SCIENCE/TECHNOLOGY
Polarimetry, Microdialysis Highlight Analytical Symposium New era starts for Eastern Analytical Symposium as it makes successful move to New Jersey, with increases in attendees and exhibits Stephen C. Stinson, C&EN Northeast News Bureau
The 29th Eastern Analytical Symposium opened in Somerset, N.J., last month big ger than ever. This showing decisively answered the question of whether EAS could successfully leave New York City, where it had held all 28 other meetings since 1959. An estimated 3600 to 3700 persons registered this year, up from about 3400 last year. At the Garden State Conven tion & Exhibit Center, 163 exhibitors took 234 booths, in comparison with 106 exhibitors and 165 booths at the New York Hilton in 1989. And at the adja cent Somerset Hilton Hotel, speakers de livered 305 papers, compared with 260 last year. Exhibitors seemed especially pleased at escaping the high labor costs of setting up booths. Requirements that outside la bor be used added an average $200 per booth in New York City. Also, there was easier access for equipment deliveries to the exhibit floor in Somerset. And larger firms could draw on local sales offices for much of their booth staffing rather than bringing people to New York. Touring the booths was easier for at tendees, because the booths were laid out in one room seven-aisles wide and 20booths long. Access to the technical ses sions was convenient, with oral papers in the first-floor ballrooms and second-floor parlors of the Hilton and posters in the exhibit area, a short walk via a covered way. There were some difficulties that can be smoothed out when EAS returns to
Somerset in 1991. For example, work shops were moved after pandemonium resulted the first day from having work shop cubicles separated from exhibits only by curtains. Also, parking was con gested at the convention center, and few restaurants were nearby. Tor the future though, the expansion of EAS has left it little room to grow in Somerset. There was only a small amount of unused space in the back of the exhibit floor that could be set up next year.
HPLC polarimetric detector advances spotlighted Reports at the Eastern Analytical Symposium by two groups on ad vances in polarimetric detectors for high-performance liquid chroma tography were in keeping with a growing interest in analyses of enantiomers of chiral drugs and oth er products. Applications specialist John D. MacFarlane of JM Science, Buffalo, N.Y., described the nonmodulated polarized beam-splitting detector that his firm is distributing for Ja pan's Showa Denko. And analytical chemistry professor Chieu D. Tran of Marquette University, Milwau kee, reported on a system not yet commercialized that uses differen tial absorption of right- and left-circularly polarized light.
Priced at $15,000, the Shodex OR1 from JM uses 780-nm near-infra red light from a light-emitting di ode. This passes through a sheet po larizer, which converts it to planepolarized light. This light next passes through the sample, which possibly rotates the plane of polarization, in a 40-μΙ, flow cell 50-mm long. The light goes on to a beam-splitting polarizer, which divides it into its horizontal ly and vertically polarized compo nents. The intensity of each component registers at two photodetectors, which transmit the signals to a dif ferential amplifier. The signals pass from this to a summing amplifier, which is also connected to a lightintensity controller. This allows in creases in light intensity in case of a fall in transmittance of the material in the flow cell. MacFarlane says the instrument is sensitive to 0.05 millidegrees of rota tion. Because the path length is so short, measured rotations are small. The detector has a range of —2° to +2° of rotation. These measured ro tations are converted to specific rota tions characteristic of the compound by the relation V0 = k[a]c, where V0 indicates the output signal, k is a constant, [a] represents the specific rotation, and c is the concentration.
Unit doubles as HPLC detector and conventional polarimeter Photo detector
Differential amplifier
iilM.
m spl [tter
1
Flow cell·
& Sheet polarizer
Lightemitting diode
Photo detector Summing amplifier
Lightintensity controller
December 3, 1990 C&EN
29
Science/Technology In addition to use as an HPLC de tector for qualitative analysis of enantiomers, the instrument can serve as a polarimeter for very accu rate determinations of specific rota tion. The user simply lets the sample into the flow cell, stops the flow, and measures the rotation. Follow ing this procedure, the 0.05 millidegree resolution of the Shodex OR-1 is superior to the ±1° of rotation performance of a conventional pola rimeter. Also, conventional polarimeters have path lengths of 10 cm and require 10 mL of sample. This in strument needs only 0.5 Mg of com pound, compared with 200 μ^ of compound needed for conventional polarimeters. The detector built by Tran at Mar quette uses a thermal lens effect, produced by differential absorption of right- and left-circularly polar ized light [Anal. Chem., 62, 2467 (1990)]. Working with graduate stu dent Minren Xu and supported by the National Institutes of Health, Tran demonstrated the detector with racemic and enantiomeric tris(ethylenediamine)cobalt(III) salt. The Mar quette chemists detected as little as 7.2 ng in a ΙΟ-μί flow cell with a 5mm path length. Tran had to use a colored cobalt complex because the only excitation source available to him was an argon ion-excitation laser (514.5 nm). Re cently, he has fitted his detector with a 350-nm laser, and he is seeking funding for a 270-nm laser that will be needed for most organic com pounds. Research Corp. Technolo gies, Tucson, has applied for patents on Marquette's behalf and will negoti ate with companies for the university. Light from the argon ion-excita tion laser passes through a prism po larizer, which converts it to planepolarized light. This plane-polarized light next passes through a Pockels cell, which converts it to circularly polarized light. The Pockels cell, w h i c h Tran bought from Conoptics of Danbury, Conn., is an electrooptic modulator whose heart is a crystal of ammoni um dihydrogen phosphate. Applica tion of an electric field to the crystal changes its refractive index in pro portion to the voltage. At two certain voltages and refrac tive indexes, the ammonium dihy30
December 3, 1990 C&EN
Ultrasensitive chiral detector developed
CZD-
Argon ionexcitation laser
Helium-neon probe laser Pump
3—1
Column Diode detector
Pockels cell Flow cell Lock-in amplifier
Polarizing prism
Dichroic filter
Signal generator
drogen phosphate crystal retards one or the other of the two counterrotating electric vectors of the planepolarized light by one quarter of the wavelength. This causes the com bined vectors to rotate clockwise or counterclockwise, resulting in rightor left-circularly polarized light. Each optical isomer of the cobalt complex absorbs one form of the circularly polarized light more strongly than the other. Absorp tion heats the sample in the light path, causing the solvent to decrease in density. Changes in density are detected by a second laser beam used as a probe (helium-neon, 632.8 nm). The probe beam is focused by a lens, passes through the sample, and en ters a photodiode detector through a pinhole. A sample of decreased den sity acts as a diverging lens, defocusing the probe beam and reducing the intensity at the detector. Tran and Xu used a signal genera tor to alternate between the two voltages about four times per sec ond, producing alternating 265-millisecond bursts of right- and left-cir cularly polarized light. A lock-in amplifier took the different signals from the photodiode and produced a chromatogram showing not only peaks for each enantiomer but also the chiral sense of each, according to whether the peaks projected above or below the baseline. The solvent through the chromatograph was 0.06M aqueous bis(^-D-tartrato)diantimonate. This an ion formed two diastereoisomeric ion pairs with the racemic cobalt complex cation, allowing them to be resolved. For the future, Tran will
Power supply
try to apply the method with organ ic solvents, which have greater re sponses in density to heating than does water. It may also be possible to use the device as a detector in capillary zone electrophoresis.
