A Handheld Electrostatic Precipitator for Sampling Airborne

Particles and Nanoparticles

Arthur Miller, Garrett Frey, Grant King, and Carl Sunderman

NIOSH Spokane Research Lab, Spokane, Washington, USA

The authors would like to thank Wayne Larson for his creative preliminary design work, the Gonzaga University student design team (Jason Graham, Elaine Markham, Alex Uffelman, and Dr. Steve Zemke) 

for developing the first prototype, Dallace Sevier and Franklin Bailey 

for their excellent EM work, and Dr. Mark Hoover of NIOSH-DRDS 

for his invaluable technical advice and encouragement. 

Address correspondence to Arthur Miller, NIOSH Spokane Research Lab, 315 E Montgomery Ave., Spokane, WA 99207, USA. 

E-mail: ALMiller@cdc.gov 

Researchers at NIOSH are developing methods for characterizing ultrafine aerosols in workplaces. One method includes the

detailed analysis of collected particles using electron microscopy

(EM). In order to collect samples for EM at remote workplaces including mining and manufacturing facilities, researchers have developed a handheld electrostatic precipitator (ESP) particle sampler capable of collecting airborne particles including nanoscale

materials, for subsequent EM analysis. The handheld ESP has been

tested in the laboratory and is currently undergoing beta testing in

the field. Gross collection efficiencies were measured with a CPC

and net efficiencies by EM analysis of collected samples. Using

laboratory-generated NaCl aerosols in the 30–400 nm size range at

a flow rate of 55 cc/min and ESP operating voltages between 5.6–

6.8 kV, both gross and net efficiencies were measured and showed

a similar correlation with voltage, with maximum efficiency of approximately 86% at 6.4 kV. EM images from samples were also

used to estimate particle size distributions of the original aerosols

and the size-dependent deposition was evaluated for upstream versus downstream locations on the sample media. Resultssuggestthat

the number concentration and particle size distribution ofsampled

aerosols may potentially be estimated from a single ESP sample,

but that the accuracy and repeatability of such quantification need

to be investigated and refined. NIOSH is planning to license the

ESP sampler for commercial manufacturing.

1. INTRODUCTION

The authors are conducting research on the characterization and mitigation of hazardous airborne pollutants in workplaces such as refineries, mills, and manufacturing facilities. 

In addition, with the advent of nanotechnologies, processes 

that produce nanoparticles or that have nanoscale waste materials are potentially exposing workers to a new generation of 

airborne hazards. Since nanomaterials have the potential for 

enhanced bioavailability and thus detrimental health effects 

(Oberdoerster 2001; Cardello et al. 2002; Ibald-Mulli et al. 

2002; Kandlikar et al. 2007), the authors and others have begun to focus research on the characterization of nanoaerosols 

in workplace environments. Much of that work is focused 

on measuring the number, size, and available surface area of 

aerosols in the workplace (in addition to their mass concentration, which is more commonly measured). There is also a 

demonstrated need for more detailed morphology characterization of airborne particles (Oberdoerster et al. 2005; Balbus 

et al. 2007) and our work therefore includes developing methods for detailed particle characterization by electron microscopy 

(EM). 

A common approach for assessing workplace aerosols is to 

do a field survey of the entire workplace, using a variety of 

instruments to gather data (Methner et al. 2007; Evans et al. 

2008). To this end, the authors are developing a portable system 

for evaluating airborne hazards in the workplace. The system 

entails making measurements using real-time direct reading instruments, along with the use of custom designed software for 

logging the data and generating 2-D tomographic maps of the 

information. Guided by the maps, spot samples are collected at 

key locations in the workplace for subsequent detailed characterization and speciation of the aerosol particles using techniques 

including transmission electron microscopy (TEM), scanning 

electron microscopy (SEM), and energy dispersive spectroscopy 

(EDS). To simplify such spot sampling under field conditions, 

the authors have developed a portable electrostatic precipitator 

(ESP) particle sampler. This article describes the design and 

evaluation of the handheld ESP. 

The basic operation of an ESP particle sampler entails the 

routing of the subject aerosol (with or without pre-charging of 

particles) through a flow channel within which an intense electric field has been created. Any particles that are charged, when 

they encounter the electric field, drift within the field toward 

a collection plate, at a velocity determined by their electrical 

mobility (Hinds 1999). In the case of a “point to plane” ESP, 

particles are simultaneously both charged and collected. This is 
due to the unique design which entails the creation of a very 

high voltage electric field using a sharp needle as the anode and 

a flat surface as the cathode (Figure 1).

