Nano-electrospray ionization mass spectrometry: addressing analytical problems beyond routine

The advent of nano-electrospray ionization (nano-ESI) has considerably extended the usability of ESI in the analytical mass spectrometric laboratory. One of the remarkable features of nano-ESI is its extremely low sample consumption. Only a few microliters of analyte solution (10(-5)-10(-8) M) are sufficient for molecular weight determination and structural investigations by MS/MS. But nano-ESI is more than just a minimized-flow ESI; the low solvent flow rate also affects the mechanism of ion formation. As a consequence, the area of ESI-MS applications is significantly enhanced. Oligosaccharides, glycosides as well as glycoproteins can be analyzed more easily than with normal ion spray. The same holds for the analysis of non-covalent complexes sprayed directly from aqueous solutions.     

Nano-electrospray ionization time-of-flight mass spectrometry of gangliosides from human brain tissue

A general approach for the detection and structural elucidation of brain ganglioside species GM1, GD1 and GT1 by nano-electrospray ionization quadrupole time-of-flight (nanoESI-QTOF) mass spectrometry (MS), using combined data from MS and MS/MS analysis of isolated native ganglioside fractions in negative ion mode and their permethylated counterparts in the positive ion mode is presented. This approach was designed to detect and sequence gangliosides present in preparatively isolated ganglioside fractions from pathological brain samples available in only very limited amounts. In these fractions mixtures of homologue and isobaric structures are present, depending on the ceramide composition and the position of the sialic acid attachment site. The interpretation of data for the entire sequence, derived from A, B, C and Y ions by nanoESI-QTOFMS/MS in the negative ion mode of native fractions, can be compromised by ions arising from double and triple internal cleavages. To distinguish between isobaric carbohydrate structures in gangliosides, such as monosialogangliosides GM1a and GM1b, disialogangliosides GD1a, GD1b and GD1c or trisialogangliosides GT1b, GT1c and GT1d, the samples were analysed after permethylation in the positive ion nanoESI-QTOFMS/MS mode, providing set of data, which allows a clear distinction for assignment of outer and inner fragment ions according to their m/z values. The fragmentation patterns from native gangliosides obtained by low-energy collision induced dissociation (CID) by nanoESI-QTOF show common behaviour and follow inherent rules. The combined set of data from the negative and positive ion mode low-energy CID can serve for the detection of structural isomers in mixtures, and to trace new, not previously detected, components.     

Recent developments in ambient ionization techniques for analytical mass spectrometry

Ambient ionization techniques enable the interrogation of a variety of samples in their native state by mass spectrometry, and are rapidly advancing all fields where screening for the presence of various analytes in a broadband and/or high-throughput fashion is desirable. This Highlight article provides an introduction to the field, and showcases the different ionization approaches reported since 2004, with an emphasis on the most recent developments.     

Nano-electrospray mass spectrometry with a modified commercial IonSpray source

Electrospray mass spectrometry is a standard tool for the investigation of biological samples. Due to the high flowrates of the standard sources, large sample amounts are required and it is almost impossible to spray physiological solutions due to their aqueous medium. The introduction of microelectrospray sources has made it possible to decrease the sample amounts needed and enabled the use of buffered solutions. In this work, a nanoES-like source based on a modification of an existing IonSpray source is introduced. In contrast to other nanoES sources available, the modification presented allows a fast change between the nanoES and the normal IonSpray modes. Copyright 2000 John Wiley & Sons, Ltd.     

Electrolytic electrospray ionization mass spectrometry of quaterthiophene-bridged bisporphyrins: beyond the identification tool

Several quaterthiophene-bridged bisporphyrins were analyzed by electrospray ionization mass spectrometry (ESI-MS). The active centers of these molecular assemblies are two porphyrins moieties complexed (Z) or not (H) with a metal ion, typically Zn(2+), and the spacer is a quaterthiophene. The two end-groups were chemically linked to the quaterthiophene spacers by (i) a C--C single bond, (ii) a trans double bond or (iii) a triple bond. The formation of charged species either by protonation ([M + H](+) and [M + 2H](2+)) or electron(s) loss (M(+) and M(2+)), account for the occurrence of electrochemical processes in the basic operation of an electrospray source acting in a non-aqueous solvent. The nature of the observed charged species is correlated with the electro-oxidation properties and proton production by electro-oxidation of residual water. The occurence of these electrochemical reaction is proposed when the electroactivity of the electrosprayed substrates is not sufficient to support the current demand of the ESI source. In this way, the results obtained from the analysed series suggest the occurrence of such a process when the interfacial potential of the metal capillary reaches a value of 0.75 V vs Ag/AgCl. The results of theoretical calculations confirm the importance of the ionization energy with regard to the protonation energy in the course of the ionization reaction. The structural differences at the porphyrin-linker junctions lead to significantly smaller ionization energy in the case of the trans double bond. The MS observation of discharged dimers from molecular assemblies, including two complexed porphyrins ZZ or two free bases HH as end-group and a triple bond as the quaterthiophene-bisporphyrin junction, indicates together with molecular modelling (carried out at the semi-empirical PM3 level), that the planar and symmetric structures favour stacking.     

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.     

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.     

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.     

