RESUMEN

An ion mobility mass spectrometry (IM-MS) investigation using a Synapt G2 mass spectrometer was conducted to separate anions generated from the three regioisomers of sulfobenzoic acid. The results revealed that the differences in arrival time distributions (ATDs) were inadequate to differentiate the isomers unambiguously. However, the ATD profiles of the product ions, generated by fragmenting the respective mass-selected m/z 201 precursor ions in the Trap region of the three-compartment traveling-wave ion guide of the Synapt G2 mass spectrometer, were distinctly different, enabling definitive differentiation of the isomers. An arrival-time peak for an ion of m/z 157 resulting from the loss of CO2 from the respective precursors was common to all three mobilograms. However, only the profile recorded from the para-isomer exhibited a unique arrival-time peak for an ion of m/z 137, originating from an SO2 loss. Such a peak corresponding to an SO2 loss was absent in the ATD profiles of the ortho- and meta-isomers. Additionally, the mobilogram of the meta-isomer displayed a unique peak at 3.42 ms. Based on its product ion spectrum, this peak was attributed to the bisulfite anion (m/z 81; HSO3-). Previously, this meta-isomer specific m/z 81 ion had been proposed to originate from a two-step process involving the intermediacy of an m/z 157 ion formed by CO2 loss. However, our detailed tandem mass spectrometric experiments suggest that the m/z 81 is not a secondary product but rather an ion that originated from a direct elimination of a benzyne derivative from the m/z 201 precursor ion.

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A peculiar phenomenon known as "breakthrough" occurs under reversed-phase ultraperformance liquid chromatography (UPLC) conditions and has been under scrutiny for decades. This effect takes place when a large volume of analyte solution, prepared in a solvent with an eluotropic strength significantly higher than that of the initial mobile phase solvent, is injected. According to the literature, under specific experimental conditions, a substantial portion of solutes is carried by the mobile phase and detected near the dead time of the chromatographic system. This phenomenon is typically observed when the injected volume of a particular analyte is sufficiently large. However, the underlying physicochemical principles governing this phenomenon have remained elusive. We present evidence demonstrating that breakthroughs can occur even when injecting a sample of a neat solvent devoid of any solute. By mass spectrometric analysis, we identified the breakthrough peak to represent the nonionic detergent Triton. When columns are equilibrated with water, Triton molecules, present as impurities in filtered water, accumulate on the nonpolar stationary phase. Upon the introduction of a solvent with a stronger elution strength, Triton molecules retained on the stationary phase are removed. As detergents, these Triton molecules aggregate into micelles featuring a hydrophobic inner core and a hydrophilic outer shell. These hydrophilic micelles are carried by the polar mobile phase and detected as the breakthrough peak at the dead time of the chromatographic system. When analytes are present, a portion of the injected solutes is captured by the micelles and transported with the breakthrough plug. This assertion was verified and confirmed by liquid chromatography-mass spectrometry (LC-MS) analysis of a methanolic solution of perfluorooctanoic acid (PFOA). The mass spectra corresponding to the breakthrough plug featured a peak for the PFOA anion (m/z 413) in addition to those for Triton.

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Molecules with multiple sites capable of accepting protons form ensembles of protomers. The manifested protomer ratios in such ensembles are influenced by many experimental conditions. In a Synapt G2 ion mobility (IM)-enabled mass spectrometry system, there are several physical locations where ion population changes can be manifested. Using APCI-generated protomers of aminonaphthalenes, we investigated its intramolecular proton transfers from the N-protomer to the C-protomer. This lossless transformation of the N-protomer to the thermodynamically favored C-protomer can take place in the ion source itself. Initially, we learned that the cone gas slows down the transformation to the C-protomer. Gaseous ions are then accelerated in the first vacuum region, where ions undergo collisional activation (heating), which facilitates the transformation to the C-protomer. Afterward, the ions are mass selected and transferred to the pre-IM (Trap)-collision cell, where ions can also be transformed to the thermodynamically favored protomers. Trap accumulated ions are then released to the IM separator via a helium-filled entry cell. The role of helium is to minimize ion activation and scattering taking place upon entry to the high-pressure T-Wave IM separator (TWIMS). The helium cell is known to increase the IM peak resolution. However, we found that significant changes occur depending on the presence or absence of helium. Without helium, source-generated protomers rapidly changed to a predominantly thermodynamically favorable ensemble protomers. Apparently, the introduction of helium into the precell induced a dramatic decrease in collisional "heating" effect, which effectively slowed down the conversion rate of the amino-protomer into the more favorable ring-protomer. The final message is that mobilograms should not be considered as direct real-time, or intrinsic, representations of the protomer ratios in the ion source.

