Mass spectrometry of labile onium ions and carbocation salts. Study by desorption-ionization mass spectrometry: Fast atom bombardment,field desorption and 252Cf plasma desorption time-of-flight mass spectrometry

In contrast to the accepted notion that mass spectrometry is impractical as an analytical tool in condensed-phase preparative carbocation chemistry, it has been shown that desorption ionization techniques (fast atom bombard-ment, field desorption and²⁵²Cf plasma desorption time-of-flight mass spectrometry) provide rapid and reliable methods for relative molecular mass determination for a wide variety of highly unstable, fragile carbocation salts and onium ions. In addition, interesting fragment ions and cluster ions (two cations and one anion) are observed. Application of these methods to study several dication ether salts has also been demonstrated.     

High-resolution field desorption mass spectrometry: Part VII. Explosives and Explosive Mixtures4

Standard explosives and technical mixtures of explosives have been investigated by field desorption mass spectrometry. The compounds investigated gave intense molecular ions or protonated molecules and structurally significant fragmentation. For comparison, the corresponding electron impact and chemical ionization mass spectrometry data are reported. Emission-controlled field desorption, photographic detection, and accurate mass measurements enabled the components of the technical mixtures to be identified. An example of the determination of an additive in a technical product by field desorption mass spectrometry and stable isotope dilution is given. The use of these techniques for quality control of explosives and for forensic investigations appears to be promising.     

Criteria for distinguishing between [M]+· and [M+H]+ ions in field desorption mass spectra

Ten criteria are introduced to distinguish between molecular ions and protonated parent molecules in field desorption mass spectrometry.     

Field desorption mass spectrometry of 1-methylpyridinium salts

Quaternary pyridinium salts were investigated by field desorption, field ionization and electron impact mass spectrometry. Thermal decomposition of the salts under field desorption conditions was found to give very abundant volatile products, identified as the dihydro analogues and the ‘methides’ of the cations. The formation of the volatile compounds, especially those with molecular weights corresponding to those of the (cation ?1), (cation +1) and (cation +13) was studied in detail by deuterium and ¹³C labelling experiments.     

Matrix effects in negative-ion field desorption mass spectrometry

The negative ion mode of field desorption mass spectrometry is matrix dependent. The matrix is used to facilitate the desolvation of ions at field strengths below the onset of field induced electron detachment. The role of the matrix in the desolvation mechanism for negative ions is briefly described and the main properties of a matrix solution, such as viscosity, solubility for complex salts and acids, and adhesion to the emitter surface, on the ion formation are discussed. The application of matrix mixtures with strong adhesion to the emitter surface even at elevated temperatures and high field strengths are considered to be important for the further improvement of this ionization method.     

Spectra of quaternary ammonium salts taken by fission-fragment and laser-induced desorption

Mass spectra of quaternary ammonium salts obtained by fission-fragment induced desorption and laser induced desorption are reported and compared with those obtained by static secondary ion mass spectrometry as well as by field desorption and by field desorption/collisional activation. The principal fragmentation pathways are the same. Certain differences are discussed.     

Field desorption mass spectrometry of oligosaccharides in the presence of metallic salts

In the field desorption mass spectrometry of oligosaccharides, traces of contaminating salts in polar solvents decrease the detection sensitivity, and in some cases no ion could be detected in the molecular range. The present work shows that the addition of alkaline salts (NaI or KI) greatly increases the sensitivity, and cationized molecular ions are obtained. The influence of the molar ratio sugar/salt on the nature and relative intensity of the desorbed species has been studied on mixtures of α-D -trehalose and NaI. At the low salt ratio (100:1), high molecular weight clusters [X M + Na]⁺ are preferentially formed. At the high salt ratio (1:10), mixed clusters [M + NaI + Na]⁺, doubly charged ions [M + 2Na]²⁺ and monomeric cationized ions [M + Na]⁺ are observed along with salt clusters [NaI + Na]⁺ and [2NaI + Na]⁺. In the range of molar ratios sugar/salt of 10:1 to 1:1, the field desorption mass spectra exhibit a cationized ion [M + Na]⁺, which contributes more than 80% of Σ₁₀₀. This cationization technique has been applied to the field desorption mass spectrometry of several oligosaccharides. In all cases, the salt effect causes the replacement of the [M + H]⁺ ion by a [M + C]⁺ ion. (Note: the term [M + C]⁺ and similar ones mean an association between the whole molecule studied (M) and an alkaline cation [C]⁺ ([K)]⁺, [Na]⁺, [Li]⁺).) Thus, di, tri- and tetrasaccharides exhibit intense [M + Na]⁺ ions in the presence of NaI and only a few fragment peaks are observed in their field desorption mass spectra. This technique has been applied successfully to the detection of the hexasaccharide ajugose in a natural sample of pentasaccharide, and has also allowed the unambiguous determination of the composition of a mixture of partially methylated trehaloses. The salt effects are dicussed in terms of selective adsorption on the emitter, pre-existing soluble complexes sugar-salt, interactions between these species in the electric field and dissociative desorption of ionic complexes.     

