The even-electron rule in electrospray mass spectra of pesticides

A study of the fragmentation and ion formation of three major families of pesticides (including herbicides, insecticides, and fungicides) by liquid chromatography/time-of-flight mass spectrometry (LC/TOF-MS) and liquid chromatography/quadrupole time-of-flight mass spectrometry (LC/Q-TOF-MS) was carried out using positive electrospray ionization (ESI) and the results compared with those by gas chromatography (GC)/TOF-MS with electron ionization (EI) in order to test the validity of the even-electron rule in electrospray ionization. First, the majority of the fragmentations by positive ion ESI were even electron (EE) ions (93% of the fragment ions). Secondly, the formation of odd-electron (OE) fragment ions was greater with EI, where the fragment ions were present in a ratio of approximately 1:2 (35% OE ions and 65% EE ions). Thirdly, in-source collision-induced dissociation (CID) fragmentation by LC/TOF-MS and CID fragmentation in the collision cell by LC/Q-TOF-MS/MS resulted in 95% of the fragment ions being identical between the two types of fragmentation. As ESI in the positive ion mode yields an EE precursor ion (normally a protonated molecule), this commonly leads to EE fragment ions by elimination of molecules - a process called the even-electron rule. Neutral radical losses were less common in ESI but were common in the EI spectra of the same compounds. The structures that did lead to OE ions in ESI (exceptions to the even-electron rule approximately 7% of all ESI ions) favored electronegative radical losses in approximately the following order: .SO(2)CH(3), .NO(2), .CH(3), .Cl, .SCH(3), .CH(2)CH, and .OH.     

Doubly charged porphyrin ion tandem mass spectrometry: Implications for structure elucidation

Under electron ionization (EI) conditions, porphyrins yield unusually high intensities of doubly charged molecular and fragment ions. These doubly charged ions offer unique opportunities for the structure elucidation of porphyrins by tandem mass spectrometry (MS/MS). First, they fragment to a greater extent than the corresponding singly charged ions under both EI/MS and EI/MS/MS conditions. Second, doubly and singly charged porphyrin ions often fragment via different pathways, and can therefore yield different structural information. This paper describes several ways in which analyses of doubly charged porphyrin ions with a triple quadrupole tandem mass spectrometer can be useful in structure elucidation of porphyrins. The effect of the metal atom on the fragmentation of metalloporphyrins in an EI source is demonstrated by correlating the extent of doubly charged fragment ion formation to a stability index. Doubly charged porphyrin ions are shown to yield predominantly doubly charged daughter ions upon collisionally activated dissociation (CAD), and are also shown to fragment to a greater extent than corresponding singly charged porphyrin ions. Advantages and disadvantages of doubly charged porphyrin ion MS/MS for structure elucidation are discussed.     

A comparative study of the fragmentation of neutral lactooligosaccharides in negative-ion mode by UV-MALDI-TOF and UV-MALDI ion-trap/TOF mass spectrometry

Structure analyses of underivatized neutral lacto oligosaccharides are systematically performed by ultraviolet matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (UV-MALDI TOF MS) and UV-MALDI ion-trap time-of-flight mass spectrometry (ion-trap/TOF MS) acquired in negative-ion mode. Interestingly, their fragmentation significantly differ each other. In postsource decay (PSD) in UV-MALDI TOF MS, cross-ring cleavage at the reducing terminal predominates. On the other hand, glycosyl bond cleavage (C-type fragmentation) takes place preferentially in collision induced dissociation (CID) in UV-MALDI ion-trap/TOF MS. The cross-ring cleavage in PSD similar to that in in-source decay occurs via a prompt reaction path characteristic of the UV-MALDI process itself. The product ion spectra of UV-MALDI ion-trap/TOF MS are similar to the electrospray ionization (ESI) ion-trap or quadrupole/TOF CID product ion spectra. During ion-trap/TOF MS experiments, the deprotonated molecular ions survive for several tens of milliseconds after CID event because the high internal energy chlorinated precursor ions are cooled by collisional cooling in the ion trap. The results obtained suggest that the PSD from the chlorinated precursor ion in UV-MALDI TOF MS might proceed as a two-step reaction; in the first, a high internal energy deprotonated molecular ion is generated as a reaction intermediate during the flight in the drift tube, and in the second, the rapid decomposition from the deprotonated molecular ion takes place.     