RESUMEN

Michler's ketone (MK) is a dimethylamino ketone that undergoes facile protonation under electrospray-ionization conditions to produce an ion of m/z 269. Initial LC-MS results showed that the collision-induced dissociation (CID) spectra of the m/z 269 ion depend heavily on the composition of the chromatographic mobile phase. Subsequent ion-mobility separation of the mass-selected m/z 269 ion revealed that protonated MK exists as two tautomeric forms. Moreover, the relative population of the two protomeric forms in the ion ensemble depends on the nature of the ambient molecules present in the atmospheric pressure ion source. For example, the ion-mobility arrival-time profile acquired from the mass-selected m/z 269 ion generated from an acetonitrile solution showed two peaks of near equal intensity. The peak with the shorter arrival time represented the O-protomer and that with the longer arrival time represented the N-protomer. However, when methanol or ammonia vapors were introduced to the ambient-pressure ion source, the intensity of the N-protomer peak decreased rapidly and that of the O-protomer signal soared until it became the dominant peak. When the introduction of methanol (or ammonia) vapors was stopped, the mobilogram signals gradually reverted back to their initial intensities. To rationalize this observation, we propose that the N-protomer of MK in the presence of methanol vapor undergoes transformation to the O-protomer by a Grotthuss-type mechanism via a methanol-based solvent bridge.

RESUMEN

Thermally desorbed 4-nitroaniline (4-NA), upon atmospheric pressure chemical ionization (APCI), generates gaseous ions for its protonated species. The APCI mass spectrum recorded under mild in-source ion-activating conditions from 4-NA showed a peak at m/z 139, whereas that acquired under high ion-activating conditions showed two additional peaks at m/z 122 (•OH loss) and 92 (•NO loss). The spectrum changed instantaneously when acetonitrile vapor was introduced to the source. In the new spectrum, both m/z 122 and 92 peaks were absent, while a new peak appeared at m/z 93. Ion-mobility separation carried out with the m/z 139 ion revealed that the initial ion represented the thermodynamically favored nitro-protonated tautomer. The ion population changed to an ensemble dominated by the less-favored amino-protomer when acetonitrile vapor was introduced to the ion source. The amino-protomer, upon collisional activation, loses •NO2 to generate an m/z 93 ion, which was confirmed to be the 4-dehydroanilinium ion. Ion mobility provided a practical way to monitor the changes secured by acetonitrile vapor because the two protomers showed different arrival times. Under spray-ionization conditions, the formation of the thermodynamically less favored protomer has been attributed to kinetic trapping. Our study demonstrated that the less favored amino-protomer could be generated by introducing acetonitrile vapor under nonspray conditions. Apparently, under APCI conditions, protonated water vapor attaches to the nitro group to generate a proton-bound heterodimer, which upon activation dissociates to yield the nitro-protomer. In contrast, protonated acetonitrile makes a tighter complex preferentially with the amino group, which upon activation breaks to the amino-protomer.

