Development of a new electron ionization/field ionization ion source for gas chromatography/time‐of‐flight mass spectrometry

We have developed a combined EI/FI source for gas chromatography/orthogonal acceleration time‐of‐flight mass spectrometry (GC/oaTOFMS). In general, EI (electron ionization) and FI (field ionization) mass spectra are complementary: the EI mass spectrum contains information about fragment ions, while the FI mass spectrum contains information about molecular ions. Thus, the comparative study of EI and FI mass spectra is useful for GC/MS analyses. Unlike the conventional ion sources for FI and EI measurements, the newly developed source can be used for both measurements without breaking the ion source vacuum or changing the ion source. Therefore, the combined EI/FI source is more preferable than the conventional EI or FI ion source from the viewpoint of the reliability of measurements and facility of operation. Using the combined EI/FI source, the complementarity between EI and FI mass spectra is demonstrated experimentally with n‐hexadecane (100 pg): characteristic fragment ions for the n‐alkane such as m/z 43, 57, 71, and 85 are obtained in the EI mass spectrum, while only the parent peak of m/z 226 (M⁺) without any fragment ions is observed in the FI mass spectrum. Moreover, the field desorption (FD) measurement is also demonstrated with poly(ethylene glycol)s M600 (10 ng) and M1000 (15 ng). Signals of [M+H]⁺, [M+Na]⁺ and [M+K]⁺ are clearly detected in the FD mass spectra. Copyright © 2009 John Wiley & Sons, Ltd.     

Infrared matrix-assisted laser desorption/ionization of polycyclic aromatic hydrocarbons with a sulfolane matrix

Infrared matrix-assisted laser desorption/ionization (IR-MALDI) of the polyaromatic hydrocarbons (PAHs) anthracene, benzo[a]pyrene, and dibenz[a,h]anthracene was performed using a 10.6-microm CO2 laser and a liquid matrix. Sulfolane (tetrahydrothiophene 1,1-dioxide) was found to be an effective matrix for PAH ionization: mass spectra obtained with a sulfolane matrix contain an intense molecular ion peak; interference from PAH fragment and matrix peaks is negligible in all cases. The main limitation of the sulfolane matrix is sample evaporation after 3 to 5 min in vacuum. This sample lifetime can be increased to between 15 and 30 min using a 2:1 (v/v) mixture of sulfolane and glycerol, but the resulting spectra have greater matrix interference and decreased shot-to-shot signal stability.     

Analysis of pesticides by gas chromatography/multiphoton ionization/mass spectrometry using a femtosecond laser

Gas chromatography/multiphoton ionization/time-of-flight mass spectrometry (GC/MPI/TOFMS) was utilized for analysis of a standard mixture sample containing 49 pesticides and 4 real samples using the third-harmonic emission (267 nm) of a femtosecond Ti:sapphire laser (100 fs) as the ionization source. A sample of a standard mixture of n-alkane was also measured for calibration of the retention time indices of the pesticides. Two photons are required for the excitation of n-alkane due to an absorption band located in the far ultraviolet region (140 nm). The n-alkane molecule in the excited state was subsequently ionized either directly or by absorbing another photon because of a high ionization potential. Due to a large excess of energy, the molecular ion was decomposed and formed many fragment ions. Compared to n-alkanes, most of the pesticides were softly ionized by the femtosecond laser; one photon was used for excitation and another was used for the subsequent ionization. The pesticides with no conjugated double bond had a lower ionization efficiency. The present analytical instrument was applied to several samples prepared from a variety of vegetables and a single fruit after pretreatment with solid-phase extraction. Three pesticides were found in these samples, although some of them were not detected by conventional GC/EI/MS–MS due to insufficient sensitivity and selectivity.     

Ultraviolet laser desorption/ionization mass spectrometry of single‐core and multi‐core polyaromatic hydrocarbons under variable conditions of collisional cooling: insights into the generation of molecular ions,fragments and oligomers

