Atmospheric Pressure Ionization Interface Research Articles

Comparison of electrospray and UniSpray, a novel atmospheric pressure ionization interface, for LC-MS/MS analysis of 81 pesticide residues in food and water matrices

In mass spectrometry, the type and design of ionization source play a key role on the performance of a given instrument. Therefore, it is of paramount importance to evaluate newly developed sources for their suitability to analyze food contaminants like pesticide residues. Here, we carried out a head-to-head comparison of key extraction and analytical performance parameters of an electrospray ionization (ESI) source with a new atmospheric pressure ionization source, UniSpray (US). The two interfaces were evaluated in three matrices of different properties (coffee, apple, and water) to determine if multiresidue analysis of 81 pesticides by QuEChERS extraction and LC-MS/MS analysis could be improved. Depending on the matrix and irrespective of the chemical class, US provided a tremendous gain in signal intensity (22- to 32-fold in peak area, 6- to 7-fold in peak height), a threefold to fourfold increase in signal-to-noise ratio, a mild gain in the range of compounds that can be quantified, and up to twofold improvement of recovery. UniSpray offered comparable linearity and precision of the analyses with ESI, and did not affect the ion ratio. A gain in sensitivity of many compounds was observed with US, but in general, the two ionization interfaces did not show significant difference in LOD and LOQ. UniSpray suffered less signal suppression; the matrix effect was in average 3 to 4 times more pronounced, but showed better values than ESI. With no effect on recovery efficiency, US improved the overall process efficiency 3 to 4 times more than ESI.Graphical abstract

Gas chromatographic quadrupole time-of-flight full scan high resolution mass spectrometric screening of human urine in antidoping analysis.

This paper presents the development and validation of a high-resolution full scan (FS) electron impact ionization (EI) gas chromatography coupled to quadrupole Time-of-Flight mass spectrometry (GC/QTOF) platform for screening anabolic androgenic steroids (AAS) in human urine samples. The World Antidoping Agency (WADA) enlists AAS as prohibited doping agents in sports, and our method has been developed to comply with the qualitative specifications of WADA to be applied for the detection of sports antidoping prohibited substances, mainly for AAS. The method also comprises of the quantitative analysis of the WADA's Athlete Biological Passport (ABP) endogenous steroidal parameters. The applied preparation of urine samples includes enzymatic hydrolysis for the cleavage of the Phase II glucuronide conjugates, generic liquid-liquid extraction and trimethylsilyl (TMS) derivatization steps. Tandem mass spectrometry (MS/MS) acquisition was applied on few selected ions to enhance the specificity and sensitivity of GC/TOF signal of few compounds. The full scan high resolution acquisition of analytical signal, for known and unknown TMS derivatives of AAS provides the antidoping system with a new analytical tool for the detection designer drugs and novel metabolites, which prolongs the AAS detection, after electronic data files' reprocessing. The current method is complementary to the respective liquid chromatography coupled to mass spectrometry (LC/MS) methodology widely used to detect prohibited molecules in sport, which cannot be efficiently ionized with atmospheric pressure ionization interface.

Hyphenation of Thermal Analysis to Ultrahigh-Resolution Mass Spectrometry (Fourier Transform Ion Cyclotron Resonance Mass Spectrometry) Using Atmospheric Pressure Chemical Ionization For Studying Composition and Thermal Degradation of Complex Materials.

