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Application of negative ions in inductively coupled plasma-mass spectrometry
Spectrochimica Acta Part B 59 (2004) 1021 – 1031 www.elsevier.com/locate/sab
Application of negative ions in inductively coupled plasma-mass spectrometry A.A. Pupyshev a,*, V.T. Surikov b a
b
Ural State Technical University, 19 Mira St., Ekaterinburg 620002, Russia Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 91 Pervomaiskaya St., Ekaterinburg 620219, Russia Received 25 June 2003; accepted 17 May 2004 Available online
Abstract The efficiency of the formation of negative background and F, Cl, Br and I ions in inductively coupled plasma (ICP) was investigated at temperatures ranging from 4000 to 9000 K using thermodynamic simulation. It was shown that the basic negative background ions in ICP, O, H, OH and probably Ar, are in accordance with experiment. The estimated total concentration of negative ions in ICP was found to be four to five orders of magnitude smaller than that of positive ions. The highest efficiency of negative ion formation should be observed for elements having high electron affinity, namely Cl, F, Br and I. However, the detection sensitivity in the negative ion detection mode may be increased slightly as compared with the positive ion detection mode only for fluorine in the temperature range 6000 – 7000 K. This is in contradiction with experimental results and may be explained by the formation of negative ions behind the skimmer and/or smaller losses of negative ions in the ion beam at low ionic current. The relationship between the efficiency of negative atomic ion formation and electron affinity was determined. This makes it possible to estimate numerically the efficiency of the formation of Ar, Na and other negative atomic ions in the ICP. The calculations performed confirmed the experimental data showing that the cation (Ba, Co, Cu, K, Na and Sr) introduced as a chloride does not affect the analytical signal of Cl. According to the calculations, high contents of halogens in samples are not expected to alter significantly the contents of F, Cl, Br and I measured in ICP as analytes. The experimentally observed significant suppression effect of high halogen concentrations may be caused by a shift of equilibrium in reactions of electron addition to halogen atoms behind the skimmer and/or increased negative ion current in the ion beam and accordingly greater losses of analytes. D 2004 Elsevier B.V. All rights reserved. Keywords: Inductively coupled plasma-mass spectrometry; Thermodynamic simulation; Negative atomic and molecular ions; Efficiency of ion formation; Matrix effects
1. Introduction The applicability of negative atomic and molecular ions for quantitative analysis by inductively coupled plasmamass spectrometry (ICP-MS) was investigated in the period between 1983 and 1992 [1 – 9]. The investigations concerned the composition of negative ion spectra [1,3 – 9], their intensity versus some plasma [3 –9] and detection [5– 7] parameters, element determination procedures [4,6 – 9] as well as effects of matrix cations and anions on analytical results [3,5,9]. In addition, simple theoretical studies of the applicability of the negative ion detection
* Corresponding author. Tel.: +7-343-3754658; fax: +7-343-3750196. E-mail address: [email protected] (A.A. Pupyshev). 0584-8547/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2004.05.011
mode for quantitative elemental analysis by ICP-MS were performed using the Saha equation [2 –4]. Comprehensive discussion of papers [1 –9] was presented in a review by Surikov and Pupyshev [10]. The major results of the investigations are as follows: – the observed high level of continuous baseline instrumental background (103 cps) is due to electrons, which reach the detector [4,5,9]; – basic background negative ions are H, O, OH, O2, NO and NO2 (the latter exists only in the presence of nitric acid) [4,5,8,9]; – negative ion spectra of the determined elements can be represented as M, MO, MO2, MO4 and HMO4 [3,4]; – the intensity of negative atomic ions increases when the plasma temperature decreases [1,2] (in accordance with
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theoretical estimations employing the Saha equation) or increases [3,4] (an experimental result obtained with enhanced applied radio-frequency power of plasma); as was found experimentally, Cl, F and Br have superior detection sensitivities as compared with the positive atomic ion detection mode [1 –4,6,9]; the detection limit (DL) for F (the most problematic element in ICP-MS) reached 110 ppb [4], a value which is much better than that obtained with modern ICP-MS spectrometers for F [0.38 ppm using dynamic reaction cell (DRC) and 10 ppm without DRC]; a linear dependence (as checked on F, Cl, Br and I) was observed between experimental intensities of negative atomic ions and electron affinity (EAM) of the corresponding elements [9] at their equal mass concentration (10 ppm) in sample; according to the calculation for Cl with the use of the Saha equation, the efficiency of negative atomic ion formation must be several orders of magnitude smaller than that of positive ions [1,2,4] of the corresponding elements; this conclusion does not agree with the experiment and in the authors’ opinion [2,4,6] may be indicative of different formation processes of negative and positive ions in inductively coupled plasma; it was supposed that negative atomic ions are likely to originate from electron capture or reactions downstream the free jet expansion rather than from plasma directly [5]; the intensity of Cl signal at concentrations of 10 Ag ml 1 is independent of the type of associated cations [9]; the intensity of O signal is influenced by CsCl at the ppb concentrations [5]; the intensities of O, F and Cl signals depend significantly on the presence of matrix halogen compounds HCl, CsCl and NaBr at ppm concentrations [3,5]; there is no agreement with respect to the charge polarity of detected Ar and Na ions in the negative ion detection mode [5,6,9]; it was supposed that positive ions Ar+ and Na+ (if their concentrations in ICP are high) may be detected in the negative ion detection mode because they penetrate through ion optics and quadrupole instead of being stopped by the optics (ion lenses) and reach the detector [5,9]; it was proposed to determine halogens in the form of negative ions using ‘‘cold’’ plasma or three-aperture interface [11].
Experimental and theoretical results on the applicability of negative atomic ions for quantitative analysis by ICP-MS are scarce and contradictory. These data fail to provide clear and comprehensive understanding and explanation of all processes occurring when negative ions are formed and detected by ICP-MS. The significant advantage of the negative ion detection mode in determining low fluorine concentrations reported in the literature is not used by
manufacturers of modern ICP-MS spectrometers in spite of real demand for such analysis. Therefore, turning back to these problems seems to be justified and useful both from the theoretical and practical viewpoints. Equilibrium thermodynamic simulation of thermochemical processes in low-temperature plasma based on the model developed for ICP-MS opens up new possibilities in investigating these problems. We employed this model for data [1– 9] analysis to check up the applicability of the model and better understand the observed processes and phenomena in case negative ions are used in ICP-MS.
2. Thermodynamic simulation The electron density and temperature in the central channel of a plasma, through which ions are extracted into the interface, change under real ICP-MS conditions with the operation parameters such as plasma power, flow rate of aerosol transporting argon and loading of plasma with aerosol. Moreover, the electron density and positive and negative ion concentrations in ICP vary as a function of the matrix in samples and the ionization energy of matrix elements. As a result, the efficiency of negative atomic ion M formation changes accordingly. These variations cannot be calculated by the Saha equation if experimental temperatures and electron density values are not known. However, such calculations can be performed using equilibrium thermodynamic simulation [12]. In this method, behavior of the complex multicomponent thermodynamic system is analyzed based on the calculation of its total equilibrium composition, including molecules, atoms, ions and electrons. For example, it may be a calculation of the composition of plasma penetrating into the interface of the mass spectrometer. The calculations are carried out by maximizing the entropy or minimizing the Gibbs energy of the system. They require the initial concentration of all components, pressure and temperature of the system, as well as thermodynamic properties of all individual substances that may be present in the equilibrium. Depending on the complexity of the elemental composition of a thermodynamic system, tens or hundreds of individual substances are usually taken into account in such calculations. The main difficulty in these calculations is to create an equilibrium model for a nonequilibrium ion source, namely, ICP. A quasi-equilibrium model for thermochemical processes in ICP, which permit obtaining the total composition of plasma in ICP-MS, was developed earlier [13 – 17]. The basic assumptions of the model are as follows: – only the central channel located on the discharge axis (analytical zone) and serving for the introduction of analyzed aerosol and ion extraction is considered; – the degree of mixing of the aerosol-introducing (transporting) argon flow with plasma-forming (outer) or intermediate argon flows is equal to zero;
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– the analytical zone is in local thermodynamical equilibrium (LTE); – the analyzed aerosol introduced into ICP evaporates completely; – components of a thermodynamic system are uniformly distributed over the analytical zone; – the ICP discharge occurs at atmospheric pressure (0.1013 MPa); – the initial composition of a thermodynamic system (working medium) is equal to the mass flow (g min 1) of all introduced plasma components: transporting argon and aerosol of the analyzed solution containing a solvent, a matrix and trace elements; – the atmosphere air does not mix with the discharge plasma; – ion extraction from the discharge plasma into the interface provides a representative probe of the plasma composition for the analytical zone; – the plasma temperature changes insignificantly upon introduction of solution aerosol into the discharge; – changes in the composition of the analyzed solution do not affect the ICP temperature; – secondary atomic ionization is possible only for the elements with the lowest sum of the first and the second ionization potentials [16]. The applicability of the model was confirmed by comparing the estimated results with a large number of experiments [15,16]. The equilibrium composition of the considered thermodynamic systems was calculated using the software package ASTRA [18] and the thermodynamic database IVTANTHERMO (Refs. [19,20], http://www.openweb.ru/ thermo/index.htm). ASTRA is based on the principle of the entropy maximization of the examined complex multicomponent thermodynamic system. Additional restrictions are imposed on the system when the maximum entropy is found: – the total internal energy of the system at equilibrium should be constant; – the mass of all elements should be conserved; – the state of the ideal gas mixture should be equated; – the law of charge conservation should be obeyed. The total equilibrium chemical composition of the system including electron density and some thermodynamic parameters are established in the calculations. For each individual substance, the thermodynamic database IVTANTHERMO gives the standard enthalpy of 0 compound formation from elements DH298 , enthalpy incre0 ment from 0 to 298.15 K [H298 H00] and numerical coefficients of the seven-valued polynomial approximating the temperature dependence of the reduced Gibbs energy. Owing to the dependences between the thermodynamic functions, these numerical coefficients make it possible to easily calculate the temperature dependences for entropy