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Gas-phase addition of dioxygen to certain ions is a well-known phenomenon in mass spectrometry. For this reaction to occur, the presence of a distonic radical site on the precursor ion is thought to be a prerequisite. Herein, we report that oxygen adduct formation can take place also with deprotonated hydroquinone, which in fact is an even-electron species without a radical site. When the product-ion spectrum of the m/z 109 ion, generated by electrospray ionization from a solution of hydroquinone in acetonitrile, was recorded under ion-mobility conditions, a new peak was observed at m/z 141. However, an analogous peak was not visible in the spectrum acquired under nonmobility conditions (i.e., without any gas introduced to the mobility cell). Presumably, traces of oxygen present in the collision gas instigate an ion-molecule reaction to produce an adduct of m/z 141, which upon activation results in CO and H2O loss to form a product ion of m/z 95. Isotope-labeling studies confirmed that one of the hydrogen atoms from the hydroxy group and another from the aromatic ring contribute to the water loss instigated from the m/z 141 adduct. Furthermore, computational methods indicated the three-dimensional structure of the ground-state deprotonated hydroquinone to be distinctly different from those of its 1,2- and 1,3-isomers. Calculations predicted that all atoms in the two m/z 109 ions generated from catechol and resorcinol lie on one plane. In contrast, the structure of the m/z 109 ion from hydroquinone was significantly different. Computations predicted that the hydrogen atom on the intact hydroxyl group of deprotonated hydroquinone protrudes out of plane from rest of the atoms. Consequently, the exposed OH group can interact with an incoming dioxygen molecule. Computations conducted at the CAM-B3LYP/6-311++g(2d,2p) level of theory detected a minimum energy crossing point (MECP) at -4.3 kJ mol-1 below the separated O2 + deprotonated hydroquinone triplet threshold. In contrast, similar calculations conducted for catechol and resorcinol yielded MECPs of +116.9 and +69.1 kJ mol-1, respectively, above the associated triplet thresholds. These results indicated that the curve crossing required to form singlet products upon reaction with triplet O2 is favorable in the case of hydroquinone and unfavorable in the cases of catechol and resorcinol. In practical terms, the selective oxygen addition appears to be a diagnostically useful reaction to differentiate hydroquinone from its ring isomers.

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Deciphering metabolomic networks has been demonstrated to provide valuable information for diagnosing and monitoring diseases. Herein, we report a technique to monitor untargeted urine metabolites to evaluate prostate cancer aggressiveness and treatment outcome. Direct chemical profiling of urine was achieved by a combined procedure of hyphenating laser diode thermal desorption with atmospheric pressure chemical ionization mass spectrometry (LDTD-APCI-MS). We describe a conceptually new approach to monitoring preoperative urinary metabolic alterations associated with prostate cancer recurrence. By evaluating mass/charge (m/z) ratios and peak intensities of ions detected by mass spectroscopy of urine samples, we revealed that intensities at m/z 313.2740 (±0.0003) and 341.3054 (±0.0006) attributable to monoacylglycerol backbone fragments from glycerides can be statistically correlated to disease progression.

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The ortho, meta, and para isomers of hydroxybenzyl alcohol can be unequivocally distinguished by the collision-induced dissociation mass spectra of their anions. The presence of a prominent peak at m/z 121 for an elimination of a dihydrogen molecule renders the ortho-isomer spectrum markedly different from those of its meta and para congeners. Investigations carried out with deuterium-labeled isotopologues of the ortho isomer verified that the labile hydrogen atom on the hydroxyl group and one of the benzylic hydrogen atoms are specifically removed in the formation of the m/z 121 ion. The ortho-isomer spectrum also showed a prominent peak at m/z 93. Experimental data indicated that the m/z 93 product ion originates either from a two-step H2 and CO elimination mechanism or from a direct loss of a HCHO molecule from the precursor anion. The intensity ratio of the m/z 93 and 94 peaks in the spectrum recorded from the m/z 124 ion generated from a sample of o-hydroxybenzyl alcohol dissolved in D2 O supported the notion that the direct HCHO loss is the more dominant pathway for the generation of the phenolate ion under low activation conditions. In contrast, the two-step mechanism becomes the more dominant pathway under high collisional activation conditions. The spectrum also showed a weak peak at m/z 105 for a water loss. Based on computational data, the m/z 105 ion generated in this way appears to be a composite generated from a common ion-neutral complex intermediate in which a hydroxyl anion is positioned equidistantly between one of the benzylic hydrogens and a nearby hydrogen atom of the benzene ring. Upon activation, the complex dissociates to form either a phenide or a quinone methide anion. The reaction forming a carbon dioxide adduct under ion-mobility conditions was used to support the proposed water-loss mechanism.

