Charge competition and the linear dynamic range of detection in electrospray ionization mass spectrometry

An experimental investigation and theoretical analysis are reported on charge competition in electrospray ionization (ESI) and its effects on the linear dynamic range of ESI mass spectrometric (MS) measurements. The experiments confirmed the expected increase of MS sensitivities as the ESI flow rate decreases. However, different compounds show somewhat different mass spectral peak intensities even at the lowest flow rates, at the same concentration and electrospray operating conditions. MS response for each compound solution shows good linearity at lower concentrations and levels off at high concentration, consistent with analyte "saturation" in the ESI process. The extent of charge competition leading to saturation in the ESI process is consistent with the relative magnitude of excess charge in the electrospray compared to the total number of analyte molecules in the solution. This ESI capacity model allows one to predict the sample concentration limits for charge competition and the on-set of ionization suppression effects, as well as the linear dynamic range for ESI-MS. The implications for quantitative MS analysis and possibilities for effectively extending the dynamic range of ESI measurements are discussed.     

Plate-model applied on concentration effects in size exclusion chromatography

Abstract

A universal procedure of modeling chromatography separation, based on the plate model, has been developed. On each plate, the equilibrium between the analyte in the mobile and stationary phases is established. This equilibrium may be described by the concentration-dependent partition coefficient. The establishment of the equilibrium on each plate is followed by the displacement of the analyte in the mobile phase by one plate, establishment of new equilibrium, etc. The result is a series of elution curves with positions of maxima dependent on injected mass of the polymer. The procedure has been tested on modeling concentration effects on the basis of dependences of the partition coefficient on local concentrations on each plate. By comparison with the experiment, the slope of the concentration dependence of the partition coefficient of polystyrene in benzene was estimated. It is in qualitative agreement with the literature data obtained by a computer simulation of chromatography separation of polymers in good solvents.     

Analyte response in laser ablation inductively coupled plasma mass spectrometry

The dependence of analyte sensitivity and vaporization efficiency on the operating parameters of an inductively coupled plasma mass spectrometer (ICPMS) was investigated for a wide range of elements in aerosols, produced by laser ablation of silicate glass. The ion signals were recorded for different carrier gas flow rates at different plasma power for two different laser ablation systems and carrier gases. Differences in atomization efficiency and analyte sensitivity are significant for the two gases and the particle size distribution of the aerosol. Vaporization of the aerosol is enhanced when helium is used, which is attributed to a better energy-transfer from the plasma to the central channel of the ICP and a higher diffusion rate of the vaporized material. This minimizes elemental fractionation caused by sequential evaporation and reduces diffusion losses in the ICP. The sensitivity change with carrier gas flow variation is dependent on m/z of the analyte ion and the chemical properties of the element. Elements with high vaporization temperatures reach a maximum at lower gas flow rates than easily vaporized elements. The sensitivity change is furthermore dependent on m/z of the analyte ion, due to the mass dependence of the ion kinetic energies. The mass response curve of the ICPMS is thus not only a result of space charge effects in the ion optics but is also affected by radial diffusion of analyte ions and the mismatch between their kinetic energy after expansion in the vacuum interface and the ion optic settings.     

Evaluation of the interrelationship between mass resolving power and mass error tolerances for targeted bioanalysis using liquid chromatography coupled to high‐resolution mass spectrometry

The determination of acceptable mass error tolerances for high‐resolution mass spectrometry based signals has been evaluated in a comprehensive way. This was achieved by using a technical approach which is based on the post‐column infusion of an analyte containing solution. This well‐known experimental setup was not used to spot signal suppression regions of a particular analyte, but to spot regions of the chromatogram where a systematic mass drift of the analyte ion can be observed (isobaric interference plot). Not the changing signal intensity but the stability of the measured analyte mass was observed. A wide range of different analytes in combinations with potentially interfering matrices has been evaluated. Furthermore, different mass resolving power settings were evaluated. Isobaric interferences between matrix compounds and analytes were common at mass resolving powers <50 000 full width at half maximum. The proposed post‐column infusion technique is a useful tool for the determination of the assay and matrix‐specific mass error tolerances. It aims to ensure the highest possible selectivity, at the same time preventing the encounter of detrimental mass error related peak deformations as well as false negative findings. Unlike conventional matrix spiking approaches, isobaric interference plots provide information of potential interferences across the whole chromatographic time range. This becomes relevant when there is a relative retention time shift between the analyte and potential interfering matrix compounds. Furthermore, the described setup can be used to study how the mass accuracy of any mass spectrometer is affected by a widely varying total ion current. Copyright © 2012 John Wiley & Sons, Ltd.     

