Measurement of Trace Metals and Metalloids

Deliberations from an ASTM Workshop

BY STEVEN VERPAELE AND OWEN BUTLER

New and revised (downward) occupational exposure limit values are emerging for metals and metalloids such as antimony, beryllium, manganese, and nickel. Validated measurement methods for workplace air measurements, based around the use in laboratories of the widely available inductively coupled plasma-atomic emission spectrometric (ICP-AES) technique for multi-elemental analysis, have long since been codified at a national level (for example, with NIOSH Method 7302 and OSHA Method ID-125G) and more recently at an international level (ASTM D7035 and ISO 15202 parts 1, 2, and 3). However, in light of these impending reductions in limit values, all aspects of such methods, including sampling, sample preparation, and instrumental analysis, may need to be reviewed. To debate these issues, a workshop on the measurement of trace metals and metalloids at workplaces, sponsored by ASTM International Committee D22 on Air Quality, was held in October 2019 in Houston, Texas. The event attracted 24 stakeholders interested in improving the science of measuring metals in workplace air. This article summarizes deliberations from this meeting.

OEL SETTING AND COMPLIANCE

Those responsible for setting exposure limit values for carcinogens, including several commonly used metals and metalloids, now recognize the concept of a threshold for carcinogenic effects and a risk-based concept for non-threshold carcinogens. (See the European Chemicals Agency Risk Assessment Committee opinion on OELs for nickel and its compounds (PDF); and this legislative text (PDF), in which the German Ministry of Labour and Social Affairs defines the concept of “acceptable cancer risks” for a number of metals and metalloids.) A great deal of epidemiological and toxicological evidence exists for metals and metalloids from studies within many metal processing workplaces from which new or revised limit values have been derived. There are new requirements for sampling specific particle size fractions (for example, in cases where a new respirable limit value has been added for an agent that previously had only an inhalable limit value). These situations require new exposure datasets, which has spurred development of samplers that can simultaneously sample inhalable and respirable fractions (discussed below). Many inhalable exposure datasets already exist, and some researchers have investigated the possibility of extrapolating likely respirable exposures from such data—that is, by deriving a respirable-to-inhalable mass ratio. In the absence, at this time, of suitable respirable exposure data, there are merits in this approach, but ratios derived in one work setting may not be translatable over time or to another work setting. Particle size distributions can vary greatly and depend on factors such as the workplace operations that generate such particles, whether those processes remain stable over time, and whether the composition and morphology of materials remain constant. At the workshop, representatives from the European metal industry presented results from workplace surveys that involved the use of cascade impactor sampling to derive metal-mass-to-particle-size data. SAMPLING CONSIDERATIONS Ongoing air sampling challenges include dealing with “wall losses” (particles not collected on the filter but deposited on some other part of the sampler inlet). Emerging challenges arising from new or reduced exposure limits include the previously mentioned need for simultaneous sampling of different health-related fractions and the increased potential for cross-contamination if smaller sample masses are collected. A paper published in the Journal of Occupational and Environmental Hygiene in 2015 has shown that, by wiping interior surfaces of samplers, wall losses can be recovered and added to a filter sample for analysis. However, this can be a laborious procedure. In recent years, an elegant solution has evolved in the form of acid-soluble capsules designed to capture all particles entering a sampling device and which are readily digestible for subsequent metal analysis. A new variant of these capsules was presented at the workshop, the disposable inhalable sampler (DIS), which is based upon the ubiquitous IOM inhalable sampler. A metal- sampling version employs a cellulose capsule bonded to a 25-mm diameter mixed cellulose ester (MCE) filter, and a polyvinyl chloride-based (PVC) version is available if gravimetric determinations are required (as PVC media exhibit better weight stability than MCE media). Comparative studies in assessing the performance of personal samplers have been undertaken, but given the numerous types of personal samplers in use, more such studies are needed. Impending new exposure limits will require both the collection of new exposure data and the sharing of existing data, hence a need for comparability. In April 2018, the Nickel Institute, a global association of primary nickel producers, proposed a new study be undertaken to assess sampler performance in both laboratory and field settings. The Institute is in the process of reaching out to relevant stakeholders to ascertain their interest in participating. (For more information about this project, email Steven Verpaele.) With reductions in exposure limits requiring reliable measurement of potentially smaller sample masses, focus has shifted to minimizing both the potential for sample contamination and ensuring that sufficient sample is collected for analysis. Endeavors here involve the development of the “single-use” sampler, as exemplified by the DIS. Other designs are in the offing as exemplified by a presentation at the workshop on the development of a new inhalable sampler. To collect sufficient sample mass, samplers operating at high flow rates (around 10 liters per minute) are available, but work to develop new samplers, running at even higher flow rates, was suggested in the workshop. Several researchers have proposed the simultaneous sampling of inhalable and respirable fractions using foams to perform the desired particle size selections. Indeed, a dual-fraction IOM sampler that employs a foam separator is now commercially available. At the workshop, a new variant of the DIS, equipped with a foam insert that allows the respirable fraction to be collected on the filter and the extra-respirable component of an inhalable sample to be trapped in the foam, was presented. Also presented was the nanoparticle respiratory deposition sampler (NRD), designed to sample those sub-micron metal particulates that deposit deep in the lung, which now also uses foams to select the required particle sizes. However, further studies on the utility of such samplers would be welcome, and there are plans to include such samplers in the comparability study proposed by the Nickel Institute.

