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, NIOSHi.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 PLANDesigning 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 SAMPLINGCONSIDERATIONS

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)

ORarea 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 metersRefer to instructions from manufacturers.

SAMPLE COLLECTION MEDIAConsult 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 SAMPLERSEach 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 ISSUESReport (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 SAMPLINGCollected 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 break-through 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 LIMITUseful limit of the analytical instrument (mg of analyte per sample).

Sample above Upper Measurement Limit, then re-dilute 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).

REMEMBERCALCULATIONS

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.