Plasma-related matrix effects in inductively coupled plasma—atomic emission spectrometry by group I and group II matrix-elements

Spectrochimica Acta Part B 58 (2003) 1301–1317

Plasma-related matrix effects in inductively coupled plasma— atomic emission spectrometry by group I and group II matrix-elements George C.-Y. Chan, Wing-Tat Chan* Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China Received 25 November 2002; accepted 25 March 2003

Abstract The effects of Na, K, Ca and Ba matrices on the plasma excitation conditions in inductively coupled plasmaatomic emission spectrometry (ICP-AES) were studied. Normalized relative intensity was used to indicate the extent of the plasma-related matrix effects. The group I matrices have no effects on the plasma excitation conditions. In contrast, the group II matrices depress the normalized relative intensities of some spectral lines. Specifically, the Group II matrices have no effects on the normalized relative intensity of atomic lines of low upper energy level (soft lines), but reduce the normalized relative intensity of some ionic lines and atomic lines of high energy level (hard lines). The Group II matrices seem to shift the Saha balance of the analytes only; no shift in the Boltzmann balance was observed experimentally. Moreover, for some ionic lines with sum of ionization and excitation potentials close to the ionization potential of argon (15.75 eV), the matrix effect is smaller than other ionic lines of the same element. The reduced matrix effects may be attributed qualitatively to charge transfer excitation mechanism of these ionic lines. Charge transfer reaction renders ionic emission lines from the quasi-resonant levels similar in characteristics of atomic lines. The contribution of charge transfer relative to excitation by other non-specific excitation mechanisms (via Saha balance and Boltzmann balance) determines the degree of atomic behavior of a quasi-resonant level. A significant conclusion of this study is that plasma-related matrix effect depends strongly on the excitation mechanism of a spectral line. Since, in general, more than one excitation mechanism may contribute to the overall excitation of an emission line, the observed matrix effects reflect the sum of the effects due to individual excitation mechanisms. Excitation mechanisms, in addition to the often-used total excitation energy, should be considered in matrix effect studies. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Inductively coupled plasma-atomic emission spectrometry; Matrix effect; Charge transfer; Excitation mechanism

1. Introduction Inductively coupled plasma-atomic emission spectrometry (ICP-AES) is a powerful analytical *Corresponding author. Fax: q852-2857-1586. E-mail address: [email protected] (W.-T. Chan).

technique for trace elemental analysis. The technique is widely used for qualitative and quantitative elemental analysis on a routine basis. However, there remains an incomplete fundamental understanding of the ICP itself. An important, yet not fully resolved, issue is the understanding

0584-8547/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0584-8547Ž03.00055-7

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and elimination of sample-dependent matrix interference. Matrix effects in ICP-AES have been widely reported in the literature and have been reviewed recently w1,2x. The extent of matrix effects is typically represented by the magnitude of relative emission intensity (or the recovery). Very often, relative emission intensities are correlated with ionization and excitation potentials of the analyte emission lines w3–9x. In the presence of matrices of easily ionizable elements, matrix effects have been found to be approximately constant w3–6x or varying linearly w7x against the total excitation potential in robust plasmas. As a result, it was suggested that matrix effects could be overcame using a single internal standard w5x or a combination of several internal standards and suitable energy potential-interference functions w7x. The introduction of two-dimensional imaging detectors (e.g. charge injection device and charge coupled device) to ICP-AES facilitates simultaneous measurement of a large number of spectral lines w10,11x. Matrix effect study is more convenient using simultaneous spectrometers with twodimensional detectors than using sequential scanning spectrometers w3x. As a result, more detailed studies of matrix effects have been reported recently. Various combinations of elements (up to 21 elements) and emission lines (up to 75 lines) have been used in the study of matrix effects w3,7,9x. However, the emission lines selected are typically the analytical (strongest) emission lines of the elements. Matrix effects that are analyte andyor spectral line specific may not be easily identified using such selection criterion. For example, only some elements can be excited via charge transfer reactions with argon w12x. A suitable charge transfer excitation route may render an ionic emission line ‘atom-like’ and matrices that interfere with the electron collision excitation mechanism may have small or no matrix effects on these analyte emission lines. In this paper, as many spectral lines as possible of each element were used to check for matrix effects that are spectral line specific, if any. In particular, elements that can undergo charge transfer reaction with argon ions are included to emphasize the line-

