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.     

Imaging of human lens lipids by desorption electrospray ionization mass spectrometry

The lipid composition of the human lens is distinct from most other tissues in that it is high in dihydrosphingomyelin and the most abundant glycerophospholipids in the lens are unusual 1-O-alkyl-ether linked phosphatidylethanolamines and phosphatidylserines. In this study, desorption electrospray ionization (DESI) mass spectrometry-imaging was used to determine the distribution of these lipids in the human lens along with other lipids including, ceramides, ceramide-1-phosphates, and lyso 1-O-alkyl ethers. To achieve this, 25 μm lens slices were mounted onto glass slides and analyzed using a linear ion-trap mass spectrometer equipped with a custom-built, 2-D automated DESI source. In contrast to other tissues that have been previously analyzed by DESI, the presence of a strong acid in the spray solvent was required to desorb lipids directly from lens tissue. Distinctive distributions were observed for [M + H]⁺ ions arising from each lipid class. Of particular interest were ionized 1-O-alkyl phosphatidylethanolamines and phosphatidylserines, PE (18:1e/18:1), and PS (18:1e/18:1), which were found in a thin ring in the outermost region of the lens. This distribution was confirmed by quantitative analysis of lenses that were sectioned into four distinct regions (outer, barrier, inner, and core), extracted and analyzed by electrospray ionization tandem mass spectrometry. DESI-imaging also revealed a complementary distribution for the structurally-related lyso 1-O-alkyl phosphatidylethanolamine, LPE (18:1e), which was localized closer to the centre of the lens. The data obtained in this study indicate that DESI-imaging is a powerful tool for determining the spatial distribution of human lens lipids.     

Rapid screening of clenbuterol in urine samples by desorption electrospray ionization tandem mass spectrometry

Rapid screening of clenbuterol in urine was performed by combining desorption electrospray ionization (DESI) and tandem mass spectrometry (MS/MS). Optimization experiments were carried out including the selection of substrates, spray solutions, nebulizing gas pressures, high-voltage power supplies and flow rates of spray solution. The limit of detection (LOD), defined as the lowest quantity that can be detected, was 5.0 pg for the pure compound. Using DESI coupled with solid-phase extraction (SPE), the linear response range was from 10 to 400 ng/mL (R(2) = 0.993) and the concentration LOD for urine sample was 2.0 ng/mL. The analysis for one spiked urine sample was achieved within 4 min. In addition to the fast analysis speed, MS/MS provided structural information for the confirmation of clenbuterol. Urine samples from different people were investigated and the recoveries were within 100 +/- 20%. The developed method can potentially be used for screening of clenbuterol in doping control.     

Direct quantification of creatinine in human urine by using isotope dilution extractive electrospray ionization tandem mass spectrometry

Urinary creatinine (CRE) is an important biomarker of renal function. Fast and accurate quantification of CRE in human urine is required by clinical research. By using isotope dilution extractive electrospray ionization tandem mass spectrometry (EESI–MS/MS) a high throughput method for direct and accurate quantification of urinary CRE was developed in this study. Under optimized conditions, the method detection limit was lower than 50 μg L⁻¹. Over the concentration range investigated (0.05–10 mg L⁻¹), the calibration curve was obtained with satisfactory linearity (R² = 0.9861), and the relative standard deviation (RSD) values for CRE and isotope-labeled CRE (CRE-d3) were 7.1–11.8% (n = 6) and 4.1–11.3% (n = 6), respectively. The isotope dilution EESI–MS/MS method was validated by analyzing six human urine samples, and the results were comparable with the conventional spectrophotometric method (based on the Jaffe reaction). Recoveries for individual urine samples were 85–111% and less than 0.3 min was taken for each measurement, indicating that the present isotope dilution EESI–MS/MS method is a promising strategy for the fast and accurate quantification of urinary CRE in clinical laboratories.     

High-throughput detection of drugs binding to proteins using desorption electrospray ionization mass spectrometry

In this paper, we present a strategy for screening drugs that bind to proteins by combining centrifugal filtration with desorption electrospray ionization mass spectrometry (DESI-MS). Membrane filtration was used to remove any unbound drugs. Then, drug–protein complexes deposited on the DESI substrate were dissociated during the DESI-MS analytical process, and the liberated drugs were measured. To validate the methodology, the screening of a series of drugs against two types of proteins was performed. Three DNA topoisomerase I (Topo I) inhibitors (camptothecin (CPT), hydroxycamptothecin (OHCPT) and 7-ethyl-10-hydroxycamptothecin (SN-38)) were screened against Topo I and the DNA-Topo I complex using DESI-MS. The results indicated that none of the inhibitors bound to Topo I, because the inhibitors had binding affinities only to the DNA-Topo I complex. Among the three drugs that bound to the DNA-Topo I complex, SN-38 had the strongest relative binding affinity, and CPT had the weakest relative binding affinity. The impact of the DESI spray solvent composition on the analysis of drug–protein complex binding was evaluated. Seven alkaloid drugs were also screened against Topo I using DESI-MS. Berberine and palmatine had good binding affinities. A screening of 21 types of drugs was carried out to determine whether the drugs bound to human serum albumin (HSA). The DESI-MS screening process could be achieved within 1.75 min. The study provides a method to qualitatively detect compounds that bind to proteins, showing great potential in drug design and screening.     

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.     