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S 0j (T), enthalpy variation [Hj(T ) Hj(0)] and heat capacity Cpj(T ) of an individual j– th substance. Individual substances (electrons, atoms, molecules, atomic and molecular ions), which may exist in equilibrium, are listed in Table 1. The negative atomic ion of argon Ar was not taken into account in the calculation of the total equilibrium composition of thermodynamic systems because the required reference thermodynamic data for this ion are not available in the literature (perhaps because of a low probability of Ar formation). In each calculation of equilibrium, we consider simultaneously the possibility of the formation of all individual substances containing all elements present in the initial components of the system. For example, in the case of the simple thermodynamic system Ar – H2O – HCl containing elements Ar, Cl, H and O (the water aerosol introduced in the plasma contained only HCl), the probability of the formation of the following individual substances is considered simultaneously (Table 1): Ar, Ar+, Cl, Cl+, Cl, Cl2, ClO, ClO2, Cl2O, e, HCl, H, H+, H, H2, H2+, H2, H3+, OH, OH+, OH, HO2, HO2, H2O, H2O2, H3O+, HOCl, O, O+, O, O2, O2+, O2 and O3. Similarly, based on the initial element composition of any thermodynamic system considered further, it is easy to determine from Table 1 what
Table 1 Individual substances taken into account in the calculations (IVTANTHERMO database) Ar, Ar+ Br, Br+, Br, Br2, BrO Ba, Ba+, Ba2 +, Ba2, BaO, BaO+, BaH, BaOH, BaOH+, Ba(OH)2, BaCl, BaCl+, BaCl2, BaOHCl Cl, Cl+, Cl, Cl2, ClO, ClO2, Cl2O, HCl, ClF, ClF3, ClF5 Co, Co+, CoO, CoH, Co(OH)2, CoCl, CoCl2, Co2Cl4 Cs, Cs+, Cs2, CsO, Cs2O, Cs2O+, Cs2O2, CsH, CsOH, Cs(OH)2, CsF, Cs2F2, CsCl, Cs2Cl2, CsBr, Cs2Br2, CsJ, Cs2J2, CsN, CsON, CsNO2, CsNO3 Cu, Cu+, Cu2, CuO, CuH, CuOH, CuCl, CuCl2, Cu2Cl2, Cu3Cl3, Cu4Cl4, Cu5Cl5 e F, F+, F, F2, FO, F2O, HF, H2F2, H3F3, H4F4, H5F5, H6F6, H7F7, HOF, F2H, FCN, F2N2, F3N, F4N2, FNO2, FNO3, F3NO + + + H, H+, H, H2, H+2, H 2 , H3, OH, OH , OH , HO2, HO2 , H2O, H2O2, H3O , HBr, HOBr, HOCl, HI, HN3, HNNH, HNO, HNO2, HNO3, HOCl, HOF I, I+, I, I2, IBr, ICl, IF, IF5, IF7, IO K, K+, K2, KO, K2O, K2O+, K2O2, KH, KOH, KCl, KCl2, K2Cl2, KN, KON, KNO2, KNO3 Mg, Mg+, Mg2 +, Mg2, MgO, MgN, MgOH, Mg(OH)2, MgCl, MgCl2, MgOHCl + + N, N+, N, N2, N+2, N 2 , N3, NO, NO , NO2, NO2, NO2 , NO3, NO3 , N2O, N2O+, N2O3, N2O4, N2O5, NH, NH+, NH2, NH3, NH+4, N2H2, N2H4, NF, NF2, NF3, N2F2, N2F4, FNO, FNO2, FNO3, F3NO, NHF, NH2F, NHF2, ClNO, ClNO2, NBr, NOBr, NOI Na, Na+, Na2, NaO, Na2O, Na2O+, Na2O2, NaH, NaOH, Na(OH)2, NaF, Na2F2, Na3F3, NaCl, Na2Cl2, Na3Cl3, NaBr, Na2Br2, NaJ, Na2J2, NaN, NaON, NaNO2, NaNO3 O, O+, O, O2, O+2, O 2 , O3 Sr, Sr+, Sr2 +, Sr2, SrO, SrO+, SrH, SrOH, SrOH+, Sr(OH)2, SrCl, SrCl+, SrCl2, SrOHCl
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individual substances were taken into account simultaneously in the calculation. Analogous calculations can be performed employing other programs such as SELECTOR [21], SOLGASMIX [22], NASA [23], etc. These programs have their own databases of thermodynamic data for individual substances. The advantage of IVTANTHERMO over other databases consists in complete mutual agreement of all given values of thermodynamic characteristics for all individual substances in the database [20]. The considered temperature range usually was 4000– 9000 K with the general calculation step 500 K. Because the local thermodynamic equilibrium conditions were assumed for the central ICP channel, temperatures in this zone of the plasma equal the equilibrium temperature. Our estimated values of electron density n(e) in argon plasma [15] are in agreement with experimental data for the central ICP channel. The estimated plasma temperatures are also consistent with experimental electron temperatures Te and possibly with ionization temperatures Ti. The initial composition of a thermodynamic system was prescribed according to experimental data: the composition was equal numerically to the mass flow rate (g min 1) of introduction of transporting argon, water, trace and matrix elements (salts, acids) into the central plasma channel. Their concentration in the analyzed solution and nebulization efficiency were also taken into account. In the experiments considered below [4,5,9], the data on nebulization efficiency are lacking. Therefore, the efficiency of solution introduction into ICP was assumed to be 0.01. (i.e., 1%). Numerical values of other experimental parameters used in the calculations, as well as qualitative and quantitative compositions of the examined thermodynamic systems for particular calculations, are given below.
3. Results and discussion 3.1. Negative background ions The calculations have been performed for experimental conditions [9]. Argon flow rate for transporting aerosol was 0.65 l min 1, sample introduction rate was 0.5 ml min 1, concentration of HNO3 was 1%. The thermodynamic system Ar – H2O – HNO3 was studied. According to the calculation results, the basic negative ions in argon plasma in the temperature range 4000 – 9000 K are O, H and OH (Fig. 1). When the temperature of plasma rises from 4000 to 9000 K, the theoretical electron density increases four orders of magnitude or more. A significant increase in the electron density causes an increase in the calculated equilibrium concentration of atomic ions O and H. The concentrations of OH and other negative polyatomic ions should decrease simultaneously. In this temperature range, the electron concentra-
Fig. 1. Estimated variations of electron density and basic negative background ion n(X) concentration logarithms in argon plasma as a function of the argon plasma temperature T simulated according to experimental conditions [9].
tion in discharge plasma must be three to five orders of magnitude greater than the total concentration of negative background atomic ions. As is seen from Fig. 1, the LTE model gives very low values of electron density for the central ICP channel at temperatures below 6500 K. For example, at 4000 K n(e) was predicted to be c 1011 cm 3. At first glance, it seems to be unlikely for Ar ICP. However, experimental measurements of the electron density and electron temperature [24] along the axis of the central ICP channel above the torch showed that the electron density can be about (2– 3)1012 cm 3 in plasma zones with Te = 1800 K. Comparison of data [24] with our calculations revealed that the estimated electron density values approach the experimental n(e) values when the equilibrium temperature T increases, and at T = Te >6500 K, they are practically the same. It is necessary to note that the experimental values of Te and gas temperature Tg in the central ICP channel are close to each other [24]. This is additional evidence that the LTE condition can be met in the central ICP [25] channel and that thermodynamic simulation is appropriate for the analytical zone. Considering the significant discrepancy between the experimental n(e) values and those calculated from the LTE condition for ‘‘cold’’ plasma, we may only estimate the character and tendencies of the thermochemical processes occurring in this temperature range. In the case of a water solution containing 1 % HNO3, it was predicted that the NO2 ions are present in plasma, but their concentration is five to six orders of magnitude smaller than that of OH over the whole temperature interval examined. When the plasma temperature increases, the concentration of NO2 must decrease sharply. NO ions
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3.2. Negative atomic ions of analytes Temperature dependences of efficiency of atomic ion formation were found based on the calculations of model argon plasma systems, in which analyzed trace elements were introduced. The formation efficiency of negative and positive atomic ions was estimated at temperature T as DðM ÞT ¼ MT =RM and DðMþ ÞT ¼ MTþ =AM ;
Fig. 2. Estimated variations of negative D(X)T and positive D(X+)T ion formation efficiency logarithms in argon plasma as a function of the plasma temperature T simulated according to experimental conditions [9].
having a very low electron affinity (EANO = 0.024 eV, EANO2 = 3.10 eV [26]) were ignored in the calculation. Negative ion spectra obtained in Ref. [9] included mainly O, OH, O2 and probably H3O ions. These spectra seem to be indicative of a high background in the region of H. Strong yet unresolved signals of O and OH and a weak signal of O2 were reported in Ref. [6]. In Ref. [5], the intensity of the O signal was three orders of magnitude higher than that of OH, strong signals were observed in the region of H and H2, whereas a signal of O2 was weak. In Ref. [4], strong signals of O, OH and O2 were observed, but the relative intensity of O and O2 was not known. The spectra reported in Ref. [1] contained strong signals of O and OH and a weak signal of O2. A very strong signal of NO2 and a weak signal of NO were observed in Ref. [4] for a 1% HNO3 water solution. In Ref. [6], the signal of NO2 from a 2% HNO3 water solution was very weak. Comparison of the estimated and experimental negative ion spectra revealed their similarity with respect to the basic background ions O, H and OH. Very high concentrations of OH and NO2 ions observed in some of the abovementioned experimental works may be due to the incomplete vaporization of aerosols or intensive gas cooling near vaporized aerosol drops. Unfortunately, in all these works [1 –9], the effects of plasma power and the transporting argon flow rate determining the temperature of the inner ICP channel are not reported. Therefore, at present we cannot compare the results of experiments and simulation in all details.
where MT and MT+ are the amounts of element M present in a thermodynamic system at temperature T as negative or positive ions respectively; AM is the total amount of element M introduced into the initial thermodynamic system. The calculations were carried out under experimental conditions [9]. The basic composition of the initial thermodynamic system (Ar –H2O – HNO3) has been given already in Section 3.1. The concentration of analytes (Br, Cl, F, I) in the experiment [9] and in our calculation was 10 Ag ml 1. Upon introduction of analytes, the composition of the thermodynamic system was Ar – H2O – HNO3 – Br –Cl – F– I. According to the calculations, the highest efficiency of negative ion formation should be observed for Cl, F, Br and I (Fig. 2), i.e., for elements having the highest electron affinity [26,27] (Fig. 3). The values of D(M)T increased almost equally for all the considered elements with the plasma temperature. Only the high-temperature (T >7000 K) region showed promise for measurements. At usual operating temperatures of contemporary ICP-MS spectrometers (6000 – 8000 K), values of D(M)T > D(M+)T can be obtained only for F. It is seen in Fig. 1 that the concentration of negative atomic ions tends to increase with temperature as a result of increased electron density. Probably, this is due
Fig. 3. Estimated variations of negative atomic ion D(M)T formation efficiency logarithms in argon plasma as a function of electron affinity energy EAM [28] simulated according to experimental conditions [9] at plasma temperature T, K: 1 – 8500, 2 – 7500, 3 – 6500. The dashed line denotes EAAr .
