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Molecular ionization in the interface of an inductively coupled plasma mass spectrometer
S ecrrochwnrca Ac$ Vol. 47B. No. 4, pp Pz nted m Great Bntain.
561-568. 1992 0
Molecular ionization in the interface of an inductively mass spectrometer
05644547/!T2 $5.00 + .otJ 1992 Pergamon Press plc
coupled plasma
H. TOGASHI, A. HASHIZUMEand Y. NIWA* NationalChemicalLaboratoryfor Industry, l-l, (Received 5 December
Higashi, Tsukuba-shi, Ibaraki 305, Japan
1990; accepted 22 October 1991)
Abstract-The ionization processes of molecules in an Ar afterglow formed in the sampling interface of an inductively coupled plasma mass spectromter were investigated by introducing organic molecules directly into the afterglow. It was found that methanol, methane and benzene are ionized in the afterglow plasma into unstable molecular ions which dissociate immediately after formation. The mechanism of the ionization was elucidated by the analysis of the mass spectra of product ions, in which breakdown graphs were used to estimate the internal energy of the nascent molecular ions. It was found that charge exchange with the Ar’ ion is the most dominant process for all the samples investigated. Neither photoionization by Ar resonance emission, nor Penning ionization by metastable Ar, makes an important contribution to the ionization. There are few electrons in the afterglow that are capable of ionizing molecules.
1. INTR~Du~~N INDWXIVELY coupled plasma mass spectrometry (ICP-MS) is an excellent technique for elemental analysis. However, spectral overlap caused by molecular ions can be a serious problem in quantitative measurement of atomic ions [1,2]. The molecular ions have been conSidered to be produced in the sampling process of the plasma into the mass spectrometer rather than in the ICP, where the temperature is too high (c. 6000 K) for molecules to exist in significant quantity. In order to introduce the plasma of the ICP under atmospheric pressure into the mass spectrometer operating at 10m5 torr, the devices are connected by a differentially pumped interface at about 1 torr. The appearance of an afterglow in the interface chamber has been suspected as giving an ionic distribution different from that in ICP [3,4]. In ICP-MS measurements on metal-containing solutions, it has been found that the intensity ratio of metal oxide ion (MO+) to metal ion (M+) increases with the bond energy of the MO+ [5-71. DOUGLAS and FRENCH [5] found experimentally that the formation temperature of the oxide ions will be 21000 K if chemical equilibrium is assumed. Since this value is much higher than the accepted temperature of the ICP, it was concluded that MO+ ions are formed through non-equilibrium processes involved in the sampling stage. Later, the above authors gave a theoretical discussion of the aerodynamics in the interface region [S] and suggested that the oxide-forming reactions will be rapidly quenched in the expansion process of the plasma sampled in the interface. Meanwhile, LAM and HORLICK [9] found that the MO+/M+ ratio decreases when the interface region is axially elongated. Although they indicated that this phenomenon can be attributed to the different directional distribution of MO+ and M+ in the expanding jet, it is likely that this discrepancy is caused by chemical processes in the interface region. However, it has not been made clear what kind of chemical reactions take place in the interface chamber. If any ionization process on molecules exists in the afterglow, it will possibly cause or enhance the spectral overlap. In order to investigate the possibility of molecular ionization, one has to know the amount of energetic particles that are capable of ionizing molecules. However, it is difficult and circuitous to estimate the population and energy distribution of the energetic species by the conventional diagnostic methods. Instead, an attempt has been made to study the ionization process in the afterglow by
* To whom correspondence
should be addressed. 561
562
H. TOGASHI et
al.
Fig. 1. Schematic of the apparatus: a, plasma torch; b, load coil; c, orifice; d, sample gas inlet; e, skimmer; f, quadrupole mass spectrometer.
the analysis of mass spectra resulting from the ionization of molecules introduced into the sampling interface. Although the plasma mixed with alien species will differ from the original one, observing it should reveal the chemical properties of the original plasma. For the analysis of mass spectra resulting from molecular ionization, knowledge of the unimolecular decomposition of molecular ions provides important information on the energies of particles in the interface and the mechanism of ionization. Ionization of a molecule is initiated by collisional energy-transfer from an energetic particle, and a molecular ion is formed with the internal energy characteristic of the ion-formation process. If the ion is in an unstable state, it may dissociate along various paths according to its internal energy. The relationship between the fraction of the resulting ions and the internal energy of the parent ion has been obtained for various molecular ions and expressed in diagrammatic form by the so-called “breakdown graph” [lo]. Thus, we can speculate, or identify if possible, the elementary process of molecular ionization by the analysis of its mass spectra with the use of the breakdown graphs. The present paper reports the investigation of the ionization of methanol, methane and benzene in the interface region, and the relative importance of ionization mechanisms possible in the Ar afterglow.
2. EXPERIMENTAL The schematic of the apparatus is shown in Fig. 1. An Ar ICP (27.12 MHz, 1.3 kW: Seiko ICP1500) generated at atmospheric pressure was extracted through a sampling orifice of 0.8 mm diameter into an interface chamber. No samples were introduced into the Ar ICP. The distance between the end of the ICP torch and the orifice was about 10 mm. The pressure in the interface chamber was maintained at about 0.5 torr by a mechanical booster pump. Ions produced by the reaction of sample molecules with an Ar afterglow in the chamber were allowed to enter a quadrupole mass spectrometer (VG SXP400) through a skimmer of 0.5 mm diameter. The electric potentials of the orifice cone and the skimmer were set at the ground. The distance between the orifice and the skimmer aperture was 19 mm. Sample gas was introduced into the interface through a stainless-steel tube with a needle valve connected to a gas cylinder or an ampoule. The flow rates of samples were known by the use of a mass flow meter for CH4 or by the measurement of the remaining liquid volume in the ampoule for CH30H and C,H,. The sample inlet tube was directed perpendicular to the axis passing through the orifice and the skimmer aperture. The exit of the inlet tube was placed at 14 mm from the orifice and at 1 mm from the axis.
