This study presents coupling of a poly(dimethylsiloxane) (PDMS) micro-chip with electrospray ionization-mass spectrometry (ESI-MS). Stable electrospray is generated directly from a PDMS micro-channel without pressure assistance. Hydrophobic PDMS aids the formation of a small Taylor cone in the ESI process and facilitates straightforward and low-cost batch production of the ESI-MS chips. PDMS chips were replicated with masters fabricated from SU-8 negative photoresist. A novel coating, an amorphous diamond-like carbon-poly(dimethylsiloxane) hybrid, deposited on the masters by the filtered pulsed plasma arc discharge technique, improved significantly the lifetime of the masters in PDMS replications. PDMS chip fabrication conditions were observed to affect the amount of background peaks in the MS spectra. With an optimized fabrication process (PDMS curing agent/silicone elastomer base ratio of 1/8 (w/w), curing at 70 degree C for 48 h) low background spectra were recorded for the analytes. The performance of PDMS devices was examined in the ESI-MS analysis of some pharmaceutical compounds and amino acids.

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A novel microchip heated nebulizer for atmospheric pressure chemical ionization mass spectrometry is presented. Anisotropic wet etching is used to fabricate the flow channels, inlet, and nozzle on a silicon wafer. An integrated heater of aluminum is sputtered on a glass wafer. The two wafers are jointed by anodic bonding, creating a two-dimensional version of an APCI source with a sample channel in the middle and gas channels symmetrically on both sides. The ionization is initiated with an external corona-discharge needle positioned 2 mm in front of the microchip heated nebulizer. The microchip APCI source provides flow rates down to 50 nL/min, stable long-term analysis with chip lifetime of weeks, good quantitative repeatability (RSD < 10%) and linearity (r(2) > 0.995) with linear dynamic rage of at least 4 orders of magnitude, and cost-efficient manufacturing. The limit of detection (LOD) for acridine measured with microchip APCI at flow rate of 6.2 muL/min was 5 nM, corresponding to a mass flow of 0.52 fmol/s. The LOD with commercial macro-APCI at a flow rate of 1 mL/min for acridine was the same, 5 nM, corresponding to a significantly worse mass flow sensitivity (83 fmol/s) than measured with microchip APCI. The advantages of microchip APCI makes it a very attractive new microfluidic detector.

Besides its particular importance as a widely used therapeutic agent, insulin (and its synthetic derivatives) has been suspected, purported, and proven to be a lethal weapon in numerous cases of attempted or successful homicide and suicide. In addition to blood and urine as common matrices for clinical diagnosis and post-mortem analysis, vitreous humour has gained considerable attention in autopsy and follow-up investigations due to its ability to provide valuable information on cause and time of death. However, post-mortem insulin analyses using such specimens have been rare due to the limited penetration of peptide hormones into the vitreous body, and immunoassays were exclusively employed in those studies. In the present communication, the determination of insulin(s) from vitreous humour by means of immunopurification combined with ultrahigh performance liquid chromatography--high resolution/high accuracy (tandem) mass spectrometry is reported. Exploiting the constantly increasing sensitivity and robustness of modern mass-spectrometry-based instruments, the option to identify insulin in post-mortem vitreous samples is demonstrated with a specimen collected from a non-diabetic victim who died from an insulin overdose. This communication represents the first successful mass-spectrometry-based analysis of post-mortem material related to an insulin poisoning case.

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