Kaveh Jorabchi

Integrated Microfluidic Device for Automated Single Cell Analysis Using Electrophoretic Separation and Electrospray Ionization Mass Spectrometry

A microfabricated fluidic device was developed for the automated real-time analysis of individual cells using capillary electrophoresis (CE) and electrospray ionization-mass spectrometry (ESI-MS). The microfluidic structure incorporates a means for rapid lysis of single cells within a free solution electrophoresis channel, where cellular constituents were separated, and an integrated electrospray emitter for ionization of separated components. The eluent was characterized using mass spectrometry. Human erythrocytes were used as a model system for this study. In this monolithically integrated device, cell lysis occurs at a channel intersection using a combination of rapid buffer exchange and an increase in electric field strength. An electroosmotic pump is incorporated at the end of the electrophoretic separation channel to direct eluent to the integrated electrospray emitter. The dissociated heme group and the alpha and beta subunits of hemoglobin from individual erythrocytes were detected as cells continuously flowed through the device. The average analysis throughput was approximately 12 cells per minute, demonstrating the potential of this method for high-throughput single cell analysis.

Mass Spectrometry of Acoustically Levitated Droplets

Containerless sample handling techniques such as acoustic levitation offer potential advantages for mass spectrometry, by eliminating surfaces where undesired adsorption/desorption processes can occur. In addition, they provide a unique opportunity to study fundamental aspects of the ionization process as well as phenomena occurring at the air-droplet interface. Realizing these advantages is contingent, however, upon being able to effectively interface levitated droplets with a mass spectrometer, a challenging task that is addressed in this report. We have employed a newly developed charge and matrix-assisted laser desorption/ionization (CALDI) technique to obtain mass spectra from a 5-microL acoustically levitated droplet containing peptides and an ionic matrix. A four-ring electrostatic lens is used in conjunction with a corona needle to produce bursts of corona ions and to direct those ions toward the droplet, resulting in droplet charging. Analyte ions are produced from the droplet by a 337-nm laser pulse and detected by an atmospheric sampling mass spectrometer. The ion generation and extraction cycle is repeated at 20 Hz, the maximum operating frequency of the laser employed. It is shown in delayed ion extraction experiments that both positive and negative ions are produced, behavior similar to that observed for atmospheric pressure matrix-assisted laser absorption/ionization. No ion signal is observed in the absence of droplet charging. It is likely, although not yet proven, that the role of the droplet charging is to increase the strength of the electric field at the surface of the droplet, reducing charge recombination after ion desorption.

Single Droplet Separations and Surface Partition Coefficient Measurements Using Laser Ablation Mass Spectrometry

Surface activity of analytes plays a significant role in many chemical and physical phenomena. We present here a mass spectrometric method to characterize surface activity and solute partitioning between bulk liquid and the gas-liquid interface in droplets. The approach employs ablation by an infrared (IR) laser from the surface of a microliter droplet deposited on a stainless steel post. The ablated material is ionized for mass spectrometric analysis by either droplet charging or by postionization in an electrospray plume. Three areas of application have been explored using this method (1) separations in a single droplet: continuous ablation by a series of many successive laser pulses results in faster depletion of more surface active analytes, effectively comprising a surface activity-based separation. (2) Partition coefficient measurements: droplet volume is held constant during ablation by continually replenishing lost solvent. This leads to analyte-specific ion signal decay curves that may be fitted to a model based on Langmuir adsorption isotherms and simple analytical expressions, yielding quantitative values for the analyte surface partition coefficients. (3) Studies of the relationship between surface partitioning and high-performance liquid chromatography (HPLC) phase partitioning: comparisons of surface activities measured by laser desorption with retention times in reversed-phase HPLC reveal that the relationship between the two partitioning processes is very sensitive to chemical structure. Poor correlation between the retention time and surface activity is also observed within a subcategory of analytes (peptides). This effect is attributed to multimodal solute-stationary phase interactions. The laser desorption approach presented here provides direct information on analyte surface activities free from the complications encountered in chromatographic methods due to chemical structure variations.

