Spectroscopic plasma diagnostics on a low-temperature plasma probe for ambient mass spectrometry

Abstract

Since the inception of ambient desorption/ionization mass spectrometry, plasma ionization sources have played an increasing role in molecular mass spectrometry. Although a variety of discharge designs and geometries, along with a range of applications, have been introduced, very little published work has focused on the characterization and fundamental examination of these discharges, especially on the desorption/ionization processes they employ. In the present work, a simple yet effective ambient desorption/ionization source based on a dielectric-barrier discharge, the low-temperature plasma (LTP) probe, was optically characterized. By means of a spatially selective detection system, maps of reactive species created in both the plasma and the afterglow regions were recorded. From these maps, the origin of impurities important in mass spectrometric analyses, such as H 2 O, N 2 , and O 2 , was deduced. Electron number densities and rotational temperatures for the LTP were found to be similar to those reported for other dielectric-barrier discharges. Lastly, the effect of plasma parameters on emission spectra was correlated with mass spectral results previously reported for the same ionization source.

Gas-Phase Peptide Sulfinyl Radical Ions: Formation and Unimolecular Dissociation

“…The data in Figure 2 compare the effect of helium gas flow rates on the formation of sulfinyl radical ions for the A chain of peptide 1. Clearly, higher intensity of sulfinyl radical ions were formed with a higher helium flow rate, presumably due to higher number densities of radicals produced from more intense plasma [60]. However, consecutive oxidation of peptide side chain or reactions of the sulfinyl radical ions became more competitive and the overall yield or purity of the sulfinyl radical ions could be adversely affected [59], as shown in Figure 2c.…”

“…Studies have shown that molecular species (OH, N 2 , N 2 + , NO, and NO 2 ) as well as atomic species (metastable He, H, and O) are abundant in an AP helium LTP [60,61]. For peptides having intra-chain disulfide bonds, one major product after the nanoESI plume interacting with LTP were ions corresponding to the addition of O and H to the intact peptide ([M + nH + OH] n•+ ) [58,59].…”

The Effects of Added Hydrogen on Noble Gas Discharges Used as Ambient Desorption/Ionization Sources for Mass Spectrometry

“…The creation of M + ions via charge transfer in pure helium discharges has been attributed to charge transfer with species such as N 2 + , NO + , and O 2 + [6,25]. However, neither N 2 + nor O 2 + has been observed in great abundance in a pure argon discharge [8], and neither ion is in the mass range observable with our instrument. Within the range that we have observed (≈35 < m/z < 110), there are no changes in the background spectra that would account for the increase in sensitivity via charge transfer ionization with the addition of H 2 to the discharge.…”

“…Based on cost and availability, argon would seem to be a good alternative to helium. However, experiments with pure argon discharges have yielded sensitivities 10-100 times worse than those recorded with helium plasmas using the LTP [8]. There is good reason to think that adding hydrogen to argon could improve its performance as an ADI plasma gas.…”

“…Of particular interest have been the plasma-based ADI sources [3][4][5] that require little to no sample preparation, enable fast analysis times, and feature simple source construction [6,7]. Plasma-based ADI sources usually use helium for the plasma gas because of its high ionization potential and the high energies of its metastable states, but other plasma gases have been utilized, including argon, nitrogen, and air [3,8]. However, little work has been done to determine the effectiveness of using gas mixtures as plasma gases for these ADI sources.…”