United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-89/028 Aug. 1989
&EPA Project Summary
Construction and Testing of
Electrochemical NO2 PSDs
M. W. Findlay, Jr., J. R. Stetter, and C. Yue
The objective of this project was to
develop, test and deliver a prototype
electrochemical instrument capable
of real-time measurement of low-ppb
levels of NO2. The unit must be small
enough for use as a personal
monitor.
The work was conducted im two
distinct tasks that were performed
concurrently: (1) improvement of the
currently available NO2 sensing tech-
nology to provide a sensor with suf-
ficiently high signal/noise ratio and
stable background signal for continu-
ous low-ppb level measurements,
and (2) the design, assembly, testing,
and delivery of a portable instrument
incorporating this sensor technology
to provide capability for monitoring
personal exposure to ambient NO2
concentrations.
During this project, the following
work was completed:
A very low background, low noise,
attitude-insensitive sensor was devel-
oped and tested to provide detection
of NO2 at levels as low as 5 ppb. A
portable, modular prototype NO2
monitor was developed and tested.
The instrument was interfaced to a
commercial data logger to allow col-
lection and storage of real-time con-
centration data, as well as a con-
tinuous display of ambient NO2
levels. Three of these units were con-
structed for evaluation. In addition,
preliminary design of a second gen-
eration data logger/controller that can
provide optimum ease of use and
maximum accuracy was completed.
Future work will address further im-
provement in the NO2 sensor stabil-
ity, reduction in NO2 instrument size,
development of the optimum data
logger/controller, expansion of the
monitor to measure other com-
pounds, and the investigation of mul-
tiple-function, multiple-compound,
sensor-array based real-time mon-
itors.
This Project Summary was devel-
oped by EPA's Atmospheric Research
and Exposure Assessment Laboratory,
Research Triangle Park, NC, to an-
nounce key findings of the research
project that is fully documented in a
separate report o.f< th& same title (see
Prefect Repoxt aedering; Information at
back.)
introduction
The Atmospheric Research and Expo-
sure Assessment Laboratory, Research
Triangle Park, North Carolina, has re-
sponsibility for: assessment of environ-
mental monitoring technology and sys-
tems for air, implementation of agency-
wide quality assurance programs for air
pollution measurement systems, and
supplying technical support to other
groups in the Agency including the Office
of Air and Radiation, the Office of Toxic
Substances, and the Office of Solid
Waste.
The monitoring of pollutant gases at
sub-ppm and low-ppb levels is of primary
concern in indoor air and non-industrial
locations such as the home. The trend
toward more airtight homes, which began
during the energy crisis of the early
I970's, has caused concern among health
experts about increased levels of indoor
pollutants such as the sulfur and nitrogen
oxides. It is the objective of this work to
develop a personal sampling device
(PSD) which will allow real-time moni-
toring of low concentrations of NO2 in the
presence of ambient levels of potential
interferents such as NH3, CO, H2S, com-
mon hydrocarbons, and SO2.
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Amperometric electrochemical sensors
provide portable real-time analysis capa-
bility and have been used for ppm level
measurement of NO2. These sensor
systems do not suffer interferences from
NO, N20, or nitrous acid but often
possess too much noise and drift to
provide accurate data at ppb levels.
Major sources of drift have been tenta-
tively identified as temperature and
relative humidity fluctuations.
The work discussed in the full report
has focused upon improvement of the
existing electrochemical technique so
that it can be used at ppb levels. This
requires gaining an understanding of the
reactions of NO2 in the sensor system.
This knowledge is required to design and
build an improved real-time portable
instrument for the measurement of NO2
in the field.
Procedure
All gases and gas mixtures were
supplied by Scott Specialty gases in
pressurized cylinders at a concentration
of 49-400 ppmv in zero air. Working
standard mixtures of NO2/air were pre-
pared fresh several times daily by
dilution of the stock gas mixture. Low
levels of NOg in air were prepared using
a calibrated permeation device (KinTek
Laboratories, Houston, TX).
Gas flow through the sensors was con-
trolled using Spectrex AS-400 pumps and
Matheson Model 602 (0-1L) and 603 (0-
5L) flow meters fitted with Clippard
needle valves. Elevated temperature
experiments were conducted in a GCA/
Precision Scientific Model 18 oven. Low
temperature experiments were performed
in a modified GE TA242TH refrigerator.