specific effects. As a result, 59 spectral lines of 9 elements were selected for the study. 2. Experimental A Perkin–Elmer Optima 3000 ICP-AES spectrometer was used. The instrument consists of a 40-MHz free-running radio-frequency generator, an Echelle grating spectrometer, and a segmented charge coupled device detector. Emission intensities of the 59 emission lines (Table 1) were measured simultaneously in groups (see below) using peak area mode. Table 1 lists the analytical wavelengths and background correction positions of the spectral lines. The operating parameters of the ICP spectrometer are typical and are listed in Table 2. The test element solution and the matrix solution were prepared separately. The 9 test elements were Mg, Mn, Pd, Zn, Cr, Cd, Cu, Be and Ni. The concentration of each element was chosen to give SyB ratio of at least 10 for the weakest spectral line of the element: 10 mgyl of Mn and Be, 25 mgyl of Mg and Zn, 100 mgyl of Pd, Cd, Cu and Ni and 150 mgyl of Cr. To check for spectral interference between these 9 test elements, the 59 emission lines (Table 1) were monitored simultaneously as individual test-element solution was aspirated into the plasma. To reduce the number of test-element solutions and experiment time, the elements were grouped to provide the least number of solutions without spectral interference among the elements. Using these criteria, the 9 test elements were grouped into three solutions for measurement: (1) Mg, Mn, Pd and Zn, (2) Cr, Cd and Cu, and (3) Ni and Be. The test-element solutions were prepared from 1000 mgyl single element standard solutions (Merck, Germany) in 2% nitric acid (Merck). Four matrix elements were used: K, Na, Ca and Ba. The matrix solutions were prepared from the corresponding chloride salts of the matrix elements in 2% nitric acid. The concentration of each matrix element was 0.1 M. The test-element and matrix solutions were pumped through two separate channels of a peristaltic pump and mixed via a glass T-joint before entering the nebulizer. Solution uptake rate of each

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Table 1 Analytical wavelength, ionization potential, excitation potential and off-peak background correction positions of the spectral lines used in this study Spectral lineynm

I.P.yeV w13x

E.P.yeV w14x

Total energyyeV

Off-peak background positionsynm

BeI 234.861* BeI 265.064 BeI 265.055 BeII 313.107 BeII 313.042

– – – 9.322 9.322

5.278 7.402 7.402 3.959 3.960

5.278 7.402 7.402 13.281 13.282

y0.020 y0.029 y0.029 y0.029 y0.029

CdI 228.802* CdI 361.051 CdII 226.502 CdII 214.438

– – 8.993 8.993

5.418 7.380 5.473 5.780

5.418 7.380 14.466 14.773

y0.020 and 0.020 0.020 y0.020 and 0.020 0.020

CuI 327.396* CuI 324.754 CuI 221.458 CuI 222.778 CuII 224.700 CuII 214.897 CuII 213.598 CuII 224.261

– – – – 7.726 7.726 7.726 7.726

3.786 3.817 6.986 7.206 8.235 8.487 8.522 8.784

3.786 3.817 6.986 7.206 15.961 16.213 16.248 16.510

y0.030 y0.030 y0.020 y0.030 y0.020 y0.029 0.020 y0.020

*

CrI 357.869 CrI 302.156 CrII 206.149 CrII 205.552 CrII 274.007 CrII 203.990 CrII 342.272 CrII 338.268 CrII 267.716 CrII 292.815 CrII 292.825 CrII 283.998

– – 6.766 6.766 6.766 6.766 6.766 6.766 6.766 6.766 6.766 6.766

3.463 5.133 6.013 6.030 6.030 6.070 6.076 6.119 6.154 7.991 8.091 8.112

3.463 5.133 12.779 12.796 12.796 12.836 12.842 12.885 12.920 14.757 14.857 14.878

y0.019 y0.019 y0.019 y0.019 y0.019 0.040 y0.019 y0.019 y0.019 y0.019 y0.032 y0.019

MgI 285.213* MgI 202.582 MgII 280.270 MgII 279.553 MgII 279.079

– – 7.646 7.646 7.646

4.346 6.119 4.423 4.434 8.864

4.346 6.119 12.069 12.080 16.510

y0.060 and 0.055 0.015 y0.026 and 0.019 y0.035 and 0.026 y0.040

MnI 403.076* MnII 260.569 MnII 257.610 MnII 294.920 MnII 347.404 MnII 347.413 MnII 270.103

– 7.435 7.435 7.435 7.435 7.435 7.435

3.075 4.757 4.812 5.378 5.378 5.401 8.295

3.075 12.192 12.247 12.813 12.813 12.836 15.730

y0.023 0.023 y0.040 y0.023 y0.023 y0.023 0.023

and 0.023

NiI 361.046 NiI 346.165* NiI 341.476 NiI 232.003 NiII 231.604 NiII 221.647 NiII 243.789

– – – – 7.635 7.635 7.635

3.542 3.606 3.655 5.343 6.393 6.633 6.765

3.542 3.606 3.655 5.343 14.028 14.268 14.400

y0.021 0.030 y0.021 y0.021 y0.021 y0.021 y0.021

and 0.021

and and and and and

0.029 0.029 0.029 0.029 0.029

and 0.030 and 0.030 and 0.030 and 0.030 and and and and

0.019 0.019 0.019 0.019

and 0.019 and 0.019 and 0.019

and 0.026 and 0.023 and 0.023

and and and and and

0.021 0.021 0.021 0.040 0.021

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Table 1 (Continued) Spectral lineynm

