Quantitative Protection Factors for Common Masks and Face Coverings.

The performance of masks, whether intended to protect the community from exhaled infectious aerosol or to protect the wearer from inhaled infectious aerosol, depends on factors such as filtration efficiency, particle size distribution, leakage, and ventilation rate. These factors depend on the activities and facial features of the mask wearer so that the mask performance for real-world applications is difficult to predict. The present work shows how protection factor, a quantity often used to describe mask performance, can be estimated without involving human volunteers. By constraining these factors to known values, mask protection factors can be compared fairly and efficiently following a series of filtration efficiency measurements performed in the laboratory. Protection factors and mask emissions for exhalation and inhalation were evaluated for masks of seven types currently in use around the world and for a hypothetical mask with 99% efficiency on all particles. The performance of reusable masks made from cotton fabric was limited by the size of the native cotton fibers. Masks that utilized finer fibers, particularly electret fibers with relatively small diameters, showed excellent performance with moderate flow resistance. Results from this work, in addition to simple guidance for mask fit and usage, can facilitate risk communication and decision-making efforts during the COVID-19 pandemic.

Are Cape Peninsula baboons raiding their way to obesity and type II diabetes? - a comparative study.

Researchers, managers and conservationists in the Cape Peninsula, South Africa, have reported cases of individual baboons (Papio ursinus) appearing overweight, lethargic and having poor teeth. Despite an intensive baboon management programme, there are certain individual baboons and troops that continue to raid human food sources. These food sources often are high in processed carbohydrates and saturated fats. As this diet is highly associated with obesity, insulin resistance and type II diabetes, the present study aimed to establish if these baboons may be at risk of developing insulin resistance. Post mortem muscle samples from 17 Cape Peninsula and 7 control adult male baboons were rapidly frozen in liquid nitrogen and analysed for insulin receptor substrate-1 (IRS-1), glucose transporter 4 (GLUT4), oxidative and glycolytic markers of metabolism (citrate synthase, 3-hydroxyacyl-CoA-dehydrogenase, lactate dehydrogenase and creatine kinase activities), and muscle fibre morphology. The sampled Peninsula baboons were heavier (33 ± 2 vs. 29 ± 2 kg, P < 0.05) and had a higher frequency of poor teeth compared to control baboons. Muscle fibre type, fibre size, GLUT4 content, oxidative and glycolytic metabolism were not different between the two groups. However, IRS-1 content, a marker of insulin sensitivity, was significantly lower (by 43%, P < 0.001) in the Peninsula baboons compared to the controls. This study provides the first indirect evidence that some Peninsula baboons with a history of raiding human food sources, may be at risk of developing insulin resistance in the wild, with long term implications for population health.

Design and performance of UPAS inlets for respirable and thoracic mass sampling.

The Ultrasonic Personal Aerosol Sampler (UPAS) is a small, lightweight, and quiet sampler that collects airborne particulate matter on a filter for gravimetric or compositional analysis. The objective of this work was to develop UPAS inlets with collection efficiencies that match criteria for respirable or thoracic mass sampling. The two-stage inlet for respirable mass described here utilizes an impaction stage and a cyclone, whereas the one-stage inlet for thoracic mass sampling utilizes a circular slot impactor. Inlet designs are based on particle collection theory used in conjunction with an optimization algorithm to predict initial inlet dimensions; these predictions were the starting points for experiments that finalized dimensions and operating conditions. Both the respirable mass inlet and the thoracic mass inlet described here are interchangeable with the UPAS, and both have efficiencies that match well with their respective standards. With either inlet, the collected sample should be within ±5% of what the standard specifies for aerosols with reasonably broad size distributions.

Personal Exposure to PM2.5 Black Carbon and Aerosol Oxidative Potential using an Automated Microenvironmental Aerosol Sampler (AMAS).

