News
Measuring the double layer
Cryogenic detectors for MALDI
The electrical double layer plays an important role in electrochemical processes at electrode surfaces, but few direct measurements of these double layers have been reported. Recently, direct measurements of the magnitude and potential dependence of the diffuse double-layer forces at electrode surfaces have been made with the atomic force Schematic diagram showing AFM force microscope (AFM). Allen J. Bard measurements between charged silica sphere and colleagues at the University of and Ti02 substrate. Texas at Austin along with Andrew C. Hillier at the University of Virginia point, the electrode potential at which the have extended this work by investigating interaction force becomes zero correthe diffuse double layer at an «-type Ti02 sponds closely to the flat-band potential. single-crystal electrode in an aqueous With this approach, at pH 5.5 a flat-band electrolyte solution potential for Ti0 2 near -0.4 V (versus the saturated calomel electrode) was deterThe double layer was studied by meamined that agreed with photoelectrochemisuring the force between a spherical silica cal measurements under similar conditions. probe placed on the end of an AFM cantiIf species are adsorbed on the surface, lever and the Ti0 2 substrate. The charge force measurements cannot be used to diwithin the diffuse double layer was calcurectly determine the flat-band potential. lated from the force between the elecHowever with a well-characterized probe trode and the AFM cantilever tip. The authe surface charge caused by adsorbed thors found that the interactions are a species can be measured at an open circuit strong function of solution pH, the presand subtracted from potential-induced ence of adsorbed anions, and the Ti0 2 curves to measure the flat-band condition electrode potential. indirecuV (I. Phvs Chem B1997 101 When the adsorption-induced surface 8298-303) charge is zero, which is the isoelectric
The upper limits of masses that can be analyzed by matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) MS are constrained by ion detector efficiencies. W. Henry Benner and coworkers at Lawrence Berkeley National Laboratory, Lawrence Livermore National Laboratory, and Conductus (Sunnyvale, CA) extend previous work with superconducting tunnel junction (STJ) devices to MALDI-TOFMS detection. (STJ devices are cryogenic detectors that respond to the energy transferred by the impact of individual particles, whether they are charged or neutral.) They used a NbAlO -Nb device operating at 1.3 K for the detection of 66-kDa human serum albumin and reported on the energy-resolving capability of the detector
What's going on inside the column? Porous media incorporated into chromatographic columns are generally opaque, making it difficult to investigate flow and transport during a separation. To develop new column designs and packing materials, flow inhomogeneities and solute dispersion in operating columns must be investigated. Methods involving tracers such as visible dyes are limited to studies on the exterior of columns or on the destructive evaluation of column packing material. Electrochemical and fluorescence methods have successfully studied dispersion and flow distribution in chromatographic columns; however both techniques are specific to columns of particular sizes and look at only a small number of locations within the bed Erik J. Fernandez and co-workers at the University of Virginia and Pharmacia
710 A
They found that the pulse height scaled approximately with the charge state of the ion. However, the discrimination between charge states is not perfect. The authors cite two reasons: The energy resolution of the particular STJ is only 20 keV, making discrimination between ions with kinetic energies of 25 (+1 charge) and 50 keV (+2 charge) difficult; and ion fragmentation could make the energy transfer inefficient and variable. (J. Am. Soc. Mass Spectrom. 1997, 8,1094-102)
techniques have recently been applied to Biotech (Sweden) investigate magnetic resomulticomponent systems. Although MRI nance imaging (MRI) for the analysis of inanalyses require that the column be nontact chromatographic columns during a typimetallic or nonferromagnetic, the author; cal separation by visualizing the desalting believe that MRI has many advantages and size-based separation of proteins. The advantage of MRI techniques is that they can over other methods and may even complement some methods, such as pulsedbe performed in an intact column during a field-gradient NMR for rharacterizing separation, and the entire bed can be visualdispersion and flow in chromatographic ized. Flow variations examined by MRI durcolumns. (J. Ckromatogr. A 1997, 7,9, ing column operation were found to be con73-89) centrated near the column inlet. Magnetic resonance visualization is limited to studying one component at a time when the measurement is based on a water signal. Multiple components can be identified by repeating the elution with a different component labeled each time. However, Three-dimensional MRI of the desalting of a protein as magnetic resonance viewed from the end of the column. (Adapted with permission. spectroscopic imaging Copyright 1997 Elsevier Science.)
Analytical Chemistry News & Features, December 1, 1997
