Ionization of Volatile Organics and Nonvolatile Biomolecules Directly from a Titanium Slab for Mass Spectrometric Analysis

Abstract

Atmospheric pressure chemical ionization (APCI)-mass spectrometry (MS) and electrospray ionization (ESI)-MS can cover the analysis of analytes from low to high polarities. Thus, an ion source that possesses these two ionization functions is useful. Atmospheric surface-assisted ionization (ASAI), which can be used to ionize polar and nonpolar analytes in vapor, liquid, and solid forms, was demonstrated in this study. The ionization of analytes through APCI or ESI was induced from the surface of a metal substrate such as a titanium slab. ASAI is a contactless approach operated at atmospheric pressure. No electric contacts nor any voltages were required to be applied on the metal substrate during ionization. When placing samples with high vapor pressure in condensed phase underneath a titanium slab close to the inlet of the mass spectrometer, analytes can be readily ionized and detected by the mass spectrometer. Furthermore, a sample droplet (~2 μL) containing high-polarity analytes, including polar organics and biomolecules, was ionized using the titanium slab. One titanium slab is sufficient to induce the ionization of analytes occurring in front of a mass spectrometer applied with a high voltage. Moreover, this ionization method can be used to detect high volatile or polar analytes through APCI-like or ESI-like processes, respectively.

Keywords: APCI; ESI; ambient ionization; nonpolar; titanium; volatile.

Figure 1

Photograph of the titanium slab-based ASAI–MS setup.

Figure 2

Examination of distance effects. ASAI mass spectra of atrazine were obtained by placing the titanium slab (0.3 cm × 0.3 cm) close to the inlet with a distance of (A) ~0.1 mm, (B) ~0.3 mm, (C) ~0.5 mm, and (D) ~0.8 mm. The voltage set on the inlet of the mass spectrometer was −4500 V.

Figure 3

Analysis of analyte vapors derived from liquid samples. ASAI mass spectra of the samples including (A) aniline, (B) azobenzene, (C) captopril, (D) indole, (E) naphthalene, (F) benzoic acid, (G) cinnamic acid, and (H) salicylic acid prepared in methanol. The concentration of the samples were 10⁻⁴ M. The vial containing the sample solution (1 mL) was placed under the titanium slab (0.3 cm × 0.3 cm) at a distance of ~1 cm from the surface of the liquid sample. The voltages set on the inlet of the mass spectrometer were −4500 V and +4500 V operated in the positive ion mode and the negative ion mode, respectively.

Figure 4

Examination of ionization mechanism. ASAI mass spectra were obtained by depositing azobenzene (0.2 nmole) on the titanium slab (0.3 cm × 0.3 cm) (A) without and (B) with boiling D₂O underneath the titanium slab. The distance between the titanium slab and the inlet of the mass spectrometer was ~0.1 mm. The voltage set on the inlet of the mass spectrometer was −4500 V.

Figure 5

Detection of aroma molecules directly from plants including (A) garlic, (B) ginger, (C) mint leaves, and (D) bananas. The samples were placed near the titanium slab (0.3 cm × 0.3 cm) which was placed close to the inlet of the mass spectrometer with a distance of ~0.1 mm. The voltage set on the inlet of the mass spectrometer was −4500 V.

Figure 6

Photographs of a sample droplet deposited on a titanium slab (0.3 cm × 0.3 cm) obtained (A) before and (B) after applying the high voltage (−4500 V) to the inlet of the mass spectrometer. ASAI mass spectra obtained from individual sample droplets (2 μL) containing (C) arginine ([M + H]⁺ = 175), (D) bradykinin ([M + 2H]²⁺ = 531), and (E) myoglobin (MW = 16,950 Da) deposited on the titanium slab (0.3 × 0.3 cm) followed by ASAI–MS analysis through the ESI-like process.