2013 年 2 巻 1 号 p. A0020
Collision-induced dissociation (CID) experiments of adducts [M+R]− with negative atmospheric ions R− (O2−, HCO3− and COO−(COOH)) produced in NO3−-free discharge area in atmospheric pressure corona discharge ionization (APCDI) method were performed using aliphatic and aromatic compounds M. The [M+R]− adducts for individual R− fragmented to form deprotonated analytes [M−H]− as well as the specific product ions which also occurred in the CID of [M−H]−, independent of analytes with several different functional groups. The results obtained suggested that the specific product ions formed in the CID of [M+R]−, as well as CID of [M−H]−, are generated due to further fragmentation of the product ions [M−H]−. It was concluded, therefore, that CID of [M+R]− formed in NO3−-free discharge area can indirectly lead to the formation of the product ions originating from [M−H]−.
The development of ambient ionization techniques has expanded the application of mass spectrometry to direct and real-time analysis of a wide variety of organic and biological molecules since desorption electrospray ionization (DESI) and direct analysis in real time (DART) were introduced in 20041) and 2005,2) respectively. The ambient ionization techniques developed to date can be divided into two groups based on the ionization process: techniques using electrospray ionization (ESI)-like processes, and atmospheric pressure chemical ionization (APCI)-like processes. APCI-like ambient ionization techniques such as DART,2) atmospheric solid analysis probe (ASAP)3) and desorption atmospheric pressure chemical ionization (DAPCI)4) utilize DC corona discharges to generate an abundance of atmospheric ions originating from common air constituents, which can efficiently ionize analytes desorbed in the gas-phase via various ion–molecule reactions. However, the corona discharge in negative-ion mode has been problematic with respect to the ionization of analyte molecules and the structural analysis of analyte ions, because of the formation of nitrogen oxide anions NOx− (especially NO3−) as typical negative atmospheric ions. The NO3− ion has a low proton affinity (1357.7±0.84 kJ mol−1),5) and resulting in less abundant formation of deprotonated analytes [M−H]− that is due to the inefficiency of proton abstraction from analytes M by NO3−.6) In contrast, the electron affinity of NO3 is high (3.94 eV),7) which inhibits the formation of molecular ions M− due to less occurrence of charge transfer from NO3− to analytes M.8) The NO3− ion can ionize analytes M as its adducts [M+NO3]−. Collision-induced dissociation (CID) of [M+NO3]−, however, provides no structural information of analyte ions, because only NO3− can form as the product ion according to a simple dissociation of M and NO3− without proton and charge transfer.9)
We have recently established an atmospheric pressure corona discharge ionization (APCDI) source that can lead to NO3−-free area only by regulating DC corona voltage and needle angle in relation to the central orifice axis in ambient air without any preparation.10) The NO3−-free area is attributed to the field lines arising from low field strength region (≈107 V m−1) at the needle tip periphery.10) It was previously reported that in the NO3−-free area analytes M can be typically ionized as the adducts [M+R]− with atmospheric ions R− having relatively high proton affinities, such as O2−, HCO3−, and COO−(COOH).6) Here we performed CID experiments of those atmospheric ion adducts [M+R]− using three different aliphatic and aromatic organic compounds with several different functional groups, arbitrary selected, and found that all the [M+R]− adducts can fragment to form the product ions originating from deprotonated analytes [M−H]−. The results obtained suggested that the ionization of analytes in the NO3−-free discharge area can be applied to structural analysis of [M−H]−, which can contribute to the development of more efficient APCI-like ambient ionization techniques using negative corona discharges.
Details of the experimental apparatus used have been described elsewhere.6) Discharge experiments using point-to-plane electrodes were performed at atmospheric pressure in unadulterated laboratory air. The laboratory temperature (298 K) and relative humidity (40–70%) were controlled using an air conditioner. The corona needle used as the point electrode was a headless stainless steel pin with a diameter of 200 μm and 20 mm in length (Shiga, Tokyo, Japan). The needle tip had a radius of curvature of ca. 1 μm and the shape of the tip surface was adequately approximated by a hyperboloid of revolution. The opposite electrode was the stainless steel orifice plate of a TSQ-7000 triple-quadrupole mass spectrometer (Thermo Fisher Scientific, San Jose, CA). The discharge gap between the electrodes was adjusted to 3 mm using a manipulator. DC corona voltage and needle angle with respect to the orifice hole axis used were −1.9 kV and π/2 rad, respectively, which were the previously optimized conditions to observe analyte ionization in the NO3−-free discharge area.6) A ceramic micro-heater was placed between the needle and the orifice plate in order to vaporize condensed-phase analytes in the discharge area. Analytes used were l-homoserine (HS; Mr 118), 5-aminosalicylic acid (5-ASA; Mr 153), and β-alanine (β-Ala; Mr 89), which have several functional groups such as alcoholic and phenolic hydroxyl groups, aliphatic and aromatic amino groups, and carboxylic acid (see Fig. 1). Those analytes were purchased from Tokyo Chemical Industry (Tokyo, Japan). The collision gas and energy (lab.) utilized in the present CID experiments were argon at 2.0×10−3 Torr and 5–40 eV, respectively.
