REPORT
INDUSTRIAL HYGIENE CHEMISTRY The Method Development Approach to Air Analysis Kim W. Baughman Southern Testing and Research Laboratories, Inc. 3709 Airport Dr. Wilson, NC 27893
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 ap plied analytical chemistry have ex panded into a wide range of a r e a s . Even though it is difficult to keep up with advances, no m a t t e r what the field, applied a n a l y t i c a l c h e m i s t s must do just that if they wish to be effective. In addition, they must con tinually evaluate the changing needs of t h e i r p a r t i c u l a r s p e c i a l t y a n d judge how best to apply analytical chemistry to solve problems. And the problems are becoming increasingly difficult. In this REPORT, we discuss a spe cific application of analytical chemis try: industrial hygiene (IH). As is the case with most applied sciences, the requirements of the IH chemist cre ate paradoxical responsibilities. The IH chemist must be a specialist, ap plying analytical c h e m i s t r y to t h e unique problems t h a t IH p r e s e n t s . The IH chemist must also be a gen eralise keeping up with the progress made in analytical chemistry so as to advance IH to the next level of so phistication.
A little background IH is most often defined as a combi nation of science and art devoted to the recognition, evaluation, and con trol of f a c t o r s or s t r e s s e s in t h e workplace that may result in illness or discomfort among workers. This
field covers a broad range of exper tise, especially considering the four a r e a s t h a t fall u n d e r this jurisdic tion: chemical, physical, ergonomie, a n d biological f a c t o r s . T h u s t h e range of expertise required of indus trial hygienists is a 3 χ 4 matrix that encompasses these four factors and each of the three primary tasks: rec ognition, evaluation, and control. It would be difficult for any one person to excel in all of the disci plines necessary to perform the du ties associated with these functions. To be fully effective, the industrial hygienist m u s t organize a t e a m of s c i e n t i s t s a n d e n g i n e e r s to fill in pieces of this matrix. If the responsi bilities of the i n d u s t r i a l hygienist are viewed in terms of this matrix, it is easy to see how the skills and spe cialties of these team members are used to perform the duties covered by IH. The IH chemist is largely re sponsible for one piece of the matrix (evaluating chemical factors or expo sures) and will also be involved in the recognition and, to a lesser de gree, control of chemical factors. Although historical references de tailing the correlation between an illness and a specific occupation date back to the fourth century B.C. (1), the date of the advent of IH chemis try cannot be traced as easily. Some times the advent of IH chemistry is e q u a t e d w i t h t h e m i n e r who first took a canary into the mine as a "de t e c t o r " for m e t h a n e g a s . In a n y event, the field of IH chemistry has advanced rapidly in the past two dec ades, primarily as a result of the Oc cupational Safety and Health Act of 1970. The purpose of this legislation is to ensure safe and healthy work ing conditions for all men and women in the workplace. The Occupational Safety and H e a l t h A d m i n i s t r a t i o n (OSHA) and the National I n s t i t u t e for Occupational Safety and Health
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(NIOSH) were formed as a result of the act. The p r i m a r y 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 a n a l y s i s of workplace atmospheres. As IH chem ists take on a more important role in t h e w o r k p l a c e , t h e r e is a g r e a t e r need for analytical chemists to sup port their activities. IH chemists are increasingly called upon to develop a n d v a l i d a t e new methods for the collection and analy sis of chemicals in the workplace. In t h e 1970s and 1980s, N I O S H a n d OSHA developed methods for deter m i n i n g h u n d r e d s of t h e s e com pounds. However, NIOSH and OSHA have been unable to address the an alytical needs for the h u n d r e d s or even thousands of other compounds t h a t w o r k e r s m a y be e x p o s e d to throughout the broad range of U.S. i n d u s t r i e s . In fact, i n d u s t r y often provides its own analytical methods for determining some compounds; in some cases, toxicology data indicate a need to limit and assess workers' exposure, b u t n e i t h e r N I O S H 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 Hygien ists (ACGIH) updates recommenda tions for threshold limit values (TLVs) in t h e workplace annually, and in 1989 OSHA updated permissible ex posure 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 recom mended or permissible exposure lev els are considerably lower than they 0003 - 2700/93/0365 -480A/$04.00/0 © 1993 American Chemical Society
REPORT were when the analytical methods were developed. T h u s t h e 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 t h e s e compounds, new m e t h o d s m u s t be developed or t h e 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 a i r s a m p l e r , w h i c h c a n often be solved by changing to a different desorption solvent. We should point out t h a t OSHA h a s been challenged in court over the new PEL s t a n d a r d s 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 R e p r e s e n t a t i v e s t h a t 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 a n d to d i s c u s s some of t h e t e c h niques 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 t h a t the sample matrix is air makes these projects unique, because the validation process is more difficult for air than for most other matrices. Standard a t m o s p h e r e s of t h e c o n t a m i n a n t of interest m u s t be generated and confirmed as part of the validation protocol.
