COMPLX: a computer algorithm for the detection of protein-ligand and other macromolecular complexes in mass spectra

A new algorithm has been designed and tested to identify protein, or any other macromolecular, complexes that have been widely reported in mass spectral data. The program takes advantage of the appearance of multiply charged ions that are common to both electrospray ionization and, to a lesser extent, matrix-assisted laser desorption/ionization (MALDI) mass spectra. The algorithm, known as COMPLX for the COMposition of Protein-Ligand compleXes, is capable of identifying complexes for any protein or macromolecule with a binding partner of molecular mass up to 100 000 Da. It does so by identifying ion pairs present in a mass spectrum that, when they share a common charge, have an m/z value difference that is an integer fraction of a ligand or binding partner molecular mass. Several additional criteria must be met in order for the result to be ranked in the output file including that all m/z values for ions of the protein or complex have progressively lower values as their assigned charge increases, the difference between the m/z values for adjacent charge states (z, z + 1) decrease as the assigned charge state increases, and the ratio of any two m/z values assigned to a protein or complex is equal to the inverse ratio of their charge. The entries that satisfy these criteria are then ranked according to the appearance of ions in the mass spectrum associated with the binding partner, the length of a continuous series of charges across any set of ions for a protein and complex and the lowest error recorded for the molecular mass of the ligand or binding partner. A diverse range of hypothetical and experimental mass spectral data were used to implement and test the program, including those recorded for antibody-peptide, protein-peptide and protein-heme complexes. Spectra of increasing complexity, in terms of the number of ions input, were also successfully analysed in which the number of input m/z values far exceeds the few associated with a macromolecular complex. Thus the program will be of value in a future goal of proteomics, where mass spectrometry already plays a central role, for the direct analysis of protein and other associations within biological extracts.     

Characterization of microchannel plate detector response for the detection of native multiply charged high mass single ions in orthogonal-time-of-flight mass spectrometry using a Timepix detector

Time-of-flight (TOF) systems are one of the most widely used mass analyzers in native mass spectrometry (nMS) for the analysis of non-covalent multiply charged bio-macromolecular assemblies (MMAs). Typically, microchannel plates (MCPs) are employed for high mass native ion detection in TOF MS. MCPs are well known for their reduced detection efficiency when impinged by large slow moving ions. Here, a position- and time-sensitive Timepix (TPX) detector has been added to the back of a dual MCP stack to study the key factors that affect MCP performance for MMA ions generated by nMS. The footprint size of the secondary electron cloud generated by the MCP on the TPX for each individual ion event is analyzed as a measure of MCP performance at each mass-to-charge (m/z) value and resulted in a Poisson distribution. This allowed us to investigate the dependency of ion mass, ion charge, ion velocity, acceleration voltage, and MCP bias voltage on MCP response in the high mass low velocity regime. The study of measurement ranges; ion mass = 195 to 802,000 Da, ion velocity = 8.4 to 67.4 km/s, and ion charge = 1+ to 72+, extended the previously examined mass range and characterized MCP performance for multiply charged species. We derived a MCP performance equation based on two independent ion properties, ion mass and charge, from these results, which enables rapid MCP tuning for single MMA ion detection.     

Automated charge state determination of complex isotope-resolved mass spectra by peak-target Fourier transform

This study describes a new algorithm for charge state determination of complex isotope-resolved mass spectra. This algorithm is based on peak-target Fourier transform (PTFT) of isotope packets. It is modified from the widely used Fourier transform method because Fourier transform may give ambiguous charge state assignment for low signal-to-noise ratio (S/N) or overlapping isotopic clusters. The PTFT algorithm applies a novel "folding" strategy to enhance peaks that are symmetrically spaced about the targeted peak before applying the FT. The "folding" strategy multiplies each point to the high-m/z side of the targeted peak by its counterpart on the low-m/z side. A Fourier transform of this "folded" spectrum is thus simplified, emphasizing the charge state of the "chosen" ion, whereas ions of other charge states contribute less to the transformed data. An intensity-dependent technique is also proposed for charge state determination from frequency signals. The performance of PTFT is demonstrated using experimental electrospray ionization Fourier transform ion cyclotron resonance mass spectra. The results show that PTFT is robust for charge state determination of low S/N and overlapping isotopic clusters, and also useful for manual verification of potential hidden isotopic clusters that may be missed by the current analysis algorithms, i.e., AID-MS or THRASH.     

