Scrubbing ions with molecules: kinetic studies of chemical noise reduction in mass spectrometry using ion-molecule reactions with dimethyl disulfide

The kinetics and product distributions of the reactions of dimethyl disulfide (DMDS) have been investigated with a group of chemical background ions commonly observed in atmospheric pressure ionization (API) mass spectrometry (MS) in order to assess the value of this molecule in filtering (or "scrubbing") these ions by changing their mass/charge (m/z) ratio. The measurements were taken with a novel electrospray ionization/selected ion flow tube/QqQ tandem mass spectrometer. The background ions studied include those with m/z 42 (protonated acetonitrile, ACN), 83 (protonated ACN dimer), 99 (protonated phosphoric acid), 117 (water cluster of m/z 99), 131 (methanol cluster of m/z 99), 149 (protonated phthalic anhydride, formed from the phthalates), and 327 (protonated triphenyl phosphate). In addition, reactions of DMDS have been studied with two model analytes--protonated caffeine and doubly protonated bradykinin--in order to assess the selectivity of DMDS reactivity. All the measurements were taken at 295 +/- 2 K in helium buffer gas at a pressure of 0.35 +/- 0.01 Torr. DMDS was observed to react efficiently with m/z 42 (ACNH+), 149 (from phthalates), and 99 (protonated phosphoric acid), with k/kc=0.91, 0.47, and 0.38, respectively. Only proton transfer was observed with ACNH+, followed by the secondary reaction of [DMDSH]+ with DMDS to yield [CH3S-S(CH3)-SCH3]+. Ligation of DMDS was the dominant primary channel observed for the reaction of the m/z 149 background ion; however, some proton transfer also was observed. Both of these primary product ions react further with DMDS to yield [CH3S-S(CH3)-SCH3]+, the structure of which we have determined computationally using DFT calculations. Only the sequential ligation with two DMDS molecules was observed for the reaction of the m/z 99 ion. Reactions of DMDS with m/z 117 [H3PO4 + H + H2O]+ and m/z 131 [H3PO4 + H + MeOH]+ were observed to proceed with k/kc=0.71 and 0.058, respectively. Ligand substitution of DMDS for H2O predominated ( approximately 94%) over DMDS ligation ( approximately 6%) in the reaction with m/z 117, while only DMDS ligation was observed for the reaction of m/z 131 with DMDS. In contrast, the reactions of DMDS with ions of m/z 83 (protonated dimer of ACN) and 327 (protonated triphenyl phosphate) were extremely inefficient, with k/kc=0.0042 and 0.0079, respectively. The higher reactivity of DMDS toward ACNH+ (m/z 42) compared to (ACN)2H+ (m/z 83) is attributed to the lower proton affinity of the unsolvated ACN. The reactivity of DMDS toward the two model analyte ions studied-protonated caffeine and doubly protonated bradykinin-was negligible, with k/kc=0.0073 and 0.010, for the respective reactions. These results suggest that, under appropriate reagent pressure conditions, DMDS can be an appropriate reagent for chemically filtering out many common API-MS background ions, without significantly affecting the observed intensity of analyte peaks.     

Multiple ionization mass spectrometry strategy used to reveal the complexity of metabolomics

