product review
The Sound of Compounds With the right application, photoacoustic spectroscopy is a chemist’s dream. Judith Handley
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t began with Alexander Graham Bell’s cigar smoke, so the story goes. On a cloudy day in Paris in the 1880s, a beam of sunlight filtered through, hit a glass tube filled with cigar smoke, and generated musical tones. Today, scientists apply the same photoacoustic (PA) technique to determine Aspergillus flavus, a toxic corn fungus, and to noninvasively measure blood glucose levels. While photoacoustic spectroscopy (PAS) is fast, sensitive, nondestructive, and selective and has a wide dynamic range, Murty Neti of California Analytical Instruments remarks that the technique in the United States was “more or less a lab curiosity,” at least through the 1970s. In some respects, PAS is a chemist’s dream. Most analytical techniques require some sample preparation, but John McClelland of MTEC Photoacoustics says that PAS “allows you to just toss them into a sample holder and measure their [PAS] absorption.” Depending on the instrument, PAS can analyze almost any physical state, whether the material is shiny, transparent, opaque, or light scattering. Samples can be quite small and analyzed qualitatively, or they can be quantitatively compared with known reference standards. The Bell story illustrates most of PAS’s basic principles. A light source has to be chosen with an appropriate wavelength for absorption by the analyte, and the light must also be interrupted. An intermittent beam causes variations in thermal energy. Consequent pressure changes are detected as
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product review
Table 1. Representative manufacturers of PAS detectors and instruments. Manufacturer
Bruker Optics, Inc. 19 Fortune Dr. Manning Park Billerica, MA 01821 978-439-9899
California Analytical Instruments, Inc. [Distributor for INNOVA AirTech Instruments (Denmark)] 1238 W. Grove Ave. Orange, CA 92865 714-974-5560
Desert Research Institute 2215 Raggio Pkwy. Reno, NV 89512 775-674-7023
Digilab LLC 68 Mazzeo Dr. Randolph, MA 02368 781-794-6400
URL
www.optics.bruker.com
www.gasanalyzers.com
photoacoustic.dri.edu
www.digilab-ftir.com
Description
Linear and step-scan FT-IR; PAS accessory available; many models; vacuum systems; general PAS and depth-profiling; linear-scan velocities continuously variable from 100 Hz to 320 kHz; step-scan phase or amplitude modulation frequencies of 5–1500 Hz; phase modulation amplitudes up to ±20 HeNe possible
PA gas analyzers; temperaturePA aerosol spectrometers and pressure-compensated; linear range equals detection limit ⫻ 105; calibration needed at ~6- to 12month intervals
acoustic signals by a microphone—“the heart of the instrument” in Neti’s view. But there are “fundamental differences in systems,” according to Pat Arnott of the Desert Research Institute (DRI), that depend on the design and operation of the instrument. This article discusses PAS instrumentation for three general applications: gases and aerosols in environmental air, depth-profiling in solids, and photoinduced processes that occur on a micro- to nanosecond timescale. Systems for other applications are also available. Table 1 lists representative commercial PAS instruments and detectors. PAS instruments come in many forms: complete systems, without a light source, or as an accessory integrated with a commercial spectrometer. Four different detectors are available, and they are chosen on the basis of the applications or the sample’s physical state. Further details are available from the companies listed in Table 1. All these instruments are described as “easy to use,” and some are turnkey. According to the manufacturers, undergraduates with a science background have been able to operate some of the instruments. Data acquisition and analysis software packages help translate signals, but generally some education and experience are important to ensure correct interpretation of the data, say the experts.
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Checking out the air PAS systems have been used for environmental air monitoring and hazardous chemical emissions detection. Complete PAS systems are available for monitoring gases or black carbon aerosols. PAS systems for gas analysis typically contain a nondispersive IR (NDIR) light source. A mechanical “chopper” interrupts the beam incident on the gas molecules, leading to variations in the absorbed energy. As a result, the speed and pressure of the molecules fluctuate, sending acoustic waves to the microphone. NDIR gas analysis instruments are equipped with up to five optical filters, each restricting light to a wavelength specific for a single analyte, says Neti. Optical filters also subtract contaminating gas and water vapor absorbances from the total signal, thus removing signals that might overwhelm the analyte spectrum. Multicomponent gas samples can be analyzed by a PAS gas detector integrated into a rapid-scan FT-IR, says Neti. With an interferometer on the FT-IR, there’s no need for filters and a chopper. The interferometer is lighter and more portable than systems with a chopper, and it delivers signals with a better S/N. One overall advantage of PAS gas analyzers is their smaller sample size. Neti says that PAS-NDIR instruments, for example, offer the same sensitivity with
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Linear and step-scan FT-IR; PAS accessory available
a 3-cm3 sample cell as a traditional NDIR system with a 25-m cell. PAS also has a wide dynamic range for gas detection, ranging from a few parts-perbillion to 105 ppb. With solid-state lasers, very small and portable PAS instruments can be constructed. Arnott built a system with a palm-size solid-state neodymium-doped yttrium–lithium–fluoride laser. The instrument also uses electronic modulation instead of a chopper, which helps keep the instrument small and easy to operate and provides a better S/N. This field instrument generates 1047-nm light to measure black carbon aerosol particles in the atmosphere. The acoustic signal process for an aerosol is a little different from that used for a gas. As the
