Focus
—NMR —EC —MS —Absorbance —Fluorescence —FTIR —ICP —PAS —Raman
LC DETECTORS The Search Is On for the Ultimate Detector Liquid chromatography (LC) does not yet have its flame ionization detector. The sensitive, universal LC detector is simply not yet available. Currently available commercial LC detectors are based on a number of detection principles, including absorbance, fluorescence, refractive index detection, electrochemical detection, and mass spectrometry. A number of these detectors are likely to be improved in the next few years. For instance, new types of LC/MS interfaces will probably appear, and currently available interfaces should be further refined. The betting is that some new detectors will also be making their commercial appearance. Nuclear magnetic resonance (NMR) "detectors" should materialize, for instance. (Here, the tail may very well turn out to wag the dog.) And detectors that were formerly used only for GC are being seriously considered for their applicability to LC detection. Ron Majors of Varian Associates recently conducted a survey of scientific papers to find out which detectors were most popular (Table I). The 0003-2700782/0351-327AS01.00/0 © 1982 American Chemical Society
table shows that optical detection is still king of the mountain. Overall, it was utilized in 86% of the papers surveyed. But what of the future? What systems can we expect to see emerging in the mid-1980s? And will we finally see the emergence of the ultimate LC detector in the next few years? Some intriguing answers to these questions were discussed at the 20th
Table I. Survey of Detector Usage, 1980-81 Absorbance detectors: 71 % (total) UV, fixed wavelength: 2 8 % UV, filter: 9 % UV, monochromator: 3 4 % Fluorescence: 15% Refractive index: 5.4% Electrochemical: 4 . 3 % Other: 4 . 3 % Courtesy Ron Majors, data from a survey of 365 published scientific papers.
Eastern Analytical Symposium (New York City, November 1981), in sessions on New Horizons in HPLC Detection, chaired by Ron Majors of Varian, and Molecular Spectroscopy in Interfacing Analytical Modes, chaired by James McDivitt of Ethicon, Inc.
Optical Detectors Optical detectors, based on both absorbance and fluorescence, are the current workhorses of LC. UV absorbance systems are especially popular, and they are already so refined that fundamental noise limitations in such systems now originate from thermal instabilities in flow cells and in optical and electronic components. Thus, explained Jim Tusa and Seth Abbott of Varian, "Future absorbance detectors may require thermostating to 0.01 °C to approach the fundamental shot noise limit of 10~ 6 absorbance units achievable with currently available high-intensity fixed wavelength lamps." A further design goal is to limit drift to 10-20 times the detector
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982 · 327 A
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Fluorescence detection is another hot area today due to the increased recognition of its inherent sensitivity by the LC community. Current research using the intense tunable laser sources now available should further increase fluorescence sensitivity. In fluorescence, the analytical signal is measured against a low background, assuming the eluent and optical components do not fluoresce. Because of this low background, fluorescence is 100 to 1000 times as sensitive as absorbance detection. And in addition to being sensitive, fluorescence detectors are selective. Most compounds absorb, but relatively few fluoresce. Of course, selectivity is a double-edged sword, and the fact that only a few compounds fluoresce also limits the applicability of fluorescence detection. The use of post-column reactions yielding fluorescent species will extend its usefulness, but fluorescence will never be that universal detection principle the LC community is looking for, the "flame ionization detector" of LC. With the advent of microcolumns in LC, eluent flow rates have descended to such low levels that the entire effluent of a microcolumn can be directed through a flame. Thus, Milos Novotny of Indiana University is investigating the application to LC of detectors heretofore used only for GC, such as the flame photometric detector. "In one case," wrote Novotny in a recent article in ANALYTICAL C H E M -
ISTRY (1981,53,1294-1308 A), "flame emission was directly measured for phosphorus-containing compounds with sensitivity down to 1 0 - 1 1 g phosphorus/s." The LC flame photometric detector, selective for compounds containing sulfur and phosphorus exclusively, is something we are bound to be seeing more of in the future.
