Ionization Capabilities of Hydronium Ions and High Electric Fields Produced by Atmospheric Pressure Corona Discharge

Atmospheric pressure corona discharge (APCD) was applied to the ionization of volatile organic compounds. The mass spectra of analytes having aromatic, phenolic, anilinic, basic and aliphatic in nature were obtained by using vapor supply and liquid smear supply methods. The vapor supply method mainly gave protonated analytes [A + H] + caused by proton transfer from hydronium ion H 3 O + , except for benzene, toluene and n -hexane that have lower proton affinity. The use of the liquid smear supply method resulted in the formation of molecular ion A · + and/or dehydride analyte [A − H] + , according to the nature of analytes used. The formation of A · + without fragment ions could be explained by the electron tunneling via high electric fields 10 8 V/m at the tip of the corona needle. The dehydride analytes [A − H] + observed in the mass spectra of n -hexane, di- and tributylamines may be explained by the hydride abstraction from the alkyl chains by the hydronium ion. The hydronium ion can play the two-roles for analytes, i.e. , the proton donor to form [A + H] + and the hydride acceptor to form [A − H] + .


INTRODUCTION
Although ambient mass spectrometry (AmbiMS) [1][2][3] has been increasingly grown up to be a practical tool for easily detecting widespread chemicals, the ion source for AmbiMS is build up from conventional ionization devices. Among the devices the needle electrode for corona discharge is simplest device and has been used for atmospheric pressure chemical ionization (APCI). We have used a needle electrode for studying the mechanism of negative-ion formation and negative-ion/molecule reactions under ambient air conditions, [4][5][6][7][8][9][10][11][12][13][14] while the corona needle are exclusively used for positive-ion APCI with solvent chemicals as the reagent gas. e corona discharge under ambient air conditions produces micro-plasma consisting of short-and longlifetime ionic species called as atmospheric ions, X + and Y − , according to the polarity of voltage applied to the electrode. e atmospheric ions X + and Y − are formed via so-called "ion-evolution," [6][7][8] which occurs by discharge-induced ion/ molecule reactions in the glow region at the tip of the needle with ca. 1 µm in radius (Fig. 1 le ) and the dri region between the tip of the needle and the ori ce of mass spectrometer ( Fig. 1 right). It is of importance to clarify the funda-mental processes of the ion formation for the application of atmospheric pressure corona discharge ionization (APCDI) to the AmbiMS of organic compounds having various physicochemical properties.
Here we examine the analyte ion formation of volatile organic compounds such as benzene and its derivatives, butylamines and hexane, by using positive-ion APCDI MS. e analytes were selected from the points of the ionization properties such as electron transfer to form molecular ion A · + , proton transfer to form protonated analyte [A+H] + and hydride transfer to form dehydride analyte [A−H] + .

Mass spectrometry
Mass spectra were acquired on a reversed geometry double-focusing mass spectrometer JMS-BU30 (JEOL, Tokyo, Japan). e mass spectrometer was operated at positive-ion mode. e ion source contained a discharge needle made of stainless steel with a diameter of 200 µm and 20 mm in length ( Fig. 1 le ), which is an insect pin with headless (Shiga, Tokyo, Japan). e needle tip with glossy surface as a point electrode has ca. 1 µm in radius (Fig. 1  le ). e gap distance between the needle tip and the orice plate was 3 mm (Fig. 2). e ori ce hole was 320 µm in diameter and 2 mm in length. e ori ce temperature was 40°C. Discharge voltage +3.9 kV was applied to the needle relative to the ori ce plate. e current value between the needle and ori ce plate were 10 µA. e ions entered into vacuum region were accelerated to 2.5 kV at the focusing lens electrode and separated by the reversed geometry double-focusing mass spectrometer. e scan speed was 5 s/ scan. e analyte was supplied as vapor molecules into a glow or dri region between the tip of the needle and the ori ce of mass spectrometer (Fig. 1a), while the liquid analyte (approximately 0.5 µL) was directly smeared on the surface of the vicinity of the tip of the needle by using a micro-pipet (Fig. 1b).
Corona discharge e positive atmospheric ions X + as reactant ion in APCI were produced by the corona discharge with a needle elec-

