Transmission mode desorption electrospray ionization

A new mode of operation for desorption electrospray ionization (DESI) analysis of liquids or solid residues from evaporated solvents is presented. Unlike traditional DESI, the electrospray is not deflected off of a surface but instead is transmitted through a sampling mesh at a 0° angle between the electrospray tip, sample mesh, and capillary inlet of a mass spectrometer. In this configuration, deposited samples can be analyzed rapidly without rigorous optimization of spray distances or angles and without the preparation time associated with solvent evaporation. The new transmission mode desorption electrospray ionization (TM-DESI) technique is not applicable to bulk materials, but instead is a method designed to simplify the sample preparation process for liquid samples and sample extracts. The technique can reduce analysis time to seconds while consuming only microliters of sample. The results presented summarize the optimization of the technique, highlight key figures of merit for several model compounds, and illustrate potential applications to high throughput screening of liquid mixtures in both extraction solvents and biological matrices.     

Redox transformations in desorption electrospray ionization

Redox changes occur in some circumstances when organic compounds are analyzed by desorption electrospray ionization mass spectrometry (DESI-MS). However, these processes are limited in scope and the data presented here suggest that there are only limited analogies between the redox behavior in DESI and the well-known solution-phase electrochemical processes in standard electrospray ionization (ESI). Positive and negative ion modes were both investigated and there is a striking asymmetry between the incidence of oxidation and of reduction. Although in negative ion mode DESI experiments, some aromatic compounds were ionized as odd-electron anion radicals, examples of full reduction were not found. By contrast, oxidation in the form of oxygen atom addition (or multiple oxygen atom additions) was observed for several different analytes. These oxidation reactions point to chemically rather than electrochemically controlled processes. Data is presented which suggests that oxidation is predominantly caused by reaction with discharge-created gas-phase radicals. The fact that common reducing agents and known antioxidants such as ascorbic acid are not modified, while a saturated organic acid like stearic acid is oxidized in DESI, indicates that the usual electrochemical redox reactions are not significant but that redox chemistry can be induced under special experimental conditions.     

Scanning electron microscopic imaging of surface effects in desorption and nano-desorption electrospray ionization

Scanning electron microscopy was used to investigate rivulets that are formed on the analyzed surface during desorption electrospray ionization (DESI) experiment. Ferromagnetic nanoparticles added to the spray solvent in a form of colloid solution functioned as an additional surface probe. The existence of the rivulets was confirmed on glass and newly demonstrated on two different types of porous polytetrafluoroethylene (PTFE). The results show that in standard DESI set-up the rivulets are arranged into very regular shapes. Same rivulets were obtained in DESI experiments without high voltage on the sprayer. However, no such rivulets or any other regular patterns were found on a surface in nano-DESI (nanospray DESI without the carrier nebulizing gas) experiments. This indicates that symmetrical rivulets are created by the hydrodynamical rather than electrostatic forces. It was also demonstrated that blocking the rivulets by a simple physical barrier did not influence known surface charging effects.     

Infrared laser-assisted desorption electrospray ionization mass spectrometry

We have used an infrared laser for desorption of material and ionization by interaction with electrosprayed solvent. Infrared laser-assisted desorption electrospray ionization (IR LADESI) mass spectrometry was used for the direct analysis of water-containing samples under ambient conditions. An ion trap mass spectrometer was modified to include a pulsed Er:YAG laser at 2.94 microm wavelength coupled into a germanium oxide optical fiber for desorption at atmospheric pressure and a nanoelectrospray source for ionization. Analytes in aqueous solution were placed on a stainless steel target and irradiated with the pulsed IR laser. Material desorbed and ablated from the target was ionized by a continuous stream of charged droplets from the electrosprayed solvent. Peptide and protein samples analyzed using this method yield mass spectra similar to those obtained by conventional electrospray. Blood and urine were analyzed without sample pretreatment to demonstrate the capability of IR LADESI for direct analysis of biological fluids. Pharmaceutical products were also directly analyzed. Finally, the role of water as a matrix in the IR LADESI process is discussed.     