Investigation of the direct injection high efficiency nebulizer for axially and radially viewed inductively coupled plasma atomic emission spectrometry

A direct injection high efficiency nebulizer (DIHEN) is explored for introduction of the sample aerosol into the central channel of the axially and radially viewed inductively coupled plasma (ICP) of a commercial ICP atomic emission spectrometer (ICPAES). The DIHEN is a micro-nebulizer that requires very low solution uptake rates (1–100 µL min−1) and nebulizer gas flow rates (<0.2 L min−1) compared to conventional nebulizer-spray chamber arrangements (∼1.0 mL min−1 and ∼1.0 L min−1, respectively). Signal-to-background ratios (SBRs), detection limits, and precision of the DIHEN are comparable or superior to those of the conventional sample introduction system, but the Mg II 280.270/Mg I 285.213 nm ratios are lower with the DIHEN, indicating that the DIHEN is more susceptible to matrix effects than the conventional nebulization system, for both the axial and radial ICPAES systems. Matrix effects are further investigated by comparing intensity ratios with and without 0.1% and 0.5% Na for several spectral lines having energy sum ranging from 7.93 to 14.79 eV. Replacement of Ar with Ar–O2 and Ar–N2 mixtures in the outer gas flow of the plasma improves SBRs and Mg II 280.270/Mg I 285.213 nm ratios of the DIHEN, and reduces matrix effects. By reducing solution uptake rate from 60 to 30 µL min−1, matrix effect is also reduced. Operation of the radial instrument at 1700 W reduces matrix effect compared to the effect noted for the axial instrument at 1500 W. Finally, the utility of the technique in practical ICPAES studies is demonstrated using a custom made organo-metallic standard for As, Hg, and Pb in xylene.

Ambient analysis by thermal desorption atmospheric pressure photoionization

Ambient mass spectrometry has attracted substantial attention in recent years. Among ambient ionization methods, thermal desorption ionization stands out because of two attributes: (1) simplicity, rendering the technique suitable for in-field applications, and (2) ability to couple with a variety of gas-phase ionization methods thereby broadening the range of molecules that can be analyzed with this method. Here, we report on improving the performance of a direct analysis in real time (DART) source by implementing atmospheric pressure photoionization (APPI) downstream of the desorption region. At identical desorption and ion sampling conditions, APPI leads to detection of radical molecular ions from non-polar compounds that are absent from the spectra generated by DART alone. Moreover, a factor of 3–5 improvement in sensitivity is observed using APPI for positive ions commonly detected by DART and DART-APPI. Using helium and nitrogen as desorption gases, APPI shows identical performance regardless of desorption gas type. In contrast, a dramatic decrease in sensitivity is observed for DART operated with nitrogen compared to DART with helium. Comparable performance for DART and DART-APPI are observed in negative ion mode, although both show a drastic improvement in the absence of the Vapur interface. This interface creates a differentially pumped chamber prior to inlet of the mass spectrometer and reduces the mass spectrometer gas load when helium is used as desorption gas.

Charge assisted laser desorption/ionization mass spectrometry of droplets

We propose and evaluate a new mechanism to account for analyte ion signal enhancement in ultraviolet-laser desorption mass spectrometry of droplets in the presence of corona ions. Our new insights are based on timing control of corona ion production, laser desorption, and peptide ion extraction achieved by a novel pulsed corona apparatus. We demonstrate that droplet charging rather than gas-phase ion-neutral reactions is the major contributor to analyte ion generation from an electrically isolated droplet. Implications of the new mechanism, termed charge assisted laser desorption/ionization (CALDI), are discussed and contrasted with those of the laser desorption atmospheric pressure chemical ionization method (LD-APCI). It is also demonstrated that analyte ion generation in CALDI occurs with external electric fields about one order of magnitude lower than those needed for atmospheric pressure matrix assisted laser desorption/ionization or electrospray ionization of droplets.

