RESUMEN
We show a practical sequential injection/zone fluidics-based analyzer that measures waterborne arsenic. The approach is capable of differentiating between inorganic As(III) and As(V). The principle is based on generating AsH3 from the sample in a confined chamber by borohydride reduction at controlled pH, sparging the chamber to drive the AsH3 to a small reflective cell located atop a photomultiplier tube, allowing it to react with ozone generated from ambient air, and measuring the intense chemiluminescence that results. Arsine generation and removal from solution results in isolation from the sample matrix, avoiding the pitfalls encountered in some solution-based analysis techniques. The differential determination of As(III) and As(V) is based on the different pH dependence of the reducibility of these species to AsH3. At pH < or =1, both As(III) and As(V) are quantitatively converted to arsine in the presence of NaBH4. At a pH of 4-5, only As(III) is converted to arsine. In the present form, the limit of detection (S/N = 3) is 0.05 microg/L As at pH < or =1 and 0.09 microg/L As(III) at pH approximately 4-5 for a 3-mL sample. The analyzer is intrinsically automated and requires 4 min per determination. It is also possible to determine As(III) first at pH 4.5 and then determine the remaining As in a sequential manner; this requires 6 min. There are no significant practical interferences. A new borohydride solution formulation permits month-long reagent stability.
RESUMEN
In many experiments, a flow-through heating arrangement is needed to reduce reaction time. Often the reaction conditions require inertness of the wetted material. Heated reactors based on polymeric tubing, notably PTFE, are the most common, and such reactors are typically used in a manner in which they are put in a heated bath or an otherwise thermally conductive potting in which a heater and a temperature sensor are embedded for heating and temperature control. Polymeric tubes are poor conductors of heat; as such, most reactors of this type have very poor energy utilization. We describe here heated flow-through reactors where a wire runs through the entire length of the tubular reactor and where the wire is directly electrically heated. The wire may or may not be electrically insulated. If the exterior of the tube is well insulated, the energy efficiency of such a reactor in heating the fluid of interest is nearly unity. This makes it most suitable for battery-powered applications. If an appropriate wire with a significant temperature coefficient of resistance is chosen, monitoring the current through the device at constant applied voltage indicates the effective mean temperature of the device and thus allows effective temperature control without an additional sensor/controller with essentially instantaneous response. Temperature control within +/- 0.4 degrees C at a mean temperature of 65 degrees C and within +/- 0.9 degrees C at 87 degrees C have been achieved.
RESUMEN
We present a new fully automated instrument for the measurement of acid gases and soluble anionic constituents of atmospheric particulate matter. The instrument operates in two independent parallel channels. In one channel, a wet denuder collects soluble acid gases; these are analyzed by anion chromatography (IC). In a second channel, a cyclone removes large particles and the aerosol stream is then processed by another wet denuder to remove potentially interfering gases. The particles are then collected by one of two glass fiber filters which are alternately sampled, washed, and dried. The washings are preconcentrated and analyzed by IC. Detection limits of low to subnanogram per cubic meter concentrations of most gaseous and particulate constituents can be readily attained. The instrument has been extensively field-tested; some field data are presented. Results of attempts to decipher the total anionic constitution of urban ambient aerosol by IC-MS analysis are also presented.