FIG. 1. Point to plane ESP configuration. 

 As aerosol particles enter 

the electric field they become charged by ions generated during corona discharge at the anode needle. The charged particles 

subsequently drift quickly in the strong electric field toward 

a grounding/collection plate onto which a sampling substrate 

(e.g., “TEM grid”) has been mounted. 

The design of a point-to-plane electrostatic precipitator particle sampler was first described in 1964 (Morrow and Mercer 1964) and in 1981 the performance of an ESP of similar 

design was rigorously evaluated in a landmark article (Cheng 

et al. 1981). Both of these works suggested that under ideal conditions, the point to plane ESP could sample without size bias. 

A more recent work challenged this result (Laskin and Cowin 

2002) and raised the question of quantification of aerosol properties based on samples collected with point to plane ESPs. Attempts to model the size-dependent collection efficiency of ESPs 

were published (Dixkens and Fissan 1999; Fierz et al. 2007) using axisymmetric designs that incorporated separate charging 

and collection regions. It was shown that such a design could 

be made field-portable and optimized to provide a fairly representative sample if high collection efficiency were sacrificed 

(Fierz et al. 2007). In that work, a scheme was demonstrated for 

quantifying size distribution and number concentration of the 

sampled aerosol by using empirically generated size-dependent 

efficiency corrections. 

Other researchers have reported successful applications of 

ESP samplers for particle collection (Morrow and Mercer 1964; 

Leith et al. 1996; Cardello et al. 2002; Ku and Maynard 2005; 

Fierz et al. 2007) but a drawback of existing ESP particle samplers is that most are bench top units and none are truly handheld, i.e., amenable for use in collecting workplace samples. 

Our goals were thus to design an ESP that could collect representative samples of an aerosol at high collection efficiency by 

combining the attributes of previous ESP designs. In addition 

it should be field portable (handheld), include a user friendly 

interface and allow for easy collection of particle samples onto 

media such as TEM grids or metallic foil substrates, under field 

conditions. 

2. DESIGN

In the first stage of the design process we identified a number of important design parameters that would be addressed to 

optimize the performance of the ESP. These included issues 

previously identified in the literature such as the charge state of 

particles, optimization of aerosol flow rate and the role of diffusion losses (Cheng et al. 1981; Fierz et al. 2007). Some of the 

more important design parameters are outlined in the following 

paragraphs, along with descriptions of how they were addressed 

in order to optimize the performance of the ESP. 

2.1. Charge Effects

In a point to plane ESP, a flux of ionized air molecules or 

“ion current” streams toward the ground plate and is countered 

by a flow of free electrons to the needle tip. As the ions travel 

toward the ground plate, they interact with the flow of aerosol 

particles, attaching to the aerosol particles and imparting a net 

charge to the particles via two mechanisms: (1) “diffusion charging,” which is due to random collisions, and (2) “field charging,” 

which is when the ions travel along electrical field lines that intersect a particle’s path. In general, the diffusion-driven charging efficiency is greater for smaller diameter particles while 

the field charging efficiency dominates for larger particles, the 

two efficiency curves crossing at around 200 nm (Hinds 1999). 

The charged aerosol particles are subsequently drawn toward the 

ground plate at a velocity determined by their electrical mobility 

(Hinds 1999). 

Our design aims at providing a unipolar charge to all particles as suggested in previous work (Dixkens and Fissan 1999). 

During the design process, as a first estimate of particle charging probability, we used calculations that assumed an electric 

field of 5 kV/cm and a modest ion concentration of 107/cm3 

as suggested for a corona charging system including both field 

and diffusion charging (Hinds 1999). Using those values over a 

one second time period yields a charge probability that is quite 

high for super-nano particles (e.g., 40 units of charge for 400 

nm particles) but drops to 1.0 for 28 nm particles and to 0.5 for 

17 nm particles. It is therefore expected that collection efficiency 

will be limited for smaller nanoparticles and that is a physical 

limitation of ESP collectors as compared to, for example, thermophoretic precipitators (Maynard 1995; Gonzalez et al. 2005). 

