Special Report: An Exciting Era In Chemistry Instrumentation

Instrumentation AUTHOR: MICHAEL ROOT, p.23 Improvements in chemical instrumentation have been accelerating within the last few years, and many of the technology advances have been further enhanced by new or derivative methods. With innovative techniques that allow chemists to study the composition and reaction dynamics of more materials with greater efficiency, accuracy, and sensitivity, it's an exciting time for the field of chemistry. There was a time when nearly all work in the field was done using so-called wet chemical processes, such as titrations for qualitative analysis of metal ions. Instrumental methods were often labor-intensive and time-consuming. But the convenience, speed, and versatility of chemical instrumentation today, especially with the addition of computer control and data handling, have facilitated a greater understanding of chemical systems. For decades, analyzing for metal ions involved extraction with an acid, neutralization, addition of an indicator, and titration with a complexing agent like EDTA. Now, chemists using atomic absorption spectroscopy or inductively coupled plasma spectroscopy rapidly measure metal ion concentrations in much lower levels, with smaller sample sizes, without the need for time-consuming titrations. Computerized systems with automatic samplers that can run overnight further reduce analysis time, freeing chemists to perform other experiments. While some equipment and techniques have evolved slowly, others have exploded onto the chemistry research scene, quickly replacing earlier methods. Chemists from around the world will be discussing these recent advances in chemical instrumentation when they convene in Atlanta April 14-19 for the spring 1991 meeting of the American Chemical Society. Although the primary use for these instruments is for analytical chemistry, the utility of many devices spans the traditional divisions of the field. Chromatography encompasses a number of techniques, many of which have been developed in the last few decades. Gas chromatography (GC), introduced in the 1950s, separates volatile compounds by pushing them through a column (packed or capillary) with a carrier gas. This sensitive and rapid method allows chemists to analyze, both qualitatively and quantitatively, a wide variety of volatile organic compounds they were unable to examine using traditional methods, such as derivitization techniques. Following the widespread use of GC, liquid chromatography (LC), a method invented in 1906, was improved to yield the now-familiar high-performance liquid chromatography (HPLC). While LC uses gravity to push the liquid solvent through the column, HPLC uses pumps to create a high-pressure solvent flow. Many inorganic and organic compounds, including polymers, are now readily analyzed with the appropriate choice of column and mobile phase (the solvent system that separates compounds on the column). Yet there are a number of chemical compounds not analyzed well by GC or LC. For example, thermally unstable organic compounds, such as isocyanates or explosives, can decompose at the temperatures used for GC. Nonionic surfactants are not easily observed by traditional LC detectors. Supercritical fluid chromatography (SFC) can overcome some of these drawbacks, however. SFC has been available since the early 1960s. At first, it never developed a market, and was little used until the mid-1980s. At that time, the technique was revitalized by the addition of capillary columns, according to marketing manager Dale Felix of Lee Scientific in Salt Lake City, a division of Sunnyvale, Calif.-based Dionex Corp., an SFC manufacturer. A supercritical fluid, like a gas, is compressible and expands to fill its container, but has the density and solvating ability of a liquid. Supercritical fluids are formed under high-temperature and high-pressure con- ditions. Says Felix, "Methods development using SFC is fairly simple [and] straightforward, similar to what you do with GC." These properties give SFC the ability to compete with both GC and HPLC, Felix notes. "It kind of fits right between the two," adds Milton Lee, professor of chemistry at Brigham Young University in Provo, Utah, one of the developers of capillary SFC. Open, tubular capillary columns may be 250 micrometers or less in diameter. Larger columns, on the order of about a millimeter, may be packed with silica or polymeric particles. HPLC product manager Cary Thrall of Isco Inc. in Lincoln, Neb., says the company's syringe pumps without check valves, which are needed to create the requisite pressures, were often used in homemade systems before the recent commercialization of SFC. Most detectors, devices that sense the presence of a chemical, normally used in GC or LC can be used with SFC. These include mass spectrometers, flame ionization detectors, and nitrogen/phosphorus detectors. Mobile phases may be carbon dioxide, freons (or their substitutes), xenon, or virtually any other chemical that forms a suitable supercritical fluid. SFC has helped chemists extend their ability to analyze complex substances like polymers, biomedical samples, waxes, coal extracts, and tars, Brigham Young's Lee says. SFC handles organic compounds with a higher molecular weight than those that can be handled by GC. Chemists in the petroleum industry are using SFC to evaluate distillation regimens of petroleum products. Lee, the founder of Lee Scientific, is examining how to effect enantiomeric separation of pharmaceutical compounds, a difficult task because enantiomers are nearly identical chemically. Environmental chemists are also benefiting from this technology. SFC is more useful than GC or LC for pesticide analysis, for example, Felix says. Pesticides may be nonvolatile or too heat-sensitive for GC. Common HPLC detectors may not detect the pesticides, thereby requiring derivitization. With