Mass spectra of benzene and its derivatives by positive-ion APCDI
Positive-ion APCDI mass spectra of benzene, toluene, phenol, aniline, 2,6-xylidine and benzylamine are shown in Fig. 3. In each data, the upper represents the mass spectrum obtained by supplying the analyte vapor (Fig. 1a), while the lower spectrum was obtained with liquid analyte smeared on the surface in the vicinity of the tip of corona needle (Fig. 1b). In the case of the vapor supply in all the spectra (upper), the ion peaks corresponding to protonated analyte [A+H] + were observed, except for benzene and toluene. In the atmospheric pressure ion evolution of corona discharge in positive polarity [6][7][8] (Scheme 1), the stable and abundant terminal ions such as H 3 O + and H 3 O + (H 2 O) are produced and staying in dri region between the tip of needle and ori ce of mass spectrometer. Consequently, the vapor analytes A arrived at the dri region collide or interact with those hydronium ions H 3 O + and H 3 O + (H 2 O). If the proton a nity of analyte PA (A) is larger than that of water (PA (H 2 O)=691.0 kJ/mol or 7.16 eV), 15) the proton transfer takes place from hydronium ions to analyte molecules as follows: Among the analytes used here, benzene and toluene having the di erence in PA between water and analyte, ∆PA values, lower than 0.97 eV did not show any analyte ions under the vapor supply condition (Table 1). Aniline and 2,6-xylidine having relatively lower values of ionization energy gave the peaks corresponding to molecular ion A · + at m/z 93 and m/z 121, respectively, under the vapor supply condition (Figs. 3d  and 3e).
In the case of the liquid smear supply condition, on the other hand, all the spectra showed molecular ions A · + as well as [A+H] + , except for toluene and benzylamine. Toluene and benzylamine gave an extraordinarily stable fragment ion at m/z 91 corresponding to the tropylium ion C 7 H 7 + , 16) though the ion at m/z 91 is originated from both a water cluster  17) as sown in Fig. 2. It is of interest to note that the corona discharge ionization produced molecular ions M · + which need electronic excitation like in electron ionization (EI).
It has been reported that the high electric elds (10 8 V/m) on the tip surface of the needle (Fig. 1 le ) bring about the electrons accelerated to 100 eV or above in kinetic energy. 7,14) e rst step of the ion evolution in Scheme 1 starts from such EI like process in the vicinity of the surface of the tip. Although the EI mass spectra of the compounds used here give intense and abundant fragment ions, 15,18) the APCDI mass spectra obtained did not show any fragment ions except for tropylium ion at m/z 91. is suggests that the formation of molecular ions A · + is due to the energy of thresholds near to the ionization energy (IE) listed in Table  1. From this, the A · + ion formation can be explained by the mechanism of eld ionization (FI) with an e ect of electron tunneling, 19,20) because the FI mass spectra are generally lacking in fragment ions. 21,22) e in uence of positivepolarity high electric eld (+hef ) on analyte molecules A may result in the induction e ects such as molecular orbital distortion and polarization. ese e ects may be leading to electron transfer reaction (3) from analyte to the tip of needle electrode without excess energy depositions for the analyte molecular ions formed.

Mass spectra of butylamines by APCDI
Butylamine having relatively high proton a nity merely gave the peak corresponding to protonated analyte [A+H] + in the mass spectrum obtained with the both vapor and liquid smear supply methods (Fig. 4a), while the spectra did not give molecular ion A · + . e formation of [A+H] + can be explained by the gas-phase proton transfer (1) due to higher proton a nity of analytes A than that of water. Although di-and tributylamines also gave preferential peaks of [A+H] + re ecting the higher proton a nity, these analytes did not show molecular ions A · + (Figs. 4b and 4c), in spite of the lower ionization energy (Table 1). Instead the mass spectra gave the peak of dehydride analyte ion [A−H] + which may be originated from the loss of hydride H − from analyte molecules. e dehydride analytes [A−H] + are o en observed in the chemical ionization (CI) mass spectra of alkane compounds. 23,24) To con rm the hydride transfer CI process in the APCDI, the mass spectra of hexane were obtained with both the vapor and liquid smear supply methods, as shown in Fig. 5. e spectra obtained with the vapor supply merely showed [A−H] + at m/z 85, while the liquid smear supply gave intense peak of [A−H] + and molecular ion A · + at m/z 86. e result obtained for n-hexane indicates that the formation For the butylamines here, consequently, the competitive reactions of the proton transfer (1) and the hydride abstraction (4) take place due to its high proton a nity and the interaction between the butyl groups and hydronium ions under APCDI conditions. e lack in the molecular ions A · + of di-and tributylamines, in spite of the lower ionization energy, may be due to the preferential interaction of butyl groups with the hydronium ions H 3 O + and H 3 O + (H 2 O) which largely exist in the dri region.

CONCLUSION
Positive-ion atmospheric pressure corona discharge ionization mass spectrometry (APCDI MS) was applied to volatile organic compounds which have aromatic, phenolic, anilinic, basic and aliphatic in nature, by using the vapor supply and liquid smear supply methods. e mass spectra obtained with the vapor supply method mainly gave protonated analytes [A+H] + caused by the proton transfer from hydronium ions H 3 O + and/or H 3 O + (H 2 O) n as the reactant ions generated by the positive corona under ambient air conditions, except for benzene, toluene and n-hexane that have lower proton a nity. On the other hand, the use of the liquid smear supply method resulted in the formation of molecular ions A · + and/or dehydride analytes [A−H] + . e resulting analyte ions A · + , [A+H] + and [A−H] + were without fragment ions, except for the formation of the tropylium ion at m/z 91 originated from the analyte ions of toluene and benzylamine. e formation of molecular ions A · + without fragment ions could be explained by the electron tunneling that occurs by high electric eld (10 8 V/m) at the tip of the corona needle. e dehydride analytes [A−H] + observed in the mass spectra of n-hexane, di-and tributylamines may be explained by the hydride abstraction from the alkyl chains by the hydronium ions H 3 O + and H 3 O + (H 2 O). From this, it can be expected that the hydronium ions can play the two-roles for the analyte ionization, i.e., the proton donor and hydride acceptor, according to the proton a nity of analytes. Further study is needed to clarify the detailed mechanism(s) of the ion formation with energetics in the APCDI MS.