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to the circumstance that the temperature effect on the equilibrium constant of reaction M ¼ M 0 þ e
ð1Þ
is smaller than the effect of electron density increase. This trend is not observed for recombination reactions of positive ions M þ þ e ¼ M 0 (Fig. 2) because of a stronger temperature dependence of the equilibrium ionization constant because the considered elements have high ionization potentials. According to the calculations, if the mass concentration of mg l 1 elements in the solution is the same, the relationship of negative ion concentrations (cm 3) should be n(F) > n(Cl)> n(Br)> n(I). This sequence of halogen ion concentrations differs from the sequence of their negative ion formation efficiencies D(Cl)T > D(F)T > D(Br)T > D(I)T because of the significant difference in the atomic masses of F and Cl. The concentration (in discharge plasma, cm 3) of negative atomic ions of other elements of the Periodical Table having electron affinity of not more than 250 kJ mol 1 at the same molar concentrations must be much smaller than for halogens. Comparison of the calculated formation efficiency logarithms of negative atomic ions and their electron affinities revealed almost linear dependences (Fig. 3). Analogous linear dependences were observed between the estimated concentrations (cm 3) of negative ions and their electron affinities if equal molar amounts of elements are introduced into the initial thermodynamic system. This also indicates that the thermodynamic data used in the calculations for all the considered elements are consistent enough. These relationships contradict analogous linear dependences obtained experimentally for Cl, F, Br and I when signals of negative ions of these elements (cps/10 Ag ml 1) corrected for their isotope abundance and instrument mass sensitivity were used to compare their electron affinity [9]. In this case, different initial experimental molar concentrations of halogens in the analyzed solution and discharge plasma should additionally affect the signals for negative ions. Then, we can expect a considerable deviation of this relationship from linearity. Indeed, the linear dependence described in Ref. [9] is no longer observed when the experimental data [9] are converted to equal molar concentrations of elements in sample. Using the linear relationship between logarithms of the estimated formation efficiency of negative atomic ions and electron affinity of elements (Fig. 3), we can resolve the contradiction concerning the polarity of detected Ar and Na ions [5,6,9]. No required reference thermodynamic data on these negative ions are available in the literature. Therefore, in the thermodynamical simulation of a multicomponent system in ICP, we could not take into account the formation of Ar and Na ions. Obviously, the absence of the
reference data is connected with a low probability of the formation of these ions in the usually examined thermodynamic systems. However, we may try to estimate the formation efficiency of Ar and Na and their concentrations in ICP using the linear dependences given in Fig. 3. The estimated electron affinity for argon atom is EAAr = 35 kJ mol 1 [28]. For example, extrapolation of the dependence between negative atomic ion formation efficiency D(M)T and electron affinity EAM shows that for the abovementioned value of EAAr at T = 7500 K, the value of D(Ar)T must be about f 1 10 7 (Fig. 3). Because the concentration of argon atoms in plasma at 7500 K is very high [n(Ar) c 1 1018 cm 3], the concentration of negative argon ions n(Ar) with allowance for the above D(Ar)T value must be nðAr Þ ¼ nðArÞ DðAr ÞT c1 1018 1 107 ¼ 1 1011 cm3 : This concentration of negative atomic ions of argon is comparable and even slightly higher than that of the basic background ions O and H (Fig. 1), which were considered in the calculations of the multicomponent equilibrium. Close values of O and Ar concentrations must provide rather close values of counting intensity of these ions. This was observed in particular for the Ar – H2O system [9]. The above result is indicative of a negative polarity of detected ions having m/z = 40, in contrast to the opinion expressed in Refs. [5,9]. Analogous estimations for sodium (EANa = 52.9 kJ mol 1 [28]) show that the polarity of detected ions with m/z = 23 may be negative at high Na concentration, also in conflict with Ref. [5]. In Ref. [4], equilibrium reaction (1) was considered and the relationship of concentrations of neutral atoms and negative ions M T0 /M T was calculated for chlorine using the Saha equation MT0 =M T ¼ ½2=nðeÞ ½ð2pme kT Þ=h2 3=2 ðQ0 =Q ÞexpðEAM =kT Þ: Here, me is the rest mass of electrons, k, Boltzmann constant, h, Planck constant, Q0 and Q are partition functions for a neutral atom and a negative ion, respectively. For T = 7500 K and n(e) = 5 1014 cm 3, the Cl 7500 / Cl07500 ratio was found [4] to be 7 10 6 ( Q, Q+, Q0 and EACl values used in the calculation were not cited). The authors [9] believe that this points to a very low formation efficiency of negative ions even for Cl having the highest electron affinity (Fig. 3, EACl = 349 kJ mol 1 0 [27]). A similar value of Cl7500 /Cl7500 was reported in Ref. [2], also without specifying the calculation details. According to the calculations by the Saha equation, very low concentrations of negative ions were expected already in the pioneering paper dealing with negative ion detection in ICP-MS [1]. Based on the comparison of electron affinity
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energy of F taken as an example and ionization energy of other elements, the authors of review [11] also concluded about low abundance of negative ions in ICP and their weak analytical possibilities. The calculation with the Saha equation at T = 7500 K and n(e) = 5.1 1014 cm 3 [4] also yielded Cl+7500 /Cl07500 = 1.2 10 2. Consequently, in the authors’ [4] opinion, the expected theoretical degradation of chlorine detection sensitivity when going from positive to negative detection mode must be four orders of magnitude or more. However, a much better detection sensitivity (cps/ppm) of Cl, F and Br was observed experimentally [4]. Based on this fact and measured negative ion energies, the authors [4] arrived at the conclusion that negative ions originate not directly in plasma but as a result of electron capture or downstream reactions of free jet expansion. In this connection, we have performed thermodynamic simulation of experimental conditions [4]: the argon flow rate for transporting aerosol, 1 l min 1, the concentration of HNO3 in the analyzed solution, 1%. In addition, in our model calculations we used the following parameters not given in Ref. [4]: solution introduction rate, 1 ml min 1, concentration of analytes (Br, Cl, F and I), 10 Ag ml 1. The considered system was Ar – H2O – HNO3 – Br – Cl –F – I. The calculations with the Saha equation and those employing thermodynamic simulation have the following differences. In the case of the Saha equation, only two physical parameters (equilibrium plasma temperature and electron density) are used. These values must be known exactly or determined experimentally because the electron concentration at a given temperature depends on the type and flow rate of the working gas, the matrix composition of the sample and efficiency of aerosol input into the plasma. In the calculations by the Saha equation, only individual ionization reaction equilibrium (for atoms and molecules) is considered, whereas the possibility of other simultaneous ionization reactions, which shift the equilibrium of the examined reaction, is not taken into account. With this restriction, the calculations permit finding the relationship of atoms and atomic ion, molecules and molecular ions only for the considered individual reactions. Any changes in the initial plasma composition (type and flow rate of the working gas, flow rate and efficiency of sample aerosol input, matrix composition of the sample) call for new estimation or measurement of the electron concentration. Thermodynamic simulation requires assigning of pressure (known), plasma temperature, total chemical composition of the simulated thermodynamic system including working gas, solvent, matrix and traces elements (which are usually known from experiments or may be assumed). In other words, these initial data adequately reproduce the real analytical operation conditions. Such calculations take into account the possibility of simultaneous formation of atoms, molecules, positive and negative atomic and molecular ions for all the elements that are present and actually interact in a multicomponent thermodynamic system. The results of the calculations make it
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possible to find the total equilibrium chemical composition of the system including the concentration of molecules, atoms, atomic and molecular ions as well as the electron number density. Thus, thermodynamic simulation of a correctly created equilibrium model provides much more quantitative information on the composition of real plasma and thermochemical processes occurring in plasma than the calculations employing the Saha equation. Moreover, for thermodynamic simulation, electron densities need not be known or found experimentally. The total composition of the simulated system at the mentioned above preassigned experimental parameters can be easily determined for other temperatures. Based on the same LTE hypothesis, the calculations performed with the Saha equation and those using thermodynamic simulation coincide completely only for binary systems, for example, argon and analyte. In this case, the electron density in ICP is determined only by the working gas (argon). When the sample solvent (hydrogen and oxygen contained in water molecules have smaller ionization potential than that of argon) and matrix elements (their ionization potential being significantly smaller than that of argon) are introduced into a thermodynamic system, the results of the Saha equation calculations and thermodynamic simulation will be different even if the same electron density is used. This is connected with different approaches to the consideration of equilibrium in a thermodynamic system: equilibrium of only one reaction in the Saha method and equilibrium of a quantity of reactions in thermodynamic simulation (as what happens in real systems). Our calculations by means of thermodynamic simulation showed that when going from positive to negative ion detection mode, the measurement sensitivity at usual ICPMS temperatures (T = 6000 – 7000 K) may be improved (five to seven times) only for fluorine. This finding is in conflict with experimental data [1– 4,6,9]. Our calculated + value of Cl7500 /Cl 7500 at T = 7500 K is about 1000, which is rather close to the estimations made in Ref. [4]. Using available experimental ion sensitivity values (cps/ ppm) for negative Se and positive S +e atomic ions [4], we have estimated the experimental factor of ion sensitivity variation when going from positive to negative ion detection mode: Fe = S +e /Se. Based on our calculated data of atomic ion formation efficiency in ICP, we determined the theoretical factor of ion sensitivity variation for the same ion detection mode transition: Ft = D(M+)T/D(M)T = M +T /MT. Comparison between experimental Fe [4] and our theoretical Ft factors of ion sensitivity variation revealed simple linear dependences (Fig. 4) for all the considered temperatures. At temperatures above 5000 K, Ft for the considered elements must be four to five orders of magnitude smaller than Fe [4]. This discrepancy between experimental and theoretical data shows that the majority of negative ions originate not directly in ICP. This conclusion closely agrees with analogous opinions [4,9]. On the other hand, it was proposed [11] that ‘‘. . .electron attachment to form negative ions should
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take into account the possibility of Ar ion formation. Then the limiting total current of negative ions through the skimmer will be f 0.15 AA, which should drastically decrease the loss of negative ions owing to the space charge effect behind the skimmer. The advantage of the three-aperture interface is that the space charge effect decreases as a result of the limiting ionic current attenuation [30,31]. The sensitivity is approximately the same as in the instrument with the two-aperture interface. This confirms the possibility of a smaller level of losses due to the charge effect in the negative ion detection mode in ICP-MS. Apparently, both reasons may be simultaneously responsible for the discrepancy between the experimental and estimated detection sensitivity of negative halogen ions. 3.3. Matrix effects
Fig. 4. Comparison of experimental Fe [4] and theoretical Ft sensitivity variation logarithms when going from positive to negative ion detection mode simulated according to experimental conditions [4] at plasma temperatures T, K: 1 – 8500, 2 – 7500, 3 – 6500. The dashed line denotes the estimated Ft for fluorine at the corresponding temperatures.