Data quality in tissue analysis using desorption electrospray ionization

There has been a recent surge in applications of mass spectrometry (MS) to tissue analysis, particularly lipid-based tissue imaging using ambient ionization techniques. This recent growth highlights the need to examine the effects of sample handling, storage conditions, and experimental protocols on the quality of the data obtained. Variables such as time before freezing after organ removal, storage time at −80 °C, time stored at room temperature, heating, and freeze/thaw cycles were investigated for their effect on the data quality obtained in desorption electrospray ionization (DESI)-MS using mouse brain. In addition, analytical variables such as tissue thickness, drying times, and instrumental conditions were also examined for their impact on DESI-MS data. While no immediate changes were noted in the DESI-MS lipid profiles of the mouse brain tissue after spending 1 h at room temperature when compared to being frozen immediately following removal, minor changes were noted between the tissue samples after 7 months of storage at −80 °C. In tissue sections stored at room temperature, degradation was noted in 24 h by the appearance of fatty acid dimers, which are indicative of high fatty acid concentrations, while in contrast, those sections stored at −80 °C for 7 months showed no significant degradation. Tissue sections were also subjected to up to six freeze/thaw cycles and showed increasing degradation following each cycle. In addition, tissue pieces were subjected to 50 °C temperatures and analyzed at specific time points. In as little as 2 h, degradation was observed in the form of increased fatty acid dimer formation, indicating that enzymatic processes forming free fatty acids were still active in the tissue. We have associated these dimers with high concentrations of free fatty acids present in the tissue during DESI-MS experiments. Analytical variables such as tissue thickness and time left to dry under nitrogen were also investigated, with no change in the resulting profiles at thickness from 10 to 25 μm and with optimal signal obtained after just 20 min in the dessicator. Experimental conditions such as source parameters, spray solvents, and sample surfaces are all shown to impact the quality of the data. Inter-section (relative standard deviation (%RSD), 0.44–7.2%) and intra-sample (%RSD, 4.0–8.0%) reproducibility data show the high quality information DESI-MS provides. Overall, the many variables investigated here showed DESI-MS to be a robust technique, with sample storage conditions having the most effect on the data obtained, and with unacceptable sample degradation occurring during room temperature storage.     

Improved imaging resolution in desorption electrospray ionization mass spectrometry

The imaging resolution of desorption electrospray ionization mass spectrometry (DESI-MS) was investigated using printed patterns on paper and thin-layer chromatography (TLC) plate surfaces. Resolution approaching 40 microm was achieved with a typical DESI-MS setup, which is approximately 5 times better than the best resolution reported previously. This improvement was accomplished with careful control of operational parameters (particularly spray tip-to-surface distance, solvent flow rate, and spacing of lane scans). In addition, an appropriately strong analyte/surface interaction and uniform surface texture on the size scale no larger than the desired imaging resolution were required to achieve this resolution. Overall, conditions providing the smallest possible effective desorption/ionization area in the DESI impact plume region and minimizing the analyte redistribution on the surface during analysis led to improved DESI-MS imaging resolution.     

Characterization of polyethylenimine by electrospray ionization and matrix-assisted laser desorption/ionization

Branched polyethylenimines (PEIs) with lower average molecular weights (600, 1200 and 1800 Da) have been studied by Electrospray Ionization (ESI) and Matrix‐Assisted Laser Desorption/Ionization (MALDI) mass spectrometry. In both, ESI and MALDI mass spectra, the main distribution arises from protonated PEI oligomers with NH₂ end groups, [PEI + H]⁺, which are observed at m/z 43n + 18. A trace of sodium contamination in the PEI samples results in the presence of a series that appears at m/z 43n + 40 [PEI + Na]⁺. However, only the MALDI mass spectra show a [PEI + K]⁺ series at m/z 43n + 56, because of matrix contamination with potassium, and a series generated by condensation of the matrix with PEI at m/z 43n + 30. Collisionally activated dissociation tandem mass spectrometry (CAD (MS/MS)) of protonated PEI oligomers is shown to yield three fragment ion series b_n, and K_n. The experiments have demonstrated the capabilities of these mass spectrometry techniques, along with CAD MS/MS to detect and characterize such polar synthetic polymers. Copyright © 2011 John Wiley & Sons, Ltd.     

