The Method Development Approach to Air Analysis Kim W. Baughman Southern Testing and Research Laboratories, Inc. 3709 Airport Dr. Wilson, NC 27693 Debra H. Love Southern Research Institute 2000 9th Avenue S. P.O. Box 55305 Birmingham, AL 35205
As the field of analytical chemistry has diversified, opportunities for applied analytical chemistry have expanded into a wide range of areas. Even though it is difficult to keep up with advances, no matter what the field, applied analytical chemists must do just that if they wish to be effective. In addition, they must continually evaluate the changing needs of their particular specialty and judge how best to apply analytical chemistry to solve problems. And the problems are becoming increasingly difficult. In this REPORT, we discuss a specific application of analytical chemistry: industrial hygiene (IH). As is the case with most applied sciences, the requirements of the IH chemist create paradoxical responsibilities. The IH chemist must be a specialist, applying analytical chemistry to the unique problems t h a t IH presents. The IH chemist must also be a generalist, keeping up with the progress made in analytical chemistry so as to advance IH to the next level of sophistication.
A little background IH is most often defined a s a combination of science and a r t devoted to the recognition, evaluation, and control of factors or stresses i n the workplace that may result in illness or discomfort among workers. This 480 A *
field covers a broad range of expertise, especially considering the four areas t h a t fall under this jurisdiction: chemical, physical, ergonomic, a n d biological factors. Thus t h e range of expertise required of industrial hygienists is a 3 x 4 matrix that encompasses these four factors and each of the three primary tasks: recognition, evaluation, and control. It would be difficult for any one person to excel i n all of the disciplines necessary to perform the duties associated with these functions. To be fully effective, the industrial hygienist must organize a team of scientists and engineers to fill i n pieces of this matrix. If the responsibilities of the industrial hygienist are viewed in terms of this matrix, it is easy to see how the skills and specialties of these team members are used to perform the duties covered by IH. The IH chemist is largely responsible for one piece of the matrix (evaluating chemical factors or exposures) and will also be involved in the recognition and, to a lesser degree, control of chemical factors. Although historical references detailing the correlation between an illness and a specific occupation date back to the fourth century B.C. (I), the date of the advent of IH chemistry cannot be traced as easily. Sometimes the advent of IH chemistry is equated with t h e miner who first took a canary into the mine as a “detector” for methane gas. In any event, the field of IH chemistry has advanced rapidly in the past two decades, primarily as a result of the Occupational Safety and Health Act of 1970. The purpose of this legislation is to ensure safe and healthy working conditions for all men and women in the workplace. The Occupational Safety and Health Administration (OSHA) and the National Institute for Occupational Safety and Health
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(NIOSH) were formed a s a result of the act. The primary function of t h e IH chemist is to assess the exposure of employees to chemical contaminants in the workplace. Traditionally this involves sampling and analysis of workplace atmospheres. As IH chemists take on a more important role in t h e workplace, there is a greater need for analytical chemists to support their activities. IH chemists are increasingly called upon to develop and validate new methods for the collection and analysis of chemicals in the workplace. In the 1970s and 19808, NIOSH and OSHA developed methods for determining hundreds of these compounds. However, NIOSH and OSHA have been unable to address the analytical needs for the hundreds o r even thousands of other compounds t h a t workers may he exposed to throughout the broad range of U.S. industries. In fact, industry often provides its own analytical methods for determining some compounds; in some cases, toxicology data indicate a need to limit and assess workers’ exposure, but neither NIOSH nor OSHA has yet developed methods for determining these compounds. A related duty of the IH chemist involves additional validation studies of methods developed in the 1970s and 1980s. The American Conference of Governmental Industrial Hygienists (ACGIH) updates recommendations for threshold limit values (TLVs) in the workplace annually, and in 1989 OSHA updated permissible exposure limits (PELS) for hundreds of compounds originally determined in 1970. (TLVs are recommended levels and PELS are mandated limits.) The result of these updates is that, for numerous compounds, the recommended or permissible exposure levels are considerably lower than they 0003-2700/93/036548OA/$04.00/0 0 1993 American Chemical Society