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The current IUPAC-recommended definition of the term "monoisotopic mass" of a chemical species is based on the most abundant isotopes of the constituent elements. It has even been proposed to constrain the definition to be based only on the atomic masses of the most abundant stable isotopes. Such an approach is flawed because in this way several elements and their compounds, in addition to isotopically enriched species, would not merit to be assigned a monoisotopic mass. Furthermore, for large molecules, such as proteins, the monoisotopic mass as currently defined loses its significance. Therefore, we propose to eliminate using the current definition altogether. Instead, the term isotopologue mass should be applied uniformly to every species denoted by a specific chemical formula.

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Of the approximately one million described insect species, ground beetles (Coleoptera: Carabidae) have long captivated the attention of evolutionary biologists due to the diversity of defensive compounds they synthesize. Produced using defensive glands in the abdomen, ground beetle chemicals represent over 250 compounds including predator-deterring formic acid, which has evolved as a defensive strategy at least three times across Insecta. Despite being a widespread method of defense, formic acid biosynthesis is poorly understood in insects. Previous studies have suggested that the folate cycle of one-carbon (C1) metabolism, a pathway involved in nucleotide biosynthesis, may play a key role in defensive-grade formic acid production in ants. Here, we report on the defensive gland transcriptome of the formic acid-producing ground beetle Harpalus pensylvanicus. The full suite of genes involved in the folate cycle of C1 metabolism are significantly differentially expressed in the defensive glands of H. pensylvanicus when compared to gene expression profiles in the rest of the body. We also find support for two additional pathways potentially involved in the biosynthesis of defensive-grade formic acid, the kynurenine pathway and the methionine salvage cycle. Additionally, we have found an array of differentially expressed genes in the secretory lobes involved in the biosynthesis and transport of cofactors necessary for formic acid biosynthesis, as well as genes presumably involved in the detoxification of secondary metabolites including formic acid. We also provide insight into the evolution of the predominant gene family involved in the folate cycle (MTHFD) and suggest that high expression of folate cycle genes rather than gene duplication and/or neofunctionalization may be more important for defensive-grade formic acid biosynthesis in H. pensylvanicus. This provides the first evidence in Coleoptera and one of a few examples in Insecta of a primary metabolic process being co-opted for defensive chemical biosynthesis. Our results shed light on potential mechanisms of formic acid biosynthesis in the defensive glands of a ground beetle and provide a foundation for further studies into the evolution of formic acid-based chemical defense strategies in insects.

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Natural product congeners serve a useful role in the understanding of natural product biosynthesis and structure-activity relationships. A minor congener with superior activity, selectivity, and modifiable functional groups could serve as a more effective lead structure and replace even the original lead molecule that was used for medicinal chemistry modifications. Currently, no effective method exists to discover targeted congeners rapidly, specifically, and selectively from producing sources. Herein, a new method based on liquid-chromatography tandem-mass spectrometry combination is evaluated for targeted discovery of congeners of platensimycin and platencin from the extracts of Streptomyces platensis. By utilizing a precursor-ion searching protocol, tandem mass spectrometry not only confirmed the presence of known congeners but also provided unambiguous detection of many previously unknown congeners of platensimycin and platencin. This high-throughput and quantitative method can be rapidly and broadly applied for dereplication and congener discovery from a variety of producing sources, even when the targeted compounds are obscured by the presence of unrelated natural products.

