Analysis of lipids with desorption atmospheric pressure photoionization‐mass spectrometry (DAPPI‐MS) and desorption electrospray ionization‐mass spectrometry (DESI‐MS)

In this article, the effect of spray solvent on the analysis of selected lipids including fatty acids, fat‐soluble vitamins, triacylglycerols, steroids, phospholipids, and sphingolipids has been studied by two different ambient mass spectrometry (MS) methods, desorption electrospray ionization‐MS (DESI‐MS) and desorption atmospheric pressure photoionization‐MS (DAPPI‐MS). The ionization of the lipids with DESI and DAPPI was strongly dependent on the spray solvent. In most cases, the lipids were detected as protonated or deprotonated molecules; however, other ions were also formed, such as adduct ions (in DESI), [M‐H]⁺ ions (in DESI and DAPPI), radical ions (in DAPPI), and abundant oxidation products (in DESI and DAPPI). DAPPI provided efficient desorption and ionization for neutral and less polar as well as for ionic lipids but caused extensive fragmentation for larger and more labile compounds because of a thermal desorption process. DESI was more suitable for the analysis of the large and labile lipids, but the ionization efficiency for less polar lipids was poor. Both methods were successfully applied to the direct analysis of lipids from pharmaceutical and food products. Although DESI and DAPPI provide efficient analysis of lipids, the multiple and largely unpredictable ionization reactions may set challenges for routine lipid analysis with these methods. Copyright © 2012 John Wiley & Sons, Ltd.     

Atmospheric pressure photoionization-mass spectrometry and atmospheric pressure chemical ionization-mass spectrometry of neurotransmitters

A group of five neurotransmitters with different properties was analyzed using atmospheric pressure photoionization (APPI) and atmospheric pressure chemical ionization-mass spectrometry (APCI-MS). The sensitivity of the techniques for the analytes was tested in six solvents and in positive and negative ion modes. APPI was found to be superior in sensitivity for all the compounds in both positive and negative ion modes. In positive ion mode, water/methanol/formic acid was found to be the best solvent, whereas in negative ion mode, water/methanol/ammonium hydroxide performed best. Detection limits using APPI were between 2.5-250 fmol, depending on the compound. The sensitivity was best for the neurosteroids dehydroepiandrosterone and beta-estradiol, and acetylcholine (LOD 2.5-10 fmol).     

Reactive desorption electrospray ionization mass spectrometry (DESI-MS) of natural products of a marine alga

Presented here is the optimization and development of a desorption electrospray ionization mass spectrometry (DESI-MS) method for detecting natural products on tissue surfaces. Bromophycolides are algal diterpene-benzoate macrolide natural products that have been shown to inhibit growth of the marine fungal pathogen Lindra thalassiae. As such, they have been implicated in antimicrobial chemical defense. However, the defense mechanisms are not yet completely understood. Precise detection of these compounds on algal tissue surfaces under ambient conditions without any disruptive sample processing could shed more light onto the processes involved in chemical defense of marine organisms. Conventional DESI-MS directly on algal tissue showed relatively low sensitivity for bromophycolide detection. Sensitivity was greatly improved by the addition of various anions including Cl⁻, Br⁻, and CF₃COO⁻ into the DESI spray solvent. Chloride adduction gave the highest sensitivity for all assayed anions. Density functional optimization of the bromophycolide anionic complexes produced during DESI supported this observation by showing that the chloride complex has the most favorable binding energy. Optimized DESI protocols allowed the direct and unambiguous detection of bromophycolides, including A, B, and E, from the surface of untreated algal tissue. Figure Desorption Electrospray Ionization, a novel technique for mass spectrometric analysis under open air conditions reveals the presence of naturally-occurring antibiotics on the surface of marine algae. Ab-initio calculations and experimental results indicate that sensitiviity could be greatly enhanced by means of dynamic complexation of these antibiotics with various small anions during the dynamic desorption process. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

