Atmospheric pressure chemical ionization mass spectrometry at low flow rates: Importance of ion source housing

Gas Dynamic Virtual Nozzle Sprayer for an Introduction of Liquid Samples in Atmospheric Pressure Ionization Mass Spectrometry

Electrospray may exhibit inadequate ionization efficiency in some applications. In such cases, atmospheric-pressure chemical ionization (APCI) and photoionization (APPI) can be used. Despite a wide application potential, no APCI and APPI sources dedicated to very low sample flow rates exist on the market. Since the ion source performance depends on the transfer of analytes from the liquid to the gas phase, a nebulizer is a critical component of an ion source. Here, we report on the nebulizer with a gas dynamic virtual nozzle (GDVN) and its applicability in APCI at microliter-per-minute flow rates. Nebulizers differing by geometrical parameters were fabricated and characterized regarding the jet breakup regime, droplet size, droplet velocity, and spray angle for liquid flow rates of 0.75-15.0 μL/min. A micro-APCI source with the GDVN nebulizer behaved as a mass-flow-sensitive detector and provided stable and intense analyte signals. Compared to a classical APCI source, an order of magnitude lower detection limit for verapamil was achieved. Mass spectra recorded with the nebulizer in dripping and jetting modes were almost identical and did not differ from normal APCI spectra. Clogging never occurred during the experiments, indicating the high robustness of the nebulizer. Low-flow-rate APCI and APPI sources with a GDVN sprayer promise new applications for low- and medium-polar analytes.

Development of a Tube Plasma Ion Source for Gas Chromatography-Mass Spectrometry Analysis and Comparison with Other Atmospheric Pressure Ionization Techniques

A tube plasma ionization (TPI) open-air source for gas chromatography-mass spectrometry (GC-MS) was developed. This source is based on an inverse low temperature plasma configuration where the pin inner electrode is applying the high voltage and the grounded electrode is the housing itself. The ionization possibilities were tested by using an EPA mix of priority contaminants, showing that 68% of the analytes could undergo both proton-transfer and charge-exchange reactions. The potential of using different discharge gases (He and Ar) to ionize the analytes and auxiliary gases (He, N₂, O_2, and synthetic air) to transport the ions toward the MS was carefully investigated. Additionally, the addition of water was also tested to show the different ionization trends in the TPI source. Finally, the ionization by TPI under both dry and wet conditions was compared with other gas-phase atmospheric pressure ionization sources showing TPI could ionize a wider range of compounds (97%) than atmospheric pressure chemical ionization (APCI, 95%) and atmospheric pressure photoionization (APPI, 87%). Besides, the detection capability of TPI was better than APCI and APPI, achieving instrumental limits of detection down to 3 fg on column, which demonstrates the great potential of this ionization source for GC-MS determinations.

In-Line Dopant Generation for Atmospheric Pressure Ionization Mass Spectrometry

A concentric trace gas permeation tube that diffuses chemical reagents to a central carrier gas stream is used to drive chemical reaction pathways and influence gas-phase chemistry for a variety of atmospheric pressure ionization sources for mass spectrometry. Tunable permeation through the reservoir-jacketed polymer membrane is triggered by the heated gas moving through the tube, evaporating the dopant into a sheath dry gas or into a sample stream in room air without diluting the analyte concentration. The permeator is used to add dopants to an electrospray plume for analyte ion charge reduction and to perform hydrogen-deuterium exchange on biomolecules in different spray conditions. Dopants are also added to atmospheric pressure chemical ionization to favor the ionization of select components of diesel fuel. Atmospheric pressure photoionization is performed with the permeation tube in line with tubing transporting sample headspace to an enclosed discharge lamp. Toluene dopant from the permeator increases the proton transfer and charge exchange signal from clove oil and mothballs many times without exposing the laboratory to reagent fumes. Water permeation is also used to humidify the sample gas stream.