not be significant in the centerline gas flow through the sampler in ICP. The short transit time of only 3 As to the skimmer also minimizes the extent of recombination and other chemical reactions in the jet.’’ Moreover, it is known that electrons are partly removed already by supersonic expansion of the plasma jet at the interface [11,25]. This decreases the probability of electron addition reactions to neutral atoms. However, it can be conceived that when ions run through ion optics tuned to the negative ion detection mode, fairly many electrons retain in the ion beam as is evidenced by the high intensity of the background continuum [4,5,9]. In this case, electrons can be attached to neutral atoms on the short way to the photon-stop lens (or to the point of ion beam deviation in spectrometers having other ion optics than on Sciex Elan 250). Perhaps this may be the reason why the concentration of detected negative atomic ions increases in comparison with their concentration in ICP. The second hypothesis may be put forward that the better experimental detection sensitivity of negative analyte ions (F, Cl, Br) as compared with positive ion detection mode is due to significantly smaller losses of negative ions when they run from the skimmer to the detector. According to calculations [11,29], the total flow of positive background ions (mainly Ar+) through the skimmer is about 1016 ions s 1, which provides the ionic current of f 1500 AA, i.e., several orders of magnitude higher than the limiting value of the space charge leading to the ion loss from the flow. The concentration of positive ions in ICP is approximately the same as the electron density, whereas the concentration of negative background ions at T = 7500 K is at least four orders of magnitude smaller (Fig. 1) even if we
To study the cation interferences theoretically, we have carried out calculations for experimental conditions [9]. The basic composition of the initial thermodynamic system (Ar – H2O – HNO3) was given in Section 3.1. The concentration of Cl analyte in the experiment [9] and in our calculation was 10 Ag ml 1. Cl was introduced in the initial composition of the thermodynamic system in form of different salts. Various thermodynamic systems Ar – H2O – HNO3 – BaCl2 (CoCl2, CuCl2, KCl, MgCl2 and SrCl2) were considered. The calculation showed that the change in the type of cation (Ba, Co, Cu, K, Na and Sr) introduced into the solution as a chloride should not alter the analytical signal of Cl when the Cl concentrations is about 10 Ag ml 1. This completely confirms data [9]. The theoretical effect of high halogen concentrations was studied using experimental data [5]. The aerosol-introducing gas flow rate was 1.0 l min 1. The solution introduction rate was assumed in the model calculation to be 1 ml min 1, the concentration of trace elements (F, Cl) in the solution was 10 Ag ml 1 (these data were not given in Ref. [5]). The concentrations of the matrix components in the analyzed solutions were varied according to Ref. [5]: HCl from 0 to 7 mol l 1, CsCl from 0 to 1500 Ag l 1, NaBr from 0 to 0.3 g l 1. The thermodynamic systems Ar –H2O – F– Br –I –HCl, Ar – H2O – F – Br – I – CsCl and Ar – H2O – F – I – HCl – NaBr were considered. According to the calculations performed, high concentrations of halogenides do not change significantly the electron density in the discharge plasma and therefore should not noticeably affect the intensity of negative ion spectra. For example, an increase in the HCl concentration in the solution to 3.5 mol l 1 should not reduce the intensity of O and F ions at all the considered temperatures (Fig. 5). This is in conflict with experimental data (Fig. 5) [5] showing decreased intensity of these ions when the concentration of HCl increases. In Ref. [5], this situation was explained hypothetically by recombination reactions occurring on the PTFE surfaces or somewhere between the plasma and the quadrupole as a result of an
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4. Conclusions
Fig. 5. Variation of normalized experimental intensity of ions (3-O, 4-F) and normalized estimated concentrations of plasma components (O, F, e, all curves coinciding) at temperatures T = 5000 K (1) and T z 6000 K (2) as a function of HCl concentration CHCl in the analyzed solution. The simulation was performed according to experimental conditions [5].
increased content of hydrogen atoms due to the growing concentration of HCl. In our opinion, the observed [5] reduction in O and F intensity can be explained also by the shift of equilibrium in the electron addition reaction to halogens (1) behind the skimmer (high Cl concentrations decrease the electron density and the probability of O and F formation) and/or by greater losses of these ions caused by a significant rise of the negative ionic current in the ion stream when the HCl concentration grows. In conformity with data [5], the addition of CsCl to the solution to the concentration of 3 Ag l 1 increased the O signal as much as two times as compared with water and stabilized it further up to CCsCl = 1.5 mg l 1. However, the graphical functions of this effect given in Ref. [5] contradict each other numerically. Our calculation shows that the electron density and the O signal should not change if such concentrations of CsCl are introduced into the solution. An addition of 0.3 g l 1 NaBr to the 3 M HCl solution results in an appreciable decrease in O and F ion intensity and increase of Cl ion intensity with its transition through a maximum (Fig. 6) [5]. The calculation performed for Br, F, Cl and O does not confirm the behavior of these ions observed experimentally. According to the calculation, only a slight linear increase in the ion intensity should take place with the growth of the electron density (Fig. 6). In our opinion, this behavior of the negative ion intensity can be explained also by a shift of equilibrium in the electron addition reaction to halogen atoms (1) behind the skimmer and/or by increased losses of these ions caused by an essential increase of the negative ion current in the ion beam when high concentrations of NaBr are introduced into the analyzed solution.
The experimental data on the basic background negative ions in Ar ICP have been confirmed by the calculations using thermodynamic simulation. The efficiency of negative atomic ion (F, Cl, Br and I) formation has been determined for real compositions of ICP. The possibility of Ar and Na formation has been estimated. Considerable quantitative discrepancy between theoretical and experimental data regarding the efficiency of negative atomic ion formation as compared with positive ions was obtained for F, Cl, Br and I, which confirms previous calculations performed earlier using the Saha equation. The calculations supported the experimental observation that the trace cation composition does not affect the Cl signal. High concentrations of halogens should not exert any substantial influence on the intensity of negative atomic ions of halogen analyte, which contradicts the experimental data. Possible reasons of this discrepancy may be an increased concentration of negative ions due to electron attachment to neutral atoms behind the skimmer or smaller losses of negative ions from the ion beam behind the skimmer, which are not taken into account in thermodynamic simulation. The supposition [11] that negative atomic ions of halogens can be detected using a three-aperture interface seems to be unlikely for the following reasons. Firstly, losses of electrons in the expanding plasma jet in this interface will be much greater than in a two-aperture interface. Hence, electron attachment to neutral atoms behind the interface
Fig. 6. Variation of normalized experimental intensity of ions (2-Br, 3-Cl, 5-F, 6-O) and estimated concentrations of some components of argon plasma (4-O, Cl, F, all curves coinciding; 1-Br) at temperature T = 5000 K as a function of NaBr concentration CNaBr in the analyzed solution. The simulation was performed according to experimental conditions [5].
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will be less. Secondly, a decrease of the ion current in this interface at the negative ion detection mode will have almost no influence on the space charge effect because of a very small limiting ion current. The second supposition [11] concerning the possibility of negative ion detection using ‘‘cold’’ plasma is apparently based on the familiar fact that negative polyatomic ions of organic and inorganic halogen compounds are formed in cold ion sources working under atmospheric pressure. The gas temperature of ‘‘cold’’ ICP is about 1450 K [32]. Large polyatomic ions can still exist at this temperature. However, the formation of such ions cannot be estimated using thermodynamic simulation until sufficient thermodynamic data on these ions are available. The present results obtained using the ASTRA software [18] are contradictory in some points to the data reported in our previous publication [33], which were estimated with the HSC program [34]. For example, it was predicted that negative atomic ions may exist within an optimal low-temperature range and that high halide concentrations have a strong impact on the intensity of negative ions of analytes and O at low ICP temperatures. The agreement between the estimated and experimental data allowed us to arrive at the erroneous conclusion that negative atomic ions may originate directly in the discharge plasma [33]. The study of the reasons of the discrepancy between some estimated data obtained at low plasma temperatures using the two thermodynamic simulation programs revealed no essential differences in the thermodynamic properties of numerous individual substances in the ASTRA-IVTANTHERMO [18] and HSC [34] databases. The calculation algorithm of the ASTRA software [18] based on the thermodynamic system entropy maximization principle permits obtaining its total equilibrium composition including the concentrations of electrons, positive and negative ions at the preassigned temperature, pressure and initial chemical composition. The calculation algorithm of the HSC program based on the Gibbs energy minimization makes it possible to obtain the total equilibrium composition of the system at the same preassigned initial parameters only when a certain initial electron concentration is introduced into the system. Our check calculations with the HSC program showed that the equilibrium concentrations of charged particles at low temperatures (3500 – 7000 K) depend strongly on the preassigned initial electron concentration. This is the main reason for some previous erroneous conclusions and the discrepancy between the data presented in this work and those reported in Ref. [33]. A fairly good agreement between the calculated and experimental concentrations of electrons and positive atomic ion was obtained when developing a thermodynamic model for thermochemical processes in ICP [13 – 17] with the ASTRA program [18], which allows us to use this software for further calculations.