A multiple ionization mass spectrometry strategy is presented based on the analysis of human serum extracts. Chromatographic separation was interfaced inline with the atmospheric pressure ionization techniques electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) in both positive (+) and negative (-) ionization modes. Furthermore, surface-based matrix-assisted laser desorption/ionization (MALDI) and desorption ionization on silicon (DIOS) mass spectrometry were also integrated with the separation through fraction collection and offline mass spectrometry. Processing of raw data using the XCMS software resulted in time-aligned ion features, which are defined as a unique m/z at a unique retention time. The ion feature lists obtained through LC-MS with ESI and APCI interfaces in both +/- ionization modes were compared, and unique ion tables were generated. Nonredundant, unique ion features, were defined as mass numbers for which no mass numbers corresponding to [M + H](+), [M - H](-), or [M + Na](+) were observed in the other ionization methods at the same retention time. Analysis of the extracted serum using ESI for both (+) and (-) ions resulted in >90% additional unique ions being detected in the (-) ESI mode. Complementing the ESI analysis with APCI resulted in an additional approximately 20% increase in unique ions. Finally, ESI/APCI ionization was combined with fraction collection and offline-MALDI and DIOS mass spectrometry. The parts of the total ion current chromatograms in the LC-MS acquired data corresponding to collected fractions were summed, and m/z lists were compiled and compared to the m/z lists obtained from the DIOS/MALDI spectra. It was observed that, for each fraction, DIOS accounted for approximately 50% of the unique ions detected. These results suggest that true global metabolomics will require multiple ionization technologies to address the inherent metabolite diversity and therefore the complexity in and of metabolomics studies.     

In-spray supercharging of peptides and proteins in electrospray ionization mass spectrometry

Enhanced charging, or supercharging, of analytes in electrospray ionization mass spectrometry (ESI MS) facilitates high resolution MS by reducing an ion mass-to-charge (m/z) ratio, increasing tandem mass spectrometry (MS/MS) efficiency. ESI MS supercharging is usually achieved by adding a supercharging reagent to the electrospray solution. Addition of these supercharging reagents to the mobile phase in liquid chromatography (LC)-MS/MS increases the average charge of enzymatically derived peptides and improves peptide and protein identification in large-scale bottom-up proteomics applications but disrupts chromatographic separation. Here, we demonstrate the average charge state of selected peptides and proteins increases by introducing the supercharging reagents directly into the ESI Taylor cone (in-spray supercharging) using a dual-sprayer ESI microchip. The results are comparable to those obtained by the addition of supercharging reagents directly into the analyte solution or LC mobile phase. Therefore, supercharging reaction can be accomplished on a time-scale of ion liberation from a droplet in the ESI ion source.     

Simple coupling of gas chromatography to electrospray ionization mass spectrometry

A simple method for direct coupling of gas chromatography (GC) with electrospray ionization mass spectrometry (ESI/MS) has been developed. The outlet of the GC capillary column was placed between the ESI needle and the atmospheric pressure ionization (API) source of a mass spectrometer. The ionization occurs via dissolution of neutral compounds into the charged ESI droplet followed by ion evaporation or via a gas-phase proton transfer reaction between a protonated solvent molecule and an analyte. The mass spectra of organic volatile compounds showed abundant protonated molecules with little fragmentation, being very similar to those produced by normal liquid ESI. The quantitative performance of the system was evaluated by determining the limit of detection (LOD), linearity ( r (2)), and repeatability (RSD). The GC-ESI/MS method was shown to be stable, providing high sensitivity and good quantitative performance.     

Investigation of the unusual behavior of metolachlor under chemical ionization in a hybrid 3D ion trap mass spectrometer

This article describes the strange behavior of the widely used herbicide metolachlor under chemical ionization conditions in a hybrid source ion trap mass spectrometer in gas chromatography/mass spectrometry (GC/MS) coupling. With the use of ammonia as the reagent gas, metolachlor provides a chlorinated ion at m/z 295/297, almost as abundant as the protonated molecule at m/z 284/286, which cannot be isolated to perform tandem mass spectrometry (MS(n)) experiments. Curiously, this ion at m/z = M + 12 is not observed for the herbicides acetochlor and alachlor, which present very similar chemical structures. The chemical structure of the m/z 295/297 ions and the explanation of the observed phenomenon based on the metastable behavior of these ions were elucidated on the basis of experiments including isotopic labeling and modifications of the operating conditions of the ion trap mass spectrometer. This work allows one to give new recommendations for an optimized use of hybrid source ion trap mass spectrometers.     