The ultraviolet laser desorption/ionization of polyaromatic hydrocarbons (PAHs) has been investigated under different background pressures of an inert gas (up to 1.2 mbar of N₂) in the ion source of a hybrid, orthogonal‐extracting time‐of‐flight mass spectrometer (oTOF‐MS). The study includes an ensemble of six model PAHs with isolated single polyaromatic cores and four ones with multiple cross‐linked aromatic and polyaromatic cores. In combination with a weak ion extraction field, the variation of the buffer gas pressure allowed to control the degree of collisional cooling of the desorbed PAHs and, thus, to modulate their decomposition into fragments. The dominant fragmentation channels observed are related to dehydrogenation of the PAHs, in most cases through the cleavage of even numbers of C―H bonds. Breakage of C―C bonds leading to the fragmentation of rings, side chains and core linkages is also observed, in particular, at low buffer gas pressures. The precise patterns of the combined fragmentation processes vary significantly between the PAHs. The highest abundances of molecular PAH ions and cleanest mass spectra were consistently obtained at the highest buffer gas pressure of 1.2 mbar. The effective quenching of the fragmentation pathways at this elevated pressure improves the sensitivity and data interpretation for analytical applications, although the fragmentation of side chains and of bonds between (poly)aromatic cores is not completely suppressed in all cases. Moreover, these results suggest that the detected fragments are generated through thermal equilibrium processes rather than as a result of rapid photolysis. This assumption is further corroborated by a laser desorption/ionization post‐source decay analysis using an axial time‐of‐flight MS. In line with these findings, covalent oligomers of the PAHs, which are presumably formed by association of two or more dehydrogenated fragments, are detected with higher abundances at the lower buffer gas pressures. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Laser desorption ionization of small molecules assisted by tungsten oxide and rhenium oxide particles

Inorganic metal oxides have shown potential as matrices for assisting in laser desorption ionization with advantages over the aromatic acids typically used. Rhenium and tungsten oxides are attractive options due to their high work functions and relative chemical inertness. In this work, it is shown that ReO₃ and WO₃, in microparticle (μP) powder forms, can efficiently facilitate ionization of various types of small molecules and provide minimized background contamination at analyte concentrations below 1 ng/µL. This study shows that untreated inorganic WO₃ and ReO₃ particles are valid matrix options for detection of protonatable, radical, and precharged species under laser desorption ionization. Qualitatively, the WO₃ μP showed improved detection of apigenin, sodiated glucose, and precharged analyte choline, while the ReO₃ μP allowed better detection of protonated cocaine, quinuclidine, ametryn, and radical ions of polyaromatic hydrocarbons at detection levels as low as 50 pg/µL. For thermometer ion survival yield experiments, it was also shown that the ReO₃ powder was significantly softer than α‐cyano‐4‐hydroxycinnaminic acid. Furthermore, it provided higher intensities of cocaine and polyaromatic hydrocarbons, at laser flux values equal to those used with α‐cyano‐4‐hydroxycinnaminic acid. Copyright © 2015 John Wiley & Sons, Ltd.     

Ambient ionization and direct identification of volatile organic compounds with microwave‐induced plasma mass spectrometry

An innovative method of volatile organic compounds analysis by using microwave‐induced plasma ionization (MIPI) source in combination with an ambient ion trap mass spectrometer is presented here. Using MIPI for direct sample vapor, analysis was achieved without any sample preparation or subsequent heating. The relative abundance of the target compounds can be obtained almost instantly within a few seconds. The ionization processes of different volatile compounds was optimized, and the limits of detection were identified in the range of 0.15–4.5 pptv or 0.73–8.80 pg ml^?1. The relative standard deviation (RSD) is in the range of 4–14%, while correlation coefficients of the working curves (R²) are better than 0.98. The new method possesses advantages of ease operation, time‐saving, high sensitivity and inexpensive setup. In addition, the ionization processes of short n‐alkane chains were investigated with the MIPI technique, and a unique [M + 13]⁺ was detected, which has not been reported in detail by any other related ionization techniques. An ionization mechanism was proposed on the basis of the experimental results obtained in this work and available information in literatures, in which the n‐alkanes in the plasma environment possibly generate protonated cyclopentadiene [M – 5]⁺ or alkyl‐substituted analogues as well as hydrous ions [M + 13]⁺ and [M + 13 + 18]⁺, as shown in Scheme 1 in the main text. Copyright © 2015 John Wiley & Sons, Ltd.     