I.P.yeV w13x

E.P.yeV w14x

Total energyyeV

Off-peak background positionsynm

NiII 233.459 NiII 228.765

7.635 7.635

6.990 8.523

14.625 16.158

y0.021 and 0.021 0.020

PdI 340.458 PdI 324.27* PdI 330.213 PdII 248.892

– – – 8.340

4.224 4.637 5.005 8.090

4.224 4.637 5.005 16.430

y0.031 and 0.031 y0.031 and 0.031 0.031 y0.031 and 0.020

ZnI 213.856* ZnI 330.259 ZnI 334.502 ZnII 206.200 ZnII 202.548

– – – 9.394 9.394

5.796 7.783 7.784 6.011 6.120

5.796 7.783 7.784 15.405 15.514

y0.020 and 0.020 y0.020 and 0.020 0.020 y0.020 and 0.020 y0.031

*

Reference spectral line.

channel was 1.0 lymin. The two channels were designated as test element channel and matrix channel, respectively, in this paper. When only one solution is required for measurement (e.g. intensity measurement of the test elements in the absence of matrix or intensity measurement of the matrix without the test elements for spectral interferences checking), a make-up solution (2% HNO3) was pumped into the other channel. The 59 emission lines of the test elements have also been examined for spectral interferences from the matrices. To check for spectral interference from the matrix, the matrix solution and the test element solution were introduced into the plasma separately. Spectral lines of the test elements that overlap with the matrix spectral lines were not used in matrix effect study of this matrix element.

In addition, shift in background intensity andyor increase in background noise by the matrix w15x were checked. If the matrix emission added to the emission intensity of a spectral line of a test element by more than 2%, the spectral line was not used for that particular matrix. The 2% threshold is within the long-term stability (RSD) of the test element intensity and is smaller than all matrix effects observed in this study. As a result of the selection criteria, the emission intensity of the matrix was typically less than 0.5% of that of the test element; matrix emission only approached 2% of the test-element emission for the weakest testelement emission lines (e.g. MgI 202.58 nm and MnII 270.10 nm). For the matrix effect study, the emission intensities of the 59 spectral lines of the 9 test elements

Table 2 Operating parameters of optima 3000 ICP forward power Plasma gas Auxiliary gas Nebulizer gas Nebulizer Injector diameter Observation height Solution intake Rate Replicates Integration time Spectral profiling Resolution

1250 W 15 lymin 0.5 lymin 1.0 lymin Cross-flow with Scott type double-pass spray chamber 2 mm 15 mm above load coil 1.0 mlymin for each solution channel 3 Auto, min 0.2 s, max 5.0 s On Normal

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were measured, in turn, in the absence and presence of Na, Ca, K and Ba matrices. Firstly, emission intensity of the test elements in the absence of matrix (standard intensity) was measured. Na matrix was then fed through the matrix channel while the feed of the test element solution continued. Emission intensities of the test element in the presence of Na were measured. After the measurement with Na matrix, Ca matrix was studied in the same manner. An interim drift check was then carried out; the ‘standard intensity’ was measured again. K and Ba matrices were studied using the same procedures. The ‘standard intensity’ was again measured for the final drift check. 3. Results and discussion 3.1. Brief review of equilibria in ICP In later sections, matrix effects are regarded as a shift in the equilibria of plasma excitation and ionization. The magnitude of matrix effects on individual spectral lines will be correlated with the excitation mechanisms of the spectral lines. Therefore, a brief review of ICP equilibrium and selected excitation mechanisms is included in the following paragraphs for the ease of later discussions. Atomic emission originates from the excited levels of the analyte atoms or ions. Excitation and ionization are naturally the two most important processes that govern the analytical characteristics of ICP-AES. At equilibrium, the ratio of the number densities of a species (atoms or ions) in two energetically bounded states is governed by the Boltzmann balance. Similarly, the distribution of ionization products (i.e. the ionization-recombination equilibrium) is governed by the Saha balance w16x. The position of Boltzmann balance and Saha balance are indicated by the excitation (Boltzmann) temperature and ionization (Saha) temperature, respectively w17x. If the kinetic energy of all particles in the plasma follows the Maxwellian distribution and shares a common temperature value with Boltzmann balance and Saha balance, then the plasma is said to be in local thermodynamic equilibrium (LTE) w18x. In ICP, LTE is maintained by detailed balance of the collisional and radiative processes such that the differential