Liquid chromatography/mass spectrometry in anabolic steroid analysis--optimization and comparison of three ionization techniques: electrospray ionization,atmospheric pressure chemical ionization and atmospheric pressure photoionization

The applicability of liquid chromatography/tandem mass spectrometry (LC/MS/MS) for the detection of the free anabolic steroid fraction in human urine was examined. Electrospray ionization (ESI), atmospheric pressure chemical ionization and atmospheric pressure photoionization methods were optimized regarding eluent composition, ion source parameters and fragmentation. The methods were compared with respect to specificity and detection limit. Although all methods proved suitable, LC/ESI-MS/MS with a methanol-water gradient including 5 mM ammonium acetate and 0.01% acetic acid was found best for the purpose. Multiple reaction monitoring allowed the determination of steroids in urine at low nanogram per milliliter levels. LC/MS/MS exhibited high sensitivity and specificity for the detection of free steroids and may be a suitable technique for screening for the abuse of anabolic steroids in sports.     

The detection and mapping of the spatial distribution of insect defense compounds by desorption atmospheric pressure photoionization Orbitrap mass spectrometry

Many insects use chemicals synthesized in exocrine glands and stored in reservoirs to protect themselves. Two chemically defended insects were used as models for the development of a new rapid analytical method based on desorption atmospheric pressure photoionization-mass spectrometry (DAPPI-MS). The distribution of defensive chemicals on the insect body surface was studied. Since these chemicals are predominantly nonpolar, DAPPI was a suitable analytical method. Repeatability of DAPPI-MS signals and effects related to non-planarity and roughness of samples were investigated using acrylic sheets uniformly covered with an analyte. After that, analytical figures of merit of the technique were determined. The spatial distribution of (E)-1-nitropentadec-1-ene, a toxic nitro compound synthesized by soldiers of the termite Prorhinotermes simplex, was investigated. Then, the spatial distribution of the unsaturated aldehydes (E)-hex-2-enal, (E)-4-oxohex-2-enal, (E)-oct-2-enal, (E,E)-deca-2,4-dienal and (E)-dec-2-enal was monitored in the stink bug Graphosoma lineatum. Chemicals present on the body surface were scanned along the median line of the insect from the head to the abdomen and vice versa, employing either the MS or MS² mode. In this fast and simple way, the opening of the frontal gland on the frons of termite soldiers and the position of the frontal gland reservoir, extending deep into the abdominal cavity, were localized. In the stink bug, the opening of the metathoracic scent glands (ostiole) on the ventral side of the thorax as well as the gland reservoir in the median position under the ventral surface of the anterior abdomen were detected and localized. The developed method has future prospects in routine laboratory use in life sciences.     

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.     

Feasibility of gas chromatography-microchip atmospheric pressure photoionization-mass spectrometry in analysis of anabolic steroids

Mass spectrometers equipped with atmospheric pressure ion sources (API-MS) have been designed to be interfaced with liquid chromatographs (LC) and have rarely been connected to gas chromatographs (GC). Recently, we introduced a heated nebulizer microchip and showed its potential to interface liquid microseparation techniques and GC with API-MS. This study demonstrates the feasibility of GC-microchip atmospheric pressure photoionization-tandem mass spectrometry (GC-μAPPI-MS/MS) in the analysis of underivatized anabolic steroids in urine. The APPI microchip provides high ionization efficiency and produces abundant protonated molecules or molecular ions with minimal fragmentation. The feasibility of GC-μAPPI-MS/MS in the analysis of six selected anabolic steroids in urine samples was studied with respect to intra-batch repeatability, linearity, linear range, and limit of detection (LOD). The method showed good sensitivity (LODs 0.2-1 ng/mL), repeatability (relative standard deviation<10%), and linearity (regression coefficient≥0.9995) and, therefore, high potential for the analysis of anabolic steroids. Quantitative performance of the method was tested with two authentic urine samples, and the results were in good agreement with those obtained with conventional GC-electron ionization-MS after derivatization.     