In this study, the hyphenation of a thermobalance to an ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometer (UHR FTICR MS) is presented. Atmospheric pressure chemical ionization (APCI) is used for efficient ionization. The evolved gas analysis (EGA), using high-resolution mass spectrometry allows the time-resolved molecular characterization of thermally induced processes in complex materials or mixtures, such as biomass or crude oil. The most crucial part of the setup is the hyphenation between the thermobalance and the APCI source. Evolved gases are forced to enter the atmospheric pressure ionization interface of the MS by applying a slight overpressure at the thermobalance side of the hyphenation. Using the FTICR exact mass data, detailed chemical information is gained by calculation of elemental compositions from the organic species, enabling a time and temperature resolved, highly selective detection of the evolved species. An additional selectivity is gained by the APCI ionization, which is particularly sensitive toward polar compounds. This selectivity on the one hand misses bulk components of petroleum samples such as alkanes and does not deliver a comprehensive view but on the other hand focuses particularly on typical evolved components from biomass samples. As proof of principle, the thermal behavior of different fossil fuels: heavy fuel oil, light fuel oil, and a crude oil, and different lignocellulosic biomass, namely, beech, birch, spruce, ash, oak, and pine as well as commercial available softwood and birch-bark pellets were investigated. The results clearly show the capability to distinguish between certain wood types through their molecular patterns and compound classes. Additionally, typical literature known pyrolysis biomass marker were confirmed by their elemental composition, such as coniferyl aldehyde (C10H10O3), sinapyl aldehyde (C11H12O4), retene (C18H18), and abietic acid (C20H30O2).

Implementation of DART and DESI Ionization on a Fieldable Mass Spectrometer

A recently developed prototype mobile laboratory mass spectrometer, incorporating an atmospheric pressure ionization (API) interface, is described. This system takes advantage of the small size, lower voltage requirements, and tandem MS abilities of the cylindrical ion trap mass analyzer. The prototype API MS uses small, low-power pumps to fit into a 0.1-m(3) self-contained package weighing <45 kg. This instrument has been adapted to allow rapid interfacing to electrospray ionization, desorption electrospray ionization, and direct analysis in real-time sources. Initial data indicate that these techniques provide rapid detection and identification of compounds for quality control, homeland security, and forensic applications. In addition, this instrument is self-contained and compact, making it ideally extensible to mobile laboratory and field analyses. Initial MS and MS/MS data for analyses of drugs, food, and explosives are presented herein.

Current Applications of Liquid Chromatography/Mass Spectrometry in Pharmaceutical Discovery After a Decade of Innovation

Current drug discovery involves a highly iterative process pertaining to three core disciplines: biology, chemistry, and drug disposition. For most pharmaceutical companies the path to a drug candidate comprises similar stages: target identification, biological screening, lead generation, lead optimization, and candidate selection. Over the past decade, the overall efficiency of drug discovery has been greatly improved by a single instrumental technique, liquid chromatography/mass spectrometry (LC/MS). Transformed by the commercial introduction of the atmospheric pressure ionization interface in the mid-1990s, LC/MS has expanded into almost every area of drug discovery. In many cases, drug discovery workflow has been changed owing to vastly improved efficiency. This review examines recent trends for these three core disciplines and presents seminal examples where LC/MS has altered the current approach to drug discovery.

A Pulsed Triple Ionization Source for Sequential Ion/Ion Reactions in an Electrodynamic Ion Trap

A pulsed triple ionization source, using a common atmosphere/vacuum interface and ion path, has been developed to generate different types of ions for sequential ion/ion reaction experiments in a linear ion trap-based tandem mass spectrometer. The triple ionization source typically consists of a nano-electrospray emitter for analyte formation and two other emitters, an electrospray emitter and an atmospheric pressure chemical ionization emitter or a second nano-electrospray emitter for formation of the two different reagent ions. The three emitters are positioned in a parallel fashion close to the sampling orifice of the tandem mass spectrometer. The potentials applied to each emitter are sequentially pulsed so that desired ions are generated separately in time and space. Sequential ion/ion reactions take place after analyte ions of interest and different set of reagent ions are sequentially injected into a linear ion trap, where axial trapping is effected by applying an auxiliary radio frequency voltage to the end lenses. The pulsed triple ionization source allows independent optimization of each emitter and can be readily coupled to any atmospheric pressure ionization interface with no need for instrument modifications, provided the potentials required to transmit the ion polarity of interest can be synchronized with the emitter potentials. Several sequential ion/ion reactions examples are demonstrated to illustrate the analytical usefulness of the triple ionization source in the study of gas-phase ion/ion chemistry.

Therapeutic drug monitoring of immunosuppressant drugs by high-performance liquid chromatography-mass spectrometry.