Comparison of in‐source collision‐induced dissociation and beam‐type collision‐induced dissociation of emerging synthetic drugs using a high‐resolution quadrupole time‐of‐flight mass spectrometer

In‐source collision‐induced dissociation (CID) is commonly used with single‐stage high‐resolution mass spectrometers to gather both a molecular formula and structural information through the collisional activation of analytes with residual background gas in the source region of the mass spectrometer. However, unlike tandem mass spectrometry, in‐source CID does not involve an isolation step prior to collisional activation leading to a product ion spectrum composed of fragment ions from any analyte present during the activation event. This work provides the first comparison of in‐source CID and beam‐type CID spectra of emerging synthetic drugs on the same instrument to understand the fragmentation differences between the two techniques and to contribute to the scientific foundations of in‐source CID. Electrospray ionization–quadrupole time‐of‐flight (ESI‐Q‐TOF) mass spectrometry was used to generate product ion spectra from in‐source CID and beam‐type CID for a series of well‐characterized fentanyl analogs and synthetic cathinones. A comparison between the fragmentation patterns and relative ion abundances for each technique was performed over a range of fragmentor offset voltages for in‐source CID and a range of collision energies for beam‐type CID. The results indicate that large fragmentor potentials for in‐source CID tend to favor higher energy fragmentation pathways that result in both kinetically favored pathways and consecutive neutral losses, both of which produce more abundant lower mass product ions relative to beam‐type CID. Although conditions can be found in which in‐source CID and beam‐type CID provide similar overall spectra, the in‐source CID spectra tend to contain elevated noise and additional chemical background peaks relative to beam‐type CID.     

Reaction of analyte ions with neutral chemical ionization gas

Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Ion-molecule reactions of neutral methane with analyte ions under normal methane chemical ionization conditions are discussed. Reactant ions can be generated by direct electron ionization (EI) fragmentation, chemical ionization (CI) fragmentation, or collision-induced dissociation (CID). Examples in which products of such reactions appear in mass spectra in both conventional CI sources in “beam” instruments and low pressure CI in a quadrupole ion trap are presented. Also shown is an example in which MS/MS product ions react with neutral methane used for CI in an ion trap. It is shown that it is relatively straightforward to recognize such reactions in a quadrupole ion trap and in certain cases to minimize or preclude them. Effects of various operating parameters have been investigated and are discussed.     

Mass-spectrometric studies of new 6-nitroquipazines—serotonin transporter inhibitors

Six synthesized 6-nitroquipazine derivatives were examined by electron ionization (EI) and electrospray ionization (ESI) mass spectrometry in positive and negative ion mode. The compounds exhibit high affinity for the serotonin transporter (SERT) and belong to a new class of SERT inhibitors. The EI mass spectra registered in negative ion mode showed prominent molecular ions for all the compounds studied. All EI mass spectra and all ESI mass spectra showed similar fragmentation pathways of molecular ions, but the pathways differed between EI and ESI. The differences were explained with the aid of theoretical evaluation of the stability of the respective radical ions (EI MS) and protonated ions (ESI MS).     

Do the aminopyridine molecular ions display aniline- or pyridine-type behaviour?

By means of collisionally induced dissociative ionization mass spectrometry it was shown that the three isomeric aminopyridine molecular ions display ion chemistry similar to aniline molecular ions; i.e. metastable HNC loss, rather than losing the more stable neutral species HCN, behaviour typical of ionized pyridine. The molecular ions do not isomerize to a common intermediate prior to fragmentation and can readily be distinguished from their collisional activation and charge-stripping mass spectra.     

A comparison of the electron ionization and electrospray behaviour of some N,N'-disubstituted hexahydropyrimidines

The behaviour of some N,N'-disubstituted hexahydropyrimidines, a class of naturally occurring compounds of biological and biomedical interest, has been studied in both electron ionization (EI) and electrospray ionization (ESI) modes coupled with collisional experiments (ESI-MSn). In both techniques, the [M-H]+ ions are highly abundant, even if their formation is generated by two different mechanisms, i.e. H. loss from the M+. species in the case of EI and hydride (H-) abstraction from the molecules in the case of ESI. Furthermore, due to the low, step-by-step internal energy deposition typical of collisional experiments performed in an ion trap mass spectrometer, different fragment ions were observed in EI and ESI-MSn collisions. In both cases, the ions can be related to the original structure and allow us to identify the positions in which the different substituents are present.     