Effect of experimental parameters on the ESI FT-ICR mass spectrum of fulvic acid

Fulvic acid (FA) is a heterogeneous mixture of organic macromolecules found in the waters, soils, and sediments of the earth's surface. The ability of electrospray ionization (ESI) to effectively transfer large ions from the solution phase to the gas phase and the coupling of ESI to the high-mass-resolution capabilities of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) provide a potential method for the mass spectrometric analysis of FA. Positive- and negative-ion ESI FT-ICR MS analyses of four reference International Humic Substances Society FAs were performed. The spray solution composition was found to have a dramatic effect on the ion distributions, with high-mass aggregates (m/z approximately 2000-4000) being formed in less polar spray solutions. Positive-ion spectra for each FA obtained under optimum conditions resulted in number-average molecular weights ranging from 1700 to 1900. The mass spectra were extremely complex, with ion distributions on the order of m/z approximately 500-3000. The presence of more than one ion at each nominal mass was routinely observed. Negative-ion ESI analysis of the FA samples resulted in the observation of multiply charged ions whose distributions could be affected by the acidification of the spray solution. Solution parameters which have been reported to affect molecular weight distributions of FA such as pH, ionic strength, and concentration of multivalent cations were found to have little or no effect on the observed m/z distributions.     

Measuring the mass of an electron by LC/TOF-MS: a study of "twin ions"

While investigating the in-source CID fragmentation of nonsteroidal antiinflammatory drugs (NSAIDs), it was noticed that the same fragment ion (nominal mass) formed in either positive or negative ion electrospray for a suite of NSAIDs. For example, ibuprofen with a molecular mass of 206, fragments to the m/z 161 ion in negative ion from its deprotonated molecule (m/z 205, [M - H]-) and fragments to the m/z 161 ion in positive ion from its protonated molecule (m/z 207, [M + H]+). This fragment ion was euphemistically called a "twin ion"because of the same nominal mass despite opposite charge. The CID fragmentation of the twin ions was confirmed also by LC/MS/MS ion trap. Accurate mass measurements in negative ion show that the loss was due to CO2 (measured loss of 43.9897 atomic mass units (u) versus calculated loss of 43.9898 u for N = 10) and in positive ion the loss is due to HCOOH (measured loss of 46.0048 u versus calculated loss of 46.0055 u, N = 10). It was realized that, in fact, the ions were not "identical mass twins of opposite charge" but separated in accurate mass by two electrons. The accurate mass measurement by liquid chromatography/time-of-flight-mass spectrometry (LC/TOF-MS) can distinguish between the two fragment ions of ibuprofen (161.13362 +/- 0.00019 and 161.13243 +/- 0.00014 for N = 20). This experiment was repeated for two other NSAIDs, and the mass of an electron was measured as the difference between the twin ions, which was 0.00062 u +/- 14.8% relative standard deviation (N = 20 analyses). Thus, the use of continuous calibration makes it possible to measure the mass of an electron within one significant figure using the NSAID solution. This result shows the importance of including electron mass in accurate mass measurements and the value of a benchmark test for LC/TOF-MS mass accuracy.     

Identification and quantification of cardiac glycosides in blood and urine samples by HPLC/MS/MS.

