Mutual storage mode ion/ion reactions in a hybrid linear ion trap

Ion/ion proton transfer reactions involving mutual storage of both ion polarities in a linear ion trap (LIT) that comprises part of a hybrid triple quadrupole/linear ion trap mass spectrometer have been effected. Mutual ion storage in the x- and y-dimensions arises from the normal operation of the oscillating quadrupole field of the quadrupole array, while storage in the z-dimension is enabled by applying unbalanced radio-frequency amplitudes to opposing sets of rods of the array. Efficient trapping (>90%) is achieved for thermalized ions over periods of several seconds. Reactions were demonstrated for multiply charged protein/peptide cations formed by electrospray with anions derived from glow discharge ionization of perfluoro(methyldecalin) (PMD) introduced from the side of the LIT rod array. Doubly and singly charged protein ions are readily formed via ion/ion reactions. The parameters that affect ion/ion reactions are discussed, including the degree of RF unbalance on the LIT rods, vacuum pressure, nature of the buffer gas, reaction time, anion abundance, and the low mass cutoff for ion/ion reaction. The present system has a demonstrated upper mass-to-charge ratio limit of at least 33,000. The system also has high flexibility with respect to defining MS(n) experiments involving both collision-induced dissociation (CID) and ion/ion reactions. Experiments are demonstrated involving beam-type CID in the pressurized collision quadrupole (Q2) followed by ion/ion reactions involving the product ions in the LIT. Ion parking experiments are also demonstrated using the mutual storage ion/ion reaction mode in the LIT, with a parking efficiency over 60%.     

Transmission mode ion/ion proton transfer reactions in a linear ion trap

A new method is described for effecting ion/ion proton transfer reactions that involves storage of analyte ions while oppositely charged ions are transmitted through the stored ion population. In this approach, the products are captured and stored in the linear ion trap for subsequent mass analysis. Charge reduction of multiply charged protein ions is used as an example to illustrate the analytical usefulness of this method. In another variation of the transmission mode ion/ion reaction approach, two charge inversion experiments, implemented by passing analyte ions through a population of multiply charged reagent ions in a LIT, are also demonstrated. A pulsed dual ion source approach coupled with a hybrid triple quadrupole/linear ion trap instrument was used to demonstrate these two methods. The results for ion/ion reactions implemented using these so-called "transmission mode" experiments were comparable to those acquired using the more conventional mutual storage mode, both in terms of efficiency and information content of the spectra. An advantage of transmission mode experiments compared with mutual storage mode experiments is that they do not require any specialized measures to be taken to enable the simultaneous storage of oppositely charged ions.     

Trapping mode dipolar DC collisional activation in the RF‐only ion guide of a linear ion trap/time‐of‐flight instrument for gaseous bio‐ion declustering

The application of dipolar direct current (DDC) to the radio frequency‐only ion guide (Q0) of a hybrid quadrupole/time‐of‐flight mass spectrometer for collision‐induced declustering of large bio‐ions is described. As a broadband technique, ion trap DDC collisional activation (CA) is employed to decluster ions simultaneously over a relatively broad mass‐to‐charge (m/z) range. Declustering DDC CA can yield significantly narrower peaks relative to those observed in the absence of declustering methods, depending upon the extent of noncovalent adduction associated with the ions, and can also be used in conjunction with other methods, such as nozzle–skimmer CA. The key experimental variables in the DDC experiment are the DDC voltage (V_DDC), V_RF, and the time over which V_DDC is applied. The V_DDC/V_RF ratio is key to the extent to which ion temperatures are elevated and also influences the upper m/z limit for ion storage. The V_DDC/V_RF ratio affects ion temperatures and the upper m/z limit in opposing directions. That is, as the ratio increases, the ion temperature also increases, whereas the upper m/z storage limit decreases. However, for a given V_DDC/V_RF ratio, the upper m/z storage limit can be increased by increasing V_RF, at the expense of the lower m/z limit for ion storage. The key value of the approach is that it affords a relatively precise degree of control over ion temperatures as well as the time over which they are elevated to a higher temperature. The utility of the method is illustrated by the application of ion trap DDC CA in Q0 to oligonucleotide, protein, and multimeric protein complex analyte ions. Copyright © 2013 John Wiley & Sons, Ltd.     

