Industrial Air Quality Monitoring. Sampling Protocols Grab vs. Integrated Personal vs. Area.

Industrial Air Quality Monitoring

Sampling Protocols

• Grab vs. Integrated

• Personal vs. Area

Grab Sampling

• Measure the concentration of an airborne substance over a short time period (e.g. less than 5 minutes)

• Effective as a screening technique to determine if more extensive monitoring is warranted

• Not usually used to calculate TWA values, since sample times are rarely representative

Integrated Sampling

• a.k.a. Continuous Sampling

• Used to estimate 8-hour and 15-minute TWA exposures by collecting one or more samples over the duration of the task period

• “Integrates” the various concentrations to which the worker has been exposed

Personal Sampling

• Preferred method of evaluating worker exposure to airborne chemicals

• Worker wears sampling device that collects sample wherever worker is, whatever they’re doing

• Aperture should be as close as possible to breathing zone (hemisphere in front of the shoulders with radius of 6-9 inches)

Area Sampling

• Evaluate background concentrations associated with a place (rather than with a worker)

• Effective in investigating suspected leaks or “hot spots”

• Not usually used to calculate TWA values, since the location is rarely representative of worker’s breathing zone

Collection Devices

• Direct reading

• Evacuated containers

• Absorption

• Adsorption

• Filters

• Passive vs. active

Direct Reading

• Provide “immediate” indication of concentration

• Example: Colorimetric (“stain”) indicator tubes (“Draeger tubes”)

• Electronic sensors available for some gases

Evacuated Containers

• Reverse aerosol can

• Plastic sampling bags

Absorption

• Gas or vapor passed through absorbing liquid (e.g. acid vapors through deionized water)

• Effective with reactive, soluble compounds

• Absorption efficiency must be considered when calculating concentration in air from concentration in absorbent liquid

Adsorption

• Alternative for insoluble or nonreactive gases

• Solid sorbent (such as granular activated carbon or silica gel) provides sites to which contaminant molecules are attracted through van der Waals forces

• Carbon adsorption tubes one of the most commonly used active collectors

Anatomy of an Adsorption Tube

• Charcoal made from coconut shell: large adsorptive surface area and nonpolar (so it prefers organic vapors rather than water &c)

• Standard tube 7cm long by 4 mm wide– 1st section contains 100 mg charcoal and a

fiberglass, glass wool, or urethane foam plug

– 2nd section (“backup”) contains 50 mg charcoal to detect “breakthrough”: > 10% back/front

Filters

• Commonly used for particulates

• Filter material chosen based upon analysis:– Weight (total particulates within capture range)– Chemical composition (need material that

dissolves or ashes to permit analysis)– Size categorization (translucence to permit

microscopy for particle sizing and counting)

Passive vs. Active Monitoring

• Passive monitors allow personal sampling without pumps

• Rely on diffusion (movement across concentration gradient

• Example: Organic vapor monitor

• Active monitors use pumps to move air through collection device

Analytical Methods

• Sources of error– Determinate error (bias)– Indeterminate error (random)

On-site Analysis

• Stain (indicator) tubes

• Direct reading instruments

Laboratory Analysis

• Gas Chromatography– Thermal Conductivity Detector– Flame Ionization Detector– Electron Capture Detector

• Atomic Absorption

• Spectrophotometry– IR, Visible, UV