Investigation of some biologically relevant redox reactions using electrochemical mass spectrometry interfaced by desorption electrospray ionization

Recently we have shown that, as a versatile ionization technique, desorption electrospray ionization (DESI) can serve as a useful interface to combine electrochemistry (EC) with mass spectrometry (MS). In this study, the EC/DESI-MS method has been further applied to investigate some aqueous phase redox reactions of biological significance, including the reduction of peptide disulfide bonds and nitroaromatics as well as the oxidation of phenothiazines. It was found that knotted/enclosed disulfide bonds in the peptides apamin and endothelin could be electrochemically cleaved. Subsequent tandem MS analysis of the resulting reduced peptide ions using collision-induced dissociation (CID) and electron-capture dissociation (ECD) gave rise to extensive fragment ions, providing a fast protocol for sequencing peptides with complicated disulfide bond linkages. Flunitrazepam and clonazepam, a class of nitroaromatic drugs, are known to undergo reduction into amines which was proposed to involve nitroso and N-hydroxyl intermediates. Now in this study, these corresponding intermediate ions were successfully intercepted and their structures were confirmed by CID. This provides mass spectrometric evidence for the mechanism of the nitro to amine conversion process during nitroreduction, an important redox reaction involved in carcinogenesis. In addition, the well-known oxidation reaction of chlorpromazine was also examined. The putative transient one-electron transfer product, the chlorpromazine radical cation (m/z 318), was captured by MS, for the first time, and its structure was also verified by CID. In addition to these observations, some features of the DESI-interfaced electrochemical mass spectrometry were discussed, such as simple instrumentation and the lack of background signal. These results further demonstrate the feasibility of EC/DESI-MS for the study of the biology-relevant redox chemistry and would find applications in proteomics and drug development research.     

Investigation on the role of pneumatic aspects in electrospray, desorption electrospray surface ionization and surface activated chemical ionization

This review reports the results of some studies carried out by us on the role of pneumatic aspects in electrospray and desorption electrospray surface ionization, with the aim to propose some relevant aspects of the mechanisms involved in these ionization methods. Electrospray ion sources, with the exception of the nano- electrospray source, operate with the concurrent action of a strong electrical field and a supplementary coaxial gas flow. The electrical field is responsible for electrospraying of the analyte solution but the use of a coaxial gas flow leads to a significant increase of the analyte signal and allows the use of higher solution flows. However, by employing capillary voltages much lower than those necessary to activate the electrospray phenomenon, analyte ions are still observed and this indicates that different mechanisms must be operative for ion production. Under these conditions, ion generation could take place from the neutral pneumatically sprayed droplet by field-induced droplet ionization. Also in the case of desorption electrospray ionization (DESI), and without any voltage on the spraying capillary as well as on the surface of interest, ions of analytes present on the surface become detectable and this shows that desorption/ionization of analytes occurs by neutral droplets impinging the surface. Consequently, the pneumatic effect of the impinging droplets plays a relevant role, and for these reasons the method has been called pneumatic assisted desorption (PAD). Some analogies existing between PAD and surface activated chemical ionization (SACI), based on the insertion of a metallic surface inside an atmospheric pressure chemical ionization source operating without corona discharge, are discussed.     

Carbohydrate and steroid analysis by desorption electrospray ionization mass spectrometry

Desorption electrospray ionization mass spectrometry (DESI-MS) is applied to the analysis of carbohydrates and steroids; the detection limits are significantly improved by the addition of low concentrations of salts to the spray solvent.     

Intact skin analysis by desorption electrospray ionization mass spectrometry

In vivo skin analysis by Desorption Electrospray Ionization was characterized on healthy human volunteers by directing pneumatically assisted electrospray directly onto their fingertips. In order to eliminate the risk of electric shock, a high ohmic resistor was built into the system. Positive ion DESI-MS analysis yields low intensity spectra, while negative ion spectra feature a number of various biogenic carboxylic acids. Compounds of external origin and excreted molecules were found to have different analysis kinetics, with the exception of highly hydrophobic species. The difference was demonstrated in the case of nicotine and cotinine. Pharmacokinetic studies were performed using a rat animal model. The kinetics of the anesthetic ketamine was followed by DESI, and results were in agreement with off-line HPLC-MS blood analysis. Using a similar approach for N,N'-dimethylthiourea (DMTU), a novel method was developed for the real-time quantification of oxidative stress. DMTU was administered to the animals, and the ratio of the molecule and its oxidized form was monitored from the skin surface. The ratio was found to be highly sensitive to experimentally induced diabetes mellitus type I and angiotensin-induced chronic oxidative stress. It was concluded that the method has a number of potential applications in the fields of forensics, pharmacology and clinical chemistry.     