Emerging challenges include the need for simultaneous sampling of different health-related fractions.

SAMPLE PREPARATION REQUIREMENTS An inherent challenge with ICP-based methodologies is that they typically require a sample to be presented for analysis in the form of a solution (that is, via digestion of an air filter sample in strong mineral acid). While the requisite procedures are codified in relevant standards, their correct use nevertheless still relies heavily upon the skill and experience of the analyst, especially those procedures that involve the use of hotplates and open beaker digestions. The use of the closed-vessel pressurized microwave-assisted digestion option has emerged as the method of choice for many laboratories undertaking trace metal analysis. Pressurized digestions enable acids to be heated to temperatures above their nominal boiling points and so allow samples to be digested most efficiently. Microwave ovens are microprocessor controlled, allowing a safe, automated, and highly repeatable operation. Furthermore, microwave power considerably shortens sample dissolution times when compared to more traditional hotplate digestion procedures. In summary, effective and consistent digestions can be performed. A microwave-assisted digestion procedure, based on EPA Method 3052, was incorporated into the published ISO 15202-2 standard. Presentations at the workshop detailed work being undertaken to include microwave-assisted digestions in future iterations of German and U.S. national standard methods. INSTRUMENTAL ANALYSIS Multi-Element Analysis Currently ICP-AES, which is codified in several national and international standards, is the technique of choice in most occupational hygiene laboratories for the multi- element analysis of air filter samples. There are concerns, however, that the sensitivity of the ICP-AES technique may be insufficient to quantify certain metals at proposed new reduced exposure limits as alluded to, for example, in a survey of laboratories that offered nickel assays (see Detection Limits in Air Quality and Environmental Measurement, ASTM International, 2019). Complimentary standards such as ASTM D7439-14 and ISO 30011 now exist, which advocate the use of the more sensitive inductively coupled plasma-mass spectrometry (ICP-MS) technique. As reported at the workshop, efforts are ongoing to incorporate ICP-MS into revisions of national standards and in particular for the analysis of elements such as antimony, beryllium, and platinum where ICP-AES indeed lacks the required sensitivity. In summary, the main challenge is potentially an economic one—that is, procurement of new ICP-MS capabilities to supplement existing ICP-AES capabilities in occupational hygiene laboratories. Single-Element Analysis Workshop participants also discussed requirements for sampling, sample preparation, and analytical determinations of beryllium, hexavalent chromium, nickel species, and platinum. Findings from an inter-laboratory study published in the Journal of Environmental Monitoring in 2012 demonstrated that either a high-boiling point acid (such as sulfuric acid) or hydrofluoric acid was required to readily dissolve beryllium oxide phases present in beryllium processing facilities. Requisite analyst skill sets have been mentioned previously but there remain concerns whether some laboratories use appropriate digestion methods and, if so, whether they follow the prescribed recipe completely (that is, use the recommended acid mixtures). An overview of a sensitive fluorescence-based beryllium assay, developed for both filter and swab samples and described in a 2017 paper in the International Journal of Analytical Chemistry, was presented at the workshop. This assay is now available as ASTM D7202 and NIOSH 7704, and is as sensitive, if not more so, than ICP-MS. Usefully, it can be made portable, so enabling timely measurements to be performed close to the workplace. Sensitive spectrophotometric protocols, discussed at the workshop, for the determination of hexavalent chromium are available, such as ASTM D6832 and ISO 16740. Nevertheless, analytical challenges remain, such as extracting insoluble hexavalent chromium species and the omnipresent potential for species interconversion between hexavalent and trivalent chromium forms during filter extraction. These considerations were detailed in a useful 2003 review paper in the Journal of Environmental Monitoring and remain relevant to this day. The assessment of health risks in the metal producing industry ideally requires the ability to distinguish quantitatively between different forms of a metal that could exist in workplace dusts. In one endeavor, the Zatka protocol—a sequential leaching method described in a 1992 paper in Environmental Science and Technology—was developed to provide a laboratory-based assay that could differentiate between “soluble,” “sulphidic,” “metallic,” and “oxidic” nickel forms present in dusts within refineries that processed sulphidic-based nickel ores. In this approach, labile nickel forms are