The currently expanding range of immunosuppressant agents has placed new challenges on therapeutic drug-monitoring (TDM) services. Many of these drugs require the measurement of concentrations with subsequent dosage adjustment to maximize efficacy while minimizing toxicity. HPLC-mass spectrometry (HPLC-MS) is a relatively new technique for drug quantification and thus TDM of immunosuppressant drugs. Although mass spectrometry relies on producing, differentiating, and detecting ions in the gas phase, the development of the atmospheric pressure ionization interface (electrospray and chemical ionization) has enabled the direct coupling of solution introduction of compounds, via HPLC, to the mass analyzer. The impetus for using HPLC-MS for immunosuppressant measurement has come from the highly potent low-dose immunosuppressant drugs tacrolimus and sirolimus, which have low nanogram per milliliter circulating concentrations. A number of strategies have been reported for sample preparation and ways to automate these processes with solid-phase extraction and 2-dimensional chromatography. The disadvantages of HPLC-MS are initial cost of equipment and availability of suitably skilled scientific staff. The advantages of HPLC-mass spectrometry are high sensitivity, specificity, small sample requirements, minimal sample preparation, rapid throughput, and simultaneous measurement. Further, scientists have the ability to develop methods to measure new immunosuppressant drugs by HPLC-MS before commercial assays become available. With potential applications increasing in immunosuppressive drug monitoring, it can be envisaged that HPLC-MS may become standard equipment in TDM laboratories of the future.

Liquid chromatography-tandem mass spectrometry analysis of urinary free cortisol.

For the analysis of urinary free cortisol (UFC), liquid chromatography (LC), LC-mass spectrometry (LC-MS), and liquid chromatography-tandem MS (LC-MS/MS) offer better specificity and accuracy of result than immunoassay-based methods (1). The high specificity of LC-MS/MS provides simplification of sample preparation, reduced costs, increased sample throughput, and a low rate of sample interference. Published approaches to LC-MS/MS analysis of UFC use either sample extraction (1) or direct sample injection (2). The direct methods reduce the labor requirements and decrease the potential for human error. We have developed a direct sample injection method similar to one published by Nassar et al. (2). The primary changes in the proposed method are the use of a guard cartridge for trapping cortisol as part of an online purification strategy and the use of step gradient rather than isocratic LC conditions. Both electrospray ionization (ESI) and atmospheric pressure ionization (APCI) are well suited for LC-MS/MS. We used APCI because preliminary experiments demonstrated a two- to threefold increase in sensitivity compared with ESI. Better sensitivity for cortisol with an APCI interface compared with ESI can be explained by the poor ionization of cortisol in solution. Cortisol and ammonium formate were purchased from Sigma, and d4-cortisol was purchased from Cambridge Isotope Laboratories. Methanol (HPLC grade) was purchased from Fisher Scientific. Patient samples were analyzed for cortisol within 1–3 days after collection. All studies with samples from humans were approved by the Institutional Review Board of the University of Utah. Sample preparation for the method was performed as follows. A 500-μL aliquot of centrifuged urine sample and 100 μL of the working internal standard (IS; d4-cortisol, 0.5 mg/L) were added to autosampler vials, and the vials were vortex-mixed. A PE series 200 HPLC system (Perkin-Elmer Analytical Instruments) was equipped with a Luna C18 column …

Ionization of dichlorophenols for their analysis by capillary electrophoresis–mass spectrometry

In order to develop an advanced analytical method using capillary electrophoresis (CE) for non-volatile environmental pollutants such as endocrine disruptors, combination with mass spectrometry (MS) is necessary for their identification. We chose dichlorophenols (DCPs) as test samples because one of their isomers, 2,4-DCP, is suspected to have endocrine disrupting effects. A preliminary study on their separation by CE–MS was performed using a laboratory-made electrospray ionization (ESI) interface. For the effective ionization of 2,4-DCP at the ESI interface, applied voltage, assisted nitrogen-gas flow-rate, salts and their concentration, and organic solvents in the sample matrix were optimized according to the signal intensity. The intensity of other DCPs under the same conditions, however, was quite different. We tried to modify the atmospheric chemical pressure ionization (APCI) interface in order to reduce the flow-rate. The intensity of directly injected DCPs was not so different from one another using the modified prototype APCI interface.

