Electroanalytical chemistry

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  •  Matched against a wide range of spectroscopic and chromatographic techniques, the techniques of electroanalytical chemistry find an important role for several reasons: – Electroanalytical methods are often specific for a particular oxidation state of an element – Electrochemical instrumentation is relatively inexpensive and can be miniaturized – Electroanalytical methods provide information about activities (rather than concentration)

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  • Analytical chemists work to improve the reliability of existing techniques to meet the demands of for better chemical measurements which arise constantly in our society They adapt proven methodologies to new kinds of materials or to answer new questions about their composition. They carry out research to discover completely new principles of measurements and are at the forefront of the utilization of major discoveries such as lasers and microchip devices for practical purposes.

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  • To help you further Chemistry references. Invite you to consult the document content "Chapter 7: Electroanalytical Techniques". The contents of the presentation materials: Fundamentals of electrolysis, electrogravimetric analysis, voltammetry. Hope this is useful references for you.

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  • Microscopic and imaging techniques: – Optical microscopy – Confocal microscopy – Electron microscopy (SEM and TEM, related methods) – Scanning probe microscopy (STM and AFM, related methods) Surface spectrometric techniques: – X-ray fluorescence (from electron microscopy) – Auger electron spectrometry – X-ray photoelectron spectrometry (XPS/UPS/ESCA)

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  • Voltammetry techniques measure current as a function of applied potential under conditions that promote polarization of a working electrode. Polarography: Invented by J. Heyrovsky (Nobel Prize 1959). Differs from voltammetry in that it employs a dropping mercury electrode (DME) to continuously renew the electrode surface. Amperometry: current proportional to analyte concentration is monitored at a fixed potential

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  •  IUPAC Definition: chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary while the other moves in a definite direction  Stationary phase (SP): common name for the column packing material in any type of chromatography  Mobile phase (MP): liquid media that continuously flows through the column and carries the analytes  Analyte: the chemical species being investigated (detected and quantitatively measured) by an analytical method...

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  • ● Adsorption chromatography • The stationary phase is an adsorbent (like silica gel or any other silica-based packing) • The separation is based on repeated adsorption-desorption steps. ● Normal-phase chromatography • The stationary bed is strongly polar in nature (e.g., silica gel), and the mobile phase is nonpolar (such as n-hexane or tetrahydrofuran). • Polar samples are retained on the polar surface of the column packing longer than less polar materials.

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  •  Infrared (IR) Spectroscopy: – First real IR spectra measured by Abney and Festing in 1880’s – Technique made into a routine analytical method between 1903- 1940 (especially by Coblentz at the US NBS) – IR spectroscopy through most of the 20th century is done with dispersive (grating) instruments, i.e. monochromators – Fourier Transform (FT) IR instruments become common in the 1980’s, led to a great increase in sensitivity and resolution  Raman Spectroscopy: – In 1928, C. V.

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  • Surface contamination: – The obvious contamination/alteration of surfaces that can be the result of less-than careful sample preparation – Solid surfaces can adsorb gases: At 10-6 torr, a complete monolayer of a gas (e.g. CO) takes just 3 seconds to form. At 10-8 torr, monolayer formation takes 1 hour. – Most studies are conducted under vaccuum – although there are newer methods that don’t require this.

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  • Mass Spectrometry (a.k.a. MS or mass spec) – a method of separating and analyzing ions by their mass-to-charge ratio MS does not involve a specific region of the electromagnetic spectrum (because it is not directly interested in the energies of emitted photons, electronic or vibrational transitions, nuclear spin transitions, etc…)

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  • LIF (laser-induced fluorescence) is a very popular CE detector – These have ~0.01 attomole sensitivity for fluorescent molecules (e.g. derivatized proteins). Direct absorbance (UV-Vis) can be used for organics. For inorganics, indirect absorbance methods are used instead, where a absorptive buffer (e.g. chromate) is displaced by analyte ions – Detection limits are in the 50-500 ppb range. Alternative methods involving potentiometric and conductometric detection are also used – Potentiometric detection: a broad-spectrum ISE – Conductometric detection: like IC...

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  • Requires improvements in the whole column: – Sub 2 μm particles Porous for optimum mass transfer New bridged hybrid particle required for pressure tolerance (up to 15000 psi) Sizing technology for narrow particle size distribution – Column hardware New frit technology to retain particles New end fittings for high pressure/low dispersion operation – Packing technology New column packing processes to optimize stability

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  • Aldehydes and Ketones – Form acetals and ketals respectively with normal methanol-containing reagents – Water formed in this reaction will then be titrated to give erroneously high water results – With aldehydes a second side reaction can take place, consuming water, which can lead to sample water content being underestimated – Replacing methanol with another solvent can solve the difficulties (commercial reagents are widely available)

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  • Deviations from Beer’s law (i.e. deviations from the linearity of absorbance vs. concentration): – Intermolecular interactions at higher concentrations – Chemical reactions (species having different spectra) – Peak width/polychromatic radiation Beer’s law is only strictly valid with single-frequency radiation Not significant if the bandwidth of the monochromator is less than 1/10 of the half-width of the absorption peak at halfheight.

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  •  UV-Visible spectra can be interpreted to help determine molecular structure, but this is presently confined to the analysis of electron behavior in known compounds.  Information from other techniques (NMR, MS, IR) is usually far more useful for structural analysis  However, UV-Vis evidence should not be ignored!

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  • The velocity at which radiation travels (or propagates) through a medium is dependent on the medium itself When radiation travels through a medium and does not undergo a frequency change, it cannot be undergoing a permanent energy transfer However, radiation can still interact with the medium – Radiation, an EM field, polarizes the electron clouds of atoms in the medium – Polarization is a temporary deformation of the electron clouds

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  • If you read March’s paper on ion traps: – What is resonant excitation? Summarize how resonant excitation is used in typical ion trap MS experiments. If you read the Russell and Edmondson paper on MALDITOF and accurate mass: – Summarize the advantages and disadvantages of MALDI-TOF (with DE and reflection) versus FTICR (including ESI-FTICR), especially in biochemical applications.

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