High-throughput quantitative analysis by desorption electrospray ionization mass spectrometry

A newly developed high-throughput desorption electrospray ionization (DESI) source was characterized in terms of its performance in quantitative analysis. A 96-sample array, containing pharmaceuticals in various matrices, was analyzed in a single run with a total analysis time of 3 min. These solution-phase samples were examined from a hydrophobic PTFE ink printed on glass. The quantitative accuracy, precision, and limit of detection (LOD) were characterized. Chemical background-free samples of propranolol (PRN) with PRN-d₇ as internal standard (IS) and carbamazepine (CBZ) with CBZ-d₁₀ as IS were examined. So were two other sample sets consisting of PRN/PRN-d₇ at varying concentration in a biological milieu of 10% urine or porcine brain total lipid extract, total lipid concentration 250 ng/μL. The background-free samples, examined in a total analysis time of 1. 5 s/sample, showed good quantitative accuracy and precision, with a relative error (RE) and relative standard deviation (RSD) generally less than 3% and 5%, respectively. The samples in urine and the lipid extract required a longer analysis time (2. 5 s/sample) and showed RSD values of around 10% for the samples in urine and 4% for the lipid extract samples and RE values of less than 3% for both sets. The LOD for PRN and CBZ when analyzed without chemical background was 10 and 30 fmol, respectively. The LOD of PRN increased to 400 fmol analyzed in 10% urine, and 200 fmol when analyzed in the brain lipid extract.     

Internal energy distributions in desorption electrospray ionization (DESI)

The internal energy distributions of typical ions generated by desorption electrospray ionization (DESI) were measured using the "survival yield" method, and compared with corresponding data for electrospray ionization (ESI) and electrosonic spray ionization (ESSI). The results show that the three ionization methods produce populations of ions having internal energy distributions of similar shapes and mean values (1.7-1.9 eV) suggesting similar phenomena, at least in the later stages of the process leading from solvated droplets to gas-phase ions. These data on energetics are consistent with the view that DESI involves "droplet pick-up" (liquid-liquid extraction) followed by ESI-like desolvation and gas-phase ion formation. The effects of various experimental parameters on the degree of fragmentation of p-methoxy-benzylpyridinium ions were compared between DESI and ESSI. The results show similar trends in the survival yields as a function of the nebulizing gas pressure, solvent flow rate, and distance from the sprayer tip to the MS inlet. These observations are consistent with the mechanism noted above and they also enable the user to exercise control over the energetics of the DESI ionization process, through manipulation of external and internal ion source parameters.     

Electrode‐assisted desorption electrospray ionization mass spectrometry

A new ion source has been developed for rapid, noncontact analysis of materials at ambient conditions. The method provides desorption of analytes under ambient conditions directly from different surfaces with little or no sample preparation. The new method, termed electrode‐assisted desorption electrospray ionization (EADESI), is on the basis of the ionization of molecules on different surfaces by highly charged droplets produced on a sharp‐edged high voltage tip, and ions produced are introduced into the mass spectrometer through a capillary. The EADESI technique can be applied to various samples including amino acids, peptides, proteins, drugs and human fluids such as urine and blood. EADESI is promising for routine analyses in different fields such as forensic, environmental and material sciences. EADESI interface can be fit to a conventional ion‐trap mass spectrometer. It can be used for various types of samples with a broad mass range. EADESI can also provide real‐time analysis which is very valuable for biomedical applications. Copyright © 2010 John Wiley & Sons, Ltd.     

Salt tolerance of desorption electrospray ionization (DESI)

The salt tolerance of desorption electrospray ionization (DESI) was systematically investigated by examining three different drug mixtures in the presence of 0, 0.2, 2, 5, 10, and 20% NaCl:KCl (1:1) from different surfaces. At physiological salt concentrations, the individual drugs in each mixture were observed in each experiment. Even at salt concentrations significantly above physiological levels, particular surfaces were effective in providing spectra that allowed the ready identification of the compounds of interest in low nanogram amounts. Salt adducts, which are observed even in the absence of added salt, could be eliminated by adding 0.1% 7 M ammonium acetate to the standard methanol:water (1:1) spray solvent. Comparison of the salt tolerance of DESI with that of electrospray ionization (ESI) demonstrated better signal/noise characteristics for DESI. The already high salt tolerance of DESI can be optimized further by appropriate choices of surface and spray solution.