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.     

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.     

Current-controlled nanospray ionization mass spectrometry

The hypothesis that direct determination of electrospray current would provide a viable method for maintaining spray stability to enable optimal nanospray analysis was tested by building a feedback apparatus capable of reading the current and readjusting the emitter voltage in real time. The apparatus consists of a current-sensing circuit that reads the voltage drop across a resistor located between the high-voltage power supply and the nanospray emitter. A low voltage proportional to the observed current is generated and sent to a data acquisition card. The information is used by a proportional-derivative-integral (PID) algorithm to calculate the magnitude of a low-voltage signal that is used to control the power supply output. Any variation of current across the sensing resistor is thus counteracted by an opposite-direction variation of the high voltage applied to the nanospray emitter. In this way, the apparatus adjusts the emitter voltage to achieve a preset value of current, which it strives to maintain over time in spite of any possible variation of the parameters influencing the spray regime. Preliminary results have shown that the feedback apparatus is capable of establishing and maintaining stable spray for samples that are usually considered challenging in traditional voltage-controlled analysis, such as those consisting of nucleic acid solutions with high salt loads. For these types of samples, the total ion count recorded in current-controlled mode was significantly more stable than that observed in voltage-controlled mode. At the same time, overall signal intensities and signal-to-noise ratios were also significantly improved. Setting the target nanospray current to a predefined value and letting the apparatus reach the target without operator intervention enabled the acquisition of viable data from solutions containing up to 2. 5 M ammonium acetate, which are ordinarily difficult by traditional manual tuning. A deeper understanding of the current-voltage relationships for samples of very different compositions is expected to enable one not only to predict the target current that should be used for a certain analysis, but also to devise algorithms to change such target as a function of predictable variations of sample properties and analytical conditions. This will allow for optimal performance to be maintained during on-line gradient chromatography in which the nature of the sprayed solution may vary very widely during the course of the analysis.     

Photoelectron resonance capture ionization mass spectrometry: a soft ionization source for mass spectrometry of particle-phase organic compounds

Photoelectron resonance capture ionization (PERCI) is a soft and sensitive ionization method, based on the attachment of low-energy (<1 eV) photoelectrons to organic analyte molecules. PERCI has been developed in our laboratory for the real-time analysis of organic particles by mass spectrometry, and is employed here to monitor the heterogeneous reaction of ozone with oleic acid. Simplified identification of the reaction products is possible as a result of the soft nature of PERCI, giving predominantly the [M--H](-) ions. The major particle-phase products are identified as: 1-nonanal, nonanoic acid, 9-oxononanoic acid, and azelaic acid, consistent with proposed mechanisms. New insight into this well-studied heterogeneous reaction is gained as additional minor particle-phase products, consistent with the Criegee mechanism, are readily detected.     

Cold-spray ionization mass spectrometry: principle and applications

A direct solution analysis method, cold-spray ionization (CSI) mass spectrometry (MS), a variant of electrospray (ESI) MS operating at low temperature (ca -80 to 10 degrees C), allows the facile and precise characterization of labile organic species, especially those in which non-covalent bonding interactions are prominent. We applied this method to investigations of the solution structures of many labile organic species, including unstable reagents and reaction intermediates, asymmetric catalysts, supramolecules and even primary biomolecules. Remarkable analytical results were obtained for highly ordered supramolecules using the CSI method. Whereas conventional ESI is not applicable to these compounds because of their instability to heat and/or air, CSI affords multiply charged molecular ions with many solvent molecules attached. Investigation of the constitution of Grignard reagents in solution is extremely challenging, but CSI-MS allowed us to identify one of the key structures in THF solution. Recently, this method was adopted for investigations of the solution structures of primary biomolecules such as nucleosides, amino acids, sugars and lipids, revealing singly charged Na(+) adducts of large clusters (chain structures), presumably linked by non-covalent interactions, including hydrogen bonding and/or hydrophobic interactions. The principle of the CSI method and applications of the method to a wide variety of labile organic species and primary biomolecules in solution are described.     

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.     

Electron impact ionization mass spectrometry and tandem mass spectrometry of phenylalkylpyridazines

The mass spectrometric fragmentation behaviour of pyridazine and four monosubstituted derivatives containing a pbenylalkyl side-chain (3- and 4-benizylpyridazine, 3- and 4-(2-pbenylethyl)pyridazine) was investigated. In the electron impact ionization mess spectra of the 3-substituted compounds abundant [M – H]⁺ peaks are observed. This allows a clear distinction between 3- and 4-substituted pyridazines, as the spectra of the latter isomers show only very weak [M – H]⁺ signals. The stability of [M – H]⁺ ions derived from 3-alkylpyridazines (deduced from only the very low abundance of further fragment ions) gives strong evidence for a cyclic structure of these ions. One fragmentation pathway typical of the parent pyridazine, the [M - N₂] fragmentation, was not detectable with any of the phenylalkylpyridazines investigated. Instead, loss of HCN, H₃CN⁺ and N²H⁺ was observed. An interesting fragmentation, observed with 3-(2-phenylethyl)pyridazine, is the loss of ⁺CH₃ from the molecular ion and also from the [M – H]⁺ ion.     

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.     

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.