Figure 1 shows the negative-ion APCDI mass spectra of three different analytes, HS, 5-ASA, and β-Ala, obtained in the NO3−-free discharge area. Those mass spectra consisted of the dominant peaks of adducts [M+R]− with atmospheric negative ions R− such as O2− (m/z 32), HCO3− (m/z 61), and COO−(COOH) (m/z 89), which can be mainly formed in the NO3−-free discharge area.6) In order to investigate fragmentation behaviors of the atmospheric ion adducts [M+R]− formed in the NO3−-free discharge area, CID experiments of all the adducts [M+R]− shown in Fig. 1 were performed. Figure 2 shows the CID spectra of atmospheric ion adducts of HS, [HS+O2]−, [HS+HCO3]−, and [HS+COO(COOH)]−, obtained at a laboratory collision energy of 10 eV. All of the [M+R]− adducts were fragmented to form predominantly deprotonated HS, i.e., [HS–H]− at m/z 118. This is due to dissociation accompanied by proton abstraction from HS by the atmospheric ions R− (Reaction (1)), because of relatively high proton affinities (PAs) of R−, i.e., PA(O2−)=ΔH° (O2−+H+→HO2)=1477.0±2.9 kJ mol−1,11) PA(HCO3−)<1551.0±9.2 kJ mol−1,12) and 1408<PA(COO−(COOH))<1412 kJ mol−1.13)
 | (1) |
In the case of the CID of [HS+COO(COOH)]−, the atmospheric ion COO−(COOH) was observed as a minor product ion (Fig. 2c), originated from a simple dissociation of HS and COO−(COOH) without proton transfer (Reaction (2)). This result suggests that the value of proton affinity of [HS–H]− is close to that of COO−(COOH) from the standpoint of the kinetic methods previously reported.13,14)
 | (2) |
With increased collision energy, the CID of [HS+R]− adducts resulted in additional product ions. The CID spectra of [HS+O2]−, [HS+HCO3]−, and [HS+COO(COOH)]− obtained at collision energy of 25 eV are shown in Figs. 3a–c, respectively. It should be noted here that all the atmospheric ion adducts were fragmented to form common product ions at m/z 72, 98, 99, and 100, marked as ▼ in Figs. 3a–c. It is of interest, furthermore, that these product ions can be also observed in the CID of deprotonated HS, [HS–H]−, generated in APCDI source (Fig. 3d).
The phenomena described above were also observed in the CID of another analytes, 5-ASA and β-Ala. Figure 4 shows the CID spectra of the atmospheric ion adducts [M+R]− (R−: O2− and HCO3−) and deprotonated analytes [M−H]− for 5-ASA and β-Ala obtained at collision energy of 25 eV. The CID of [5-ASA+R]− and [β-Ala+R]− resulted in the formation of individual [M−H]− and the product ions at m/z 108, and m/z 16, 32, 44, and 59, respectively, which were independent of the reactant ions R− (Figs. 4a, b, d, and e). The occurrence of the latter product ions (marked as ▼ in Figs. 4a, b, d, and e) was consistent with the results of the CID of deprotonated analytes [5-ASA–H]− (Fig. 4c) and [2(β-Ala)–H]− (Fig. 4f).
According to the results described above, the CID of the adduct ions [M+R]− for all the analytes M and atmospheric ions R− can result in the formation of the two type product ions depending on collision energy, i.e.,
1) Deprotonated analytes [M−H]− dominantly formed under low collision energy.
2) Specific product ions formed at high collision energy.
The product ions described in 2 also occur in the CID of [M−H]− generated in APCDI source. Those facts suggest that the product ions [M−H]− described in 1 have the same structure of [M−H]− generated in APCDI source, and that the specific product ions mentioned in 2 originate from the further fragmentation of the product ions [M−H]−. It is concluded, therefore, that CID of atmospheric ion adducts [M+R]− formed in the NO3−-free discharge area indirectly leads to the formation of the product ions originating from deprotonated analytes [M−H]− generated in APCDI source. This conclusion can be useful for structural analysis of [M−H]− when CID of [M−H]− cannot be performed due to less abundant formation of [M−H]− in the discharge area in negative-ion APCDI method.
Fragmentation behaviors of [M+R]− adducts with negative atmospheric ions R− (O2−, HCO3−, and COO−(COOH)) formed in the NO3−-free discharge area were investigated using negative-ion APCDI method and three different organic compounds M. All the [M+R]− adducts fragmented to form deprotonated analytes [M−H]− and the specific product ions which also occur in CID of [M−H]− generated in APCDI source, independent of analytes with several functional groups. These results indicate that the product ions formed in CID of [M+R]−, as well as CID of [M−H]−, can be formed via further fragmentation of the product ion [M−H]−, and that [M−H]− formed in CID of [M+R]− has the same structure of [M−H]− generated in APCDI source. It is concluded, therefore, that CID of atmospheric ion adducts [M+R]− formed in the NO3−-free discharge area indirectly leads to the generation of the product ions originating from deprotonated analytes [M−H]−, which can be useful for the structural analysis of [M−H]− in APCI-like ambient ionization techniques using negative corona discharges.
This work was supported by Grants-in-Aid for Scientific Research (C) (23550101 and 24619005) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