3. RESULTS
AND
DISCUSSION
When CH,OH was used as a sample gas with a flow rate of 9 kg s-l (= 1.7 X 1017 molecule s-l), HCO+, CH*OH+ and CH3+ were observed, as shown in Fig. 2. These
Ionization in ICP-MS
io
563
20
3b
m/z
Fig. 2. Mass spectra of ions produced by the reaction of CH,OH interface of ICP-MS.
and Ar afterglow at the
observations demonstrate that the ionization of molecules definitely takes place in the interface chamber. The molecular ion CH30H+ was not observed. The weak ion at m/z 32 is not assigned to CH30H+, but to 13CH,0H+ because the intensity,ratio 32+/31+ =l x lo-* coincides with the ratio of natural abundance of carbon isotopes i3C/‘*C= 1.11 x lo-*. The CH30H2+ ion at m/z 33 is probably produced from the secondary reaction of CH30H with CH20H+ or CHO+ [ll] since the ratio of 33+ to 15+, 29+ and 31+ uniformly increased by a factor of about 5 when the flow rate of the sample was doubled. The absence of the molecular ion CH,OH+ in the mass spectrum indicates that nascent CH,OH+ formed in the Ar afterglow is in an excited state leading to spontaneous dissociation into an ion and a neutral species. The energy of the excited state can be estimated by the use of breakdown graphs if the relative abundance of the product ions is known. Figure 3 shows the breakdown graph of the CH,OH+ ion that was obtained in the study of photoelectron-photoion coincidence spectroscopy on CH30H [12,13]. The abscissa shows the internal energy of CH30H+ measured from its ground state. In the experiment, sample molecules were ionized by monochromatic light, and ions and electrons produced in the single photoionization events were
, I
0
CH2OH++ ;
1
2 INTERNAL
3
\ \
4
5 ENERGY
6
7 (eW
Fig. 3. Breakdown graph of CH,OH+ obtained by photoelectron-photoion coincidence spectroscopy. (+, CH,OH+; 0, CH,OH+; n , HCO+; A, CH,‘). (T. Nishimura, Y. Niwa, T. Tsuchiya and H. Nozoye, 1. Chem. Phys. 72, 2222 (1980); reprinted with permission of the American Institute of Physics).
564
H. TOGASHI et al.
detected by the use of a mass spectrometer and an electron spectrometer using the coincidence technique. The measurement of the electron energy and the identification of a fragment ion gave the internal energy of the molecular ion and its fragmentation path, respectively. The following processes can be considered as possible mechanisms of ionization in the interface region: (i) electron impact ionization; (ii) photoionization by Ar resonance emission (hv) (11.62, 11.83 eV); (iii) Penning ionization by Ar atoms in a metastable state (Arm) (11.55, 11.72 eV); and (iv) charge exchange with Ar+ ions (15.76 eV). The values in parentheses are the internal energies of the energetic particles or photon energies. Among these processes, only electron impact ionization is a non-resonant process, i.e. the energy of the electron is not entirely transformed into molecular internal energy. The energy of the photon, the metastable Ar atom and the Ar+ ion is resonantly transferred to the molecule, and then an excited molecule with a definite internal energy is produced. In photoionization and Penning ionization, however, the ejected electron takes away part of this internal energy. The energy of the resulting molecular ion is then less than the energy of the photon or the metastable Ar atom. Alternatively, the charge exchange process produces an ion with an internal energy equal to that of the At-+ ion, since the ejected electron combines with Ar+ to form Ar in the ground state. Since the ionization potential (ZP) of CHaOH is 10.85 eV, photoionization and Penning ionization will place CH30H+ with an internal energy of 0.77 and 0.98 eV [E(hv)-IP(CH,OH)], or 0.70 and 0.87 eV [E(Arm)-IP(CH30H)] at a maximum. Figure 3 shows that such molecular ions will remain stable or dissociate exclusively into CH,OH+. If ionization is brought about by electron impact, the molecular ion CH,OH+ should be comparable with CH,OH+ and HCO+ in intensity, which has been evidenced by electron-impact mass spectra of CH30H [14] normally measured with an electron energy of about 70 eV. Since the molecular ion CHaOH+ is absent in mass spectra, and HCO+ and CH,+ are observed in comparable amounts with CH20H+, it is clear that electrons, photons and Ar metastables make a minor contribution to the ionization of CH30H in the Ar afterglow. The charge exchange process will yield a molecular ion with an internal energy of 4.91 eV [ZP(Ar)-ZP(CH,OH)]. This ion dissociates into HCO+, CH*OH+ or CH3+ according to the breakdown graph (Fig. 3). The observed abundance of the ions is in good agreement with those obtained from the diagram on the assumption of the chargeexchange mechanism, as seen from Table 1. Therefore, it is concluded that the ionization of CH30H is caused by charge exchange with the Ar+ ion. Methane was investigated in order to further corroborate the insignificance of electron-impact ionization when compared with the charge-exchange process. Since the ZP of CH4 (12.75 eV) is higher than the photon energy of Ar resonance emission and the internal energy of the metastable Ar atom, neither photoionization nor Penning ionization takes place for CH,+ Then the electron-impact ionization can be directly contrasted with the charge-exchange process. When the sample was introduced into the interface at a flow rate of 5 seem (standard cubic centimeters per minute), which equals 2.2 X 1018 molecule s-i, CH3+, CH,+ and a small amount of CH+ were observed in the mass spectra, as shown in Fig. 4(a). The ions C2H3+, C,H,+ and C2H5+ were observed in relatively small amounts. However, their abundance increased with the amount of CH4 [Fig. 4(b)], indicating that these ions result from the reaction of CH4 with CH3+, CH,+ or CH+ [15]. The peak at m/z 16 is not assigned to CH,+ but to 13CH3+, since the intensity ratio 16+/15+ = 1 x lo-’ agrees with the natural abundance as described above. The absence of CH,+ in the mass spectra indicates that the CH4+ ion is formed in an unstable state leading to immediate dissociation. Moreover, this result is confirmed by the fact that CH,+ is not observed in the mass spectra. If CH,+ survived long enough to react with CH4, CHS+ would be produced predominantly by the reaction of CH,+ with CH, [16]. The CH,+ ion was not detected, even though the flow rate of the
Ionization in ICP-MS
565
Table 1. Relative ion abundance obtained from experimental data (Exp.) and breakdown graphs (B.G.) with the assumption of charge~xchange ionization by Ar+ Sample
Product ion*
Exp. (%)
B.G. (%I
CH,OH
CX,+ HCO’ CH,OH+ CH,+ CX,+ C&+ C&f,+ C&L+ CJ%+ C&+
17 51 25 22 76 24 41 1 9 3
28t 42 30 32$ 68 148 47 8 29 2
CH, C86
* Only included. t Ref. $ Ref. 5 Ref.
