News of an FT mass spectrometer, the fragments of unlabeled lysozyme appear as multiplets because of the presence of naturally abundant isotopes. The fragment at 584.4 has isotopic peaks separated by 0.5 mass units indicating a charge of +2 and a mass of 1168.8 Da. Comparing the unlabeled spectrum with peaks in the corresponding spectra of isotopically labeled lysozymes provided a count of 53 carbons and 18 nitrogens for this fragment, and only one possibility with the correct carbon and nitrogen count is found within 50 ppm of the experimental m/z. With this general approach 20 of the 24 fragments found in the 500-2000 for the lysozvme were uniquely identified (/. Am Chem Soc 1199 118 7402-03)
Measuring ascorbic acid kinetics
Getting more information from MALDI-TOFMS Since its introduction a decade ago, matrix-assisted laser desorption/ionization MS (MALDI-MS) has grown in importance as a means to determine the molecular weight of biomolecules even in crude extracts or in peptide mixtures. However, MALDI is not only a technique for producing intact molecular ions. Depending on the matrix, irradiance, and ion source conditions, MALDI-MS experiments C3n also be used to generate ion fragments. For molecular weight screening, especially for mixtures of proteins, this fragmentation is undesirable and contributes to the degradation of mass resolution for larger proteins For smaller analytes on the other hand the fragmentation can facilitate the determination of the chemical structure a technique labeled postsource decay
Jianru Stahl-Zeng and Franz Hillenkamp of the University of Munster (Germany) and Michael Karas of the University of Frankfurt (Germany) have investigated metastable-ion analysis using a gridless reflection time-of-flight MS instrument, applying three different acceleration potentials: 5 kV, , 7V, and 10 kV. The gridless ion mirrors allow detectors with smaller active areas to be used compared with two-stage reflection instruments. The authors found that ion mirror tuning has a strong influence on instrument performance; they also describe a simple window-independent mass calibration function to calibrate fragment ions to a mass accuracy of 5 x 10"4. The full fragmentation pattern was observed even at low acceleration potential, but with a reduced mass resolution. Delayed ion extraction can be used to improve the resolution of precursor ion selection. {Eur. Mass Spectrom. 1996,2, 23-32)
Although ascorbic acid (vitamin C) is probably the most widely used vitamin supplement, there is no international consensus on daily requirements and the amount required for antioxidant protection. Nutritional surveys provide limited information, but tracer studies are necessary for detailed kinetic information. Adrian P. Izzard and colleagues at the Dunn Nutritional Laboratory (U.K.) have evaluated using stable isotopes rather than the commonly used radioactive 14C-labeled material for in vivo mcssurement. of ciscor* bic acid pharmacokinetics. They used electron impact GC/MS to measure the enrichment of ascorbic acid in plasma for a 24-h period after oral adPost source decay-MALDI of the peptide KA6-K in 2,5-DHB, with a wavelength of ministration of 13C-labeled material. (Be337 nm and prompt extraction. (Adapted with permission of IM Publications.) cause ascorbic acid absorption is known to be dose-dependent, with excess material excreted in the urine, the researchers limited the dose of 13C-labeled ascorbic acid to 50 mg, about the amount in one The ascorbic acid in the plasma samstudy, and that first-order kinetics may not large orange.) ples was derivatized with tetramethylsiadequately describe the metabolic clearlane (TMS), and the resulting tetra-TMS ance of ascorbic acid in humans. (J. Mass ascorbate was subjected to GC/MS using Spectrom. .199,31, 741-48) electron impact and positive-ion chemical ionization. Although they obtained only Crank up the pH partial chromatographic resolution, the researchers were able to use selected-ion Reversed-phase separation of basic commonitoring to quantitate the ascorbic acid pounds at pH < 3 often gives the best peak without interference from the glucose shape and column efficiency, although enantiomers that flank it. solute instability or band-spacing problems can result. Separation at pH 4-8 can Their results, although comparable to those reported in other published studies, solve these problems, but it also creates new ones. At pH 9 or above, separation indicate that the U.KL's recommended becomes very attractive because basic daily intake of 40 mg/day of ascorbic acid Typical chromatogram of sugars in plasma. compounds can be separated as free for normal adults may be insufficient to (Adapted wiih permission of John Wiley & bases, which minimizes interactions with maintain the body levels found in this Sons.) 596 A
Analytical Chemistry News & &eatures, October 1, 1,96