Field desorption mass spectrometry of quaternary ammonium salts: Cluster ion formation

The field desorption mass spectra of salts such as quaternary ammonium and carbenium salts with organic cations in addition to high cation intensities show signals for cluster ions composed of the salt cation + salt molecule, i.e. [C + nM]⁺, n = 1–5, thus allowing determination of the molecular weights of salts. In some cases cluster ions of the type [nM – 1]⁺ are detected. Conditions for the formation of cluster ions are discussed.     

The analysis of industrial synthetic polymers by electrospray droplet impact/secondary ion mass spectrometry

Electrospray droplet impact (EDI)/secondary ion mass spectrometry (SIMS) is a new desorption/ionization technique for mass spectrometry in which highly charged water clusters produced from the atmospheric‐pressure electrospray are accelerated in vacuum by several kV and impact the sample deposited on the metal substrate. In this study, several industrial synthetic polymers, e.g. polystyrene (PS) and polyethylene glycol (PEG) were analyzed by EDI/SIMS mass spectrometry. For higher molecular weight analytes, e.g. PS4000 and PEG4600, EDI/SIMS mass spectra could be obtained when cationization salts are added. For the polymers of lower molecular weights, e.g. PEG300 and PEG600, they could be readily detected as protonated ions without the addition of cationization agents. Anionized PS was also observed in the negative ion mode of operation when acetic acid was added to the charged droplet. Compared to matrix‐assisted laser desorption/ionization (MALDI), ion signal distribution with lower background signals could be obtained particularly for the low‐molecular weight polymers. Copyright © 2009 John Wiley & Sons, Ltd.     

Field Desorption Mass Spectrometry of Standard Organophosphorus Pesticides and Their Identification in Waste Water

The characteristic behaviour of four groups of commonly used organophosphorus pesticides such as phosphates, phosphorothionates, phosphorothiolates and phosphorodithioates has been investigated by field desorption mass spectrometry. Their spectra show molecular ions of high abundance and characteristic fragmentation patterns. The phosphates and phosphorothionates usually show the α-cleavage with respect to the P-atom with and without proton transfer. In contrast, the phosphorothiolates and phosphorodithioates usually show the β-cleavage from the P-atom. The fragments with the charge retention on the phosphorus moiety were also observed in the field desorption mass spectra, although their abundances were often relatively low.

The analyses of standard mixtures as well as some waste water samples indicate that field desorption mass spectrometry is suitable for the identification of organophosphorus pesticides at nanogram level in mixtures and environmental samples without preliminary separation and purification.     

Field desorption mass spectra of involatile sulphonic acids,sulphonates and dyestuff materials

Abstract-Useful field desorption mass spectra can be obtained from involatile sulphonic acids and sulphonates, even when large dyestuff molecules are involved. Unusual cluster ions are commonly found, which can be singly or doubly charged. Most ions can be assigned, however, and the great potential of field desorption mass spectrometry in the identification of these involatile species is clearly established.     

Applications of high-pressure liquid chromatography and field desorption mass spectrometry in studies of natural porphyrins and chlorophyll derivatives.

Mixtures of porphyrins derived from natural sources can be readily separated by high-pressure liquid chromatography both analytically and on a preparative scale. A variety of procedures have been developed not only for the esters but also for free acids, and on the analytical scale quantitation is easily achieved by visible absorption. The retention times are largely characteristic of the number of carboxylic acid side chains (or other polar groups) but further information can be obtained by mass spectrometric studies of the various fractions. Field desorption mass spectrometry is particularly useful for this purpose because the emitter wire can be dipped directly into the eluates. The field desorption spectra of porphyrin free acids and esters as well as their metal complexes give essentially molecular ions with little or no fragmentation in most cases, whereas electron-impact mass spectrometry, particularly of free acids, is impeded by the low volatility of porphyrins. Mixtures can also be analysed by field desorption mass spectrometry, and this provides not only a rapid qualitative assessment of the components of a mixture, but also a check on the subsequent chromatographic separations.     

Fragment ion formation of quaternary ammonium salts under fast atom bombardment and field desorption collisionally activated dissociation conditions

Comparing fast atom bombardment and field desorption collisionally activated dissociation mass spectra of tetrabutyl ammonium salts it becomes evident that under fast atom bombardment conditions, besides desorption of intact salt cations, the ionization of intact salt molecules must be assumed. Alkyl ions formed under fast atom bombardment and not under field desorption collisionally activated dissociation conditions are ionization products of tetraalkyl ammonium salt molecules. Radical cations are produced by both techniques.     

Field desorption ‘without fields’?

A field independent thermal desorption of ions has been suggested to occur under conditions of field desorption mass spectrometry by several authors. It is shown that these speculations are incorrect for theoretical reasons and inconsistent with experimental facts.     