SFC, lower temperatures can be used and detectors are more versatile. Capillary electrophoresis (CE) is another technology that has "really advanced in the last three to four years," according to Isco CE product manager Kevin Patt. Like gel electrophoresis (GE), its predecessor, CE is primarily a tool of biochemists and biomedical scientists (The Scientist, Sept. 3, 1990, page 27). The distinction is that CE is faster, less labor-intensive, and more versatile than GE, and it can do many of the tasks normally done by HPLC. Since capillary columns are used, the volume of buffer solutions required for CE is a fraction of that for HPLC, translating into reduced costs and less solvent waste. CE is relatively new and may eventually replace GE entirely. GE separates biochemicals such as peptides and oligonucleotides by applying a high voltage across an ionically conducting gel bed. However, if the voltage is too great, the gel overheats, potentially damaging the sample or gel. CE uses capillary columns--filled with gel, solids, a buffer solution, or other liquid--that dissipate heat more efficiently so higher voltages can be used. This results in better resolution and faster analysis time--typically 10 to 30 minutes for CE, contrasted with two hours to three days for GE, Patt explains. Other advantages over GE include small sample volumes (less than 1 microliter, compared with 1 to 100 microliters for GE and 10 to 50 microliters for HPLC), and low detection limits (picogram levels). CE is more quantitative and is more easily automated than GE. In addition, a variety of detectors can be used. Ian Jardine, director of analytical biochemistry for Finnigan MAT in San Jose, Calif., is exploring mass spectrometry (MS) as a detector for CE. With MS, it's often difficult to detect the chemical of interest when it is dissolved. But Jardine credits differential pumping and the recently developed electrospray technology with rendering MS useful for GE and many other techniques in which the chemical of interest is dissolved in solution. CE instrumentation has improved the ease with which scientists identify and measure complex biochemicals. DNA mapping for the Human Genome Project, for example, is one area in which CE is being used. The technique has increased the speed of DNA sequencing, an important feature in light of the fact that the human genome contains 3 billion DNA bases. According to Joseph Olechno, CE product manager at Dionex, CE goes well beyond proteins and oligonucleotides. It is much faster and less expensive than HPLC for molecules with molecular weights less than a few hundred daltons. CE can also separate organic acids and amines, vitamins, and even inorganic ions like lanthanides and alkali metals. It measures inorganic species at about 1 part per million, much less sensitive than other chromatographic methods, but still within a concentration range commonly used in the laboratory. Raman spectroscopy measures the light scattered by molecules irradiated with monochromatic light. It is particularly useful for the structural analysis of organic compounds with multiple bonds and those containing sulfur atoms. Ever since its underlying principle was discovered in 1928, Raman spectroscopy has been used primarily in academic research laboratories and has never enjoyed the popularity of its counterpart, infrared (IR) spectroscopy. One reason that the technique has remained relatively uncommon is that, up until the late 1980s, visible wavelength lasers were generally used. This limits the practicality of Raman to compounds that do not fluoresce, since these emissions obscure the relatively weak Raman signal. "Ninety percent of the time you didn't get a spectrum," says Sam White, IR senior product specialist at Perkin Elmer Corp. of Norwalk, Conn. In the past five years, however, with the development of Fourier transform Raman spectroscopy (FTR) equipped with near IR lasers, Raman spectroscopy has become more practical. According to Ken Kempfert, product development manager at Nicolet Instrument Corp. in Madison, Wis., FTR samples are irradiated with a relatively high-powered neodymium YAG (yttrium aluminum garnet) laser at a wavelength of 1,064 nanometers. This eliminates a majority of the fluorescence problems. Further, FTR has excellent signal-to-noise characteristics, resolution, and precision. White adds, "Unless [the sample is] black you can pretty much be guaranteed of getting a spectrum." Mary Ann Finch, national marketing manager at Bruker Instruments Inc. of Billerica, Mass., agrees, noting that with FTR, because of reduced fluorescence, "you can look at 98 percent of the world, instead of just 10 percent [with regular Raman]." Although FTR is still a research instrument at this time, both Kempfert and White say they believe that analytical methods for the technology will be developed in the future. White notes that glass and water, strong IR absorbers, tend to be invisible in FTR. "You can have pills in a glass bottle, hold it up to the laser, and run a spectrum" to identify the drug, binding agents, and other ingredients in the pills, he says. The most frequent application of vibrational spectroscopies like FTR is for organic compounds. Thomas Brill, professor of chemistry at the University of Delaware in Newark, is working on FTR methods to study the reactivity of environmentally hazardous organic compounds in water at high temperature and high pressure, typically 230oC to 250oC and up to 5,000 pounds per square inch. "It's an absolute virgin area," Brill says. At high temperatures, up to 500oC, even extremely stable chlorohydrocarbons are readily decomposed to essentially carbon dioxide, hydrochloric acid, and water. This may provide an innocuous way to destroy toxic organic chemical wastes like PCBs and dioxin. Brill is looking to use FTR to probe the dynamics of the decomposition reactions of organic compounds