Temperature was monitored with a 0-
110°C mercury thermometer.
Commercial sensors were obtained
from the Energetics Sciences Division of
National Drager (ND or ESI). Solid elec-
trolyte sensors using Nafion (El DuPont
TM) cation exchange material and aque-
ous electrolyte sensors with low surface
area (LSA) and high surface area (HSA)
sensing electrodes were designed and
built by TRI under carefully controlled
conditions.
Sensors were tested in the drawn-air
("pump-after-sensor") flow configuration
recommended in earlier work. A low
volume exposure chamber was used to
test the ND sensors. The volume of the
chamber used with the TRI sensors was
decreased by 50% from the prior work in
order to increase the linear flow rate of
analyte past the working electrode. This
design change should improve response
time and signal magnitude.
All of the bench-top experiments were
performed at room temperature (ap-
proximately 22°C) and the potential of the
NO2 gold sensing electrode was main-
tained constant at +800 mV vs. SHE
(-200 mV vsi Pt/air reference electrode)
throughout'the tests. The sample
(analyte) flow; was 100 cc/min through the
sensor and, since the gas mixtures were
prepared from laboratory air, most tests
were performed at 30.-60% RH. Only
tests of the 49 ppm NO2/air commercial
mixture were1 dry ("approximately" 0%
RH). i
Results and Discussion
The Proposed Instrument
After discussion of the specifications of
the "ideal" instrument, the hardware de-
signs for the NO2 instrument were pre-
pared. A block diagram of the N02 unit is
as shown in Figure 1. The components of
the instrument were identified as:
1. Pump/sensor - module 1: This
component is the center of the unit. It
contains Jthe sensor, pump, poten-
tiostat cirpuit, pump motor control
circuit, circuit to provide a stable
power supply, temperature circuit,
and all signal measurement circuits.
There are' also provisions for an alarm
circuit and LED display for con-
tinuous readout, but these were not
be included in the prototype instru-
ments. !
2. Data logger/controller - module 2:
This component of the unit will con-
tain the A/D channels for signal input
and digital input/output channels for
instrument control. The input chan-
nels will monitor the sensor output,
temperature, battery voltages, sensor
bias, and [other pertinent parameters.
The output channels will allow control
of the pump motor circuit and analog
switch for auto-gain set and auto-zero
measurements. An evaluation of
existing and possible data loggers
relative to these specifications was
completed as a part of this work.
3. Battery pack - module 3: This module
contains jthe rechargeable Ni-Cad
batteries used for pump power. To
provide the voltage to operate the
pump motor circuit as well as power
the pump, a 7.2 V pack was originally
specified.1 A 9V battery will be in-
cluded in module 1 to keep the
sensor on standby and ready for use
even when the instrument was not
deployed.
Sensor Development
TRI has obtained excellent response
from catalyst films vapor deposited onto
porous membranes. By controlling the
nature of this film, the signal can be
optimized. Commercially available high
surface area (HSA) sensors and the TRI
Nafion sensor were used as "perform-
ance standards" for evaluation of the
"improved" responses from the TRI LSA
sensors tested in this work.
Figure 2 compares the response of a
commercial NO2 sensor to that of the TRI
Nafion sensor and the acid electrolyte
low surface area working electrode sen-
sor. The work outlined briefly below was
focused on optimizing the sensor
performance further, and characterizing
its response and lifetime behavior.
Membrane Selection:
Choice of a membrane for the sensing
electrode is critical and the membrane
can have a profound effect upon analyti-
cal performance. For use in a personal
monitor, it is necessary that the mem-
brane be rugged and possess a very high
resistance to water flow through the
pores. A series of tests were performed
to compare the properties of several por-
ous membranes to determine the one
most suited for the NO2 monitor.
The membrane properties evaluated
included tensile, or tear, strength,
porosity, weeping pressure (the pressure
required to force electrolyte through the
membrane), and response character-
istics. Several of the membranes tested
provide suitable substrates. The Gortex
(0.45 iim and 1.0 pm pore size), Celgard
(0.02 and 0.04 pm) as well as the Zitex
material (E606122 and E606-223) were
tested in TRI sensor designs.