Fragmentation of organic ions bearing fixed multiple charges observed in MALDI MS

Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI TOF MS) was used to analyze a series of synthetic organic ions bearing fixed multiple charges. Despite the multiple intrinsic charges, only singly charged ions were recorded in each case. In addition to the pseudo‐molecular ions formed by counterion adduction, deprotonation and electron capture, a number of fragment ions were also observed. Charge splitting by fragmentation was found to be a viable route for charge reduction leading to the formation of the observed singly charged fragment ions. Unlike multivalent metal ions, organic ions can rearrange and/or fragment during charge reduction. This fragmentation process will evidently complicate the interpretation of the MALDI MS spectrum. Because MALDI MS is usually considered as a soft ionization technique, the fragment ion peaks can easily be erroneously interpreted as impurities. Therefore, the awareness and understanding of the underlying MALDI‐induced fragmentation pathways is essential for a proper interpretation of the corresponding mass spectra. Due to the fragment ions generated during charge reduction, special care should be taken in the MALDI MS analysis of multiply charged ions. In this work, the possible mechanisms by which the organic ions bearing fixed multiple charges fragment are investigated. With an improved understanding of the fragmentation mechanisms, MALDI TOF MS should still be a useful technique for the characterization of organic ions with fixed multiple charges.     

Fragment ion formation from n-alkanes and cycloalkanes by plasma desorption

The plasma desorption of hydrogen loss fragment ions from frozen films of several classes of aliphatic volatile hydrocarbons was investigated with ²⁵²Cf plasma desorption mass spectrometry. A time-of-flight instrument suitable for the reproducible sampling and analysis of volatile hydrocarbons in the solid state was designed for this study. Representative compounds from the n-alkanes and cycloalkanes were analyzed. For all the compounds studied, radical molecular ions. M^+., and hydrogen loss fragment ions, [M ? mH]⁺ with m varying from 1 to 10, were produced. The length of the alkane chain, the size of the alkane rings and the stability of the resulting H-loss ions affected the number and abundance of these ions. Smaller molecules underwent proportionally more hydrogen loss than larger molecules of the same class.     

New MALDI matrices based on lithium salts for the analysis of hydrocarbons and wax esters

Lithium salts of organic aromatic acids (lithium benzoate, lithium salicylate, lithium vanillate, lithium 2,5‐dimethoxybenzoate, lithium 2,5‐dihydroxyterephthalate, lithium α‐cyano‐4‐hydroxycinnamate and lithium sinapate) were synthesized and tested as potential matrices for the matrix‐assisted laser desorption/ionization (MALDI)‐mass spectrometry analysis of hydrocarbons and wax esters. The analytes were desorbed using nitrogen laser (337.1 nm) and ionized via the attachment of a lithium cation, yielding [M + Li]⁺ adducts. The sample preparation and the experimental conditions were optimized for each matrix using stearyl behenate and n‐triacontane standards. The performance of the new matrices in terms of signal intensity and reproducibility, the mass range occupied by matrix ions and the laser power threshold were studied and compared with a previously recommended lithium 2,5‐dihydroxybenzoate matrix (LiDHB) (Cva?ka and Svato?, Rapid Commun. Mass Spectrom. 2003, 17, 2203). Several of the new matrices performed better than LiDHB. Lithium vanillate offered a 2–3 times and 7–9 times higher signal for wax esters and hydrocarbons, respectively. Also, the signal reproducibility improved substantially, making this matrix a suitable candidate for imaging applications. In addition, the diffuse reflectance spectra and solubility of the synthesized compounds were investigated and discussed with respect to the compound's ability to serve as MALDI matrices. The applicability of selected matrices was tested on natural samples of wax esters and hydrocarbons. Copyright © 2014 John Wiley & Sons, Ltd.     

Laser desorption ionization of red phosphorus clusters and their use for mass calibration in time‐of‐flight mass spectrometry

Phosphorus clusters P_n (n = 1–89) are easily formed from red phosphorus by laser desorption ionization (LDI) and they cover a range of up to approx. m/z 3000 in both positive and negative ion mode. The clusters are singly charged and the spectra are simple because phosphorus is monoisotopic. The mass spectra can be measured with an acceptable resolution and intensity. The use of positively charged P_n clusters for calibration in mass spectrometry was examined and it was demonstrated that in external calibration a standard deviation of ±0.04 m/z units can be achieved even when using a common commercial matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) instrument. When used as internal standards the P_n clusters react with some analytes – C₆₀ and C₇₀ fullerenes and cucurbituril[8], for example. It was also found that red phosphorus is a suitable MALDI matrix for peptides and proteins, illustrated by the examples of a Calmix mixture of bradykinin, angiotensin, renin, adrenocorticotropic hormone ACTH fragment 18‐359 and insulin, and of insulin alone. Copyright © 2009 John Wiley & Sons, Ltd.     

