Ambient desorption ionization mass spectrometry

Ambient desorption ionization mass spectrometry (MS) allows for the direct analysis of ordinary objects in the open atmosphere of the laboratory or in their natural environment. Analyte desorption usually accompanies the ionization step and these processes are often concerted, multi-step processes. Ambient desorption ionization methods typically require little or no sample preparation, offer a much simplified work flow and deliver unprecedented ease of use to MS analyses.

Since the introduction of desorption electrospray ionization (DESI [Z. Takats, J.M. Wiseman, B. Gologan, R.G. Cooks, Science (Washington, D. C.) 306 (2004) 471]) in 2004 and the direct analysis in real time (DART [R.B. Cody, J.A. Laramee, H.D. Durst, Anal. Chem. 77 (2005) 2297]) in 2005, this new field of MS has developed rapidly. Numerous permutations of the various options for analyte desorption and ionization have been demonstrated. Desorption steps, such as momentum transfer, dissolution into ricocheting droplets and thermal desorption, have been combined with ionization steps, including ESI, atmospheric pressure chemical ionization and photo-ionization. The large number of possible combinations of desorption and ionization components that have already been applied is creating a proliferation of techniques and acronyms that is becoming ever more complex.

Here, we provide a logical framework for the classification of these related experiments, based on the desorption and ionization processes involved in each.     

Chemical ionization mass spectrometry of nitrosamines

The mass spectra of eight nitrosamines have been recorded, with excitation by chemical ionization (CI) and electron impact (EI). Comparison of the intensities of the base peaks under CI and El conditions gives intensity ratios in the range 1.4-1.9 for low resolution measurements and up to 10 for high resolution measurements, confirming the enhanced sensitivity available in the CI mode.     

Electrospray ionization mass spectrometry fingerprinting of propolis

Crude ethanolic extracts of propolis, a natural resin, have been directly analysed using electrospray ionization mass (ESI-MS) and tandem mass spectrometry (ESI-MS/MS) in the negative ion mode. European, North American and African samples have been analyzed, but emphasis has been given to Brazilian propolis which displays diverse and region-dependent chemical composition. ESI-MS provides characteristic fingerprint mass spectra, with propolis samples being divided into well-defined groups directly related to their geographical origins. Chemometric multivariate analysis statistically demonstrates the reliability of the ESI-MS fingerprinting method for propolis. On-line ESI-MS/MS tandem mass spectrometry of characteristic [M - H](-) ion markers provides an additional dimension of fingerprinting selectivity, while structurally characterizing the ESI-MS marker components of propolis. By comparison with standards, eight such markers have been identified: para-coumaric acid, 3-methoxy-4-hydroxycinnamaldehyde, 2,2-dimethyl-6-carboxyethenyl-2H-1-benzopyran, 3-prenyl-4-hydroxycinnamic acid, chrysin, pinocembrin, 3,5-diprenyl-4-hydroxycinnamic acid and dicaffeoylquinic acid. The negative mode ESI-MS fingerprinting method is capable of discerning distinct composition patterns to typify, to screen the sample origin and to reveal characteristic details of the more polar and acidic chemical components of propolis samples from different regions of the world.     

Field ionization mass spectrometry—III: Cardenolides

The electron impact (EI) and field ionization (FI) mass spectra of some representative underivatized cardenolides have been studied together for the first time, with the objective of assessing the structural and in particular the sequence information afforded by this promising analytical method. Two series of cardenolides have been examined, each consisting of a mono-, di- and trisaccharide glycoside. The first was based on the aglycone digitoxigenin and comprised neriifolin, thevebioside and cerberoside; the second was based on the genin strophantidin and included cymarin, K-strophantin-β and K-strophantoside. Other cardiac glycosides investigated and discussed include digitoxin and helveticoside. Important and structurally diagnostic fragmentations have been uncovered, especially in FI mode. In addition, the EI and FI spectra of individual components of the cardenolides have been recorded, e.g. the genins digitoxigenin and strophantidin, and the mono-saccharides L-thevetose, D-cymarose and D-glucose.     

Methane chemical ionization mass spectrometry of flavonoids

The chemical ionization mass spectra of a representative selection of flavones, flavonols, flavanones, and flavanols have been examined, using methane as the reagent gas. The flavones and flavonols showed no significant fragmentation under the conditions employed, but the flavanones and flavanols showed characteristic fragmentation which could be of use in structural elucidation of these compounds.     

The chemical ionization mass spectrometry of RDX

The fragmentation pathways of RDX in chemical ionization mass spectrometry have been rationalized, using data from different reagent gases, including CD₄ and iso-C₄D₁₀. The dependence of spectra taken with different gases on the acid strength of the reactant ions in the gases is accounted for.     

Multiphoton ionization detected by fourier-transform mass spectrometry

Ions produced by multiphoton ionization (MPI) of naphthalene, fluoranthene and triphenylene have been detected by Fourier-transform mass spectrometry (FT MS). Paret ions are produced very efficiently at 250 and 222 nm with pulse energies as low as 1 mJ. With FT MS a complete, high-resolution mass spectrum is obtained for each laser pulse.     