References [1] D.J. Douglas, G. Rosenblatt, E.S.K. Quan, J.B. French, Mass spectrometric sampling of inductively coupled plasmas: progress and recent results, ICP Inf. Newsl. 9 (1983) 390. [2] D.J. Douglas, R.S. Houk, Inductively coupled plasma mass-spectrometry (ICP-MS), Prog. Anal. At. Spectrosc. 8 (1985) 1 – 18. [3] J.E. Fulford, E.S.K. Quan, Negative ions in inductively coupled plasma mass spectrometry (ICP-MS), ICP Inf. Newsl. 13 (1987) 221. [4] J.E. Fulford, E.S.K. Quan, Negative ions in inductively coupled plasma mass spectrometry, Appl. Spectrosc. 42 (1988) 425 – 428. [5] M. Chtaib, J.-P. Schmit, Negative-ion mode in inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 3 (1988) 315 – 318. [6] M.E. Chisum, Applications of negative ion analyses on the ELAN250 ICP-MS, Atom Spectrosc. 12 (1991) 155 – 159. [7] M.E. Chisum, Applications of negative ion analyses on the ELAN250 ICP-MS, ICP Inf. Newsl. 17 (1992) 642. [8] D.A. Wilson, G.H. Vickers, G.M. Hieftje, Analytical characteristics of an inductively coupled plasma - mass spectrometers, Spectrochim. Acta, Part B 42 (1987) 29 – 38. [9] G.H. Vickers, D.A. Wilson, G.M. Hieftje, Detection of negative ions by inductively coupled plasma mass spectrometry, Anal. Chem. 60 (1988) 1808 – 1812. [10] V.T. Surikov, A.A. Pupyshev, Mass spectrometry of negative ions of inductively coupled plasma, Analitika i Kontrol 7 (2003) 3 – 8 (in Russia). [11] H. Niu, R.S. Houk, Fundamental aspects of ion extraction in inductively coupled plasma mass spectrometry, Spectrochim. Acta Part B 51 (1996) 779 – 815. [12] G.K. Moiseev, Thermodynamic simulation in materials science and technology, Thermochim. Acta 245 (1994) 21 – 43. [13] A.A. Pupyshev, A.K. Lutsak, V.N. Muzgin, Thermodynamic simulation of thermochemical processes in inductively coupled plasma, J. Anal. Chem. Russ. (Engl. transl. of Zh. Anal. Khim.) 53 (1998) 627 – 637. [14] A.A. Pupyshev, A.K. Lutsak, Thermodynamic simulation of ionization effects in inductively coupled plasma, J. Anal. Chem. Russ. (Engl transl. of Zh. Anal. Khim.) 53 (1998) 987 – 999. [15] A.A. Pupyshev, V.N. Muzgin, A.K. Lutsak, Thermochemical processes and ion transport in inductively coupled plasma mass spectrometry: theoretical description and experimental confirmation, J. Anal. At. Spectrom. 14 (1999) 1485 – 1492. [16] A.A. Pupyshev, E.V. Semenova, Formation of doubly charged atomic ions in the inductively coupled plasma, Spectrochim. Acta Part B 56 (2001) 2397 – 2418. [17] A.A. Pupyshev, A.K. Lutsak, Extending the capabilities of the thermodynamic simulation of thermochemical processes in inductively coupled plasma discharge, J. Anal. Chem. Russ. (Engl. transl. of Zh. Anal. Khim.) 57 (2002) 672 – 680. [18] N.A. Vatolin, G.K. Moiseev, B.G. Trusov, Thermodynamic Modeling in High Temperature Inorganic Systems (Termodinamicheskoe modelirovanie v vysokotemperaturnyh neorganicheskih sistemah), Metallurgia Publishers, Moscow, 1994, 352 pp. (in Russian). [19] L.V. Gurvich, Thermodynamic properties of individual substances. V.1 – 4. Hemisphere Purb. Corp. Press, N.Y. et al. 1989 – 1994. [20] L.V. Gurvich, V.S. Iorish, V.D. Chekhovskoi, V.S. Yungman, IVTANTHERMO. A Thermodynamic Database and Software System for the Personal Computer, CRC Press. Inc, Boca Raton, 1993. [21] I.K. Karpov, Physical – Chemical Modeling with Computers in Geochemistry, Nauka, Novosibirsk, 1981, 247 pp. (Phisiko-khimicheskoe modelirovanie na EVM v geokhimii, in Russian). [22] W.B. White, S.M. Johnson, G.B. Dantzig, Chemical equilibrium in complex mixtures, J. Chem. Phys. 28 (1958) 751 – 755. [23] W.R. Smith, R.W. Missen, Chemical Reaction Equilibrium Analysis: Theory and Algorithm, Wiley, New York, 1982, 364 pp. [24] N.N. Sesi, G.M. Hieftje, Studies into the interelement matrix effect in
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inductively coupled plasma spectrometry, Spectrochim. Acta Part B 51 (1996) 1601 – 1628. A. Montaser (Ed.), Inductively Coupled Plasma Mass Spectrometry, Wiley-VCH, New York, 1998, 964 pp. L.V. Gurvich, G.V. Karachevtsev, V.N. Kondratyev, et al., Chemical Bonds Rupture Energies. Ionization Potentials and Electron Affinity (Energii razryva khimicheskikh svyazei. Potentsialy ionizatsii i srodstvo k electronu), Nauka Publishers, Moscow, 1974, 351 pp. (in Russian). B.M. Smirnov, Negative Ions, McGraw-Hill, New York, 1982, 210 pp. J. Emsley, The Elements, Clarendon Press, Oxford, 1991, 256 pp. G.R. Gillson, D.J. Douglas, J.E. Fulford, H.W. Halligan, S.D. Tanner, Nonspectroscopic interelement interferences in inductively coupled plasma mass spectrometry, Anal. Chem. 60 (1988) 1472 – 1474. S.D. Tanner, L.M. Cousins, D.J. Douglas, Reduction of space charge
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effects using a three-aperture gas dynamic vacuum interface for inductively coupled plasma - mass spectrometry, Appl. Spectrosc. 48 (1994) 1367. S.D. Tanner, D.J. Douglas, J.B. French, Gas and ion dynamic of the three-aperture vacuum interface for inductively coupled plasma-mass spectrometry, Appl. Spectrosc. 48 (1994) 1373. S.D. Tanner, Characterization of ionization and matrix suppression in inductively coupled ‘‘cold’’ plasma mass spectrometry, J. Anal. At. Spectrom. 10 (1995) 1485. A. Pupyshev, V. Surikov, Thermodynamic simulation of applicability of negative ions in ICP-MS, Progr. and Abstr. of the Scientific Contributions of 2003 Europ Winter Conf. on Plasma spectrochemistry, Garmisch-Partenkirchen, Germany, Januare 12 – 17, 2003, pp. 145. HSC Chemistry for Windows, Version 1.10, Outokumpu research, Finland, 1993.
Application of negative ions in inductively coupled plasma-mass spectrometry A.A. Pupyshev a,*, V.T. Surikov b a
b
Ural State Technical University, 19 Mira St., Ekaterinburg 620002, Russia Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 91 Pervomaiskaya St., Ekaterinburg 620219, Russia Received 25 June 2003; accepted 17 May 2004 Available online
Abstract The efficiency of the formation of negative background and F, Cl, Br and I ions in inductively coupled plasma (ICP) was investigated at temperatures ranging from 4000 to 9000 K using thermodynamic simulation. It was shown that the basic negative background ions in ICP, O, H, OH and probably Ar, are in accordance with experiment. The estimated total concentration of negative ions in ICP was found to be four to five orders of magnitude smaller than that of positive ions. The highest efficiency of negative ion formation should be observed for elements having high electron affinity, namely Cl, F, Br and I. However, the detection sensitivity in the negative ion detection mode may be increased slightly as compared with the positive ion detection mode only for fluorine in the temperature range 6000 – 7000 K. This is in contradiction with experimental results and may be explained by the formation of negative ions behind the skimmer and/or smaller losses of negative ions in the ion beam at low ionic current. The relationship between the efficiency of negative atomic ion formation and electron affinity was determined. This makes it possible to estimate numerically the efficiency of the formation of Ar, Na and other negative atomic ions in the ICP. The calculations performed confirmed the experimental data showing that the cation (Ba, Co, Cu, K, Na and Sr) introduced as a chloride does not affect the analytical signal of Cl. According to the calculations, high contents of halogens in samples are not expected to alter significantly the contents of F, Cl, Br and I measured in ICP as analytes. The experimentally observed significant suppression effect of high halogen concentrations may be caused by a shift of equilibrium in reactions of electron addition to halogen atoms behind the skimmer and/or increased negative ion current in the ion beam and accordingly greater losses of analytes. D 2004 Elsevier B.V. All rights reserved. Keywords: Inductively coupled plasma-mass spectrometry; Thermodynamic simulation; Negative atomic and molecular ions; Efficiency of ion formation; Matrix effects
1. Introduction The applicability of negative atomic and molecular ions for quantitative analysis by inductively coupled plasmamass spectrometry (ICP-MS) was investigated in the period between 1983 and 1992 [1 – 9]. The investigations concerned the composition of negative ion spectra [1,3 – 9], their intensity versus some plasma [3 –9] and detection [5– 7] parameters, element determination procedures [4,6 – 9] as well as effects of matrix cations and anions on analytical results [3,5,9]. In addition, simple theoretical studies of the applicability of the negative ion detection
* Corresponding author. Tel.: +7-343-3754658; fax: +7-343-3750196. E-mail address: [email protected] (A.A. Pupyshev). 0584-8547/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2004.05.011
mode for quantitative elemental analysis by ICP-MS were performed using the Saha equation [2 –4]. Comprehensive discussion of papers [1 –9] was presented in a review by Surikov and Pupyshev [10]. The major results of the investigations are as follows: – the observed high level of continuous baseline instrumental background (103 cps) is due to electrons, which reach the detector [4,5,9]; – basic background negative ions are H, O, OH, O2, NO and NO2 (the latter exists only in the presence of nitric acid) [4,5,8,9]; – negative ion spectra of the determined elements can be represented as M, MO, MO2, MO4 and HMO4 [3,4]; – the intensity of negative atomic ions increases when the plasma temperature decreases [1,2] (in accordance with
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theoretical estimations employing the Saha equation) or increases [3,4] (an experimental result obtained with enhanced applied radio-frequency power of plasma); as was found experimentally, Cl, F and Br have superior detection sensitivities as compared with the positive atomic ion detection mode [1 –4,6,9]; the detection limit (DL) for F (the most problematic element in ICP-MS) reached 110 ppb [4], a value which is much better than that obtained with modern ICP-MS spectrometers for F [0.38 ppm using dynamic reaction cell (DRC) and 10 ppm without DRC]; a linear dependence (as checked on F, Cl, Br and I) was observed between experimental intensities of negative atomic ions and electron affinity (EAM) of the corresponding elements [9] at their equal mass concentration (10 ppm) in sample; according to the calculation for Cl with the use of the Saha equation, the efficiency of negative atomic ion formation must be several orders of magnitude smaller than that of positive ions [1,2,4] of the corresponding elements; this conclusion does not agree with the experiment and in the authors’ opinion [2,4,6] may be indicative of different formation processes of negative and positive ions in inductively coupled plasma; it was supposed that negative atomic ions are likely to originate from electron capture or reactions downstream the free jet expansion rather than from plasma directly [5]; the intensity of Cl signal at concentrations of 10 Ag ml 1 is independent of the type of associated cations [9]; the intensity of O signal is influenced by CsCl at the ppb concentrations [5]; the intensities of O, F and Cl signals depend significantly on the presence of matrix halogen compounds HCl, CsCl and NaBr at ppm concentrations [3,5]; there is no agreement with respect to the charge polarity of detected Ar and Na ions in the negative ion detection mode [5,6,9]; it was supposed that positive ions Ar+ and Na+ (if their concentrations in ICP are high) may be detected in the negative ion detection mode because they penetrate through ion optics and quadrupole instead of being stopped by the optics (ion lenses) and reach the detector [5,9]; it was proposed to determine halogens in the form of negative ions using ‘‘cold’’ plasma or three-aperture interface [11].
Experimental and theoretical results on the applicability of negative atomic ions for quantitative analysis by ICP-MS are scarce and contradictory. These data fail to provide clear and comprehensive understanding and explanation of all processes occurring when negative ions are formed and detected by ICP-MS. The significant advantage of the negative ion detection mode in determining low fluorine concentrations reported in the literature is not used by
manufacturers of modern ICP-MS spectrometers in spite of real demand for such analysis. Therefore, turning back to these problems seems to be justified and useful both from the theoretical and practical viewpoints. Equilibrium thermodynamic simulation of thermochemical processes in low-temperature plasma based on the model developed for ICP-MS opens up new possibilities in investigating these problems. We employed this model for data [1– 9] analysis to check up the applicability of the model and better understand the observed processes and phenomena in case negative ions are used in ICP-MS.
