 mg/m3)
MCEF
0.8 m, 37mm & 25 mm used for Not for Gravimetric
855 and 385 mm2 used for Metals, asbestos
100
Glass
Analytical finish requires strong chemicals
Benzidine, aldrine
32
PVC
5 m typical, used for gravimetric tests
Dust, Cr (IV)
15
PTFE
5m typical, used for extraction/desorption
Alkaline dust, formic acid
25
Silver
Sample support medium for x-ray analysis
2
Types of Asbestos
- Amosite
- Chrysotile - Most Common
- Tremolite
- Crocidolite - Blue Asbestos, Most Dangerous
Inhailable particulate Mass (IPM)
any size within the respiratory tract (up to 100 m) (Inertial Impaction)
Thoracic Particulate Mass (TPM)
Airway of lungs or gas exchange region (5-15 m) (Top Impaction, bottom diffusion and impaction)
Reparable Particulate Mass (RPM)
Gas exchange region 5 m, sedimentation or diffusion dependent on size of particle (0.1 m)
