State-ofithe-Art: Counting Molecules-Approaching the Limits of Chemical ~ n a l y s s It was clear even in the very early days of laser development that the laser re~resenteda lieht source that would orovide extraordinary sensitivity to l& concentrations of materials. I t was not really appreciated how low one could go until Hurst, Nayfeh, and Young published their classic work in 1977 demonstrating detection of sinele atoms. This achievement is the ultimale in a chemical measurement and was viewed with meat interest by. . people . in analstical chemistw. It is the of this symposium to bring together experts in the various areas of chemical measurement to present their vision of the present status in ultra-trace measurement methods and of the future promises toexcend the methods to the ultimate limits. Although the flashy idea is always to go to lower concentrations, a practicing chemist quickly finds that the prohlem they encounter is usually not having enough signal hut rather having too many other materials that are also contributing to the signal. Methods are usually not limited by their detection limits hut by their specificity for the particular compound of interest. As Fred Lytle noted during the symposium, a less sensitive method can actually get hetter detection limits in an analysis of a real sample if it has hetter selectivity than a competitive method. Thus a recurring theme during this symposium is the inherent specificity of the various methods. Two general classes of techniques which are presently capable of extremely low detection limits are laser methods and mass spectroscopic methods. The symposium begins with an exploration of different laser techniques followed by an exploration of different mass spectroscopic techniques. It concludes with a presentation which unites the two methods into a common one which has the promise of using the best features of hoth to make a very sensitive and versatile method. Sam Hurst (page 895) begins the symposium with a discussion of single atom detection. He introduces the idea that ionization can he detected with much higher efficiency than fluorescence and that laser-excited ionization can approach almost unit efficiency. Thus single atoms within a laser beam can he measured easily. However, to take a real sample and convert it into its atomic constituents is a demanding task that introduces many complications. Gary Hieftje (page 900) approaches this prohlem and discusses the various approaches that are used now to solve it. Gary concentrates on the different methods that are used to bring a sample into the gas phase in an atomic state and the use of laser-excited fluorescence from these atoms. John Travis (page 909) concentrates instead on a complementary technique which uses flames to
atomize and then ionize a sample. The ionization is enhanced by using a laser to excite the atom closer to the ionization continuum. Many timesone is not interested in the atomic composition hut in the molecular composition. Fluorescence is a very sensitive wav to see low concentrations of molecules that fluoresce. ~ & one e immediately runs into the problem of the lack of s~ecificitvhecause d l samoles contain im~uritiesthat fluoresce. red-Lytle (page 9i5) discusses fluorescence methods and concentrates particularly on ways of increasing the specificity for an analysis. Not all molecules fluoresce, however. A very general analytical approach that can measure all molecules is mass spectroscopy. A mass spectrometer is capable of very low detection limits and of providing detailed information about a molecule. For most samples however, it does not have sufficient specificity. Mike Gross (page 921) discusses applications where gas chromatography is used as a separation step prior to detection by mass spectroscopy. He uses the important example of measuring dioxins in the environment to illustrate this approach. Graham Cooks and K. L. Busch (page 926) look a t many of the other methods to increase specificity in mass spectroscopy. They concentrate on approaches where another mass spectrometer provides a separation step fur a mass sDectrometer. FinAly, Rick Smalley (page 934) unites these two general areas in his talk on mass selective photoionization. A laser is used to initiate photoionization by absorption of several ohotons. Such a orocess leads to both ions and fraements that can he examined by a mass spectrometei. The combined use of laser wavelength. intensitv. and f r a w e n t mass ~rovides a method with specificxy as wellas very low detection limits. I t is always difficult to see into the future, hut this symposium presents one set of perspectives on present and future developments. The areas are currently being explored with great enthusiasm and it is likely that new ideas will open up completely new directions ,not even considered here for reaching the ultimate limits. Nevertheless, the reader will get a sense of the excitement through reading the articles in this issue and perhaps will be the person who brings the key idea that allows us truly to reach the ultimate limit. ~
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John C. Wright University of Wisconsin-Madison Madison. WI 53706
Offprints of "Counting Molecules" Available
Journal of Chemical Education
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Symposium Organizer
The seven papers comprising this fifth State-of-the-Art Symposium provide a unique package of teaching materials on a topic of current interest, and it is felt that there will be a high demand for reprints of the whole Symposium. Therefore, offprints containing all of the articles in the series bound together and covered have been made and are available for $5.00 each ($4.00 each for 10 or more to the same address), including postage. In order to keep processing costs at a minimum, f d payment must accompany all orders. Send check or money order to Molecule Offprint, Journal of Chemical Education, 238 Old Kent Road, Springfield, PA 19064.
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