Development of cVSSI-APCI for the Improvement of Ion Suppression and Matrix Effects in Complex Mixtures

Abstract

The new ionization technique termed vibrating sharp-edge spray ionization (cVSSI) has been coupled with corona discharge to investigate atmospheric pressure chemical ionization (APCI) capabilities. The optimized source was evaluated for its ability to enhance ion signal intensity, overcome matrix effects, and limit ion suppression. The results have been compared with state-of-the-art ESI source performance as well as a new APCI-like source. In methanol, the ion signal intensity increased 10-fold and >10-fold for cocaine and the suppressed analytes, respectively. The ability to overcome ion suppression was improved from 2-fold to 16-fold for theophylline and vitamin D2, respectively. For aqueous samples, ion signal levels increased by two orders of magnitude for all analytes. In both solvent systems, the signal-to-noise ratios also increased for all suppressed analytes. One example of the characterization of low-ionizing (by ESI or cVSSI alone) species in the presence of high-ionizing species by direct analysis from a cotton swab is presented. The work is discussed with respect to the advantages of cVSSI-APCI for direct, in situ, and field analyses.

Figure 1.

Schematic diagram of the cVSSI-APCI system setup. This includes the waveform generator, RF amplifier, high-voltage power supply, and syringe pump. Separate components are labeled.

Figure 2.

Ion chronograms and mass spectra for the methanol mixture sample. Panel A shows the ion chronogram recorded over a 6.5-minute time frame during which the voltage applied to the HESI needle was ramped (see text for details). Panel B shows the mass spectrum recorded at the voltage setting for which maximum intensities were observed for the lower-intensity ions. The corresponding time frame for recording this spectrum is shown as a red trace in the ion chronogram (Panel A). Panel C shows the ion chronogram recorded over a 5-minute time frame during which the voltage applied to the APCI needle was ramped (see text for details) for the cVSSI-APCI setup. Panel D shows the mass spectrum recorded at the voltage setting for which maximum intensities were observed for the lower-intensity ions. The corresponding time frame for recording this spectrum is shown as a red trace in the ion chronogram (Panel C). The peak labels in panel B and D correspond with the numbered molecules in Table 1.

Figure 3.

Mass spectra of 5 μM caffeine. Panel A shows the mass spectrum obtained when only cVSSI is activated. Panel B show the mass spectrum obtained when both cVSSI and APCI are activated. Dataset features are labeled with the compound number designation in Table 1.

Figure 4:

Ion chronograms and mass spectra for the water mixture sample. Panel A shows the ion chronogram recorded over 6.5-minute time frame during which the voltage applied to the HESI needle was ramped (see text for details). Panel B shows the mass spectrum recorded at the voltage setting for which maximum intensities were observed for the lower-intensity ions. The corresponding time frame for recording this spectrum is shown as a red trace in the ion chronogram (Panel A). Panel C shows the ion chronogram recorded over 5-minute time frame during which the voltage applied to the APCI needle was ramped (see text for details) for the cVSSI-APCI setup. Panel D shows the mass spectrum recorded at the voltage setting for which maximum intensities were observed for the lower-intensity ions. The corresponding time frame for recording this spectrum is shown as a red trace in the ion chronogram (Panel C). The peak labels in panel B and D correspond with the numbered molecules in Table 1.

Figure 5.

Standard curves for thymine (A, B) and theophylline (C, D). The left and right panels show the data that were recorded for the HESI and cVSSI-APCI ionization sources, respectively. Data are reported as a ratio of the average intensity of [M+H]⁺ ions of the analytes and caffeine. The red line indicates the average of the non-linear region of the curve.