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reaction rates of each microscopic process and the corresponding reverse process are equal w17x. However, equilibrium in the plasma is sometimes not achieved, even in the absence of matrix. For example, highly excited atomic levels that are closely spaced to the ionization continuum are in equilibrium with the ground state ions due to the small energy gap between the levels and the large reaction rate constants of collisional excitationy de-excitation and ionizationyrecombination w19x. Other processes, such as radiative decay, are not rapid enough to re-distribute the balance significantly w20x. The distribution of the upper energy levels may, therefore, depart from the Boltzmann balance between the lower and upper energy levels. The high atomic levels follow the Saha balance of the corresponding ion rather than the Boltzmann balance of the atom. For example, the excitation temperature of FeI lines is not a constant. Instead, the measured temperature increases with upper energy levels of the FeI lines w21–23x. The contribution of Boltzmann balance and Saha balance to the overall excitation process, therefore, depends on the energy level of the analyte. Similarly, Fey and co-workers reported that the lower excitation levels of MgI and MgII are dominated by Boltzmann balance and the contribution from Saha balance increases with increasing excitation levels w24x. Apart from ion-electron recombination, charge transfer reaction may also disturb the Boltzmann and Saha balance in ICP. Charge transfer is a onestep process that controls the balance between an ion state of one element and the ground state of another element w24x. The levels of the analyte that can be directly excited from ground state through charge transfer are called ‘quasi-resonant levels’. For some excitation levels of some elements, charge transfer reaction between argon ions and the analyte atoms contribute as a selective ionization and excitation mechanism. Ar has two levels capable of undergoing charge transfer reaction for the excitation of an analyte in the ICP: 2 P3y2 level (ground state) of energy of 15.75 eV and 2P1y2 level of 15.93 eV. Charge transfer in the ICP has been studied using Mg as a test element w20,24–27x. Direct experimental evidences of charge transfer to excite Mg atoms to the MgII 3d

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D and 4s 2S states are well documented in the literature w20,24–27x. The charge transfer reaction rate between Mg atoms and Ar ions is reported to be higher than competing excitationyde-excitation processes w25,26x and leads to an overpopulation of the quasi-resonant levels of Mg compared to Saha–Boltzmann balance. Such overpopulation has been experimentally observed w20,25,28x. Significantly, Farnsworth and co-workers have expanded the charge transfer study to third row elements from Ca to Cu and Y w12,27x. Although only a few analytes have been studied as candidates of charge transfer reactions in the ICP, some rules of charge transfer reaction are known. Firstly, the energy difference (energy defects) between the ionization potential of argon and the sum of ionization and excitation potentials of the product analyte ion must be small for effective charge transfer w12x. (Curiously, a positive energy defect means that the sum of excitation potential and ionization potential of the product analyte ion is smaller than the ionization potential of argon). There is qualitative evidence that positive energy defect of 2 eV or less provides the largest collisional cross-section w29x. Charge transfer with negative energy defect is also possible. For example, the well-known Mg–Ar system has a negative energy defect. Secondly, the charge transfer reaction should obey the Wigner spin rule w30x. (Briefly, the spin rule requires that the sum of the spins of the two interacting species must be conserved before and after the charge transfer reaction.) However, energy levels that do not satisfy the spin rule can still be excited indirectly by charge transfer in ICP w12,27x. Farnsworth and co-workers found that the magnitude of energy defect is the major rate-determining factor of charge transfer reaction w12x. They observed fluorescence emission from both doublet and quartet states at nearby levels (with ;2 eV energy gap) when they excited only the doublet state of Cu atoms in an ICP with a dye laser w12x. Fluorescence emission from the quartet was indirectly excited via frequent and efficient collision of the doublet state atoms and electrons. Collision between atoms and electrons in ICP rapidly spreads any excess population in a given state to adjacent levels w12x. As a result, energy levels that

do not satisfy the Wigner spin rule may also be excited indirectly by charge transfer reaction if there is a nearby energy level that satisfies the spin rule. 3.2. Normalized relative intensity Relative intensity is often used to indicate the extent of matrix-effects w3–9x. The intensity of the emission line in the presence of a matrix (Ia,M) is compared to those without the matrix (Ia). Matrix effect is expressed either as the percentage of the original signal intensity (i.e. recovery, Ia,M yIa) or the percentage change in signal intensity w(Ia,M – Ia)yIax. Relative intensity reflects the sum of two matrix effects: a change in plasma characteristics (plasma-related matrix effects) and a change in sample introduction efficiency, e.g. nebulization efficiency (non-plasma-related matrix effects). Since the objective of this paper is to study plasmarelated matrix effects, the contribution from nonplasma-related matrix effects will be ignored. To concentrate on the study of plasma-related matrix effects, ‘normalized relative intensity’ will be used in latter discussion. A reference spectral line is chosen for each test element. The relative intensities of the spectral lines of an element (Ia,M yIa) are divided by the relative intensity of the reference line of this element (Ir,M yIr ) to obtain the normalized relative intensities. The effect due to changes in the number density of the test element in the plasma, if any, is cancelled in the ratio. The normalized relative intensity would only reflect plasma-related matrix effects. The reference line, ideally, should suffer from little or no plasmarelated matrix effects. In this paper, the spectral line with the lowest excitation potential is chosen as the reference spectral line. If it happens that any of the four matrices posed spectral interference on the spectral line with the lowest excitation potential, then the spectral line with the second lowest excitation potential will be used as the reference. From another point of view, the normalized relative intensity w(Ia,M yIa)y(Ir,M yIr )x is the recovery of the emission intensity ratio of these two lines w(Ia,M yIr,M)y(Ia yIr )x in the presence of matrix. Intensity ratio of two emission lines, depending on

