INDUSTRIAL HYGIENE -

PREPARATION OF AIR CONTAMINANT

CONCENTRATIONSUNIVERSITY 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.

PURPOSES

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.

VALIDATED METHODS

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.

TWO SYSTEMS

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.

BATCH MIXTURES

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.

SEALED CHAMBERS

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.

BOTTLES

Figure 16.1; Table 16.1. (Third Edition)Figure 14.1; Table 14.1. (Second Edition)

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).

PLASTIC BAGS

Figure 16.3 (14.3 – Second Edition)

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.

PRESSURE CYLINDERS

Values in appropriate UNITS; for constants in equation, numerical value corresponds to the UNITS required.

Ambient pressures commonly measured with barometer and expressed as mm of mercury;

Air volumes measured in liters; and, Temperature as degrees Celsius.

CALCULATION INFORMATION

SI metric units: meter; kg, degrees C; bar/Pascal, etc.

1 M3 = 1000 Liters; 1 atmosphere = 760 mm Hg

Ideal gas law; molar gas volume; partial vapor pressures

Concentrations – mg/M3 vs. ppm (ug/L)

Standard conditions: temperature of 25 degrees C and 760 mm Hg pressure.

CALCULATION INFORMATION

See: Third Edition: Chapter 16, pages 385 – 386Second Edition: Chapter 14, pages 248 – 250.

Equation 16-2

Equation 16-3

Equations 16-4 and 16-4a

Equation 16-5

Equation 16-6 and 16-6a

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.

FLOW-DILUTION SYSTEMS

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.

GAS-METERING DEVICES

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 METER CALIBRATION CORRECTIONS

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.

MIXING SYSTEMS

Figure 16.4 – all-glass mixer (3rd Edition)Figure 14.4 (2nd Edition)

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.

SOURCE DEVICES

Figure 16.5 – self-dilution device (3rd Edition)Figure 14.5 (2nd Edition)

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

OTHER SOURCE DEVICES

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.

PERMEATION TUBE SOURCES

Also See: Third Edition – Chapter 16, pages 392 – 396.Second Edition – Chapter 14, pages 254 - 257

Figure 16.14 - Permeation Tube Flow System

Variables involved include: Source strength Desired concentration Flow rates

Examples of Calculations – refer to Chapter 16 (2nd Edition – Chapter 14) and also Appendix.

FLOW-DILUTION CALCULATIONS

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).

SOURCE DEVICES FOR AEROSOLS

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 mono-disperse. 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.

PARTICLE SIZES

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

AEROSOL PRODUCTION METHODS

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.

MULTIPURPOSE CALIBRATION COMPONENTS AND SYSTEMS

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.

APPENDIX A CALCULATIONS

Equation A16.1 – Ideal Gas Eq.

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.

TEMPERATURE AND PRESSURE EFFECTS

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 AND PRESSURE EFFECTS ON VOLUME

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.

RANGE OF TEMPERATURES FOR VOLUME 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.

RANGE OF PRESSURES FOR VOLUME 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.

TEMPERATURE AND PRESSURE CORRECTION TO MASS