Traditional methods for measuring personal exposure to fine particulate matter (PM2.5) are cumbersome and lack spatiotemporal resolution; methods that are time-resolved are limited to a single species/component of PM. To address these limitations, we developed an automated microenvironmental aerosol sampler (AMAS), capable of resolving personal exposure by microenvironment. The AMAS is a wearable device that uses a GPS sensor algorithm in conjunction with a custom valve manifold to sample PM2.5 onto distinct filter channels to evaluate home, school, and other (e.g., outdoors, in transit, etc.) exposures. Pilot testing was conducted in Fresno, CA where 25 high-school participants ( n = 37 sampling events) wore an AMAS for 48-h periods in November 2016. Data from 20 (54%) of the 48-h samples collected by participants were deemed valid and the filters were analyzed for PM2.5 black carbon (BC) using light transmissometry and aerosol oxidative potential (OP) using the dithiothreitol (DTT) assay. The amount of inhaled PM2.5 was calculated for each microenvironment to evaluate the health risks associated with exposure. On average, the estimated amount of inhaled PM2.5 BC (µg day-1) and OP [(µM min-1) day-1] was greatest at home, owing to the proportion of time spent within that microenvironment. Validation of the AMAS demonstrated good relative precision (8.7% among collocated instruments) and a mean absolute error of 22% for BC and 33% for OP when compared to a traditional personal sampling instrument. This work demonstrates the feasibility of new technology designed to quantify personal exposure to PM2.5 species within distinct microenvironments.

Pores and Void in Asclepiades' Physical Theory.

This paper examines a fundamental, though relatively understudied, aspect of the physical theory of the physician Asclepiades of Bithynia, namely his doctrine of pores. My principal thesis is that this doctrine is dependent on a conception of void taken directly from Epicurean physics. The paper falls into two parts: the first half addresses the evidence for the presence of void in Asclepiades' theory, and concludes that his conception of void was basically that of Epicurus; the second half focuses on the precise nature of Asclepiadean pores, and seeks to show that they represent void interstices between the primary particles of matter which are the constituents of the human body, and are thus exactly analogous to the void interstices between atoms within solid objects in Epicurus' theory.

Conducting environmental health research in the Arabian Middle East: lessons learned and opportunities.

BACKGROUND: The Arabian Gulf nations are undergoing rapid economic development, leading to major shifts in both the traditional lifestyle and the environment. Although the pace of change is brisk, there is a dearth of environmental health research in this region. OBJECTIVE: We describe challenges and successes of conducting an environmental epidemiologic study in the United Arab Emirates (UAE), a Gulf nation in the Middle East, with an inter-disciplinary team that includes in-country academic and government collaborators as well as U.S. academic collaborators. DISCUSSION: We present several issues, including study and data collection design, exposure assessment, scheduling and time coordination, quality assurance and quality control, and institutional review board protocols. These topics are considered in a cultural context. Benefits of this research included building linkages among multinational, interdisciplinary team members, generating data for local environmental decision making, and developing local epidemiologic research capacity. The Middle Eastern culture of hospitality greatly benefited the project team. CONCLUSION: Cultural differences impact multiple aspects of epidemiologic research and should be respectfully addressed. Conducting international population-based environmental research poses many challenges; these challenges can be met successfully with careful planning, cultural knowledge, and flexibility. Lessons learned are applicable to interdisciplinary research all over the world. The research conducted will benefit the environmental and public health agencies of the UAE and provide the nation's leadership with country-specific environmental health data that can be used to protect the public's health in a rapidly changing environment.

Indoor air pollutants and health in the United Arab Emirates.

BACKGROUND: Comprehensive global data on the health effects of indoor air pollutants are lacking. There are few large population-based multi-air pollutant health assessments. Further, little is known about indoor air health risks in the Middle East, especially in countries undergoing rapid economic development. OBJECTIVES: To provide multifactorial indoor air exposure and health data, we conducted a population-based study of indoor air pollution and health in the United Arab Emirates (UAE). METHODS: We conducted a cross-sectional study in a population-based sample of 628 households in the UAE. Indoor air pollutants [sulfur dioxide (SO2), nitrogen dioxide (NO2), hydrogen sulfide (H2S), formaldehyde (HCHO), carbon monoxide (CO), and particulate matter] were measured using passive samplers over a 7-day period. Health information was collected from 1,590 household members via in-person interviews. RESULTS: Participants in households with quantified SO2, NO2, and H2S (i.e., with measured concentrations above the limit of quantification) were twice as likely to report doctor-diagnosed asthma. Participants in homes with quantified SO2 were more likely to report wheezing symptoms {ever wheezing, prevalence odds ratio [POR] 1.79 [95% confidence interval (CI) 1.05, 3.05]; speech-limiting wheeze, POR 3.53 (95% CI: 1.06, 11.74)}. NO2 and H2S were similarly associated with wheezing symptoms. Quantified HCHO was associated with neurologic symptoms (difficulty concentrating POR 1.47; 95% CI: 1.02, 2.13). Burning incense daily was associated with increased headaches (POR 1.87; 95% CI: 1.09, 3.21), difficulty concentrating (POR 3.08; 95% CI: 1.70, 5.58), and forgetfulness (POR 2.68: 95% CI: 1.47, 4.89). CONCLUSIONS: This study provides new information regarding potential health risks from pollutants commonly found in indoor environments in the UAE and other countries. Multipollutant exposure and health assessments in cohort studies are needed to better characterize health effects of indoor air pollutants.