Implementation of dipolar direct current (DDC) collision-induced dissociation in storage and transmission modes on a quadrupole/time-of-flight tandem mass spectrometer

Means for effecting dipolar direct current collision-induced dissociation (DDC CID) on a quadrupole/time-of-flight in a mass spectrometer have been implemented for the broadband dissociation of a wide range of analyte ions. The DDC fragmentation method in electrodynamic storage and transmission devices provides a means for inducing fragmentation of ions over a large mass-to-charge range simultaneously. It can be effected within an ion storage step in a quadrupole collision cell that is operated as a linear ion trap or as ions are continuously transmitted through the collision cell. A DDC potential is applied across one pair of rods in the quadrupole collision cell of a QqTOF hybrid mass spectrometer to effect fragmentation. In this study, ions derived from a small drug molecule, a model peptide, a small protein, and an oligonucleotide were subjected to the DDC CID method in either an ion trapping or an ion transmission mode (or both). Several key experimental parameters that affect DDC CID results, such as time, voltage, low mass cutoff, and bath gas pressure, are illustrated with protonated leucine enkephalin. The DDC CID dissociation method gives a readily tunable, broadband tool for probing the primary structures of a wide range of analyte ions. The method provides an alternative to the narrow resonance conditions of conventional ion trap CID and it can access more extensive sequential fragmentation, depending upon conditions. The DDC CID approach constitutes a collision analog to infrared multiphoton dissociation (IRMPD).     

Effects of salt concentration on analyte response using electrospray ionization mass spectrometry

The effect of salt concentration on analyte response using electrospray ionization mass spectrometry (ESI-MS) was measured and compared to that predicted by Enke's equilibrium partitioning model. The model predicts that analyte response will be proportional to concentration and that the response factor will decrease with increasing electrolyte concentration. The measured analyte response is proportional to concentration over four orders of magnitude when the electrolyte concentration is below 10(-3) M, as the model predicts. The concentration of excess charge ([Q]) generated by the ESI process increases significantly at 10(-3) M ionic concentration, but the response factor decreases at this concentration. Changes in shape of the spray that cause a loss of ion transmission efficiency may be the basis for the decrease in response. An increase in the analyte response factor with increasing electrolyte concentration is observed for electrolyte concentrations below 10(-3) M. An explanation for this based on the electrical double layer is proposed.     

Mechanism of production of ions in electrospray mass spectrometry

A mechanism for the production of multiply charged molecular ion species in electrospray mass spectrometry (ES-MS) is still required. A concise discussion of a recently published ionic solution equilibrium model offering a partial mechanism is presented. That publication proposed that the ion abundance–charge profile could be fitted by one or a series of superimposed Gaussian functions, in accord with a solution equilibrium model. It is shown that indeed a simulated mass spectrum based on a solution odel can compare well with the observed spectrum. However, some new and recently published experimental evidence is presented which shows that the ES mass spectra of many proteins give rise to multiply charged molecular ions which carry higher charges than those calculated by the model. Further, the ion abundance–charge profile is very sensitive to some experimental parameters, e.g. cone voltage; it does not necessarily reflect the solution or gaseous ion populations in the mass spectrometer source. Therefore, the concept that gaseous multi-charged ions originate from equivalently charged solvated ions in electrically neutral solution must be treated with caution.     

Application of the headspace analysis to systems with unknown partition coefficients

Summary Possibilities for the quantitative determination of volatile constituents in solutions by headspace analysis are investigated in cases where the partition coefficients (K) depend on the composition of the object under study. The timeconsuming work of revealing and taking into account the effect of the analytes and the other present constituents on the magnitude of K can be eliminated if simultaneously with determining the analyte content in the equilibrium gas one also measures the partition coefficients characteristic for the solution under study. These quantities can be obtained by repeated analysis of the equilibrium gas after partial removal of the volatile solution constituents. At small K (0.1 to 10) the required change of analyte concentration in the solution can be attained by single replacement of the equilibrium gas by pure gas, and at large K, by passing a known volume of pure gas through the solution under study under equilibrium conditions. The paper establishes the range of applicability of each version, provides the necessary equations and presents the results of an experimental check in the case of determining alcohol admixtures in water solutions.     