Portable electrospray ionization mass spectrometry (ESI-MS) for analysis of contaminants in the field

Hitherto analysis of chemicals in the field using mass spectrometry (MS) has been limited to analysis of non-polar and thermally stabile organic compounds using either a direct gas leak or a membrane inlet as MS interface. Recently, Professor R. Graham Cooks’ group demonstrated that miniature mass spectrometers operating at elevated pressures (>0.13?Pa (1?·?10^?3??Torr)) can be combined with electrospray ionization (ESI) for analysis of polar as well as thermally labile organic compounds. We present a simple miniaturized ESI unit for analysis of small liquid samples using miniature mass spectrometry. The ESI unit operates without pumps and supplementary sheath gases, which makes it very simple to handle in the field. 20–30?µL of sample solution is simply dropped into a small cavity in the ESI unit, where after the spray is initiated by applying high voltage to it. The miniaturized ESI unit was tested in combination with a miniature mass spectrometer (the Mini 10 developed by Professor R. Graham Cooks, Purdue University, IN) and we found that common herbicides (Atrazine, Prometryne, Terbutryne and Triadimefone) could be detected with detection limits around 1?mg?L^?1 and with a quantitative reproducibility of +/?30%. These characteristics, although high for environmental samples, are comparable to detection limits obtained with other ESI units used with miniature mass spectrometers and represent an early step forward towards a future field instrument. A major advantage of the capillary spray cell is its direct compatibility with micro extraction techniques for sample pre-concentration.     

Atmospheric pressure chemical ionization and atmospheric pressure photoionization for simultaneous mass spectrometric analysis of microbial respiratory ubiquinones and menaquinones

An atmospheric pressure photoionization (APPI) source and an atmospheric pressure chemical ionization (APCI) source were compared for the selective detection of microbial respiratory ubiquinone and menaquinone isoprenologues using tandem mass spectrometry. Ionization source- and compound mass-dependent parameters were optimized individually for both sources, using the available quinone standards. Detection levels for the two ion sources were determined with ubiquinone-6 (UQ6) and menaquinone-4 (MK4, vitamin K2) standards using flow injection analysis and selected reaction monitoring (SRM). With APPI the calculated lower limit of detection (LLOD) was 1.7 fmol microl(-1) for UQ6 and 2.2 fmol microl(-1) for MK4 at a signal-to-noise ratio of 3. These LLODs were at least three times lower than with APCI. The selectivity of detection afforded by SRM detection reduced complex mixture analysis to 3 min per sample by eliminating the need for chromatographic separations. The detection method was successfully applied to quinone quantification in a variety of environmental samples and cell cultures. Adequate amounts of respiratory quinones can be extracted and quantified from samples containing as low as 2 x 10(7) cells.     

Rapid analysis of metabolites and drugs of abuse from urine samples by desorption electrospray ionization-mass spectrometry

Urine samples obtained from drug abusers were screened for drugs of abuse and their metabolites using DESI-MS and the results obtained were compared to results obtained from GC-MS experiments. The detected analyte classes included amphetamines, opiates, cannabinoids and benzodiazepines. The compounds detected were codeine, morphine, oxymorphone, 11-nor-9-carboxy-Delta(9)-tetrahydrocannabinol, Delta(9)-tetrahydrocannabinol, alprazolam, temazepam, oxazepam, N-desmethyldiazepam (nordiazepam) and hydroxytemazepam. Identities of all the analytes were confirmed by tandem mass spectrometry, matching MS/MS spectra with authentic standard compounds. The concentrations of the analytes in the samples were obtained from semi-quantitative GC-MS studies and were in the range of 270-22,000 ng mL(-1). The analytes could be detected by DESI even after a hundred-fold dilution indicating that the sensitivity of DESI was more than adequate for this study. Selectivity in the DESI-MS measurements for different kinds of analytes could be increased further by optimizing the spray solvent composition: the use of an entirely aqueous solvent enhanced the signal of polar analytes, such as the benzodiazepines, whereas the use of a spray solvent with a high organic content increased the signal of less polar analytes, such as codeine and morphine.     