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According to current consensus, structures of protomeric (or deprotomeric) tautomers of gaseous ions generated by electrospray ionization depend primarily on the nature of the spray solvent. To probe the effect of the spray solvent on protonation, 4-aminobenzoic acid (PABA) has often been selected as the model compound. It is widely accepted that the protonation in the gas phase takes place primarily on the carbonyl oxygen atom when the sample is sprayed in methanol and on the nitrogen atom when acetonitrile is used as the spray solvent. Although this observation is valid, our current results indicate that the determination of the predominant protomer in the gas phase by the spray solvent is an indirect effect moderated by the solvent vapor molecules present in the ambient ion source. To investigate real-time changes in protomer distributions due to solvents, we used ion-mobility mass spectrometry (IM-MS). Initially, when a PABA solution in methanol was electrosprayed, the ion-mobility arrival-time profile recorded showed essentially one peak for the O-protomer. However, when acetonitrile or acetone vapors were introduced to the ambient-pressure ion source via the flowing desolvation gas, the intensity of the O-protomer peak diminished rapidly, and the N-protomer signal became dominant. The moment the acetonitrile (or acetone) vapors were removed from the ion source, the protomer-distribution signals began gradually reverting back to their original intensities. Furthermore, when PABA samples in methanol and acetonitrile were electrosprayed separately via a dual-sprayer setup, which allowed for the selective blocking of the gaseous ion-generation cascade of charged droplets from either sprayer, the predominant signal corresponded only to the N-protomer, irrespective of the position of the mechanical barrier. Because the mechanical barrier prevents only the gaseous ion formation, but not the physical access of solvent vapors to the ion source, it is evident that the solvent vapor that engulfs the ion source is the governing factor that decides the protomer distribution, not the nature of the spray solvent. Noticeably, acetonitrile wields a stronger effect on the manifested protomer distribution than many other solvents, including methanol, water, hexanes, and toluene. Apparently, the so-called "memory" of the solution-phase structures and the phenomenon described as "kinetic trapping" are both due to indirect effects caused by the solvent vapor engulfing the atmospheric-pressure ion source. Moreover, the so-called "memory" effect can either be "saved" or "erased" by exposing the initially formed gaseous ions to different solvent vapors from an alternative source.

RESUMEN

Ferrocene and its derivatives and nickelocene undergo facile ionization when exposed directly to the ionizing plasma of a helium-plasma ionization (HePI) source. Mass spectra recorded from such samples under ambient positive-ion-generating conditions show intense peaks for the respective molecular ions [M+•] and protonated species [(M + H)+]. The protonation process occurs most efficiently when traces of water are present in the heated nitrogen used as the "heating gas." In fact, the relative population of the two categories of ions generated in this way can be manipulated by regulating the heating-gas flow. Moreover, rapid and highly efficient gas-phase hydrogen-deuterium exchange (HDX) reactions can be performed in the ion source by passing the heating gas through a vial with D2O before it reaches the HePI source. Moreover, the ionized species generated in this way can be subjected to in-source CID fragmentation in the QDa-HePI source very efficiently by varying the sampling-cone voltage. By this procedure, ions generated from ferrocene and nickelocene could be stripped so far as to ultimately generate the bare-metal cation. Other typical fragment-ions produced from protonated metallocenes included the M(cp)1+ ions (M = Fe or Ni), by elimination of a cyclopentadiene molecule, or the molecular cation, by loss of a H• radical. Moreover, H/D exchanges and subsequent tandem mass spectrometric analysis indicated that the central metal core participates in the initial protonation process of ferrocene under HePI conditions. However, in compounds such as ferrocene carboxaldehyde and ferrocene boronic acid, the protonation takes place at the peripheral functional group.