Reduction of chemical noise in electrospray ionization mass spectrometry by supplemental IR activation

Supplemental infrared (IR) activation was applied to reduce background chemical noise and increase analyte ion signal in a linear ion trap mass spectrometer. Peptides, proteins, and small molecules were all introduced by electrospray ionization, and when regions of chemical noise were isolated and subjected to IR irradiation, protonated analyte molecules were observed in the product ion mass spectra. By isolating the entire mass range (e.g., m/z 400–2000) and then irradiating all ions in the trap, supplemental IR activation increased the signal of singly protonated peptides by almost 70% and by 40%–55% for the lower charge states of cytochrome c. This increase in analyte ion signal was less dramatic for the higher charge states of peptides and proteins. The chemical noise present in the mass spectra is attributed to incomplete desolvation of the electrospray, as the abundance of the protonated peptides observed upon supplemental IR activation of the chemical noise decreased with higher inlet capillary temperatures. Collision activation was not as effective for desolvating the ions present in the chemical noise.     

An extended description of the effect of detergent monomers on migration in micellar electrokinetic chromatography

Three equilibria determine the interaction of a neutral analyte with the detergent in micellar electrokinetic chromatography and therefore its migration: (i) that of the free analyte in the aqueous phase with the micelle, (ii) its association with free detergent monomers in the aqueous phase, and (iii) the partition of the associate of analyte and monomer between the aqueous solution and the micelle. For the first equilibrium, non-stoichiometric partitioning between two phases is preferred in the present work over the assumption of complex formation between one molecule of the analyte with one micelle. The second equilibrium is described by the formation of a 1:1 associate of the analyte and monomer. In this paper, thirdly an additional equilibrium is introduced, namely, the distribution of the analyte-monomer associate between the aqueous and the micelle phase; it is expressed by the according partition coefficient. The three equilibrium constants are interrelated. Mobility data for a lipophilic fluorescent compound and a series of n-alkylphenones (differing in chain length) were measured as a function of the SDS concentration below and above the critical micellar concentration. Curve fitting enabled the derivation of the equilibrium constants. It was found that the association constants of the analytes with the detergent monomers are between 2 and 75 M(-1). Interestingly, the partition coefficient of the analyte-monomer associate between the aqueous and micellar phase is by a factor of 5-200 larger than that of the free analyte.     

Improved detection of low vapor pressure compounds in air by serial combination of single-sided membrane introduction with fiber introduction mass spectrometry (SS-MIMS-FIMS)

The use of two methods in tandem, single-sided membrane introduction mass spectrometry (SS-MIMS) and fiber introduction mass spectrometry (FIMS), is presented as a technique for field analysis. The combined SS-MIMS-FIMS technique was employed in both a modified commercial mass spectrometer and a miniature mass spectrometer for the selective preconcentration of the explosive simulant o-nitrotoluene (ONT) and the chemical warfare agent simulant, methyl salicylate (MeS), in air. A home-built FIMS inlet was fabricated to allow introduction of the solid-phase microextraction (SPME) fiber into the mass spectrometer chamber and subsequent desorption of the trapped compounds using resistive heating. The SS-MIMS preconcentration system was also home-built from commercial vacuum parts. Optimization experiments were done separately for each preconcentration system to achieve the best extraction conditions prior to use of the two techniques in combination. Improved limits of detection, in the low ppb range, were observed for the combination compared to FIMS alone, using several SS-MIMS preconcentration cycles. The SS-MIMS-FIMS response for both instruments was found to be linear over the range 50 to 800 ppb. Other parameters studied were absorption time profiles, effects of sample flow rate, desorption temperature, fiber background, memory effects, and membrane fatigue. This simple, sensitive, accurate, robust, selective, and rapid sample preconcentration and introduction technique shows promise for field analysis of low vapor pressure compounds, where analyte concentrations will be extremely low and the compounds are difficult to extract from a matrix like air.     

