Integrated liquid chromatography-heated nebulizer microchip for mass spectrometry

Single-particle-frit-based packed columns for microchip chromatographic analysis of neurotransmitters

The fabrication of effective microchip liquid chromatography (LC) systems tends to be limited by the availability of suitable chromatographic columns. Herein, we developed a glass microchip LC system in which porous single-particle silica was adopted as frits and a freeze-thaw valve was utilized to achieve sample injection without interfering with sampling. The fabrication of single-particle-frit-based packed columns did not require an additional packing channel, thus avoiding dead volumes within the channel interface that can influence chromatographic separation. Further, the length of the packed column could be adjusted using the location of single-particle frits within the column channel. The fabricated frits exhibited high mechanical strength, good permeability, and tolerance for high pressures during chromatographic separation. In particular, the developed microchip LC system was able to withstand high separation pressures of more than 5000 psi. The microchip LC system was applied to the separation of neurotransmitters. Three different monoamine neurotransmitters were completely separated within 5 min with theoretical plate numbers on the order of 100,000 plates m^-1. The microchip LC system has a potential for application in a variety of fields including environmental analysis, food safety, drug analysis, and biomedicine.

Applicability of low-flow atmospheric pressure chemical ionization and photoionization mass spectrometry with a microfabricated nebulizer for neutral lipids

RATIONALE

Mass spectrometry with atmospheric pressure chemical ionization (APCI) or photoionization (APPI) is widely used for neutral lipids involved in many fundamental processes in living organisms. Commercial APCI and APPI sources operate at high flow rates compatible with conventional high-performance liquid chromatography (HPLC). However, lipid analysis is often limited by a small amount of sample, which requires low flow rate separations like capillary or micro-HPLC. Therefore, APCI and APPI suitable for microliter-per-minute flow rates need to be developed and applied for neutral lipids.

METHODS

A micro-APCI/APPI source with a heated chip nebulizer was assembled and mounted on a Thermo ion trap instrument. The ion source operated in APCI, APPI or dual mode was optimized for low microliter-per-minute sample flow rates. The source performance was investigated for squalene, wax esters, fatty acid methyl esters, triacylglycerols, and cholesterol.

RESULTS

The ion source behaved as a mass-flow-sensitive detector. Direct infusion of methyl oleate showed superior analytical figures of merit when compared with high-flow ion sources. A detection limit of 200 pmol/mL and a linear dynamic range spanning three orders of magnitude were measured for micro-APCI. The mass spectra of most lipids differed from high flow rate spectra. Unlike micro-APCI, micro-APPI spectra were complicated by odd-electron species. Dual APCI/APPI mode did not show any benefits for neutral lipids. Applications for lipid samples were demonstrated.

CONCLUSIONS

Micro-APCI-MS is a useful detection technique for neutral lipids at microliter-per-minute flow rates. It offers high sensitivity and high quality of spectra in direct infusion mode and promises successful utilization in capillary and micro-HPLC applications.

Fast analysis of 29 polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs with ultra-high performance liquid chromatography-atmospheric pressure photoionization-tandem mass spectrometry

Polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs are ubiquitous in the environment. Some of them are probable carcinogens and some are source markers. This work presents an ultra-high performance liquid chromatography-atmospheric pressure photoionization-tandem mass spectrometry (UHPLC-APPI-MS/MS) method for simultaneous analysis of 20 PAHs and nine nitro-PAHs. These compounds are separated in 15 minutes in the positive mode and 11 minutes in the negative mode, one half of GC/MS analysis time. Two pairs of precursor/product ions are offered, which is essential for confirmation. This method separates and quantifies benzo[a]pyrene (the most toxic PAHs) and non-priority benzo[e]pyrene (isomers, little toxicity) to avoid overestimation of toxin levels, demonstrating its importance for health-related researches. With 0.5% 2,4-difluoroanisole in chlorobenzene as the dopant, limits of detection of PAHs except acenaphthylene and those of nitro-PAHs except 2-nitrofluoranthene are below 10 pg and 3 pg, respectively, mostly lower than or comparable to those reported using LC-related systems. The responses were linear over two orders of magnitude with fairly good accuracy and precision. Certified reference materials and real aerosol samples were analyzed to demonstrate its applicability. This fast, sensitive, and reliable method is the first UHPLC-APPI-MS/MS method capable of simultaneously analyzing 29 environmentally and toxicologically important PAHs and nitro-PAHs.