2. Thermodynamic simulation The electron density and temperature in the central channel of a plasma, through which ions are extracted into the interface, change under real ICP-MS conditions with the operation parameters such as plasma power, flow rate of aerosol transporting argon and loading of plasma with aerosol. Moreover, the electron density and positive and negative ion concentrations in ICP vary as a function of the matrix in samples and the ionization energy of matrix elements. As a result, the efficiency of negative atomic ion M formation changes accordingly. These variations cannot be calculated by the Saha equation if experimental temperatures and electron density values are not known. However, such calculations can be performed using equilibrium thermodynamic simulation [12]. In this method, behavior of the complex multicomponent thermodynamic system is analyzed based on the calculation of its total equilibrium composition, including molecules, atoms, ions and electrons. For example, it may be a calculation of the composition of plasma penetrating into the interface of the mass spectrometer. The calculations are carried out by maximizing the entropy or minimizing the Gibbs energy of the system. They require the initial concentration of all components, pressure and temperature of the system, as well as thermodynamic properties of all individual substances that may be present in the equilibrium. Depending on the complexity of the elemental composition of a thermodynamic system, tens or hundreds of individual substances are usually taken into account in such calculations. The main difficulty in these calculations is to create an equilibrium model for a nonequilibrium ion source, namely, ICP. A quasi-equilibrium model for thermochemical processes in ICP, which permit obtaining the total composition of plasma in ICP-MS, was developed earlier [13 – 17]. The basic assumptions of the model are as follows: – only the central channel located on the discharge axis (analytical zone) and serving for the introduction of analyzed aerosol and ion extraction is considered; – the degree of mixing of the aerosol-introducing (transporting) argon flow with plasma-forming (outer) or intermediate argon flows is equal to zero;
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– the analytical zone is in local thermodynamical equilibrium (LTE); – the analyzed aerosol introduced into ICP evaporates completely; – components of a thermodynamic system are uniformly distributed over the analytical zone; – the ICP discharge occurs at atmospheric pressure (0.1013 MPa); – the initial composition of a thermodynamic system (working medium) is equal to the mass flow (g min 1) of all introduced plasma components: transporting argon and aerosol of the analyzed solution containing a solvent, a matrix and trace elements; – the atmosphere air does not mix with the discharge plasma; – ion extraction from the discharge plasma into the interface provides a representative probe of the plasma composition for the analytical zone; – the plasma temperature changes insignificantly upon introduction of solution aerosol into the discharge; – changes in the composition of the analyzed solution do not affect the ICP temperature; – secondary atomic ionization is possible only for the elements with the lowest sum of the first and the second ionization potentials [16]. The applicability of the model was confirmed by comparing the estimated results with a large number of experiments [15,16]. The equilibrium composition of the considered thermodynamic systems was calculated using the software package ASTRA [18] and the thermodynamic database IVTANTHERMO (Refs. [19,20], http://www.openweb.ru/ thermo/index.htm). ASTRA is based on the principle of the entropy maximization of the examined complex multicomponent thermodynamic system. Additional restrictions are imposed on the system when the maximum entropy is found: – the total internal energy of the system at equilibrium should be constant; – the mass of all elements should be conserved; – the state of the ideal gas mixture should be equated; – the law of charge conservation should be obeyed. The total equilibrium chemical composition of the system including electron density and some thermodynamic parameters are established in the calculations. For each individual substance, the thermodynamic database IVTANTHERMO gives the standard enthalpy of 0 compound formation from elements DH298 , enthalpy incre0 ment from 0 to 298.15 K [H298 H00] and numerical coefficients of the seven-valued polynomial approximating the temperature dependence of the reduced Gibbs energy. Owing to the dependences between the thermodynamic functions, these numerical coefficients make it possible to easily calculate the temperature dependences for entropy
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S 0j (T), enthalpy variation [Hj(T ) Hj(0)] and heat capacity Cpj(T ) of an individual j– th substance. Individual substances (electrons, atoms, molecules, atomic and molecular ions), which may exist in equilibrium, are listed in Table 1. The negative atomic ion of argon Ar was not taken into account in the calculation of the total equilibrium composition of thermodynamic systems because the required reference thermodynamic data for this ion are not available in the literature (perhaps because of a low probability of Ar formation). In each calculation of equilibrium, we consider simultaneously the possibility of the formation of all individual substances containing all elements present in the initial components of the system. For example, in the case of the simple thermodynamic system Ar – H2O – HCl containing elements Ar, Cl, H and O (the water aerosol introduced in the plasma contained only HCl), the probability of the formation of the following individual substances is considered simultaneously (Table 1): Ar, Ar+, Cl, Cl+, Cl, Cl2, ClO, ClO2, Cl2O, e, HCl, H, H+, H, H2, H2+, H2, H3+, OH, OH+, OH, HO2, HO2, H2O, H2O2, H3O+, HOCl, O, O+, O, O2, O2+, O2 and O3. Similarly, based on the initial element composition of any thermodynamic system considered further, it is easy to determine from Table 1 what
Table 1 Individual substances taken into account in the calculations (IVTANTHERMO database) Ar, Ar+ Br, Br+, Br, Br2, BrO Ba, Ba+, Ba2 +, Ba2, BaO, BaO+, BaH, BaOH, BaOH+, Ba(OH)2, BaCl, BaCl+, BaCl2, BaOHCl Cl, Cl+, Cl, Cl2, ClO, ClO2, Cl2O, HCl, ClF, ClF3, ClF5 Co, Co+, CoO, CoH, Co(OH)2, CoCl, CoCl2, Co2Cl4 Cs, Cs+, Cs2, CsO, Cs2O, Cs2O+, Cs2O2, CsH, CsOH, Cs(OH)2, CsF, Cs2F2, CsCl, Cs2Cl2, CsBr, Cs2Br2, CsJ, Cs2J2, CsN, CsON, CsNO2, CsNO3 Cu, Cu+, Cu2, CuO, CuH, CuOH, CuCl, CuCl2, Cu2Cl2, Cu3Cl3, Cu4Cl4, Cu5Cl5 e F, F+, F, F2, FO, F2O, HF, H2F2, H3F3, H4F4, H5F5, H6F6, H7F7, HOF, F2H, FCN, F2N2, F3N, F4N2, FNO2, FNO3, F3NO + + + H, H+, H, H2, H+2, H 2 , H3, OH, OH , OH , HO2, HO2 , H2O, H2O2, H3O , HBr, HOBr, HOCl, HI, HN3, HNNH, HNO, HNO2, HNO3, HOCl, HOF I, I+, I, I2, IBr, ICl, IF, IF5, IF7, IO K, K+, K2, KO, K2O, K2O+, K2O2, KH, KOH, KCl, KCl2, K2Cl2, KN, KON, KNO2, KNO3 Mg, Mg+, Mg2 +, Mg2, MgO, MgN, MgOH, Mg(OH)2, MgCl, MgCl2, MgOHCl + + N, N+, N, N2, N+2, N 2 , N3, NO, NO , NO2, NO2, NO2 , NO3, NO3 , N2O, N2O+, N2O3, N2O4, N2O5, NH, NH+, NH2, NH3, NH+4, N2H2, N2H4, NF, NF2, NF3, N2F2, N2F4, FNO, FNO2, FNO3, F3NO, NHF, NH2F, NHF2, ClNO, ClNO2, NBr, NOBr, NOI Na, Na+, Na2, NaO, Na2O, Na2O+, Na2O2, NaH, NaOH, Na(OH)2, NaF, Na2F2, Na3F3, NaCl, Na2Cl2, Na3Cl3, NaBr, Na2Br2, NaJ, Na2J2, NaN, NaON, NaNO2, NaNO3 O, O+, O, O2, O+2, O 2 , O3 Sr, Sr+, Sr2 +, Sr2, SrO, SrO+, SrH, SrOH, SrOH+, Sr(OH)2, SrCl, SrCl+, SrCl2, SrOHCl
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individual substances were taken into account simultaneously in the calculation. Analogous calculations can be performed employing other programs such as SELECTOR [21], SOLGASMIX [22], NASA [23], etc. These programs have their own databases of thermodynamic data for individual substances. The advantage of IVTANTHERMO over other databases consists in complete mutual agreement of all given values of thermodynamic characteristics for all individual substances in the database [20]. The considered temperature range usually was 4000– 9000 K with the general calculation step 500 K. Because the local thermodynamic equilibrium conditions were assumed for the central ICP channel, temperatures in this zone of the plasma equal the equilibrium temperature. Our estimated values of electron density n(e) in argon plasma [15] are in agreement with experimental data for the central ICP channel. The estimated plasma temperatures are also consistent with experimental electron temperatures Te and possibly with ionization temperatures Ti. The initial composition of a thermodynamic system was prescribed according to experimental data: the composition was equal numerically to the mass flow rate (g min 1) of introduction of transporting argon, water, trace and matrix elements (salts, acids) into the central plasma channel. Their concentration in the analyzed solution and nebulization efficiency were also taken into account. In the experiments considered below [4,5,9], the data on nebulization efficiency are lacking. Therefore, the efficiency of solution introduction into ICP was assumed to be 0.01. (i.e., 1%). Numerical values of other experimental parameters used in the calculations, as well as qualitative and quantitative compositions of the examined thermodynamic systems for particular calculations, are given below.
3. Results and discussion 3.1. Negative background ions The calculations have been performed for experimental conditions [9]. Argon flow rate for transporting aerosol was 0.65 l min 1, sample introduction rate was 0.5 ml min 1, concentration of HNO3 was 1%. The thermodynamic system Ar – H2O – HNO3 was studied. According to the calculation results, the basic negative ions in argon plasma in the temperature range 4000 – 9000 K are O, H and OH (Fig. 1). When the temperature of plasma rises from 4000 to 9000 K, the theoretical electron density increases four orders of magnitude or more. A significant increase in the electron density causes an increase in the calculated equilibrium concentration of atomic ions O and H. The concentrations of OH and other negative polyatomic ions should decrease simultaneously. In this temperature range, the electron concentra-
Fig. 1. Estimated variations of electron density and basic negative background ion n(X) concentration logarithms in argon plasma as a function of the argon plasma temperature T simulated according to experimental conditions [9].
tion in discharge plasma must be three to five orders of magnitude greater than the total concentration of negative background atomic ions. As is seen from Fig. 1, the LTE model gives very low values of electron density for the central ICP channel at temperatures below 6500 K. For example, at 4000 K n(e) was predicted to be c 1011 cm 3. At first glance, it seems to be unlikely for Ar ICP. However, experimental measurements of the electron density and electron temperature [24] along the axis of the central ICP channel above the torch showed that the electron density can be about (2– 3)1012 cm 3 in plasma zones with Te = 1800 K. Comparison of data [24] with our calculations revealed that the estimated electron density values approach the experimental n(e) values when the equilibrium temperature T increases, and at T = Te >6500 K, they are practically the same. It is necessary to note that the experimental values of Te and gas temperature Tg in the central ICP channel are close to each other [24]. This is additional evidence that the LTE condition can be met in the central ICP [25] channel and that thermodynamic simulation is appropriate for the analytical zone. Considering the significant discrepancy between the experimental n(e) values and those calculated from the LTE condition for ‘‘cold’’ plasma, we may only estimate the character and tendencies of the thermochemical processes occurring in this temperature range. In the case of a water solution containing 1 % HNO3, it was predicted that the NO2 ions are present in plasma, but their concentration is five to six orders of magnitude smaller than that of OH over the whole temperature interval examined. When the plasma temperature increases, the concentration of NO2 must decrease sharply. NO ions
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3.2. Negative atomic ions of analytes Temperature dependences of efficiency of atomic ion formation were found based on the calculations of model argon plasma systems, in which analyzed trace elements were introduced. The formation efficiency of negative and positive atomic ions was estimated at temperature T as DðM ÞT ¼ MT =RM and DðMþ ÞT ¼ MTþ =AM ;
Fig. 2. Estimated variations of negative D(X)T and positive D(X+)T ion formation efficiency logarithms in argon plasma as a function of the plasma temperature T simulated according to experimental conditions [9].