Interferometric droplet imaging for in situ aerosol characterization in an inductively coupled plasma

Size, velocity and evaporation rate of droplets in an Ar inductively coupled plasma (ICP) are simultaneously measured for the first time using a novel laser based imaging technique. In interferometric droplet imaging (IDI), an interference pattern created by the reflected and refracted rays from a droplet are collected in an out-of-focus image. The droplet diameter is determined by counting the number of fringes in the collected interference pattern. Combination of IDI and particle tracking velocimetry (PTV) provides the capability of monitoring droplet properties during the journey inside ICP. Using a demountable-direct injection high efficiency nebulizer, droplets in the range of 3–30 μm in diameter traveling at 15–70 m s−1 are observed in the analytical zone of the ICP. The upper velocity threshold for surviving droplets is determined by the nebulizer gas flow rate, whereas the lower threshold is mainly influenced by thermal expansion of the plasma gas. Droplet evaporation rates (0.26–0.36 mm2 s−1) are in good agreement with other reports and theoretical simulations for droplets in a 3000 K Ar environment.

Plasma-Assisted Reaction Chemical Ionization for Elemental Mass Spectrometry of Organohalogens

AbstractWe present plasma-assisted reaction chemical ionization (PARCI) for elemental analysis of halogens in organic compounds. Organohalogens are broken down to simple halogen-containing molecules (e.g., HBr) in a helium microwave-induced plasma followed by negative mode chemical ionization (CI) in the afterglow region. The reagent ions for CI originate from penning ionization of gases (e.g., N2) introduced into the afterglow region. The performance of PARCI-mass spectrometry (MS) is evaluated using flow injection analyses of organobromines, demonstrating 5–8 times better sensitivities compared with inductively coupled plasma MS. We show that compound-dependent sensitivities in PARCI-MS mainly arise from sample introduction biases. Figureᅟ

Gas Chromatography Plasma-Assisted Reaction Chemical Ionization Mass Spectrometry for Quantitative Detection of Bromine in Organic Compounds

We have recently introduced plasma-assisted reaction chemical ionization mass spectrometry (PARCI-MS) for elemental analysis of halogens in organic compounds. Here, we utilize gas chromatography (GC) coupled to PARCI-MS to investigate the mechanism of Br(-) ion generation from organobromines and to evaluate analytical performance of PARCI for organobromine analysis. Bromine atoms in compounds eluting from GC are converted to HBr in a low-pressure microwave induced helium plasma with trace amounts of hydrogen added as a reaction gas. Ionization is achieved by introducing nitrogen into the afterglow region of the plasma, liberating electrons via penning ionization and leading to formation of negative ions. We demonstrate that N2 largely affects the ionization process, whereas H2 affects both the ionization process and in-plasma reactions. Our investigations also suggest that dissociative electron capture is the main ionization route for formation of Br(-) ions. Importantly, GC-PARCI-MS shows a uniform response factor for bromine across brominated compounds of drastically different chemical structures, confirming PARCIs ability to quantify organobromines in the absence of compound-specific standards. Over 3 orders of magnitude linear dynamic range is demonstrated for bromine quantification. We report a detection limit of 29 fg of bromine on-column, ~4-fold better than inductively coupled plasma-MS.

Heat-Assisted Argon Electrospray Interface for Low-Flow Rate Liquid Sample Introduction in Plasma Spectrometry

A heated (approximately 90 degrees C) laminar flow interface has been designed to assist in the development of an argon electrospray sample introduction system for low-flow rate applications using inductively coupled plasma (ICP) spectrometry. Previously, the stability and robustness of the ICP were compromised by the entrainment of air, N2, or gas mixtures (e.g., Ar-N2) from the electrospray source. Also, more concentrated organic solvents (e.g., 50% (v/v) methanol-water), typically introduced by electrospray, could generate carbon deposits that obstruct the entrance lens to an ICP optical emission spectrometer (ICP-OES) or the sampler/skimmer cone interface in an ICP mass spectrometer (ICP-MS), decreasing analyte sensitivity. With the new interface design, a stable spray of 5% (v/v) methanol-water in a pure argon environment is achieved, eliminating the aforementioned problems. The turbulence and the consequent droplet loss caused by high gas velocity around the electrospray capillary are mitigated by the use of a laminar-flow gas with the aid of a flow diffuser. The argon electrospray interface is successfully installed on an ICP-OES and an ICP-MS for the first time.