Selective self-ion/molecule reactions in both external and internal source ion trap mass spectrometers

Novel results on the selective self-ion/molecule reactions (SSIMR) in both external and internal source ion trap mass spectrometers are demonstrated. Selective self-ion/molecule reaction product ions were produced between the oxygenated and nitrogenated crown ethers. For the oxygenated crown ethers, self-ion/molecule reactions lead to the formation of the protonated ions, adduct ions of fragments ([M + F](+)) and [M + H(3)O](+), while the nitrogenated crown ethers produce [M + H](+), [M + CH](+) and [M + C(2)H(3)](+) ions.     

Ion/molecule reactions for detecting ammonia using miniature cylindrical ion trap mass spectrometers

Gaseous ammonia, a common toxic industrial compound, is not detected readily in ion trap mass spectrometers because its molecular ion falls below the low-mass cutoff (~m/z 40) normally used when examining organic compounds. Instead, reactions of ammonia with halobenzene radical cations were used with internal electron ionization in two cylindrical ion trap miniature mass spectrometers to create a characteristic product ion by which to identify and quantify ammonia. Ammonia showed a linear response over the concentration range studied (parts per million [ppm] to parts per billion [ppb]) with limits of detection of 17 ppm and 220 ppb for experiments involving direct introduction and thermal desorption after pre-concentration, respectively. These values are comparable to ammonia's permissible exposure limit (50 ppm) and odor threshold (5 ppm). Receiver operating characteristic (ROC) curves were used to describe the method sensitivity, the probability of true positives, and the false positive rate for ammonia. A customized reaction scan function was created to select the species available for the ion/molecule reaction and set the amount of time the product ion could be accumulated in the trap. Product ion identity was verified using tandem mass spectrometry. Similar reactions with methylamine, ethylamine and the two nitriles, acetonitrile and benzonitrile, were explored.     

Identification of the aromatic tertiary N-oxide functionality in protonated analytes via ion/molecule reactions in mass spectrometers

A mass spectrometric method is presented for the rapid identification of compounds that contain the aromatic N-oxide functional group. This method utilizes a gas-phase ion/molecule reaction with 2-methoxypropene that yields a stable adduct for protonated aromatic tertiary N-oxides (and with one protonated nitrone) in different mass spectrometers. A variety of protonated analytes with O- or N-containing functional groups were examined to probe the selectivity of the reaction. Besides protonated aromatic tertiary N-oxides and one nitrone, only three protonated amines were found to form a stable adduct but very slowly. All the other protonated analytes, including aliphatic tertiary N-oxides, primary N-oxides, and secondary N-oxides, are unreactive toward or react predominantly by proton transfer with 2-methoxypropene.     

Characterization of beam-induced reactions occurring in liquid secondary ion mass spectrometry/fast-atom bombardment by tandem mass spectrometry

Department of General and Physical Chemistry, University of Liege, Liege, Belgium A specific beam-induced secondary reaction involving the condensation of hydroxylic matrices with some organic groups (aldehydes, ketones, etc.) accompanied by the loss of a water molecule was investigated by liquid secondary ion mass spectrometry/fast-atom bombardment (LSIMS/FAB). A mechanistic scheme and a structure of the induced product are proposed. The features of this secondary reaction were studied and the influence of the types of solutes, acidic additives, and matrices analyzed. Rather than a drawback, LSIMS/FAB mass spectrometry can take advantage of this matrix effect to infer analytical information through tandem mass spectrometry experiments. Specific neutral loss scans can be conducted to highlight beam-induced reactive molecules, even when the detection of these species is prevented in normal scan spectra by other surface-active components.     

On the high‐resolution mass analysis of the product ions in tandem time‐of‐flight (TOF/TOF) mass spectrometers using a time‐dependent re‐acceleration technique

The time‐dependent reacceleration of product ions produced as a result of dissociation of a single precursor ion in a tandem time‐of‐flight mass spectrometer is considered for the first time. Analytical expressions for the shapes of electric pulses bringing all the kinetic energies of the product ions to the same value are derived for two cases: forward acceleration mode and deceleration, followed by re‐acceleration in the reversed direction (reversed mode). Secondary time‐of‐flight focusing resulting from the re‐acceleration in the reversed mode is shown to be mass‐dependent and, when averaged over a wide mass range, the focusing is tight enough to provide mass resolution exceeding 10 000. After time‐dependent re‐acceleration, additional compression of the ion packet width leading to better mass resolution can be obtained by decelerating the ions in a constant field. Copyright © 2009 John Wiley & Sons, Ltd.     