LC/MS The albatross around the neck of LC/MS has always been the fundamental mismatch between LC flow rate and the vacuum requirements of the mass spectrometer. The two interfaces currently available handle this problem in different ways. In the system manufactured by Hewlett-Packard, the effluent from the chromatograph is split, with only a small fraction being directed into the ion source of the mass spectrometer. In systems marketed by Finnigan and other manufacturers, the effluent is deposited onto a moving belt or wire, the solvent is vaporized, and the belt or wire enters the ion source, where the eluites are desorbed, ionized, and analyzed. Both techniques will no doubt undergo refinement in the next few years.
But there are also some brand new LC/MS techniques knocking at the door, such as the two spray techniques introduced by Marvin Vestal at the EAS session. In the first of these, the so-called thermospray technique, the LC effluent is fed into a microfurnace, where it is vaporized. This vapor is then directed into the ion source. In one of the newest versions of this technique, the whole vapor jet traverses the ion source and is pumped out on the other side. Two modes of ionization are possible: chemical ionization (with the solvent as chemical ionization reagent), or thermospray ionization. The latter refers to the formation of ions coincident with vaporization when the effluent is heated in the thermospray furnace. "We have found we don't really need a source of electrons to produce ions," explained Vestal. "And the ionization efficiency of the sample is generally much higher than that of the solvent in the thermospray process, so we get a rather large gain." According to Vestal, this particular detector may well see the commercial light of day within the next year or two. Still in its infancy, the second spray technique Vestal described involves spraying the effluent onto a belt. "What we'd like to do is get a nice uniform coating of the sample by spraying it on, and get rid of the solvent simultaneously, so we don't have to heat the belt to remove the solvent," said Vestal. After that, the belt would be transferred into the ion source, and the sample would be ionized and detected. Vestal is particularly interested in the idea of using soft ionization techniques, such as laser desorption, fast atom bombardment, or secondary ion mass spectrometry, to gently ionize samples once they are on the belt.
Electrochemical Detection Thin-layer electrochemical detectors for LC, now commonplace in many biochemical and pharmaceutical laboratories, were first devised in 1972. Famed far and wide for their sensitivity (in many cases a picomole or less can be detected), electrochemical detectors also provide considerable selectivity, since only a few components in a complex sample are likely to be electroactive. (The detector will ignore those that aren't.) Ronald Shoup of Bioanalytical Systems, Inc., suggested trying a multiple electrode detector to further increase the specificity of electrochemical detection. Shoup discussed two multielectrode modes at the EAS session.
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982 · 329 A
Mossbauer Spectroscopy and Its Chemical Applications
Advances in Chemistry Series No. 194 John G. Stevens, Editor University of North Carolina at Asheville Gopal K. Shenoy, Editor Argonne National Laboratory Based on a symposium jointly sponsored by the Divisions of Nuclear Chemistry and Technology and Inorganic Chemistry of the American Chemical Society. Latest research on an interdisciplinary technique useful to chemists, geologists, metallurgists, engineers, and biologists This 29-chapter volume focuses on the current state of the art of Mossbauer spectroscopy and deals specifically with the study of chemical bonding; environmental, biological, energy, and catalyst applications; and phase analysis. CONTENTS Halogen-Containing Compounds · Inorganic and Organometallic Compounds · Conversion Electron Mossbauer Spectroscopy · IonImplanted Alloys · Europium-151 and Thulium'169 · lron-57 Mossbauer Spectroscopy · Coal Characterization and Utilization · Pyrite and Coal · Victorian Brown Coal · Fossil Fuels and Petroleum Source Rock · Iron Oxides in Soil · The Steel Industry · Iron-Sulfur Clusters · Mossbauer Effect in Zinc-67 « Neptunium-237 Mossbauer Spectroscopy * Antimony-121 Mossbauer Spectroscopy · Iodine-127 Mossbauer Spectroscopy · Tellurium-125 and lodine-129 Study · Spin Crossover · Magnetic Phase Transitions · High-PressUre Spin-State Transformation · Magnetic Phase Transitions · Ferroelectric Phase Transition · Hydrogen Storage Materials • Battery Materials · Colloidal Catalyst Solutions · Mixed-Metal Catalysts · Medium-Pore Zeolite-Iron Catalysts · Oxidation Catalysts ·