Cardiac glycosides (CG) are of forensic importance because of their toxicity and the fact that very limited methods are available for identification of CG in biological samples. In this study, we have developed an identification and quantification method for digoxin, digitoxin, deslanoside, digoxigenin, and digitoxigenin by high-performance liquid chromatography tandem mass spectrometry (HPLC/MS/MS). CG formed abundant [M + NH4]+ ions and much less abundant [M + H]+ ions as observed with electrospray ionization (ESI) source and ammonium formate buffer. Under mild conditions for collision-induced dissociation (CID), each [M + NH4]+ ion fragmented to produce a dominant daughter ion, which was essential to the sensitive method of selected reaction monitoring (SRM) quantification of CG achieved in this study. SRM was compared with selected ion monitoring (SIM) regarding the effects of sample matrixes on the methodology. SRM produced lower detection limits with biological samples than SIM, while both methods produced equal detection limits with CG standards. On the basis of the HPLC/MS/MS results for CG, we have proposed some generalized points for conducting sensitive SRM measurements, in view of the property of analytes as well as instrumental conditions such as the type of HPLC/MS interface and CID parameters. Analytes of which the molecular ion can produce one abundant daughter ion with high yield under CID conditions may be sensitively measured by SRM. ESI is the most soft ionization source developed so far and can afford formation of the fragile molecular ions that are necessary for sensitive SRM detection. Mild CID conditions such as low collision energy and low pressure of collision gas favor production of an abundant daughter ion that is essential to sensitive SRM detection. This knowledge may provide some guidelines for conducting sensitive SRM measurements of very low concentrations of drugs or toxicants in biological samples.     

Implementation of ion/ion reactions in a quadrupole/time-of-flight tandem mass spectrometer

A commercial quadrupole/time-of-flight (QqTOF) tandem mass spectrometer has been adapted for ion/ion reaction studies. To enable mutual storage of oppositely charged ions in a linear ion trap, the oscillating quadrupole field of the second quadrupole of the system (Q2) serves to store ions in the radial dimension while auxiliary radio frequency is superposed on the end lenses of Q2 during the reaction period to create barriers in the axial dimension. A pulsed dual electrospray (ESI) source is directly coupled to the instrument interface for the purpose of proton transfer reactions. Singly and doubly charged protein ions as high in mass as 66 kDa are readily formed and observed after proton-transfer reactions. For the modified instrument, the mass resolving power is approximately 8000 for a wide m/z range, and the mass accuracy is approximately 20 ppm for external calibration and approximately 5 ppm for internal calibration after ion/ion reactions. Parallel ion parking is demonstrated with a six-component protein mixture, which shows the potential application of reducing spectral complexity and concentrating certain charge states. The current system has high flexibility with respect to defining MS(n) experiments involving collision-induced dissociation (CID) and ion/ion reactions. Protein precursor and CID product masses can be determined with good accuracy, providing an attractive platform for top-down proteomics. Electron transfer dissociation ion/ion reactions are implemented by using a pulsed nano-ESI/atmospheric pressure chemical ionization dual source for ionization. The reaction between protonated peptide ions and radical anions of 1,3-dinitrobenzene formed exclusively c- and z-type fragment ions.     

Characterization of triacetone triperoxide by ion mobility spectrometry and mass spectrometry following atmospheric pressure chemical ionization

The atmospheric pressure chemical ionization of triacetone triperoxide (TATP) with subsequent separation and detection by ion mobility spectrometry has been studied. Positive ionization with hydronium reactant ions produced only fragments of the TATP molecule, with m/z 91 ion being the most predominant species. Ionization with ammonium reactant ions produced a molecular adduct at m/z 240. The reduced mobility value of this ion was constant at 1.36 cm(2)V(-1)s(-1) across the temperature range from 60 to 140 °C. The stability of this ion was temperature dependent and did not exist at temperatures above 140 °C, where only fragment ions were observed. The introduction of ammonia vapors with TATP resulted in the formation of m/z 58 ion. As the concentration of ammonia increased, this smaller ion appeared to dominate the spectra and the TATP-ammonium adduct decreased in intensity. The ion at m/z 58 has been noted by several research groups upon using ammonia reagents in chemical ionization, but the identity was unknown. Evidence presented here supports the formation of protonated 2-propanimine. A proposed mechanism involves the addition of ammonia to the TATP-ammonium adduct followed by an elimination reaction. A similar mechanism involving the chemical ionization of acetone with excess ammonia also showed the formation of m/z 58 ion. TATP vapors from a solid sample were detected with a hand-held ion mobility spectrometer operated at room temperature. The TATP-ammonium molecular adduct was observed in the presence of ammonia and TATP vapors with this spectrometer.     