High-performance liquid chromatography on glass chips using precisely defined porous polymer monoliths as particle retaining elements

A stable and permanent integration of miniature packed bed separation columns into microfluidic systems is a major issue in nano liquid chromatography. Various approaches like differently shaped retaining elements or the use of key stone effect have been investigated. We show a flexible integration of miniature packed bed separation columns into microfluidic chips utilising common HPLC material achieved by laser-assisted generation of narrow, photopolymerised frits. The generated retaining elements serve as an in- and outlet frits for the columns. An optimised pre-polymeric solution, consisting of butyl acrylates and a porogen, allows a precise fabrication of frit-type structures with lengths of less than 100 m and the capability to withstand common slurry packing pressures of more than 250 bar. The separation of seven polycyclic aromatic hydrocarbons by pressure-driven, reversed-phase chromatography proves the high quality of the created chromatographic column inside a glass chip. Plate heights down to 2.9 were achieved and extremely fast separations with sub-second peak widths were performed in isocratic and gradient elution modes on very short columns (≤ 25 mm).

Remote detection NMR imaging of gas phase hydrogenation in microfluidic chips

The heterogeneous hydrogenation reaction of propene into propane in microreactors is studied by remote detection (RD) nuclear magnetic resonance (NMR). The reactors consist of 36 parallel microchannels (50 × 50 μm(2) cross sections) coated with a platinum catalyst. We show that RD NMR is capable of monitoring reactions with sub-millimeter spatial resolution over a field-of-view of 30 × 8 mm(2) with a steady-state time-of-flight time resolution in the tens of milliseconds range. The method enables the visualization of active zones in the reactors, and time-of-flight is used to image the flow velocity variations inside the reactor. The overall reaction yields determined by NMR varied from 10% to 50%, depending on the flow rate, temperature and length of the reaction channels. The reaction yield was highest for the channels with the lowest flow velocity. Propane T1 relaxation time in the channels, estimated by means of RD NMR images, was 270 ± 18 ms. No parahydrogen-induced polarization (PHIP) was observed in experiments carried out using parahydrogen-enriched H2, indicating fast spreading of the hydrogen atoms on the sputtered Pt surface. In spite of the low concentration of gases, RD NMR made imaging of gas phase hydrogenation of propene in microreactors feasible, and it is a highly versatile method for characterizing on-chip chemical reactions.

Chip-based separation devices coupled to mass spectrometry

The hyphenation of miniaturized separation techniques like chip electrophoresis or chip chromatography to mass spectrometry (MS) is a highly active research area in modern separation science. Such methods are particularly attractive for comprehensive analysis of complex biological samples. They can handle extremely low sample amounts, with low solvent consumption. Furthermore they provide unsurpassed analysis speed together with the prospect of integrating several functional elements on a single multifunctional platform. In this article we review the latest developments in this emerging field of technology and summarize recent trends to face current and future challenges in chip-based biochemical analysis.

A practical guide for the fabrication of microfluidic devices using glass and silicon

This paper describes the main protocols that are used for fabricating microfluidic devices from glass and silicon. Methods for micropatterning glass and silicon are surveyed, and their limitations are discussed. Bonding methods that can be used for joining these materials are summarized and key process parameters are indicated. The paper also outlines techniques for forming electrical connections between microfluidic devices and external circuits. A framework is proposed for the synthesis of a complete glass/silicon device fabrication flow.