FINGERPRINTING OF THE DEVELOPMENT OF AEROBIC COMPOSTING PROCESSES OF AGRICULTURAL WASTES BY ON-LINE COMBINATION OF LIQUID CHROMATOGRAPHY AND MASS SPECTROMETRY

Liquid chromatography (LC) was employed in conjunction to mass spectrometry (MS) to develop a straightforward LC-MS procedure for monitoring the variation of functionalized nonpolar organic compounds during the development of an aerobic composting process of agricultural wastes. The LC separation was performed by a reversed-phase microbore HPLC octadecyl-silica column, which was eluted by a linear acetonitrile gradient in water. The reversed-phase HPLC column was connected on-line to a triploquadrupole mass spectrometer by an atmospheric pressure ionization interface. Weighed amounts of agricultural wastes sampled at different time intervals during the aerobic composting process were extracted by dichloromethane and analyzed by LC-MS. The resulting fingerprinting offered a means to monitor the variations of functionalized nonpolar compounds during the aerobic process and indicate the presence of compounds that are not degraded by the biotranformation, such as ursolic acid and β-sitosterol.

Quantitation and characterization of phospholipids in pharmaceutical formulations by liquid chromatography–mass spectrometry

A simple and fast method for phospholipid analysis was developed using high-performance liquid chromatography–mass spectrometry with an atmospheric pressure ionization interface. Separation of the phospholipid molecular species was achieved using a linear gradient of a mixture of chloroform–10 m M ammonium acetate–methanol (30:5:65) on a silica column. Optimization of the mass spectrometer conditions has allowed the method to separate and detect the phospholipids mainly as protonated molecular species. In comparison to existing LC–MS methods, improvement in the total analysis time and sensitivity were achieved. Separation of all major phospholipid molecular classes was achieved in less than 6 min. Marked improvement was observed in the linearity of the response of the phospholipids studied providing a linear response over three orders of magnitude. Data supporting the validation of this method for the characterization of major phospholipids molecular species are also presented.

Simultaneous Determination of N-Acetylaspartic Acid, N-Acetylglutamic Acid, and N-Acetylaspartylglutamic Acid in Whole Brain of 3-Mercaptopropionic Acid-Treated Rats Using Liquid Chromatography-Atmospheric Pressure Chemical Ionization Mass Spectrometry

The measurement of N-acetylaspartic acid (NAA), N-acetylglutamic acid (NAG), and N-acetylaspartylglutamic acid (NAAG) in the whole brain of 3-mercaptopropionic acid (3-MPA)-treated rats has been developed using liquid chromatography-mass spectrometry with an atmospheric pressure ionization interface system. The recoveries of these compounds were 90.85 ± 3.43% for NAA, 91.62 ± 5.47% for NAG, and 92.29 ± 4.44% for NAAG. The detection limits for NAA, NAG, and NAAG were 12, 15, and 20 μg/ml, respectively. After administration of 3-MPA, the concentrations of NAA, NAG, and NAAG in the whole brain over 10 min increased 177.25, 134.23, and 127.70%, respectively. These concentrations then decreased over the next 60 min. The simultaneous determination of NAA, NAG, and NAAG using this method was found to be very useful for studies of metabolism of NAA, NAG, and NAAG in biological samples.

Simultaneous determination of γ-aminobutyric acid and glutamic acid in the brain of 3-mercaptopropionic acid-treated rats using liquid chromatography–atmospheric pressure chemical ionization mass spectrometry

The measurement of γ-aminobutyric acid (GABA) and glutamic acid (Glu) in the whole brain and in various regions of the brain in 3-mercaptopropionic acid (3-MPA)-treated rats has been developed using liquid chromatography–mass spectrometry with an atmospheric pressure ionization interface system. The recoveries of these compounds were 94.90±4.18% for GABA, 95.60±2.86% for Glu after ion-exchange treatment. The detection limits for GABA and Glu were 2.5±0.3 μg/ml and 5.0±0.8 μg/ml, respectively, when 20μl sample were injected. GABA concentration in the whole brain decreased gradually to 5 min and reached 63% of normal value after administration of 3-MPA, and the concentration increased gradually thereafter until 60 min. Conversely, the concentration of Glu in the whole brain increased gradually to 10 min and reached 154% of normal value, and after that decreased gradually and reached almost normal level at 60 min after administration of 3-MPA. GABA concentration in various regions of brain decreased to 5 min in all regions after administration of 3-MPA, and reached normal levels at 60 min as in the whole brain. This method was found to be useful for studies of metabolism of GABA and Glu in biological samples.

