Low pressure chemical ionization in ion trap mass spectrometry

This article presents the specificities of low pressure chemical ionization in ion trap mass spectrometry. One main feature is the ability to perform chemical ionization with liquid reagents as readily as with "conventional" gases (methane, isobutane and ammonia). The reactivities and analytical applications of gas and liquid reagents are summarized from literature data and are compared when possible.     

Mass-selected reagent ion chemical ionization and multiple tandem mass spectrometry techniques in an ion trap mass spectrometer for structural analysis

Mass-selected reagent ion chemical ionization (CI) performed in an ion trap instrument is an efficient tool to investigate gas-phase ion reactivities and therefore to find out new and/or optimized applications for structural analysis. For instance, it was shown that the C₃H₆O^{+ .} (58 mass units) molecular ion originated from vinyl methyl ether (VME) should necessarily be used alone (i.e. unit-mass selected) to produce significant diagnostic-ions for double bond location in aliphatic alkenes. Regarding the assignment of epoxides, the previous NO⁺/CI method was adapted for an optimal use in the trap through isolation of NO⁺ cation from N₂O (instead of NO) plasma and production of the acylium diagnostic-ions via CID of [M − H]⁺ formed by NO⁺-induced hydride abstraction. New alkylation ion-products, e.g. RCH = O⁺-al , were also found to characterize isomeric epoxides as a result of either an initial electrophilic addition of the C₂H₅⁺ cation (with saturated epoxides) or a methyl-transfer from [VME]^{+ .} (with α,β-unsaturated epoxides). The multiple tandem mass spectrometry (MSⁿ) capabilities of the ion trap were essential to achieve reagent ion mass-selection, structural assignment of the diagnostic-ions, or to provide further selectivity. © 1997 John Wiley & Sons, Ltd.     

Atmospheric pressure chemical ionization of fluorinated phenols in atmospheric pressure chemical ionization mass spectrometry, tandem mass spectrometry, and ion mobility spectrometry

Atmospheric pressure chemical ionization (APCI)-mass spectrometry (MS) for fluorinated phenols (C6H5-xFxOH Where x = 0-5) in nitrogen with Cl- as the reagent ion yielded product ions of M Cl- through ion associations or (M-H)- through proton abstractions. Proton abstraction was controllable by potentials on the orifice and first lens, suggesting that some proton abstraction occurs through collision induced dissociation (CID) in the interface region. This was proven using CID of adduct ions (M Cl-) with Q2 studies where adduct ions were dissociated to Cl- or proton abstracted to (M-H)-. The extent of proton abstraction depended upon ion energy and structure in order of calculated acidities: pentafluorophenol > tetrafluorophenol > trifluorophenol > difluorophenol. Little or no proton abstraction occurred for fluorophenol, phenol, or benzyl alcohol analogs. Ion mobility spectrometry was used to determine if proton abstraction reactions passed through an adduct intermediate with thermalized ions and mobility spectra for all chemicals were obtained from 25 to 200 degrees C. Proton abstraction from M Cl- was not observed at any temperature for phenol, monofluorophenol, or difluorophenol. Mobility spectra for trifluorophenol revealed the kinetic transformations to (M-H)- either from M Cl- or from M2 Cl- directly. Proton abstraction was the predominant reaction for tetra- and penta-fluorophenols. Consequently, the evidence suggests that proton abstraction occurs from an adduct ion where the reaction barrier is reduced with increasing acidity of the O-H bond in C6H5-xFxOH.     

Structural characterization and identification of dibenzocyclooctadiene lignans in Fructus Schisandrae using electrospray ionization ion trap multiple-stage tandem mass spectrometry and electrospray ionization Fourier transform ion cyclotron resonance multiple-stage tandem mass spectrometry