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Fig. 1. Relative intensity in the presence of 0.1 M Na and Ca matrices vs. total excitation potential of the test-element spectral lines.

the nature (ionic or atomic) of the lines, can be used to monitor the plasma excitation conditions (temperature andyor electron number density) under the assumption of local thermodynamic equilibrium (LTE) w31x. Since all the reference lines were atomic lines in this study, the normalized relative intensities of atomic and ionic lines are actually the relative emission intensity ratios of an atomic–atomic line pair and an ionic–atomic line pair, respectively. Emission intensity ratio of an atomic–atomic line pair reflects the temperature of the plasma under LTE and ratio of ionic–atomic line pair reflects both temperature and electron number density of the plasma w31x. It should be stressed that normalized relative intensity is an indicator of matrix effects that relate to changes in plasma excitation conditions, rather than an indicator of all plasma-related matrix effects. Some plasma-related matrix effects do not change the plasma excitation conditions. For example, the matrix may affect the rate of the atomization process of the analyte and results in lateral

diffusion interference w32–35x. Such interference is plasma-related but may not change the normalized relative intensity. Therefore, a change in normalized relative intensity surely indicates the existence of plasma-related matrix effects but the reverse may not be true. 3.3. Matrix effect—normalized relative intensity 3.3.1. Overview The emission intensity of 59 spectral lines of 9 elements (Table 1) was measured. Fig. 1 shows the relative intensity vs. the total excitation energy of the spectral lines in the presence of 0.1 M Na and Ca matrices. Figs. 2–5 show the normalized relative intensity vs. total excitation energy in the presence of 0.1 M of Na, K, Ca and Ba matrices, respectively. The normalized relative intensities of the reference spectral lines (Table 1) are not included in Figs. 2–5 since the values are 100% by definition, which give no information on matrix effects. The gap between 8 and 10 eV distinguishes

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Fig. 2. Normalized relative intensity in the presence of 0.1 M Na matrix vs. total excitation potential of the test-element spectral lines. The insert shows the enlarged portion of the graph from 14 to 17 eV.

atomic and ionic spectral lines, i.e. the excitation energy of all the atomic spectral lines used in this study is below 8 eV and the total excitation energy (sum of excitation and ionization potentials) of all the ionic lines is larger 10 eV. The scattering of the normalized relative intensity is "3% (Fig. 2) which is of similar order of magnitude of the plasma drift (1–3%). There is no experimental evidence of matrix effects for Na on plasma excitation conditions, within experimental uncertainty. It is possible that change in sample introduction efficiency (non-plasma related matrix effect) may contribute to the slight reduction in relative intensity vs. total excitation energy (Fig. 1) for Na matrix w5,6x. In agreement with previous study, the effects of Na and K matrices are different from the effects of Ca and Ba matrices w36x. There is no matrix

effect of Na and K on plasma excitation conditions; the normalized relative intensity shows a flat response vs. total excitation energy and does not differ (within experimental uncertainty) from 100% for all the spectral lines studied (Figs. 2 and 3). On the other hand, for the Ca or Ba matrices, the normalized relative intensity of spectral lines of high excitation energy ()7 eV) is reduced. The effects of Ca and Ba matrices are similar (Figs. 4 and 5). Interestingly, as the total excitation potential increases to 15.7 eV or higher, the extent of reduction in normalized relative intensity of some spectral lines becomes smaller. Some lines even have no reduction in normalized relative intensity, (i.e. no matrix effects on plasma excitation conditions). Matrix effects for emission lines of total excitation energy of 15.7 eV or higher is element specific (see discussions in later sections).

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Fig. 3. Normalized relative intensity in the presence of 0.1 M K matrix vs. total excitation potential of the test-element spectral lines. The insert shows the enlarged portion of the graph from 14 to 17 eV.