On-line investigation of the generation of nonaqueous intermediate radical cations by electrochemistry/mass spectrometry

Anodic oxidation of triphenylamine (TPA) in acetonitrile was investigated by electrochemistry (EC) combined online with mass spectrometry (MS) through a particle beam (PB) interface, EC/PB/MS. Electrooxidation of TPA generates a TPA*+ radical cation (m/z 245) which dimerizes to tetraphenylbenzidine (TPB, MW 488). TPB is readily oxidized to TPB*+ (m/z 488) and TPB2+ (m/z 244) at the oxidation potential of TPA. In EC/PB/MS, direct monitoring of the oxidation of TPB to TPB*+ radical cation as a function of the electrode potential was achieved via selective ion monitoring of the ion peak at m/z 488. By using the relative intensity ratio of ions at m/z 244 (TPB2+) to 245 (TPA*+), the formation of TPB2+ as a function of the electrode potential was also monitored. EC/PB/MS showed a maximum rate of formation of TPB*+ at +1.2 Vvs Pd, while TPB2+ is generated at a maximum rate at +1.6 V vs Pd. The effect of spectral interference from the electron impact ionization of TPA, on EC/PB/MS results, is also discussed. Finally, a significant signal enhancement is observed in the presence of tetrabutylammonium perchlorate (TBAP) and is reported for the first time. Compatibility of coupling of EC with MS via PB interface for EC/MS studies in nonaqueous solvents is demonstrated. The observation of significant signal enhancement in the presence of TBAP may facilitate other applications of LC/MS.     

Parent and neutral loss monitoring on a quadrupole ion trap mass spectrometer: screening of acylcarnitines in complex mixtures

A novel and practical technique for performing both parent and neutral loss (P&NL) monitoring experiments on a quadrupole ion trap mass spectrometer is presented. This technique is capable of performing scans analogous to the parent and neutral loss scans routinely applied on tandem-in-space instruments and allows for the screening of a sample to detect analytes of a specific compound class on a chromatographic time-scale. Acylcarnitines were chosen as the model compound class to demonstrate the analytical utility of P&NL monitoring because of their amenability to electrospray ionization (ESI), their unique and informative MS/MS fragmentation pattern, and their importance in biological functions. The [M + H]+ ions of all acylcarnitines dissociate to produce neutral losses of 59 and 161 amu and common product ions at m/z 60, 85, and 144. Both the neutral loss monitoring of 59 amu and the parent ion monitoring of m/z 85 are shown to be capable of identifying acylcarnitine [M + H]+ ions in a synthetic mixture and spiked pig plasma. The neutral loss monitoring of 59 amu is successful in detecting acylcarnitines in an unspiked pig plasma sample.     

Electrospray ionization tandem mass spectrometry for structural elucidation of protonated brevetoxins in red tide algae

Brevetoxins, the toxic components of "red tide" algae, all share one of two robust polycyclic ether backbone structures, but they are distinguished by differing side-chain substituents. Electrospray ionization mass spectrometry analyses of brevetoxins have shown that the polyether structure invariably has a very high affinity for sodium cations that results in the production of abundant (M + Na)+ ions even when sodium cations are only present as impurities. Because the ionic charge tends to remain localized on the sodium atom and because at least two bonds must be broken in order to produce polycyclic backbone fragmentation, it is extremely difficult to obtain abundant product ions (other than Na+) from (M + Na)+ brevetoxin precursor ions in low-energy collision-induced dissociation (CID) MS/MS experiments. This report establishes that acid additives (oxalic acid, trifluoroacetic acid, and particularly hydrochloric acid) in aqueous methanol solutions can promote high yields of protonated brevetoxin molecules (MH+ ions) for Btx-1, -2, and -9 brevetoxins. Most importantly, unlike their (M + Na)+ counterparts, MH+ precursor ions offer readily detectable product ions in CID MS/MS experiments, even under low-energy collisions. This direct structural characterization approach has provided decomposition information from brevetoxins that was previously inaccessible, including the identification of diagnostic product ions for "type A" brevetoxins (m/z 611) and "type B" brevetoxins (m/z 779, 473, 179) and characteristic ions for Btx-1 (m/z 221, 139), Btx-2 (m/z 153), and Btx-9 (m/z 157, 85). Precursor ion scans and constant neutral loss scans are proposed to enable screening of individual type A or type B brevetoxins present in naturally occurring mixtures.     