rb
ions appearing in the breakdown graphs are [12]. [19]. [20].
i0
m/z
fb)
10
20
m/z
Fig. 4. Mass spectra of ions produced by the reaction of CH., and Ar afterglow: (a) sample flow rate 5 seem; (b) 25 seem.
sample was increased [Fig. 4(b)], whereas other secondary ions, C2H3+, GH4+ and C2H5+, increased in intensity. It has been confh-rned that CH,’ is the most abund~t product ion of electronimpact ionization of CH, with electron energies up to 2000 eV [17], which is obviously much higher than the energy of electrons in the afterglow. The complete depletion of CH4+ in the observation clearly shows that the ionization is not caused by electrons. Charge exchange between Ar+ and CH4 will produce CH,+ with an internal energy of 3.01 eV [ZP(Ar)-ZP(CH,)]. The breakdown graph of CH4+ [l&19] indicates that those CH4+ ions completely dissociate into CH,+ or CH2+. In Table 1, the relative abundance of these ions expected from a breakdown graph is compared with that
566
H.
TOGASHI et al.
70
80
Fig. 5. Mass spectra of ions produced by the reaction of C,H, and Ar afterglow.
obtained from Fig. 4(a). From the good agreement between the corresponding values, one may conclude that the ionization of CH4 is brought about by charge exchange with the Ar+ ion. Finally, this conclusion was confirmed by a similar experiment on C6H6 (ZP = 9.24 eV). As shown in Fig. 5, with a flow rate of 18 pg s-l (= 1.4 x 10” molecule s-l), a small amount of C,H,+ was observed together with relatively intense fragment ions. The breakdown graph of CsH6+ [20] shows that most of the CsH6+ produced by charge exchange with At-+, with an internal energy of 6.52 eV [ZP(Ar)-ZP(C&Q], will dissociate and only about 2% of it will remain stable. Table 1 shows that the prediction by the breakdown graph accurately reproduces the relative ion abundance obtained from Fig. 5. Although C4H2+, C4H3+ and CSH3+ shown in Fig. 5 are not included in the breakdown graph [20], their formation has been confirmed in the study of charge-exchange mass spectrometry [21]. The electron temperature in the Ar afterglow at the interface was estimated to be 7000-15000 K [4] by the Langmuir probe method, in which a Maxwellian distribution was assumed for the electron velocity. At these temperatures, only about 0.01% of the electrons have an energy higher than the ZP of benzene and even fewer electrons are available to ionize methanol and methane molecules. Thus, the insignificance of electron impact ionization in the interface chamber is consistent with the result of the probe study. If the cross sections of charge exchange and Penning processes are of the same order, one may infer the numerical superiority of Ar+ ions over Ar metastables in the afterglow. The cross section of the Penning process of metastable Ar with C6H6 has been obtained as 160 A’ [22], whereas the ionization cross-section of Ar+ with C,H, has been determined as 280 A’ [23]. For the ICP, on the other hand, the abundant existence of Ar+ ions compared to Ar metastables has already been established. The density of Ar+ ions has been estimated as 5 x 1014-5 X 1015 cmm3 from measurements of the electron density [24], whereas the density of Ar metastables has been found to be 2 x 10” cmp3 [25]. Thus, it is possible that the predominance of Ar+ ions over Ar metastables in the ICP is not significantly altered by the sampling process. Although the significance of the charge exchange mechanism in ionization has been confirmed in the above discussion by the good agreement of the mass spectra with the predictions of the breakdown graphs, there remains the possibility that the sample molecules are already excited before ionization, or the product ions are further dissociated by collisions with energetic particles. Then, the mass spectra must have accidentally imitated the patterns of charge exchange ionization. In order to show that such secondary collisions can be neglected in the present experiments, the feasibility of such collisions will be discussed. The Ar+ ion is obviously not important in the secondary collisions because it is not capable of colliding with positive ions or exciting sample molecules to any stable states. However, although the other energetic particles are insignificant in ionization compared with the Ar+ ion, it does not follow that those particles are not important in secondary
Ionization in ICP-MS
567
collisions. Then, to consider the influence of those energetic particles, the number of collisions of a molecule or product ion with Ar in the interface chamber is estimated. Consider now the gas dynamics of the afterglow plasma in the interface chamber. It has been pointed out [8] that, in the interface of ICP-MS, the place where the sampled plasma exhibits free expansion is limited to a region surrounded by a concentric barrel-shaped shock wall and terminated by a perpendicular plane shock wall named Mach disc, whereas turbulance or unpredictable motion of gas may exist in the rest of the chamber. The position of the Mach disc (xM,)measured from the orifice can be estimated by the equation: .xIVIDIDO = 0.67(P@‘J’*,
(1)
