Analytical Currents Synopses of significant analytical articles from other publications
Screening . Pap smears -V* with near-IR The Papanicolaou (Pap) smear is a commonly used cervical cancer screening technique. Pap smears are traditionally evaluated visually, which can lead to both false negatives and false positives. Chris W. Brown and colleagues at the University of Rhode Island have developed a rapid method for analyzing Pap smears with near-IR spectroscopy. The researchers obtained near-IR spectra from Pap smears collected from 50 female patients, including 30 with normal cells, 9 with atypical cells, and 11 with cervical cancer. They found that ordinary near-IR spectra show minor differences among the normal, atypical, and abnormal Pap smears, whereas second-derivative spectra show more obvious differences. Principal component analysis of the second-derivative spectra can be used to classify the normal and abnormal Pap smears, but cannot completely separate the normal and atypical Pap smears. The authors note that because the available samples were limited, resulting in a small data set for classification by PCA and discriminant analysis, further studies with a larger data set are necessary to establish a diagnostic method for cervical cancer. (Appl. Spectrosc. 1995, 49, 432-36)
Taking advantage of antibody cross-reactions Cross-reactions between an antibody and nontarget analytes can spell disaster for clinical assays that are intended to be specific, but in cases where an immunoassay is designed to screen for a general class of analytes, cross-reactivity is actually desirable. However, few accurate methods exist to quantitate the contributions of individual analytes to the overall signal in these cases. Bruce D. Hammock and colleagues at the University of California-Davis developed a mathematical model for simultaneous quantitation of four cross-reacting atrazine herbicides in a multiantibody immunoassay designed for atrazine determination in drinking water.
Both mono- and polyclonal antibodies were used in the immunoassay to provide a variable degree of cross-reactivity for individual atrazine analogues. The crossreactivity profiles for each of the analytes with the array of antibodies were used to characterize the four herbicide analogues. Individual analyte concentrations were estimated from these profiles and from the assay signals by using a mathematical model that was an extension of the empirical four-parameter log-logistic fit. The four analytes could be quantitated simultaneously at concentrations in the low to sub-part-per-billion range. (Anal. Chim. Acta 1995, 304, 339-52)
Large peptide sequences by MS/MS Tandem MS has been used successfully for amino acid sequencing of peptides smaller than ~ 3500 Da, but for larger peptides, FAB and similar single-charge ionization methods for precursor ions yield incomplete fragmentation of the peptide backbone. Electrospray ionization (ESI) produces abundant multiply-protonated intact peptide precursor ions, which appear to fragment more easily than singly-charged ions with a given CAD energy. Ron Orlando and V. S. Kumar Kolli of the University of Georgia used ESI with high-energy CAD in a four-sector mass spectrometer to achieve complete fragmentation of several peptides in the MW range of 4000-5000 Da. The high mass resolution of the sector instrument permitted the researchers to assign charge states to the fragment ions by identifying their isotopic spacing. MSI had a resolution of ~ 500 and MS2 had a resolution of ~ 3600 for most of the experiments. CAD was performed in the third field-free region using helium as the collision gas. The use of ESI in this study reduced the amount of sample required for tuning and mass spectral acquisition as compared with FAB methods from 10-100 nmol down to 500 pmol of peptide. The authors concluded that ESIMS/MS using a four-sector mass spectrometer simplified the peptide sequencing process considerably as compared with Edman degradation and extended the mass range of sequenceable peptides for MS methods. (/. Am. Soc. Mass Spectrom. 1995, 6, 234-41)
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Determining two-electron reaction currents
In the stepwise oxidation or reduction of a molecule, the removal or addition of the first electron typically occurs more readily (with lower potential) than that of the second and subsequent electrons. Dennis H. Evans and Zhou Rongfeng have determined that the common assumption that the current of a two-electron reaction is always twice that of a oneelectron reaction can lead to significant errors when removal of the second electron is more difficult than that of the first electron. The researchers used chronoamperometric digital simulation, normal pulse voltammetry, and cyclic voltammetry to determine that in such cases, the comproportionation reaction between the final product and the original reactant to give two molecules of the one-electron intermediate is favored and can occur in the diffusionreaction layer. Thus, when the diffusion coefficients of the three species are different, the current observed at potentials at which the two-electron reaction occurs will not necessarily be twice that seen at potentials at which the one-electron reaction occurs. (J. Electroanal. Chem. 1995, 385, 201-7)
Mediated electron transfer across Nafion interfaces Nafion perfluorosulfonated ionomer coatings for electrodes that contain Ru(II) cationic complexes undergo electron transfer with anionic oxidants in solution. One possible application for these coated electrodes might be electrocatalysis. However, the mediator couples in the coatings frequently display non-ideal Nernstian behavior that could be interpreted as mediator-molecule interactions in the coating or as the result of a Gaussian distribution of formal potentials from individual redox couples within the coating. Fred Anson and Eyal Sabatani of the California Institute of Technology developed a mathematical model based on this Gaussian distribution to examine the kinetics of electron-transfer cross-reactions Analytical Chemistry, July 1, 1995
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A n a l y t i c a I Currents at interfaces between mediator coatings and reactants in solution. Expanding on preliminary work using a small number of mediator-substrate combinations with high rate constants, the authors applied their model to crossreactions between Ru(II) complexes in the Nation coatings and Fe(III) or Ru(III) complexed with chelators in solution and with several iron-substituted heteropolytungstate complexes. The heteropolytungstate complexes were size excluded from the coating, and reactions for them occurred exclusively at the coatingsolution interface. The model was compared with Marcus theory to show that the linear correlation between the rate constants and the driving forces of the electron transfer cross-reactions was not affected when one reactant exhibited a distribution of formal potentials instead of a single value. (/. Electroanal. Chem. 1995, 386,111-19) What's going on at the interface? The development of in situ spectroscopic and spatial microscopic methods provides information at the atomic and molecular levels for ordered metal-solution interfaces that is comparable to that available for metal-ultrahigh vacuum (UHV) systems. This situation has encouraged the examination of electrode-adsorbate systems that offer direct comparisons with metal-UHV analogues. Michael J. Weaver and colleagues at Purdue University used UHV experiments to examine the IR spectral and surface potential properties of Pt(lll) dosed with carbon monoxide, water, and potassium with the goal of understanding the various electrostatic and other environmental factors that influence the structure and bonding of carbon monoxide and water at these electrochemical interfaces. They chose this UHV-based ternary coadsorption system in light of the availability of in situ IR reflection-absorption spectroscopy data for the electrodesolution interface at negative electronic charges, thereby enabling the validity of the UHV electrochemical modeling approach to be assessed. Variations in the metal-UHV surface potential attending alterations in the interfacial composition were evaluated with a Kelvin probe. In addition to providing insights into surface solvation, the measurements provided the required link to the in situ electrode potential scale. (/. Phys. Chem. 1995, 99, 7677-88) 412 A
Analytical Chemistry, July 1, 1995
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