Since the transit time of particles through our ESP is relatively 

short and since particle charging is a function of the N-t product, 

in order to optimize the charging probability for our ESP, one 

of our main design goals was to maximize ion concentration by 
maximizing ion current, while avoiding arcing. A related goal 

aimed at maximizing particle deposition velocity and thus allowing shorter transit times (higher flow speeds) was to increase 

field strength by limiting standoff and maintaining a high corona 

voltage. 

2.2. Flow Field

The first requirement of the flow field is that it is laminar, to 

prevent unwanted mixing which might influence the trajectories 

of particles as they drift toward the collection plate. Ideally the 

flow must be slow enough so that charged particles with the least 

electrical mobility have trajectories that end on the collection 

plate. Preliminary estimates based on our design geometry and 

published particle charging rates (Hinds 1999) suggested that a 

flow of approximately 100 cc/min would be appropriate. This 

was used as a starting point, verified experimentally and the flow 

rate further adjusted for optimum performance as described in 

section 3.3. 

Another flow related issue is the relationship between flow 

channel geometry, the shape of the sampling volume and the 

shape of the electric field. Flow channel geometry was chosen after comparative testing of collection efficiency using flow 

profiles with cross sections that were circular, square, and triangular, of dimensions similar to the size of a typical collection 

substrate (TEM grid). The geometry that produced the best efficiency was the triangular cross section with a base width of 

3 mm. This supported the hypothesis that a flow profile that 

closely contained the electric field would perform best. The current design thus incorporates a flow path that is triangular in 

cross section (Figure 2).

FIG. 2. Schematic of the collection region of the prototype ESP. 

 The flow path is fabricated by milling a 

V-groove in a solid block of CPVC that is mated with a second 

(flat) block to create the flow path. A channel is milled into the 

flat block to accept the “key” onto which the sampling media 

is affixed. The blocks are fastened together before the inlet and 

outlet are machined since those must be axisymmetric across 

the mating plane. The CPVC material was chosen for its low 
conductivity and permissivity and acts as an insulator to prevent 

the high voltage at the needle from leaking to a ground point. 

The aerosol in the region between the needle and the media 

holder thus becomes the ground path as the corona is initiated. 

Other issues related to the flow field are diffusion losses, 

electrophoretic losses, and particle blow-by, i.e., particles at the 

periphery of the electrical field that either do not get charged or 

do not get fully entrained by the electrical field and thus pass 

through the sampling region without being collected. Our design 

minimizes the effects of diffusion losses by minimizing the 

residence time of aerosol in the sampler prior to collection and 

minimizes electrophoretic losses by not pre-charging particles 

as in some other designs (Dixkens and Fissan 1999; Fierz et al. 

2007). Blow-by is minimized through the flow channel design 

and placement of the tip of the corona needle at the apex of the 

triangular cross section. 

2.3. Corona Initiation and Ion Current

Sharpness of the anode needle was previously shown to be a 

critical ESP performance parameter at operating conditions of 

lower voltage and/or current (Cheng et al. 1981). To address this 

issue, our prototypes incorporated a robust but relatively sharp 

gold plated needle as an anode. To ensure it did not degrade 

over time, it was observed occasionally with a microscope and 

the EDS spectra of samples were examined for the presence 

of gold. 

Cheng et al. also showed that efficiency was reduced at higher 

ion current. Since for that work, high ion currents were generated by increasing the voltage and because more recent results 

(Fierz et al. 2007) demonstrated the relationship between voltage (field strength) and particle trajectory, we hypothesized that 

the effect noted by Cheng et al. may have been partly due to the 

narrowing of the electric field lines within the sampling volume. 

We therefore changed the shape of the flow channel to accommodate a narrower electric field, which was aimed at obviating 

the potential collection losses due to narrowing of the field. 

During operation of the prototype ESP, when voltage is applied to the corona needle via the solid-state high voltage generator, an electric field is created around the needle tip. The 

sharper the tip (smaller radius of curvature), the stronger the 

gradient of the electric field in the region around the tip. As 

the voltage to the needle is increased, the strength of the electric field eventually gets so high that it causes air molecules to 

ionize. This initiates a corona, made up of ions emitting from 

the region around the needle tip. We call this onset the “corona 

threshold” (Figure 3).

FIG. 3. Measured ion current versus corona voltage during prototype ESP 

testing. 

