INTERNATIONAL SYMPOSIUM
ON DETECTION IN LIQUID CHROMATOGRAPHY AND FLOW INJECTION ANALYSIS
Gary Christian Department of Chemistry University of Washington Seattle, WA 98195
Ira Krull Northeastern University Barnett Institute 341 MU, 360 Huntington Ave. Boston, MA 02115
Julian Tyson Department of Chemistry University of Massachusetts at Amherst Amherst, MA 01003
High-performance liquid chromatography (HPLC) and flow injection analysis (FIA) are the hydrodynamic techniques of greatest impact in contemporary analytical chemistry. They feature major technical differences (e.g., in the pressures and separatory columns used and scope of application) as well as a gap of more than 15 years between development and commercialization. Nevertheless, HPLC and FIA are complementary techniques in terms of speed, capability, sensitivity, and certain modular elements. The essential features of the continuous detection systems, in particular, are remarkably similar for both methods. This coincidence prompted the late Roland Frei to suggest a meeting on detection at which specialists in HPLC and FIA could discuss common problems and experiences. 0003-2700/90/0362-455A/$02.50/0 © 1990 American Chemical Society
Last year the International Symposium on Detection in HPLC and FIA was held September 20-22 in Cordoba, Spain. Final organization was handled by Miguel Valcarcel and Delores Luque de Castro of the University of Cordoba. The International Association of Environmental Analytical Chemistry, Hewlett-Packard, and the University of Cordoba (the symposium's host) sponsored the event. More than 200 scientists from 24 countries attended the meeting. Topics included luminescence, hyphenated techniques, electrochemistry, pre- and postcolumn derivatization, fast UV-vis detectors, and atomic spectroscopy.
FOCUS Whether the object is to determine many components in a complex mixture or to measure just one, detection system requirements are very similar, and many of the speakers took this as a framework for their lectures. Josef Huber of the University of Vienna pointed out that in FIA the chemical reactor design must primarily meet the requirements of the measuring system, whereas the requirements of the separation system have to be considered for HPLC reaction detection. The critical criterion is the reactor efficiency, N, which describes the longitudinal mixing per unit reactor volume. The aim is to design a flow reactor that minimizes
longitudinal mixing. Reactor volume, on the other hand, is dictated by reaction kinetics (i.e., the required reaction time). From Huber's and subsequent lectures it soon became apparent that detection is more than just hardware. Whether chromatography or FIA, it is the judicious combination of on-line chemical derivatization with an appropriate detector that provides the basis for a versatile analytical methodology. One area of common interest was multichannel detector systems. These systems have made a major impact on the information power of HPLC, and that capability is just now being explored in FIA. Anthony Fell of the University of Bradford in England described diode array detection in HPLC, emphasizing detection techniques, diode array advances, interfacing HPLC with diode array detectors, and methods for two- or three-dimensional data representations. Several methods have been established for the qualitative and quantitative validation of chromatographic peak points: multiple absorbance-ratio correlation (MARC), second- and higher order derivative chromatograms, multiple-wavelength spectral suppression, principal component analysis, iterative target-testing reactor analysis, and multiple-component analysis. In addition, peak recognition techniques based on archive retrieval have been developed using spectral and time domain information.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 7, APRIL 1, 1990 · 455 A
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Nel Velthorst from the Free University of The Netherlands discussed recent advances in laser applications, especially for fluorescence in microcolumn chromatography and capillary electrophoresis (CE). According to Velthorst, low-temperature fluorescence line narrowing has very high selectivity for identification purposes. It can be used with thin-layer chromatography or as an off-line detection method in micro LC. A different type of detection scheme was discussed by Douglas Westerlund from Sweden's University of Uppsala. He reviewed methods for the indirect detection (photometric) of organic and inorganic analytes in reversed-phase HPLC. Indirect detection requires a probe compound with a large UV-absorbance or fluorescence, or strong electrochemical response as a mobilephase component. An injected sample produces migrating zones where the local mobile phase deviates from the bulk. Analytes will thus coelute where deficiencies or excesses of the probe—depending on charge and hydrophobicity—result in respectively negative or positive peaks (1,2). Velthorst also discussed indirect methods, describing results from Frei's laboratory on indirect UV-vis and fluorescence detection. Analyte ions affect the concentration of absorbing/ emitting compounds in the mobile phase or in a postcolumn addition, resulting in a negative peak. Luminescence-quenching techniques were also presented, wherein an added detector species with a long luminescence lifetime is efficiently quenched by the analyte. In addition, work was presented from C. Gooijer's laboratory at the Free University of The Netherlands on novel luminescence detection approaches, especially luminescence quenching of a Tb-acetylacetonate complex to determine inorganic ions under ion-pair, reversed-phase HPLC conditions. An overview of conventional, on-line HPLC electrochemical (EC) and fastscanning EC detection for normal and microcolumn modes was given by L. J. Nagels from the Rijksuniversitair Centrum Antwerpen in Belgium. He emphasized using very slow flow rates for microcolumn LC-EC interfacing and characterized a disposable copolymer EC detector for analyses at flow rates of < 10 /ig/min. Several symposium speakers described EC detection schemes. L. Huber from Hewlett-Packard presented a flow-through solid electrode for EC detection in reversed-phase HPLC. This particular commercial unit combines a disposable, glassy carbon thinlayer electrode with optional pulsed
456 A · ANALYTICAL CHEMISTRY, VOL. 62, NO. 7, APRIL 1, 1990
