Weiterentwicklung und Charakterisierung der halo-FAPA als Anregungs- und Ionisationsquelle für die Emissions- bzw. Massenspektrometrie
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Abstract
The halo-FAPA (halo-shaped flowing atmospheric pressure afterglow) ionization and excitation source is based on a direct current, low power glow discharge at ambient conditions (120 V – 250 V, 2 mA – 80 mA). Helium (10 mL min-1 – 1000 mL min-1) flows through two concentrically aligned, stainless steel, capillary electrodes (i. d. 2.4 mm and 1.0 mm, respectively), extending the plasma beyond the discharge zone (afterglow). Sampling into the outer or inner channel facilitates ionization via electron transfer or proton transfer reactions, respectively. A sample introduced into the outer channel interacts with species generated immediately by the discharge (e. g., e-, He+, He*). Sampling into the inner channel, however, results in sample interaction with reaction products of the aforementioned species with atmospheric components such as water cluster ions (H(H2O)n+). The latter facilitate soft ionization of polar and medium polar molecules via proton transfer reactions.
Halo-FAPA afterglow sampling has proven to be very powerful in molecular ion mass spectrometry. The halo-FAPA source is simple, inexpensive, and has low sample uptake. Samples may be introduced via the ADI-MS principle or in continuous flow. The halo-FAPA construction has been revised, improving or eliminating deficiencies in aerosol transfer, risk of contamination, and heat resistance. This way, discharge dimensions may also be varied. This revised halo-FAPA implementation has been characterized regarding its electric properties. Also, several heavy-duty components underwent microscopic examination. An experimental setup allowing immediate switching of mercury vapor uptake has been developed. Using the mercury atomic emission signal, the halo-FAPA has been characterized and plasma diagnostics were conducted. The mercury signal strongly increases with increasing discharge current and smaller discharge dimensions. Signal increase is smaller for higher gas flow rates in the afterglow region while the signal is most strongly dependent on gas flow rates in the discharge region. There is an optimum of about 200 mL min-1 as lower flow rates increase the discharge temperature at the cost of emerging discharge instability.
Rotational temperature is well suited to measure the influence a modified experimental setup or altered operating parameters have. Using two different molecular species, rotational temperatures depending on operating parameters were determined for both halo-FAPA experimental setups, old and new. Excitation temperatures were determined using helium emission lines. Ranging from 2600 K to 2950 K, they are typical for this kind of glow discharge, though large uncertainties of up to 400 K did exist. Unlike rotational temperatures, excitation temperatures have been almost independent from water introduction into the discharge via a DOD aerosol generator. Electron number densities were measured from both STARK broadening of the BALMER series Hβ line at 486.132 nm (Ne = 1,2 · 1020} m-3) as well as the SAHA-EGGERT equation using magnesium atomic and ion lines (Ne = 1014 m-3). Different results from these measurements suggest non-thermal ionization mechanisms such as PENNING ionization, which was confirmed by ionization temperature measurements of about 5400 K -- 5500 K using SAHA-EGGERT equation.
Aqueous mercury solutions were also introduced into the halo-FAPA source and detected via optical emission spectrometry. Sample introduction via conventional, low-flow pneumatic nebulization and DOD aerosol generation were compared. The latter showed far superior results in optical emission spectrometry. Using the DOD aerosol generator, the halo-FAPA-OES system could be calibrated based on a dosing frequency-based strategy, resulting in an absolute limit of detection of (21 ± 14) pg s-1 (U; k=2).