Positive ion spectra

Fig. 50. High-resolution positive ion spectra of hd-PE after electrochemical treatment with nitric acid for I min at 60°C in the pre.sence of silver ions. Peak components in order of increasing mass wi/z =43 - C H,0+, C3HI w/z = 57 - CiHsO+, C4H+ wi/z = 71 - CiHiOt, C4H7O+, C5H+ mjz. = 85 - C4HsOt, CsH.)0, C6H/ 3 m/z = 97 - C5H202 CzH/, m /z =111-C6H70t, C7H 0, CsH 5. Reprinted by permission of John Wiley and Sons from Ref. [58].

Experimental. The mass spectra in Figures 1-8 are positive-ion spectra produced by electron impact and were obtained from a single-focusing, magnetic deflection Atlas CH4 Mass Spectrometer. The ionizing potential was 70 e.v. and the ionizing current 18/a a. An enamel reservoir heated to 120°C. was used from which the sample was leaked into the ion source. 

Furlei and coworkers44 studied the negative ion mass spectra of several cyclic sulfones (82-98) upon dissociative electron capture and concluded that the negative molecular ions were notably stabilized by the introduction of electron-withdrawing substituents and/or unsaturation. Some difference was found in the negative ion mass spectra of configurational isomers (85 vs. 86 and 87 vs. 88) in contrast to the situation in their positive ion spectra. A strong S02 ion (m/z 64) was observed also for all the compounds studied. 

Secondary ion mass spectra were measured using a Perkin-Elmer+PHI 3500 instrument. Experiments were carried out with 4 kV Ar ions at beam currents of 3 and 300 nanoamps. Spectra were measured to at least 500 daltons (d). Samples were prepared in the manner used for the XPS studies. For measurements on the pure complexes, sample charging occurred, as evidenced by the inability to record secondary ion mass spectra. To reduce charging, a low energy electron beam (50-400 eV) was rastered across the sample during SIMS analysis. Positive and negative ion SIMS spectra were recorded however, only positive ion spectra are of interest for this discussion. In the spectra only unipositive ions were detected, so that the mass numbers detected correspond to combinations of the various isotopes of the elements in the ion. Thus an ion at m/z 17 d is assigned to... 

LC-ISP-MS has been also successfully applied for the assay of 21 sulfonamides in salmon flesh (121). Separation was achieved in a reversed-phase LC system with gradient elution. Simple positive-ion spectra with an intense protonated molecule and no fragment ions of relevant abundance were displayed by all analytes by operating in the full-scan acquisition and SIM modes. Further application of tandem MS using SRM for increased sensitivity could overcome the lack of structural information presented by the ISP mass spectra. 

For the confirmation of PMFs in Valencia orange peel oil and juice, an HPLC method coupled with a thermospray mass spectrometry (HPLC-TSP-MS) detection system was utilized (112). A C 8 column (/zBondapak, 300 X 6-mm ID) was used with a mobile phase of H20-ACN (60 40, v/v) at a flow rate of 1.0 ml/min. Extract (20 fi1) was injected into the HPLC-TSP-MS system, and positive-ion spectra from m/z 100 to 700 were recorded at 1360 ms. Mass spectro-metric identification was done using positive chemical ionization (ICP). This technique allowed confirmation of the presence of eight flavones in the peel oils and seven flavones in the juice. 

Ions of both polarities are generated, usually at comparable abundances. For atomic ions, their yield is governed in first order by the ionization potential or electron affinity. For acidic compounds, specific molecular ions (e.g. (M-H) ) are found mostly in the negative ion spectra for basic compounds specific ions (e.g. (M+H), (M+Alkali) ) are more easily identified in the positive ion spectra. The somewhat surprising results obtained for organic salts will be discussed further down. 

U. Relative sensitivity factors for atomic ions out of a given matrix vary by only one, in extreme cases by about two, orders of magnitude at least for the high irradiance techniques (J ). This holds even for ions f only+one polarity, i.e. under suitable conditions ions such as F or Cl are detected in positive ion spectra. This contrasts favourably with e.g. SIMS where sensitivity factors vary by several orders of magnitude. 

Figure 3. Positive ion spectra (up) of phthalic acid and of neutrals (down) from metastable decay of positive ions.

Positive ion spectra were not particularly helpful, showing only Cu+ and trace alkali metal ions (Na+, K+). However, the negative ion spectra were more complex, and a comparison between the spectrum of the stained copper (Figure 9a) with the unstained copper (Figure 9b) shows several additional peaks in the spectrum of the stained copper. Examination of these peaks revealed them to result from contamination of the surface by sulfur, with prominent peaks identified as ... 

The positive ion spectra typical of many organic species being dominated by the CnHm clusters, particularly and... 

The recognition of a molecular species ion in a spectrum may be difficult. Hence, a scan of neutral losses with mass 132 Da allows one to detect selectively the protonated molecular ions of the various nucleosides contained in the mixture, because the production of an intense BH2+ ion from MH+ by the loss of a sugar molecule (132 Da for a ribose) is an important characteristic of positive ion spectra. A modified nucleoside, /V6-isopentenyladenosine, was identified in tRNATyr of E.coli using this method [210] (Figure 8.39). 

Lastly, it has been shown that negative ion electron-impact spectra of some compounds contain no skeletal reaiTangement ions, in contrast to the corresponding positive ion spectra (Bowie et al., 1969a). 

The lower panel displays the negative ion trace from the PPINICI analysis and contains essentially only the peaks associated with fecal metabolites, whereas the positive ion trace (upper panel) contains numerous peaks associated with normal fecal constituents. The two monoacids 2 and 2 were the only pyridine-derived metabolites which could be identified from the positive ion chromatogram. The positive ion spectra were useful nonetheless, because they provided confirmation of molecular weights, and in some cases provided molecular weight information for peptide conjugates which underwent substantial fragmentation after electron capture. 

Figure 2. Positive ion spectra using filament ionization. Continued on next pa e.

Scheme 1. Retro Diels-Alder fragmentations in positive ion spectra.

Figure 5. Positive ion spectra of a. 6-Demethyltetracycline and b. Tetracycline. Continued on next page.

Figure 5. Continued. Positive ion spectra of c. Declomycin and d. Chlortetracycline.

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. 

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