Use of passive diffusion tubes to monitor air pollutants.

Monitoring gas-phase pollutants is essential to understand exposure patterns and to establish a link between exposure and health. Measurement of the low concentrations found outdoors or in indoor living space normally requires large, expensive instruments that use electrical power. In this study, colorimetric passive diffusion tubes, normally used to monitor high concentrations of airborne contaminants in the workplace for sampling periods of a few hours, were evaluated to measure much lower concentrations of the same pollutants for periods of up to 1 wk. These tubes are small, inexpensive, and require no electrical power. Responses of diffusion tubes for carbon monoxide (CO), hydrogen sulfide (H2S), nitrogen dioxide (NO2), sulfur dioxide (SO2), and benzene were studied. Low pollutant concentrations measured with passive diffusion tubes matched reasonably well with true concentrations for all pollutants except NO2. These results suggest that passive diffusion tubes can provide an inexpensive, unobtrusive, and effective method to monitor low pollutant concentrations. Passive diffusion tubes may be particularly useful in surveys where the spatial variability in concentrations is high and where the cost of traditional monitoring instruments is a concern.

THE QUALITATIVE STATUS OF THE ONKOI IN ASCLEPIADES' THEORY OF MATTER.

The medical and philosophical system of Asclepiades of Bithynia (fl. later second century BC)(1) has been the subject of considerable controversy.(2) His physical theory of anarmoi onkoi in particular has seen intense debate, and although many of its broader features appear to be fairly well established, many of its most fundamental details remain obscure. Perhaps somewhat paradoxically, some of the most important work carried out on Asclepiades has been explicitly focused instead on Heraclides of Pontus,(3) the reconstruction of whose physical theory has often proceeded on the assumption that this was largely replicated by Asclepiades some two centuries later. But to a great extent the Asclepiadean debate has been framed in terms of the question of his intellectual debts to ancient atomism, and Epicureanism in particular, and in this respect the present study will be no different.(4) The most recent scholarship has been sharply divided over this question. Vallance has emphasized the principally medical context of Asclepiades' system, and made the case that the frangibility of the onkoi marks such a fundamental divergence from Epicurus' atomism that any influence from Epicurean physics should be rejected, and that we should look instead especially to Erasistratus.(5) Casadei, however, following on to a certain extent from the work of Pigeaud, has rightly drawn attention to the tendency in Vallance's exposition to suppress a number of fundamental elements of Asclepiades' doctrine which are undeniably also distinguishing features of Epicurean philosophy.(6) The most significant of these include his particulate theory of matter, his antiteleological conception of nature, and his rejection of any theory of qualitative change. But these correspondences would certainly not be sufficient to qualify Asclepiades' system simply as a reproduction of Epicureanism, and there is clear evidence that Asclepiades stood in opposition to Epicurus in certain fundamental respects. In a recent study which has done much to establish Asclepiades' credentials as a philosopher, focusing especially on his philosophy of mind, Polito has underlined certain distinctly non-Epicurean elements in his system, such as his radical determinism and his denial of a localized ruling-part-of-the-soul.(7) It thus seems clear that, despite some important parallels between their systems, Asclepiades cannot be regarded as an Epicurean physician. The evidence we have for his doctrine, and the authority which was accorded him by later writers, clearly attests to his status as an independent and innovative thinker in his own right. While Asclepiades' theory must, in my view, be analysed within the context of the Epicurean atomistic tradition, it must equally be acknowledged that any identifiable relationship between Epicurus and Asclepiades is likely to be one of considerable complexity.In this paper I shall attempt to explore further the nature of the relationship between Epicurus and Asclepiades by examining some aspects of the latter's theory of matter. Given the widespread disagreement about his theory in general, I propose to focus on a fundamental question which I believe the extant evidence allows us to answer with a satisfactory degree of certainty, namely what Asclepiades' position was on the qualitative status of his onkoi. In Section I I shall analyse four passages which have a direct bearing on this question, from Caelius Aurelianus, Galen, Sextus Empiricus, and Calcidius respectively. I shall argue here that this position was in its details substantially the same as Epicurus' with regard to his atoms. It must be stressed that it is only in details that we can make such comparisons, since we have no surviving testimony which recounts Asclepiades' arguments or broader reasons for holding such a position. Nevertheless, in Section II I shall argue that these identifiable similarities in their respective doctrines on the qualities of their elements were more than superficial or incidental, and strongly suggest that Asclepiades and Epicurus shared certain premisses which were fundamental to their physics, which might then be used to contextualize and elucidate some of the more idiosyncratic and apparently unique parts of Asclepiades' system. This will lead me to suggest an interpretation of an important piece of evidence which may confirm that Asclepiades was reacting in a direct and critical way to certain aspects of Epicurus' physical doctrine.

