Appl. Phys. B 85, 285–288 (2006)
Applied Physics B
DOI: 10.1007/s00340-006-2295-x
Lasers and Optics
m. angelmahru
a. miklós
p. hess
Photoacoustic spectroscopy of formaldehyde
with tunable laser radiation
at the parts per billion level
Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253,
69120 Heidelberg, Germany
Received: 31 March 2006/Revised version: 24 April 2006
Published online: 24 June 2006 • © Springer-Verlag 2006
ABSTRACT Selective and sensitive detection of formaldehyde
(CH2 O) was achieved using a grazing-incidence optical parametric oscillator (GIOPO), pumped at high repetition rate, as
a light source for photoacoustic spectroscopy. The photoacoustic spectrum of formaldehyde was measured in the range from
2785 to 2840 cm−1 and an absorption line at 2805.0 cm−1
was selected for detection. Concentrations down to 20 ppbv
(parts per billion by volume) formaldehyde in nitrogen were
recorded. The detection limit determined by background fluctuations was 3 ppbv (S/N = 1) for 3 s lock-in time constant and
3 min acquisition time. The 2805.0 cm−1 absorption line of the
ν1 vibrational mode was chosen because of the absence of interference with water and carbon-dioxide bands. This allowed
the direct detection of formaldehyde in laboratory air without
filtering.
PACS 42.62.Fi;
1
formaldehyde are of the order of a few parts per billion by
volume (ppbv). Over the mid-Atlantic formaldehyde concentrations in the range of 0.1 – 0.3 ppbv have been recorded [9],
while urban concentrations tend to be substantially higher,
typically 5 – 40 ppbv [10, 12]. Besides natural emission of
formaldehyde, the incomplete combustion of fuel, especially
reformulated fuels, such as methanol with relatively high oxygen content, is one of the major man-made formaldehyde
sources [13]. This is giving rise to concerns about formaldehyde in the atmosphere, since methanol is expected to be one
of the preferred sources of energy in the near future. Recently,
methanol has been used extensively as a substitute fuel in several countries, especially Japan and Brazil. Thus, the concentration in the atmosphere should be observed very carefully.
2
Experimental
2.1
Setup
42.68.Ca; 42.65.Yj; 07.07.Df
Introduction
Formaldehyde (CH2 O) is often considered as one
of the most dangerous toxins that can be found in living
space. Since cheap chipboards, clothes, dyes, and carpets all
emit formaldehyde, this gas has been responsible for serious problems of indoor air quality for several years already.
Even at low concentrations, formaldehyde causes health problems and may be associated with various diseases, such as
bronchial asthma, atopic dermatitis and sick building syndrome [1–3]. Additionally, there is still discussion about the
potential of formaldehyde to cause cancer [4]. In spite of all
attempts to reduce the concentration of formaldehyde in living spaces, the number of patients allergic to formaldehyde is
still increasing in many industrial countries with air pollution
problems [5, 6].
Besides indoor air quality, an additional problem with
higher concentrations of formaldehyde in the atmosphere
is being encountered, because formaldehyde gives rise to
other toxic compounds by generating free radicals, resulting
in photochemical smog [7, 8]. Typical background levels of
u Fax: +49 6221 544255, E-mail: [email protected]
The experimental setup, with a grazing-incidence
optical parametric oscillator (GIOPO), a periodically poled
lithiumniobate crystal (PPLN), and a diode-pumped
Q-switched Nd:YAG laser of ∼ 6 W average power and
a pulse duration of 40 ± 10 ns, has been described in detail
previously [14, 15]. For nonlinear optical wave generation,
five periodically poled domains with a period between 29.0
and 31.0 µm were available in the PPLN crystal. For the
formaldehyde experiments the 29.5 µm domain was chosen. Rough spectral tuning of the GIOPO was performed
by changing the temperature of the PPLN. Fine tuning over
a range of 15 cm−1 was realized by moving the tuning mirror with a micro-stepping motor controlled by a PC. The
maximum power of the idler after the germanium filter was
typically ∼ 65 mW (17 µJ energy per pulse). An iris aperture
was placed 14 cm from the grating and its diameter was adjusted to 3.0 mm to reduce the linewidth of the output idler
beam. The full width at half maximum (FWHM) value of
the line profile of the OPO was 0.10 cm−1 . Two gold-coated
spherical copper mirrors were used to focus the infrared beam
into the photoacoustic (PA) detector. Since the light diffracted
by the iris aperture produced a relatively large background
signal in the PA cell, a second iris aperture was placed in front
of the PA cell. In this way, the voltage of the background signal
could be decreased from about 3 µV to 0.6 µV. The maximum
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Applied Physics B – Lasers and Optics
idler power transmitted through the PA cell was reduced to
45 mW.
