United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S3-90/088 Jan. 1991
&EPA Project Summary
Development of Real-Time
Monitors for Gaseous
Formaldehyde
Thomas J. Kelly and Russell H. Barnes
The objective of this work
assignment was to conceive,
construct, and test prototype
monitors for gaseous formaldehyde
(HCHO) based on (a) a novel
spectroscopic method for direct
formaldehyde measurement in air,
and (b) a separate wet chemical
method involving collection of
formaldehyde from air into aqueous
solution for subsequent analysis.
Specific requirements of the
prototype monitors were that they be
portable, real-time, continuous
instruments having time response on
the order of one minute, and that they
have sensitivity sufficient to allow
monitoring of formaldehyde in
ambient air at single-digit part per
billion by volume (ppbv)
concentrations.
The spectroscopic method
developed in this study is based on
direct fluorescence detection of
gaseous formaldehyde following
excitation with UV light. This method
has been developed to the prototype
stage by modifications of a
commercial fluorescence SO2
detector to convert it to
formaldehyde detection. The
modifications include changing
optical filtering and focussing
elements to match the wavelengths
appropriate for formaldehyde, and
adjusting the gain of the signal
amplifying electronics. The prototype
spectroscopic formaldehyde monitor
exhibits a detection limit of < 30
ppbv, with a time response of about
one minute. The instrument is
portable and fully self-contained.
The wet chemical method is based
on derivatization of formaldehyde in
aqueous solution via the Hantzsch
reaction, the cyclization of a p-
diketone, an amine, and an aldehyde
to form a fluorescent product. In this
study, the detection of fluorescent
product was made more sensitive by
using intense 254 nm light from a
mercury lamp for excitation, rather
than the 410 nm light used in
previous studies employing the
Hantzsch reaction. The increased
sensitivity of aqueous detection
allowed use of a simple and efficient
glass coil scrubber for collection of
gaseous formaldehyde. The wet
chemical formaldehyde monitor
incorporating these improvements
exhibits a detection limit for gaseous
formaldehyde of 0.2 ppbv and for
aqueous formaldehyde of 0.02 nM,
with time response of about one
minute, following a lag time of two
minutes. This instrument also is
portable, requiring only occasional
replenishment of simple aqueous
reagent solutions.
Both instruments were tested in the
laboratory with gaseous
formaldehyde standards. In addition,
the aqueous scrubbing/analysis
method was field tested by
continuous operation over a 10-day
period in which outdoor and indoor
air were sampled for alternate half-
hour periods. Concentrations
observed in the field study were 0.2
to 7 ppbv, and 10 to >50 ppbv, in
outdoor and indoor air, respectively.
This study also provided data for a
comparison of real-time (aqueous
Printed on Recycled Paper
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scrubbing/analysis) and integrated
measurements, using dinitro-
phenylhydrazine (DNPH) impingers.
This comparison showed agreement
between the real-time and DNPH data,
even at concentrations as low as 1
ppbv.
This Project Summary was
developed by EPA's Atmospheric
Research and Exposure Assessment
Laboratory, Research Triangle Park,
NO, to announce key findings of the
research project that Is fully
documented Jn a .separate report of
the same title (see Project Report
ordering Information at back).
Introduction
Formaldehyde (HCHO) is the most
abundant aldehyde in the ambient
atmosphere, originating both from
primary emissions in combustion sources
and from atmospheric oxidation of
hydrocarbons. Formaldehyde produces
free radicals upon photolysis, contributing
to the formation of ozone and other
oxidants. Concentrations of formaldehyde
in the ambient atmosphere range from
below 1 ppbv in rural areas to several
tens of ppbv in urban areas such as the
Los Angeles basin. A pronounced diurnal
variation is observed in Los Angeles due
to the impact of both local sources and
photochemistry, and a pronounced
seasonal variation is observed in rural
areas due to seasonal changes in
photochemical activity. Formaldehyde is
also found in industrial environments and
in indoor air, originating from a variety of
processes and materials of construction.
