New Dimensions In Gas Phase Analysis
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290 300 310 320 330 340 W A V E L E N G T H nm You are looking at a Second Derivative Absorption Spectrum of Formaldehyde vapor at a concentration of 11 PPM. The following features of a derivative Spectrum are worth noticing: 1. A peak is located at the wavelength where the compound exhi bits narrow band absorption. Therefore, the spectrum displays that unique characteristic physical property of the specific compound. 2. The height of any peak is linear with the concentration of the compound, this linearity holding from concentrations of percent to parts per billion. 3. The derivative signal is bi-polar. It does pass through zero at different wavelengths. The derivative spectrum of a mixture of gasses is the algebraic sum of each single gas spectrum at any particular wavelength. Therefore, absorption of two or more compounds in the same wavelength region which would create inseparable interference in direct absorption spectroscopy can often be resolved in the derivative mode by choosing a wave length where one compound's derivative signal goes through zero. Write for detailed information regarding the SM4 and also des criptions of special purpose single component analyzers using the same technique of derivative spectroscopy.
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LSI · SPECTROMETRICS One Inverness Drive Englewood, Colorado 80110 303-770-3300
The SM4 is an electro-optical gas analyzer which is used to specifically identify and ac curately measure the concentration of indivi dual gases in a complex mixture. Identification is accomplished by sensing a physical property of the compound — narrow-band absorption of ultraviolet or visi ble light. A large number of compounds exhi bit characteristic narrow-band absorption spectra either in the uv or visible spectral region and can therefore be detected and measured using the SM4 Analyzer. Such com pounds include most aromatic hydrocarbons, aldehydes, some metals in a vapor state and many common air pollutants. Each individual compound absorbs radiation at different wavelengths and therefore "writes its signa ture" in its second derivative absorption spectrum. Measurement sensitivity to this narrow band absorption is achieved using second deriva tive spectroscopy. This technique provides high sensitivity to changes in spectral curva ture over narrow (.1 to 5 nm) wavelength regions due to narrow band absorption of radiation but little or no sensitivity to broad curvature changes resulting from broad band absorption. The result is a highly specific analysis tool with a quantitative output linear over several orders of magnitude. Usable measurement range is variable from pure gas to parts per billion concentrations with an electronic dynamic range of 1000 : 1 and an optical dynamic range of 25,000 : 1. A variety of accessory hardware is designed to make the SM4 a versatile laboratory tool, readily adaptable to your specific analysis re quirement. ,
CIRCLE 149 ON READER SERVICE CARD A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 8, JULY 1974 • 695 A