The differential PA detector employed was designed for
fast time response, low electronic and acoustic noise, and
high sensitivity. The device had a fully symmetrical design
with dual acoustic resonator tubes sandwiched between two
acoustic filters. The resonator tubes had an inside diameter of
5.5 mm and a length of 40 mm. A more detailed description
of the PA detector can be found elsewhere [16]. To reduce adsorption and desorption processes, the surface of the PA cell
was coated with a 30 µm thick teflon layer. Despite the surface
coating, a continuous mass flow had to be applied to reduce
adsorption–desorption effects.
The repetition rate of the Nd:YAG pump laser at 1064 nm
was adjusted to match the longitudinal acoustic resonance of
the resonator tube near 3.8 kHz. The Q-factor of the acoustic
amplification in the teflon-coated PA cell at ambient pressure
was determined to be 31 ± 2.
The preamplified PA signal was measured using a digital
lock-in amplifier (Stanford model SR850) and recorded with
a PC. The time constant of the lock-in amplifier was 1 s for
the measurement of infrared spectra and 3 s for the calibration
measurements.
2.2
Gas supply
By using a certified gas mixture of 11.1 ppmv
formaldehyde in nitrogen and pure nitrogen, mixtures with
small concentrations of formaldehyde down to 20 ppbv could
be prepared by using suitable mass flow controllers. In addition, online measurements of formaldehyde in laboratory air
were also possible with this setup. In these experiments the
laboratory air was sucked through the PA cell by a vacuum
pump. To provide a constant gas flow with low acoustic noise,
a buffer volume, an acoustic filter and a mass flow controller
were placed between the PA cell and the pump.
2.3
Measurement method
The main purpose of the present work was to
develop a method for the detection of formaldehyde in laboratory air. Since strong interference with water and carbon
dioxide absorption lines was expected in the selected spectral region, the absorption spectra of these molecules had to be
taken into consideration when searching for an interferencefree formaldehyde line. Therefore, formaldehyde and water
spectra were measured in the range between 2785 cm−1 and
2840 cm−1 . To record the formaldehyde spectrum a concentration of 10 ppmv with a constant mass flow of 300 sccm
(standard cubic centimeter per minute) was studied at ambient pressure. Rough tuning of the GIOPO was performed
– as mentioned above – by changing the temperature of
the PPLN. To measure the spectrum of formaldehyde in
the range 2785– 2840 cm−1 , the temperature of the PPLN
crystal was increased from 50 ◦ C to 90 ◦ C in steps of two degrees. Fine tuning of the GIOPO over a range of 15 cm−1 ,
at a fixed temperature, was performed by moving the tuning mirror with the micro-stepping motor. The separately
recorded overlapping spectra, which were normalized to
45 mW, were combined to obtain a continuous spectrum
of formaldehyde.
The spectrum of water was measured in the same way. To
generate a known amount of water within a gas flow, nitrogen
was bubbled through a vessel of distilled water at room temperature with a mass flow of 300 sccm. The water spectrum
was recorded in the range between 2785 cm−1 and 2840 cm−1
by the method described above.