Searching for anthropogenic contaminants in human breast adipose tissues using gas chromatography‐time‐of‐flight mass spectrometry

The potential of gas chromatography‐time‐of‐flight mass spectrometry (GC‐TOF MS) for screening anthropogenic organic contaminants in human breast adipose tissues has been investigated. Initially a target screening was performed for a list of 125 compounds which included persistent halogen pollutants [organochlorine (OC) pesticides, polychlorinated biphenylss (PCBs), polybrominated diphenyl ethers (PBDEs)], polyaromatic hydrocarbons (PAHs), alkylphenols, and a notable number of pesticides from the different fungicide, herbicide and insecticide families. Searching for target pollutants was done by evaluating the presence of up to five representative ions for every analyte, all measured at accurate mass (20‐mDa mass window). The experimental ion abundance ratios were then compared to those of reference standards for confirmation. Sample treatment consisted of an extraction with hexane and subsequent normal‐phase (NP) High performance liquid chromatography (HPLC) or SPE cleanup. The fat‐free LC fractions were then investigated by GC‐TOF MS. Full‐spectral acquisition and accurate mass data generated by GC‐TOF MS also allowed the investigation of nontarget compounds using appropriate processing software to manage MS data. Identification was initially based on library fit using commercial nominal mass libraries. This was followed by comparing the experimental accurate masses of the most relevant ions with the theoretical exact masses with calculations made using the elemental composition calculator included in the software. The application of both target and nontarget approaches to around 40 real samples allowed the detection and confirmation of several target pollutants including p,p′‐DDE, hexachlorobenzene (HCB), and some polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs). Several nontarget compounds that could be considered anthropogenic pollutants were also detected. These included 3,5‐di‐tert‐butyl‐4‐hydroxy‐toluene (BHT) and its metabolite 3,5‐di‐tert‐butyl‐4‐hydroxybenzaldehyde (BHT‐CHO), dibenzylamine, N‐butyl benzenesulfonamide (N‐BBSA), some naphthalene‐related compounds and several PCBs isomers not included in the target list. As some of the compounds detected are xenoestrogens, the methodology developed in this paper could be useful in human breast cancer research. Copyright © 2008 John Wiley & Sons, Ltd.     

Photochemical and Thermal Stability of Some Dihydroxyacetophenones Used as UV‐MALDI‐MS Matrices

2,4‐, 2,5‐, 2,6‐ and 3,5‐dihydroxyacetophenone (DHA) used as matrices in matrix‐assisted ultraviolet laser desorption/ionization mass spectrometry (UV‐MALDI‐MS) were studied by steady‐state and transient absorption spectroscopy, together with DFT calculations at the B3LYP level of theory. All compounds have low fluorescence quantum yields, possibly due to an efficient excited‐state intramolecular proton transfer (ESIPT). Laser flash photolysis (LFP) results showed that, only for 2,4‐DHA, a phototautomer could be detected at λ = 400 nm. Their photochemical stability in solution at different wavelengths and conditions was analyzed by UV–Vis and ¹H nuclear magnetic resonance spectroscopy (¹H‐NMR), together with thin layer chromatography and ultraviolet laser desorption/ionization mass spectrometry (UV‐LDI‐MS). Only 3,5‐DHA showed decomposition when irradiated, probably because phototautomerization is not possible. Thermal stability studies of these compounds in solid state were also conducted.     

Structure-type separation of diesel fuels by solid phase extraction and identification of the two- and three-ring aromatics by capillary GC-mass spectrometry

A simple and effective solid phase extraction (SPE) method using silica gel micro glass columns has been developed for the separation of diesel fuel into groups of aliphatic, and mono-, di- and polyaromatic hydrocarbons. It is based on a stepwise gradient of dichloromethane in n-pentane. The resulting fractions were analyzed by capillary gas chromatography with a flame ionization detector and coupled gas chromatography-mass spectrometry. Commercially available standards, and retention indices and mass spectra were used for identification of individual aromatic compounds. The principal polycyclic aromatic hydrocarbons (PAHs) in diesel fuel are naphthalene, biphenyl, fluorene, phen-anthrene and their alkylated derivatives. Sulfur-containing PAHs are mainly represented by methyl-substituted dibenzo-thiophenes.     