Document authentication at molecular levels using desorption atmospheric pressure chemical ionization mass spectrometry imaging

Molecular images of documents were obtained by sequentially scanning the surface of the document using desorption atmospheric pressure chemical ionization mass spectrometry (DAPCI‐MS), which was operated in either a gasless, solvent‐free or methanol vapor‐assisted mode. The decay process of the ink used for handwriting was monitored by following the signal intensities recorded by DAPCI‐MS. Handwritings made using four types of inks on four kinds of paper surfaces were tested. By studying the dynamic decay of the inks, DAPCI‐MS imaging differentiated a 10‐min old from two 4 h old samples. Non‐destructive forensic analysis of forged signatures either handwritten or computer‐assisted was achieved according to the difference of the contour in DAPCI images, which was attributed to the strength personalized by different writers. Distinction of the order of writing/stamping on documents and detection of illegal printings were accomplished with a spatial resolution of about 140 µm. A Matlab® written program was developed to facilitate the visualization of the similarity between signature images obtained by DAPCI‐MS. The experimental results show that DAPCI‐MS imaging provides rich information at the molecular level and thus can be used for the reliable document analysis in forensic applications. © 2013 The Authors. Journal of Mass Spectrometry published by John Wiley & Sons, Ltd.     

Liquid chromatography/atmospheric pressure ionization-mass spectrometry in drug metabolism studies

The study of the metabolic fate of drugs is an essential and important part of the drug development process. The analysis of metabolites is a challenging task and several different analytical methods have been used in these studies. However, after the introduction of the atmospheric pressure ionization (API) technique, electrospray and atmospheric pressure chemical ionization, liquid chromatography/mass spectrometry (LC/MS) has become an important and widely used method in the analysis of metabolites owing to its superior specificity, sensitivity and efficiency. In this paper the feasibility of LC/API-MS techniques in the identification, structure characterization and quantitation of drug metabolites is reviewed. Sample preparation, LC techniques, isotope labeling, suitability of different MS techniques, such as tandem mass spectrometry, and high-resolution MS in drug metabolite analysis, are summarized and discussed. Automation of data acquisition and interpretation, special techniques and possible future trends are also the topics of the review.     

Quantitative analysis of polycyclic aromatic hydrocarbons using high-performance liquid chromatography-photodiode array-high-resolution mass spectrometric detection platform coupled to electrospray and atmospheric pressure photoionization sources

Polycyclic aromatic hydrocarbons (PAHs) are common pollutants present in atmospheric aerosols and other environmental mixtures. They are of particular air quality and human health concerns as many of them are carcinogenic toxins. They also affect absorption of solar radiation by aerosols, therefore contributing to the radiative forcing of climate. For environmental chemistry studies, it is advantageous to quantify PAH components using the same analytical technics that are commonly applied to characterize a broad range of polar analytes present in the same environmental mixtures. Liquid chromatography coupled with photodiode array and high-resolution mass spectrometric detection (LC-PDA-HRMS) is a method of choice for comprehensive characterization of chemical composition and quantification of light absorption properties of individual organic compounds present in the environmental samples. However, quantification of non-polar PAHs by this method is poorly established because of their imperfect ionization in electrospray ionization (ESI) technique. This tutorial article provides a comprehensive evaluation of the quantitative analysis of 16 priority pollutant PAHs in a standard reference material using the LC–MS platform coupled with the ESI source. Results are further corroborated by the quantitation experiments using an atmospheric pressure photoionization (APPI) method, which is more sensitive for the PAH detection. The basic concepts and step-by-step practical guidance for the PAHs quantitative characterization are offered based on the systematic experiments, which include (1) Evaluation effects of different acidification levels by formic acid on the (+)ESI-MS detection of PAHs. (2) Comparison of detection limits in ESI+ versus APPI+ experiments. (3) Investigation of the PAH fragmentation patterns in MS² experiments at different collision energies. (4) Calculation of wavelength dependent mass absorption coefficient (MAC_λ) of the standard mixture and its individual PAHs using LC-PDA data. (5) Assessment of the minimal injected mass required for accurate quantification of MAC_λ of the standard mixture and of a multi-component environmental sample.     