where Do is the orifice diameter, and PO and PI are the pressures at the outside and inside of the chamber, respectively. Insertion of Do = 0.8 mm, PI = 0.5torr and PO = 760 torr gives XMD= 21 mm, which indicates that the Mach disc is placed close to or penetrated by the skimmer tip in the present apparatus. Hence, the flow of the afterglow plasma can be regarded as free expansion in the region where the sample molecules are injected and carried toward the skimmer by the Ar jet. In the trajectory of a sample molecule (M), starting at the injection point and ending at the skimmer aperture, the collision frequency with Ar is highest at the starting point, where the density of Ar is higher than in the remaining part. On the assumption that M, immediately after injection, moves straight in the direction perpendicular to the Ar jet, the probability (q) that M comes into collision with Ar per unit length along the straight path of M, is simply given by:
(2)
q = unAvA/vM,
where vM and VA are the velocity of M and Ar, respectively, IJ is the collisional crosssection of M with Ar, and nA is the density of Ar. The value of ItA at x downstream from the orifice is given [8] by: nA = 0.161 x (xlD~J-2no,
(3)
where no is the density of Ar in the ICP. Inserting x = 14 mm and n,, = 1.2 x 1018 cmp3 for a typical ICP (6000 K, 760 torr) into Eqn (3), nA = 6.4 x 1Or4 cmp3 is obtained. Further use is made of the following approximation: vA/vM = (mMT.&ATM)
I’*,
(4)
where mM and mA are the masses of M and Ar, and TM and TA are the temperatures in the sample reservoir of M and the ICP, respectively. If TA = 6000K and TM = 300 K (room temperature), and u = 50 A2 as a typical collisional cross-section in gas kinetics [26], q = 1.3 mm-’ when methanol is assumed to be the sample molecule. Since the distance between the injection point and the skimmer tip is about 5 mm, the total number of collisions of a sample molecule with Ar at the interface should be about equal to or less than 1.3 x 5 = 6.5, which indicates that the probability of ionization of a sample molecule is small compared to unity since the density of Ar+, as well as other energetic particles, is much lower than that of Ar. It is therefore obvious that the occurrence of secondary collisions of a sample molecule or product ion with energetic particles can be neglected when compared with the primary ionization of the sample molecule by an Ar+ ion.
4. CONCLUSIONS The ionization mechanisms of organic molecules externally introduced into the interface chamber of an ICP mass spectrometer have been investigated. The elementary
568
H. TOGASHIet al.
processes of the formation of nascent molecular ions were examined by the analysis of the mass spectra of their fragment ions with the aid of breakdown graphs. It has been revealed that charge exchange with the Ar+ ion is the most important ionization process. Neither Penning ionization by metastable Ar atoms, nor photoionization by Ar resonance emission noticeably takes place, even if these processes are energetically allowed. There are few electrons possessing sufficient energy to ionize these molecules. REFERENCES [l] M. A. Vaughan and G. Horlick, Appl. Spectrosc. 40, 434 (1986). [2] G. Horlick, S. H. Tan, M. A. Vaughan and Y. Shao, Inductively Coupled Plasmas in Analytical Atomic Spectrometry, Eds A. Montaser and D. W. Golightly, chap. 10. VCH, New York (1987). [3] R. S. Houk and H. B. Lim, Analyt. Chem. 58, 3244 (1986). [4] H. B. Lim and R. S. Houk, Spectrochim. Acta 45B, 453 (1990). [5] D. J. Douglas and J. B. French, Spectrochim. Acta 41B, 197 (1986). [6] R. C. Hutton and A. N. Eaton, J. Analyt. Atom. Spectrom. 2, 595 (1987). [7] A. L. Gray and J. G. Williams, /. Analyt. Atom. Spectrom. 2, 599 (1987). [8] D. J. Douglas and J. B. French, J. Analyt. Atom. Specfrom. 3, 743 (1988). [9] J. W. H. Lam and G. Horlick, Spectrochim. Acta 45B, 1327 (1990). [lo] J. H. D. Eland, Photoelectron Spectroscopy, chap. 7. Buttenvorths, London (1974). [ll] P. Wilmenius and E. Lindholm, Ark. Pys. 21, 97 (1962). [12] T. Nishimura, Y. Niwa, T. Tsuchiya and H. Nozoye, J. Chem. Phys. 72, 2222 (1980). [13] Y. Niwa and T. Tsuchiya, Adv. Mass Spectrom. 8, 56 (1980). [14] See for example Spectrum database system (SD&S); No. 3302: 0. Yamamoto, K. Someno, N. Wasada, J. Hiraishi, K. Hayamixu, K. Tanabe, T. Tamura and M. Yanagisawa, Analyt. Sci. 4, 233 (1988). [15] N. G. Adams and D. Smith, Chem. Phys. Len. 47, 383 (1977). [16] W. A. Chupka, Zen-Molecule Reactions, Ed. J. L. Franklin, Vol. 1, chap. 3. Plenum Press, New York (1972). [17] B. Adamcxyk, A. J. H. Boerboom, B. L. Schram and J. Kistemaker, J. Chem. Phys. 44, 4640 (1966). [18] B. Brehm and E. von Puttkamer, Z. Naturforsch. 22a, 8 (1967). [19] R. Stockbauer, J. C/rem. Phys. 58, 3800 (1973). [20] J. H. D. Eland, R. Frey, H. Schulte and B. Brehm, Int. J. Mass Spectrom. Ion Phys. 21, 209 (1976). [21] B.-G. Jonsson and E. Lindholm. Ark. Fys. 39, 65 (1969). [22] M. Bourbne and J. Le Calve, J. Chem. Phys. 58, 1452 (1973). [23] J. M. Tedder and P. H. Vidaud, Chem. Phys. Lett. 64, 81 (1979). [24] T. Hasegawa and H. Haraguchi, Inductively Coupled Plasmas in Analytical Atomic Spectrometry, Eds A. Montaser and D. W. Golightly, chap. 8. VCH, New York (1987). (251 Y. Nojiri, K. Tanabe, H. Uchida, H. Haraguchi, K. Fuwa and J. D. Winefordner, Spectrochim. Acta 38B, 61 (1983). [26] E. H. Kennard, Kinetic Theory of Gases. McGraw-Hill, New York (1938).