Field desorption mass spectrometry,fast atom bombardment mass spectrometry and fast atom bombardment tandem mass spectrometry of echinacoside,the main caffeoyl-glycoside from Echinacea angustifolia roots (Asteraceae)

The mass spectral fragmentation of echinacoside, a pharmacologically active caffeoyl-glycoside isolated from Echinacea angustifolia roots, has been investigated using different soft-ionization techniques, field desorption and fast atom bombardment (positive and negative ions) mass Spectrometry. Both ionization modes are successful in molecular mass determination and furnish approximately equivalent structural information. A fast atom bombardment tandem mass spectrometry approach (negative ions) was developed for the study of the fragmentation pathways and for the detection of echinacoside in crude plant extracts. The results demonstrate the usefulness of this technique for the rapid search of this important caffeoyl-glucoside directly in natural complex matrices.     

Approaches for the analysis of low molecular weight compounds with laser desorption/ionization techniques and mass spectrometry

This review summarizes various approaches for the analysis of low molecular weight (LMW) compounds by different laser desorption/ionization mass spectrometry techniques (LDI-MS). It is common to use an agent to assist the ionization, and small molecules are normally difficult to analyze by, e.g., matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) using the common matrices available today, because the latter are generally small organic compounds themselves. This often results in severe suppression of analyte peaks, or interference of the matrix and analyte signals in the low mass region. However, intrinsic properties of several LDI techniques such as high sensitivity, low sample consumption, high tolerance towards salts and solid particles, and rapid analysis have stimulated scientists to develop methods to circumvent matrix-related issues in the analysis of LMW molecules. Recent developments within this field as well as historical considerations and future prospects are presented in this review.     

Laser desorption mass spectrometry of squaric acid and its salts

The laser desorption mass spectrometry of the oxocarbon squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) and its salts of the form A₂C₄O₄ (A = cation) is described. Both positive and negative ion spectra were obtained. The positive ion spectrum of the acid is characterized by an ion corresponding to loss of CO from [M + H]⁺. The negative ion spectrum shows an intense [M ? H]^? peak in addition to a dimer species. The alkali salt spectra contain [M + A]⁺ in the positive mode and [M ? A]^? and an intense [C₄HO₄]^? in the negative mode. The smaller alkali salts also have an [M + H]⁺ adduct ion. Unlike the alkali squarates, the ammonium salt shows ions corresponding to losses of neutrals from the molecular adduct in the positive ion spectrum and a dimer species in the negative ion spectrum. Molecular weight information was obtained in all cases. A (bis) dicyanomethylene derivative of potassium squarate was also studied. Some field desorption mass spectrometry results are presented for comparison.     

Matrix-Assisted in-Beam Mass Spectra of Thermally Fragile Molecules

In-beam and matrix-isolation techniques have been used in the mass spectral studies of several categories of biologically significant compounds. These include amino acids, quaternary ammonium salts, vitamins and nucleosides. Molecular ions and/or (M+H)⁺ions are obtained, together with useful fragmentations, all of which are valuable in structural elucidations. Spectra obtained by this version of the in-beam technique are similar but not identical with those obtained by field desorption and secondary ion mass spectrometry. Ammonium and sodium chlorides, ammonium sulfate, p-toluenesulfonic and hydrochloric acids can all be used as a room temperature matrix. The detection limits for vitamin E and 2′-deoxyguanosine have been determined as 1 ngand 5 μg, respectively.     

Analysis of organic salts by laser ionization

Alkali salts of 1-hexyl sulfonate were ionized by laser irradiation and the positive ions were analyzed by time-of-flight mass spectrometry. The sodium, potassium, rubidium and cesium salts produce relatively simple spectra. The major organic ions observed are due to the salt plus a cation or the dimer plus a cation. No molecular or fragment ions were detected. The instrumentation and the method of interpreting these data are described.     

A study of the applicability of various ionization methods and tandem mass spectrometry in the analyses of triphenyltin compounds

Triphenyltin compounds are widely introduced into the Dutch aquatic environment. To be able to detect them in environmental samples, the ionization methods of electron ionization, chemical ionization, fast atom bombardment, field desorption, thermospray and electrospray have been applied to triphenyltin acetate, chloride, fluoride and hydroxide to find out which of these methods is best suited to obtain molecular weight information on the intact molecules. For this purpose, field desorption is shown to be the most appropriate method giving, without fragmentation, molecular ion peaks, with the exception of triphenyltin hydroxide. The latter compound gives rise to the base peak at m/z 716, due to the formation of bis(triphenyltin)oxide. Field desorption tandem mass spectrometry, applied to the molecular ions, has shown that the main decomposition pathway corresponds to the loss of a phenyl radical. Subsequently, sediment and surface water samples from the Dutch inland water, without and with the use of clean-up procedures, have been analyzed by the application of field desorption in combination with tandem mass spectrometry. Within the limits of detection, no signals for the presence of triphenyltin compounds in these environmental samples has been found. Upon spiking these samples with triphenyltin acetate, chloride, fluoride and hydroxide, it has appeared that the covalently bonded non-aromatic substituent of the molecules is exchanged for hydroxyl.