Electrospray ionization tandem mass spectrometric study of salt cluster ions. Part 1--investigations of alkali metal chloride and sodium salt cluster ions

Salt cluster ions of alkali metal chlorides ACl (A = Li(+), Na(+), K(+), Rb(+) and Cs(+)) and sodium salts NaB (B = I(-), HCOO(-), CH(3)COO(-), NO(2)(-), and NO(3)(-)), formed by electrospray ionization, were studied systematically by mass spectrometry. The influences on the total positive ion and negative ion currents of variation of solvent, solution concentration, desolvation temperature, solution flow-rate, capillary voltage and cone voltage were investigated. Only cone voltage was found to influence dramatically the distribution of salt cluster ions in the mass spectra observed. Under conditions of normal cone voltage of approximately 70 V, cluster ions having magic numbers of molecules are detected with high relative signal intensity. Under conditions of low cone voltage of approximately 10 V, the distribution of cluster ions detected is characterized by a relatively low average mass/charge ratio due to the presence of multiply charged cluster ions; in addition, there is a marked reduction in cluster ions having a magic number of molecules. Product ion mass spectra obtained by tandem mass spectrometry of cluster ions are characterized by a base peak having a magic number of molecules that is less than and closest to the number of molecules in the precursor ion. Structures have been proposed for some dications and some quadruply charged ions. At pH 3 and 11, the mass spectra of NaCl clusters show the presence of mixed clusters of NaCl with HCl and NaOH, respectively. The effects of ionic radius on 20 distributions of cluster ions for 10 salts were investigated; however, the fine structure of these effects is not readily discerned.     

Observation of apigenin anionic clusters in the gas phase

The ubiquity and favorable medicinal properties of flavonoids make essential the determination of flavonoid levels in various matrices. While developing a liquid chromatography/tandem mass spectrometry method for the analysis of the flavonoid, apigenin, anionic oligomers and nitrate- and chloride-bound clusters of this compound were observed. Tandem mass spectrometry of these oligomers and cluster ions showed the cleavage of apigenin molecules from the precursor. The observation of these cluster ions shows the possibility of post-column derivatization techniques to enhance specificity in analysis. Copyright 2000 John Wiley & Sons, Ltd.     

Observation of Na(+)-bound oligomers of quercetin in the gas phase

While developing a liquid chromatography/tandem mass spectrometry method for the analysis of the flavonoid quercitin, it was observed that quercetin (3,3',4',5,7-pentahydroxyflavone) exhibited clustering in both the positive and negative ion mode. Two series of positive ion clusters were observed; the first series corresponds to singly charged [2M + Na](+) at m/z 627.2 to [13M + Na](+) at m/z 3947.5, while the second series corresponds to doubly charged [7M + 2Na](2+) at m/z 1080.4 to [25M + 2Na](2+) at m/z 3798.5. In the negative ion mode, the behavior of quercetin parallels that of apigenin (4',5,7-trihydroxyflavone) in that [M + NO(3)](-), [2M + NO(3)](-), and [3M + NO(3)](-) were observed at m/z 364.1, 666.0, and 968.9, respectively; in addition, quercitin clusters with chloride ions ([2M + Cl](-) at m/z 638.9 and [3M + Cl](-) at m/z 940. 9) were observed. The results of tandem mass spectrometric examination of several cluster ions are reported.     

The Chromatographic Role in High Resolution Mass Spectrometry for Non-Targeted Analysis

Resolution improvements in time-of-flight instrumentation and the emergence of the Orbitrap mass spectrometer have researchers using high resolution mass spectrometry to determine elemental compositions and performing screening methods based on the full-scan data from these instruments. This work is focused on examining instrument performance of both a QTOF and a bench-top Orbitrap. In this study, the impact of chromatographic resolution on mass measurement accuracy, mass measurement precision, and ion suppression is examined at a fundamental level. This work was extended to a mixture of over 200 pesticides to determine how well two different software algorithms componentized and correctly identified these compounds under different sets of chromatographic conditions, where co-elution was expected to vary markedly.     

Determination of ion structures in structurally related compounds using precursor ion fingerprinting

Structurally-related alkaloids were analyzed by electrospray ionization/multiple stage mass spectrometry (ESI/MS ⁿ ) at varying collision energies to demonstrate a conceptual algorithm, precursor ion fingerprinting (PIF). PIF is a new approach for interpreting and library-searching ESI mass spectra predicated on the precursor ions of structurally-related compounds and their matching product ion spectra. Multiple-stage mass spectra were compiled and constructed into “spectral trees” that illustrated the compounds’ product ion spectra in their respective mass spectral stages. The precursor ions of these alkaloids were characterized and their spectral trees incorporated into an MS ⁿ library. These data will be used to construct a universal, searchable, and transferable library of MS ⁿ spectra. In addition, PIF will generate a proposed structural arrangement utilizing previously characterized ion structures, which will assist in the identification of unknown compounds.     

