The metal under study was evaporated on the crystal disk to form circular electrodes 5 mm in diameter and 150-nm thick. The filtering device protected the probe from coarse dust and mechanical damage. The field station consisted of an outdoor mast supporting the probes, meteorological equipment, and connective cabling, and the indoor portion consisted of a signal conditioning box, with power supply and multiplexer, and a desktop computer for data acquisition and transfer. This system had a resolution of ± 10 ng/cm2. A crystal with gold electrodes exposed for 12 days in an urban atmosphere showed a mass increase of 850 ng/cm2. A copper electrode exposed for approximately the same length of time showed a mass increase of ~ 2000 ng/ cm2. Analysis of the electrodes in the lab by X-ray photoelectron spectroscopy allowed characterization of the results in terms of salt deposition, water adsorption and desorption, and corrosion effects. (J. Electrochem. Soc. 1.96,143, 839-44)
Determining methylated species of Sn, Pb, and Hg
Investigating mass shifts
Variation in compensated mass (Am_tot) and irreversible mass gain (Am_irr) of a gold electrode vs. time. (Adapted with permission from The Electrochemical Society.)
ment. Freddy C. Adams and Michiel Ceulemans of the University of Antwerp (Belgium) have optimized a purge-andtrap injection-GC/microwave-induced plasma-atomic emission spectrometry (MIP-AES) system to simultaneously perThe methylated species of tin, lead, and form sample preparation and detection mercury can be anthropogenically introduced or can form naturally in the environ- of inorganic mercury and methylated tin, lead, and mercury species in water at trace ment by biomethylation processes. The levels. highly toxic nature of these compounds The ionic species were volatilized from necessitates the development of accurate the sample after ethylation using NaBEt4 and sensitive analytical methods for meain acetate buffer at pH 5 and preconcensuring these compounds in the environtrated on a capillary cryogenic trap. The analytes were desorbed by linear heating of the trap, separated by capillary GC, and detected by MIP-AES. Precision of 2-3% was obtained for tin and lead species, but the poorer precision of 7-8% obtained for mercury species was probably caused by degradation of the species during the linear heating of the trap. Using a 10-mL water sample the researchers obtained detection limits of 0 15 ng/L for methylated tin 0 20 ng/L for methylated lead 0 60 ng/L for methylated mercurv and 2 ne/L for inorganic mercurv (7 Anal Schematic of the purge-and-trap injection-GC/ Atom Sbectrom 1996 11 MIP-AES system. (Adapted with permission from 201-06") The Royal Society of Chemistry.)
Mass shifts in the quadrupole ion-trap mass spectrometer and the quadrupole mass filter have perplexed researchers, and explanations of the mechanisms for this phenomenon have been offered. John F. J. Todd and Wenjun Mo of the University of Kent (U.K.) have elaborated on one of these mechanisms, proposing an ioncoupling model to predict and interpret the shifts induced by the presence of ions of other m/z ratios in the ion-trap mass spectrometer. They dynamically scanned the dipolar tickle ffequency to determine the mass shifts for two ion species involved in a coupling interaction. When ions have close m/z values, there is a shift toward lower values. They repeated the experiments with ions that have distant m/z values and obtained frequency shifts of about the same order as those obtained in the previous experiments. Their data also indicate the existence of coupling effects when ions are ejected at a high q Their results, when examined in conjunction with earlier work, support the existence of ion-coupling effects that subsequently give rise to a mass shift (Rapid Commun Mass Spectrom 1199 10 424-28)
The coupling effects between 61~ and 62~ ions: (a) and (b) show unperturbed secular frequencies, and (c) shows the shifted frequencies caused by ion coupling. (Adapted with permission from John Wiley & Sons.)
Analytical Chemistry News & Features, June 1, 1996 3 4 7 A