Negative ion electrospray high‐resolution tandem mass spectrometry of polyphenols

Representative compounds with a 1,3‐dihydroxybenzene substructure belonging to different important polyphenol classes (stilbenes, flavones, isoflavones, flavonols, flavanones, flavanols, phloroglucinols, anthraquinones and bisanthraquinones) were investigated based on detailed high‐resolution tandem mass spectrometry measurements with an Orbitrap system under negative ion electrospray conditions. The mass spectral behaviour of these compound classes was compared among each other not only with respect to previously described losses of CO, CH₂CO and C₃O₂ but also concerning the loss of CO₂ and successive specific fragmentations. Furthermore, some unusual fragmentations such as the loss of a methyl radical during mass spectral decomposition are discussed. The obtained results demonstrate both similarities and differences in their mass spectral fragmentation under MSⁿ conditions, allowing a characterization of the corresponding compound type. Copyright © 2015 John Wiley & Sons, Ltd.     

Ion/molecule reactions investigated by using tandem mass spectrometry in an ion trap: halomethylation

The ion-trap mass spectrometer has several features which make it a useful device for the study of ion/molecule reactions, viz., the ability to store ions for long periods, mass-selective storage, access to time and pressure-resolved data, and MS/MS capabilities in which the fragmentation behavior of selected ions may give insight into ion structure. These capabilities are used to study the gas-phase halomethylation of a variety of organic compounds with CH₂Cl⁺ as the reagent ion. The ion/molecule reaction of greatest interest involves addition of CH₂Cl⁺, followed by the elimination of HCl, resulting in a net addition of methyne. This methyne-addition reaction is observed in many aromatic compounds as well as such compounds as cycloheptatriene and cyclo-octene. The structures of the product ions were probed using collision-activated dissociation.     

Travelling‐wave ion mobility mass spectrometry and negative ion fragmentation of hybrid and complex N‐glycans

Nitrogen collisional cross sections (CCSs) of hybrid and complex glycans released from the glycoproteins IgG, gp120 (from human immunodeficiency virus), ovalbumin, α1‐acid glycoprotein and thyroglobulin were measured with a travelling‐wave ion mobility mass spectrometer using dextran as the calibrant. The utility of this instrument for isomer separation was also investigated. Some isomers, such as Man₃GlcNAc₃ from chicken ovalbumin and Man₃GlcNAc₃Fuc₁ from thyroglobulin could be partially resolved and identified by their negative ion fragmentation spectra obtained by collision‐induced decomposition (CID). Several other larger glycans, however, although existing as isomers, produced only asymmetric rather than separated arrival time distributions (ATDs). Nevertheless, in these cases, isomers could often be detected by plotting extracted fragment ATDs of diagnostic fragment ions from the negative ion CID spectra obtained in the transfer cell of the Waters Synapt mass spectrometer. Coincidence in the drift times of all fragment ions with an asymmetric ATD profile in this work, and in the related earlier paper on high‐mannose glycans, usually suggested that separations were because of conformers or anomers, whereas symmetrical ATDs of fragments showing differences in drift times indicated isomer separation. Although some significant differences in CCSs were found for the smaller isomeric glycans, the differences found for the larger compounds were usually too small to be analytically useful. Possible correlations between CCSs and structural types were also investigated, and it was found that complex glycans tended to have slightly smaller CCSs than high‐mannose glycans of comparable molecular weight. In addition, biantennary glycans containing a core fucose and/or a bisecting GlcNAc residue fell on different mobility‐m/z trend lines to those glycans not so substituted with both of these substituents contributing to larger CCSs. Copyright © 2016 John Wiley & Sons, Ltd.     

Rapid detection of cyanobacterial toxins in precursor ion mode by liquid chromatography tandem mass spectrometry

We established an analytical method based on liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) in the precursor ion mode for simultaneous qualitative monitoring of various groups of cyanobacterial toxins. The toxin groups investigated were paralytic shellfish poisoning (PSP) toxins, anatoxins (ANAs), cylindrospermopsins (CYNs), microcystins (MCs), and nodularins (NODs), including rare and uncharacterized derivatives found in plankton and water matrices. Alternative analytical methods based on tandem mass spectrometry commonly operate in multiple reaction monitoring (MRM) mode and depend on prior knowledge of putative toxigenicity of the cyanobacterium species and strain, and the expected toxin variants. In contrast, the precursor ion mode yields diagnostic mass fragments for the detection of characteristic compounds of the different toxin classes and thus allows monitoring of a large set of unspecified cyanotoxins of various groups, even when the species composition is undetermined or uncertain. This rapid method enables screening for a wide spectrum of toxic cyanobacterial metabolites and degradation products in a single chromatographic separation with detection limits at nanogram levels. The precursor ion technique is a valuable adjunct to existing mass spectrometric methods for cyanotoxins, although it is not a complete replacement for detailed quantitative analysis requiring comprehensive sample cleanup.     

