News
The pluses and minuses by near-IR Enantiomeric differentiation is particularly important in the pharmaceutical industry, because enantiomerically pure drugs can lower the necessary dose and the cost. Shift reagents selectively interact with an analyte of interest to differentiate it from the rest of a mixture and can thus be used to determine enantiomeric purity. Gary M. Hieftje and Stacy A. MacDonald of Indiana University used (3-cyclodextrin and silica gel as shift reagents to distinguish between (!/?)-(+)- and (lS)-(-)-cc-pinene by near-IR transmission spectroscopy. The shift reagents did not enable a physical separation of the enantiomers; instead, they caused changes in the spectra that could be determined by principal component analysis (PCA). To eliminate baseline fluctuations due to differences in scattering, PCA was performed on the first derivatives of the spectra. (3-Cyclodextrin was used as the shift reagent by depositing a solution that contained both (3-cyclodextrin and a-pinene on a polycarbonate film; to use silica gel as the shift reagent, 4 uL of a-pinene, which is a liquid, was deposited directly on the silica gel layer of a glass-backed TLC plate. PCA indicated that there were differences between the two enantiomers when |3-cyclodextrin was used as the shift reagent; depending on the method of film deposition and drying, the enantiomers overlapped to varying degrees in the principal component plot. Silica gel was also able to distinguish between the enantiomers by PCA. (Appl. Spectrosc. 1996, 50,1161-64)
Principal component plot of the first derivative of the spectra (1100-2498 nm) of a-pinene/ $-cyclodextrin solutions spotted on a polycarbonate membrane supported on a quartz plate. The solutions contained 9.5 mM $-cyclodextrin, 9.4 mM a-pinene, and 20% DMF. (Adapted with permission from the Society for Applied Spectroscopy.) 12 A
Viewing the depths
~ 25°, represented the response of the sysConfocal microscopy provides depth reso- tem to an "infinitely" thin sample. Diamond, which is an efficient Raman scatterer, was lution through the use of a pinhole at the used to test the system's depth response. back imaging plane of the microscope, an To demonstrate the potential application of arrangement that allows measurement of light originating from a small focal volume this system to more complex samples, they devised a three-component layered sample to be differentiated from light from out-ofcomposed of diamond, 5P4E lubricant, and focus regions. In most confocal designs, polyester film. the focal volume cannot be changed without changing lenses or the size of the pinDepth resolution increased in both hole. Dor Ben-Amotz and co-workers at confocal and superfocal methods as the Purdue University investigated using a focal length of the lens focusing the light fiber-optic bundle as a variable-size pinon thefiberbundle decreased. The superhole for confocal and "superfocal" Raman focal method provided spectra at greater spectroscopy. depths than did the confocal method, with The scattered light is back-collected and no accompanying loss of spectral resolution. (Appl. Spectrosc. 1996,50,1150-55) collimated through the microscope objective. After it exits the microscope, the light is focused on the fiber bundle, which has an 18around-1 round-to-slit configuration. The round end of the bundle is placed in the image plane of the lens, and the linear array is placed at the entrance slit of the monochromator; the signal is detected with a liquid nitrogencooled charge-coupled device detector. For confocal measurements, the outer 18 fibers are physically masked, and only the signal from the centerfiberis collected. The light from all 19 fibers is collected and integrated for superfocal measurements. The researchers evaluated Block diagram of the confocal/superfocal Raman their system with three differsetup. (Adapted with permission from the Society for ent samples. A mirror, tilted at Applied Spectroscopy.)
Detecting contaminants in HF It's nasty stuff, but the microelectronics industry has found lots of routine uses for hydrofluoric acid. When the acid is used in assembly-line type processes, however, it inevitably accumulates metallic and organic contaminants. Certain metallic ions, in particular, are known to plate out from HF solutions onto silicon substrates such as chips, affecting device performance and the overall process yield. Oliver M. R Chyan and colleagues from the University of North Texas, along with Jennifer Sees and a co-worker from Texas Instruments, have developed a novel sensor for detecting low levels of metallic impurities in HF that is based on the observation that noble metals outplating on silicon wafers form nanometersized particles on the wafer surface.
Analytical Chemistry News & Features, January 1, 1997
Time-dependent potentiometric responses of a (100) n-Si-based sensor toward various Ag+ concentrations in 0.01% HF. (Adapted with permission from the Electrochemical Society.)
