The vapor pressure of water in equilibrium with sorption sites within a Nafion membrane is given by log P(WN) = -3580/T + 10.01, where P(WN) is expressed in Torr and T is the membrane temperature, in kelvin. The efficiency of dryers based on selective permeation of water through Nafion can thus be enhanced by cooling the membrane. Residual water in effluents exceeds equilibrium levels if insufficient time is allowed for water to diffuse to the membrane surface as gas passes through the dryer. For tubular configurations, this limitation can be avoided if L > or = Fc(10(3.8)/120 pi D), where L is the length of the tubular membrane, in centimeters, Fc is the gas flow rate, in mL/ min, and D is the diffusion coefficient for water in the carrier gas at the operating temperature of the dryer, in cm2/s. An efficient dryer that at room temperature dries gas to a dew point of -61 degrees C is described; the same dryer maintained at 0 degrees C yields a dew point of -80 degrees C and removes water as effectively as Mg(ClO4)2 or a dry ice/acetone slush. The use of Nafion membranes to construct devices capable of delivering gas streams with low but precisely controlled humidities is discussed.
We report a pilot study of high-precision differential isotope ratio measurements made on replicate samples of pure carbon dioxide using three instruments of identical manufacture. Measurement protocols were designed to explore the effects of sample size, ion source conductance, and inlet changeover equilibration time on the raw measurements. Our goal was better understanding of factors that influence these measurements in order to establish procedures for highly reproducible and accurate determinations of Reference Material (RM) isotopic compositions. Evaluation and modeling of reported data illuminated effects consistent with two instrumental memory sources--one short-lived (t((1/2)) approximately 10 s) and the other long-lived (t((1/2)) approximately 6-10 min), uncompensated by normal background measurements--that can significantly influence measurements made by the dual inlet method. These biases, proportional to the difference in isotopic compositions between the measured sample and reference gases, decrease in magnitude with increasing sample size, source conductance, and equilibration time. We observed biases as high as 0.1 per thousand per 10 per thousand difference between sample and reference gases. These memory sources may be responsible for measured delta(13)C values of RMs generally being highly reproducible within any single laboratory but less reproducible among independent laboratories. The magnitude of the bias is consistent with the ranges of delta(13)C values reported in prior laboratory intercomparisons. Uncertainties are most likely due to high and variable long-lived memory among the instruments tested.
Formation of HCO2+ from CO2 and background H2O in isotope ratio mass spectrometers has been examined in detail. The process is troublesome because its product is not resolved from 13C16O2+. The resulting, artifactual enhancement of the mass 45 ion current (and analogous enhancement of the mass 46 ion current by transfer of hydrogen to mass 45 species) can cause systematic errors in analyses of 13C based on measurement of ion current ratios in the mass spectrum of CO2. Such errors are neutralized when isotopic analyses are based on differential comparisons in which ion currents and background water levels are precisely equal during admission and ionization of both sample and standard gases. In continuous-flow systems, however, that requirement is generally not met. The resulting systematic error is proportional to the 18/44 ion current ratio. When the widely used MAT252 mass spectrometer is tuned to yield maximum sensitivity, the constant of proportionality is 26 +/- 2/1000 (i.e., the error will be 0.26/1000 if the mass 18 ion current is 100 times smaller than that at mass 44). Errors can be reduced 5-fold when the ion-source residence time of CO2+ is decreased by use of stronger ion-extraction potential gradients. Under those same conditions, sensitivity is decreased by 60%. For operation at highest sensitivity, carrier gas dew points on the order of -70 degrees C are required to obtain errors < or = 0.1/1000 for samples yielding mass 44 ion currents of 10 nA. Carrier gas dew points < or = -80 degrees C are conveniently reached by use of a Nafion dryer operated at approximately 0 degree C.
The invasive zebra mussel (Dreissena polymorpha) has become an accepted biomonitor organism for trace elements, but it has yet to be studied along the Lake Michigan shoreline. Likewise, the relationships between tissue concentrations of elements, organism size, and sediment concentrations of elements have not been fully explained. The present study found that a variety of allometric variables such as length, dry tissue mass, shell mass, organism condition indices, and shell thickness index were useful in explaining intrasite variability in elemental concentrations. The flesh condition index (grams of tissue dry mass per gram of shell mass) explained variability at the most sites for most elements. Once allometric intrasite variability was taken into account, additional significant differences were found between sites, although the net effect was small. Significant positive relationships between sediment and tissue concentrations were found for Pb and Zn, with a significant negative relationship for Cd. It was also found that Cu and Zn concentrations in tissues increased significantly along the shoreline in the southeasterly direction, whereas Hg increased in a northwesterly direction. Opportunistic sampling found that zebra mussels accumulate significantly higher concentrations of nearly all elements analyzed compared to Asian clams (Corbicula fluminea) at the same site. The present study demonstrates the need to fully explain natural sources of variability before using biomonitors to explain spatial distributions of trace elements.
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