Atmospheric pressure ionization mass spectrometry

Atmospheric pressure ionization has been used to great effect by mass spectrometrists in widely diverse fields. The aim of this brief review is to set down the advantages of API/MS to fundamental and analytical chemists alike through discussion of the basic processes of ionisation and through its application.     

Characterization of mouse switch variant antibodies by matrix-assisted laser desorption ionization mass spectrometry and electrospray ionization mass spectrometry

The amino acid sequences of mouse monoclonal antibodies have been characterized completely by mass spectrometry. Antibodies used in the present study were derived from mouse switch variant cell lines that produce four kinds of immunoglobulin Gs (IgGs). The amino acid sequences of these antibodies had not been estimated from the corresponding DNA sequence, so the sequences of IgGs derived from other strains were used as references in this study. Intra- and interchain disulfide bonds of the IgGs were reduced and carboxymethylated and the products were subjected to proteolytic digestion. The existence of N-linked oligosaccharides also was taken into account. The capabilities and limitations of matrix-assisted laser desorption ionization-time-of-flight mass spectrometry and capillary liquid chromatography-electrospray ionization mass spectrometry are discussed in the structural characterization of the antibodies. Based on our results, allotypes of the antibodies examined are discussed. This study shows that amino acid sequences of proteins, such as IgG, can be investigated without information about the corresponding DNA sequence if appropriate reference sequences derived from other strains can be used.     

Low-resolution accurate mass measurement in self-chemical ionization mass spectrometry

Low-resolution (2000, 10% valley definition) accurate mass measurements using self-chemical ionization (self-CI) have been evaluated as an alternative to the conventional chemical ionization (CI) method. In conventional CI experiments a high pressure of reagent gas is required to induce ionization while in self-CI no reagent gas is used and the self-CI is produced presumably by molecular/fragment ion–molecule reactions. Nine compounds ranging in mass from 50–500 daltons were examined. Results obtained by the self-CI method indicate that the elemental composition assignment can be obtained simultaneously for the protonated molecule and/or molecular ion. It is also shown that perfluorokerosene can be used routinely in self-CI as an internal reference standard over a broad mass range (50–500 daltons). It is sometimes difficult to use a single reference standard in conventional low-resolution CI accurate mass spectrometry over a similar mass range.     

Atmospheric-pressure Penning ionization mass spectrometry

A preliminary study on the atmospheric-pressure Penning ionization (APP(e)I) of gaseous organic compounds with Ar* has been made. The metastable argon atoms (Ar*: 11.55 eV for (3)P(2) and 11.72 eV for (3)P(0)) were generated by the negative-mode corona discharge of atmospheric-pressure argon gas. By applying a high positive voltage (+500 to +1000 V) to the stainless steel capillary for the sample introduction (0.1 mm i.d., 0.3 mm o.d.), strong ion signals could be obtained. The ions formed were sampled through an orifice into the vacuum and mass-analyzed by an orthogonal time-of-flight mass spectrometer. The major ions formed by APP(e)I are found to be molecular-related ions for alkanes, aromatics, and oxygen-containing compounds. Because only the molecules with ionization energies less than the internal energy of Ar* are ionized, the present method will be a selective and highly sensitive interface for gas chromatography/mass spectrometry.     

Ambient ionization mass spectrometry: a tutorial

Ambient ionization is a set of mass spectrometric ionization techniques performed under ambient conditions that allows the direct analysis of sample surfaces with little or no sample pretreatment. Using combinations of different types of sample introduction systems and ionization methods, several novel techniques have been developed over the last few years with many applications (e.g., food safety screening; detection of pharmaceuticals and drug abuse; monitoring of environmental pollutants; detection of explosives for antiterrorism and forensics; characterization of biological compounds for proteomics and metabolomics; molecular imaging analysis; and monitoring chemical and biochemical reactions). Electrospray ionization and atmospheric pressure chemical ionization are the two main ionization principles most commonly used in ambient ionization mass spectrometry. This tutorial paper provides a review of the publications related to ambient ionization techniques. We describe and compare the underlying principles of operation, ionization processes, detecting mass ranges, sensitivity, and representative applications of these techniques.     

Negative chemical ionization mass spectrometry of explosives

The negative chemical ionization mass spectra of nitrobenzene, ethylene glycol dinitrate and nitroglycerine have been obtained using various reagent ions. For nitrobenzene, [OH]^? gives the [M ? H]^?, together with [M]^?˙ ions formed by electron capture, but other reagent ions gave relatively low intensity adduct peaks. Ethylene glycol dinitrate and nitroglycerine gave abundant [M + X]^? ions (X = NO₂, NO₃, Cl, Br, I), together with ions arising from the thermal decomposition of the samples in the heated inlet system. The rate of anion attachment to these compounds is much greater than that to related compounds having only one functional group, and it is suggested that this is due to the participation of the adjacent groups in the bonding between the substrate and anion.     