At a temperature of 64.0 ◦ C of the PPLN crystal the idler
output was centered close to 2805.0 cm−1 with maximum
power. Since in previous investigations, it had been shown
that the shape of the PA spectrum of formaldehyde is sensitive
to the optimum alignment of the GIOPO [14], this was carefully optimized before the formaldehyde measurements were
started.
3
Results
The PA spectrum of 10 ppmv formaldehyde in
nitrogen was measured in the range 2785– 2840 cm1 . The
measured PA spectrum and a calculated spectrum are compared in Fig. 1. The calculated spectrum was obtained by
convolving the HITRAN spectrum of CH2 O with the lineshape function of the GIOPO. The agreement between the
measured and calculated spectra is good. While the overall
structure of both spectra is quite similar, characteristic differences can be seen in the strength of individual absorption
lines.
For formaldehyde detection an absorption line was needed
with negligible interference with water absorption. Therefore,
the photoacoustic spectrum of water vapour was also measured in the same wavenumber range. The measured formaldehyde and water spectra are compared in Fig. 2. The smallest
overlap of a relatively strong CH2 O absorption peak with water lines was found at 2805.0 cm−1 . According to the assignment given in the literature this absorption line belongs to the
ν1 vibrational mode of formaldehyde [17]. The PA signal of
the saturated water vapour (∼ 2.3%) corresponded to 63 ppbv
formaldehyde. Taking into account the ∼ 40% humidity in
the laboratory, the water vapour absorption of the laboratory
air corresponded to an absorption of about 25 ppbv CH2 O at
2805.0 cm1 .
(a) Measured PA spectrum of the ν1 , ν5 and (ν3 + ν6 ) bands of
10 ppmv formaldehyde in nitrogen. (b) Calculated HITRAN spectrum. The
absorption was estimated for a 1 cm path length
FIGURE 1
ANGELMAHR et al.
Photoacoustic spectroscopy of formaldehyde with tunable laser radiation at the parts per billion level
287
noise equivalent (NNEA), can be used to characterize the
performance of the PA detector. The NNEA is given by the
minimum optical absorption coefficient detectable ( S/N = 1)
multiplied by the optical power and divided by the detector
bandwidth [18]. For the lock-in time constant of 3 s used in the
present experiments the NNEA of the acoustical system was
6.2 × 10−9 W cm−1 (Hz)−1/2 .
Figure 4 provides a comparison of the spectra obtained
with pure formaldehyde (10 ppmv) and water (∼ 2.3%) samples with the spectrum measured for laboratory air in the spectral range between 2798 cm−1 and 2811 cm−1 . From the signal at 2805.0 cm−1 a concentration of approximately 50 ppbv
could be estimated.
Measured PA spectrum of 10 ppmv formaldehyde and ∼ 2.3%
water. The selected absorption line at 2805.0 cm−1 is indicated by an arrow
FIGURE 2
Additionally, the corresponding CO2 spectrum was calculated with HITRAN by assuming a CO2 concentration of
300 ppmv. For this concentration of CO2 in air no relevant absorption was found at 2805.0 cm−1 .
Calibration measurements were performed at 2805.0 cm−1
by using the calibrated gas mixture. The results obtained from
20 ppbv to 250 ppbv formaldehyde concentrations are presented in Fig. 3. One hundred samples were taken within
3 min from the lock-in output and the mean value and deviations are plotted in Fig. 3. The minimum concentration that
can be detected by the photoacoustic detector is determined
by the S/N ratio, where S is the PA signal and N the noise
level. By regarding the standard deviation of the measured PA
signal in pure nitrogen as noise (background), a sensitivity of
3 ppbv formaldehyde at S/N = 1 was found. This assumption
is reasonable, because the background of the PA cell, which
was measured several times during calibration measurements,
was essentially constant.