under these conditions. FTR offers an excellent way to monitor these chemicals, since they are in aqueous solution. One of the trends in chemical instrumentation is the combination of two techniques. GC/MS is a common example. About eight years ago, the first Fourier transform infrared (FTIR) microscopes were introduced, marrying optical microscopy with FTIR spectroscopy. FTIR microscopy represents a significant advance for materials analysis. Using this technique, chemists may obtain an IR spectrum on samples as small as 10 micrometers in diameter. Other methods capable of that kind of spatial resolution, such as energy dispersive spectroscopy or Auger spectroscopy, will provide only elemental analysis, whereas FTIR offers more chemical information and unique "fingerprints" for various compounds. The first FTIR microscopes were constructed from light microscopes, which were then converted to an FTIR instrument, according to White. Perkin Elmer took a different approach, however, by building optics around an IR instrument. "We started from the other end," White explains, but "ultimately you end up at the same place." "There is a broad range of applications" for FTIR, says Andre Sommer of Miami University in Ohio. As a spectroscopist in the university's Molecular Microspectroscopy Laboratory (MML), an instrument development laboratory, he has seen numerous instances in which FTIR microscopy has solved materials problems, such as identification of particulate impurities and analysis of polymer laminates used in packaging. Sommer sees FTIR microscopy as a valuable tool in examining archeological artifacts and art authentication, since only small samples are needed and it is nondestructive. Analysis of the chemicals used to dye fibers found at archeological sites has been used to surmise how advanced civilizations were and to infer trade routes for dyes that were not indigenous to the region. Patricia Lang, formerly a graduate student at MML, helped to expose an art forgery for a Chicago art museum using FTIR microscopy. One of the more highly publicized breakthroughs in instrumentation within the last five years has been the development of scanning tunneling microscopy (STM). IBM scientists Gerd Binnig and Heinrich Rohrer in Zurich, Switzerland, invented STM in 1982, for which they were awarded the 1986 Nobel Prize in physics. The power of STM lies in its ability to image surfaces on the atomic level in air, solution, or vacuum. Since the development of the technique, several companies have jumped into STM production (The Scientist, June 12, 1989, page 18). STM basically consists of a fine, conducting probe that is brought within a few nanometers of a material's surface. When a voltage is applied between the specimen and probe, a current flows between them. Although conductivity differences can influence the current, it generally corresponds to the height of the tip above the sample surface. Computer-controlled micropositioning devices move the probe up or down as it is scanned across the surface in order to maintain a constant current. Alternatively, changes in the current can be recorded. In this way, the topology of a surface can be mapped. According to Jerome Wiedmann, marketing director for Digital Instruments Inc. of Santa Barbara, Calif., the piezoelectric micropositioning devices and probe tips used in all STM instruments do not in themselves represent any significant advance. Rather, the important developments have occurred in the supporting systems that control these tools. "People have known about piezoelectrics for some time," he says. And scientists can simply cut a wire with scissors to make the probes. "The name of the game is feedback electronics and software control," says Wiedmann. Like antilock brakes for automobiles and swept-wing aircraft, STM had to wait for microprocessor development to catch up, in order to control the micropositioning devices carefully enough. STM has helped chemists visualize the surfaces of materials in a way not available through other techniques. Some of the major uses are investigating thin films, such as those on computer hard disk drives, or silver halide coatings on photographic film. STM instruments sold today include a potentiostat capable of controlling voltage with respect to a reference electrode. This feature permits chemists to study electrochemical reactions, such as electroplating and corrosion, at a sample's surface. "Rust may be boring," Wiedmann says, but "as far as industry is concerned, it is very important." Corrosion is a crucial factor in industries in which metals are used, such as packaging and electronics. Some researchers are looking into STM and related methods to study biological specimens, including DNA, proteins, and viruses. Arizona State University solid state physicist Stuart Lindsay, for example, used STM to determine that calf thymus DNA was kinked, as Francis Crick and Aaron Klug had predicted in 1975. Lindsay also found that synthetic DNA has a more linear conformation than natural DNA, which may influence its reactivity. Computer control has become an essential ingredient in new chemical instrumentation. Even older instruments, such as atomic absorption spectroscopy and nuclear magnetic resonance spectroscopy systems, are now sold with computers to run experiments and manipulate data. With the greater efficiency and speed that computers provide, along with new technological advances, today's chemists can detect smaller levels of chemicals, probe individual atoms on surfaces, elucidate the structure of complex molecules, and study faster reaction kinetics. It is likely that these trends will continue as new generations of instruments are introduced in the future. Michael Root is a research chemist and science writer based in Verona, Wis.

Special Report: An Exciting Era In Chemistry Instrumentation

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