Positive ion electrospray ionization mass spectrometry of oligonucleotides

Positive ion electrospray ionization mass spectra have been obtained of deoxyribonucleic acids (DNA) and ribonucleic acids (RNA), including transfer RNAs (77-mer, ～ 25 kDa). For several different solution conditions, the charge state distributions of DNA and RNA molecules were determined. It is postulated that the production of the multiply charged positive ions results from gas phase dissociation of complexes between nitrogen-containing bases and oligonucleotides.     

Some tenets pertaining to electrospray ionization mass spectrometry

The field of electrospray ionization mass spectrometry is reviewed with emphasis placed upon advances in the elucidation of fundamental mechanistic aspects of the ionization process that have been reported over the past 10 years. The analytical consequences of these findings are also examined. Eight central conclusions or 'tenets' are presented, as deduced from the body of work contained in 80 references.     

Electrospray ionization mass spectrometry of

Lysoglycerophosphocholine lipids (lyso-GPC) are important intermediates in the synthesis and metabolism of glycerophosphocholine lipids which are major components of the cellular lipid bilayer. Significant differences in the collisional induced decomposition (CID) behavior were observed for each of the four different subtypes of lyso-GPC in both positive and negative ions. A major difference was observed in the initial CID product ions derived from lyso-GPC [M + H]+ with the loss of water that was very abundant for acyl lyso-GPC which have a fatty acid ester substituent at either the sn-1 or sn-2 positions. Loss of neutral water was not very prominent in the case of plasmenyl and plasmanyl lyso-GPC species. The mechanism responsible for this difference in behavior of lyso-GPC subtypes was consistent with a higher proton affinity of carboxyl carbonyl oxygen atoms and vinyl ether oxygen atoms found in acyl and plasmenyl lyso-GPC lipids, respectively, as compared to the carbinol oxygen atom common to all lyso-GPC species. Collisional activation of lyso-GPC negative ions [M - 15]- also revealed distinctive differences in product ions derived from acyl and ether lyso-GPC species. The acyl compounds showed the facile elimination of a highly stable carboxylate anion, whereas plasmenyl species underwent fragmentation with loss of a neutral aldehyde, likely a result of rearrangement involving the double bond in the vinyl ether moiety. The alkyl ether species (plasmanyl lyso-GPC lipids) did not undergo either decomposition reaction observed for the other lyso-GPC subtypes which permitted differentiation of acyl, plasmenyl, and plasmanyl lyso-GPC subtypes.     

Matrix-assisted laser desorption/ionization mass spectrometry of taxanes

Taxanes are biologically active compounds that have been extensively used in pharmacology for their powerful anticancer properties. High specificity and low level sensitivity for analysis of these compounds have been obtained with reversed-phase high-pressure liquid chromatography/mass spectrometry (RP-HPLC/MS), but the number of applications of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for low molecular weight analytes is rapidly growing. A new MALDI-MS approach for the rapid screening of a variety of taxanes and a tandem mass spectrometric (MS/MS) analysis of the most important and diagnostic taxane fragmentation pathways are proposed. A solid-phase extraction method followed by preliminary quantification is also reported.     

Ultraviolet femtosecond laser ionization mass spectrometry

For this study, multiphoton ionization/mass spectrometry using an ultraviolet (UV) femtosecond laser was employed for the trace analysis of organic compounds. Some of the molecules, such as dioxins, contain several chlorine atoms and have short excited-state lifetimes due to a "heavy atom" effect. A UV femtosecond laser is, then, useful for efficient resonance excitation and subsequent ionization. A technique of multiphoton ionization using an extremely short laser pulse (e.g., <10 fs), referred to as "impulsive ionization," may have a potential for use in fragmentation-free ionization, thus providing information on molecular weight in mass spectrometry.     

Electrohydrodynamic ionization mass spectrometry of sulfonates

Electrohydrodynamic ionization mass spectrometry has been applied to the analysis of 2-naphthol-3,6-disulfonic acid, disodium salt and 4,5-dihydroxy-2,7-naphthalenedisulfonic acid, disodium salt. The negative ion mass spectra indicate ions characteristic of the sample and no fragment ions are observed. Degrees of dissociation of the sulfonates and the ion-solvent interactions are reflected in the negative ion spectra.     

Pyrolysis/chemical ionization mass spectrometry of polymers

Pyrolysis/mass spectrometric studies have been made on polystyrene, poly(vinyl chloride), and poly(methyl methacrylate) with electron ionization (EI) and chemical ionization (CI) mass spectrometry and a variable temperature probe for direct insertion into the source of the mass spectrometer. Similar results obtained with EI and CI mass spectrometry are in agreement with previous experiments. Advantages of the simplification of spectra in the CI made, as well as the advantages of using both techniques for identification of pyrolysis products, are discussed.