Since no matrix effect was observed for Na and K matrices for all the spectral lines studied (Figs. 2 and 3), only the data related to the Ca and Ba matrices will be discussed. Figs. 4 and 5 are divided into three regions for discussion: atomic spectral lines of excitation energy below 8 eV, ionic lines of excitation energy between 10 and 15.7 eV (the ionization potential of argon), and ionic lines of energy higher than 15.7 eV. Matrix effects in these three regions are distinctly different. 3.3.2. Ca and Ba matrix effects on atomic spectral lines Spectral lines of excitation potential of 7 eV or smaller (Mg, Cu, Ni, Pd and Cr atomic lines)

show no matrix effects of Ca and Ba; the normalized relative intensity is 100%. However, for atomic spectral lines of excitation potential larger than 7 eV (Zn, Be and Cd atomic lines), the normalized relative intensity decreases sharply with excitation potential, indicating matrix effects of Ca and Ba on these lines. The reduction in normalized relative intensity of the high level Zn, Be and Cd atomic lines in the presence of Ca and Ba matrices cannot be explained by a change in excitation temperature of the plasma. Assuming that the distribution of excited species of the same state (atomic or ionic) is governed by Boltzmann distribution, the change in the normalized relative intensity (DRB) for a small change in excitation temperature (DT) is w37x

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Fig. 4. Normalized relative intensity in the presence of 0.1 M Ca matrix vs. total excitation potential of the test-element spectral lines. The insert shows the enlarged portion of the graph from 14 to 17 eV.

DRB DE s 2 DT RB kT where RB is the normalized relative intensity of the atomic line, T is the excitation temperature, k is the Boltzmann constant and DE is the difference in the upper energy levels of the two lines. Under LTE conditions, for a fixed change in excitation temperature, the change in normalized relative intensity for an atomic emission line should be directly proportional to the energy difference in the upper levels of this emission line and the reference emission line. Table 3 lists the DE values of some atomic spectral lines of Mg, Ni, Cr, Cu, Zn, Be and Cd. Clearly, no correlation was found on the extent of matrix effect and DE. The depression in normalized relative intensity of high level ZnI, CdI and BeI lines in the presence

of Ca and Ba matrices is probably related to hard line behavior of these high energy atomic levels. Emission lines can be classified into two basic categories according to the response of the net line intensity to a change in ICP power. ‘Soft’ lines have spatial emission behavior that is very dependent on power, aerosol flow rate and analyte excitation and ionization characteristics. ‘Hard’ lines have spatial emission behavior that is relatively insensitive to these parameters w38,39x. Atomic lines of elements with low to medium ionization potential (I.P. F8 eV) are, in general, soft lines w15x. Ionic lines and atomic lines of elements with high ionization potential (I.P.)8 eV) are, in general, hard lines w15,40x. The excitation mechanisms of soft line and hard line are believed to be different w15x. Soft lines are mainly excited by collisional mechanisms w15x. Atomic hard lines

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Fig. 5. Normalized relative intensity in the presence of 0.1 M Ba matrix vs. total excitation potential of the test-element spectral lines. The insert shows the enlarged portion of the graph from 14 to 17 eV.

may be significantly populated by ion–electron recombination reaction w38,41x, with character similar to the ionic lines. In fact, the spatial profiles

of atomic hard lines are similar to their corresponding ionic lines. The population of these highenergy atomic excited levels is also more

Table 3 Normalized relative intensity (N. rel. int.) in the presence of Ca and Ba matrix for some atomic emission lines and the difference in upper energy levels of the atomic emission lines and their corresponding reference emission line Spectral line and upper energy level

Reference line and upper energy level

DEyeV

N. rel. int. in Ca matrix

N. rel. int. in Ba matrix

MgI 202.582 (6.119 eV) NiI 232.003 (5.343 eV) CrI 302.156 (5.133 eV) CuI 221.458 (6.986 eV) CuI 222.778 (7.206 eV) ZnI 330.259 (7.783 eV) BeI 265.064 (7.402 eV) CdI 361.051 (7.380 eV)

MgI 285.213 (4.346 eV) NiI 346.165 (3.606 eV) CrI 357.869 (3.463 eV) CuI 327.396 (3.786 eV) CuI 327.396 (3.786 eV) ZnI 213.856 (5.796 eV) BeI 234.861 (5.278 eV) CdI 228.802 (5.418 eV)