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REPORT were when the analytical methods were developed. Thus the methods may not cover the range necessary to evaluate exposure levels at the new limits. In many cases, the original methods are satisfactory and all that is needed is validation at the lower levels; however, for some of these compounds, new methods must be developed or the original methods must be modified substantially to allow determination at the lower levels of interest. Because of the advances in analytical chemistry in the past decade, the original methods generally can be extended to lower levels. One problem we have encountered in trying to extend current methods to lower levels is poor recovery from the air sampler, which can often be solved by changing to a different desorption solvent. We should point out that OSHA has been challenged in court over the new PEL standards set in 1989.The latest ruling failed to uphold the new standard, leaving industry to decide for itself whether to roll back to the old limits. However, there is proposed legislation in the House of Representatives that would restore the 1989 PELS. Qualified analytical chemists are needed to address the deficiencies in air-sampling and analysis methods for workplace monitoring. One purpose of this article is to review the procedures used to develop and validate sampling and analysis methods and t o discuss some of t h e techniques we have used to facilitate the process. NIOSH has developed criteria for evaluating air-sampling and analysis methods (2).Like most analytical criteria, these deal primarily with the accuracy and precision of the method. However, the fact that the sample matrix is air makes these projects unique, because the validation process is more difficult for air than for most other matrices. Standard atmospheres of the contaminant of interest must be generated and confirmed as part of the validation protocol. Determlnatlon of personal exposure levels Before examining the validation process in detail, we want to look at the features of these methods, some of which are unique to IH chemistry and some of which are universal. One of the most important features is the focus on determining personal exposure levels. We are actually trying to determine the amount of a contaminant that workers are exposed to during the time they are on the job. Therefore the sampler should
The worker wears a portable sampling pump with afilter, held in a casseffe, and a charcoal sorbent tube behind it. follow workers everywhere they go throughout the day. Most often, the industrial hygienist is trying to evaluate the amount of exposure attributable to inhalation: in most situations, this is the route of greatest exposure. The primary mechanism used to evaluate this type of exposure is the portable air-sampling pump that hooks on the belt or is placed in the pocket of the worker. One type of collection device is clipped to the worker’s lapel and attached to the pump with plastic tubing (see photo). This sampling configuration is used when the analytes in question may be in both the particulate and the vapor. The sample i s collected by drawing a i r through the sampling device during the exposure period. The type of collection device used depends on the analyte of interest. The most common types are sorbent tubes, filters, and impingers. Sorbent tubes, consisting of two separated sections of a solid-trapping medium in a 6-8-cm glass tube, are used to collect vapors and gases. For many gases and volatile liquids, the sorbent is coated with a reactive substrate to enhance trapping efficiency. The most common sorbent material is granular activated charcoal, which is very effective for trapping organic vapors. The use of other sorbent types, however, is becoming more common. Particulates are usually collected on 25-37-mm diameter filters, the
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most common of which are glass fiber, Teflon, or polymeric membranes. To detect a n analyte believed t o be present in both the particulate and the vapor, a combination sampling train consisting of a filter followed by a sorbent tube may be used. The other common type of collection device is an impinger filled with 5-25 mL of an appropriate solvent or solution for trapping the analyte. Impingers are difficult to handle in the field, and many industrial hygienists would like to see them replaced with sorbent tubes or filters. Some of the other devices used to collect air samples, such as sampling bags or evacuated bulbs, are not practical because of the need to collect personal samples. After the exposure period is over, the sampler is sent to the laboratory for analysis. The choice of analytical method depends on the nature of the contaminant but covers the entire range of trace analytical techniques: GC, HPLC, UV-vis spectroscopy, atomic absorption and emission spectroscopy, fluorescence and luminescence spectroscopy, electrochemistry, light and electron microscopy, X-ray diffraction, and classical wet chemical methods. Method development and evaluation Although NIOSH and OSHA have invested millions of dollars in the development of analytical methods over the past 20 years, they will never be able to develop and evaluate methods for all of the compounds used or produced by U.S. industry. NIOSH has established a set of evaluation criteria by which new methods can be validated, allowing industrial and research IH laboratories to evaluate methods developed in their own laboratories with confidence t h a t the methods will meet NIOSH criteria. NIOSH methods are recommendedthey a r e not mandated (3). Any method that meets NIOSH criteria can be used to monitor a particular contaminant. The NIOSH evaluation protocol is based on the miterion that the result should be within 25% of t h e true concentration for 95% of the measurements (3, 4). To meet this criterion, the precision over the working range of the method is determined by collecting several samples. The pooled relative standard deviations (RSDs) of the analytical recoveries must be < 0.105.NIOSH has also determined, as the criterion for overall bias of the method, that the mean concentration must he within 10% of