Passive aerosol sampler for particle concentrations and size distributions.

This research evaluated the UNC passive aerosol sampler as a tool to measure particle mass concentrations and size distributions. The exposure scenario represented high concentrations and exposure periods of a few hours. Mass concentrations measured with the passive sampler were compared to concentrations measured using both a dichotomous sampler and an aerodynamic particle sizer (APS). In addition, the size distributions measured with the passive sampler were compared to those measured using the APS. Mass concentrations measured using the dichotomous sampler and the APS agreed well. The passive sampler tracked, but tended to overestimate, mass concentrations measured by the other two instruments. Size distributions measured with the passive sampler followed the general pattern of those measured using the APS. Overall, the passive sampler demonstrated both its utility and its limitations in these tests. The concentration measurements and size distributions found using passive samplers were more variable than those of the other instruments, but generally followed the data taken using the other methods. The advantages of low cost and ease of use offset the limitations in data quality with the passive sampler; these advantages are particularly welcome for sampling situations where aerosol properties vary over space or time.

Passive sampler for PM10-2.5 aerosol.

This study investigates the use of a small passive sampler for aerosol particles to determine particulate matter (PM)10-2.5 concentrations in outdoor air. The passive sampler collects particles by gravity, diffusion, and convective diffusion onto a glass coverslip that is then examined with an optical microscope; digital images are processed with free software and the resultant PM10-2.5 concentrations determined. Both the samplers and the analyses are relatively inexpensive. Passive samplers were collocated with Federal Reference Method (FRM) samplers in Chapel Hill, NC; Phoenix, AZ; and Birmingham, AL; for periods from 5 to 15 days. Particles consisted primarily of inorganic dusts at some sites and a mix of industrial and inorganic materials at other sites. Measured concentrations ranged from < 10 microg/m3 to approximately 40 microg/m3. Overall, PM10-2.5 concentrations measured with the passive samplers were within approximately 1 standard deviation of concentrations measured with the FRM samplers. Concentrations determined with passive samplers depend on assumptions about particle density and shape factors and may also depend somewhat on local wind speed and turbulence; accurate values for these parameters may not be known. The degree of agreement between passive and FRM concentrations measured here suggests that passive measurements may not be overly dependent on accurate knowledge of these parameters.

A Medical Treatise On Remedies? P.Turner 14 Revised.

P.Turner 14 preserves a fragment of a Greek medical text in question-and-answer format dated to the later second century AD. This paper offers some new supplements to the text, and, in the light of these, a re-interpretation of its subject matter and medical genre, as well as some observations on the possible doctrines which may have influenced it.

Method to evaluate the dustiness of pharmaceutical powders.