1.773 1.737 1.670 3.200 3.420 1.987 2.124 1.962

102.3 100.8 100.5 99.0 98.3 94.1 96.0 –

– 100.6 101.8 – 99.8 88.1 95.1 92.8

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‘Saha-like’ w38x. The observation of suppression of the normalized relative intensity of the atomic hard lines by the Ca and Ba matrices is in parallel to the suppression of the Mg, Cu, Ni, Cr and Mn ionic lines (next section). Therefore, the matrix effects of Ba and Ca on ZnI, CdI and BeI lines may be attributed to the atomic hard line behavior of these lines. The behavior of ZnI and CdI lines has been reported to be more similar to ionic lines than to atomic lines w38,42x. 3.3.3. Ca and Ba matrix effects on ionic spectral lines of excitation potential -15.7 eV In the presence of Ca and Ba matrices, the normalized relative intensity was less than 100% for all ionic spectral lines of total excitation potential -15.7 eV, indicating a change in plasma excitation conditions w36x. As discussed in Section 3.2, the normalized relative intensity of ionic lines is the emission intensity ratio of an ionic-atomic emission line pair. The ratio is governed by both Saha and Boltzmann balance. Matrix effects may shift both the Boltzmann and the Saha balance. However, the relative effects of the matrix on the balances can differ significantly as shown below. In Figs. 4 and 5, the normalized relative intensity appears to be similar for all emission lines. There is no correlation of suppression in the normalized relative intensity with the excitation potential. For example, the two CrII lines at 12.8 and 14.8 eV (DE ;2eV) shows the same (within experimental uncertainty) decrease in normalized relative intensity. If there is a change in the Boltzmann balance, spectral line with higher excitation potentials will suffer more severe matrix effects. Therefore, there is no evidence of a shift in Boltzmann balance by the Ba and Ca matrices for the ionic lines. The same conclusion can be drawn for the atomic emission lines as there was no suppression in normalized relative intensity of the atomic lines (Section 3.3.2). It seems that the Ca and Ba matrices affect only the Saha balance but not the Boltzmann balance. The mechanism by which Ca and Ba matrices affect the Saha equilibrium is not known. However, The matrix effect, although related to Saha balance, is probably not caused by a shift in electron number density of the plasma. Since the first

ionization potential of all matrices studied (Na, K, Ca or Ba) is low, ionization of the matrix elements in the ICP should be complete. In other words, all matrices should contribute the same number of electrons and should lead to similar changes in electron number density in the ICP. Although the second ionization potential of Ca and Ba is relatively low (lower than first ionization potential of Ar), contribution of electrons from second ionization of Ca and Ba should be insignificant w43x. It can be inferred that the change in Saha equilibrium by the Ca and Ba matrices is probably a matrixelement specific effect, rather than a global change in temperature or electron number density as predicted by LTE model. Similar matrix effects have been observed for matrix elements of low second ionization potential w36x. 3.3.4. Ca and Ba matrix effects on ionic spectral line with excitation potential )15.7 eV The matrix effects for ionic emission lines with excitation potential close to the ionization potential of argon (15.75 eV) are more complex than other ionic lines. There is no correlation or trend between the normalized relative intensity and the excitation potential. The MgII line at 16.51 eV (MgII 279.08 nm) has ;100% normalized relative intensity in the presence of Ca matrix (Ba posed spectral interference for this MgII line). The CuII line at 15.96 eV (CuII 224.70 nm) has a significantly higher normalized relative intensity than the CuII lines at 16.21–16.51 eV (CuII 213.60, 214.90 and 224.26 nm) (Table 1). NiII line at 16.16 eV (NiII 228.77 nm) also has significantly higher normalized relative intensity than NiII lines at 14.03 to 14.63 eV (Table 1). However, MnII line at 15.73 eV (MnII 270.10 nm) has the same reduced normalized relative intensity as other MnII lines. The behavior of spectral lines in this range of excitation potential (15.7 eV to 16.5 eV) may be attributed to charge transfer reaction with the ground state Ar ions. The absence of matrix effect of Ca on the MgII 279.08 nm spectral line (EPs16.51 eV) may be attributed to the dominant excitation mechanism of this MgII line via charge transfer reaction with argon ions. It is well known that charge transfer reaction contributes significantly to the excitation

