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Presentation on theme: "INDUSTRIAL HYGIENE - SAMPLING OF GASES AND VAPORS"— Presentation transcript:


2 PURPOSE Introduce the techniques available for Industrial Hygienists to evaluate EXPOSURES to gases and vapors arising in or from the workplace. Also: - be aware of technology available for assessment of traditional environments - indoor and ambient air, and - capabilities and limitations of methods.

When developing a particular sampling strategy, review sampling and analytical methods available for the contaminants of interest. Select most suitable for the specific application. e.g. OSHA, NIOSH i.e. published and validated methods EPA methods used for lower level indoor air pollutants and toxic compounds in ambient air media.

4 SAMPLING METHOD Select a method that meets the sampling and analytical ACCURACY and PRECISION requirements of the standard in its unique field conditions. Usually stipulate measurement at the PEL within a +/- 25% of the “true” value at a 95% confidence level. EPA – indoor air pollutants and toxic compounds associated with ambient air.

Select and consult with a qualified analytical laboratory, e.g. AIHA that participates in Laboratory Accreditation Programs. Labs can assist in choosing methods that meet the sensitivity and specificity criteria for the environment being evaluated. Choose sampling media and strategy compatible with method selected and advise on special handling. Two key factors: knowledge of occupational environment AND overall perspective of the limitation of the chemistry of sampling/analysis.

6 SENSITIVITY Exercise caution when using a traditional workplace sampling method for measuring contaminants in indoor or ambient air because the expected concentrations may be below the working range. To obtain sensitivity, NIOSH recommends exceeding the recommended air volume while observing the recommended maximum flow rate. The conservative value protects against breakthrough (e.g. primary vs. back-up sections of sampler) under “worst-case” conditions of high %RH and/or concentrations.

7 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.

8 SAMPLING PLAN Designing a sampling plan involves consideration of the following: location of samples, the number of workers to be sampled, and the duration of sampling. Also consider other factors – noise, equipment, size, flow rate, and security. Two basic types of samples are used to assess employee exposure to gases and vapors in the workplace: - integrated - grab.

9 INTEGRATED SAMPLING For gases and vapors, involves passage of a known volume of air through an absorbing or adsorbing medium to remove the desired contaminants from the air during a specified period of time. Contaminants of interest are collected and concentrated over a period of time to obtain the average exposure levels during the entire sampling period.

10 GRAB SAMPLING This technique involves the 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.

11 WHOLE AIR SAMPLING This technique involves the collection of air into a sealable container (e.g. stainless steel canister or sampling bag) for subsequent analysis. Can be collected over a short period of time as grab samples or integrated over a longer period of time to obtain Time-Weighted Average (TWA) concentrations.

12 INTEGRATED SAMPLING This type of sampling to cover the entire period of exposure is 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 to compare with OSHA PELs, ACGIH TLVs and NIOSH RELs.

Appropriate sample duration and flow rate need to be chosen relative to the purpose of sampling, the sensitivity of the analytical method, and the expected concentration of the contaminant of interest. It is also essential that the flow rate and time be accurately measured. The accuracy depends on the precise determination of the mass of contaminant collected as well as the volume of air sampled.

14 ACTIVE SAMPLING Means of collecting an airborne substance that employs a mechanical device such as an 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.

15 AIR SAMPLING PUMPS Integrated methods require a relatively constant source of suction that can be calibrated to the recommended flow rate (within +/- 5% with collection media in-line). Personal sampling within the worker’s breathing zone or can be used as area samplers. Features – constant flow capabilities/back pressure; intrinsically safe; electromagnetic susceptibility, etc. Must be capable of maintaining the desired flow rate over the entire sampling period with the sample collection device in-line. Pressure drop; constant flow vs. constant pressure

16 CALIBRATION Pump flow must be calibrated with the entire sampling train assembled as it will be used in the field. Good IH practice requires both pre- and post-pump calibration on the same day under pressure and temperature conditions similar to those at site. Should not be done with built-in rotameters (not precision devices and will not give a quantitative measure of the rate of airflow).

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 meters. Refer to instructions from manufacturers.

Consult 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 as well as also humidity/temperature effects, proper measuring range; physical state of the contaminant being sampled; multiple phases (i.e. particulate and vapor phase).

- Select method to be 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 breakthrough. - Multiple phases can be assessed by a series of samplers.

- Cumbersome equipment and may interfere with job of workers throughout shift. - Pump calibration is time consuming and requires technical training on tasks. - Pump may become somewhat less reliable at maintaining constant flow over the entire sampling period, and more frequent calibration may be necessary.

21 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 an air sampler. Operate on principle of diffusion.

22 DIFFUSIVE SAMPLERS Diffusive samplers rely on the 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 which serves to collect the chemicals of interest, but also to maintain the concentration as close to zero as possible at the end of the diffusion path. Each 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).