The trend among pharmaceutical companies to develop selective drugs of high potency has pushed the industry to consider the potential of each hazardous ingredient to become airborne. Dustiness issues are not unique to the pharmaceutical industry, but are relevant to any industry where powdered materials are mixed, transferred and handled. Interest in dustiness is also driven by concerns for worker health, the potential for plant explosions and the prevention of product loss. Unlike other industries, the pharmaceutical industry is limited by the milligram quantity of powdered material available for testing during product development. These needs have led to the development of a bench-top dustiness tester that requires only 10 mg of powder and fully contains the generated aerosol. The powder is dispersed within a 5.7 liter glass chamber that contains a respirable mass sampler and a closed-face sampler to quantify the respirable and total dust that are generated with a given energy input. The tester distinguished differences in dustiness levels of five different powders. Finer powders were dustier, and the respirable dust percentage was always less than that for total dust. Four testers have been built and evaluated using pharmaceutical grade lactose. Dustiness measurements determined using all four testers were comparable. The pharmaceutical industry uses surrogates such as lactose to represent active compounds in tests that estimate the dust concentration likely to occur in a new manufacturing operation. Differences between the dustiness of the active compound and its surrogate challenge the relevance of the surrogate tests to represent true exposures in the workplace. The tester can determine the dustiness of both the active compound and its surrogate, and the resultant ratio can help to interpret dust concentrations from surrogate tests. Further, dustiness information may allow the pharmaceutical researcher to select powder formulations that present low airborne concentrations in the workplace.

Particle deposition in industrial duct bends.

A study of particle deposition in industrial duct bends is presented. Particle deposition by size was measured by comparing particle size distributions upstream and downstream of bends that had geometries and flow conditions similar to those used in industrial ventilation. As the interior surface of the duct bend was greased to prevent particle bounce, the results are applicable to liquid drops and solid particles where duct walls are sticky. Factors investigated were: (i) flow Reynolds number (Re = 203 000, 36 000); (ii) particle Reynolds number (10 < Repinfinity < 200); (iii) particle Stokes number (0.08 < Stk < 16); (iv) bend angle (theta = 45 degrees, 90 degrees, 180 degrees ); (v) bend curvature ratio (1.7 < R0 < 12); (vi) orientation (horizontal-to-horizontal and horizontal-to-vertical); and (vii) construction technique (smooth, gored, segmented). Measured deposition was compared with models developed for bends in small diameter sampling lines (Re < 20 000; Repinfinity < 13). Whereas deposition measured in this work generally agreed with that estimated with models for particles <30 microm (Stk < 0.7), it was significantly lower than that estimated for larger particles. As the flow around larger particles became increasingly turbulent, the models progressively under-represented drag forces and over-estimated deposition. For particles >20 microm, deposition was slightly greater in the horizontal-to-horizontal orientation than in the horizontal-to-vertical orientation due to gravitational settling. Penetration was not a multiplicative function of bend angle as theory predicts, due to the developing nature of turbulent flow in bends. Deposition in a smooth bend was similar to that in a gored bend; however, a tight radius segmented bend (R0 = 1.7) exhibited much lower deposition. For more gradual bends (3 < R0 < 12), curvature ratio had negligible effect on deposition.

A personal sampler for aircraft engine cold start particles: laboratory development and testing.

Industrial hygienists in the U.S. Air Force are concerned about exposure of their personnel to jet fuel. One potential source of exposure for flightline ground crews is the plume emitted during the start of aircraft engines in extremely cold weather. The purpose of this study was to investigate a personal sampler, a small tube-and-wire electrostatic precipitator (ESP), for assessing exposure to aircraft engine cold start particles. Tests were performed in the laboratory to characterize the sampler's collection efficiency and to determine the magnitude of adsorption and evaporation artifacts. A low-temperature chamber was developed for the artifact experiments so tests could be performed at temperatures similar to actual field conditions. The ESP collected particles from 0.5 to 20 micro m diameter with greater than 98% efficiency at particle concentrations up to 100 mg/m(3). Adsorption artifacts were less than 5 micro g/m(3) when sampling a high concentration vapor stream. Evaporation artifacts were significantly lower for the ESP than for PVC membrane filters across a range of sampling times and incoming vapor concentrations. These tests indicate that the ESP provides more accurate exposure assessment results than traditional filter-based particle samplers when sampling cold start particles produced by an aircraft engine.

Sampling and analysis of aircraft engine cold start particles and demonstration of an electrostatic personal particle sampler.