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FOCUS amperometric detection methods. The electrode surface can be cleaned between actual current measurements. Joseph Wang, from New Mexico State University, discussed biocatalyst-modified amperometric electrodes for detection in flowing streams. According to Wang, membranes for size or charge exclusion enhance the selectivity and stability of the electrodes. Cellulose acetate, polyaniline, and polyphenol coatings provide the desired size exclusion effect, whereas charged poly(4-vinylpyridine) or Nafion coatings are used for appropriate charge exclusion. Multifunctional operation (selectivity, catalysis, protection, etc.) is obtained with a multilayer or composite (mixed) electrode coating. Some specialized applications of EC detectors were also described. Gyorgy Marko-Varga, from the University of Lund in Sweden, analyzed fermentation broth sugars using postcolumn, coimmobilized enzyme reactors and modified electrodes; and José Lima from the University of Oporto in Portugal discussed a tetraoctylammonium barbiturate-based PVC potentiometric detector for FIA that is sensitive to barbiturate ions. Ultraminiaturized detection for microcolumn HPLC or CE methods was reviewed by Hans Poppe from the University of Amsterdam. A key consideration for these detectors is the smallest volume needed for detection, because that ultimately dictates the absolute detection limit and how far miniaturization can be taken. Poppe described a number of miniaturized systems for detection on the nanoliter scale, including liquid-liquid extraction, sheath flow reactor, enzyme-moderated reaction, and electrochemical systems. Additional detection power, through on-line derivatization, was illustrated for heterogenous systems. Gary Christian and Jaromir Ruzicka, both from the University of Washington, reported on optosensing in FIA using the reversible sorption of hydrophobic reagents and analytes onto hydrophobic particles to selectively measure analytes. They described a porous, hydrophobic membrane detector that allows gases to diffuse (eg., NH 3 , HC1) but blocks passage or detection of nongaseous species such as NaOH. The gases are detected by the changing color of an indicator incorporated into the membrane. The implications of the complementary nature of FIA and HPLC (3) were presented. New polymeric reagents for off- and on-line derivatization of nucleophiles were described by Ira Krull. Solidphase, heterogenous derivatization leads to improved specificity and sensi-
Still using these tools tivity in HPLC, regardless of detection mode. KrulFs group has designed a mixed-bed, solid-phase reactor containing three different tagging reagents, thereby leading to three derivatives with different chromatographic and detector properties. A novel approach was described for off- and online tagging of enantiomeric amines for Pirkle-type chiral separations. In the future, according to Krull, all approaches to chiral recognition in HPLC should be possible via single or mixedbed type polymeric reagents. Several hyphenated techniques were also presented. Richard Browner from the Georgia Institute of Technology discussed the idea of using a particle beam (MAGIC) interface with HPLC/ FT-IR. In his system a vapor aerosol of analyte particles is deposited on a salt disk/plate and rotated into the FT-IR beam. It appears that this new approach for LC/FT-IR interfacing may have some significant advantages over current practices. Julian Tyson described detectors for atomic spectrometers. Because these spectrometers either operate intermittently (electrothermal atomizers) or produce a steady-state signal (nebulizers for flames or plasmas), some form of on-line chemistry will probably be needed to improve FIA and atomic spectrometry detection limits. Alternatively, nebulizers and spray chambers will have to be specifically designed for use with FIA introduction. According to Tyson, the single-well stirred tank model indicates that there should be some benefit from designs with reduced spray chamber volumes. Some particularly relevant research by the symposium's organizers presented the dual use of an FIA manifold as a screening system and postcolumn reactor-detector in HPLC. Valcarcel and Luque de Castro exploited the rapid measurement frequency of FIA for screening large numbers of samples for total analyte content—in this case, toxic substances. Only elevated samples are then subjected to time-consuming chromatography, employing the same FIA configuration as a postcolumn reactor-detector. Most of the participants acknowledged the high scientific level of the meeting and left Cordoba with the hope for future meetings on the same subject. The seed has been planted. References (1) Denkert, M.; Hackzell, L.; Schill, G.; Sjogren, E. J. Chromatogr. 1981,218, 31. (2) Arvidsson, E.; Commen, J.; Schill, G.; Westerlund, D.; J. Chromatogr. 1989,461, 429. (3) Ruzicka, J.; Christian, G. D. Analyst, in press.
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Gary Christian earned his B.S. degree (1959) from the University of Oregon and a Ph.D. (1964) from the University of Maryland. In 1972 he joined the faculty at the University of Washington, where he is professor of chemistry. Christian's research interests include electroanalytical chemistry, atomic spectroscopy, process analysis, and flow injection analysis. He received the 1988 ACS Division of Analytical Chemistry Award for Excellence in Teaching. He has served on the Advisory Board of ANALYTICAL CHEMISTRY and is joint Editor-in-Chief of Talanta and presently chairs the ACS Division of Analytical Chemistry.
Ira Krull received his B.S. degree from the City College of New York (1962) and an M.S. degree (1966) and a Ph.D. (1968) from New York University. Krull is now associate professor of chemistry and a faculty fellow at Northeastern University's Burnett Institute. His research interests include trace metal analysis and speciation via chromatography with element-selective detection, solid-phase derivatization for detection in GC and HPLC, EC detection via photochemical reactions or photoelectrochemical detection in flowing streams, and biopolymer determinations with linear diode array and LALL spectroscopies in HPLC and FIA.
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Julian Tyson obtained a B.Sc. degree (1971) from the University of Aberdeen in Scotland and his Ph.D. (1975) from London University (Imperial College). Last year he joined the faculty at the University of Massachusetts at Amherst after 13 years at the University of Technology, Loughborough, in the United Kingdom. His research focuses on flow injection and continuous flow techniques for enhancing analytical spectrometries (especially atomic spectrometry), solid sample introduction for atomic spectrometries, atomic fluorescence spectrometry, and spectroelectrochemistry.
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