 Further increase in voltage causes the ion 

concentration to increase and the corona may become visible 

to the naked eye. The ensuing flux of ionized air molecules 

or “ion current,” results in charging of the aerosol particles in 

the flow channel and they are subsequently accelerated toward 

the ground plate by the electric field. As the needle voltage 

is raised above the corona threshold, the ion current increases 

steeply with voltage potential and the ensuing increase in ion 

concentration in the collection region along with the stronger 

electric field, result in better particle charging and better collection efficiency. However, as voltage is raised above the corona 

threshold, the ion current becomes more and more unstable. This 

is due to the gap between the needle and the plate acting as a 

variable resistor that fluctuates based on temperature, humidity 

and the localized concentrations of ions and particles. There is 

thus a “transition region” of voltage, where the current is unstable (and pulses/surges at high frequency). The voltage at which 

this happens is dependent mainly on the gap distance. If voltage 

is raised still further the fluctuating current eventually stabilizes 

to a steady arcing as indicated by the dotted line in Figure 3 (this 

is akin to an arc welder). Since the power supply and high voltage generator in our ESP cannot withstand such high current, 

we discontinued tests before this level was reached. 

For the tests of Figure 3, the fluctuating current was measured indirectly using a digital oscilloscope, by monitoring 

the voltage drop across a small (100 kOhm) resistor placed 

in the grounding path. A series of capacitors were used to filter 

out the high frequency noise and the oscilloscope displayed the 

average voltage drop during each test which was then used to 

calculate an average current for that test. The collection efficiency during those tests was reduced for voltages above about 

6.5 kV, as will be shown in section 3.3. A similar result was 

reported previously (Cheng et al. 1981) but the reason for reduced efficiency was not confirmed. It is our hypotheses that 

the surging of current and/or sparking allow the ions to follow 

a more narrow path to ground and also reduce the strength of 

the electrical field by reducing the voltage potential between the 

needle and collection plate. Both these effects would reduce the 

charging efficiency and thus the collection efficiency of the ESP. 

Avoiding sparking is also important because it can potentially 

damage the sampling media or result in creation of new particles 

as debris from larger particles or the sample media. To stabilize 

the ion current during operation, a resistor of approximately 25 
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mega-ohms was placed in the corona circuit and the operating 

voltage limited to about 6.3 kV. 

2.4. User Interface

The operator interface on the current prototype comprises an 

LCD screen that displays menu choices accessed by the user 

via a keypad on the front panel. The most important parameter 

selected by the user is the sampling time (duration). In order 

to best estimate sampling time, the user should know the approximate concentration of particles in the air. Since not all 

users will have this information, a “cheat sheet” is provided 

and attached to the back of the ESP. The suggested sampling 

times were estimated by calculating the time needed to capture 

an ideal sample based upon the stated aerosol concentration, a 

flow rate of 55 cc/min, an assumed 85% collection efficiency 

and a desired average deposition spacing of approximately one 

micrometer between particles collected on the sample media. 

As an example, the suggested sampling time for an aerosol of 

106 particles/cc is approximately 8 seconds. 

2.5. Sampling Media and Basic Operation

One of the primary design challenges was developing a 

method for inserting the TEM grid, or alternatively a thin metallic disk for SEM analysis, into the sampler. TEM grids are somewhat delicate (3 mm diameter and coated with films that are tens 

of nanometers in thickness) and require care when handling. 

They are typically stored in small slots in sealed, preferably 

static-free plastic boxes and handled with tweezers (Figure 4). 

FIG. 4. (a) Tweezers are used to handle grids and a radiation source (background) helps to prevent static charge. (b) Sample media is placed onto the ground 

post of a key for use in the ESP (sealable case for keys in background). 

For ease of inserting the grids into the ESP under field conditions, we designed a removable “key” onto which a grid or foil 

disk media is attached. Tweezers are used to place the media 

onto the ground post of the key (Figure 4b) and the media is subsequently affixed to the key (e.g., with a small piece of adhesive 

tape). This approach was chosen with the intent that such keys 

could be pre-loaded with sample media under lab conditions 

and taken to the field in a sealed case. 

While the ESP was developed as a tool for aerosol characterization with the primary goals of maximizing particle collection 

efficiency and sample quality for subsequent EM analysis, the 

design also focused on portability and user friendliness. Several 

prototypes were built and tested and the results used to iteratively 

refine the design. The current pre-manufacturing prototype is 

shown in Figure 5. The main features are the 5-key touch-pad 

user interface control, an LCD screen, user menus for sample 

setup, on-screen user feedback messages and insertable keys for 

loading the TEM grids or other sample media. 