The electrospray ionization ion trap multiple-stage tandem mass spectrometry (ESI-MSⁿ) and electrospray ionization Fourier transform ion cyclotron resonance multiple-stage tandem mass spectrometry (ESI-FT-ICR-MSⁿ) have been applied successfully to the direct investigation of a number of dibenzocyclooctadiene lignan constituents from the methanol extracts of the Fructus Schisandrae in the positive ion mode. The detailed structural characterization of the same skeleton and different peripheral substituents had been studied and the precise elemental compositions of ions at high mass resolution had been obtained. So the fragmentation mechanisms could be clarified. And the lignan components in Schisandra chinensis (Turcz.) Baill. fruits (SCF) and Schisandra sphenanthera Rehd. et Wils. fruits (SSF) were identified by comparing the structural information and fragmentation mechanisms. Then a pair of isobaric compounds was differentiated. Meanwhile these two similar fruits were distinguished. The research results demonstrated that ESI-MSⁿ technique is a sensitive, selective and effective tool for the direct analysis and rapid determination of constituents in complex mixtures from nature products. And these should be useful for the identification of similar compounds and differentiation of similar species from Chinese herbs.     

Boundary-effect activated dissociation in ion trap tandem mass spectrometry

The potential of boundary-effect activated dissociation (BAD) in ion trap tandem mass spectrometry (MS/MS) is discussed. Several classes of compounds were investigated and the BAD product ion tandem mass spectra were compared with those from collisionally activated dissociation (CAD) where an auxilliary r.f. ‘tickle’ voltage is used. The energy deposition in BAD MS/MS is generally lower than that for optimized CAD, but the experiments are easier to perform. An example of the BAD MS/MS of C₁₀ alkylbenzenes and benzothiophene in diesel fuel using gas chromatographic introduction and predicted r.f. and d.c. voltages is presented.     

Enhanced signal generation for use in the analysis of synthetic pyrethroids using chemical ionization tandem quadrupole ion trap mass spectrometry

Synthetic pyrethroids fragment extensively under electron ionization (EI) conditions to give low mass ions, most of them with the same m/z ratios. This fragmentation is primarily due to the labile ester linkage found in these compounds. In this research we established the best gas chromatography (GC) conditions in the EI mode that served as a benchmark in the development of a chemical ionization (CI) protocol for ten selected synthetic pyrethroids. Based on proton affinity data, several reagent gases were evaluated in the positive CI ionization mode. Methanol was found to produce higher average ion counts relative to the other gases evaluated, which led to the development of an optimized method consisting of selective ejection chemical ionization (SECI) and MS/MS. Standard stainless steel ion trap electrodes produced significant degradation of chromatographic performance on late eluting compounds, which was attributed to electrode surface chemistry. A dramatic improvement in signal-to-noise (S/N) ratios was observed when the chromatographically inert Silcosteel® coated electrodes were used. The resulting method, that has significant S/N ratio improvements resulting from a combination of septum programmable injections (SPI), optimized CI and inert Silcosteel®-coated electrodes, was used to determine instrument detection limits.     

Study of atmospheric pressure chemical ionization of PETN by ion mobility spectrometry/tandem mass spectrometry

We present the results of studying the effects of temperature and humidity of the reaction medium and the intensity of ultraviolet radiation on the atmospheric pressure chemical ionization of Penthrite. The peculiarities of the ion mobility spectra of this compound obtained by ion mobility spectrometry-tandem mass spectrometry are analyzed.     

Comprehensive quality evaluation of Fructus Schisandrae using electrospray ionization ion trap multiple-stage tandem mass spectrometry coupled with chemical pattern recognition techniques

Electrospray ionization ion trap multiple-stage tandem mass spectrometry (ESI-MS(n)) was used to evaluate Fructus Schisandrae of similar species (Schisandra chinensis (Turcz.) Baill. fruits and Schisandra sphenanthera Rehd. et Wils. fruits) and different growth characteristics (color, shape, etc.). The application of chemical pattern recognition in the ESI-MS(n) data analysis was carried out by principal component analysis (PCA), hierarchical cluster analysis (HCA) and linear discriminant analysis (LDA). Then the antioxidant activity of different Fructus Schisandrae samples were determined by an LC-ESI-MS method and ferric reducing antioxidant power (FRAP) assay. Using the ESI-MS(n) method coupled with chemical pattern recognition analysis and correlated with the antioxidant activity evaluation, the two similar species were successfully distinguished, thus improving the therapeutic safety and effectiveness. The superior characteristics of Schisandra chinensis (Turcz.) Baill. fruits were obtained and made the selection and breeding of Chinese medicine materials more scientific. This study indicates that ESI-MS(n) is a valuable tool for the authentication of botanical origin and can also be useful for the quality control of Chinese medicinal herbs.     

