Focusing Plasma Desorption/Ionization Mass Spectrometry

Abstract

A plasma-based source named focusing plasma desorption/ionization (FPDI) is described, which applies a high direct current voltage between a metal wire inside a polymeric hollow truncated cone and a piece of a one-sided coated conducting paper substrate. The conducting paper acts as both the counter electrode and the sample carrier. Upon the generation of a visible plasma beam, it would directly ionize the samples spotted on the conducting paper substrate or located around the plasma beam. The signal intensity of target analytes in mass spectrometric analysis is dependent highly on whether the conducting paper substrate is grounded or not, the type of conducting paper substrate, the inside diameter of the polymeric hollow truncated cone tip, the metal wire tip-topolymer tip distance, the polymer tip-to-paper substrate distance, the applied voltage, and the helium flow rate. Based on the experimental observation, a plausible mechanism is proposed for the generation of the plasma beam from FPDI. Compared to the available low-temperature plasma, flowing atmospheric-pressure afterglow, and helium plasma ionization sources, FPDI has demonstrated higher sensitivity and better compatibility with commercial mass spectrometers without any extra power supplies. As a proof of concept, FPDI coupled with a mass spectrometer has also been applied for the discrimination of different brands of gasoline and determination of solid tablets and pesticides with limits of detection in the range of 2.2 to 30.7 ng mL −1 .

Surface charge-induced electrospray for high-throughput analysis of complex samples and electrochemical reaction intermediates using mass spectrometry

“…Among them, the residual voltage on the polypropylene tip surface was much higher than the others, although they all demonstrated a fading trend with time. As is well known, the surface polarity of those materials has an order of quartz > resin > polypropylene, 31 proportional to their ionization efficiencies and surface residual voltages. This in turn reveals that the transfer of voltage from the Cu conductive tape to the paper substrate for the SCIESI is associated with the pipette tip material.…”

Improvement in the performance of focusing plasma desorption ionization by altering its counter electrode

“…The experimental procedure of the FPDI source used in the current work was similar to that of our recent report 32 except for the paper counter electrode. In particular, the pesticide standards used in these experiments, including alachlor, acetochlor, metolachlor, pretilachlor, and benzeneacetamide, were prepared by diluting the corresponding stock solutions into methanol/water (1:1, v/v).…”

“…Similar to other plasma‐based ionization sources, 6,9–12 the helium gas flow not only served as the medium to generate active species upon high voltage for ionizing target analytes in samples but also was the carrier to deliver charged analytes into mass spectrometer inlet. However, as conducting materials were coated on the surface of the paper substrate, 32 it would deteriorate the transfer efficiency of helium gas flow‐through‐coated paper, and a certain percentage of helium gas moved around the outsides of the paper, which would make charged analytes in helium flow far away from the MS inlet. To address this issue, the current study employs a piece of paper drilled with holes as both a counter electrode and a sample carrier.…”

“…In the recent study, 32 we introduced a focusing plasma desorption ionization (FPDI), which required only a helium flow rate as low as 0.3 L min −1 for generating visible plasma upon a low current DC power supply (a few microamperes). In addition, its performance was superior to that of both LTP and FAPA sources in analyzing diverse organic compounds.…”