Elucidation of urinary metabolites of fluoxymesterone by liquid chromatography-tandem mass spectrometry and gas chromatography-mass spectrometry

The suitability of liquid chromatography tandem mass spectrometry (LC-MS/MS) and gas chromatography mass spectrometry (GC-MS) for the elucidation of fluoxymesterone metabolism has been evaluated. Electrospray ionization (ESI) and collision induced dissociation (CID) fragmentation in LC-MS/MS and electron impact spectra (EI) in GC-MS have been studied for fluoxymesterone and two commercially available metabolites. MS(n) experiments and accurate mass measurements performed by an ion-trap analyser and a QTOF instrument respectively have been used for the elucidation of the fragmentation pathway. The neutral loss scan of 20 Da (loss of HF) in LC-MS/MS has been applied for the selective detection of fluoxymesterone metabolites. In a positive fluoxymesterone doping control sample, 9 different analytes have been detected including the parent compound. Seven of these metabolites were also confirmed by GC-MS including 5 previously unreported metabolites. On the basis of the ionization, the CID fragmentation, the accurate mass of the product ions and the EI spectra of these analytes, a tentative elucidation as well as a proposal for the metabolic pathway of fluoxymesterone has been suggested. The presence of these compounds has also been confirmed by the analysis of five other positive fluoxymesterone urine samples.     

Study of the fragmentation pattern of ketamine‐heptafluorobutyramide by gas chromatography/electron ionization mass spectrometry

Ketamine is an anaesthetic compound used in human and veterinary medicine with hallucinogen properties that have resulted in its increased illicit use by teenagers at rave parties. Although several gas chromatography/mass spectrometry (GC/MS) methods have been reported for the quantification of the drug both in urine and in hair, its electron ionization (EI) fragmentation after derivatization with different reagents has been not yet fully investigated. The present work reports the study of the fragmentation of ketamine, derivatized with heptafluorobutyric anhydride (HFBA‐Ket), using gas chromatography/electron ionization mass spectrometry (GC/EI‐MS). The complete characterization of the fragmentation pattern represented an intriguing exercise and required tandem mass spectrometry (MSⁿ) experiments, high‐resolution accurate mass measurements and the use of deuterated d⁴‐ketamine to corroborate the proposed structures and to characterize the fragment ions carrying the unchanged aromatic moiety. Extensive fragmentation was observed, mainly located at the cyclohexanone ring followed by rearrangement of the fragment ions, as confirmed by the mass spectra obtained from the deuterated molecule. The GC/EI‐MS analysis of HFBA‐Ket will represent a useful tool in forensic science since high‐throughput analyses are enabled, preserving both the GC stationary phase and the cleanliness of the mass spectrometer ion optics. Copyright © 2009 John Wiley & Sons, Ltd.     

Mass spectra of new substituted 2-amino-4H-pyrans: a retro-Diels-Alder reaction pattern

New substituted 2-amino-3-cyano-4H-pyrans have been studied by electron ionization (EI), chemical ionization (CI) and electrospray ionization (ESI) mass spectrometry. The retro-Diels-Alder reaction (RDA) is the main fragmentation pattern observed in the EI spectra forming an unsaturated ketone as the diene fragment. In contrast, a different RDA reaction takes place yielding an unsaturated amide as diene fragment together with the unsaturated ketone in the CI spectra. The MS/MS spectra obtained using an ESI source reveal that the favoured fragmentation by collision induced dissociation (CID) is the elimination of the substituent at the C4 position with formation of a stable pyrilium cation.     