Catalyst Loading:
The magnitude of the signal from a
given concentration of analyte gas is pro-
portional to the catalyst loading on the
membrane until the reaction in the sensor
(at the working electrode) is no longer
catalyst limited. The optimum loading for
a sensor is the one that produces con-
stant response and for which slight varia-
tions in catalyst activity (due to small
changes in temperature or specific sur-
face area) do not substantially affect the
sensor signal.
A series of tests were performed to es-
tablish the relationship between catalyst
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NO2 Analyzer
NO2 Analyzer
+ 888.88
a n
Battery
Pack
1.2 V Ni-Cd
Instrument
(Sensor, Pump,
Electronics)
Data Logger
(4 Channel)
Figure 1. NO2 Instrument at the end of this phase of the project, showing the three
modules and connectors.
loading and sensor response to NO2 for
each of the potential membrane mater-
ials. These included the three different
porous membrane materials and several
catalyst loadings for each membrane:
600, 900, 1200, 2100, and 3000 Ang-
stroms. The array of sensors constructed
from the different electrode and mem-
branes were evaluated for signal magni-
tude (pA/ppm), background current
(ppm), noise level (ppb), and response
time.
Based upon signal magnitude and
physical properties, the G-1.0 membrane,
was chosen as the most suitable mem-
brane for the sensor. Initial tests of the
precision and stability of sensors using
this material were encouraging. Precision
of the sensor response was typically
within ± 5%, and baseline drift less than
20 nA,(> 100 ppb) over 4 hrs of testing
at 1-5 ppm N02 levels. The data indicate
that the performance of the sensor could
be improved even further with a higher
loading of electro-catalyst.
Attitude Sensitivity:
In order to use the sensor for a porta-
ble monitor, it is necessary to make it
position and attitude insensitive. An
hydrophilic polypropylene wick material
was packed behind the sensing electrode
membrane so that it extended down into
the electrolyte. This wick insures continu-
ous contact between the sensing elec-
trode and the electrolyte; even if the sen-
sor were turned upside down.
Characterization of TRI NO2
Sensor
Based upon the data from the optimi-
zation study described above, several
sensors were constructed for testing in
the personal modular prototype monitors
that were built. A series of tests were per-
formed to characterize the response of
the new TRI sensor described above.
These tests included evaluation of the
following characteristics:
Signal Magnitude:
The signal for NO2 from the low
surface area sensor is typically 0.2-0.5
liA/ppm. Signal noise is less than 5 nA,
allowing the detection of about 5 ppb
NO2. The improvements realized in the
signal to background ratio with the LSA
sensor should allow the optimum moni-
toring for N02 at the lowest levels. How-
ever, the versatility of the monitor will
allow use of many different sensors in the
unit. The HSA sensors may provide a
better response at extremely high levels
of pollutant while the LSA sensors are
optimized for performance at low NO2
concentrations.
Response of Potential
Interferents:
Several compounds may be present in
significant concentrations in ambient air.
Table 1 lists a number of compounds
likely to be encountered during the use of
the instrument. Of the compounds tested,
only H2S yielded a signal large enough to
be of concern.
The use of a selective filter to remove
the H2S from the sample was studied.
Initial tests of the filter at NO2 levels of 1
ppm indicate little scrubbing NO2 by the
H2S filter. This filter needs to be studied
in detail at low concentrations of NO2, to
determine the effect on response time
and calibration of the NO2 personal
monitor.
Effects of Flow Rate:
For a sensor operated in a flowing air
stream, it is necessary to determine the
effect of the rate of air flow upon the re-
sponse of the sensor. The LSA sensor is
independent of flow rate in the range 45-
300 cc/min. Since the air flow in the in-
strument is set at 100 cc/min, any fluctu-
ations in pump speed will not greatly
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M. W. Findlay, Jr., J. R. Stetter, and C. Yue are with Transducer Research, Inc.,
Naperville. IL 60540.
James D. Mulik is the EPA Project Officer (see below).
The complete report, entitled "Construction and Testing, of Electrochemical NO2
PSDs," (Order No. PB 89-169 874/AS; Cost: $15.95, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penally for Private Use S300
EPA/600/S3-89,'028
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