Methyl chloride, mass spectra

Naturally occurring isotopes of any element are present in unequal amounts. For example, chlorine exists in two isotopic forms, one with 17 protons and 18 neutrons ( Cl) and the other with 17 protons and 20 neutrons ( Cl). The isotopes are not radioactive, and they occur, respectively, in a ratio of nearly 3 1. In a mass spectrum, any compound containing one chlorine atom will have two different molecular masses (m/z values). For example, methyl chloride (CH3CI) has masses of 15 (for the CH3) plus 35 (total = 50) for one isotope of chlorine and 15 plus 37 (total = 52) for the other isotope. Since the isotopes occur in the ratio of 3 1, molecular ions of methyl chloride will show two molecular-mass peaks at m/z values of 50 and 52, with the heights of the peaks in the ratio of 3 1 (Figure 46.4). 

A diagrammatic illustration of the effect of an isotope pattern on a mass spectrum. The two naturally occurring isotopes of chlorine combine with a methyl group to give methyl chloride. Statistically, because their abundance ratio is 3 1, three Cl isotope atoms combine for each Cl atom. Thus, the ratio of the molecular ion peaks at m/z 50, 52 found for methyl chloride in its mass spectrum will also be in the ratio of 3 1. If nothing had been known about the structure of this compound, the appearance in its mass spectrum of two peaks at m/z 50, 52 (two mass units apart) in a ratio of 3 1 would immediately identify the compound as containing chlorine. 

Reduction (LiAlH4) of the pachystermines-A and -B gives the pachystermine-diol (232), which was methylated to (233). Treatment of (232) with methane-sulphonyl chloride in pyridine gives the azetidine (234), characterised by an intense peak at m/e 138 in its mass spectrum. Under the same conditions, (233)... 

Fig. 5 shows a comparison based on extracted ion current profiles of ionization efficiencies for Nifedipine. Included are positive ion APCI and ESI, along with negative ion chloride ion attachment APCI. As predicted. Nifedipine can be readily ionized by all three LC/MS ionization processes. The mass spectra of Nifedipine obtained using these three ionization processes are shown in Fig. 6. Positive ion ESI and APCI both show [M + H] at m/z 347 (note the [M + Na] ion at m/z 369 in the ESI spectrum) with some fragmentation (the ion at m/z 315 likely results from loss of methanol from one of the methyl ester groups). The negative ion chloride ion attachment spectrum shows the anticipated [M + Cl] at m/z 381, along with a [M + Cl - HCl] ion at m/z 345. Note that there is also a relatively small [M + TFA] (TFA) ion at m/z 459. If TFA is present in the HPLC...

Hgure 5 Positive ion ToF-SIMS spectrum from poly(methyl methacrylate) in the mass range miz=0-200. (Reproduced with permission from Briggs D and Fletcher IW (1997) Cs" ion beam damage of poly(vinyl chloride) and poly(methyl methacrylate) studied by high mass resolution ToF-SIMS. Surface and Interface Analysis 25 167-176 John Wiley Sons Ltd.)... 

Parallel results were found when bryostatins 4 and 10 were allowed to react with m-chloroperbenzoic acid in methylene chloride at room temperature. Introduction of one oxygen atom required two days. The SP-SIMS spectra were consistent with an additional 16 mass units, but while epoxidation of bryostatin 4 occurred at the A -olefin, epoxidation of bryostatin 10 occurred at the A -olefin, a conclusion reached by considering the H-NMR spectra. The spectrum of bryostatin 10 epoxide retained the olefinic resonances of the A double bond but the H-30 olefinic resonance at 85.658 was shifted to higher field at 53.358, a shift characteristic of hydrogen on an epoxide. In addition, the C-36 methyl... 

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