Precise detection of trace amount of molecules, such as the disease biomarkers present in biofluids or explosive residues, requires high sensitivity detection. electrospray ionization–mass spectrometry (ESI-MS) is a common and effective technique for sensitive trace molecular detection in small-volume liquid samples. In ESI-MS, nano-liter volume samples are ionized and aerosolized by ESI, and fed into MS for mass analysis. ESI-MS has proven to be a reliable ionization technique for coupling liquid phase separations like liquid chromatography (LC) and capillary zone electrophoresis (CE) with the highly specific resolving power of MS. While CE and ESI can be performed on a microfluidic chip having a footprint of a few cm2, MS is typically at least 100 times bigger in size than a micro-chip. A reduced size, weight, and power profile would enable semi-portable applications in forensics, environmental monitoring, defense, and biological/pharmaceutical applications. To achieve this goal, we present an initial study evaluating the use of mid-infrared absorption spectroscopy (MIRAS) in place of MS to create a ESI-MIRAS system. To establish feasibility, we perform ESI-MIRAS on phospholipid samples, which have been previously demonstrated to be separable by CE. Phospholipids are biomarkers of degenerative neurological, kidney, and bone diseases and can be found in biofluids such as blood, urine and cerebrospinal fluid. To establish sensitivity limits, calibration samples of 100 μM concentration are electrospray deposited on to a grounded Si wafer for different times (1 minutes to 4 minutes with a 1 minute step). The minimum detectable concentration-time product, where a FTIR globar is used as the MIR source, is found ~200 μM·s.
Tahsin Ahmed, Erick Foster, Paul Bohn, and Scott Howard, "Feasibility study of mid-infrared absorption spectroscopy using electrospray ionization," Proc. SPIE 9930, Biosensing and Nanomedicine IX, 99300U (Presented at SPIE Nanoscience + Engineering: August 30, 2016; Published: 27 September 2016); https://doi.org/10.1117/12.2237958.
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