Determination of carnitine and acylcarnitines in urine by high-performance liquid chromatography-electrospray ionization ion trap tandem mass spectrometry

A high-performance liquid chromatography-mass spectrometry method has been developed for the simultaneous determination of native carnitine and eight acylcarnitines in urine. The procedure uses a solid-phase extraction on a cation-exchange column and the separation is performed without derivatization within 17 min on a reversed-phase C8 column in the presence of a volatile ion-pairing reagent. The detector was an ion trap mass spectrometer and quantification was carried out in the MS-MS mode. Validation was done for aqueous standards at ranges between 0.75 and 200 micromol/l, depending on the compound. Carnitine was quantified in urine and comparison with a radioenzymatic assay gave a satisfactory correlation (R2 = 0.981). The assay could be successfully applied to the diagnostic of pathological acylcarnitines profile of metabolic disorders in urines of patients suffering from different organic acidurias.     

Laser desorption lamp ionization source for ion trap mass spectrometry

A two‐step laser desorption lamp ionization source coupled to an ion trap mass spectrometer (LDLI‐ITMS) has been constructed and characterized. The pulsed infrared (IR) output of an Nd:YAG laser (1064 nm) is directed to a target inside a chamber evacuated to ~15 Pa causing desorption of molecules from the target's surface. The desorbed molecules are ionized by a vacuum ultraviolet (VUV) lamp (filled with xenon, major wavelength at 148 nm). The resulting ions are stored and detected in a three‐dimensional quadrupole ion trap modified from a Finnigan Mat LCQ mass spectrometer operated at a pressure of ≥ 0.004 Pa. The limit of detection for desorbed coronene molecules is 1.5 pmol, which is about two orders of magnitude more sensitive than laser desorption laser ionization mass spectrometry using a fluorine excimer laser (157 nm) as the ionization source. The mass spectrum of four standard aromatic compounds (pyrene, coronene, rubrene and 1,4,8,11,15,18,22,25‐octabutoxy‐29H,31H‐phthalocyanine (OPC)) shows that parent ions dominate. By increasing the infrared laser power, this instrument is capable of detecting inorganic compounds. Copyright © 2015 John Wiley & Sons, Ltd.     

Selective chemical ionization of nitrogen and sulfur heterocycles in petroleum fractions by ion trap mass spectrometry

A procedure is reported for the selective ammonia chemical ionization of some nitrogen and sulfur heterocycles in petroleum fractions using ion trap mass spectrometry (ITMS). The ion trap scan routine is designed to optimize the population of ammonium reagent ions and eject from the trap (by radio frequency/direct current isolation) electron ionization products formed during reagent ion formation prior to ionization of the sample. The ITMS procedure is compared with standard ion trap detector and conventional quadrupole ammonia chemical ionization for the determination of nitrogen and sulfur heterocycles in gas oil and kerosine samples. Greatly enhanced selectivity is shown for the ITMS procedure by suppression of competing charge-exchange processes.     

Selective adduct formation by furan chemical ionization reagent in gas chromatography ion trap mass spectrometry

The analytical potential of furan as a chemical ionization (CI) reagent was evaluated for selectivity with nine monosubstituted naphthalene compounds. The ion-molecule reactions of furan and tetrahydrofuran (THF) were compared with those of methane, methanol and acetonitrile (prominently producing [M + H](+) ion base peaks) with naphthalene compounds in chemical ionization mass spectrometry (CI-MS). Reactions with furan predominantly show M(+) and [M + 39](+) ions. Based on this phenomenon, investigations were carried out for some of the molecular factors such as proton affinity, substituent effects and the preferred site of [C(3)H(3)](+) ion attachment that influence reactivity in furan CI. High selectivity with different substituents is observed in the formation of [M + 39](+) adduct ion, suggesting its usefulness as selective ionization reagent liquid. The selectivity and sensitivity are illustrated in the analysis of mixture of amino acids. Furthermore, the structure determination and reaction mechanism study is characterized by collision-activated dissociation experiments in CI-MS/MS and CI-MS/MS/MS.     