Simulation of ion trajectories in a quadrupole ion trap: a comparison of three simulation programs

An attempt has been made to compare the performance, design and operation of three simulators, ISIS, ITSIM and SIMION-3D, when applied to the calculation of ion trajectories in a quadrupole ion trap. For the simulation of the trajectory of a single ion in a collision-free system, the calculated spatial trajectory components, kinetic energies and secular frequencies from the three simulators were virtually identical. It is concluded that, despite the various approaches to electrode design, calculation of fields, integration methods and ion generation tactics, there is a remarkable degree of consistency among the products of the simulators when dealing with collision-free conditions. The results of the ion injection simulations under collisional conditions were indicative of the complexity that can be introduced into the simulations with little effort. Random effects such as collisions of ions with He buffer gas and accumulated calculation errors together with the different collision model settings and the different approaches to field calculation are thought to have contributed to the somewhat minor differences in trapping efficiency. SIMION is the simulator of choice for the simulation of ion trajectories in hybrid instruments and in custom-designed assemblies of electrodes; and ITSIM would appear to be the best choice on the basis of computational speed for running multiparticle simulations and user friendliness. Both ISIS and ITSIM are adept at providing detailed information of collision events. Copyright 1999 John Wiley & Sons, Ltd.     

Ultra-performance liquid chromatography/mass spectrometry of intact proteins

Given that numerous small molecule applications of ultra-performance liquid chromatography (UPLC) have been published, efforts were made to examine the potential of UPLC to enhance the separation of intact proteins. Beginning with typically employed conditions, column temperature and organic solvent were optimized followed by an HPLC vs. UPLC comparison. When applied to a mixture of 10 protein standards, the optimized method yielded improved chromatographic resolution, enhanced sensitivity, and a threefold increase in throughput. Subsequent cell lysate analysis demonstrated no compromise in chromatographic or mass spectral data quality at 1/3 of the original run time.     

Mass spectrometric study of persistent acid metabolites of nonylphenol ethoxylate surfactants

A mass spectrometric study was carried out on two nonylphenoxycarboxylic acids, NP1EC and NP2EC (where 1 and 2 indicate the number of ethoxylate units attached to the nonylphenoxy moiety), that are persistent metabolites of widely used nonionic surfactant nonylphenol ethoxylates. In a gas chromatographic/mass spectrometric (GC/MS) study of the methyl esters of NP1EC and NP2EC, two series of fragment ions were observed in electron ionization (EI) mass spectra; m/z (179 + 14n, n = 0-7) and m/z (105 + 14n, n = 0-4) for NP1ECMe and m/z (223 + 14n, n = 0-7) and m/z (107 + 14n, n = 0-5) for NP2ECMe. Similarity indices were used to compare quantitatively the mass spectra of isomers. The mass spectra of two isomers were found to be similar whereas those of the remaining isomers were readily distinguishable from each other. The abundant fragment ions of the two NPECMes were investigated further by GC/MS/MS; product ions resulting from cleavage in the alkyl moiety, cleavage in the ECMe moiety and cleavage in both moieties were detected. Possible structures of the nonyl groups in the two esters were inferred. GC/chemical ionization (CI) mass spectra of the NPECMes with isobutane as reagent gas showed characteristic hydride ion-abstracted fragment ions shifted by 1 Da from those in the corresponding EI mass spectra. The sensitivity of a selected ion monitoring quantitation method for the NPECMes is enhanced under CI conditions compared with that under EI conditions. With electrospray ionization MS/MS, [M - H](-) ions of NP1EC (m/z 277) and NP2EC (m/z 321) were observed and, upon collision-induced dissociation of [M - H](-) of each of the two acids, fragment ions of m/z 219 corresponding to deprotonated nonylphenol, were observed in each case. Based on this observation, a rapid, simple and reliable selected product ion quantitation method is proposed for NP1EC and NP2EC.     

Study of the neurotransmitter dopamine and the neurotoxin 6-hydroxydopamine by electrospray ionization coupled with tandem mass spectrometry

Electrospray ionization combined with tandem mass spectrometry has been applied to a study of dopamine and 6-hydroxydopamine, an important neurotransmitter and a well-known neurotoxin, respectively. Both protonated and deprotonated molecules were observed for the two compounds. Upon collision-induced dissociation of protonated and deprotonated 6-hydroxydopamine molecules, the number of fragmentation pathways observed was greater than that observed with protonated and deprotonated dopamine molecules; the greater proclivity to fragment of the former is due to the 6-substituted hydroxyl group, which is para to the 3-OH group and ortho to the CH2CH2NH2 group. Furthermore, 6-hydroxydopamine showed a greater propensity to oxidize than did dopamine when sample solutions were kept uncovered in the air for 24 h prior to mass spectrometric examination. Radical structures of the four main oxidation products of 6-hydroxydopamine have been suggested on the basis of their product ion mass spectra; one or more of these oxidation products may be responsible for the cytotoxic property of 6-hydroxydopamine.     

Rapid and sensitive detection of cholera toxin using gold nanoparticle-based simple colorimetric and dynamic light scattering assay

Herein, a rapid and simple gold nanoparticle based colorimetric and dynamic light scattering (DLS) assay for the sensitive detection of cholera toxin has been developed. The developed assay is based on the distance dependent properties of gold nanoparticles which cause aggregation of antibody-conjugated gold nanoparticles in the presence of cholera toxin resulting discernible color change. This aggregation induced color change caused a red shift in the plasmon band of nanoparticles which was measured by UV–Vis spectroscopy. In addition, we employed DLS assay to monitor the extent of aggregation in the presence of different concentration of cholera toxin. Our assay can visually detect as low as 10 nM of cholera toxin which is lower than the previously reported colorimetric methods. The reported assay is very fast and showed an excellent specificity against other diarrhetic toxins. Moreover, we have demonstrated the feasibility of our method for cholera toxin detection in local lake water.