The Photoacoustic Mercury Analyser and Antarctica
Updated 22nd April 2015
Mount Horseshoe and old reversed footprints.
I joined the Department of Scientific and Industrial Research in 1974 where I initially worked on Atomic Spectrometry. This was mostly atomic absorption spectrometry and atomic emission spectrometry of solutions to determine their elemental composition. Later I became interested in other applications of atomic absorption spectrometry. This included the measurement of low concentrations of arsenic and mercury. Arsenic determination was by the release of the hydride into a heated quartz cell which produced arsenic vapour. Arsenic vapour concentration was measured by atomic absorption from an electrodeless discharge arsenic lamp. Mercury determination was by the release of cold mercury vapour into a long pyrex cell at room temperature. In both cases the atomic absorption was measured along the axis of the cells. My colleague Dr Byron Weissberg specialised in the latter method to determine mercury in rainfall.
I had an interest in the photoacoustic effect and I applied it to the visible and infrared absorption spectrometry of solids. I built a number of photoacoustic cells and the associated electronics for recording pulse shapes and amplitudes on a chart recorder. A Xenon strobe light source was used for illumination. A cell was also briefly mounted in a FTIR spectrometer. The signal from the microphone was able to be directly processed by the instrument electronics to obtain a spectrum. The setup could be used for FTIR spectrometry of opaque solids. Byron suggested I try mercury vapour in one of my cells with a mercury light source and it worked. Gradually I developed the technique and the geometry of the cells until it could respond to less than 1 picogram of mercury present in the sample cell. This proved the be sufficiently sensitive for routine trace mercury determinations. The prototype instrument is shown at right.
The Photoacoustic Effect
When a sample is heated it absorbs energy and expands. When the heat source is removed the sample cools and contracts as energy is conducted and radiated away. By turning the source of energy on and off rapidly, say at 50Hz, the expansion and contraction also occurs rapidly and is heard as sound. In the case of solids the rapid heating and cooling affects a boundary layer of air causing it to expand and contract making sound. In gaseous samples absorption of modulated light causes expansion and contraction of the gas at the modulation frequency. This signal is easily detected as sound by a standard electret microphone. A suitable electret microphone costs about $3.50.
Electronic filtering discriminates against ambient noise and a second microphone and cell cancels more external noise. Amplification and rectification results in an analog signal with an amplitude proportional to the sample concentration. A sample and hold circuit produces a stable output for measurement with a digital volt meter.
The photoacoustic mercury detector uses a germicidal mercury light source modulated at 50Hz. The photoacoustic cell is illuminated transversely and is of a similar length to this light source. A microphone is positioned in a branch at one end of the cell. Cigarette filters partially block each end while allowing the passage of the carrier gas and mercury vapour. The filters allow some pressure to be transmitted to the microphone as atomic absorption takes place. The presence of a diatomic carrier gas such as nitrogen or air quenches any fluorescence, producing heat, and therefore sound.
The references below cover the development and use of this technique.
The Photoacoustic Mercury Analyser was used for several seasons in Antarctica. Air and snow were sampled to establish baseline levels of mercury. Changes in the future may arise from inputs of industrial and volcanic pollutants. The instrument was able to operate at extremely low temperatures, as well as in a laboratory setting
I am trying to build a low cost analyser using basic components. The photos at right show my current progress.
A model AM4011 electret microphone or similar from Jaycar is used as detector. It costs $3.50. This will perform the role of an expensive electron multiplier or silicon UV detector in more conventional instruments. A LM324 quad operational amplifier at $2.50 provides all of the signal processing required for amplification, filtering and an analog output to a data logger.
The lamp in this version is an aquarium steriliser used for cleaning water in aquariums. In this application we use just the lamp, enclosed in it's plastic cell. This can be purchased complete as a spare part. The plastic cell is used directly as the photoacoustic cell. This arrangement avoids exposure of the user to UV-C light. The cell interior is painted white. The cell, complete with filters and microphone, is sealed with RTV and epoxy resin This geometry is still under development. A simple silica cell may be better as the signal amplitude varies inversely with cell volume. The optimum volume just holds the mercury vapour sample long enough for measurement, without dilution.
The lamp will be driven by a solid state switching circuit at 50Hz. Note that normally the lamp is on twice per cycle which means that light is produced at 100Hz. Here it is on just once per cycle giving a pulsed output at 50Hz. The distinct on-off cycle produces a stronger sound. The lamp only needs to be on long enough for the change in pressure to stabilise so the duty-cycle can be reduced. The exit filter is replaced with closed cell foam. A 5mm hole in the foam is filled with an embedded cigarette filter. The entrance port shares the microphone and a plastic carrier gas elbow, which is a standard item from an aquarium supplier. The microphone connects to the electronics described above. The carrier gas elbow also contains a filter.
The carrier gas flows past a septum where mercury vapour can be admitted. The carrier gas can be nitrogen or air. With air a reducing vapour needs to be added to prevent oxidation of the mercury vapour in the presence of UV light. The reducing vapour can be a hydrocarbon or hydrogen gas. In this application hydrogen, at low volume, is added to the air supplied from an aquarium pump. A reservoir and filter is used to remove the pump noise. Hydrogen is derived from a fuel cell running as an electroliser. Only small quantities are needed so the cell is run at a much reduced current.
Photoacoustic mercury analyser working in igloo laboratory, Polar plateau.
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Photoacoustic cell for solids.
Photoacoustic mercury analyser prototype.
Photoacoustic mercury analyser final prototype.
Photoacoustic mercury analyser final prototype.
Open photoacoustic cell with germicidal lamp.
White painted photoacoustic cell.
Photoacoustic cell with microphone and filters. Sampling air for mercury at Lake Vanda.
Clean power for lab.
Photoacoustic mercury analyser commercial instrument.
Entrance to snow sampling pit.
Steps in snow sampling pit.
Spence on top of Mount Horseshoe.