Test generation system The diagram below is a schematic of a typical laboratory-built test generation system for vapors. Normally, test generators are dynamicflow systems consisting of a primary generation source of t h e desired analyte, one or more dilution stages, a sampling chamber, and a sampling-and-analysis system to monitor the generator effluent. Many types of primary generation sources can be used for organic vapors: delivery rate injection of certified gas mixtures with calibrated flowmeters, syringe injection of a liquid or a solution of the liquid with a calibrated syringe pump, or temperature-controlled permeation and diffusion. A dilution stage is usually required to lower the concentration of the analyte produced by the primary generation source to realistic test levels in air. The dilution stage typically consists of a mixing chamber for combining the analyte with air. Calibrated flowmeters are used
to regulate the dilution ratio. Before air is introduced into the mixing chamber, dust particles and oil droplets are removed from the air with conventional particle filters and oil traps. The air supplied to the mixing chamber is cleansed of organic vapor impurities with charcoal and is dried with molecular sieves. The dry air is then humidified to the desired level by saturating the stream with water vapor in a gaswashing tower. To regulate the humidity level in the chamber, the filtered, dry a i r i s split into two streams, one of which is directed to the water tower. The dry and humidified airstreams are then recombined in the mixing chamber at a ratio appropriate for the production of the desired humidity. The design of the sampling chamber varies, depending on the type and number of sampling devices to be evaluated. The chamber, constructed of glass and Teflon, is
e
sized to facilitate production of a homogeneous dynamic atmosphere and to aceommodate the sampling devices as well a s pressure and temperature gauges. When the system is used in the development of methods for solidsorbent tubes, the sampling chamber is usually a mounted glass cylinder - 15 ern in length and 10 cm in diameter with Teflon disks on the ends. At one end, the disk has an axial opening for introduction of the analyte and air mixture as well as additional openings for the insertion of devices to measure temperature and pressure. At the other end, the disk has an axial opening for chamber exhaust and numerous smaller openings along its periphery for the insertion of sorbent tubes and sampling lines to eontinuous monitors. The temperature of the sampling chamber is controlled over a range from ambient to about 35 "C with electric heating tape and insulation.
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the concentration found by a reference method. Specific criteria for recovery, capacity, stability, and sampling interferences are also given and will be discussed as each step of the evaluation protocol is addressed. We have patterned a method development and evaluation protocol consisting of seven distinct stages that are described below. Selection and evaluation of candidate analytical procedures. The logic used here is the same as for any method development protocol. The analysis method depends on the chemical and physical properties of the contaminant and most likely will involve one of the standard instrumental techniques listed earlier. The literature is reviewed and the investigators consider their own personal analytical experience with similar compounds. Candidate methods are then chosen and evaluated. A candidate solvent also is chosen, although the choice may change based on later experiments. Evaluation of the candidate methods begins with the preparation of a standard calibration curve for the analyte. Data are evaluated with respect to instrumental detection limits, linearity, and reproducibility. The method judged to be the best may then be optimized. Internal standards may also be evaluated. After the method has been optimized, the linear range and detection limit are determined. The fmal product is a calibration curve that defines the operating limits of the method.
Selection of candidate airs a m p l i n g methods. This step is perhaps the one in which the experienced IH chemist can accelerate the method development process. When selecting an air-sampling method, several factors are considered. First, what is the physical state of the compound of interest? The sampler must be able to quantitatively collect the analyte as it exists in the workplace. In some cases, provisions must be made to capture the analyte in two Merent physical states. The capacity of the sampler must also be considered. How much of the analyte or, more likely, how much air volume can be drawn through the device before losses occur because of breakthrough? Can we effectively remove the analyte from the collection medium to prepare it for determination? Even though we have separated the first two tasks (analysis method selection and air-sampling method selection), they are always considered in tandem because they must be compatible.
As mentioned earlier, t h e most common collection media for vapor samples are sorbent tubes, which are desorbed with solvent following collection. Of the many available sorbent materials, charcoal is the most common, primarily because of the excellent collection efficiency attributable to its adsorbing character. One problem associated with using charcoal as a sorbent, however, is its tendency to hold on to some compounds too strongly, particularly those that are semivolatile. Recoveries are poor, especially at low levels. Recall that many methods were validated a t levels based on the 1970 PELs, which for many compounds are orders of magnitude greater than the current PELS. Thus the desorption efficiency from charcoal may no longer be acceptable because t h e range of interest has changed. Sometimes this problem can be overcome by choosing another solvent, but for many compounds new sampling methods are needed. The list of alternative sorbents is lengthy. Common sorbents are listed in the box below. When the analyte is expected to be present as a particulate or aerosol, a filter normally is used in place of or in front of the sorbent tube. Common filter selections are also shown in the box.
