A dual-beam detection strategy with automatic balancing is described for ultrasensitive spectroscopy. Absorbances of 2 × 10(-7) Hz(-½) in free-space configurations and 5 × 10(-6) Hz(-½) in fiber-coupled configurations are demonstrated. With the dual-beam technique, atmospherically broadened absorption transitions may be resolved with InGaAsP, AlGaAs, and AlGaInP single-longitudinal-mode diode lasers. Applications to trace measurements of NO(2), O(2), and H(2)O are described by the use of simple, inexpensive laser and detector systems. Small signal gain measurements on optically pumped I(2) with a sensitivity of 10(-5) are also reported.
Abstract.Recommended best practices in monitoring of product status during pharmaceutical freeze drying are presented, focusing on methods that apply to both laboratory and production scale. With respect to product temperature measurement, sources of uncertainty associated with any type of measurement probe are discussed, as well as important differences between the two most common types of temperature-measuring instruments-thermocouples and resistance temperature detectors (RTD). Two types of pressure transducers are discussed-thermal conductivity-type gauges and capacitance manometers, with the Pirani gauge being the thermal conductivity-type gauge of choice. It is recommended that both types of pressure gauge be used on both the product chamber and the condenser for freeze dryers with an external condenser, and the reasoning for this recommendation is discussed. Developing technology for process monitoring worthy of further investigation is also briefly reviewed, including wireless product temperature monitoring, tunable diode laser absorption spectroscopy at manufacturing scale, heat flux measurement, and mass spectrometry as process monitoring tools.KEY WORDS: heat flux measurement; mass spectrometry; pressure measurement; process analytical technology; temperature measurement; tunable diode laser absorption spectroscopy.
Three-body rate coefficients for reactions of H + 0 2 + M where M = Nz and HzO have been measured in a high-temperature flow reactor as a function of temperature up to 750 K. Room-temperature rate coefficients for M = Ar have also been quantified. The rate coefficients are measured by the flash photolysis method where atomic hydrogen is produced by excimer laser photolysis of precursor molecules (HzS or HzO) and then probed by either laser-induced fluorescence or resonance absorption. The rate coefficient measurements for nitrogen (298-580 K) can be described by the Arrhenius expression exp[(825 f 130)/T] cm6 molecuk2 s-l. Rate coefficients measured for water are k~~o ( S 7 5 ) = (1.2 f 0.3) X k~,o(650) = (1.0 f 0.3) X 10-31, and k~~o ( 7 5 0 ) = (1.22;) X cm6 molecule-2 s-'. Room-temperature measurements for Ar gave a rate coefficient of kk(298) = (2.1 f 0.2) X 10-32.= (2.9 f 0.8) X
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