Quantitative determination of a nonpeptide antithrombotic in dog plasma by microbore high-performance liquid chromatography-tandem mass spectrometry utilizing pneumatically assisted electrospray ionization

A method has been developed and is described for the quantitative determination of a nonpeptide antithrombotic in dog plasma. The assay employs reversed phase microbore high-performance liquid chromatography in conjunction with tandem mass spectrometry utilizing pneumatically assisted electrospray ionization. The analyte and internal standard are isolated from the plasma matrix by solid-phase extraction. The mass spectrometer is operated in the positive ion multiple reaction monitoring mode and is set to detect the presence of a precursor-product ion pair for both the analyte and internal standard to generate product ion chromatograms for both species. The analyte is quantified by using weighted least-squares regression of the peak height ratio of drug:internal standard. The method provides linear response for plasma concentrations ranging from 5 ng/mL (25 pg on-column) to 2500 ng/mL. Statistical evaluation and examples of authentic sample assays are also presented.     

Remote mass spectrometric sampling of electrospray- and desorption electrospray-generated ions using an air ejector

A commercial air ejector was coupled to an electrospray ionization linear ion trap mass spectrometer (LTQ) to transport remotely generated ions from both electrospray (ESI) and desorption electrospray ionization (DESI) sources. We demonstrate the remote analysis of a series of analyte ions that range from small molecules and polymers to polypeptides using the AE-LTQ interface. The details of the ESI-AE-LTQ and DESI-AE-LTQ experimental configurations are described and preliminary mass spectrometric data are presented.     

A brief overview of the present status of the mechanisms involved in electrospray mass spectrometry

A brief account of the mechanisms by which ions in solution are converted to ions in the gas phase is given on the basis of information available in the literature and the four companion articles on electrospray mass spectrometry (ESMS) in this issue. The following stages/phenomena are described: (a) production of the charged droplets at the ES capillary tip; (b) evolution of the charged droplets due to solvent evaporation and droplet fission caused by Coulombic repulsion of the charges on the droplets; production of the gas phase ion from very small charged droplets by the charge residue model (CRM) or the ion evaporation method (IEM); (c) dependence of the sensitivity in ESMS on the chemical nature of the analyte and its concentration as well as on the concentration of other electrolytes that are present in the solution; qualitative predictions on the sensitivity of the analyte based on the surface activity of the analyte ions; (d) relationship between ions produced in the gas phase and original ions present in the solution; and (e) globular proteins. Much of the information presented in (a)-(e) has been available for some time in the literature. However some significant advances are relatively recent. Recent results by de la Mora and co-workers, including their contribution in this Special Feature, provide very strong evidence that small ions (in distinction from macroions such as bio-macroions) are produced by IEM. On the other hand, macroions and particularly the polyprotonated globular proteins are produced by CRM. Also noteworthy is the development of an equation by Enke with which the observed relative ion signal intensities of the gas-phase ions produced can be predicted on the basis of the ion concentration in solution over a wide concentration range. The recognition that the sensitivity of organic analyte ions can be qualitatively predicted on the basis of the hydrophilicity or hydrophobicity of the part of the molecule that is not part of the charged (ionic) group and affects the surface activity of the ionic species is also noteworthy and a very useful relatively recent development.     

Coupling a particle beam interface directly to a quadrupole ion trap mass spectrometer

A particle beam interface has been coupled to a quadrupole ion trap mass spectrometer. The system allows the collection of electron ionization mass spectra from analyte in solution. The interface incorporates a pneumatic nebulizer, a heated desolvation chamber, and a three-stage separator region. Additional helium, for improved performance, is added through stage 3. The particles formed in the interface are separated from solvent molecules and are transferred directly to the ion trap where they are expected to collide with the hot hyperbolic surface of the end cap. The end cap serves both as a heated target used to vaporize the particles and as an ion-trapping electrode. Mass analysis is achieved with the mass-selective instability scan supplemented with resonance ejection. Electron ionization spectra from 100 ng of caffeine [molecular weight (MW) = 1941; 1-naphthalenol methylcarbamate (carbaryl) (MW = 2011, 17α-hydroxyprogesterone (MW = 330), and reserpine (MW = 608) are shown using sampling by a segmented flow analysis. Some charge exchange is evident with methanol as well as self-chemical ionization at higher analyte levels. The interface shows a nonlinear caffeine calibration curve for analyte amounts below 30 ng and a more linear response at higher amounts. Caffeine was detected at 25 pmol (5 ng), with a signal-to-noise ratio of 50, 20-μL loop, full scan.     