561-568. 1992 0
Molecular ionization in the interface of an inductively mass spectrometer
05644547/!T2 $5.00 + .otJ 1992 Pergamon Press plc
coupled plasma
H. TOGASHI, A. HASHIZUMEand Y. NIWA* NationalChemicalLaboratoryfor Industry, l-l, (Received 5 December
Higashi, Tsukuba-shi, Ibaraki 305, Japan
1990; accepted 22 October 1991)
Abstract-The ionization processes of molecules in an Ar afterglow formed in the sampling interface of an inductively coupled plasma mass spectromter were investigated by introducing organic molecules directly into the afterglow. It was found that methanol, methane and benzene are ionized in the afterglow plasma into unstable molecular ions which dissociate immediately after formation. The mechanism of the ionization was elucidated by the analysis of the mass spectra of product ions, in which breakdown graphs were used to estimate the internal energy of the nascent molecular ions. It was found that charge exchange with the Ar’ ion is the most dominant process for all the samples investigated. Neither photoionization by Ar resonance emission, nor Penning ionization by metastable Ar, makes an important contribution to the ionization. There are few electrons in the afterglow that are capable of ionizing molecules.
1. INTR~Du~~N INDWXIVELY coupled plasma mass spectrometry (ICP-MS) is an excellent technique for elemental analysis. However, spectral overlap caused by molecular ions can be a serious problem in quantitative measurement of atomic ions [1,2]. The molecular ions have been conSidered to be produced in the sampling process of the plasma into the mass spectrometer rather than in the ICP, where the temperature is too high (c. 6000 K) for molecules to exist in significant quantity. In order to introduce the plasma of the ICP under atmospheric pressure into the mass spectrometer operating at 10m5 torr, the devices are connected by a differentially pumped interface at about 1 torr. The appearance of an afterglow in the interface chamber has been suspected as giving an ionic distribution different from that in ICP [3,4]. In ICP-MS measurements on metal-containing solutions, it has been found that the intensity ratio of metal oxide ion (MO+) to metal ion (M+) increases with the bond energy of the MO+ [5-71. DOUGLAS and FRENCH [5] found experimentally that the formation temperature of the oxide ions will be 21000 K if chemical equilibrium is assumed. Since this value is much higher than the accepted temperature of the ICP, it was concluded that MO+ ions are formed through non-equilibrium processes involved in the sampling stage. Later, the above authors gave a theoretical discussion of the aerodynamics in the interface region [S] and suggested that the oxide-forming reactions will be rapidly quenched in the expansion process of the plasma sampled in the interface. Meanwhile, LAM and HORLICK [9] found that the MO+/M+ ratio decreases when the interface region is axially elongated. Although they indicated that this phenomenon can be attributed to the different directional distribution of MO+ and M+ in the expanding jet, it is likely that this discrepancy is caused by chemical processes in the interface region. However, it has not been made clear what kind of chemical reactions take place in the interface chamber. If any ionization process on molecules exists in the afterglow, it will possibly cause or enhance the spectral overlap. In order to investigate the possibility of molecular ionization, one has to know the amount of energetic particles that are capable of ionizing molecules. However, it is difficult and circuitous to estimate the population and energy distribution of the energetic species by the conventional diagnostic methods. Instead, an attempt has been made to study the ionization process in the afterglow by
* To whom correspondence
should be addressed. 561
562
H. TOGASHI et
al.
Fig. 1. Schematic of the apparatus: a, plasma torch; b, load coil; c, orifice; d, sample gas inlet; e, skimmer; f, quadrupole mass spectrometer.
the analysis of mass spectra resulting from the ionization of molecules introduced into the sampling interface. Although the plasma mixed with alien species will differ from the original one, observing it should reveal the chemical properties of the original plasma. For the analysis of mass spectra resulting from molecular ionization, knowledge of the unimolecular decomposition of molecular ions provides important information on the energies of particles in the interface and the mechanism of ionization. Ionization of a molecule is initiated by collisional energy-transfer from an energetic particle, and a molecular ion is formed with the internal energy characteristic of the ion-formation process. If the ion is in an unstable state, it may dissociate along various paths according to its internal energy. The relationship between the fraction of the resulting ions and the internal energy of the parent ion has been obtained for various molecular ions and expressed in diagrammatic form by the so-called “breakdown graph” [lo]. Thus, we can speculate, or identify if possible, the elementary process of molecular ionization by the analysis of its mass spectra with the use of the breakdown graphs. The present paper reports the investigation of the ionization of methanol, methane and benzene in the interface region, and the relative importance of ionization mechanisms possible in the Ar afterglow.
2. EXPERIMENTAL The schematic of the apparatus is shown in Fig. 1. An Ar ICP (27.12 MHz, 1.3 kW: Seiko ICP1500) generated at atmospheric pressure was extracted through a sampling orifice of 0.8 mm diameter into an interface chamber. No samples were introduced into the Ar ICP. The distance between the end of the ICP torch and the orifice was about 10 mm. The pressure in the interface chamber was maintained at about 0.5 torr by a mechanical booster pump. Ions produced by the reaction of sample molecules with an Ar afterglow in the chamber were allowed to enter a quadrupole mass spectrometer (VG SXP400) through a skimmer of 0.5 mm diameter. The electric potentials of the orifice cone and the skimmer were set at the ground. The distance between the orifice and the skimmer aperture was 19 mm. Sample gas was introduced into the interface through a stainless-steel tube with a needle valve connected to a gas cylinder or an ampoule. The flow rates of samples were known by the use of a mass flow meter for CH4 or by the measurement of the remaining liquid volume in the ampoule for CH30H and C,H,. The sample inlet tube was directed perpendicular to the axis passing through the orifice and the skimmer aperture. The exit of the inlet tube was placed at 14 mm from the orifice and at 1 mm from the axis.