Using the regular silicon wafer as a sensing electrode, the researchers were able to directly measure the silicon open-
circuit potential shift generated by the charge-transfer reaction between metal ions and the surface. The sensitivity of the potentiometric sensor is in the parts-pertrillion range. For example, silver ions in HF produce a detection sensitivity of about +150 mV shift per decade change of [Ag+]. The new sensor concept can be easily incorporated into the actual silicon device fabrication sequences. (J. Electrochem. Soc. 1996,143, L235-37)
Novel diode-like material Royce W. Murray, R Mark Wightman, and a team of electroanalytical chemists at the University of North Carolina at Chapel Hill have discovered a solid-state material that exhibits diode-like current-voltage and light-emitting behavior under novel condi-
A better introduction for long-lived radioisotopes ICPMS is emerging as the technique of choice for determining long-lived, lowintensity-radiation radionuclides and aparticle-emitting radioisotopes that normally require laborious separations. However, ICPMS of these analytes can be plagued by sample introduction problems. Many problems can be avoided by using electrothermal vaporization (ETV), but this approach still suffers from poor reproducibility and interferences from refractory molecular species involving the furnace or cup material. Jorge S. Alvarado and Mitchell D. Erickson of Argonne National Laboratory have introduced a general method for using ETV-ICPMS to determine long-lived radioisotopes by vaporizing samples in the presence of CHF3. Samples vaporized with the fluorocarbon present are converted to metal halides, which have much lower boiling points than do oxides or carbides. In addition, several matrix effects are eliminated and the sample requires no pretreatment. The authors report that their method yielded absolute detection limits ranging from 0.6 fg for 226Ra to 5 fg for 238U; analytical calibration plots were linear over 2-3 orders of magnitude. They also describe ways to minimize matrix effects from Group IA and IIA elements without sacrificing analytical performance. (J. Anal. Atom. Spectrom. 1996,11,923-28)
tions. The material consists of thin (~ 200 nm) solvent-free films of the fixed-site redox polymer poly[Ru(vbpy)3](PF6)2 (vbpy stands for 4-vinyl-4'methyl-2,2'-bipyridine) that have been electrochemically deposited on Ft interBimolecular hopping transport of electrons between Ru2* digitated array electrodes. 3 + 2 It is already known that and Ru * sites (left) and Ru and Ru * sites (right) under the forward-bias condition. ruthenium film and other similar fixed-site redox polymers can transport electrons by hopstates of +1 through +3; thus, this is the ping or self-exchange between donor and first report of a redox polymerfilmstrucacceptor sites. In this case, emission arose ture that, once prepared, does not require from *Ru2+, which is produced by eleca net material compositional change to tron-transfer reactions between Ru3+ and turn forward bias current and light emisRu+ states within the film. However, there sion on and off. Instead, the magnitude is a novel aspect to this film's diode behav- and potential bias dependency of the curior. The dry films containfixedconcentra- rent flow and emission intensity in the dry tion gradients of ruthenium in oxidation polymer are primarily governed by and rise with sustained voltage gradients in the polymer film. Moreover, thefilmsdo not exhibit the limiting current plateaus characteristic of charge transport processes governed by Fick's law. Unfortunately, the ruthenium film suffers from numerous limitations such as low quantum efficiency, which probably excludes practical applications of this material. However, the basic mechanism should be operant in other redox films with mixed valent layers containing frozen concentration gradients, say the researchers, and a search is now under way for other chemical systems that behave similarly. (J. Am. Chem. Soc. 1996,118, 10609-616)
Proteins by blotting membrane
Thorium-232 with argon (top), and argon and CHF3 (bottom) as the carrier gas. (Adapted with permission from the Royal Society of Chemistry.)
In microchromatography, the uniform packing and good separating power of continuous-bed microcolumns are an alternative to the difficulties encountered in packed columns. However, the problems in detecting microamounts of proteins by UV monitoring prompted Stellan Hjerten and colleagues at Uppsala University (Sweden) to develop a more sensitive method based on adsorbing proteins onto a moving blotting membrane. For cation exchange, 5-, 10-, and 15-um i.d. chromatographic tubes were filled with a solution containing piperazine diacrylamide, Ar,A',A'',Ar'-tetramethylethylenediamine, and methacrylamide that was allowed to polymerize. A section of the coating was burned off by a heated tungsten wire to
Analytical Chemistry News & Features, January 1, 1997 13 A
