Negative ion spectra

In analyses where molecular masses are being matched, more accurate mass measurements provide more reliable matches and identifications.26,65,66 In a reference laboratory the mass accuracy to several decimal points, provided by a Fourier transform ion cyclotron resonance mass analyzer, may be desirable. In field or portable systems there is usually a trade-off in mass accuracy for size and ruggedness. Reliable identifications can be made with moderate mass accuracy, even 1 Da, if a large enough suite of molecular ions is recorded and used to search the database. If both positive ion and negative ion spectra are... 

NdjlCufpballj 23H2O, 43 208-210 Negative ion spectra, 28 7 Neocuproine copper complexes, 45 289 Neodymium carbides, 11 200 dibromide, 20 4 dichloride, 20 4 preparation of, 20 8 properties of, 20 15-18 diiodide, 20 4... 

Figure 9.55a shows the results of single-particle analysis (see Chapter ll.B.4a) of a typical particle in the upper troposphere (Murphy et al., 1998). In the negative ion spectra, a variety of fragments due to organics are observed, along with sulfates and some halogens. In other particles, soot and minerals were also common constituents. For comparison, Fig. 9.55b shows that a typical particle in the stratosphere is primarily sulfate (see Chapter 12.C.5). 

Figure 3 shows the negative SSIMS spectra obtained from Si and Cr surfaces at room temperature. Table 2 shows the chemical formulae for the peaks identified in Fig. 3. The negative ion spectra from these surfaces are again found to be very similar. Comparison of Figs 3(a) and (b) shows that the Cr and Si surfaces yield similar 3-APTHS related ions with roughly comparable intensities. Negative ions consistent with a chromium native oxide were also observed for the Cr surface. Parent ions due to 3-APTHS were absent from both the positive and negative SSIMS spectra. No dimer, trimer, or higher molecular weight...

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. 

The spectrum of Co2(CO)8 shows no Co2(CO) ion, suggesting that the bridge unit is symmetrically homolytically cleaved (247). No doubly charged ions were found. The negative-ion spectra of both Co2(CO)8 and Mn2(CO)10 have been recorded (247), the predominant ions being tetrahedral Co(CO)4, and trigonal bipyramidal Mn(CO)j. 

Negative-ion spectra were also recorded, and showed ions C5HxM(CO) (M = Co, = 1, 2 M = Mn, n = 0-3 M = V, n = 2, 3) but no ions M(CO) " were observed. The errors in measurements ( 3 m/e units) precluded an accurate determination of the value of x. These ions were probably formed by an ion-pair production process... 

This approach was employed, since spectra of single spots on the surface might have local variations in composition which would hinder a comparison of the two samples. The spot size at the focal point was approximately 150 microns in diameter. The instrument was operated in dual-cell mode, with ions detected in the analyzer side immediately after the laser was fired, in order to minimize undesirable ion-molecule reactions which might occur at the higher pressures in the source cell. Both positive and negative ion spectra were collected for each sample. 

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 ... 

Phthalic anhydride 225 has been used in the environmental analysis of alcohol ethoxylates, which are widely found surfactants. Samples are derivatized with phthalic anhydride converting the ethoxylates into phthalate half-esters. Under analysis by electrospray LCMS, the derivatized alcohols gave stronger signals with less background in the negative ion spectra <2005JCH39>. 

Both positive and negative ion mass spectrometry are useful for fluorine compounds and often are complimentary. Fluorocarbons rarely show an M+ peak in positive ion El spectra but often exhibit intense M in negative ion spectra. Chemical ionization techniques routinely offered in inexpensive mass spectrometer systems greatly enhance the abihty to observe molecular ions in both positive and negative ion spectra. Most mass spectral data on simple main group... 

A successful technique has been published by Yinon and Boettger (Refs 56a 57) who modified an Associated Electrical Industries Model MS9 instrument to perform negative ion analysis over a wide range of electron energies and source pressures. They obtained negative ion spectra of nitrobenzene and TNT. Because of its selectivity and sensitivity, this method is well suited to detect TNT in concealed areas such as airline baggage (Ref 56a)... 

An alternative method for producing mass spectra of solid samples which shows promise for the analysis of zeolites and related materials is the plasma desorption technique recently reported by Schwiekert et al. [71,72]. This technique uses a less energetic means of ionizing the solid than laser ablation, and initial indications are that negative ion spectra from materials like zirconium phosphate may reflect the connectivity as well as the stoichiometry of the solid analyzed to a greater degree than laser ablation. 

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