Determination 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 u n i q u e to IH c h e m i s t r y a n d some of which a r e u n i v e r s a l . One of the most important features is the focus on determining personal exposure levels. We are actually trying to d e t e r m i n e t h e a m o u n t of a c o n t a m i n a n t t h a t workers are exposed to during the time they are on the job. Therefore the sampler should
The worker wears a portable sampling pump with a filter, held in a cassette, 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 a t t r i b u t a b l e to i n h a l a tion; in most situations, this is the route of greatest exposure. The prim a r y m e c h a n i s m used to e v a l u a t e this type of exposure is the portable a i r - s a m p l i n g p u m p t h a t 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 is collected by d r a w i n g 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 t u b e s , filters, and i m p i n g e r s . Sorbent tubes, consisting of two separ a t e d sections of a s o l i d - t r a p p i n g medium in a 6 - 8 - c m 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 v a p o r s . T h e u s e of o t h e r s o r b e n t t y p e s , however, is becoming more common. Particulates are usually collected on 2 5 - 3 7 - m m diameter filters, the
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most common of which are glass fiber, Teflon, or polymeric membranes. To detect an analyte believed to be present in both the particulate and the vapor, a combination sampling t r a i n 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 - 2 5 mL of an appropriate solvent or solution for t r a p p i n g t h e a n a l y t e . I m pingers 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 evacu a t e d b u l b s , 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 c o n t a m i n a n t b u t covers the e n t i r e range of trace analytical techniques: GC, H P L C , U V - v i s 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 N I O S H a n d OSHA h a v e 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 h a s 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 labor a t o r i e s w i t h confidence t h a t t h e methods will meet NIOSH criteria. NIOSH methods are recommended— t h e y a r e n o t m a n d a t e d (3). A n y method t h a t meets NIOSH criteria can be used to monitor a particular contaminant. The NIOSH evaluation protocol is based on the criterion that the result should be w i t h i n 25% of t h e t r u e concentration for 95% of the m e a surements (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 t h e analytical recoveries must be < 0.105. NIOSH has also determined, as the criterion for overall bias of t h e method, t h a t the m e a n concentration must be within 10% of
REPORT 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 t h a t are described below. S e l e c t i o n a n d e v a l u a t i o n 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 l i t e r a t u r e 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 s t a n d a r d calibration curve for t h e analyte. Data are evaluated with respect to instrumental detection limits, linearity, a n d reproducibility. The m e t h o d j u d g e d to be t h e best m a y t h e n be o p t i m i z e d . I n t e r n a l standards may also be evaluated. After the method has been optimized, the linear range and detection limit are determined. The final product is a calibration curve t h a t defines the operating limits of the method. S e l e c t i o n of c a n d i d a t e a i r s a m p l i n g m e t h o d s . T h i s s t e p is perhaps the one in which the experienced IH chemist can accelerate the method development process. When selecting a n a i r - s a m p l i n g 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 m u s t be made to capture the analyte in two different 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 t h o u g h we h a v e s e p a r a t e d the first two t a s k s (analysis method selection and air-sampling method selection), they are always considered in t a n d e m because they must be compatible.