Determination of polymer additives by liquid chromatography coupled with mass spectrometry. A comparison of atmospheric pressure photoionization (APPI), atmospheric pressure chemical ionization (APCI), and electrospray ionization (ESI)

A method for the determination of polymer additives like antioxidants, UV absorbers and processing stabilizers using liquid chromatography (LC) coupled with atmospheric pressure photoionization mass spectrometry (APPI-MS) is presented. Ion source parameters were optimized regarding temperatures, gas flow rates, and voltages applied. Detection limits were determined using APPI with or without dopant and were compared with electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Differences between APPI, ESI and APCI are pointed out and the effect of the dopant toluene and acetone is discussed. The optimized method yielded detection limits between 0.001 mg L⁻¹ and 0.022 mg L⁻¹ for 15 different analytes. Linear calibration plots could be obtained for all solutes over a wide concentration range showing satisfying repeatability with standard deviations of peak areas between 3.4% and 7.6%. The results indicate that the developed method can be regarded as suitable for the quantitative determination of polymer additives even at low concentration levels.     

Comparison of electrospray ionization and atmospheric pressure chemical ionization techniques in the analysis of the main constituents from Rhodiola rosea extracts by liquid chromatography/mass spectrometry

Rhodiola rosea L. (Golden Root) has been used for a long time as an adaptogen in Chinese traditional medicine and is reported to have many pharmacological properties. A liquid chromatographic (LC) method with mass spectrometric (MS) detection based on selected ion monitoring (SIM) was developed for determining salidroside, sachaliside 1, rosin, 4-methoxycinnamyl-O-beta-glucopyranoside, rosarin, rosavin, cinnamyl-(6'-O-beta-xylopyranosyl)-O-beta-glucopyranoside, 4-methoxy-cinnamyl-(6'-O-alpha-arabinopyranosyl)-O-beta-glucopyranoside, rosiridin and benzyl-O-beta-glucopyranoside from the callus and plant extracts in one chromatographic run. Good linearity over the range 0.5-500 ng ml(-1) for salidroside, 2-2000 ng ml(-1) for rosavin and 2-500 ng ml(-1) for benzyl-O-beta-glucopyranoside was observed. The intra-assay accuracy and precision within quantitation ranges varied between -10.0 and +13.2% and between 0.7 and 9.0%, respectively. Optimization of the ionization process was performed with electrospray and atmospheric pressure chemical ionization techniques using four different additive compositions for eluents in the LC/MS scan mode, using both positive and negative ion modes. The best ionization sensitivity for the compounds studied was obtained with electrospray ionization when using pure water without any additives as the aqueous phase.     

Gas chromatography coupled to atmospheric pressure ionization mass spectrometry (GC-API-MS): Review

Although the coupling of GC/MS with atmospheric pressure ionization (API) has been reported in 1970s, the interest in coupling GC with atmospheric pressure ion source was expanded in the last decade. The demand of a “soft” ion source for preserving highly diagnostic molecular ion is desirable, as compared to the “hard” ionization technique such as electron ionization (EI) in traditional GC/MS, which fragments the molecule in an extensive way. These API sources include atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), atmospheric pressure laser ionization (APLI), electrospray ionization (ESI) and low temperature plasma (LTP). This review discusses the advantages and drawbacks of this analytical platform. After an introduction in atmospheric pressure ionization the review gives an overview about the history and explains the mechanisms of various atmospheric pressure ionization techniques used in combination with GC such as APCI, APPI, APLI, ESI and LTP. Also new developments made in ion source geometry, ion source miniaturization and multipurpose ion source constructions are discussed and a comparison between GC-FID, GC-EI-MS and GC-API-MS shows the advantages and drawbacks of these techniques. The review ends with an overview of applications realized with GC-API-MS.     

