Field desorption protonation

Mass Spectrometry. Field desorption mass spectrometry has been used to analy2e PPO (179). Average molecular weight parameters (M and could be determined using either protonated (MH + ) or cation attachment (MNa + ) ions. Good agreement was found between fdms and data supphed by the manufacturer, usually less than 5% difference in all cases up to about 3000 amu. Laser desorption Fourier transform mass spectrometry was used to measure PPG ion and it was claimed that ions up to m/2 9700 (PEG) can be analy2ed by this method (180). 

Hufford et al [57] used proton and 13C NMR spectrometric data to establish the novel sulfur-containing microbial metabolite of primaquine. Microbial metabolic studies of primaquine using Streptomyces roseochromogenus produced an A-acety-lated metabolite and a methylene-linked dimeric product, both of which have been previously reported, and a novel sulfur-containing microbial metabolite. The structure of the metabolite as an S-linked dimer was proposed on the basis of spectral and chemical data. The molecular formula C34H44N604S was established from field-desorption mass spectroscopy and analytical data. The 1H- and 13C NMR spectra data established that the novel metabolite was a symmetrical substituted dimer of primaquine A-acetate with a sulfur atom linking the two units at carbon 5. The metabolite is a mixture of stereoisomers, which can equilibrate in solution. This observation was confirmed by microbial synthesis of the metabolite from optically active primaquine. 

In this presentation, we report the characterization of HTE liquid polymers free from the interference of cyclic oligomers. Cyclic oli omers of the higher molecular weight HTE liquid polymers (Mn>1200), including commercial Hydrin 10 liquid polymers, were removed by extraction. The functionality of the liquid polymer was then determined, and structures are proposed as determined by infrared, carbon-13 and proton NMR, and field desorption mass spectroscopic analyses. 

The precursor model of FAB applies well to ionic analytes and samples that are easily converted to ionic species within the liquid matrix, e.g., by protonation or deprotonation or due to cationization. Those preformed ions would simply have to be desorbed into the gas phase (Fig. 9.6). The promoting effect of decreasing pH (added acid) on [M+H] ion yield of porphyrins and other analytes supports the precursor ion model. [55,56] The relative intensities of [Mh-H] ions in FAB spectra of aliphatic amine mixtures also do not depend on the partial pressure of the amines in the gas phase, but are sensitive on the acidity of the matrix. [57] Furthermore, incomplete desolvation of preformed ions nicely explains the observation of matrix (Ma) adducts such as [M+Ma+H] ions. The precursor model bears some similarities to ion evaporation in field desorption (Chap. 8.5.1). 

M ammonium acetate, which yields [NH4] +, clustered [NH4] + and [CH3COO] ions protonation to [M + H] + or ammonium addition to [M + NH4] + is observed and the spectra are similar to ammonia Cl or field desorption spectra327,374 possible reactions between gas-phase ions and analyte molecules have been investigated374. The ionization mechanism is still not entirely understood, nor is the influence of a number of experimental parameters such as the nature and concentration of the electrolyte, the flow rate and the thermospray temperature375. Thermospray ionization exhibits better sensitivity than conventional Cl and is widely used in pesticide residue analyses327,336,337,340. 

Ionization Methods/Processes. The recent development of several new ionization methods in mass spectrometry has significantly improved the capability for the analysis of nonvolatile and thermally labile molecules [18-23]. Several of these methods (e.g., field desorption (FD), Californiun-252 plasma desorption (PD), fast heavy ion induced desorption (FHIID), laser-desorption (LD), SIMS, and fast atom bombardment (FAB) or liquid SIMS) desorb and ionize molecules directly from the solid state, thereby reducing the chance of thermal degradation. Although these methods employ fundamentally different excitation sources, similarities in their mass spectra, such as, the appearance of protonated, deprotonated, and/or cationized molecular ions, suggest a related ionization process. 

Although the determination of HA or HB selectivity is relatively straightforward the techniques for isolation of pyridine nucleotides from the reaction mixtures are tedious and time consuming. Two more recent techniques use either proton magnetic resonance or electron impact and field desorption mass spectrometry. The technique of Kaplan and colleagues requires a 220 MHz nuclear magnetic resonance spectrometer interfaced with a Fourier transform system [104], It allows the elimination of extensive purification of the pyridine nucleotide, is able to monitor the precise oxidoreduction site at position 4, can be used with crude extracts, and can be scaled down to /nmole quantities of coenzyme. The method can distinguish between [4-2H]NAD+ (no resonance at 8.95 8) and NAD+ (resonance at 8.95—which is preferred) or between [4A-2H]NADH (resonance at 2.67 8, 75 4B = 3.8 Hz) and [4B-2H]NADH (resonance at 2.77 8, J5 4A = 3.1 Hz). 

Features common to the spectra of glucuronic acid conjugates analysed by FAB, laser and field desorption were summarized several years ago (15). These appear to hold as well as for plasma desorption and thermospray spectra more recently examined. The situation with thermospray is somewhat more complicated as will be discussed later. Generally speaking, positive ion spectra contain protonated, natri ted or analogous molecular ions species, and usually (M+H-176) ions formed by the elimination of neutral dehydroglucuronic acid. 

Arginine is known to be a delicate thermosensitive sample. Mass spectra of the intact molecule neither upon electron impact nor upon chemical ionization are known, since this molecule undergoes a neutral decompositon reaction upon heating. A field desorption mass spectrum of the protonated molecule has been measured/26/. 

Figure 20-6c is a field desorption spectrum for glutamic acid. It is even simpler than the spectrum from field ionization and consists of only the protonated molecular ion peak at miz = 148 and an isotope peak at iniz = 149. 

Degradation products from base treatment of cefaclor have not been isolated for characterization. A piperazine-2,5-dione (133) was obtained from refluxing the p-nitrobenzyl ester of cefaclor (132) in benzene. Cephalexin p-nitrobenzyl ester (130) in a parallel experiment had been shown to give 131. Quite unexpectedly, this degradation product of cefaclor contained no chlorine in elemental analysis the field desorption mass spectrum suggested the loss of HCl. The structure for 133 was derived from proton and C-NMR analyses. 

Pyrazofurin was obtained courtesy of Eli Lilly Co., Indianapolis, Ind. (Lot y/f CT-2940-4B). Each ampoule contained 300 mg PF as lyophylized powder and was reconstituted with 10 ml of sterile 0.15 M NaCl prior to use the vials were stored in a refrigerator. Pyrazofurin 5 -monophosphate (PFP) was obtained courtesy of Eli Lilly Co., Indianapolis, Inc. PFP was synthesized by the 5 --phosphorylation of a 2 ,3isopropylidene derivative of PF, followed by removal of the ketal protecting group (5). The field desorption mass spectrum of PFP revealed a strong signal at m/e 340, corresponding to the protonated molecular ion (12). 

The FD spectra of a number of sulphonic acid alkali sal s have been reported to show intense M + C y ions besides M / ions (26). Protonation can also be an ion generating process in the field desorption of an organic alkali salt as is demonstrated in Fig. 6 for 3-deoxyfluoro-D-glucose-6-disodium phosphate. 

Field desorption spectra are usually simple since the field imparts little excess internal energy to the desorbed ions. The mass spectra often contain only the molecular ion (M ) or protonated molecular ion ([M +... 

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