Liquid chromatography–mass spectrometric analysis of phenylurea herbicides and their degradation products using an atmospheric pressure ionization interface

A method was developed for the determination of two phenylurea herbicides and their degradation products using liquid chromatography–atmospheric pressure chemical ionization (APCI)-mass spectrometry without any additives in the mobile phase. The APCI detection sensitivity of these compounds was maximized when the vaporizer temperature was set at 250 °C and a positive-ion detection mode was chosen. Under these conditions, the detection limits (S/N = 3) of these compounds in total-ion and selected-ion monitoring were 2.5–3.0 and 0.12–0.26 ng, respectively. In a biodegradability test on diuron, small quantities of degradation products could be determined by the proposed method.

A new technique for decomposition of selected ions in molecule ion reactor coupled with ortho-time-of-flight mass spectrometry.

A molecule ion reactor (MIR), i.e. a gas filled radio-frequency only quadrupole with a longitudinal electrical field (RFQLEF), is used as an atmospheric pressure ionization interface for an orthogonal time-of-flight mass spectrometer (O-TOFMS). A new phenomenon of selective ion 'heating' in a MIR near Mathieu's instability threshold was found and confirmed by computer simulation. The 'heating' in collisions with buffer gas molecules leads to ion decomposition. In the case of multicharged ions, fragments with an m/z value higher than that of the parent ion have a stable motion and can be analysed by an O-TOFMS. Fragmentation of bradykinin and beta-endorphin molecular ions having a selected charge state is demonstrated. The spectra show clear 'ladder' structure. The phenomenon may be used as an alternative to tandem mass spectrometry (MS/MS) for molecule structure analysis.

Analyses of iminodipeptides containing C-terminal proline or hydroxyproline in biological samples by liquid chromatography/mass spectrometry

Analyses of authentic iminodipeptides containing an N-terminal proline (Pro-X) or C-terminal proline (X-Pro), and iminodipeptides in the urine and serum of patients with prolidase deficiency, have been demonstrated using liquid chromatography/mass spectrometry with an atmospheric pressure ionization interface system. The separation of iminodipeptides was carried out on a reversed phase high-performance liquid Chromatographic column using 0.1% aqueous trifluoroacetic acid-methanol (75:25, v/v) as mobile phase. Very intense protonated molecular ions [ M+ H] + of various synthetic iminodipeptides, Pro-Gly, Gly-Pro, Pro-Ala, Ala-Pro, Pro-Val, Val-Pro, Pro-Leu and Leu-Pro, were observed. ProGly (Pro-X) and Gly-Pro (X-Pro) have the same protonated molecular ion (m/z 173), but the peaks of these compounds on the mass chromatograms were clearly distinguished by the differences of the retention times and mass spectra. The quasimolecular ions [ M+ H] + of various iminodipeptides were observed in the urine samples of patients with prolidase deficiency, and Gly-Pro, Ala-Pro, Val-Pro, Leu-Pro, Ile-Pro, Ser-Pro, Thr-Pro, Glu-Pro, Asp-Pro, His-Pro, Lys-Pro, Pro-Pro and Tyr-Pro as iminodipeptides containing proline with C-terminal residues and Glu-Hyp, Pro-Hyp, Ile-Hyp and Gly-Hyp as iminodipeptides containing hydroxyproline with C-terminal residues were identified in the urine of patients with prolidase deficiency. The quasi-molecular ions [ M+ H] + of various iminodipeptides containing with C-terminal proline and hydroxyproline also were observed in the sera of patients with prolidase deficiency, using selected ion monitoring. The quasi-molecular ions [ M+ H] + of iminodipeptides containing C-terminal proline were not observed in the sera of normal subjects or the patients' mother, but the latter contained various iminodipeptides containing C-terminal hydroxyproline. This method proved useful for the determination of iminodipeptides in the sera of patients with prolidase deficiency.