3. PERFORMANCE EVALUATION

The design goals for the ESP focused on the primary user 

needs, which are to get a “representative sample” in a timely 

manner, under field conditions. The performance criteria of most 

interest to the eventual end-user are thus collection efficiency 

and deposition uniformity. Tests were conducted to evaluate 

these parameters using well-characterized aerosols under laboratory conditions as described in the following paragraphs. The 

performance goals were for the ESP to have maximum collection efficiency and to produce samples that were uniformly 

loaded with particles, for optimum EM analysis. 

3.1. Experimental Setup

An experimental setup was designed to provide a source of 

well characterized aerosols that was constant and repeatable. 
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The setup allows real-time evaluation of ESP collection efficiency using either a fast mobility particle sizer (FMPS) (model 

3091, TSI Inc., St. Paul, MN) or a condensation particle counter 

(CPC) (model 3007, TSI Inc., St. Paul, MN). ESP samples of 

the lab-generated aerosols can be simultaneously collected onto 

TEM grids or other sample media during each test. 

FIG. 5. A prototype handheld ESP. 

While conducting experiments, the aerosols are generated 

using an atomizer (Figure 6), typically from dilute solutions 

of NaCl or KI in deionized water, producing micrometer sized 

droplets of the solutions.

FIG. 6. Schematic of experimental setup for evaluating collection efficiency 

of the ESP. 

 A portion of the aerosol is subsequently 

dried in a cylindrical diffusion dryer yielding a polydisperse 

aerosol of sub micrometer salt particles, while the excess is 

vented at atmospheric pressure. The particle size distribution of 

the dried aerosols is dependent on the solution concentration 

which is typically mixed to provide polydisperse aerosols with 

mean diameters in the 40–200 nm range. The sub micrometer 

aerosols are routed through the ESP at a pre-determined flow 

rate in the range of 40–200 cc/min, measured by a laminar flow 

element (LFE) and controlled by a needle valve. Since the FMPS 

and the CPC require flows greater than 200 cc/min, the aerosol 

exiting the ESP is diluted with HEPA-filtered air to provide the 

flow required for the instruments (Figure 6). The dilution factor 

is calculated from the measured flow rates and used to correct 

the particle concentration measurements as needed to determine 

actual concentrations in the ESP during tests. 

Using this setup, the “gross” collection efficiency of the ESP 

(fraction of particles removed from the flow) is determined by 

measuring the reduction in particle concentration while the ESP 

is on and dividing that number by the baseline concentration 

measured with the ESP off. As noted previously (Laskin and 

Cowin 2002) this method of measurement may be subject to 

errors due to diffusion and electrophoresis. For this reason, additional measurements of “net” efficiency (fraction of particles 

deposited onto the sample substrate) were conducted as noted 

in section 3.2. For a typical measurement of gross efficiency, 

the concentration is first measured for a few minutes with the 

ESP off, then while measurements continued, it is switched on 

for about one minute, then off again for the remainder of the 

test. Since both instruments have relatively fast response time 

(1 s) versus the length of typical tests, the particle concentration values for “ESP on” and “ESP off” are taken as the average of tens/hundreds of individual measurements taken during 

the tests. The gross efficiency is then calculated using these 

average values. 

3.2. Evaluating Collection Efficiency

In order to get a representative sample of an aerosol using an 

ESP collector, it would be optimal for the collection efficiency 

to be 100% and the particle deposition to be uniform across 

the surface of the sampling media, i.e., no sized-biased deposition. It has been shown (Fierz et al. 2007) that by modeling the 

theoretical deposition (for an axial flow ESP), and using empirical, size-dependent correction factors, it is possible to design an 

ESP that offers uniform deposition albeit at reduced collection 

efficiencies. In contrast, our approach is to try and demonstrate 

a technique which relies on high collection efficiency over a 

small surface area to provide quasi-uniform deposition onto the 

media over very short collection times. While it may provide a 

less uniform sample than the above method, such a technique 

would have the benefit of shorter collection times, which is an 

important advantage when taking field samples. 