Analysis of wastewater samples by direct combination of thin-film microextraction and desorption electrospray ionization mass spectrometry

An analysis method for aqueous samples by the direct combination of C18/SCX mixed mode thin-film microextraction (TFME) and desorption electrospray ionization mass spectrometry (DESI-MS) was developed. Both techniques make analytical workflow simpler and faster, hence the combination of the two techniques enables considerably shorter analysis time compared to the traditional liquid chromatography mass spectrometry (LC-MS) approach. The method was characterized using carbamazepine and triclosan as typical examples for pharmaceuticals and personal care product (PPCP) components which draw increasing attention as wastewater-derived environmental contaminants. Both model compounds were successfully detected in real wastewater samples and their concentrations determined using external calibration with isotope labeled standards. Effects of temperature, agitation, sample volume, and exposure time were investigated in the case of spiked aqueous samples. Results were compared to those of parallel HPLC-MS determinations and good agreement was found through a three orders of magnitude wide concentration range. Serious matrix effects were observed in treated wastewater, but lower limits of detection were still found to be in the low ng L(-1) range. Using an Orbitrap mass spectrometer, the technique was found to be ideal for screening purposes and led to the detection of various different PPCP components in wastewater treatment plant effluents, including beta-blockers, nonsteroidal anti-inflammatory drugs, and UV filters.     

Rapidly switchable matrix-assisted laser desorption/ionization and electrospray quadrupole-time-of-flight mass spectrometry for protein identification

We describe a new interface for a prototype quadrupole-quadrupole-time-of-flight (TOF) mass spectrometer (Centaur, Sciex) that allows rapid switching between electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) modes of operation. Instrument performance in both modes is comparable (i.e., resolution approximately 10,000 FWHM, mass accuracy <10 ppm, sensitivity approximately 1 fmol) because the ion source is decoupled from the TOF mass analyzer by extensive gas collisions in the quadrupole stages of the instrument. The capacity to obtain side-by-side high quality ESI and MALDI mass spectra from a single proteolytic mixture greatly facilitates the identification of proteins and elucidation of their primary structures. Improved strategies for protein identification result from this ability to measure spectra using both ionization modes in the same instrument and to perform MS/MS on singly charged as well as multiply charged ions. Examples are provided to demonstrate the utility and performance of the modified instrument.     

Rapid identification of pesticides in human oral fluid for emergency management by thermal desorption electrospray ionization/mass spectrometry

Self‐poisoning with pesticides accounts for approximately one‐third of all suicides worldwide. To expedite rescue in the emergency department, it is essential to develop a point‐of‐care analytical method for rapid identification of ingested pesticides. In this study, five of the most common pesticides ingested by self‐poisoning patients in Taiwan were analyzed from oral fluid samples. Pesticide‐oral fluid mixtures were applied on a cotton swab and then transferred into methanol. A metallic probe was used to sample the methanol solution for subsequent thermal desorption‐electrospray ionization mass spectrometry analysis. Altogether, pesticide sampling, transfer, desorption, ionization, and detection took less than 1 min. The reproducibility of this method (n = 6) was shown in the observed low‐relative standard deviation (<7%) in the detection of pesticide in oral fluid. The detection limits of the pesticides in oral fluid obtained from four human subjects by thermal desorption‐electrospray ionization mass spectrometry were between 1–10 ppb with relative standard deviation 10.7%. Moreover, in this study, linear responses of five pesticides in oral fluid with concentrations between 1 ppb–1 ppm (R2 between 0.9938 and 0.9988) were observed. As the whole analytical process is extremely short, this technique allows for early non‐invasive point‐of‐care identification of pesticides in the oral fluid of self‐poisoning patients in the emergency room, providing important toxicological information for decision‐making during critical resuscitation. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

Applications of electrospray laser desorption ionization mass spectrometry for document examination

We have employed electrospray laser desorption ionization mass spectrometry (ELDI‐MS) to rapidly characterize certain classes of compounds – the inks within the characters made by inks and inkjet printer on regular paper and the chemical compounds within thermal papers. This ELDI‐MS approach allowed the ink and paper samples to be distinguished in terms of their chemical compositions. Sample pretreatment was unnecessary and the documents were practically undamaged after examination. The ink chemicals on the documents were desorbed through laser irradiation (sampling spot area: <100 µm²); the desorbed molecules then entered an electrospray plume – prepared from an acidic methanol/water solution (50%) – where they became ionized through fusion or ion‐molecule reactions with the charged solvent species and droplets in the plume. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

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.     

Characterization of mouse switch variant antibodies by matrix-assisted laser desorption ionization mass spectrometry and electrospray ionization mass spectrometry

The amino acid sequences of mouse monoclonal antibodies have been characterized completely by mass spectrometry. Antibodies used in the present study were derived from mouse switch variant cell lines that produce four kinds of immunoglobulin Gs (IgGs). The amino acid sequences of these antibodies had not been estimated from the corresponding DNA sequence, so the sequences of IgGs derived from other strains were used as references in this study. Intra- and interchain disulfide bonds of the IgGs were reduced and carboxymethylated and the products were subjected to proteolytic digestion. The existence of N-linked oligosaccharides also was taken into account. The capabilities and limitations of matrix-assisted laser desorption ionization-time-of-flight mass spectrometry and capillary liquid chromatography-electrospray ionization mass spectrometry are discussed in the structural characterization of the antibodies. Based on our results, allotypes of the antibodies examined are discussed. This study shows that amino acid sequences of proteins, such as IgG, can be investigated without information about the corresponding DNA sequence if appropriate reference sequences derived from other strains can be used.