Mass spectral study of meso-alkyl and meso-cycloalkyl calix(4)pyrroles under electron impact conditions

A series of meso-cycloalkyl calix(4)pyrroles (I) and meso-dialkyl calix(4)pyrroles (II) has been studied under electron ionization (EI) mass spectral conditions. All the calix(4)pyrroles showed prominent molecular ions. The cleavage of the C--C bond linked at position 2 of the pyrrole ring (beta-cleavage) is the foremost and dominant fragmentation process. The beta-cleavage process, either through ring opening or directly, results in the loss of an alkyl radical from the molecular ion. The collision-induced dissociation (CID) spectra of I showed specific sequential expulsion of pyrrole and/or cycloalkyl rings from the molecular ion with or without hydrogen migrations, revealing more information on the structure of individual compounds than was available from the EI spectra. The isomeric cycloalkyl calix(4)pyrroles showed distinguishable CID spectra, indicating structure specificity in initial ring opening whereas, in the case of II, the EI mass spectrum contains all the structure-indicative fragment ions. The CID spectra of II resulted in a dominant [M-R]+ ion, with other characteristic ions being less abundant.     

Mass spectral studies of N,N-dialkylaminoethanols

Some dialkylaminoethanols, precursors of chemical warfare agents such as V-agents and nitrogen mustards, were analyzed by electron impact (EI) and electrospray ionization (ESI) mass spectrometry. The fragmentation pathways in EI and ESI-MS/MS methods are rationalized. The collision-induced dissociation (CID) spectra of [M+H](+) ions of aminoethanols in ESI mode are clearly distinguishable from one another, including those of isomeric normal and branched chain dialkylaminoethanols. Structures can be proposed based on the general fragmentation pathways of these molecules.     

Structural analysis of 2,6-[bis(alkyloxy)methyl]-phenyltin derivatives using electrospray ionization mass spectrometry

Electrospray ionization (ESI) mass spectrometry was applied to the structural analysis of 23 2,6-[bis(alkyloxy)methyl]phenyltin(IV) derivatives. The mass spectra were measured in both polarity modes and multistage tandem mass spectrometric (MS(n)) measurements were performed on the ion trap analyser for positively charged tin-containing ions. The sum of complementary ions observed in the positive-ion mode (i.e. [M-R(3)](+) ion) and in the negative-ion mode (i.e. [R(3)](-) ion) permits molecular mass determination in spite of the fact that the molecular adducts were often missing even in the first-order mass spectra. The subsequent fragmentation of [M-R(3)](+) ions studied by MS(n) and the correlation of observed fragment ions with the expected structures of synthesized organotin(IV) compounds allowed us to understand the fragmentation behaviour and the mechanism of the ion formation for studied compounds. The typical neutral losses are alkenes, alcohols and aldehydes. The fragmentation pattern of one selected compound was supported by MS(n) measurements of an isotopically labelled analogue to confirm unusual ion-molecule reactions of some fragment ions with water in the ion trap.     

红霉素类抗生素的自身化学电离质谱研究

采用自身化学电离离子化(SCI)/四极质谱法,测定了5个红霉素类抗生素,不仅得到了丰度较高的准分子离子(MH)^+,而且也得到了丰富的碎片离子,并对其裂解规律进行了总结。结果表明对于在电离过程中不稳定的化合物,SCI/四极质谱法既可确定其分子量,也可提供丰富的分子结构信息。此方法简便、快速,且图谱简单有助于结构分析。     

Structural elucidation of disinfection by-products in treated drinking water

Two unusual disinfection by-products have been detected periodically in the gas chromatography/mass spectrometry (GC/MS) characterization analyses of semi-volatile organics in treated drinking water. The electron ionization (EI) mass spectra contained mono-chlorinated and mono-brominated ions at m/z 107/109 and 151/153, respectively. Library searching techniques suggested mono-halogenated butanol structures but no matches could be found. Positive ion chemical ionization (CI) spectra contained mono-chlorinated and mono-brominated ions at m/z 105/107 and 149/151, respectively. No [M + H]+ ions were initially observed. Accurate mass measurements confirmed the empirical formulae for the significant ions in the EI spectra and the mono-halogenated butanol structures. Further CI experiments with other reagent gases and instruments revealed possible molecular weights of 139 and 183 Da, respectively. Tandem mass spectrometry (MS/MS) experiments in EI and CI were used to elucidate the fragmentation schemes. The two compounds have been tentatively identified as 1-aminoxy-1-chlorobutan-2-ol and 1-aminoxy-1-bromobutan-2-ol, respectively.     