Generation of highly charged peptide and protein ions by atmospheric pressure matrix-assisted infrared laser desorption/ionization ion trap mass spectrometry

We show that highly charged ions can be generated if a pulsed infrared laser and a glycerol matrix are employed for atmospheric pressure matrix-assisted laser desorption/ionization mass spectrometry with a quadrupole ion trap. Already for small peptides like bradykinin, doubly protonated ions form the most abundant analyte signal in the mass spectra. The center of the charge-state distribution increases with the size of the analyte. For example, insulin is detected with a most abundant ion signal corresponding to a charge state of four, whereas for cytochrome c, the 10 times protonated ion species produces the most intense signal. Myoglobin is observed with up to 13 charges. The high m/z ratios allow us to use the Paul trap for the detection of MALDI-generated protein ions that are, owing to their high molecular weight, not amenable in their singly protonated charge state. Formation of multiple charges critically depends on the addition of diluted acid to the analyte-matrix solution. Tandem mass spectra generated by collision-induced dissociation of doubly charged peptides are also presented. The findings allow speculations about the involvement of electrospray ionization processes in these MALDI experiments.     

In vivo analysis and spatial profiling of phytochemicals in herbal tissue by matrix-assisted laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) was developed for spatial profiling of phytochemicals and secondary metabolites in integrated herbal tissue without solvent extraction. Abundant alkaloid ions, including (+)-menisperine (m/z 356), magnoflorine (m/z 342), stepharanine (m/z 324), protonated sinomenine (m/z 330), protonated sinomendine (m/z 338), and a metabolite at m/z 314, could be directly desorbed from alpha-cyano-4-hydroxycinnamic acid- (CHCA-) coated stem tissue of Sinomenium acutum upon N2 laser (337 nm) ablation, while the ion signals desorbed from sinapinic acid- (SA-) coated and 2,5-dihydroxybenzoic acid- (DHB-) coated stem tissue were at least 10 times weaker. Solvent composition in the matrix solution could have significant effects on the ion intensity of the metabolites. Under optimized conditions that maximize the ion intensity and form homogeneous matrix crystals on the tissue surface, spatial distributions of the metabolites localized in different tissue regions, including cortex, phloem, xylem, rim, and pith, and their relative abundances could be semiquantitatively determined. The three metabolites detected at m/z 356, 342, and 314 showed specific distributions in the herbal samples collected from different growing areas, while others were not. By applying principal component analysis (PCA), the characteristic metabolites in specific tissue regions could be easily determined, allowing unambiguous differentiation of the herbal samples from different geographic locations.     

Electron-transfer reagent anion formation via electrospray ionization and collision-induced dissociation

A strategy is described and demonstrated for the formation of reagent anions via electrospray ionization (ESI) for electron-transfer dissociation (ETD). To circumvent difficulties associated with formation of high mass-to-charge ratio (m/z) reagent anions, it is desirable to form ETD reagents via means other than those that require reagent molecule vaporization. ESI is a candidate method, but anions that are generally generated efficiently by ESI tend to react with multiply protonated polypeptides via proton transfer. The strategy described herein involves the use of a precursor reagent molecule that ionizes efficiently via electrospray ionization and that can subsequently be converted to an ETD reagent via gas-phase dissociation. The approach is demonstrated with arenecarboxylic acids that yield strong signals associated with the deprotonated molecule and that subsequently undergo collision-induced dissociation (CID) by loss of CO(2). In the present work, triply protonated KGAILKGAILR served as a test substrate for the CID product ions to give rise to ETD. Several precursor molecules were shown to be capable of generating ETD reagents via ESI followed by CID. These included 9-anthracenecarboxylic acid, 2-fluoro-5-iodobenzoic acid, and 2-(fluoranthene-8-carbonyl)benzoic acid. The latter molecule has the most attractive set of characteristics as a precursor for a relatively high m/z ratio ETD reagent.     