Detection of pharmaceutical compounds in tissue by matrix-assisted laser desorption/ionization and laser desorption/chemical ionization tandem mass spectrometry with a quadrupole ion trap

A novel quadrupole ion trap mass spectrometer laser microprobe instrument with an external ionization source was constructed and used to investigate the matrix-assisted laser desorption/ionization (MALDI) detection of pharmaceutical compounds in intact tissue. In addition to MALDI, laser desorption coupled with chemical ionization (LD/CI) was investigated. MALDI, using 2,5-dihydroxybenezoic acid (DHB) as a matrix, was employed to detect the anticancer drug paclitaxel from a thin section of rat liver tissue which had been incubated in a solution of paclitaxel. The results of that experiment showed that the ability to perform tandem mass spectrometry (MS/MS) with the quadrupole ion trap was crucial in the identification of drug compounds at trace levels in the complex tissue matrix. MALDI MS/MS was then used to detect the presence of paclitaxel in a human ovarian tumor at a concentration of approximately 50 mg/kg. Finally, the drug spiperone was detected in incubated rat liver tissue at an approximate level of 25 mg/kg using LD/CI (no MALDI matrix). Again, the MS/MS capability of the quadrupole ion trap was crucial in the identification of the drug at trace levels in the complex tissue matrix.     

Desorption electrospray ionization mass spectrometric analysis of organophosphorus chemical warfare agents using ion mobility and tandem mass spectrometry

Desorption electrospray ionization mass spectrometry (DESI‐MS) has been applied to the direct analysis of sample media for target chemicals, including chemical warfare agents (CWA), without the need for additional sample handling. During the present study, solid‐phase microextraction (SPME) fibers were used to sample the headspace above five organophosphorus CWA, O‐isopropyl methylphosphonofluoridate (sarin, GB), O‐pinacolyl methylphosphonofluoridate (soman, GD), O‐ethyl N,N‐dimethyl phosphoramidocyanidate (tabun, GA), O‐cyclohexyl methylphosphonofluoridate (cyclohexyl sarin, GF) and O‐ethyl S‐2‐diisopropylaminoethyl methyl phosphonothiolate (VX) spiked into glass headspace sampling vials. Following sampling, the SPME fibers were introduced directly into a modified ESI source, enabling rapid and safe DESI of the toxic compounds. A SYNAPT HDMS? instrument was used to acquire time‐aligned parallel (TAP) fragmentation data, which provided both ion mobility and MSⁿ (n = 2 or 3) data useful for the confirmation of CWA. Unique ion mobility profiles were acquired for each compound and characteristic product ions of the ion mobility separated ions were produced in the Triwave? transfer collision region. Up to six full scanning MSⁿ spectra, containing the [M + H]⁺ ion and up to seven diagnostic product ions, were acquired for each CWA during SPME fiber analysis. A rapid screening approach, based on the developed methodology, was applied to several typical forensic media, including Dacron sampling swabs spiked with 5 µg of CWA. Background interference was minimal and the spiked CWA were readily identified within one minute on the basis of the acquired ion mobility and mass spectrometric data. Copyright © 2010 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.     

Electrospray ionization and tandem ion trap mass spectrometry for the confirmation of seven beta-lactam antibiotics in bovine milk

The feasibility of a technique to confirm the presence of residues from seven beta-lactam antibiotics in bovine milk has been demonstrated. The technique makes use of electrospray ionization and tandem ion trap mass spectrometry. Residues are first extracted from milk by reversed-phase solid phase extraction. Target analytes are separated by on-line reversed-phase liquid chromatography and ionized in the electrospray interface. The product ion mass spectra are acquired following collision-induced dissociation of protonated molecules. Confirmation is based on comparison of full scan spectra between unknowns and bona fide standards. The feasibility of this technique has been demonstrated for the six beta-lactams currently approved for use in lactating dairy cattle (penicillin G, ampicillin, amoxicillin, cloxacillin, cephapirin and ceftiofur) and a drug not approved for animal use, cefazolin. The technique has been applied to control milk fortified at 5 ng/mL of penicillin G and 10 ng/mL of the other six drugs.     