Gas‐phase fragmentation reactions of protonated benzofuran‐ and dihydrobenzofuran‐type neolignans investigated by accurate‐mass electrospray ionization tandem mass spectrometry

We have investigated gas‐phase fragmentation reactions of protonated benzofuran neolignans (BNs) and dihydrobenzofuran neolignans (DBNs) by accurate‐mass electrospray ionization tandem and multiple‐stage (MSⁿ) mass spectrometry combined with thermochemical data estimated by Computational Chemistry. Most of the protonated compounds fragment into product ions B ([M + H–MeOH]⁺), C ([ B –MeOH]⁺), D ([ C –CO]⁺), and E ([ D –CO]⁺) upon collision‐induced dissociation (CID). However, we identified a series of diagnostic ions and associated them with specific structural features. In the case of compounds displaying an acetoxy group at C‐4, product ion C produces diagnostic ions K ([ C –C₂H₂O]⁺), L ([ K –CO]⁺), and P ([ L –CO]⁺). Formation of product ions H ([ D –H₂O]⁺) and M ([ H –CO]⁺) is associated with the hydroxyl group at C‐3 and C‐3′, whereas product ions N ([ D –MeOH]⁺) and O ([ N –MeOH]⁺) indicate a methoxyl group at the same positions. Finally, product ions F ([ A –C₂H₂O]⁺), Q ([ A –C₃H₆O₂]⁺), I ([ A –C₆H₆O]⁺), and J ([ I –MeOH]⁺) for DBNs and product ion G ([ B –C₂H₂O]⁺) for BNs diagnose a saturated bond between C‐7′ and C‐8′. We used these structure‐fragmentation relationships in combination with deuterium exchange experiments, MSⁿ data, and Computational Chemistry to elucidate the gas‐phase fragmentation pathways of these compounds. These results could help to elucidate DBN and BN metabolites in in vivo and in vitro studies on the basis of electrospray ionization ESI‐CID‐MS/MS data only.     

Simple extraction method using syringe filter for detection of ethephon in tomatoes by negative‐ion mode liquid chromatography with tandem mass spectrometry

In this study, a simple, rapid, and sensitive method was developed for the extraction of ethephon from homogenized tomatoes that does not require a cleanup procedure. In a syringe filter, three distinct layers – aqueous, acetonitrile, and n‐hexane – are clearly separated after storage at ?80 °C for 5–10 min. A Dionex IonPac column was used to separate the analyte before detection using negative‐ion mode liquid chromatography with tandem mass spectrometry (LC/MS/MS). The matrix effect of the tested analyte was negligibly small and the matched calibration showed a good linearity over a concentration range of 0.01–1.0 mg/kg with a correlation coefficient (R²) of 0.9998. The recovery at three fortification levels (0.1, 0.5 and 1.0 mg/kg) was between 82.9 and 108.6% with relative standard deviations (RSDs) <5.0%. The limit of quantification (0.03 mg/kg) was lower than the maximum residue limit (3 mg/kg) set by the Ministry of Food and Drug Safety, Republic of Korea. From a field trial, the method developed herein was applied to calculate the decline pattern and predict the pre‐harvest residue limits of ethephon in tomatoes. In conclusion, the proposed sample preparation is feasible for the detection of hydrophilic analytes in tomatoes. Copyright © 2015 John Wiley & Sons, Ltd.     

Development of a tandem time‐of‐flight mass spectrometer with an electrospray ionization ion source

A new tandem time‐of‐flight mass spectrometer with an electrospray ionization ion source ‘ESI‐TOF/quadTOF’ was designed and constructed to achieve the desired aim of structural elucidation via high‐energy collision‐induced dissociation (CID), and the simultaneous detection of all fragment ions. The instrument consists of an orthogonal acceleration‐type ESI ion source, a linear TOF mass spectrometer, a collision cell, a quadratic‐field ion mirror and a microchannel plate detector. High‐energy CID spectra of doubly protonated angiotensin II and bradykinin were obtained. Several fragment ions such as a‐, d‐, v‐ and w‐type ions, characteristic of high‐energy CID, were clearly observed in these spectra. These high‐energy CID fragment ions enabled confirmation of the complete sequence, including leucine–isoleucine determinations. It was demonstrated that high‐energy CID of multiply protonated peptides could be achieved in the ESI‐TOF/quadTOF. Copyright © 2010 John Wiley & Sons, Ltd.