For most measurements of gross collection efficiency, the 

CPC was used, and logged data showed that the number concentration dropped suddenly when the ESP was powered on 

(Figure 7). For some gross efficiency measurements, the FMPS 

was used, to allow real time measurements of both size distribution and total number concentration, at a sampling rate of 1 

Hz. The measured efficiencies for the FMPS tests were similar 

to those measured using the CPC, but the measurable size range 

varies somewhat for the two instruments, so direct comparison 

of results is not made. Since the FMPS measures all “size bins” 

simultaneously (Tammet 1998), the data can be used to estimate 

the size-dependent collection efficiency (Figure 8). The data for 

each curve in Figure 8 are from a single test run lasting about 
five minutes. The collection efficiencies were calculated for each 

FMPS channel and are averaged from a few hundred data records 

over the duration of a test. The data of Figure 8 indicate that for 

these operating conditions (100 cc/min flow and 6.4 kV corona 

voltage) the larger sub micrometer particles are collected very 

efficiently, that the collection efficiency drops off for smaller 

particles, reaching a minimum for particles in the 100–200 nm 

size range and that it improves for nanoparticles down to around 

30 nm. This result is similar to that published previously (Cheng 

et al. 1981) and reflects the size dependent behavior of combined 

field and diffusion charging (Hinds 1999). Further experimental work is called for to elucidate the collection efficiency for 

smaller particles (<30 nm). It is notable that nanoparticles are 

notoriously difficult to charge (Baron and Willeke 2001) and it 

would be expected that collection efficiency would thus drop off 

steeply for particles smaller than about 30 nm. 

FIG. 7. Particle concentration versus time for a typical test, measured with 

the CPC. 

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FIG. 8. Size-dependent gross collection efficiency of NaCl particles, measured with the FMPS, for a prototype ESP similar to that in Figure 5, operating 

at 100 cc/min flow rate and 6.4 kV corona voltage. 

With the goal of comparing gross efficiency measurements 

with net efficiency values derived from ESP samples, a rudimentary procedure was used to estimate the net efficiency of 

collection. First, the total number of particles collected onto 

aluminum foil substrate samples was estimated by counting the 

number of particles per unit area on 8–10 SEM images from 

locations across the substrate. The average number per unit area 

from all the images was then multiplied by the deposition area 

to get an estimate of the total number of particles on the sample. 

The net efficiency was calculated as the ratio of this number of 

deposited particles divided by the maximum number of particles 

possible on the sample (derived from the flow rate, collection 

time, and the original aerosol concentration as measured by a 

CPC). 

To compare gross and net efficiencies using this method, pairs 

of back-to-back tests were conducted using identical conditions, 

i.e., NaCl aerosols in the 40–300 nm size range at 55 cc/min 

flow rate and 6.4 kV corona voltage. For the first test of each 

pair, gross efficiency was measured with the CPC and for the 

second test, net efficiency was determined by SEM analysis 

of the collected sample. Results indicate that both gross and 

net efficiency have a similar correlation with ESP voltage, with 

variations on the order of 5–15% (Figure 9). 

FIG. 9. Comparison of gross and net efficiencies from back to back tests using 

the ESP of Figure 5 operating at 55 cc/min flow rate and 6.4 kV corona voltage. 

Such variations 

are not unusual for ESP tests. In addition to inherent variations 

in test conditions, the CPC-based efficiency measurements are 

potentially compromised by electrophoretic losses, while the 

EM-derived efficiencies are subject to error induced by nonuniform deposition across the sample media (since counting of 

every particle is unrealistic). The measurement or estimation of 

“true efficiency” is thus elusive. 

In addition to the deficiencies of the simplified counting 

method, there are other possible reasons why the EM-derived 

particle concentrations might not match those measured by 

the CPC or FMPS. One concern is that the absolute accuracy of particle sizers/counters is in question and there is no 
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standard reference method for their calibration (Fierz et al. 2007; 

Rodrigue et al. 2007). There is also the possibility of particle 

losses within the collection system due to diffusion of particles 

onto surfaces of the flow channel or due to charged particles that 

blow-by or whose trajectories miss the sampling media but are 

caught by surfaces surrounding the media. 