Electrospray mass and tandem mass spectrometry of homologous and isomeric singly, doubly, triply and quadruply charged cationic ruthenated meso-(phenyl)m-(meta- and para-pyridyl)n (m + n = 4) macrocyclic porphyrin complexes

Ten homologous or isomeric singly, doubly, triply and quadruply charged cationic macrocyclic complexes I-Va, bn+ (n = 1-4) formed by the coordination of [Ru(bipy)2Cl]+ to the pyridyl N-atoms of a series of meso-(phenyl)m-(meta or para-pyridyl)n-porphyrins (m + n = 4) were transferred to the gas phase and structurally characterized by electrospray ionization (ESI) mass (MS) and tandem mass (MS/MS) spectrometry. Previously known to be stable in solution and in the solid state, I-Va, bn+ are found to constitute also a new class of stable, long-lived multiply charged gas-phase ions with spatially separated charge sites. Increasing intramolecular electrostatic repulsion from Ia, b+ to IVa, b3+ facilitates in-source and tandem collision-induced dissociation (CID). However, for the quadruply charged ions Va, b4+, electrostatic repulsion is alleviated mainly by ion pairing with the CF3SO3- counterion forming the salt clusters [Va,b/CF3SO3]3+ and [Va,b/(CF3SO3)2]2+ with reduced charge states. Ion-pairing that yields [IVa,b/CF3SO3]2+ is also observed as a minor ESI process for the triply charged ions IVa, b3+. The gaseous ions I-Va, bn+ (n = 2, 3 or 4) dissociate by sequential 'charge partitioning' with the formation of two cationic fragments by the release of [Ru(bipy)2Cl]+. The meta (a) and para (b) isomers and the positional isomers II2+ and III2+ display nearly identical ESI-MS and ESI-MS/MS spectra. ESI-MS/MS of I-Va, bn+ shows that the Ru-py(P) is, intrinsically, the weakest bond since this bond breaks preferentially upon CID.     

Charge state separation for protein applications using a quadrupole time-of-flight mass spectrometer

A novel method for separating ions according to their charge state using a quadrupole time-of-flight mass spectrometer is presented. The benefits of charge state separation are particularly apparent in protein identification applications at low femtomole concentration levels, where in conventional TOF MS spectra peptide ions are often lost in a sea of chemical noise. When doubly and triply charged tryptic peptide ions need to be filtered from singly charged background ions, the latter are suppressed by two to three orders of magnitude, while from 10-50% of multiply charged ions remain. The suppression of chemical noise reduces the need for chromatography and can make this experimental approach the electrospray equivalent of conventional MALDI peptide maps. If unambiguous identification cannot be achieved, MS/MS experiments are performed on the precursor ions identified through charge separation, while the previously described Q2-trapping duty cycle enhancement is tuned for approximately 1.4 of the precursor m/z to enhance intensities of ions with m/z values above that of the precursor. The resulting product ion spectra contain few fragments of impurities and provide quick and unambiguous identification through database search. The multiple charge separation technique requires minimal tuning and may become a useful tool for analysis of complex mixtures.     

Radial stratification of ions as a function of mass to charge ratio in collisional cooling radio frequency multipoles used as ion guides or ion traps

Collisional cooling in radio frequency (RF) ion guides has been used in mass spectrometry as an intermediate step during the transport of ions from high pressure regions of an ion source into high vacuum regions of a mass analyzer. Such collisional cooling devices are also increasingly used as 'linear', two-dimensional (2D) ion traps for ion storage and accumulation to achieve improved sensitivity and dynamic range. We have used the effective potential approach to study m/z dependent distribution of ions in the devices. Relationships obtained for the ideal 2D multipole demonstrate that after cooling the ion cloud forms concentric cylindrical layers, each of them composed of ions having the same m/z ratio; the higher the m/z, the larger is the radial position occupied by the ions. This behavior results from the fact that the effective RF focusing is stronger for ions of lower m/z, pushing these ions closer to the axis. Radial boundaries of the layers are more distinct for multiply charged ions, compared to singly charged ions having the same m/z and charge density. In the case of sufficiently high ion density and low ion kinetic energy, we show that each m/z layer is separated from its nearest neighbor by a radial gap of low ion density. The radial gaps of low ion population between the layers are formed due to the space charge repulsion. Conditions for establishing the m/z stratified structure include sufficiently high charge density and adequate collisional relaxation. These conditions are likely to occur in collisional RF multipoles operated as ion guides or 2D ion traps for external ion accumulation. When linear ion density increases, the maximum ion cloud radius also increases, and outer layers of high m/z ions approach the multipole rods and may be ejected. This 'overfilling' of the multipole capacity results in a strong discrimination against high m/z ions. A relationship is reported for the maximum linear ion density of a multipole that is not overfilled.     