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The Peruvian stick insect Oreophoetes peruana is the only known animal source for unsubstituted quinoline in nature. When disturbed, these insects discharge a defensive secretion containing quinoline. Analysis of samples obtained from l-[2',4',5',6,'7'-2H5]tryptophan-fed stick insects demonstrated that the insects convert it to [5,6,7,8-2H4]quinoline by removing the 2'-CH moiety in the indole ring of tryptophan. Analogous experiments using l-[1'-15N]tryptophan and l-[1'-15N,15NH2]tryptophan showed that the indole-N atom is retained while the α-amino group is eliminated during the biosynthesis. Mass spectra recorded from quinoline derived from [2-13C1]tryptophan-fed insects indicated that the α-carbon atom of tryptophan is incorporated as the C-2 atom of the quinoline ring.

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Surface-enhanced Raman scattering (SERS) is highly sensitive and label-free analytical technique based on Raman spectroscopy aided by field-multiplying plasmonic nanostructures. We report the use of SERS measurements of patient urine in conjunction with biostatistical algorithms to assess the treatment response of prostate cancer (PCa) in 12 recurrent (Re) and 63 nonrecurrent (NRe) patient cohorts. Multiple Raman spectra are collected from each urine sample using monodisperse silver nanoparticles (AgNPs) for Raman signal enhancement. Genetic algorithms-partial least squares-linear discriminant analysis (GA-PLS-LDA) was employed to analyze the Raman spectra. Comprehensive GA-PLS-LDA analyses of these Raman spectral features (p = 3.50 × 10-16 ) yield an accuracy of 86.6%, sensitivity of 86.0%, and specificity 87.1% in differentiating the Re and NRe cohorts. Our study suggests that SERS combined with multivariate GA-PLS-LDA algorithm can potentially be used to detect and monitor the risk of PCa relapse and to aid with decision-making for optimal intermediate secondary therapy to recurred patients.

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Bombardier beetles are well-known for their remarkable defensive mechanism. Their defensive apparatus consists of two compartments known as the reservoir and the reaction chamber. When challenged, muscles surrounding the reservoir contract sending chemical precursors into the reaction chamber where they mix with enzymes resulting in an explosive discharge of a hot noxious chemical spray containing two major quinones: 1,4-benzoquinone and 2-methyl-1,4-benzoquinone (toluquinone). Previously, it has been speculated that the biosynthesis of all benzoquinones originates from one core precursor, 1,4-hydroquinone. Careful ligation of the base of the reservoir chamber enabled us to prevent the explosive reaction and sample untransformed reservoir fluid, which showed that it accumulates significant quantities of 1,4-hydroquinone and 2-methyl-1,4-hydroquinone. We investigated the biosynthetic mechanisms leading to quinone formation by injecting or feeding Brachinus elongatulus beetles with stable-isotope-labeled precursors. Chemical analysis of defensive secretion samples obtained from 1,4-hydroquinone-d6-administered beetles demonstrated that it underwent conversion specifically to 1,4-benzoquinone. Analogously, results from m-cresol-d8 injected or fed beetles confirmed that m-cresol is metabolized to 2-methyl-1,4-hydroquinone, which is then oxidized to 2-methyl-1,4-benzoquinone in the hot spray. Our results refute the previous claim that 1,4-hydroquinone is the precursor of all substituted benzoquinones in bombardier beetles and reveal that they are biosynthetic products of two independent pathways. Most likely, the aforementioned biosynthetic channel of hydroxylation of appropriate phenolic precursors and subsequent oxidation is not restricted to bombardier beetles; it could well be a general pathway that leads to the formation of all congeners of benzoquinones, one of the most widely distributed groups of defensive compounds in arthropods. Graphical abstract.