A national database on concentrations of
volatile organic compounds indicates that
indoor formaldehyde concentrations are
typically several times higher than
outdoor concentrations. Formaldehyde is
emitted from motor vehicles, and this
source may become more important due
to the proposed increasing reliance on
vehicles using alternate fuels (e.g.,
methanol). In either indoor or outdoor air,
the presence of formaldehyde is
important because of the toxicity of this
chemical, including suspected
carcinogenesis in humans.
Because of the importance of gaseous
formaldehyde from both an atmospheric
chemistry and a toxicology viewpoint,
several methods have been developed
for measurement of formaldehyde in air,
and intercomparisons of methods have
been performed. Spectroscopic methods
include Fourier transform infrared
absorption (FTIR), differential optical
absorption spectroscopy (DOAS), and
tunable diode laser absorption
spectroscopy (TOLAS). All are capable of
real-time HCHO measurement, which is
of importance in studying the short-term
variations in ambient HCHO which
convey information about its sources and
sinks. However, all three spectroscopic
devices are large, complex, and
expensive, and only the TOLAS method
appears to have sensitivity adequate for
measurement of HCHO at the sub-ppbv
levels characteristic of rural air. Smaller
and less complex real-time HCHO
detectors have also been developed,
based on continuous collection of HCHO
in aqueous solution for subsequent
analysis by colorimetry, fluorescence, or
enzyme-catalyzed fluorescence. These
methods can provide high sensitivity, but
they are subject to some operational
difficulties. Integrated collection and
derjvatization of HCHO with 2,4-
dinitrophenyl-hydrazine can also provide
high sensitivity but is not amenable to
real-time analysis.
This report describes efforts to improve
capabilities for formaldehyde
measurement, by developing two new
real-time HCHO monitors for use in
ambient and indoor environments. The
primary purpose of this work was to
develop prototype monitors based on (a)
a novel spectroscopic method for direct
measurement of HCHO in air, and (b) a
separate wet chemical method involving
collection of HCHO from air into aqueous
solution for subsequent analysis. The
methods were required to be portable,
real-time continuous monitors, having
time response on the order of one
minute, and sensitivity sufficient to allow
monitoring of single-digit ppbv
concentrations of formaldehyde in
ambient air.
Results
Spectroscopic Monitor - A literature
search on the spectral characteristics of
formaldehyde led to the choice of direct
gas^phase "fluorescence' as the basis for
the spectroscopic monitor, because of
the inherently high sensitivity of this
technique. Extensive feasibility
calculations indicated that application of
state-of-the-art optical and electronic
approaches to the fluorescence
technique could achieve sensitivity
sufficient for ambient measurements. For
practical reasons, it was decided to
assemble a prototype formaldehyde
detector by modification of a commercial
fluorescence SO2 detector. The Thermo
Environmental Model 43-S S02 detector
was selected for this purpose because of
its high sensitivity (detection limit for SO2
of « 0.1 ppbv) and its performance in
field studies. Initial modifications to the
43-S involved changing optical filter
elements to match the wavelengths
appropriate for, formaldehyde detection
(260-350 nm excitation; 380-550 nm
emission), and adjusting the gain of the
electronics. The excitation region used
for formaldehyde is well removed from
that (190-230 nm) used for SO2.
Performance of the initial modifications
and testing of the spectroscopic
prototype showed good sensitivity
despite a high background signal.
Subsequently, improved selection of low-
fluorescence optical filters for the emitted
light greatly reduced the background
signal, with the result that a detection
limit of < 30 ppbv formaldehyde was
achieved with the spectroscopic monitor.
Further modifications are planned which
^will increase sensitivity andLJower the
"detection limit.""The" sp'ect'rosc'opic
monitor is fully self-contained, requiring
no external supplies or pumps, and has
time response of about one minute.