The Reduction of Chemical Noise in an Atmospheric Pressure Ionization/Ionspray Interface for Mass Spectrometry

The reduction of chemical noise and the consequent gain in sensitivity on an ionspray interface to an ion-trap mass spectrometer for capillary electrophoresis/mass spectrometry (CE/MS) applications is described. The principal sources of chemical noise were found in ionized airborne contaminants and in solvent cluster adducts. The reduction of these unwanted signals was achieved by the application of a cover to the heated-shield atmospheric pressure ionization interface and the removal of all plastic components near the heated zones. By the use of a clean bath gas and curtain gas, the ionization region was constantly purged with nitrogen gas to prevent the deposition of contaminants on the inside surfaces and to protect the orifice from obstruction. The cover of the heated-shield interface together with the presence of the bath and curtain gas also increased the desolvation of solvent clusters, most likely by the application of more concentrated heat during the ionization process.

Determination of various steroids in normal adrenal glands and adrenocortical tumours by liquid chromatography/atmospheric pressure ionization mass spectrometry

AbstractSeven steroids in normal adrenal gland and those of patients with aldosteronoma or Cushing's adenoma were measured by liquid chromatography/mass spectrometry with an atmospheric pressure ionization interface system. Using this system, quasi‐molecular ions [M + H]+ of standard cortisol, corticosterone, 17α‐hydroxyprogesterone, 11β‐hydroxyandrostenedione and Δ4‐androstenedione and [(M + H) − H2O]+ ions of aldosterone and dehydroe‐piandrosterone were observed as their base peaks, and quasi‐molecular ions [M + H]+ and [(M + H) − H2O]+ ions of seven steroids were clearly observed in chromatograms of biological samples. The ratio of glucocorticoid (cortisol) to mineralocorticoid (corticosterone) content was measured, and found to be significantly lower for aldosteronomas and higher for Cushing's adenomas than for normal adrenals. These findings indicate that the method had suffficient sensitivity for the identification and simultaneous measurement of a variety of adrenal gland steroids.

Determination of Vitamin D3 and D2 in Multi-Vitamin Tablets by High-Performance Liquid Chromatography/Atmospheric Pressure Chemical Ionization Mass Spectrometry

Liquid chromatography/atmospheric pressure chemical ionization mass spectrometry was used to determine vitamin D3 and its analogs. Vitamin D3 and its analogs were separated on a reversed phase ODS column. The eluent was fed into the atmospheric pressure ionization interface and vitamins were then quantitated in the selected ion monitoring mode. The detection limit of vitamin D3 and its analogs were ca. 0.5 ng (on-column) at a signal-to-noise ratio of 2. This method was used to determine vitamin D3 and D2 in multi-vitamin tablets. The flow injection mode was also effective for the determination of VD3 in multi-vitamin tablets.

Construction and characterization of a small-bore electrospray ionization source

A simple, economical, and efficient electrospray ionization (ESI) source has been constructed in the configuration of a probe that makes use of a standard 13 mm vacuum lock. The principal components have been placed inside a glass tube making use of the electrical insulating properties of the glass while allowing for visual adjustments to be readily made. The ESI source, a variation of an atmospheric pressure ionization interface, is a modified version of designs published by Chait et al. ( Rapid Comm. Mass Spectrom. 1990, 4, 81–87) and Knapp et al. ( Anal. Chem. 1991, 63, 1658–1660) wherein a heated metal capillary is used for desolvation. The ESI probe has been tested on three different Extrel quadrupole mass spectrometers, with removable ion volumes, using polypeptides and small proteins. No modifications to the standard electron ionization/chemical ionization lens assembly were required to obtain excellent results other than removal of the ion volume. The spectra acquired were in excellent agreement with those previously published.