In the medical field, rapid and less-invasive techniques for diagnosing diseases are required. The chemical components of sweat or sebum might reflect the health conditions of the living body. For example, a lack of insulin causes an increase of blood ketone-based derivative. Since they are secreted by human skin, detection of these components will become useful for the early stage diagnosis of diabetes. In addition, information about drugs taken or cosmetics adhered to the skin could also be acquired. The amount of adhered samples was quite small, almost conventional sampling method was not suitable and sample preparation method without sample losses should be developed. The laser ablation or high-temperature gas sampling might be overcome these problems, however, these techniques gave a thermal damage to targets, so it was difficult to apply them to heat sensitive materials like human skin.
In our laboratory, Atmospheric Plasma Soft Ablation method (APSA) has been developed for desorption/ionization of skin-surface adhesive compounds1. In APSA, sample on the surface is desorbed and ionized simultaneously by high-energy species in low temperature plasma. This plasma does not give thermal or discharge damage to the compounds. In our previous study, a touchable helium high-power pulsed microplasma2 (HPPM) as low temperature plasma source was irradiated to pharmaceutical samples. As a result, each sample on the glass plate was successfully analyzed with the lower limit of detection (LOD) of pmol level. In practical use, the plasma source for desorption/ionization should be handled remotely from mass spectrometer. However, analytical sensitivity of this system may be spoiled with increasing the distance between the desorbing region and the mass spectrometer.
In this study, a new desorption/ionization system was developed without decreasing sensitivity with stretching distance between respective regions. This system had two individual low temperature plasmas. The schematic of our system is shown in Fig1. Skin-surface adhesive compounds were desorbed by a high-power pulsed low temperature microplasma and the desorbed molecules were transported to the ionization region with the gas flow, and then ionized by a dielectric barrier discharge using helium gas flow. Plasma source condition can be optimized for desorption and ionization respectively. Therefore high sensitive analysis could be expected nevertheless the desorption region was remote from the inlet of the mass spectrometer. Phenyl salicylate (1,000 ppm in 5 µL acetonitorile, m/z=215) on the palm was used as real sample. Although the distance between desorption region and the inlet of mass spectrometer (300 mm) was approximately 30 times longer than the previous system, signal intensity of sample was almost the same, and then sample was successfully analyzed with the LOD of 2.5 pmol (Fig.2). To improve analytical sensitivity further, hydrogen was mixed into dielectric barrier discharge (DBD) gas for increasing production of proton in ionization region and increasing the amount of protonated sample molecules. As a result, the signal intensity of phenyl salicylate increased by 4 times when hydrogen was mixed with DBD gas at a flow rate of less than 1%. Since analytical performance of this system was evaluated further by changing the plasma gas and the discharge parameters, they will be reported.
受賞情報
The Best Poster Award in the 6th Asia-Pacific Winter Conference on Plasma Spectrochemistry
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