Compatibility of electron ionization and soft ionization methods in gas chromatography/orthogonal time‐of‐flight mass spectrometry

Orthogonal acceleration time‐of‐flight (oa‐TOF) mass spectrometry (MS) was coupled to gas chromatography (GC) to measure ion yields (ratio of ion counts to number of neutrals entering the ion source) and signal‐to‐noise (S/N) in the electron ionization (EI) mode (hard ionization) as well as in the soft ionization modes of chemical ionization (CI), electron capture negative ion chemical ionization (NICI) and field ionization (FI). Mass accuracies of the EI and FI modes were also investigated. Sixteen structurally diverse volatile organic compounds were chosen for this study. The oa‐TOF mass analyzer is highly suited for FI MS and provided an opportunity to compare the sensitivity of this ionization method to the more conventional ionization methods. Compared to the widely used quadrupole mass filter, the oa‐TOF platform offers significantly greater mass accuracy and therefore the possibility of determining the empirical formula of analytes. The findings of this study showed that, for the instrument used, EI generated the most ions with the exception of compounds able to form negative ions readily. Lower ion yields in the FI mode were generally observed but the chromatograms displayed greater S/N and in many cases gave spectra dominated by a molecular ion. Ion counts in CI are limited by the very small apertures required to maintain sufficiently high pressures in the ionization chamber. Mass accuracy for molecular and fragment ions was attainable at close to manufacturer's specifications, thus providing useful information on molecular ions and neutral losses. The data presented also suggests a potentially useful instrumental combination would result if EI and FI spectra could be collected simultaneously or in alternate scans during GC/MS. Copyright © 2009 John Wiley & Sons, Ltd.     

Formation of 2-substituted benzofuran fragment ions from 6-alkyl- and 6-aryldibenzo(d,f)(1,3)dioxepine derivatives under electron ionization-a useful precursor ion for isomeric differentiation

Tandem mass spectrometry has been applied to differentiate three sets of o-, m- and p-methyl, -methoxy and -nitro-substituted-6-phenyl-dibenzo(d,f)(1,3)dioxepines. Collision-induced dissociation (CID) experiments have been carried out on 2-phenylbenzo[b]furan fragment ions, which originate from the decomposition of the molecular ions after their EI-induced isomerization to spirocyclic structures. With the exception of m- and p-methylphenylbenzo[b]furan isomers, which display identical CID mass spectra, the three isomeric methoxy- and nitrophenylbenzo[b]furan fragment ions display very characteristic CID behavior which allows unequivocal differentiation of the 6-phenyl-dibenzo(d,f)(1,3)dioxepine isomers. 6-(o-nitrophenyl)-dibenzo(d,f)(1,3)dioxepine isomer, does not form a 2-(o-nitrophenyl)benzo[b]furan ion and, therefore, it can be differentiated from the m- and p- isomers based on the mere EI mass spectra. Furthermore, it shows a characteristic ion most likely due to an ortho effect between the nitro group and the dioxepine ring. Multiple stage mass spectrometric techniques (MSn), labeled derivatives and reference compounds were used in order to gain additional information on the structures of product ion from the CID fragmentation.     

一些双电荷离子的气相碎裂反应

借助质量分析离子动能谱和串联质谱研究了由电子轰击产生的双电荷离子的单分子亚稳碎裂及碰撞诱导分解过程,讨论了两种实验方法导致的差别因素.此外,根据质量分析离子动能谱提供的双电荷离子电荷分离反应的动能释放值计算了两电荷中心间距的最小值,以判别按不同电荷分离方式碎裂的双电荷离子的过渡态结构.     

两种氨基酸中[MH-CO2H2]+的特征质谱碎裂

运用低能碰撞诱导解离（ＣＩＤ）研究了电子轰击（ＥＩ）、快原子轰击（ＦＡＢ）电离条件下质子化亮氨酸与异亮氨酸解离产生亚稳离子「ＭＨ－ＣＯ２Ｈ２」的单分子质谱破裂，二种异构体呈现出了各自不同的解离特征，根据ＣＩＤ的特征碎片离子和氘代同位素标记实验，提出了其破裂过程的存在离子／中性（碎片）复合物中间体机理，并对有关的特征离子的形成进行了讨论。