3. RESULTS
AND
DISCUSSION
When CH,OH was used as a sample gas with a flow rate of 9 kg s-l (= 1.7 X 1017 molecule s-l), HCO+, CH*OH+ and CH3+ were observed, as shown in Fig. 2. These
Ionization in ICP-MS
io
563
20
3b
m/z
Fig. 2. Mass spectra of ions produced by the reaction of CH,OH interface of ICP-MS.
and Ar afterglow at the
observations demonstrate that the ionization of molecules definitely takes place in the interface chamber. The molecular ion CH30H+ was not observed. The weak ion at m/z 32 is not assigned to CH30H+, but to 13CH,0H+ because the intensity,ratio 32+/31+ =l x lo-* coincides with the ratio of natural abundance of carbon isotopes i3C/‘*C= 1.11 x lo-*. The CH30H2+ ion at m/z 33 is probably produced from the secondary reaction of CH30H with CH20H+ or CHO+ [ll] since the ratio of 33+ to 15+, 29+ and 31+ uniformly increased by a factor of about 5 when the flow rate of the sample was doubled. The absence of the molecular ion CH,OH+ in the mass spectrum indicates that nascent CH,OH+ formed in the Ar afterglow is in an excited state leading to spontaneous dissociation into an ion and a neutral species. The energy of the excited state can be estimated by the use of breakdown graphs if the relative abundance of the product ions is known. Figure 3 shows the breakdown graph of the CH,OH+ ion that was obtained in the study of photoelectron-photoion coincidence spectroscopy on CH30H [12,13]. The abscissa shows the internal energy of CH30H+ measured from its ground state. In the experiment, sample molecules were ionized by monochromatic light, and ions and electrons produced in the single photoionization events were
, I
0
CH2OH++ ;
1
2 INTERNAL
3
\ \
4
5 ENERGY
6
7 (eW
Fig. 3. Breakdown graph of CH,OH+ obtained by photoelectron-photoion coincidence spectroscopy. (+, CH,OH+; 0, CH,OH+; n , HCO+; A, CH,‘). (T. Nishimura, Y. Niwa, T. Tsuchiya and H. Nozoye, 1. Chem. Phys. 72, 2222 (1980); reprinted with permission of the American Institute of Physics).
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detected by the use of a mass spectrometer and an electron spectrometer using the coincidence technique. The measurement of the electron energy and the identification of a fragment ion gave the internal energy of the molecular ion and its fragmentation path, respectively. The following processes can be considered as possible mechanisms of ionization in the interface region: (i) electron impact ionization; (ii) photoionization by Ar resonance emission (hv) (11.62, 11.83 eV); (iii) Penning ionization by Ar atoms in a metastable state (Arm) (11.55, 11.72 eV); and (iv) charge exchange with Ar+ ions (15.76 eV). The values in parentheses are the internal energies of the energetic particles or photon energies. Among these processes, only electron impact ionization is a non-resonant process, i.e. the energy of the electron is not entirely transformed into molecular internal energy. The energy of the photon, the metastable Ar atom and the Ar+ ion is resonantly transferred to the molecule, and then an excited molecule with a definite internal energy is produced. In photoionization and Penning ionization, however, the ejected electron takes away part of this internal energy. The energy of the resulting molecular ion is then less than the energy of the photon or the metastable Ar atom. Alternatively, the charge exchange process produces an ion with an internal energy equal to that of the At-+ ion, since the ejected electron combines with Ar+ to form Ar in the ground state. Since the ionization potential (ZP) of CHaOH is 10.85 eV, photoionization and Penning ionization will place CH30H+ with an internal energy of 0.77 and 0.98 eV [E(hv)-IP(CH,OH)], or 0.70 and 0.87 eV [E(Arm)-IP(CH30H)] at a maximum. Figure 3 shows that such molecular ions will remain stable or dissociate exclusively into CH,OH+. If ionization is brought about by electron impact, the molecular ion CH,OH+ should be comparable with CH,OH+ and HCO+ in intensity, which has been evidenced by electron-impact mass spectra of CH30H [14] normally measured with an electron energy of about 70 eV. Since the molecular ion CHaOH+ is absent in mass spectra, and HCO+ and CH,+ are observed in comparable amounts with CH20H+, it is clear that electrons, photons and Ar metastables make a minor contribution to the ionization of CH30H in the Ar afterglow. The charge exchange process will yield a molecular ion with an internal energy of 4.91 eV [ZP(Ar)-ZP(CH,OH)]. This ion dissociates into HCO+, CH*OH+ or CH3+ according to the breakdown graph (Fig. 3). The observed abundance of the ions is in good agreement with those obtained from the diagram on the assumption of the chargeexchange mechanism, as seen from Table 1. Therefore, it is concluded that the ionization of CH30H is caused by charge exchange with the Ar+ ion. Methane was investigated in order to further corroborate the insignificance of electron-impact ionization when compared with the charge-exchange process. Since the ZP of CH4 (12.75 eV) is higher than the photon energy of Ar resonance emission and the internal energy of the metastable Ar atom, neither photoionization nor Penning ionization takes place for CH,+ Then the electron-impact ionization can be directly contrasted with the charge-exchange process. When the sample was introduced into the interface at a flow rate of 5 seem (standard cubic centimeters per minute), which equals 2.2 X 1018 molecule s-i, CH3+, CH,+ and a small amount of CH+ were observed in the mass spectra, as shown in Fig. 4(a). The ions C2H3+, C,H,+ and C2H5+ were observed in relatively small amounts. However, their abundance increased with the amount of CH4 [Fig. 4(b)], indicating that these ions result from the reaction of CH4 with CH3+, CH,+ or CH+ [15]. The peak at m/z 16 is not assigned to CH,+ but to 13CH3+, since the intensity ratio 16+/15+ = 1 x lo-’ agrees with the natural abundance as described above. The absence of CH,+ in the mass spectra indicates that the CH4+ ion is formed in an unstable state leading to immediate dissociation. Moreover, this result is confirmed by the fact that CH,+ is not observed in the mass spectra. If CH,+ survived long enough to react with CH4, CHS+ would be produced predominantly by the reaction of CH,+ with CH, [16]. The CH,+ ion was not detected, even though the flow rate of the
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Table 1. Relative ion abundance obtained from experimental data (Exp.) and breakdown graphs (B.G.) with the assumption of charge~xchange ionization by Ar+ Sample
Product ion*
Exp. (%)
B.G. (%I
CH,OH
CX,+ HCO’ CH,OH+ CH,+ CX,+ C&+ C&f,+ C&L+ CJ%+ C&+
17 51 25 22 76 24 41 1 9 3
28t 42 30 32$ 68 148 47 8 29 2
CH, C86
* Only included. t Ref. $ Ref. 5 Ref.