Wet Chemical Method - A survey of
existing collection devices for gaseous
HCHO and analytical approaches for
aqueous HCHO indicated the feasibility of
an improved wet chemical monitor based
on the Hantzsch reaction, the cyclization
of a p-diketone, an amine, and an
aldehyde. Spectral data on products of
the Hantzsch reaction suggested that
sensitivity mjight be improved by use of
intense 254 nm light from an Hg lamp for
excitation, rather than the 410 nm light
used previously. Lab tests confirmed that
sensitivity was increased by over a factor
of three with 254 nm excitation. Using
this approach with a highly sensitive
commercial fluorometer achieved
detection limits of 0.02 nM aqueous
HCHO. This high sensitivity for aqueous
formaldehyde thus allowed use of a
simple, reliable, and efficient glass coil
collection for gaseous HCHO, resulting in
a detection limit of 0.2 ppbv HCHO at a
"sample airflowrate of"2"L/min. Selectivity
towards formaldehyde is very high, e.g.,
> 10,000 to 1 relative to acetaldehyde.
The wet chemical monitor was field
tested by continuous operation over a 10-
day period in which indoor and outdoor
air were sampled for alternate half-hour
periods at an occupied residence in
Columbus, Ohio. The monitor operated
very reliably, despite variations in
ambient temperature and power outages
due to severe weather. Approximately
200 hours of indoor and outdoor data
were obtained. Indoor concentrations
ranged from 10 to over 50 ppbv,
averaging over 30 ppbv, and varied with
the degree of ventilation of the home.
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Outdoor concentrations ranged from 0.2
to 7 ppbv, averaging 3.3 ppbv, and varied
with meteorological conditions and time
of day. A comparison was also performed
between 3-hour integrated DNPH data for
formaldehyde in outdoor air and the real-
time data averaged over corresponding
intervals. This comparison showed close
agreement between the two methods,
even though the range of concentrations
compared was very Ipw, i.e., 1 to 5 ppbv.
Conclusions and
Recommendations
Two new continuous, real-time
monitors for formaldehyde in air have
been developed. One is a spectroscopic
method employing gas-phase
fluorescence which has been developed
to the prototype stage, exhibiting a
detection limit of < 30 ppbv
formaldehyde with one-minute time
response. The other employs aqueous
scrubbing and subsequent continuous
analysis of the collected formaldehyde.
Gaseous and aqueous detection limits of
this device are 0.2 ppbv and 0.02 pM,
respectively, with time response similar
to that of the spectroscopic device.
The aqueous scrubbing/analysis
method was shown in extensive field
testing to be capable of continuous
monitoring of gaseous formaldehyde at
concentrations characteristic of both
indoor and outdoor air. This method also
provided data on formaldehyde in
outdoor ambient air which agreed well
with simultaneous data from
dinitrophenylhydrazine (DNPH) impinger
sampling, even- at concentrations as low
as 1 ppbv. The spectroscopic method
shows promise for detecting these low
concentrations as well, by means of
further modifications to the present
prototype.
It is recommended that the gas-phase
fluorescence method originated here be
explored further, by additional
modifications to the prototype monitor.
Additional modifications should include
further reduction of background
fluorescence by selection of optical
filters, and increasing excitation light
intensity by increasing the flashlamp
output. Detection limits below 5 ppbv
should ultimately be achievable with the
spectroscopic approach. Improvements
in the wet chemical method are also
possible, by reducing the reagent
background signal. Two approaches
suggested are greater purification of
reagents, and improved selection of
optical filters used in the fluorometer.
It is further recommended that both
monitors be considered for deployment
in monitoring networks. The capability
and reliability of the wet chemical method
for continuous 'field measurement of
ambient HCHO have been demonstrated
in this study. Little maintenance of the
wet chemical method is required beyond
replenishment of simple aqueous
reagents. The spectroscopic HCHO
monitor should be especially useful in
network operations, because of its fully
self-contained design.
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Thomas J. Kelly and Russell H. Barnes are with Battelle Columbus Division,
Columbus, OH 43201-2693.
William A. McClenny is the EPA Project Officer (see below).
The complete report, entitled "Development of Real-Time Monitors for Gaseous
Formaldehyde," (Order No. PB91-126 029/AS; Cost: $17.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4850
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
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S3-90/088
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