News
Interfacing CE-ICPMS Trace element analysis has evolved from quantifying the elements in a sample to determining various oxidation states and chemical forms—information that is essential to understanding the role and fate of elements in the environment and biological systems. One powerful approach to determining elemental speciation is to combine HPLC with inductively coupled plasma detection. However, replacing HPLC with CE
could provide analyses with greater separation efficiency (especially with high molecular mass species such as metalloproteins), smaller analysis volumes, faster analysis times, and lower reagent consumption. Barry L. Sharp and colleagues at Loughborough University and Norwich Research Park (both in the United Kingdom) describe a new CEICPMS interface for elemental speciation and test it by studying metallothionein, a heavy-metal binding protein. The interface is based on a commercial microconcentric nebulizer that has a home-built cyclonic spray chamber. The spray chamber is fabricated from glass, and it has a general spherical shape with a drain at the base and a horizontal mounting for the nebulizer on the side. Internal volume is 21 mL. Negattve pressure was the preferred method for counterbalancing the nebulizer suction. Absolute metal detection limits for m C d , muCd (1''Zn, and w Zn in the low femtogram
Schematic of the CE-ICPMS interface. (Adapted with permission. Copyright 1998 Royal Society of Chemistry.)
Sweeping for enhanced sensitivity Micellar electrokinetic chromatography (MEKC) is not known for its sensitivity. At best, micromolar levels can be detected with on-line photometric detection. Alternative modes of detection, such as laserinduced fluorescence, improve sensitivity. Such detectors, however, have limited applicability, and many laboratories cannot afford them. Joselito P. Quirino and Shigeru Terabe of Himeji Institute of Technology Japan) report on a new technique based on a special concentration effect, called "sweeping", for increasing detection sensitivity in MEKC. In theory, unlimited increases in sensitivity can be gained by narrowing neutral analyte zones in MEKC. For zone-narrowing to occur, several conditions must be met: a constant electric field along the capillary column, negligible electroosmotic flow, and the absence of the pseudostationary phase in the analyte mixture. The sweeping phenomenon is analogous to carefully moving crumbs along the floor with a broom. Neutral analytes are 18 A
(/. Anal At Spectrom 1998 13 1095-100)
essentially picked up and accumulated by the pseudostationary phase—in this case, sodium dodecyl sulfate (SDS)—and swept into narrow zones when voltage is applied. The method works well for analytes that have a strong affinity for SDS. Several test analytes, including alkyl phenyl ketones, naphthalene derivatives and phenanthrene, dialkyl phthalates, steroids, and a few biologically active compounds, showed an enhancement in sensitivity from 100- to 5000-fold under the sweeping conditions. Enhancement increased with increasing retention factor k. Detection limits were lower than those obtained with HPLC; for example, 9-18 ppb of a racemic herbicide spiked in lake water was separated by MEKC using' the sweeping technique and was detected by UV-absorption The authors recommend using a clean-UD step to eliminate unwanted components in complicated sample matrixes; however relatively clean liquid sample's can be filtered and injected directly into the capillarv provided they have the same conductivity as the background solution drienrp 1 9 9 8 ?R? ifiS-fiSI
Analytical Chemistry News & Features, January 1, 1999
In the clouds Aqueous H 2 S0 4 aerosol particles contribute to the formation of cirrus and polar stratospheric clouds (PSCs). Their actual role in the process, however, is not well understood. Mario J. Molina and co-workers at Massachusetts Institute of Technology have gained insight into the mechanisms of cloud formation by investigating liquid-solid phase transitions in micrometer-sized H 2 S0 4 /H 2 0 aerosol droplets (0-35 wt %)) which range in temperature from 273 to 170 K Data on the nucleation of ice from the aerosols are directly obtained by using a new optical microscope technique. The aerosol particles (5-20-um diameter) are deposited onto a quartz plate and are observed by using an optical microscope while they cool. Altogether, 1214 H 2 S0 4 aerosol droplets were examined to determine their compositions and freezing/melting-point temperatures. The observed freezing points are lower (up to a 30 K difference for the more concentrated aerosols) than those points previously reported in the literature. The relationship between composition and freezing point fits a well-defined curve—supercooling with respect to the melting point is found to increase strongly at higher concentrations. The data reveal that a supercooling of up to 66 K (26 wt %) )i secessary for the formation of ice. Based on a thermodynamic model, the new freezing-point temperature data are applied to the formation of clouds in the upper troposphere and lower stratosphere. The results suggest that the nucleation of ice in cirrus clouds requires saturation ratios of up to 1.6 with respect to ice. In addition, the formation of ice PSCs is predicted to occur about 3 K below the ice frost point. (J. Phys. Chem. A 1998,102, 8924-31)
Optical microscope for studying aerosol phase transitions.