The sensitivity of the detector was determined to be D =
1.1 × 10−9 Wcm−1 for S/N = 1. Since the noise power density can be considered as constant within the detector bandwidth, a laser-independent quantity, namely the normalized
Photoacoustic calibration curve measured for different
formaldehyde–nitrogen mixtures between 20 ppbv and 250 ppbv
FIGURE 3
4
Review of previous works
In the early photoacoustic studies of trace gas analysis pulsed UV lasers have been applied to identify spectral
signatures of the formaldehyde molecule between 302.5 nm
and 303.7 nm [19]. Despite the fact that quite sensitive detection could be demonstrated, current investigations concentrate
on the application of tunable IR lasers, allowing the excitation
of characteristic narrowband and interference-free absorption
lines to improve selectivity in mixtures and to avoid photochemical interferences.
Recently a novel compact formaldehyde sensor was
presented that is based on quartz-enhanced photoacoustic
spectroscopy (QEPAS) [20]. By using an interband cascade laser (ICL) operated at 78 K with 12 mW output power
the minimum detectable concentration of formaldehyde at
2832.483 cm−1 was determined as 600 ppbv for a lock-in
time constant of 10 s. The corresponding sensor consisted
of a small quartz tuning fork with a very small volume (of
∼ 1 mm3 ) and a very high Q-factor of 16 725. The overall cell size was comparable to that of the present photoacoustic cell. Since the resonance frequency was about
32.7 kHz, acoustic or vibrational noise suppression was not
required. The NNEA of the QEPAS system was meas-
FIGURE 4 (a) Comparison of spectra measured for formaldehyde
(10 ppbv) and water (∼ 2.3%) in nitrogen with (b) spectrum detected in
ambient laboratory air
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Applied Physics B – Lasers and Optics
ured as 2.2 × 10−8 W cm−1 (Hz)−1/2 in comparison with
6.2 × 10−9 W cm−1 (Hz)−1/2 achieved with the present setup.
Note that the NNEA of the QEPAS could only be realized at
reduced pressures, e.g., 200 Torr, where optimum performance was recorded and a strong dependence of the sensitivity
on the V-T relaxation rate of the particular species in the host
gas was observed.
The same research group developed also a differencefrequency-based tunable absorption spectrometer with a diode-laser-pumped, fiber-coupled, periodically poled LiNbO3
radiation source and 2f wavelength modulation [21, 22]. With
this spectrometer detection of atmospheric formaldehyde
was performed at 2831.6417 cm−1 and 2831.6987 cm−1 with
a replicate precision of 0.24 ppbv using a 100 m Herriott cell.
In these experiments a rather complicated nonlinear laser light
source has been used, as in the present work, but in addition,
a cryogenically cooled InSb or mercury cadmium telluride
(MCT) detector was needed to measure the light intensity
after the multipass cell.
5
Summary
With the setup described here, extended spectra of
the species of interest can be measured in the very wide spectral range of the GIOPO from 2670 cm−1 to 4560 cm−1 with
a narrow linewidth. At 2805.0 cm−1 a strong absorption line
of formaldehyde with almost no interference with H2 O and
CO2 bands was found, suitable for atmospheric measurements
without using gas filters. At this excitation frequency calibration measurements were performed and a detection sensitivity
for formaldehyde of 3 ppbv ( S/N = 1) was established. Employing a compact photoacoustic detector the device needs no
cryogenic cooling and works at normal pressure. By increasing the idler power of the GIOPO and further improving the
setup, the sub-ppbv level can be reached with this GIOPObased photoacoustic sensor.
With the expected future development of thermoelectrically cooled ICLs with substantially higher laser power
(>100 mW), the combination of such a light source with stateof-the-art photoacoustic monitoring, which scales linearly
with laser power, will provide a versatile compact trace-gas
sensor, suitable for field applications, working at the sub-ppbv
level without cryogenic cooling.
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