Schlieren visualization of fluid dynamics effects in direct analysis in real time mass spectrometry.

RATIONALE: The success of ambient analysis using plasma-based ion sources depends heavily on fluid dynamics and mass transport efficiency in the sample region. To help characterize the influence of these determining factors, visualization of the gas flow profile for a Direct Analysis in Real Time (DART) ion source at the mass spectrometer atmospheric pressure (AP) interface was performed using the Schlieren technique. METHODS: The DART helium flow pattern was imaged in model systems incorporating different interface designs, i.e. skimmer or capillary inlet, and for sampling strategies using several types of traditional DART sample probes including a glass capillary, swab, and drug tablet. Notably, Schlieren experiments were conducted on instruments equipped with the gas-ion separator tube (GIST) adapter and Vapur® pump, and on setups featuring the transmission mode (TM) DART module used in standard practice. RESULTS: DART sources were seen to expel a collimated, highly laminar helium stream across interface distances up to ~8 cm. The helium stream was robust to the influence of gas temperature (50-500 °C) and flow rate (≤3.5 L min(-1) ), but considerable DART gas deflection or full disruption was observed in each sampling scenario. The severity of the flow disturbance depended on probe size and placement, the GIST/Vapur® settings, or counter-current gas movements present at the interface. CONCLUSIONS: The real-time Schlieren visualizations introduced in this work provide new insight on the fluid dynamics within the DART-MS sample gap while also helping to identify those experimental parameters requiring optimization for improved transmission.

Plasma-spray ionization (PLASI): a multimodal atmospheric pressure ion source for liquid stream analysis.

A new ion generation method, named plasma-spray ionization (PLASI) for direct analysis of liquid streams, such as in continuous infusion experiments or liquid chromatography (LC), is reported. PLASI addresses many of the analytical limitations of electrospray ionization (ESI) and has potential for real time process stream analysis and reaction monitoring under atmospheric conditions in non-ESI friendly scenarios. In PLASI-mass spectrometry (MS), the liquid stream is pneumatically nebulized and partially charged at low voltages; the resultant aerosol is thus entrained with a gaseous plasma plume from a distal glow discharge prior to MS detection. PLASI-MS not only overcomes ESI-MS limitations but also generates simpler mass spectra with minimal adduct and cluster formation. PLASI utilizes the atomization capabilities of an ESI sprayer operated below the ESI threshold to generate gas-phase aerosols that are then ionized by the plasma stream. When operated at or above the ESI threshold, ionization by traditional ESI mechanisms is achieved. The multimodal nature of the technique enables readily switching between plasma and ESI operation. It is expected that PLASI will enable analyzing a wide range of analytes in complex matrices and less-restricted solvent systems, providing more flexibility than that achievable by ESI alone.

Head group-functionalized poly(ethyleneglycol)-lipid (PEG-lipid) surface modification for highly selective analyte extractions on capillary-channeled polymer (C-CP) fibers.

Polypropylene (PP) capillary-channeled polymer (C-CP) fibers were modified by adsorption of a head group-functionalized lipid to generate analyte-specific surfaces for application as a stationary phase in high performance liquid chromatography (HPLC) or solid phase extraction (SPE). The aliphatic moiety of the lipid adsorbs strongly to the hydrophobic PP surface, with the hydrophilic active head groups orienting themselves toward the more polar mobile phase, thus allowing for interactions with the desired solutes. Initial proof-of-concept was achieved by adsorbing a biotin-poly(ethylene glycol)-functionalized lipid to the surface of the PP C-CP fibers. Surface modification and uniformity was evaluated by binding streptavidin labeled with Texas Red (SAv-TR) to the biotin moiety. Isolation of SAv-TR from a mixture in neat buffer and in cleared lysate demonstrated the capability of the modified fibers to extract an analyte of interest from a complex viscous mixture. It is believed that this surface modification approach is generally applicable to a diversity of selective protein immobilization applications, including clinical diagnostics and preparative scale HPLC on C-CP fibers as well as to other hydrophobic supports.

Capillary-channeled polymer (C-CP) films as processing platforms for protein analysis by matrix-assisted laser/desorption ionization mass spectrometry (MALDI-MS).

Polypropylene (PP) capillary-channeled polymer (C-CP) films have parallel, µm-sized channels that induce solution wicking via capillary action. Efficient mass transport from the solution phase to the channel surface leads to adsorption of hydrophobic protein solutes. The basic premise by which C-CP films can be used as media to manipulate analyte solutions (e.g., proteins in buffer), for the purpose of desalting or chromatographic separation prior to MALDI-MS analysis is presented here. Cytochrome c and myoglobin prepared in a Tris-HCl buffer, and ribonuclease A, lysozyme, and transferrin prepared in phosphate buffered saline (PBS), are used as the test solutions to demonstrate the desalting concept. Protein analysis is performed after deposition on a C-CP film with and without a water washing step, followed by spray deposition of a typical sinapinic acid matrix. Extracted MALDI mass spectra exhibit much improved signal-to-noise characteristics after water washing. A mixture of cytochrome c and myoglobin (2 µL of 2.5 µM each in Tris-HCl buffer) was applied, washed with water and spatially separated via simple capillary action (wicking) using a reversed-phase solvent composition of 0.1% trifluoroacetic acid (TFA) in 50:50 acetonitrile (ACN):H(2)O. Subsequent application of sinapinic acid followed by imaging of the film using MALDI-MS reveals that as the protein solution is wicked down the film, separation occurs.