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Selected-ion recording (SIR) or multiple-reaction monitoring (MRM) protocols are widely employed for the quantification of targeted analytes by liquid chromatography-mass spectrometry (LC-MS). After chromatographic separation, analytes are desolvated and converted to gaseous ions usually by electrospray-ionization. The chromatographic peaks generated in this way are then integrated for quantification. It is generally assumed that the chromatographic peak intensities are dependent only on the selected MRM-transition protocols and the instrumental parameters set on the mass spectrometer. Using p-aminosalicylic acid (PAS) as a model compound, we demonstrate that the nature of the LC mobile phase exerts a significant effect on the chromatographic peak intensities. Under identical mass spectrometric conditions, chromatographic peak intensities recorded with methanol as the mobile phase were drastically different from those acquired using acetonitrile as the eluent. In fact, the product-ion mass spectra recorded with protonated PAS under different solvent conditions were qualitatively different. The observed differences were attributed to the existence of different protomers of PAS in the gas phase in dissimilar ratios under different solvent-spray conditions. Results from ion-mobility mass spectrometry experiments confirmed this hypothesis. For example, when PAS was sprayed from an acetonitrile solution, the arrival-time profile recorded from the mass-selected m/z 154 ion for protonated PAS showed essentially one arrival-time peak for the N-protonated tautomer. In contrast, the profile recorded from a methanolic PAS solution showed a different arrival-time peak for a more mobile protomer, which was recognized as the carbonyl-protonated PAS. The coexistence of protomers in different and variable ratios in an ensemble of ions generated by electrospray ionization of a single pure compound wields strong ramifications on the identification and quantification of analytes by LC-MS. However, the inclusion of an ion-mobility separator before the mass-selected ions are fragmented and detected by mass spectrometry ameliorates the complications rendered by the coexistence of different protomers and deprotomers.

RESUMEN

Upon collisional activation under mass spectrometric conditions, protonated 2-, 3-, and 4-iodoanilines lose an iodine radical to generate primarily dehydroanilinium radical cations (m/z 93), which are the distonic counterparts of the conventional molecular ion of aniline. When briefly accumulated in the Trap region of a Triwave cell in a SYNAPT G2 instrument, before being released for ion-mobility separation, these dehydroanilinium cations react readily with traces of oxygen present in the mobility gas to form peroxyl radical cations. Although all three isomeric dehydroanilinium ions showed avid affinity for O2, their reactivities were distinctly different. For example, the product-ion spectra recorded from mass-selected m/z 93 ion from 3- and 4-iodoanilines showed a peak at m/z 125 for the respective peroxylbenzenaminium ion. In contrast, an analogous peak at m/z 125 was absent in the spectrum of the 2-dehydroanilinium ion generated from 2-iodoaniline. Evidently, the 2-peroxylbenzenaminium ion generated from the 2-dehydroanilinium ion immediately loses a •OH to form protonated ortho-quinonimide (m/z 108).

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Hydrocyanic acid (HCN) is a well-known defensive allomone in the chemical arsenal of millipedes in the order Polydesmida. The presence of HCN in the headspace vapor of adult Xystocheir dissecta (Wood, 1867), a common millipede from the San Francisco Bay Area, was traced by laser desorption/ionization-mass spectrometry (LDI-MS). To accomplish this, the headspace vapor surrounding caged, live millipedes was allowed to diffuse passively over gold-nanoparticle (AuNP) deposits placed at various distances from the emitting source. The stainless steel plates with AuNP deposits were removed and irradiated by a 355-nm laser. The gaseous ions generated in this way were detected by time-of-flight mass spectrometry. The intensity of the mass spectrometric peak detected at m/z 249 for the Au(CN)2- complex anion was compared to that of the residual Au- signal (m/z 197). Using this procedure, HCN vapors produced by the live millipedes could be detected up to 50 cm away from the source. Furthermore, the addition of H2O2, as an internal oxygen source for the gold cyanidation reaction that takes place in the AuNP deposits, significantly increased the detection sensitivity. Using the modified H2O2 addition procedure, HCN could now be detected at 80 cm from the source. Moreover, we found a decreasing intensity ratio of the Au(CN)2-/Au- signals as the distance from the emitting source increased, following an exponential-decay distribution as predicted by Fick's law of diffusion. Graphical abstract.

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