Solvent-induced conformational changes of polypeptides probed by electrospray-ionization mass spectrometry.

Electrospray-ionization (ESI) mass spectrometry is used to monitor higher order structural changes of polypeptides induced by alteration of the pH or organic solvent composition in the protein solution environment. A bimodal charge-state distribution is observed in the ESI mass spectrum of ubiquitin (relative molecular mass 8565) in solutions containing small amounts (less than 20%) of organic solvents. The distribution of peaks at high m/z (low-charge state) is found to represent the protein in its native, globular state; the higher-charge-state distribution is characteristic for a more extended conformation. Addition of methanol denaturant in excess of 40% v/v is needed to eliminate the low-charge-state distribution completely. Lesser amounts of acetonitrile, acetone, or isopropanol (approximately 20%) are required to denature the ubiquitin protein. Other proteins showing conformational effects in their ESI mass spectra are also illustrated. While the ESI spectra are related to solution phase structure, ESI-tandem mass spectrometry of multiply charged molecular ions of different conformation is suggested as a probe of gas-phase protein three-dimensional structure.     

Characterisation of humic substances using atmospheric pressure chemical ionisation and electrospray ionisation mass spectrometry combined with size-exclusion chromatography

Humic substances were analysed by atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI) mass spectrometry in positive and negative modes. Using APCI the average m/z range of humic substances was reduced 5-fold compared to ESI. High-resolution time-of-flight mass spectrometry revealed the formation of multiply charged molecules in the ESI mode. Moreover, it was possible to obtain daughter ion mass spectra of humic substances by nanospray tandem mass spectrometry. The size-exclusion chromatography elution profile of humic substances was highly influenced by the pH of the analyte solution. By contrast, the pH had no significant influence on the observed mass spectra of humic substances.     

Charge Transfer and Metastable Ion Dissociation of Multiply Charged Molecular Cations Observed by Using Reflectron Time-of-Flight Mass Spectrometry

A multiply charged molecule expands the range of a mass window and is utilized as a precursor to provide rich sequence coverage; however, reflectron time-of-flight mass spectrometer has not been well applied to the product ion analysis of multiply charged precursor ions. Here, we demonstrate that the range of the mass-to-charge ratio of measurable product ions is limited in the cases of multiply charged precursor ions. We choose C₆F₆ as a model molecule to investigate the reactions of multiply charged molecular cations formed in intense femtosecond laser fields. Measurements of the time-of-flight spectrum of C₆F₆ by changing the potential applied to the reflectron, combined with simulation of the ion trajectory, can identify the species detected behind the reflectron as the neutral species and/or ions formed by the collisional charge transfer. Moreover, the metastable ion dissociations of doubly and triply charged C₆F₆ are identified. The detection of product ions in this manner can diminish interference by the precursor ion. Moreover, it does not need precursor ion separation before product ion analysis. These advantages would expand the capability of mass spectrometry to obtain information about metastable ion dissociation of multiply charged species.     

Characterization of cysteinylation of pharmaceutical-grade human serum albumin by electrospray ionization mass spectrometry and low-energy collision-induced dissociation tandem mass spectrometry