Electrospray ionization mass spectrometric detection of low polar compounds by adding NaAuCl4

Liquid chromatography electrospray ionization mass spectrometry (LC/ESI/MS) has been widely used for various analyses. However, it is difficult to use LC/ESI/MS for the analysis of low polar compounds, such as polycyclic aromatic hydrocarbons. It is well known that AuCl₄^? ion decomposes to AuCl₃ by heating, and AuCl₃ is a strong π‐electrophilic Lewis acid. Low polar compounds (pyrene, benzo[a]pyrene, perylene, benzo[ghi]perylene, dibenzothiophene and p‐dimethoxybenzene) were detected by ESI/MS in the positive ion mode by adding NaAuCl₄. The low polar compound interacts with AuCl₃ formed at the ESI interface, and undergoes electron transfer to AuCl₃. The radical cation of the low polar compound was then detected by MS. In addition, the LC/ESI/MS determination of polycyclic aromatic hydrocarbons by the post‐column addition of NaAuCl₄ was studied. © 2016 The Authors Journal of Mass Spectrometry Published by John Wiley & Sons Ltd     

Water CI+, a new selective and highly sensitive method for the detection of environmental components using ion trap mass spectrometers

A new method for mass spectrometric trace analysis using an ion trap detector is presented based on chemical ionization with water as a reactant. The technique has advantages over the methods commonly used with regard to selectivity and detection limit and it facilitates the detection of unstable organic compounds. Specific applications in the trace analysis of polyaromatic hydrocarbons, trinitroaromatics as well as glycols and their derivatives are described.     

Water CI+, a new selective and highly sensitive method for the detection of environmental components using ion trap mass spectrometers

A new method for mass spectrometric trace analysis using an ion trap detector is presented based on chemical ionization with water as a reactant. The technique has advantages over the methods commonly used with regard to selectivity and detection limit and it facilitates the detection of unstable organic compounds. Specific applications in the trace analysis of polyaromatic hydrocarbons, trinitroaromatics as well as glycols and their derivatives are described.     

Mass spectrometry of rhenium complexes: a comparative study by using LDI‐MS,MALDI‐MS,PESI‐MS and ESI‐MS

A group of rhenium (I) complexes including in their structure ligands such as CF₃SO₃‐, CH₃CO₂‐, CO, 2,2′‐bipyridine, dipyridil[3,2‐a:2′3′‐c]phenazine, naphthalene‐2‐carboxylate, anthracene‐9‐carboxylate, pyrene‐1‐carboxylate and 1,10‐phenanthroline have been studied for the first time by mass spectrometry. The probe electrospray ionization (PESI) is a technique based on electrospray ionization (ESI) that generates electrospray from the tip of a solid metal needle. In this work, mass spectra for organometallic complexes obtained by PESI were compared with those obtained by classical ESI and high flow rate electrospray ionization assisted by corona discharge (HF‐ESI‐CD), an ideal method to avoid decomposition of the complexes and to induce their oxidation to yield intact molecular cation radicals in gas state [M]^+. and to produce their reduction yielding the gas species [M]^–.. It was found that both techniques showed in general the intact molecular ions of the organometallics studied and provided additional structure characteristic diagnostic fragments. As the rhenium complexes studied in the present work showed strong absorption in the UV–visible region, particularly at 355 nm, laser desorption ionization (LDI) mass spectrometry experiments could be conducted. Although intact molecular ions could be detected in a few cases, LDI mass spectra showed diagnostic fragments for characterization of the complexes structure. Furthermore, matrix‐assisted laser desorption ionization (MALDI) mass spectra were obtained. Nor‐harmane, a compound with basic character, was used as matrix, and the intact molecular ions were detected in two examples, in negative ion mode as the [M]^–. species. Results obtained with 2‐[(2E)‐3‐(4‐tert‐buthylphenyl)‐2‐methylprop‐2‐enylidene] malononitrile (DCTB) as matrix are also described. LDI experiments provided more information about the rhenium complex structures than did the MALDI ones. Copyright © 2012 John Wiley & Sons, Ltd.     

The Synthesis and STM/AFM Imaging of ‘Olympicene’ Benzo[cd]pyrenes

H‐Benzo[cd]pyrene (‘Olympicene′) is a polyaromatic hydrocarbon and non‐Kekulé fragment of graphene. A new synthetic method has been developed for the formation of 6H‐benzo[cd]pyrene and related ketones including the first time isolation of the unstable alcohol 6H‐benzo[cd]pyren‐6‐ol. Molecular imaging of the reaction products with scanning tunnelling microscopy (STM) and non‐contact atomic force microscopy (NC‐AFM) characterised the 6H‐benzo[cd]pyrene as well as the previously intangible and significantly less stable 5H‐benzo[cd]pyrene, the fully conjugated benzo[cd]pyrenyl radical and the ketones as oxidation products.     

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.