rb
ions appearing in the breakdown graphs are [12]. [19]. [20].
i0
m/z
fb)
10
20
m/z
Fig. 4. Mass spectra of ions produced by the reaction of CH., and Ar afterglow: (a) sample flow rate 5 seem; (b) 25 seem.
sample was increased [Fig. 4(b)], whereas other secondary ions, C2H3+, GH4+ and C2H5+, increased in intensity. It has been confh-rned that CH,’ is the most abund~t product ion of electronimpact ionization of CH, with electron energies up to 2000 eV [17], which is obviously much higher than the energy of electrons in the afterglow. The complete depletion of CH4+ in the observation clearly shows that the ionization is not caused by electrons. Charge exchange between Ar+ and CH4 will produce CH,+ with an internal energy of 3.01 eV [ZP(Ar)-ZP(CH,)]. The breakdown graph of CH4+ [l&19] indicates that those CH4+ ions completely dissociate into CH,+ or CH2+. In Table 1, the relative abundance of these ions expected from a breakdown graph is compared with that
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TOGASHI et al.
70
80
Fig. 5. Mass spectra of ions produced by the reaction of C,H, and Ar afterglow.
obtained from Fig. 4(a). From the good agreement between the corresponding values, one may conclude that the ionization of CH4 is brought about by charge exchange with the Ar+ ion. Finally, this conclusion was confirmed by a similar experiment on C6H6 (ZP = 9.24 eV). As shown in Fig. 5, with a flow rate of 18 pg s-l (= 1.4 x 10” molecule s-l), a small amount of C,H,+ was observed together with relatively intense fragment ions. The breakdown graph of CsH6+ [20] shows that most of the CsH6+ produced by charge exchange with At-+, with an internal energy of 6.52 eV [ZP(Ar)-ZP(C&Q], will dissociate and only about 2% of it will remain stable. Table 1 shows that the prediction by the breakdown graph accurately reproduces the relative ion abundance obtained from Fig. 5. Although C4H2+, C4H3+ and CSH3+ shown in Fig. 5 are not included in the breakdown graph [20], their formation has been confirmed in the study of charge-exchange mass spectrometry [21]. The electron temperature in the Ar afterglow at the interface was estimated to be 7000-15000 K [4] by the Langmuir probe method, in which a Maxwellian distribution was assumed for the electron velocity. At these temperatures, only about 0.01% of the electrons have an energy higher than the ZP of benzene and even fewer electrons are available to ionize methanol and methane molecules. Thus, the insignificance of electron impact ionization in the interface chamber is consistent with the result of the probe study. If the cross sections of charge exchange and Penning processes are of the same order, one may infer the numerical superiority of Ar+ ions over Ar metastables in the afterglow. The cross section of the Penning process of metastable Ar with C6H6 has been obtained as 160 A’ [22], whereas the ionization cross-section of Ar+ with C,H, has been determined as 280 A’ [23]. For the ICP, on the other hand, the abundant existence of Ar+ ions compared to Ar metastables has already been established. The density of Ar+ ions has been estimated as 5 x 1014-5 X 1015 cmm3 from measurements of the electron density [24], whereas the density of Ar metastables has been found to be 2 x 10” cmp3 [25]. Thus, it is possible that the predominance of Ar+ ions over Ar metastables in the ICP is not significantly altered by the sampling process. Although the significance of the charge exchange mechanism in ionization has been confirmed in the above discussion by the good agreement of the mass spectra with the predictions of the breakdown graphs, there remains the possibility that the sample molecules are already excited before ionization, or the product ions are further dissociated by collisions with energetic particles. Then, the mass spectra must have accidentally imitated the patterns of charge exchange ionization. In order to show that such secondary collisions can be neglected in the present experiments, the feasibility of such collisions will be discussed. The Ar+ ion is obviously not important in the secondary collisions because it is not capable of colliding with positive ions or exciting sample molecules to any stable states. However, although the other energetic particles are insignificant in ionization compared with the Ar+ ion, it does not follow that those particles are not important in secondary
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collisions. Then, to consider the influence of those energetic particles, the number of collisions of a molecule or product ion with Ar in the interface chamber is estimated. Consider now the gas dynamics of the afterglow plasma in the interface chamber. It has been pointed out [8] that, in the interface of ICP-MS, the place where the sampled plasma exhibits free expansion is limited to a region surrounded by a concentric barrel-shaped shock wall and terminated by a perpendicular plane shock wall named Mach disc, whereas turbulance or unpredictable motion of gas may exist in the rest of the chamber. The position of the Mach disc (xM,)measured from the orifice can be estimated by the equation: .xIVIDIDO = 0.67(P@‘J’*,
(1)