Three samples of albumin derived from human plasma (pharmaceutical grade, HSA) obtained from different commercial sources were investigated for their micro-heterogeneities by means of electrospray ionization (ESI) ion trap mass spectrometry (ITMS). The study covered MS analyses of the intact proteins as well as on the tryptic peptide level. The intact protein samples were analyzed without any separation step except for simple desalting. With these samples we observed in the positive ion ESI mass spectra that the multiply charged ion signals of HSA consisted of a number of fully or partly resolved peaks with relative intensities depending on the analyzed sample. The non-modified form of HSA was detected in the three HSA preparations at m/z values of 66448 +/- 3.6, 66450 +/- 0.6 and 66451 +/- 3.2 ([MH]+), respectively. The value calculated from the amino acid sequence was 66439. The second compound present with high intensity (in two cases the base peak in the deconvoluted mass spectrum) is interpreted as a modified HSA, and the molecular mass increase in relation to the unmodified HAS was between 116 and 118 Da (m/z of 66 564, 66 567 and 66 569), suggesting the presence of a covalently bound cysteine residue. A further peak in the deconvoluted ESI spectra was found in all three samples with rather low signal/noise ratio at m/z 66 619, 66 621 and 66 613, respectively, which may correspond to a non-enzymatic glycation described in the literature. The verification of the proposed covalent HSA modifications was subsequently done on the peptide level using high-performance liquid chromatography (HPLC)/ESI-MS and HPLC/ESI-MS/MS including low-energy collision-induced dissociation (CID). Prior to the tryptic digestion, the HSA samples were alkylated without a prior reduction step. Following this procedure we detected peptides of the sequence T21-41 that included the Cys-34 residue in both forms: cysteinylated (m/z 639.15 [M+4H]4+) as well as vinylpyridine-alkylated (m/z 635.69 [M+4H]4+, which means in its previously native free SH form). In the next step on-line LC/ESI low-energy CID MS/MS experiments were performed to verify these two proposed structures. By means of MS/MS analysis of the mentioned ions the described modification (cysteinylation) at the Cys-34 residue could be proven. This abundant modification of HSA in pharmaceutical-grade preparations could be unambiguously identified as cysteinylation at the Cys-34 residue. On the other hand, the proposed non-enzymatic glycation was not detectable on the peptide level in the on-line HPLC/ESI-MS mode, maybe due to the low concentration in the three samples under investigation.     

Profiling and imaging of tissues by imaging ion mobility-mass spectrometry

Molecular profiling and imaging mass spectrometry (IMS) of tissues can often result in complex spectra that are difficult to interpret without additional information about specific signals. This report describes increasing data dimensionality in IMS by combining two-dimensional separations at each spatial location on the basis of imaging ion mobility-mass spectrometry (IM-MS). Analyte ions are separated on the basis of both ion-neutral collision cross section and m/z, which provides rapid separation of isobaric, but structurally distinct ions. The advantages of imaging using ion mobility prior to MS analysis are demonstrated for profiling of human glioma and selective lipid imaging from rat brain.     

The fundamentals of applying electrospray ionization mass spectrometry to low mass poly(methyl methacrylate) polymers

Electrospray ionization (ESI) is capable of ionizing many soluble polymers. The ESI spectra are complex because of overlap of the multiply charged ions of the oligomer distribution, causing current computer transform programs to fail. However, it is possible to determine the origin of the multiply charged ions, making it feasible to write a program designed to transform ESI polymer spectra. To assess the value of such a program for polymer analysis, isolated monodisperse methyl methacrylate (MMA) oligomers (25 and 50 repeat units) were used to determine molar signal response and propensity for fragmentation. The sum of the peak areas for the multiply charged MMA 50-mer was found to be only about 66% of the summed peak areas for the 25-mer for the same molar concentration. However, conversion of the multiply charged peak areas to the singly charged representations, with peak area compression taken into account, gave equal signal responses for the 25-and 50-mers. Signal response variations due to the tacticity of the MMA oligomers were not observed. Fragmentation of the MMA oligomers also was shown not to occur under normal ESI conditions. Therefore, transformation of the polymer spectra to the singly charged molecular ion distribution should allow accurate calculation of average molecular weights, polydispersity, end group mass, and repeat unit mass.     

Proton transfer reaction studies of multiply charged proteins in a high mass-to-charge ratio quadrupole mass spectrometer

Proton transfer reaction of multiply charged ions at high mass-to-charge ratios were explored with a low frequency quadrupole mass spectrometer. This instrument enabled a qualitative comparison of proton transfer reaction rates at low charge states for ions generated by electrospray ionization (ESI) from different solution conformations and for disulfide-linked versus disulfide-reduced protein ions. Proton transfer reactions that efficiently reduced the number of charges for ESI-generated ions to approximately the number of arginines in the polypeptide sequence were observed. No significant differences in gas-phase reaction rates were noted between different solution conformers. Differences in reaction rates between “native” and disulfide-reduced proteins were much smaller than those observed below m/z 2000 with lower proton affinity reagents or by using lower reagent concentrations. These smaller differences in reaction rates are thought to reflect the reduced electrostatic contributions from widely spaced charge sites and thus, the reduced sensitivity to an ion's three-dimensional structure or “compactness.”     