23 OSHA ISSUES Research report in 1998 that 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 in that use of the sampling rate variation for a passive sampler along with the analytical error component allowed the 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.

24 PASSIVE SAMPLERS Commercially available for a variety of airborne contaminants. Some samplers are designed to collect a broad range of compounds, whereas others because of their collection media preferentially collect a single chemical or family of chemicals. Examples: activated charcoal sorbent – organic vapors and GC analysis; chemical treated sorbents or filter paper for preferential collection for HPLC analysis. Direct-reading passive samplers based on colorimetric techniques. May not be as accurate as lab analytical methods. Discuss examples.

- Easy to use, allowing samples to be collected by personnel with less technical training. - Less expensive. - Less obtrusive to wearer for monitoring. - For most applications, the mass of contaminant collected by passive samplers is not significantly affected by temperature or pressure.

- May not be OSHA/NIOSH methods to reference in order to insure reliability of data. - Sampling rate, if theoretically calculated, may not prove to be 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 reverse diffusion effects.

27 GRAB SAMPLING This type of collected sample measures gas and vapor concentrations AT A POINT IN TIME and are used to evaluate “PEAK” exposures for comparison to “Ceiling” limits. Can be used to identify unknown contaminants, to evaluate contaminant sources, or to measure contaminant levels from intermittent processes or other sources. Collected using syringes, canisters, or sampling bags. Instantaneous (as well as integrated) measurements of gases/vapors also may be performed using detector tubes or direct-reading instruments.

- After collection, can frequently be analyzed immediately by GC or direct- reading instruments. - Therefore, quick decisions can be made in field or at the site about source of leaks, Confined Space Entry (CSE), PPE, etc.

- 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. - Using multiple grab samples to assess full-shift exposures is time-consuming and subject to error.

Inherent limitations of method: Sampler capacity Limit of Detection (LOD) Limit of Quantification (LOQ) Upper measurement limits which define the useful range of the method. These factors determine the minimum, maximum, or optimum volume of air to be sampled and may determine the confidence that can be placed in the results. Discuss with lab before sampling!

31 SAMPLER CAPACITY Predetermined conservative estimate of the total mass of contaminant that can be collected on the sampling medium without loss of overloading. NIOSH definition of 2/3 of the experimental breakthrough capacity of the solid sorbent, that is 67% of the mass of contaminant on the sorbent at the breakthrough volume. Breakthrough volume is defined as that 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%.

Lowest concentration level that can be 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.

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.

34 DETECTION LIMIT Described both in terms of detection limit of the analytical procedure, and the detection limit of the overall procedure. OSHA reports, in general, that detection limits are defined as the amount of analyte that gives a response that is significantly different (three SD) from the background response.

Useful limit of the analytical instrument (mg of analyte per sample). Sample above Upper Measurement Limit, then re-dilute and re-analyze. Discuss with analytical lab.

36 TARGET CONCENTRATION Preliminary estimate of the airborne concentration of the contaminant of interest 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.

37 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 the working range is defined by a sample that has a mass of contaminant equal to the LOQ. The upper boundary is defined by sampler capacity. Refer to Page 278 (Third Edition) for formulas.

38 SOLID SORBENT MEDIA Adsorb onto surface; Effectiveness determined by:
Trap and retain nearly all contaminant from air Amenable to desorption from sorbent Sufficient capacity to retain quantity of contaminant to facilitate analysis without creating large pressure drop across sample media Not cause chemical change of contaminant except by analytical method as needed Absorb contaminant of interest in presence of other contaminants, possibly in higher concentrations than the contaminant of interest.

Various Factors: Temperature Humidity Sampling Rate Other Contaminants Sample Breakthrough - 25% - Migration

40 DESORPTION - Solvent extraction - extract contaminants of interest from the adsorbent materials Examples: carbon disulfide; mixtures - Thermal desorption – drive contaminant off sorbent by subjection to high temperature; entire mass of contaminant collected introduced directly into analytical instrument with no dilution; can measure low airborne concentrations.

Measure of how much analyte can be recovered from the sorbent tube; determined typically at 0.1, 0.5, 1, and 2 times the target concentration based on the recommended air volume and expressed as a percentage of analyte spike on the sorbent tube. Should be determined for each lot number of solid sorbent used for sampling and should be done in the concentration range of interest.

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

Organic Polymers - selectivity to particular applications, and the stability of some polymers 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.

44 OTHER MEDIA Chemically treated filters – derivatize/desorb
Liquid absorbers Gas washing bottles – e.g. impingers Fritted glass bubblers Sampling bags/partially evacuated rigid containers (canisters) Situations: use of direct-reading instruments; leaks/emergencies; peak concentrations; highly volatile compounds Precautions – storage time; reaction; diffusion Whole Air Sampling [HDS – helium] Cold Traps

45 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 Potential Work Shift Adjustments Formulas on Page (Third Edition).

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.


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