Separation and identification of compounds in Adinandra nitida by comprehensive two-dimensional liquid chromatography coupled to atmospheric pressure chemical ionization source ion trap tandem mass spectrometry

A comprehensive two-dimensional liquid chromatographic (2D-LC) separation system based on the combination of a CN column and a Merck Chromolith Flash reversed-phase column was developed for the separation of components in Adinandra nitida, one type of traditional Chinese medicine (TCM). The two dimensions were connected by a ten-port, dual-position valve controlled automatically by software written in-house. The effluents were detected by both ultraviolet and atmospheric pressure chemical ionization source ion trap tandem mass spectrometry (MS). The calculated peak capacity of the 2D-LC–MS/MS system was above 1240. More than 57 components were resolved in the methanol extract from Adinandra nitida leaves, and five of these were identified based on their relative retention times, molecular weights and MS/MS spectra.     

Surface-activated chemical ionization ion trap mass spectrometry in the analysis of amphetamines in diluted urine samples

A new ionization method, named surface-activated chemical ionization (SACI), was employed for the analysis of five amphetamines (3,4-methylenedioxyamphetamine (MDA), 3,4-methylenedioxymethamphetamine (MDMA), 3,4-methylenedioxyethylamphetamine (MDE), amphetamine and methamphetamine) by ion trap mass spectrometry. The results so obtained have been compared with those achieved by using atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) using the same instrument, clearly showing that SACI is the most sensitive of the three. The limit of detection and linearity range for SACI were compared with those obtained using APCI and ESI, showing that the new SACI approach provides the best results for both criteria. SACI was used to analyze MDA, MDMA MDE, amphetamine and methamphetamine in four urine samples, and the quantitation results are compared with those achieved using ESI.     

Top-down tandem mass spectrometry of tRNA via ion trap collision-induced dissociation

Transfer RNA is a class of highly modified and structured non-coding RNA molecules generally comprised of 74–95 nucleotides. In this study, tandem mass spectrometry of intact multiply charged tRNA anions of roughly 25 kDa in mass has been demonstrated using a quadrupole/time-of-flight tandem mass spectrometer adapted for ion/ion reaction studies. The sample proved to be a mixture of tRNA molecules. The mass of the most abundant component of the mixture was not consistent with that of the nominal identity of the tRNA from the supplier, viz., tRNA^phe; rather, the mass was consistent with tRNA^Phe bearing an incomplete 3′-terminus. Multiply-charged anions from the major components were isolated in the gas phase and subjected to ion trap collision-induced dissociation without subsequent ion/ion reactions. Abundant fragments from the 5′- and 3′-termini of the molecule could be used to identify the major component as tRNA^phe-3′adenosine (without 3′-phosphorylation). Roughly 15% of the primary sequence of the intact tRNA was unambiguously reflected in the product ion spectrum. The existence of a possible tRNA^Phe variant and the intact tRNA^Phe was also supported by ion trap CID data. The multiply-charged fragment ions derived from tRNA^Phe-3′adenosine were further charge-reduced to mostly singly- and doubly-charged species via proton transfer ion/ion reactions with benzoquinoline cations. The resulting reduction in spectral overlap and charge state ambiguity simplified interpretation of the product ion spectrum and allowed for the identification of product ions from roughly 60% of the sequence.     

Analysis of a commercial preparation of erythromycin estolates by tandem mass spectrometry and high performance liquid chromatography/electrospray ionization tandem mass spectrometry using an ion trap mass spectrometer

Because of the lack of a UV chromophore and their much smaller abundances in comparison with the major component, the minor components in erythromycin estolate preparations are difficult to analyze by high performance liquid chromatography ultraviolet (HPLC-UV). Tentative assignment of the major and minor components can be achieved with the combination of full scan and ZoomScan using an ion trap mass spectrometer. Tandem mass spectrometry (MS/MS) provided an effective method to quickly identify most components without chromatographic separation, and all the related compounds, except the isobaric pair ECE and PdMeEA, could be identified in this way. The best result was obtained by using liquid chromatography/tandem mass spectrometry (LC/MS/MS) operated in selected reaction monitoring mode. The major compound, the estolate of erythromycin A (EAE), and seven other minor components, could be separated and identified, with semiquantitative estimates of relative concentrations.