There are other potential reasons why the accurate measurement, modeling, or estimating of collection efficiency for 

ESP samplers is an elusive goal. As described previously 

(Friedlander 2000), the roles of diffusion charging and field 

charging vary for different sized particles. They also vary as a 

function of corona strength, electric field strength, and geometry 

of the collection region. The method of efficiency measurement 

can also vary. It was shown previously that the combined uncertainties involved in efficiency estimation and measurement 

are quite high and that differences in aerosol concentration 

derived from ESP samples analyzed by TEM versus concentrations measured by a mobility particle sizer can be as high 

as 40%, primarily due to the statistical uncertainty of deposition uniformity (Fierz et al. 2007). For these reasons, published 

data on the collection efficiency of ESP samplers varies somewhat (Cheng et al. 1981; Laskin and Cowin 2002; Fierz et al. 

2007). 

There are, however, commonalities in the data sets. For example, in the efficiency data of Figure 8, we see a region of 

minimum collection between 100 and 200 nm, similar to that 

previously reported (Cardello et al. 2002). We also see an improvement in efficiency between about 30 and 100 nm as reported previously (Cardello et al. 2002; Fierz et al. 2007). In 

addition to these similarities there are also differences in the 

data. This is somewhat expected and is potentially due to the 

variations in design, including corona parameters and differences in geometries of the flow field, electric field, and operating 

parameters. Future research is warranted to further investigate 

efficiency optimization and measurement. 

3.3. Optimizing Flow Rate and Corona Voltage

The experimental setup was used to investigate the influence 

of flow rate on collection efficiency. Results show a reduction 

in gross efficiency with increased flow (Figure 10) as reported 

previously (Cheng et al. 1981).

FIG. 10. Gross efficiency (measured with CPC) versus flow rate for prototype 

ESP operating at 6.4 kV. 

 While lower flow rates increase 

residence time and thus collection efficiency, there is a trade off 

between lower versus higher flow rate. While reducing the flow 

rate may improve collection efficiency, it can have the effect 

of depositing more particles near the leading edge of the sample substrate, which is undesirable since one of the goals is to 

have the particles well dispersed across the surface of the substrate for optimum analysis. Low flow rates also increase the 

potential for diffusion losses especially for small particles. On 

the other hand, if the flow rate is too high, residence time will 

be short and some particles may not gather enough electrical 

charge to offset their inertia and thus their trajectories will not 

end on the plate and they will not be collected. 

The experimental setup was also used to investigate the influence of corona voltage on gross collection efficiency (measured 

with the CPC), using an ESP operating at a flow rate of 55cc/min 

(Figure 11). 

FIG. 11. Gross collection efficiency (measured by CPC) versus corona voltage using the ESP of Figure 5 operating at 55 cc/min flow rate. 

The data are based on 3–9 tests per voltage setting 

and suggest that collection efficiency improves in direct correlation with voltage up to approximately 6.8 kV and then begins 

to degrade. It is notable that the standard deviation among tests 

at each voltage setting is relatively large. This is reflective of 

the variability in ion current and field strength that is associated 

with the unstable nature of the gap resistance, compounded by 

factors such as temperature, humidity, flow rate fluctuations, 

particle concentration, media type, and media contact on the 

ground post. It is also notable that the transition after 6.8 kV 

parallels the steep increase in average ion current reflected in 

the data of Figure 3. This supports the hypothesis that the higher 

and more fluctuating ion currents lead to reduced collection efficiency, possibly due to effects such as narrowing of the corona 

and fluctuations in the electric field. 
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Comparing the data of Figure 11 to Figure 9, all based on 

NaCl aerosols at 55 cc/min flow and similar levels of corona 

voltage, it is apparent that the gross and net efficiencies have a 

somewhat repeatable and similar trend, albeit a significant standard deviation. Given that no two tests are exactly the same and 

that both measurement methods have potential inaccuracies, the 

data suggest that for this prototype, expected collection efficiencies would likely be in the range of 76–94% at these operating 

conditions. 

FIG. 12. Examples of NaCl particle deposition onto: (a) TEM grid (b) aluminum foil media (SEM image). 

FIG. 13. Comparing particle size distribution of the original aerosol (FMPS measured) with results of TEM analysis of particles at upstream, center and 

downstream locations on the sample. 

3.4. Deposition Uniformity

Uniformity of deposition and spacing of particles are especially important when attempting to use EM images to re-create 

particle size distributions and/or determine the relative concentration of various particle types in a given sample (Whitehead 

and Leith 2008). Optimal deposition uniformity is that which 

results in no size-dependent deposition across the grid surface. 