Feasibility of desorption atmospheric pressure photoionization and desorption electrospray ionization mass spectrometry to monitor urinary steroid metabolites during pregnancy

Steroids have important roles in the progress of pregnancy, and their study in maternal urine is a non-invasive method to monitor the steroid metabolome and its possible abnormalities. However, the current screening techniques of choice, namely immunoassays and gas and liquid chromatography–mass spectrometry, do not offer means for the rapid and non-targeted multi-analyte studies of large sample sets. In this study, we explore the feasibility of two ambient mass spectrometry methods in steroid fingerprinting. Urine samples from pregnant women were screened by desorption electrospray ionization (DESI) and desorption atmospheric pressure photoionization (DAPPI) Orbitrap high resolution mass spectrometry (HRMS). The urine samples were processed by solid phase extraction for the DESI measurements and by enzymatic hydrolysis and liquid–liquid-extraction for DAPPI. Consequently, steroid glucuronides and sulfates were detected by negative ion mode DESI–HRMS, and free steroids by positive ion mode DAPPI–HRMS. In DESI, signals of eleven steroid metabolite ions were found to increase as the pregnancy proceeded, and in DAPPI ten steroid ions showed at least an order of magnitude increase during pregnancy. In DESI, the increase was seen for ions corresponding to C18 and C21 steroid glucuronides, while DAPPI detected increased excretion of C19 and C21 steroids. Thus both techniques show promise for the steroid marker screening in pregnancy.     

Established and emerging atmospheric pressure surface sampling/ionization techniques for mass spectrometry

The number and type of atmospheric pressure techniques suitable for sampling analytes from surfaces, forming ions from these analytes, and subsequently transporting these ions into vacuum for interrogation by MS have rapidly expanded over the last several years. Moreover, the literature in this area is complicated by an explosion in acronyms for these techniques, many of which provide no information relating to the chemical or physical processes involved. In this tutorial article, we sort this vast array of techniques into relatively few categories on the basis of the approaches used for surface sampling and ionization. For each technique, we explain, as best known, many of the underlying principles of operation, describe representative applications, and in some cases, discuss needed research or advancements and attempt to forecast their future analytical utility. Copyright (c) 2008 John Wiley & Sons, Ltd.     

Differentiation of human kidney stones induced by melamine and uric acid using surface desorption atmospheric pressure chemical ionization mass spectrometry

Clinically obtained human kidney stones of different pathogenesis were dissolved in acetic acid/methanol solutions and then rapidly analyzed by surface desorption atmospheric pressure chemical ionization mass spectrometry (SDAPCI-MS) without any desalination treatment. The mass spectral fingerprints of six groups of kidney stone samples were rapidly recorded in the mass range of m/z 50-400. A set of ten melamine-induced kidney stone samples and nine uric acid derived kidney stone samples were successfully differentiated from other groups by principal component analysis of SDAPCI-MS fingerprints upon positive-ion detection mode. In contrast, the mass spectra recorded using negative-ion detection mode did not give enough information to differentiate those stone samples. The results showed that in addition to the melamine, the chemical compounds enwrapped in the melamine-induced kidney stone samples differed from other kidney stone samples, providing useful hints for studying on the formation mechanisms of melamine-induced kidney stones. This study also provides useful information on establishing a MS-based platform for rapid analysis of the melamine-induced human kidney stones at molecular levels.