The analysis of polystyrene and polystyrene aggregates into the mega dalton mass range by cryodetection MALDI TOF MS

Mass spectra of atactic polystyrene were collected into the mega-dalton mass range with a matrix-assisted laser desorption ionization time of flight (MALDI TOF) mass spectrometer, which incorporates a cryodetector comprised of an array of 16 superconducting tunnel junctions (STJ). The STJ cryodetector, theoretically, has no loss in signal response at any mass compared with the reduced signal found at high mass when using a conventional secondary-ionization detector. Since ion detection at high m/z is one of the fundamental limitations of mass spectrometry (MS), the cryodetector was used to explore the high m/z limit of the MALDI TOF technique for the analysis of two polymer types. Mass spectra were collected for polystyrene at Mn 170, 400, 900, and 2000 kDa and polymethyl methacrylate (PMMA) at Mn 62.6 kDa and 153.7 kDa. For polystyrene, the data showed a trend toward increased aggregation and charge state with mass. The Mn 2 MDa polystyrene data revealed a peak at m/z 2.2 MegaTh and a charge state analysis revealed that these ions were primarily polystyrene aggregates with a mass of approximately 4 MDa. This aggregate assignment was possible because the cryodetector response allows for the determination of a charge state up to about four. The contribution of each charge state for a selected peak can be determined in this fashion. This analysis revealed the preferential formation of doubly charged even-numbered aggregates over odd-numbered aggregates for high molecular mass polystyrene. A potential mechanism for the aggregation process for doubly charged species is discussed.     

Fragmentation studies on the antibiotic avilamycin A using ion trap mass spectrometry

A comprehensive study on the fragmentation pattern of the antimicrobial growth promoter avilamycin A was conducted in a quadrupole ion trap mass spectrometer equipped with an electrospray ionization (ESI) source. Performing multiple-stage experiments on the deprotonated molecule (m/z 1401) and its principal product ions showed that a sequential shortening of the oligosaccharide backbone took place, which can be attributed to the localization of the negative charge in the terminal dichloroisoeverninic acid. Under (+)-ESI conditions, avilamycin A readily formed an intense sodium-cationized molecule, [M + Na](+) (m/z 1425). Structural elucidation of the second-, third- and fourth-generation fragment ions revealed that all of the structures shared a common molecular portion comprising a central monosaccharide. This observation allowed us to assign confidently the complexation site of the alkali metal cation. In addition to the monosodiated molecule, the full-scan mass spectral acquisition also yielded a less abundant disodiated molecule, [M - H + 2Na](+) (m/z 1447). Multiple-stage experiments on this ion indicated that the second sodium ion compensates for the negative charge located at either of two positions within the molecule. While deprotonation of the phenolic hydroxyl group in the dichloroisoeverninic acid moiety was suggested to be driven by charge stabilization in the aromatic ring (in analogy with the deprotonated molecule in the (-)-ESI mode), the deprotonation at an alpha-carbon of an ester side-chain substituent in the oligosaccharide part was believed to provide a stable chelation-like coordination site for the cation.     

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

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

Analysis of carrageenans by matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry. I. kappa-Carrageenans.

Matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) mass spectra of small kappa-carrageenans are reported and discussed. MALDI spectra can be obtained in both positive and negative ion mode. In the absence of extraneous metal ions, positive ions are formed by the attachment of one Na(+) ion to the carrageenan, whereas for negative ions one Na(+) ion is detached from the sulfate group. Multiply charged species are not observed in MALDI. Intense ESI spectra can be obtained in negative ion mode and now multiply charged species are seen. Alkali exchange experiments show that in these small carrageenan anions one, but only one, alkali metal ion is bound in a bidentate coordination with two ionic sulfate groups. G2-type ab initio calculations on model ions HO(-) [M(+)] (-)OH (M = Li, Na, K, Cs), as well as arguments based on a simple Coulombic interaction model, show that the bidentate stabilization energy drops rapidly as the size of the alkali cation increases. Exchange of Na(+) with Li(+) leads to expulsion of the Na(+) ion generating, in ESI, intense multiply charged anions. An attempt is made to rationalize this behavior in terms of hydration effects.     

