Polymer Probing Accelerated
The utility of high-throughput screening (HTS) extends well beyond the environs of the pharmaceutical laboratory. Several other industries have embarked on accelerated combinatorial chemistry endeavors, and the need for rapid analysis techniques is ever growing.

Polymer manufacturing is a prime example of a sector in which automated synthesis is providing large amounts of products for the development of new materials. Now, Arno Tuchbreiter and colleagues from Universität Freiburg in Germany have demonstrated the efficacy of a technique called attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy for fast and high-accuracy analysis of important polymer compounds (J. Comb. Chem. 2001, 3 (6), 598–603).

Olefin copolymers, such as ethene/propene, have numerous materials applications, including plastics, optics, and adhesives. Different polymerization parameters can lead to a range of possible composition ratios with varying properties. Traditionally, polymers have been characterized using ¹³C nuclear magnetic resonance (NMR), which is very cost-intensive and time-consuming, taking approximately 5 h per sample. Conventional transmission FTIR spectroscopy is effective for polymer analysis as well and a lot faster and cheaper than NMR, but the involved sample preparation that is often required (e.g., compression molding of KBr pellets) is not ideal for HTS. ATR-FTIR spectroscopy, on the other hand, works via IR reflection, as opposed to transmission, from the sample, which doesn’t require any premanipulation.

“Golden Gate” ATR-FTIR schematic.

The researchers used a “Golden Gate” setup (see figure) in which the polymer sample is immobilized on top of a dense crystal platform and an IR beam is directed to the underside of the crystal at an angle greater than the system’s critical angle, which depends on the refractive properties of both the crystal and the polymer. This process causes a total reflection of the wave, minus what is absorbed by the sample, off the polymer surface and back through the crystal to a detector.

Using this ATR approach, the team was able to generate IR absorption spectra for several copolymers—ethene/propene, ethane/1-hexene, and ethane/1-octene—at a rate that would allow for 40 probes per h (as opposed to about 2 measurements per h for conventional FTIR spectroscopy). Using multivariate calibration on the spectra, content ratios (e.g., ethene vs. propene) were determined such that they correlated very closely to results from ¹³C characterization.

Thus, ATR-FTIR spectroscopy shows strong promise for allowing a combinatorial approach in searching for new and useful polymerization parameters.

In the case of nitrogen oxide (NO_x) emissions, this second benefit can be of utmost importance—NO_x can interact with volatile organic compounds (VOCs) already in the atmosphere to create ozone, an environmental contaminant known to be harmful to human health. The amount of VOCs varies depending on location. Thus, as the theory goes, allowing plants that must release more NO_x only in areas with low concentrations of VOCs could keep ozone concentrations low.

Recently, David T. Allen and his team from the University of Texas at Austin developed a test to actually analyze the utility of location-dependent NO_x emissions trading for ozone control.

The researchers studied data from ozone concentrations around 51 plants in eastern Texas that emit NO_x from time periods in 1993, 1995, and 1996 (Environ. Sci. Technol. 2001, 35 (22), 4397–4407). The team then sorted the plants into worst case scenarios, trades resulting in plants with large outputs of NO_x being located in areas with high VOC concentrations, and best case scenarios, in which large output was paired with lower concentrations. The researchers found that between all possible trades, approximately 20% fit into the best case scenario category. Through statistical simulations, it was determined that these trades, if allowed to take place, could reduce ozone production by at least 25% on any given day. Additional simulations that were performed indicated that the average amount of ozone production could be reduced and the variability of day-to-day ozone production could be decreased if a small group of high-impact facilities (those producing more NO_x in an area of high VOC concentration) were allowed to sell, but not buy, emissions credits.

Thus, smarter trades between plants could greatly decrease ozone levels. And studies such as this one could help government officials set up a framework for trading rules.

Many beer producers—and beer consumers —have been aware of this light problem for some time (hence the common use of brown glass for beer bottles), and recent experiments have pointed to the major bittering agents of beer, the six isomerized -acid compounds that are derived from the hops, as the photosensitive precursors to the repugnant sulfur compound, 3-methylbut-2-ene-1-thiol, found to be a major contributor to the skunked flavor.

Now, scientists from the University of North Carolina (Chapel Hill) and the University of Ghent (Belgium), led by Malcolm D. Forbes and Denis De Keukeleire, have uncovered significant details regarding the photochemical, free radical pathway toward the thiol (Chem.—Eur. J. 2001, 7 (21), 4553–4561) by investigating the unpaired electrons that arise. The researchers used time-resolved electron paramagnetic resonance (TREPR) spectroscopy, a technique that measures, at a very fast timescale, transitions between energy levels created from interactions of electron spin states with a magnetic field. TREPR on irradiated samples of the iso--acids allowed the team to directly monitor the creation of short-lived free radicals during the photodegradation. The two major (resonance-stabilized) radical intermediates produced were pinpointed, on a timescale of about 400 ns, by overlaying the TREPR spectra on computer simulations of potential radical species’ spectra.

On the basis of the general properties—strength, timescale, and coupling constants—of the TREPR signals, the researchers were able to make several solid conclusions on the photophysical states that must be passed through for those radicals to be formed (see figure). Additionally, another group of iso--acids not commonly found in beer, with the side-chain carbonyl (adjacent to the radical formation site) reduced to a hydroxyl group, were investigated and found to not result in any TREPR spectrum at the experimental wavelength. This, according to the study report, “is the most conclusive proof to date that the photochemistry leading to lightstruck flavor in beer originates at this [the side-chain carbonyl] site.”

With the current trend in the beer industry to switch from brown glass to cheaper clear glass bottles, a greater understanding of what causes the so-called “lightstruck” flavor is essential to preventing it.

Agilent Sends Columns Packing. Agilent Technologies (Palo Alto, CA) will transfer its packed gas chromatography (GC) column sales to Supelco, a division of Sigma-Aldrich (St. Louis, MO). Agilent will continue to sell its other GC products, services, and consumables (Business Wire).

Funding Pharma at Purdue. Purdue University (West Lafayette, IN) has received $5 million to support pharmaceutical education and the construction of a good manufacturing practice manufacturing facility. The facility would make Purdue one of only a few universities able to make drug products for human consumption. The gift was from Allen Chao, CEO of Watson Pharmaceuticals Inc. (Corona, CA), and his wife, Lee Hwa-Chao (Purdue University press release).

Cornell Digital Library Group Gets Grant. The National Science Foundation awarded $1.56 million to Cornell University’s (Ithaca, NY) Digital Research Group to develop the core infrastructure of a National Science Digital Library that will be a source of high-quality science information for students from kindergarten through college (for more information, see www.cs.cornell.edu/cdlrg).

CheMatch Surpasses 5 million. The Global Trading Network of CheMatch.com, an Internet-based marketplace and information resource for commodity chemicals, polymers, feedstocks and fuel products, surpassed 5 million metric tons in transaction volumes in the third quarter of fiscal year 2001 (Business Wire).