Microdialysis probe technique offers avenue to new drugs Hair-thin, precisely surgically im plantable microdialysis probes have been developed by chemists in West Lafayette, Ind., to aid in analyses for tiny amounts of substances in tissue fluids of live, unanesthetized ani mals over long periods. The devices may find use in development of new drugs and in basic biochemical research. Peter T. Kissinger, who is both professor of analytical chemistry at Purdue University and president of Bioanalytical Systems, told an audi ence at the Eastern Analytical SymProbe allows long-term analysis in live animals
Outer shaft
Inner tube
Dialysis membrane Laser-burned holes
Science/Technology
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posium that the probes may be espe cially useful in the neurosciences, be cause they can be implanted in specific brain structures. Bioanalytical Systems is a supplier of high-per formance chromatographs and auxil iaries for biological samples, and of electroanalytical workstations. Kissinger described use of the probes with HPLC and as universal sampling devices that can be used with gas chromatography, mass spectrometry, capillary zone electro phoresis, ion-specific electrodes, mass spectrometry, radioimmunoas say, chemiluminescence, and elec troanalytical techniques. Each probe consists of an inner tube contained within an outer shaft such that the inner tube extends 1 to 5 mm beyond the end of the outer shaft. An outer, cylindrical dialysis membrane extends from the end of the shaft to the end of the inner tube. The membrane tip of the probe has a diameter of 240 μπι or less. After implanting the probe surgi cally into an animal's blood vessel or
target tissue, the investigator uses an external syringe pump to inject a perfusion solution at about 1 /*L per minute into the inner tube. The so lution should have the same pH, ionic strength, and nutrient content as the animal's own tissue fluid. The solution flows down to the end of the inner tube, passes out of it through minute, laser-burned holes almost at the end, enters the space between the inner tube and dialysis membrane, flows up the in ner surface of the cylindrical mem brane, and out the shaft for analysis. The investigator can introduce a drug or other test compound into the perfusion solution. Because the concentration of the test compound is greater on the inside of the mem brane, it diffuses out into the ani mal's tissue fluid. Similarly, concen trations of hormones, neurotrans mitters, and any metabolites of the test compound will be greater in tis sue fluid, so they diffuse through the membrane into the solution, which carries them out for analysis.
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Science/Technology Kissinger showed liquid chromatograms from one experiment that detected 100-femtogram (10~13 g) amounts of aspartic, glutamic, and 7-aminobutyric acids, glutamine, serine, glycine, and taurine simultaneously, and from a second experiment that detected similar amounts of epinephrine, norepinephrine, dopamine, homovanillic acid (a dopamine metabolite), serotonin, and 5-hydroxyindoleacetic acid (a serotonin metabolite). Kissinger says his collaborators at Bioanalytical Systems had demonstrated implantation of probes into fat tissue, adrenal glands, blood vessels, brains, spinal cords, eyes, gastric mucosa, hearts, skeletal muscles, and uteruses of animals as well as subcutaneously. For brain implantation, the West Lafayette researchers first inserted a guide tube, anchoring it to the outside of the skull with dental acrylic adhesive. The guide tube contained a dummy insert to keep it closed and sterile until researchers were ready to insert the probe. Kissinger says such membranetipped probes are preferable to traditional perfusion studies with open tubes. This is because microorganisms cannot pass through the membrane into the animal. The membrane also segregates metabolites and other low molecular weight compounds from enzymes that might degrade them. Also, there is no need to sacrifice animals to determine contents of tissue fluids, thus conserving animals. It also avoids changes in tissue fluids that can occur after death. By selecting among syringe pumps and refrigerated fraction collectors, users can do studies over six to 24 hours. Thus they can see effects of sleep-wake and light-dark cycles and follow tissue metabolism over time. The only limitation is that one must choose a phenomenon for study that has the same cycle time as the analytical technique. In Kissinger's liquid chromatography, this was the 10-minute interval between sampling. Each animal can serve as its own control. Users can monitor tissue fluids for a period before instilling the test compound to see differences in the same animal. D
