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Thermal Analysis in Materials Characterization Γτττττπτττττα An intensive short course sponsored by the American Chemical Society Dr. Edith A. Turi, Course Director • Gain an overview of the basic principles of thermal techniques • Learn to eliminate costly largescale trial and error runs • Be able to analyze and interpret thermal data, predict performance, improve quality, increase productivity • Receive personal guidance from experts in the field
Saturday-Sunday March 7 - 8 , 1 9 9 2 New Orleans, LA For more information call the Continuing Education Short Course Office at (800) 227-5558 or at (202) 872-4508. Or, use the coupon below to request a free descriptive brochure on this dynamic course. American Chemical Society Dept. of Continuing Education Meeting Code TAMC9203 1155 16th St., N.W. Washington, DC 20036
INSTRUMENTATION trochemical oxidation at +0.50 V is characterized by a 134 + /184 + inten sity ratio of 0.30 and a 175-/182" in tensity ratio of 0.25, whereas enzy matic oxidation for 1 min yields a 134 + /184 + intensity ratio of 1.8 and a 175-/182" intensity ratio of 0.45. These intensity ratios indicate that the chemical reactions of the electro chemical oxidation i n t e r m e d i a t e s have proceeded to a greater extent in the enzymatic oxidation, leading to greater relative abundances of the fi nal products. In fact, the enzymatically generated intermediates have a much longer residence time in solution (reaction time of ~ 1 min in the reactor) than do the electrochemical i n t e r m e d i a t e s (500 ms); this accounts for the differ ent intensity ratios. During the 1-min enzymatic reaction period, some of the imine alcohol and imine amine can de cay to the bicyclic imidazolone as well as to the final products (Figure 7). Greater relative abundances of the 139" and the 141 + ions, which corre spond to the [M - H]" and [M + H] + of the bicyclic imidazolone, indicate that this in fact occurs. Because the electrochemical and enzymatic thermospray mass spectra are virtually identical, both reactions must follow similar reaction path ways. Therefore, it is reasonable to postulate that the structures of the intermediates and products formed and identified by E C / T S P / M S / M S
reflect those of the enzymatically generated compounds of the same molecular weights. Hence, the obser vation of the imine alcohol and the imine amine intermediates during the peroxidase-catalyzed oxidation supports the formation of a similar primary intermediate in the enzy matic and electrochemical oxidation of uric acid. Advantages and limitations of the thermospray interface for EC/MS Successful analysis by EC/MS r e quires that the component of interest in the condensed phase be t r a n s formed into gas-phase ions. In ther m o s p r a y , h e a t is applied, which quickly vaporizes the LC eluent and ionizes the analyte (Figure 2). This heating process has two major conse quences for on-line EC/MS work. First, reaction rate constants for the disappearance of intermediates can not be readily compared with values obtained by room-temperature meth ods. The rapid temperature changes t h a t occur during the thermospray vaporization process can alter the ki netics of the reactions responsible for product formation. Second, although the thermospray technique is gener ally known as a "soft" ionization technique, some thermal decomposi tion can occur and complicate mass spectral interpretation of a redox re action if the analysis is performed in a flow injection mode. Temperature
~\f£±£ f Please send me JL ( ? ι 9 · information on the ACS Short Course, Thermal Analysis in Materials Characterization, to be held March 7-8, 1992, in New Orleans, LA. Name Title Organization Address City. State. ZIP
Figure 10. EC/HPLC/MS positive-ion mass chromatograms of the oxidation products of 6-thiopurine. (Adapted with permission from Reference 30.)
30 A · ANALYTICAL CHEMISTRY, VOL. 64, NO. 1, JANUARY 1, 1992