having a very low electron affinity (EANO = 0.024 eV, EANO2 = 3.10 eV [26]) were ignored in the calculation. Negative ion spectra obtained in Ref. [9] included mainly O, OH, O2 and probably H3O ions. These spectra seem to be indicative of a high background in the region of H. Strong yet unresolved signals of O and OH and a weak signal of O2 were reported in Ref. [6]. In Ref. [5], the intensity of the O signal was three orders of magnitude higher than that of OH, strong signals were observed in the region of H and H2, whereas a signal of O2 was weak. In Ref. [4], strong signals of O, OH and O2 were observed, but the relative intensity of O and O2 was not known. The spectra reported in Ref. [1] contained strong signals of O and OH and a weak signal of O2. A very strong signal of NO2 and a weak signal of NO were observed in Ref. [4] for a 1% HNO3 water solution. In Ref. [6], the signal of NO2 from a 2% HNO3 water solution was very weak. Comparison of the estimated and experimental negative ion spectra revealed their similarity with respect to the basic background ions O, H and OH. Very high concentrations of OH and NO2 ions observed in some of the abovementioned experimental works may be due to the incomplete vaporization of aerosols or intensive gas cooling near vaporized aerosol drops. Unfortunately, in all these works [1 –9], the effects of plasma power and the transporting argon flow rate determining the temperature of the inner ICP channel are not reported. Therefore, at present we cannot compare the results of experiments and simulation in all details.
where MT and MT+ are the amounts of element M present in a thermodynamic system at temperature T as negative or positive ions respectively; AM is the total amount of element M introduced into the initial thermodynamic system. The calculations were carried out under experimental conditions [9]. The basic composition of the initial thermodynamic system (Ar –H2O – HNO3) has been given already in Section 3.1. The concentration of analytes (Br, Cl, F, I) in the experiment [9] and in our calculation was 10 Ag ml 1. Upon introduction of analytes, the composition of the thermodynamic system was Ar – H2O – HNO3 – Br –Cl – F– I. According to the calculations, the highest efficiency of negative ion formation should be observed for Cl, F, Br and I (Fig. 2), i.e., for elements having the highest electron affinity [26,27] (Fig. 3). The values of D(M)T increased almost equally for all the considered elements with the plasma temperature. Only the high-temperature (T >7000 K) region showed promise for measurements. At usual operating temperatures of contemporary ICP-MS spectrometers (6000 – 8000 K), values of D(M)T > D(M+)T can be obtained only for F. It is seen in Fig. 1 that the concentration of negative atomic ions tends to increase with temperature as a result of increased electron density. Probably, this is due
Fig. 3. Estimated variations of negative atomic ion D(M)T formation efficiency logarithms in argon plasma as a function of electron affinity energy EAM [28] simulated according to experimental conditions [9] at plasma temperature T, K: 1 – 8500, 2 – 7500, 3 – 6500. The dashed line denotes EAAr .
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to the circumstance that the temperature effect on the equilibrium constant of reaction M ¼ M 0 þ e
ð1Þ
is smaller than the effect of electron density increase. This trend is not observed for recombination reactions of positive ions M þ þ e ¼ M 0 (Fig. 2) because of a stronger temperature dependence of the equilibrium ionization constant because the considered elements have high ionization potentials. According to the calculations, if the mass concentration of mg l 1 elements in the solution is the same, the relationship of negative ion concentrations (cm 3) should be n(F) > n(Cl)> n(Br)> n(I). This sequence of halogen ion concentrations differs from the sequence of their negative ion formation efficiencies D(Cl)T > D(F)T > D(Br)T > D(I)T because of the significant difference in the atomic masses of F and Cl. The concentration (in discharge plasma, cm 3) of negative atomic ions of other elements of the Periodical Table having electron affinity of not more than 250 kJ mol 1 at the same molar concentrations must be much smaller than for halogens. Comparison of the calculated formation efficiency logarithms of negative atomic ions and their electron affinities revealed almost linear dependences (Fig. 3). Analogous linear dependences were observed between the estimated concentrations (cm 3) of negative ions and their electron affinities if equal molar amounts of elements are introduced into the initial thermodynamic system. This also indicates that the thermodynamic data used in the calculations for all the considered elements are consistent enough. These relationships contradict analogous linear dependences obtained experimentally for Cl, F, Br and I when signals of negative ions of these elements (cps/10 Ag ml 1) corrected for their isotope abundance and instrument mass sensitivity were used to compare their electron affinity [9]. In this case, different initial experimental molar concentrations of halogens in the analyzed solution and discharge plasma should additionally affect the signals for negative ions. Then, we can expect a considerable deviation of this relationship from linearity. Indeed, the linear dependence described in Ref. [9] is no longer observed when the experimental data [9] are converted to equal molar concentrations of elements in sample. Using the linear relationship between logarithms of the estimated formation efficiency of negative atomic ions and electron affinity of elements (Fig. 3), we can resolve the contradiction concerning the polarity of detected Ar and Na ions [5,6,9]. No required reference thermodynamic data on these negative ions are available in the literature. Therefore, in the thermodynamical simulation of a multicomponent system in ICP, we could not take into account the formation of Ar and Na ions. Obviously, the absence of the
reference data is connected with a low probability of the formation of these ions in the usually examined thermodynamic systems. However, we may try to estimate the formation efficiency of Ar and Na and their concentrations in ICP using the linear dependences given in Fig. 3. The estimated electron affinity for argon atom is EAAr = 35 kJ mol 1 [28]. For example, extrapolation of the dependence between negative atomic ion formation efficiency D(M)T and electron affinity EAM shows that for the abovementioned value of EAAr at T = 7500 K, the value of D(Ar)T must be about f 1 10 7 (Fig. 3). Because the concentration of argon atoms in plasma at 7500 K is very high [n(Ar) c 1 1018 cm 3], the concentration of negative argon ions n(Ar) with allowance for the above D(Ar)T value must be nðAr Þ ¼ nðArÞ DðAr ÞT c1 1018 1 107 ¼ 1 1011 cm3 : This concentration of negative atomic ions of argon is comparable and even slightly higher than that of the basic background ions O and H (Fig. 1), which were considered in the calculations of the multicomponent equilibrium. Close values of O and Ar concentrations must provide rather close values of counting intensity of these ions. This was observed in particular for the Ar – H2O system [9]. The above result is indicative of a negative polarity of detected ions having m/z = 40, in contrast to the opinion expressed in Refs. [5,9]. Analogous estimations for sodium (EANa = 52.9 kJ mol 1 [28]) show that the polarity of detected ions with m/z = 23 may be negative at high Na concentration, also in conflict with Ref. [5]. In Ref. [4], equilibrium reaction (1) was considered and the relationship of concentrations of neutral atoms and negative ions M T0 /M T was calculated for chlorine using the Saha equation MT0 =M T ¼ ½2=nðeÞ ½ð2pme kT Þ=h2 3=2 ðQ0 =Q ÞexpðEAM =kT Þ: Here, me is the rest mass of electrons, k, Boltzmann constant, h, Planck constant, Q0 and Q are partition functions for a neutral atom and a negative ion, respectively. For T = 7500 K and n(e) = 5 1014 cm 3, the Cl 7500 / Cl07500 ratio was found [4] to be 7 10 6 ( Q, Q+, Q0 and EACl values used in the calculation were not cited). The authors [9] believe that this points to a very low formation efficiency of negative ions even for Cl having the highest electron affinity (Fig. 3, EACl = 349 kJ mol 1 0 [27]). A similar value of Cl7500 /Cl7500 was reported in Ref. [2], also without specifying the calculation details. According to the calculations by the Saha equation, very low concentrations of negative ions were expected already in the pioneering paper dealing with negative ion detection in ICP-MS [1]. Based on the comparison of electron affinity
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energy of F taken as an example and ionization energy of other elements, the authors of review [11] also concluded about low abundance of negative ions in ICP and their weak analytical possibilities. The calculation with the Saha equation at T = 7500 K and n(e) = 5.1 1014 cm 3 [4] also yielded Cl+7500 /Cl07500 = 1.2 10 2. Consequently, in the authors’ [4] opinion, the expected theoretical degradation of chlorine detection sensitivity when going from positive to negative detection mode must be four orders of magnitude or more. However, a much better detection sensitivity (cps/ppm) of Cl, F and Br was observed experimentally [4]. Based on this fact and measured negative ion energies, the authors [4] arrived at the conclusion that negative ions originate not directly in plasma but as a result of electron capture or downstream reactions of free jet expansion. In this connection, we have performed thermodynamic simulation of experimental conditions [4]: the argon flow rate for transporting aerosol, 1 l min 1, the concentration of HNO3 in the analyzed solution, 1%. In addition, in our model calculations we used the following parameters not given in Ref. [4]: solution introduction rate, 1 ml min 1, concentration of analytes (Br, Cl, F and I), 10 Ag ml 1. The considered system was Ar – H2O – HNO3 – Br – Cl –F – I. The calculations with the Saha equation and those employing thermodynamic simulation have the following differences. In the case of the Saha equation, only two physical parameters (equilibrium plasma temperature and electron density) are used. These values must be known exactly or determined experimentally because the electron concentration at a given temperature depends on the type and flow rate of the working gas, the matrix composition of the sample and efficiency of aerosol input into the plasma. In the calculations by the Saha equation, only individual ionization reaction equilibrium (for atoms and molecules) is considered, whereas the possibility of other simultaneous ionization reactions, which shift the equilibrium of the examined reaction, is not taken into account. With this restriction, the calculations permit finding the relationship of atoms and atomic ion, molecules and molecular ions only for the considered individual reactions. Any changes in the initial plasma composition (type and flow rate of the working gas, flow rate and efficiency of sample aerosol input, matrix composition of the sample) call for new estimation or measurement of the electron concentration. Thermodynamic simulation requires assigning of pressure (known), plasma temperature, total chemical composition of the simulated thermodynamic system including working gas, solvent, matrix and traces elements (which are usually known from experiments or may be assumed). In other words, these initial data adequately reproduce the real analytical operation conditions. Such calculations take into account the possibility of simultaneous formation of atoms, molecules, positive and negative atomic and molecular ions for all the elements that are present and actually interact in a multicomponent thermodynamic system. The results of the calculations make it
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possible to find the total equilibrium chemical composition of the system including the concentration of molecules, atoms, atomic and molecular ions as well as the electron number density. Thus, thermodynamic simulation of a correctly created equilibrium model provides much more quantitative information on the composition of real plasma and thermochemical processes occurring in plasma than the calculations employing the Saha equation. Moreover, for thermodynamic simulation, electron densities need not be known or found experimentally. The total composition of the simulated system at the mentioned above preassigned experimental parameters can be easily determined for other temperatures. Based on the same LTE hypothesis, the calculations performed with the Saha equation and those using thermodynamic simulation coincide completely only for binary systems, for example, argon and analyte. In this case, the electron density in ICP is determined only by the working gas (argon). When the sample solvent (hydrogen and oxygen contained in water molecules have smaller ionization potential than that of argon) and matrix elements (their ionization potential being significantly smaller than that of argon) are introduced into a thermodynamic system, the results of the Saha equation calculations and thermodynamic simulation will be different even if the same electron density is used. This is connected with different approaches to the consideration of equilibrium in a thermodynamic system: equilibrium of only one reaction in the Saha method and equilibrium of a quantity of reactions in thermodynamic simulation (as what happens in real systems). Our calculations by means of thermodynamic simulation showed that when going from positive to negative ion detection mode, the measurement sensitivity at usual ICPMS temperatures (T = 6000 – 7000 K) may be improved (five to seven times) only for fluorine. This finding is in conflict with experimental data [1– 4,6,9]. Our calculated + value of Cl7500 /Cl 7500 at T = 7500 K is about 1000, which is rather close to the estimations made in Ref. [4]. Using available experimental ion sensitivity values (cps/ ppm) for negative Se and positive S +e atomic ions [4], we have estimated the experimental factor of ion sensitivity variation when going from positive to negative ion detection mode: Fe = S +e /Se. Based on our calculated data of atomic ion formation efficiency in ICP, we determined the theoretical factor of ion sensitivity variation for the same ion detection mode transition: Ft = D(M+)T/D(M)T = M +T /MT. Comparison between experimental Fe [4] and our theoretical Ft factors of ion sensitivity variation revealed simple linear dependences (Fig. 4) for all the considered temperatures. At temperatures above 5000 K, Ft for the considered elements must be four to five orders of magnitude smaller than Fe [4]. This discrepancy between experimental and theoretical data shows that the majority of negative ions originate not directly in ICP. This conclusion closely agrees with analogous opinions [4,9]. On the other hand, it was proposed [11] that ‘‘. . .electron attachment to form negative ions should
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take into account the possibility of Ar ion formation. Then the limiting total current of negative ions through the skimmer will be f 0.15 AA, which should drastically decrease the loss of negative ions owing to the space charge effect behind the skimmer. The advantage of the three-aperture interface is that the space charge effect decreases as a result of the limiting ionic current attenuation [30,31]. The sensitivity is approximately the same as in the instrument with the two-aperture interface. This confirms the possibility of a smaller level of losses due to the charge effect in the negative ion detection mode in ICP-MS. Apparently, both reasons may be simultaneously responsible for the discrepancy between the experimental and estimated detection sensitivity of negative halogen ions. 3.3. Matrix effects
Fig. 4. Comparison of experimental Fe [4] and theoretical Ft sensitivity variation logarithms when going from positive to negative ion detection mode simulated according to experimental conditions [4] at plasma temperatures T, K: 1 – 8500, 2 – 7500, 3 – 6500. The dashed line denotes the estimated Ft for fluorine at the corresponding temperatures.