Solid sorb Charcoal Silica gel Florisil Alumina Porapak 0 (also N,R,T, i Molecular sieve XAD-2 (also 4, 7, and 8) Tenax Ambersorb XE 340 (also Carbasieve Chromosorb 101-108 Filters Teflon Glass fiber Cellulose ester Polyvinyl chloride Silver membrane Polycarbonate Passive samplers Solid adsorbe Liquid media Chemically impregnated tape Reagent-coated sorbents in diffusion tubes Imninrnam and hiihhlarc
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At this point, combinations of samp l e r s a n d solvent systems a r e screened for desorption efficiency by spiking several samplers with the analyte of interest and desorbing them for analysis. It is desirable to fwst evaluate samplers that are common IH stock items; readily available equipment is likely to provide the best access to the developed methodology. From t h e collected data, a sampling and analysis method is chosen for validation studies. Determination of recovery. At this p i n t in the protocol, the NIOSH criteria are applied to a series of experiments for performance evaluation. For extraction from sampling media, recoveries of > 90% are required for membrane filters and are preferred for solid sorbent samplers. Recoveries as low as 75%, however, may be acceptable for sorbent tubes (2,3).The recovery is determined by spiking six samples of the designated sampling medium at each of four analyte levels, allowing the samples to equilibrate for 16-24 h, and analyzing the samples to determine the recovery. The levels generally include quantities representative of exposures a t 0.1, 0.5, 1.0, and 2.0 times the target value (PEL, TLV, or internal exposure limit), but some analysts prefer to evaluate a broader range, particularly a t low levels. The evaluation of recovery involves the use of a microliter syringe to spike the sampling medium with standard solutions of the compound. Spiking levels are calculated on the basis of the anticipated sample volume of the method. E v a l u a t i o n of b r e a k t h r o u g h . Also referred to as sampler capacity, breakthrough can be determined in several ways. The object is to determine how large a sample volume can be collected before sample loss becomes significant. It is recommended that breakthrough studies be performed using the most severe environmental field conditions likely to be encountered. Sampling a t outdoor facilities in Houston or Baton Rouge, for example, presents a significant monitoring challe High humidity and high t e m p e r k t e n d to increase t h e breakthrough of some compounds through samplers. To evaluate breakthrough properly, a test generator capable of producing stable test atmospheres of the analyte of interest is needed. Construction of a test generator is described on p. 483 A. (Note: For filters, test generators can also be built, but capacity is often defined on the basis of increased pressure drop a t high load-
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REPOR7 ing.) The experimental portion of this task involves generation of a standard test atmosphere at a concentration t 2.0 times the target level, a t least 80% relative humidity (RH), and a temperature of a t least 25 OC. Breakthrough is determined in two different ways, depending on the analyte and its concentration. If the concentration is high enough, a continuous monitor may be used to measure the combined effluent from several samplers. When the effluent concentration reaches a specified level, usually 5% of the generator concentration, breakthrough of the analyte is said to occur. A n example of a breakthrough curve is shown in Figure 1. We monitored the breakthrough of a 200-ppm atmosphere of benzene by directing the effluent from the sorbent tube directly into a photoionization detector. The detector output was monitored on a strip chart recorder. Benzene was initially detected a t 3 h, and by 10 h the effluent level had risen to 5% of the chamber concentration. When the concentration of analyte is too low for Continuous monitoring, or if a continuous monitor is not available for that particular compound, several sets of two samplers are connected in series. Each set is exposed for an increasing length of time until a breakthrough curve can be constructed. As a passlfail test, samplers can be exposed to 1.5 times the recommended sample volume a t 2.0 times the target level. If breakthrough is not observed, the sampler passes the breakthrough challenge. The NIOSH criterion for an acceptable method is that no more than 20% of the total analyte should be found in the backup collector. However, for most methods, a sampler that has e 1% breakthrough can be found. Determination of accuracy and precision. This step of the protocol is also based on the NIOSH method evaluation procedure. Again, a test atmosphere of the analyte is generated in a chamber of 80% RH. Six samples are collected at each of four levels corresponding to 0.1, 0.6, 1.0, and 2.0 times the target level. (Eighteen samples are actually collected a t each level, but 12 are used for the stability study.) The samples are analyzed to determine the accuracy and precision of the complete sampling and analysis method. According to NIOSH guidelines, no greater than 10% bias is acceptable. An RSD of