Aircraft engines emit an aerosol plume during startup in extremely cold weather that can drift into areas occupied by flightline ground crews. This study tested a personal sampler used to assess exposure to particles in the plume under challenging field conditions. Area and personal samples were taken at two U.S. Air Force (USAF) flightlines during the winter months. Small tube-and-wire electrostatic precipitators (ESPs) were mounted on a stationary stand positioned behind the engines to sample the exhaust. Other ESPs were worn by ground crews to sample breathing zone concentrations. In addition, an aerodynamic particle sizer 3320 (APS) was used to determine the size distribution of the particles. Samples collected with the ESP were solvent extracted and analyzed with gas chromatography-mass spectrometry. Results indicated that the plume consisted of up to 75 mg/m(3) of unburned jet fuel particles. The APS showed that nearly the entire particle mass was respirable, because the plumes had mass median diameters less than 2 micro m. These tests demonstrated that the ESP could be used at cold USAF flightlines to perform exposure assessments to the cold start particles.

Control methods for mineral oil mists.

Effective mist collection is important, but it is not the only determinant of mist concentration in plant air. Oil-based metalworking fluids such as straight and soluble oils contain semivolatile hydrocarbons. When these fluids form a mist, their semivolatile components partition between the vapor and mist phases depending on the makeup of the mist and on local conditions. This article addresses the relationship between the concentrations of semivolatile hydrocarbons in the vapor and mist phases using theory for partitioning developed in the field of atmospheric chemistry. Mist can be removed effectively in a collector that uses a HEPA filter as its final collection stage. Acceptable HEPA lifetime requires effective upstream stages that reduce mist loading to the HEPA; furthermore, acceptable HEPA performance requires that it be installed and maintained properly. Collectors designed to remove mist do not remove vapor, and as collector exhaust mixes into cooler plant air that already contains some mist, vapor from the collector can repartition to increase the mist concentration in the plant. Assessing the effect of vapor-to-mist repartitioning is complicated; however, repartitioning may be important for many of the compounds contained in oil-based metalworking fluids. Conditions that minimize vapor-to-mist repartitioning, such as ventilating the plant with clean outdoor air, increasing plant temperature, or controlling the release of vapor, may also be expensive, uncomfortable to plant occupants, or impractical from an engineering standpoint. As a result, very low mist concentrations in plant air may be difficult to attain.

Partitioning theory for respiratory deposition of semivolatile aerosols.

The objective of this work is to model the deposition of semivolatile aerosols in the lungs based on gas-particle and tissue-air partitioning theory. Semivolatile compounds exist in air as both particles and gases simultaneously. Mass distributes between the two phases according to a gas-particle partitioning ratio, R(pg) = K(p)(TSP). Particle deposition in the lungs is a function of aerodynamic diameter, whereas gas deposition is a function of tissue solubility, which is related to the air-lung partitioning ratio, K(la). Therefore, deposition to the lungs will vary with R(pg) and K(la). These and other parameters determine a dimensionless deposition number, D, which indicates whether particles or gases are most responsible for deposition of semivolatile chemicals in the lung. The deposition number allows industrial hygienists to design effective air sampling strategies and control measures that will minimize risks associated with exposure to semivolatiles. Examples of deposition numbers for common semivolatile pollutants are provided, including alkanes, polycyclic aromatic hydrocarbons, pesticides and polychlorinated biphenyls.

Electrostatic sampler for semivolatile aerosols: chemical artifacts.

Electrostatic precipitators (ESPs) show promise as an alternative sampling method for semivolatile aerosols because they are less susceptible to adsorptive and evaporative artifacts than filter based methods. However, the corona discharge may after the chemical composition of a sampled aerosol. Chemical artifacts associated with electrostatic precipitation of semivolatile aerosols were investigated in the laboratory. ESPs and filters sampled both particles and vapors of alkanes, polycyclic aromatic hydrocarbons, and alkenes across varying concentrations. Gravimetric measurements between the two sampling methods were well correlated. Ozone generated by the ESP corona was the primary cause of alkene reactions in the gas phase. Particles collected within the corona region were vulnerable to irradiation by corona ions overtime. Particles collected outside the corona region did not react. Vapors passing through the corona reacted to a lesser extent. Vapors captured after passing through the ESP reacted with ozone that was not removed by the vapor trap. Chemical speciation of highly reactive compounds (i.e., alkenes or other compounds with relatively short half-lives outdoors) is not appropriate with ESPs. Electrostatic precipitation of these compounds is appropriate, however, when total organic carbon is of interest as the ESP does not alter the amount of mass measured gravimetrically. ESPs can make accurate measurements of more persistent semivolatile compounds, such as alkanes and PAHs.