Molecular weight distributions of heavy aromatic petroleum fractions by Ag+ electrospray ionization mass spectrometry

The ability of electrospray ionization mass spectrometry (ESI MS) to analyze heavy aromatic petroleum fractions using silver nitrate as a reagent compound to form characteristic adduct ions has been examined. The complexation of aromatic compounds containing long alkyl substituents with the silver ion leads to the formation of abundant adduct ions such as [M + Ag]+ and [2M + Ag]+. The concentration of the [2M + Ag]+ ions can be reduced by increasing the sampling cone voltage. Molecular ions and other adduct ions may also be formed depending on the structure of the aromatic molecule. Results obtained from the analysis of representative heavy petroleum fractions and vacuum residues by the Ag+ ESI MS method and conventional ionization methods were in good agreement. The current method extends the applicability of electrospray ionization to the analysis of neutral hydrocarbons in heavy aromatic petroleum fractions. It is simple and compatible with widely available LC/MS instrumentation. The extreme complexity of the Ag     

Choosing between atmospheric pressure chemical ionization and electrospray ionization interfaces for the HPLC/MS analysis of pesticides

An evaluation of over 75 pesticides by high-performance liquid chromatography/mass spectrometry (HPLC/MS) clearly shows that different classes of pesticides are more sensitive using either atmospheric pressure chemical ionization (APCI) or electrospray ionization (ESI). For example, neutral and basic pesticides (phenylureas, triazines) are more sensitive using APCI (especially positive ion). While cationic and anionic herbicides (bipyridylium ions, sulfonic acids) are more sensitive using ESI (especially negative ion). These data are expressed graphically in a figure called an ionization-continuum diagram, which shows that protonation in the gas phase (proton affinity) and polarity in solution, expressed as proton addition or subtraction (pKa), is useful in selecting APCI or ESI. Furthermore, sodium adduct formation commonly occurs using positive ion ESI but not using positive ion APCI, which reflects the different mechanisms of ionization and strengthens the usefulness of the ionization-continuum diagram. The data also show that the concept of "wrong-way around" ESI (the sensitivity of acidic pesticides in an acidic mobile phase) is a useful modification of simple PKa theory for mobile-phase selection. Finally, this finding is used to enhance the chromatographic separation of oxanilic and sulfonic acid herbicides while maintaining good sensitivity in LC/MS using ESI negative.     

Infrared multiple photon dissociation spectroscopy and computational studies of O-glycosylated peptides

The infrared multiple photon dissociation (IRMPD) spectra of O-glycosylated peptides in the gas phase were studied in the IR scanning range of 5.7-9.5 μm. Fragmentation of protonated and sodiated O-glycopeptides was investigated using electrospray ionization (ESI) Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry (MS) with a free electron laser (FEL). FEL is used in the IRMPD technique as a tunable IR light source. In the IRMPD spectroscopic analysis of the protonated O-glycopeptide, fragment ions of the b/y and B/Y types were observed in the range of 5.7-9.5 μm, corresponding to the cleavage of the backbone in the parent amino acid sequence and glycosyl bonds, whereas the spectra of the sodiated glycopeptide showed major peaks of photoproducts of the B/Y type in the range of 8.4-9.5 μm. The IRMPD spectra of the O-glycopeptides were compared with simulated IR spectra for the structures obtained from the molecular dynamics.