not be significant in the centerline gas flow through the sampler in ICP. The short transit time of only 3 As to the skimmer also minimizes the extent of recombination and other chemical reactions in the jet.’’ Moreover, it is known that electrons are partly removed already by supersonic expansion of the plasma jet at the interface [11,25]. This decreases the probability of electron addition reactions to neutral atoms. However, it can be conceived that when ions run through ion optics tuned to the negative ion detection mode, fairly many electrons retain in the ion beam as is evidenced by the high intensity of the background continuum [4,5,9]. In this case, electrons can be attached to neutral atoms on the short way to the photon-stop lens (or to the point of ion beam deviation in spectrometers having other ion optics than on Sciex Elan 250). Perhaps this may be the reason why the concentration of detected negative atomic ions increases in comparison with their concentration in ICP. The second hypothesis may be put forward that the better experimental detection sensitivity of negative analyte ions (F, Cl, Br) as compared with positive ion detection mode is due to significantly smaller losses of negative ions when they run from the skimmer to the detector. According to calculations [11,29], the total flow of positive background ions (mainly Ar+) through the skimmer is about 1016 ions s 1, which provides the ionic current of f 1500 AA, i.e., several orders of magnitude higher than the limiting value of the space charge leading to the ion loss from the flow. The concentration of positive ions in ICP is approximately the same as the electron density, whereas the concentration of negative background ions at T = 7500 K is at least four orders of magnitude smaller (Fig. 1) even if we
To study the cation interferences theoretically, we have carried out calculations for experimental conditions [9]. The basic composition of the initial thermodynamic system (Ar – H2O – HNO3) was given in Section 3.1. The concentration of Cl analyte in the experiment [9] and in our calculation was 10 Ag ml 1. Cl was introduced in the initial composition of the thermodynamic system in form of different salts. Various thermodynamic systems Ar – H2O – HNO3 – BaCl2 (CoCl2, CuCl2, KCl, MgCl2 and SrCl2) were considered. The calculation showed that the change in the type of cation (Ba, Co, Cu, K, Na and Sr) introduced into the solution as a chloride should not alter the analytical signal of Cl when the Cl concentrations is about 10 Ag ml 1. This completely confirms data [9]. The theoretical effect of high halogen concentrations was studied using experimental data [5]. The aerosol-introducing gas flow rate was 1.0 l min 1. The solution introduction rate was assumed in the model calculation to be 1 ml min 1, the concentration of trace elements (F, Cl) in the solution was 10 Ag ml 1 (these data were not given in Ref. [5]). The concentrations of the matrix components in the analyzed solutions were varied according to Ref. [5]: HCl from 0 to 7 mol l 1, CsCl from 0 to 1500 Ag l 1, NaBr from 0 to 0.3 g l 1. The thermodynamic systems Ar –H2O – F– Br –I –HCl, Ar – H2O – F – Br – I – CsCl and Ar – H2O – F – I – HCl – NaBr were considered. According to the calculations performed, high concentrations of halogenides do not change significantly the electron density in the discharge plasma and therefore should not noticeably affect the intensity of negative ion spectra. For example, an increase in the HCl concentration in the solution to 3.5 mol l 1 should not reduce the intensity of O and F ions at all the considered temperatures (Fig. 5). This is in conflict with experimental data (Fig. 5) [5] showing decreased intensity of these ions when the concentration of HCl increases. In Ref. [5], this situation was explained hypothetically by recombination reactions occurring on the PTFE surfaces or somewhere between the plasma and the quadrupole as a result of an
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4. Conclusions
Fig. 5. Variation of normalized experimental intensity of ions (3-O, 4-F) and normalized estimated concentrations of plasma components (O, F, e, all curves coinciding) at temperatures T = 5000 K (1) and T z 6000 K (2) as a function of HCl concentration CHCl in the analyzed solution. The simulation was performed according to experimental conditions [5].
increased content of hydrogen atoms due to the growing concentration of HCl. In our opinion, the observed [5] reduction in O and F intensity can be explained also by the shift of equilibrium in the electron addition reaction to halogens (1) behind the skimmer (high Cl concentrations decrease the electron density and the probability of O and F formation) and/or by greater losses of these ions caused by a significant rise of the negative ionic current in the ion stream when the HCl concentration grows. In conformity with data [5], the addition of CsCl to the solution to the concentration of 3 Ag l 1 increased the O signal as much as two times as compared with water and stabilized it further up to CCsCl = 1.5 mg l 1. However, the graphical functions of this effect given in Ref. [5] contradict each other numerically. Our calculation shows that the electron density and the O signal should not change if such concentrations of CsCl are introduced into the solution. An addition of 0.3 g l 1 NaBr to the 3 M HCl solution results in an appreciable decrease in O and F ion intensity and increase of Cl ion intensity with its transition through a maximum (Fig. 6) [5]. The calculation performed for Br, F, Cl and O does not confirm the behavior of these ions observed experimentally. According to the calculation, only a slight linear increase in the ion intensity should take place with the growth of the electron density (Fig. 6). In our opinion, this behavior of the negative ion intensity can be explained also by a shift of equilibrium in the electron addition reaction to halogen atoms (1) behind the skimmer and/or by increased losses of these ions caused by an essential increase of the negative ion current in the ion beam when high concentrations of NaBr are introduced into the analyzed solution.
The experimental data on the basic background negative ions in Ar ICP have been confirmed by the calculations using thermodynamic simulation. The efficiency of negative atomic ion (F, Cl, Br and I) formation has been determined for real compositions of ICP. The possibility of Ar and Na formation has been estimated. Considerable quantitative discrepancy between theoretical and experimental data regarding the efficiency of negative atomic ion formation as compared with positive ions was obtained for F, Cl, Br and I, which confirms previous calculations performed earlier using the Saha equation. The calculations supported the experimental observation that the trace cation composition does not affect the Cl signal. High concentrations of halogens should not exert any substantial influence on the intensity of negative atomic ions of halogen analyte, which contradicts the experimental data. Possible reasons of this discrepancy may be an increased concentration of negative ions due to electron attachment to neutral atoms behind the skimmer or smaller losses of negative ions from the ion beam behind the skimmer, which are not taken into account in thermodynamic simulation. The supposition [11] that negative atomic ions of halogens can be detected using a three-aperture interface seems to be unlikely for the following reasons. Firstly, losses of electrons in the expanding plasma jet in this interface will be much greater than in a two-aperture interface. Hence, electron attachment to neutral atoms behind the interface
Fig. 6. Variation of normalized experimental intensity of ions (2-Br, 3-Cl, 5-F, 6-O) and estimated concentrations of some components of argon plasma (4-O, Cl, F, all curves coinciding; 1-Br) at temperature T = 5000 K as a function of NaBr concentration CNaBr in the analyzed solution. The simulation was performed according to experimental conditions [5].
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will be less. Secondly, a decrease of the ion current in this interface at the negative ion detection mode will have almost no influence on the space charge effect because of a very small limiting ion current. The second supposition [11] concerning the possibility of negative ion detection using ‘‘cold’’ plasma is apparently based on the familiar fact that negative polyatomic ions of organic and inorganic halogen compounds are formed in cold ion sources working under atmospheric pressure. The gas temperature of ‘‘cold’’ ICP is about 1450 K [32]. Large polyatomic ions can still exist at this temperature. However, the formation of such ions cannot be estimated using thermodynamic simulation until sufficient thermodynamic data on these ions are available. The present results obtained using the ASTRA software [18] are contradictory in some points to the data reported in our previous publication [33], which were estimated with the HSC program [34]. For example, it was predicted that negative atomic ions may exist within an optimal low-temperature range and that high halide concentrations have a strong impact on the intensity of negative ions of analytes and O at low ICP temperatures. The agreement between the estimated and experimental data allowed us to arrive at the erroneous conclusion that negative atomic ions may originate directly in the discharge plasma [33]. The study of the reasons of the discrepancy between some estimated data obtained at low plasma temperatures using the two thermodynamic simulation programs revealed no essential differences in the thermodynamic properties of numerous individual substances in the ASTRA-IVTANTHERMO [18] and HSC [34] databases. The calculation algorithm of the ASTRA software [18] based on the thermodynamic system entropy maximization principle permits obtaining its total equilibrium composition including the concentrations of electrons, positive and negative ions at the preassigned temperature, pressure and initial chemical composition. The calculation algorithm of the HSC program based on the Gibbs energy minimization makes it possible to obtain the total equilibrium composition of the system at the same preassigned initial parameters only when a certain initial electron concentration is introduced into the system. Our check calculations with the HSC program showed that the equilibrium concentrations of charged particles at low temperatures (3500 – 7000 K) depend strongly on the preassigned initial electron concentration. This is the main reason for some previous erroneous conclusions and the discrepancy between the data presented in this work and those reported in Ref. [33]. A fairly good agreement between the calculated and experimental concentrations of electrons and positive atomic ion was obtained when developing a thermodynamic model for thermochemical processes in ICP [13 – 17] with the ASTRA program [18], which allows us to use this software for further calculations.
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