Negative-ion mass spectra

Stemmier, E.A. and Hites, R.A., Electron Capture Negative Ion Mass Spectra of Environmental Contaminants and Related Compounds, VCH, Weinheim, Germany, 1988. 

Quasiequilibrium statistical theory was applied to the negative ion mass spectra of diphenylisoxazoles. Electron capture by the isoxazole leads to molecular ions having excited vibrations of the ring and of bonds attached to it. The dissociation rate constants were also calculated (77MI41615, 75MI416U). 

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. 

Mass Spectrometry. Mass spectrometry holds great promise for low-level toxin detection. Previous studies employed electron impact (El), desorption chemical ionization (DCI), fast atom bombardment (FAB), and cesium ion liquid secondary ion mass spectrometry (LSIMS) to generate positive or negative ion mass spectra (15-17, 21-23). Firm detection limits have yet to be reported for the brevetoxins. Preliminary results from our laboratory demonstrated that levels as low as 500 ng PbTx-2 or PbTx-3 were detected by using ammonia DCI and scans of 500-1000 amu (unpublished data). We expect significant improvement by manipulation of the DCI conditions and selected monitoring of the molecular ion or the ammonia adduction. 

Principles and Characteristics With sample spots in a developed thin-layer chromatogram, there are no constraints on the operation of the mass spectrometer. Depending on the analytical information required, either low- or high-resolution mass-spectral data can be recorded, and both positive- and negative-ion mass spectra can be obtained from the same sample spot. 

A comparison between the positive and negative ion mass spectra of 3-amino-5-methylthio-l,2,4-thiadiazole and a study of the positive ion mass spectrum of 3-amino-5-methylthio-l,2,4-thiadiazole using 1SN isotopes appeared in Cl I EC-11(1996) <1996CHEC-II(4)307>. Since the publication of CHEC-II(1996), no new studies focusing on the mass spectra of 1,2,4-thiadiazoles have appeared. 

Dougherty, R.C. Weisenbeiger, C.R. Negative Ion Mass Spectra of Benzene, Naphthalene, and Anthracene. A New Technique for Obtaining Relatively Intense and Reproducible Negative Ion Mass Spectra. J. Am. Chem. Soc. 1968, 90, 6570-6571. 

Fig. 4.3 Primary screening and deconvolution stage mass spectra. (A) The region of the negative ion mass spectra containing the ions of interest is shown for two compounds, in two replicate primary screening experi-ments. Full-scale y-axis intensity values are normalized to 308 counts per second for compound 4 (m/z 436) and 162 counts per second for compound 5 (m/z 498).

Although negative-ion mass spectra are easily obtained in most mass spectrometers by relatively simple reversal of magnetic and electric field polarities, frequent absence of molecular ions and low negative-ion intensities [about 10 that of positive ionization (244)] have meant, until recently, few negative-ion studies. 

Stemmler EA, Hites RA. 1987. The electron capture negative ion mass spectra of 2,6-dinitroaniline and 2,4-dinitrophenol herbicides and related nitrobenzene derivatives. Biomed Environ Mass Spectrom 14 417-434. 

Fossil organic particles in situ usually release positive ions at masses 23,24, 39, 41, 54, 56 thus indicating Na, Mg, K and Fe. But other signals also occur (Fig. 19). The negative ion mass spectra are dominated by carbon clusters with zero, one or two hydrogen atoms attached. They resemble spectra obtained from polymer foils (Gardella et al., 1980)42> rather than those from coals and carbon films (Fiirstenau et al., 1979)43). Consequently, the material contains more long hydrocarbon chains rather than aromatic constituents. Peaks at m/e = 79 (benzylium) and 90 (tropylium) indicate aromatic constituents. Unspecific ions like CN, CNO, and Cl are commonly present (Fig. 11). 

There are clear similarities in the negative-ion mass spectra of the M(trifluoro-acetylacetonate)3 complexes (M = Vra or Al111).379 The major ion peak corresponds to (ML3)- and this is followed in intensity by one for (L) changes in oxidation state are evident for the metals in the fragment ions. 

Figure 6. Scanned API positive and negative ion mass spectra for Tetradijon (A).

Hunt et al (12) demonstrated the possibility of obtaining positive and negative ion mass spectra at the same time by employing an electrical field (quadrupole) mass analyzer with rapid switching. This may prove to be a valuable technique in many applications, as indicated by Hunt. 

The negative ion mass spectra of 21 was studied by an electron im-pact/Fourier transform ion cyclotron resonance mass spectrometry method (95MI13). 

Procedures based on monitoring the ubiquitous (in organochlorine negative ion mass spectra) m/z 70-73 ions, i.e., Cl2 and HCl2 [59], present the problem that many other persistent OCs fragment to yield such ions, e. g., p,p -DDT, p,p -DDE, lindane, dieldrin, aldrin, and endrin, to name a few [60-62]. Thus, if these contaminants are not selectively removed from the sample matrix during the extraction or clean-up procedures, they would ultimately contribute to the response of the quantitation ion, Cl2 (m/z 70), and lead to an overestimation in the level of PCAs in samples. 

Tomy GT, Tittlemier SA, Stern GA, Muir DCG, Westrnore JB (1998) Eifects of temperature and sample amount on the electron capture negative ion mass spectra of polychloro-n-alkanes. Chemosphere 37 1395-1410... 

Rankin PC (1971) Negative ion mass spectra of some pesticidal compounds. J Assoc Off Anal Chem 54 1340 -1348... 

Stemmier EA, Hites RA (1988) Electron capture negative ion mass spectra of environmental contaminants and related compounds. VCH,Weinheim (ISBN 0-89573-708-6)... 

The processing software searches the summed diode-array chromatogram for any impurity peak > 8% threshold. Positive- and negative-ion mass spectra from the TIC and corresponding to the same UV peak retention time are summed, subtracted and plotted. 

Figure 25. Laser ablation negative ion mass spectra of (a) NaMOR, (b) KLTL, (c) NaLTA zeolites. Reproduced with permission from reference 69.

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