As m e n t i o n e d e a r l i e r , t h e m o s t 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, p r i m a r i l y because of t h e excellent collection efficiency attribu t a b l e to its a d s o r b i n g c h a r a c t e r . One problem associated with using charcoal as a sorbent, however, is its t e n d e n c y to hold on to some comp o u n d s too s t r o n g l y , p a r t i c u l a r l y those that are semivolatile. Recoveries are poor, especially at low levels. Recall that many methods were valid a t e d at levels b a s e d on t h e 1970 P E L s , which for m a n y compounds are orders of magnitude greater than the current PELs. Thus the desorption efficiency from charcoal may no l o n g e r be a c c e p t a b l e b e c a u s e t h e range of interest has changed. S o m e t i m e s t h i s problem can be overcome by choosing a n o t h e r solvent, but for many compounds new sampling methods are needed. The list of alternative sorbents is lengthy. Common sorbents are listed in t h e 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.
IH samplers and media Solid sorbents Charcoal Silica gel Florisil Alumina Porapak Q (also N,R,T, and QS) Molecular sieve XAD-2 (also 4, 7, and 8) Tenax Ambersorb XE 340 (also 348) Carbosieve Chromosorb 101-108 Filters Teflon Glass fiber Cellulose ester Polyvinyl chloride Silver membrane Polycarbonate Passive samplers Solid adsorbents Liquid media Chemically impregnated tape Reagent-coated sorbents in diffusion tubes Impingers and bubblers
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At this point, combinations of samplers and solvent systems are screened for desorption efficiency by spiking several s a m p l e r s w i t h t h e a n a l y t e of i n t e r e s t a n d desorbing them for analysis. It is desirable to first evaluate samplers t h a t are common IH stock items; readily available e q u i p m e n t is likely to provide the best access to the developed methodology. F r o m t h e collected d a t a , a s a m p l i n g a n d a n a l y s i s m e t h o d is chosen for validation studies. D e t e r m i n a t i o n of r e c o v e r y . At this point in the protocol, the NIOSH criteria are applied to a series of exp e r i m e n t s for performance evaluation. For extraction from sampling media, recoveries of > 90% are r e quired 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 q u a n t i t i e s r e p r e s e n t a t i v e of exposures at 0.1, 0.5, 1.0, and 2.0 times the target value (PEL, TLV, or internal exposure limit), b u t some a n a lysts prefer to e v a l u a t e a b r o a d e r range, particularly at 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 t h a t b r e a k t h r o u g h studies be performed using the most severe environmental field conditions likely to be encountered. Sampling at outdoor facilities in Houston or Baton Rouge, for example, presents a significant monitoring challenge. High humidity a n d h i g h t e m p e r a E i r r e ^ e n d to i n c r e a s e t h e b r e a k t h r o u g h of some c o m p o u n d s t h r o u g h s a m p l e r s . To evaluate b r e a k t h r o u g h 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 at high load-
REPORT ing.) The experimental portion of this task involves generation of a standard test atmosphere at a concen tration > 2.0 times the target level, at least 80% relative humidity (RH), and a temperature of at least 25 °C. Breakthrough is determined in two different ways, depending on the an alyte and its concentration. If the concentration is high enough, a con tinuous monitor may be used to mea sure the combined effluent from sev eral samplers. When the effluent concentration reaches a specified level, usually 5% of the generator concentration, breakthrough of the analyte is said to occur. An example of a breakthrough curve is shown in Figure 1. We monitored the break through of a 200-ppm atmosphere of benzene by directing the effluent from the sorbent tube directly into a photoionization detector. The detec tor output was monitored on a strip chart recorder. Benzene was initially detected at - 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 com pound, 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 pass/fail test, samplers can be exposed to 1.5 times the recommended sample volume at 2.0 times the target level. If break through is not observed, the sampler passes the breakthrough challenge. The NIOSH criterion for an accept able method is that no more than 20% of the total analyte should be found in the backup collector. How ever, for most methods, a sampler that has < 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 gener ated in a chamber of 80% RH. Six samples are collected at each of four levels corresponding to 0.1, 0.5, 1.0, and 2.0 times the target level. (Eight een samples are actually collected at each level, but 12 are used for the stability study.) The samples are an alyzed 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