Transmission mode ion/ion reactions in the radiofrequency‐only ion guide of hybrid tandem mass spectrometers

Transmission mode ion/ion reactions have been performed within the first quadrupole, the Q0 radiofrequency (RF)‐only quadrupole, of two types of hybrid tandem mass spectrometers (viz., triple quadrupole/linear ion trap and QqTOF instruments). These transmission mode reactions involved the storage of either the reagent species and the transmission of the analyte species through the Q0 quadrupole for charge inversion reactions or the storage of the analyte ions and transmission of the reagent ions as in charge reduction experiments. A key advantage to the use of transmission mode ion/ion reactions is that they do not require any instrument hardware modifications to provide interactions of oppositely charged ions and can be implemented in any instrument that contains a quadrupole or linear ion trap. The focus of this work was to investigate the potential of using the RF‐only quadrupole ion guide positioned prior to the first mass‐resolving element in a tandem mass spectrometer for ion/ion reactions. Two types of exemplary experiments have been demonstrated. One involved a charge inversion reaction and the other involved a charge reduction reaction in conjunction with ion parking. Ion/ion reactions proved to be readily implemented in Q0 thereby adding significantly greater experimental flexibility in the use of ion/ion reaction experiments with hybrid tandem mass spectrometers. Copyright © 2009 John Wiley & Sons, Ltd.     

Calibration laws based on multiple linear regression applied to matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry

Operation of any mass spectrometer requires implementation of mass calibration laws to translate experimentally measured physical quantities into a m/z range. While internal calibration in Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) offers several attractive features, including exposure of calibrant and analyte ions to identical experimental conditions (e.g. space charge), external calibration affords simpler pulse sequences and higher throughput. The automatic gain control method used in hybrid linear trap quadrupole (LTQ) FT-ICR-MS to consistently obtain the same ion population is not readily amenable to matrix-assisted laser desorption/ionization (MALDI) FT-ICR-MS, due to the heterogeneous nature and poor spot-to-spot reproducibility of MALDI. This can be compensated for by taking external calibration laws into account that consider magnetic and electric fields, as well as relative and total ion abundances. Herein, an evaluation of external mass calibration laws applied to MALDI-FT-ICR-MS is performed to achieve higher mass measurement accuracy (MMA).     

Surface effects and electrochemical cell capacitance in desorption electrospray ionization

Time resolved measurements show that during a desorption electrospray ionization (DESI) experiment, the current initially rises sharply, followed by an exponential decrease to a relatively steady current. When the high voltage on the spray emitter is switched off, the current drops to negative values, suggesting that the direction of current flow in the equivalent DESI circuit is reversed. These data demonstrate that the DESI source behaves as a dc capacitor and that the addition of a surface between the sprayer and the counter electrode in DESI introduces a new electrically active element into the system. The charging and discharging behavior was observed using different surfaces and it could be seen both by making current measurements on a plate at the entrance to the mass spectrometer as well as by measuring ion current in the linear ion trap within the vacuum system of the mass spectrometer. The magnitude of the steady state current obtained without analyte present on the surface is different for different surface materials, and different capacitor time constants of the equivalent RC circuits were calculated for different DESI surfaces. The PTFE surface has by far the greatest time constant and is also able to produce the highest DESI currents. Surface properties play a crucial role in charge transfer during DESI in addition to the effects of the chemical properties of the analyte. It is suggested that surface energy (wettability) is an important factor controlling droplet behavior on the surface. The experimental data are correlated with critical surface tension values of different materials. It is proposed, based on the results presented, that super-hydrophobic materials with extremely high contact angles have the potential to be excellent DESI substrates. It is also demonstrated, using the example of the neurotransmitter dopamine, that the surface charge that develops during a DESI-MS experiment can cause electrochemical oxidation of the analyte.     

Laserspray ionization (LSI) ion mobility spectrometry (IMS) mass spectrometry

A simple device is described for desolvation of highly charged matrix/analyte clusters produced by laser ablation leading to multiply charged ions that are analyzed by ion mobility spectrometry-mass spectrometry. Thus, for example, highly charged ions of ubiquitin and lysozyme are cleanly separated in the gas phase according to size and mass (shape and molecular weight) as well as charge using Tri-Wave ion mobility technology coupled to mass spectrometry. This contribution confirms the mechanistic argument that desolvation is necessary to produce multiply charged matrix-assisted laser desorption/ionization (MALDI) ions and points to how these ions can be routinely formed on any atmospheric pressure mass spectrometer.