where Do is the orifice diameter, and PO and PI are the pressures at the outside and inside of the chamber, respectively. Insertion of Do = 0.8 mm, PI = 0.5torr and PO = 760 torr gives XMD= 21 mm, which indicates that the Mach disc is placed close to or penetrated by the skimmer tip in the present apparatus. Hence, the flow of the afterglow plasma can be regarded as free expansion in the region where the sample molecules are injected and carried toward the skimmer by the Ar jet. In the trajectory of a sample molecule (M), starting at the injection point and ending at the skimmer aperture, the collision frequency with Ar is highest at the starting point, where the density of Ar is higher than in the remaining part. On the assumption that M, immediately after injection, moves straight in the direction perpendicular to the Ar jet, the probability (q) that M comes into collision with Ar per unit length along the straight path of M, is simply given by:
(2)
q = unAvA/vM,
where vM and VA are the velocity of M and Ar, respectively, IJ is the collisional crosssection of M with Ar, and nA is the density of Ar. The value of ItA at x downstream from the orifice is given [8] by: nA = 0.161 x (xlD~J-2no,
(3)
where no is the density of Ar in the ICP. Inserting x = 14 mm and n,, = 1.2 x 1018 cmp3 for a typical ICP (6000 K, 760 torr) into Eqn (3), nA = 6.4 x 1Or4 cmp3 is obtained. Further use is made of the following approximation: vA/vM = (mMT.&ATM)
I’*,
(4)
where mM and mA are the masses of M and Ar, and TM and TA are the temperatures in the sample reservoir of M and the ICP, respectively. If TA = 6000K and TM = 300 K (room temperature), and u = 50 A2 as a typical collisional cross-section in gas kinetics [26], q = 1.3 mm-’ when methanol is assumed to be the sample molecule. Since the distance between the injection point and the skimmer tip is about 5 mm, the total number of collisions of a sample molecule with Ar at the interface should be about equal to or less than 1.3 x 5 = 6.5, which indicates that the probability of ionization of a sample molecule is small compared to unity since the density of Ar+, as well as other energetic particles, is much lower than that of Ar. It is therefore obvious that the occurrence of secondary collisions of a sample molecule or product ion with energetic particles can be neglected when compared with the primary ionization of the sample molecule by an Ar+ ion.
4. CONCLUSIONS The ionization mechanisms of organic molecules externally introduced into the interface chamber of an ICP mass spectrometer have been investigated. The elementary
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processes of the formation of nascent molecular ions were examined by the analysis of the mass spectra of their fragment ions with the aid of breakdown graphs. It has been revealed that charge exchange with the Ar+ ion is the most important ionization process. Neither Penning ionization by metastable Ar atoms, nor photoionization by Ar resonance emission noticeably takes place, even if these processes are energetically allowed. There are few electrons possessing sufficient energy to ionize these molecules. REFERENCES [l] M. A. Vaughan and G. Horlick, Appl. Spectrosc. 40, 434 (1986). [2] G. Horlick, S. H. Tan, M. A. Vaughan and Y. Shao, Inductively Coupled Plasmas in Analytical Atomic Spectrometry, Eds A. Montaser and D. W. Golightly, chap. 10. VCH, New York (1987). [3] R. S. Houk and H. B. Lim, Analyt. Chem. 58, 3244 (1986). [4] H. B. Lim and R. S. Houk, Spectrochim. Acta 45B, 453 (1990). [5] D. J. Douglas and J. B. French, Spectrochim. Acta 41B, 197 (1986). [6] R. C. Hutton and A. N. Eaton, J. Analyt. Atom. Spectrom. 2, 595 (1987). [7] A. L. Gray and J. G. Williams, /. Analyt. Atom. Spectrom. 2, 599 (1987). [8] D. J. Douglas and J. B. French, J. Analyt. Atom. Specfrom. 3, 743 (1988). [9] J. W. H. Lam and G. Horlick, Spectrochim. Acta 45B, 1327 (1990). [lo] J. H. D. Eland, Photoelectron Spectroscopy, chap. 7. Buttenvorths, London (1974). [ll] P. Wilmenius and E. Lindholm, Ark. Pys. 21, 97 (1962). [12] T. Nishimura, Y. Niwa, T. Tsuchiya and H. Nozoye, J. Chem. Phys. 72, 2222 (1980). [13] Y. Niwa and T. Tsuchiya, Adv. Mass Spectrom. 8, 56 (1980). [14] See for example Spectrum database system (SD&S); No. 3302: 0. Yamamoto, K. Someno, N. Wasada, J. Hiraishi, K. Hayamixu, K. Tanabe, T. Tamura and M. Yanagisawa, Analyt. Sci. 4, 233 (1988). [15] N. G. Adams and D. Smith, Chem. Phys. Len. 47, 383 (1977). [16] W. A. Chupka, Zen-Molecule Reactions, Ed. J. L. Franklin, Vol. 1, chap. 3. Plenum Press, New York (1972). [17] B. Adamcxyk, A. J. H. Boerboom, B. L. Schram and J. Kistemaker, J. Chem. Phys. 44, 4640 (1966). [18] B. Brehm and E. von Puttkamer, Z. Naturforsch. 22a, 8 (1967). [19] R. Stockbauer, J. C/rem. Phys. 58, 3800 (1973). [20] J. H. D. Eland, R. Frey, H. Schulte and B. Brehm, Int. J. Mass Spectrom. Ion Phys. 21, 209 (1976). [21] B.-G. Jonsson and E. Lindholm. Ark. Fys. 39, 65 (1969). [22] M. Bourbne and J. Le Calve, J. Chem. Phys. 58, 1452 (1973). [23] J. M. Tedder and P. H. Vidaud, Chem. Phys. Lett. 64, 81 (1979). [24] T. Hasegawa and H. Haraguchi, Inductively Coupled Plasmas in Analytical Atomic Spectrometry, Eds A. Montaser and D. W. Golightly, chap. 8. VCH, New York (1987). (251 Y. Nojiri, K. Tanabe, H. Uchida, H. Haraguchi, K. Fuwa and J. D. Winefordner, Spectrochim. Acta 38B, 61 (1983). [26] E. H. Kennard, Kinetic Theory of Gases. McGraw-Hill, New York (1938).