ESIprot: a universal tool for charge state determination and molecular weight calculation of proteins from electrospray ionization mass spectrometry data

Electrospray ionization (ESI) ion trap mass spectrometers with relatively low resolution are frequently used for the analysis of natural products and peptides. Although ESI spectra of multiply charged protein molecules also can be measured on this type of devices, only average spectra are produced for the majority of naturally occurring proteins. Evaluating such ESI protein spectra would provide valuable information about the native state of investigated proteins. However, no suitable and freely available software could be found which allows the charge state determination and molecular weight calculation of single proteins from average ESI‐MS data. Therefore, an algorithm based on standard deviation optimization (scatter minimization) was implemented for the analysis of protein ESI‐MS data. The resulting software ESIprot was tested with ESI‐MS data of six intact reference proteins between 12.4 and 66.7 kDa. In all cases, the correct charge states could be determined. The obtained absolute mass errors were in a range between ?0.2 and 1.2 Da, the relative errors below 30 ppm. The possible mass accuracy allows for valid conclusions about the actual condition of proteins. Moreover, the ESIprot algorithm demonstrates an extraordinary robustness and allows spectral interpretation from as little as two peaks, given sufficient quality of the provided m/z data, without the necessity for peak intensity data. ESIprot is independent from the raw data format and the computer platform, making it a versatile tool for mass spectrometrists. The program code was released under the open‐source GPLv3 license to support future developments of mass spectrometry software. Copyright © 2010 John Wiley & Sons, Ltd.     

Performance of the computer algorithm COMPLX for the detection of protein complexes in the mass spectra of simulated biological mixtures

The performance of the algorithm COMPLX for detecting protein-ligand or other macromolecular complexes has been tested for highly complex data sets. These data contain m/z values for ions of proteins of the SWISS-PROT database within simulated biological mixtures where each component shares a similar molecular weight and/or isoelectric point (pI). As many as 1600 ion signals were entered to challenge the algorithm to identify ion signals associated with a single protein complex that has been ionised and detected within a mass spectrometer. Despite the complexity of such data sets, the algorithm is shown to be able to identify the presence of individual bimolecular complexes. The output data can be re-evaluated by the user as necessary in light of any additional information that is known concerning the nature of predicted associations, as well as the quality of the data-set in terms of errors in m/z values as a direct consequence of the mass calibration or resolution achieved. The data presented illustrates that the best results are obtained when output results are ranked according to the largest continuous series of ion pairs detected for a protein or macromolecule and its complex for which the ligand mass is assigned the lowest mass error.     

Polarization induced electrospray ionization mass spectrometry for the analysis of liquid,viscous and solid samples

In this study, a polarization‐induced electrospray ionization mass spectrometry (ESI‐MS) was developed. A micro‐sized sample droplet was deposited on a naturally available dielectric substrate such as a fruit or a stone, and then placed close to (~2 mm) the orifice of a mass spectrometer applied with a high voltage. Taylor cone was observed from the sample droplet, and a spray emitted from the cone apex was generated. The analyte ion signals derived from the droplet were obtained by the mass spectrometer. The ionization process is similar to that in ESI although no direct electric contact was applied on the sample site. The sample droplet polarized by the high electric field provided by the mass spectrometer initiated the ionization process. The dielectric sample loading substrate facilitated further the polarization process, resulting in the formation of Taylor cone. The mass spectral profiles obtained via this approach resembled those obtained using ESI‐MS. Multiply charged ions dominated the mass spectra of peptides and proteins, whereas singly charged ions dominated the mass spectra of small molecules such as amino acids and small organic molecules. In addition to liquid samples, this approach can be used for the analysis of solid and viscous samples. A small droplet containing suitable solvent (5–10 µl) was directly deposited on the surface of the solid (or viscous) sample, placed close the orifice of mass spectrometer applied with a high voltage. Taylor cone derived from the droplet was immediately formed followed by electrospray processes to generate gas‐phase ions for MS analysis. Analyte ions derived from the main ingredients of pharmaceutical tablets and viscous ointment can be extracted into the solvent droplet in situ and observed using a mass spectrometer. Copyright © 2015 John Wiley & Sons, Ltd.