Optimal particle spacing is that which insures that few, if any, 

particles are deposited such that they overlap on other particles. 

This is achieved by adjusting the sampling time to result in optimum loading of the sample media, such that particle spacing is 

on the order of a few times the diameter of typical particles. The 

deposition uniformity issue is more complex, since it involves 

several variables including particle charging rates, variations 

in electrical mobility of particles and size-dependent particle 

trajectories. 

To assess the spacing and deposition uniformity, tests were 

conducted using NaCl polydisperse aerosol collected onto TEM 

grids and aluminum foil substrates. Results indicate that for 

properly selected sampling times, the spacing of sampled 
particles on the substrates is relatively even (Figure 12). The 

density of deposited particles varies from highest at the upstream centerline of the flow path to lowest at downstream locations away from the centerline. The uniformity of deposition 

(spatial size-biased collection) on TEM grid samples was determined by counting and sizing particles on TEM images taken 

along the centerline at upstream, middle, and downstream locations. All “size bins” of the resulting particle size distributions 

were multiplied by a correction factor and then compared to the 

size distribution measured with the FMPS during the sample 

collection process (Figure 13). The correction factor used was 

calculated as the ratio of maximum possible number of particles 

on the sample based on CPC data, flow rate, and collection time, 

divided by the number of particles counted. 

In the data of Figure 13, it is apparent that there is a slight 

variation in the size distribution at the different locations, but 

that the combined data from all three locations matches reasonably well with the original particle size distribution. While 

the x-axis is labeled simply “Dp,” it is notable that the FMPS 

measures electrical mobility diameter while the EM-based data 

is derived from the average diameter of the 2-D cross sectional 

area of the particle images. This may in part explain the slight 

rightward shift of the EM-derived data compared to the FMPS 

data. These limitations not withstanding, the results suggest that 

the lower collection efficiency for some particle sizes (shown 

in Figure 8) does not affect the EM-derived particle size distribution significantly for this aerosol. Further research is needed 

to better understand, control and measure deposition uniformity 

and to use this information to optimize the ESP for collection 

of representative samples of polydisperse aerosols. 

4. CONCLUSIONS

A handheld ESP particle sampler was designed, using previously published results to aid in optimization of collection 

efficiency and deposition uniformity. Results of sample analyses suggest that particle size distributions can be approximately 

reconstructed using images taken along the flow centerline at upstream, middle, and downstream locations. Using lab-generated 

NaCl aerosol at a flow rate of 55 cc/min and ESP corona voltage 

of 6.4 kV, gross collection efficiencies of 76–94% for particles 

in the 30–400 nm range were measured by the FMPS. Using the 

same aerosol, net deposition/collection efficiencies were determined by particle counts on SEM images and compared with 

measured gross efficiencies for a series of back to back tests 

using corona voltages in the range of 5.6–6.8 kV. The gross 

and net efficiencies both showed a similar correlation with ESP 

voltage, with variations on the order of 5–15%. Since similar 

or larger discrepancies have been reported for comparison of 

nanoparticle measurements made with different brands of particle sizers (Rodrigue et al. 2007), that level of accuracy may 

well be deemed acceptable by the industry. 

It should be noted that not all aerosols are amenable to sampling with an ESP. Explosive atmospheres, for example, should 

obviously be avoided. When used for collecting volatile and 

semi-volatile materials, evaporation of particles/droplets may 

occur during sample handling or when exposed to the electron 

beam during electron microscopy analysis. In some cases this 

can be remedied by careful handling of samples and analysis with an “environmental” (low vacuum) electron microscope 

(Miller et al. 2008). Another issue which must be considered 

is the potential for droplets or particles to undergo reactions 

during/after collection onto the TEM grid, especially at surface 

interfaces, including potential reactions due to ozone created by 

the corona. Some prior knowledge of the source aerosol will 

thus be advantageous in the effective use of this technique. 

While further work to improve the quantitative analysis of 

ESP samples by EM is needed, this work is a significant step 

toward the potential routine application of single particle analyses to provide information regarding the properties of sampled 

aerosols. The handheld ESP addresses the need for a tool that 

can take samples quickly and effectively for assessment of ultrafine workplace aerosols. Beta versions of the handheld ESP are 

currently being evaluated with the goal of making this device 

commercially available. 

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