sequentially removed (and analyzed) by subjecting air filter samples to leaching agents of increasing strength, thereby exploiting solubility differences between nickel compounds. In a workshop presentation, use of this same sequential leaching approach to elucidate the possible forms of nickel present in particles within a stainless-steel mill was presented. Here both the original Zatka protocol and a modified version were utilized alongside instrumental particle characterization techniques such as X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and Quantitative Evaluation of Materials by Scanning Electron Microscopy (QEMSCAN). Given the diverse particle morphologies and sizes that could be encountered in metal-handling workplaces, an approach employing a battery of analytical techniques not only better characterizes workplace particles but can ascertain whether the use of sequential leach-type procedures in other work settings has merit. The International Platinum Group Metal Association (IPA) has published a harmonized methodology for the sampling of platinum in workplace atmospheres that subsequently involves the extraction of filter samples in dilute hydrochloric acid for determining the soluble platinum fraction. Researchers at the University of Wisconsin, working on behalf of IPA, presented their highly sensitive ICP-MS assay for measurements at concentrations less than 1 ng/m3. Purity of reagents and cleanliness of apparatus are critical in achieving such sensitivities. QUALITY ASSURANCE AND CONTROL REQUIREMENTS Proficiency testing (PT) allows a laboratory to compare its analytical performance against those of its peers and demonstrate that it can obtain reliable results. While the performance of laboratories analyzing simulated (spiked aqueous metal solutions) test filters remains satisfactory, such samples may not provide a realistic examination because they are not truly representative of air filter samples collected from many workplaces. Indeed, a 2008 paper in The Annals of Occupational Hygiene, drawing upon results from a PT scheme that offered realistic welding fume on filter samples, concluded that, despite the availability of standard methods, use of inappropriate or incorrectly performed digestions was indeed a major source of analytical bias, which remains apparent to this day. If available, analysis of matrix reference materials (RMs), with a known elemental composition, could help laboratories determine whether they are following a published procedure correctly or verify whether the performance of their in-house digestion method is satisfactory. Ideally such RMs would be representative of the wide range of dusts and fumes encountered in the workplace and preferably are available as particle-on-filter samples. Producing large batches on near-identical RM filters, however, remains challenging, and to date, as far as is known, only two metal-on-filter RMs have been produced, a welding fume material certified only for its chromium content and a beryllium oxide material. At the workshop, endeavors in developing new RMs were presented. These included a new approach for producing metal oxide on filter materials (described in two publications in the journal Gefahrstoffe - Reinhaltung der Luft) and the development and validation of two bulk welding fume RMs, characterized for elements such as chromium, iron, manganese, nickel, and zinc, detailed in a 2014 publication in the Journal of Occupational and Environmental Hygiene. EMERGING MEASUREMENT SCIENCE A widely shared aspiration is the development of field-based methods for the determination of metals and metalloids in the workplace. While analyzing air filter samples at the workplace is currently possible—for example, through use of portable X-ray fluorescence (XRF) analyzers—new portable instruments that can sample and quantify airborne elemental concentration data in near real time are ideally required. At the workshop, emerging research into the development of a portable spark emission spectrometer was presented. Ongoing endeavors such as development of new air sampler designs; new filter digestion approaches; approaches to boost the sensitivity of the ICP-AES technique (for example, through use of high efficiency nebulizers); and evaluation of new ICP-MS technologies (such as new triple-cell systems for the improved removal of isobaric interferences) will, in time, feed into future iterations of national and international (ASTM and ISO) standards. STEVEN VERPAELE is an industrial hygienist with the Nickel Institute in Brussels, Belgium, and a member of the AIHA Sampling and Laboratory Analysis Committee. OWEN BUTLER is an analytical chemist and proficiency testing specialist with the U.K. Health and Safety Executive in Harpur Hill, Buxton, United Kingdom. Acknowledgment: The authors thank the ASTM workshop delegates for their contributions, without which this summary report would not have been possible.

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RESOURCES

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