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of the Mg atoms to the MgII 3d 2D and 4s 2S states, but not for other Mg excited states w20,24– 27x. Charge transfer involves excitation from atomic ground state directly to an ionic excited state in one step. Among other factors, the rate of charge transfer is proportional to the atomic ground state number density of the analyte. In other words, the emission intensity of MgII 279.08 nm should be related directly to the Mg atomic ground state population. (The transition concerned is from MgII 3d 2D3y2 to 3p 2P1y2.) Ogilvie and Farnsworth reported positive correlation for the MgI 285.21 nm and MgII 279.08 nm line and negative correlation for the MgI 285.21 nm and MgII 279.55 nm line in their correlation spectroscopy experiment w27x. The MgI 285.21 nm emission intensity represents the population of the low level excited states of Mg which in turn is related to the atomic ground state population w26x because of the Boltzmann balance w24x. In this study, the absence of matrix effect of Ca on the MgII 279.08 nm emission (reference line for the normalized relative intensity is also the MgI 285.21 nm) is therefore, probably due to the dominant excitation mechanism of the emission line via charge transfer reaction between argon ions and the Mg atoms. In other words, the line does not follow Saha balance like other ionic lines. It is interesting to note that smaller matrix effects of Ca on the MgII 279.08 nm line compared to other MgII line has also been observed using axial viewing mode. Brenner et al. w9x reported relative intensities 1.02, 0.85 and 0.92 for the MgI 285.21 nm, the MgII 280.27 nm and the MgII 279.08 nm lines, respectively in the presence of 0.1% Ca matrix in a robust plasma. Some Cu lines are excited in the ICP by charge transfer reaction with the argon ions. Farnsworth et al. reported positive correlation between the intensities of CuII 211.21 nm (16.84 eV), CuII 213.60 nm (16.24 eV) and CuII 224.70 nm (15.95 eV) with a reference CuI line at 324.75 nm. The correlation suggests that charge transfer contribute either directly or indirectly to the excitation of these CuII levels w12x. Although both Mg and Cu can be excited via charge transfer, the behaviors of normalized relative intensity of the quasi-resonant emission lines

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of these two elements in the presence of Ba and Ca matrices are different. The Mg quasi-resonant line showed no depression in normalized relative intensity but the Cu quasi-resonant lines (e.g. CuII 224.70 nm and CuII 213.60 nm) are depressed. The difference may be attributed to the relative contribution, as compared to other excitation mechanisms, of charge transfer reaction to the excitation of these quasi-resonant states. If charge transfer is the dominant excitation route, the quasiresonant emission lines should follow atomic emission lines of low excitation potentials closely w12,24,27x. In contrast, if charge transfer only contributes slightly to the overall excitation and other excitation mechanisms (via Saha balance and Boltzmann balance) dominate the overall excitation, the quasi-resonant line would behave more like a ‘normal’ ionic emission lines. Although there is no report that compares the contribution of charge transfer to the overall excitation processes of the quasi-resonant states of Mg and Cu, nor is there any reference that provides data on the charge transfer reaction rate or collisional cross-section of the reactions of Cu, the results from Farnsworth’s correlation spectroscopy experiment may provide some hints on the relative contribution of charge transfer to excitation of Mg and Cu. In Farnsworth’s first correlation spectroscopy experiment, the authors reported clear positive correlation between the quasi-resonant MgII levels and the MgI level w27x. However, the quasiresonant CuII line at 224.70 nm had a positive correlation with CuI level that was barely distinguishable from baseline noise w27x. In a follow-up paper, the authors reported the results of correlation spectroscopy for the third row metals from calcium through copper w12x. Again, positive correlations for the Cu quasi-resonant levels were clearly distinguished (positive evidence of charge transfer reaction), but the correlation magnitude was small. The correlation magnitude for CuII 224.70 nm was only 0.04 at its peak while the correlation magnitude of some other elements were much larger (e.g. 0.30 and 0.50 for Sc and V, respectively) w12x. Their results indicate that the contribution of charge transfer reaction to the excitation of the quasi-resonant levels of Cu is relatively small compared to other excitation mechanisms. The

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difference in the contribution of excitation mechanisms is a possible explanation of the different behavior of Mg and Cu quasi-resonant levels in the presence of Ca and Ba matrices. Similarly, the smaller depression in normalized relative intensity of CuII 224.70 nm (EPs15.95 eV) than the other three CuII lines of similar energy levels (EPs16.21, 16.25 and 16.51 eV) can be explained by the relative contribution of charge transfer to the overall excitation processes of these quasi-resonant levels. Farnsworth reported that charge transfer directly or indirectly contributed to the excitation of Cu quasi-resonant levels of total excitation energy from 15.95 to 16.84 eV. The contribution decreased with increasing energy levels due to increasing energy defects (the degree of correlation of the correlation spectroscopy experiments decreased with increasing energy defects) w12x. The same conclusion has also been suggested in the work of Goldwasser and Mermet w44x. They found that the shape of the vertical profile of intensity ratios of CuII 213.60 nm (16.24 eV)yCuII 219.23 nm (16.20 eV) and CuII 213.60 nmyCuII 224.70 nm (15.95 eV), are different w44x. Since the differences in upper energy levels (DE) of the line pairs are small, the ratios should be practically constant along the central channel for emission lines of ‘Boltzmann-like’ characteristics. The intensity ratio of CuII 213.60y219.23 was relatively constant along the vertical profile while the ratio CuII 213.60y224.70 showed a large variation along the vertical profile. They concluded that the CuII 224.70 nm line (15.95 eV) showed stronger emission intensity than if equilibrium existed in the studied energy range. The authors suggested that charge transfer reaction enhanced the line intensity of the CuII 224.70 nm line w44x. It is reasonable to assume that the contribution of charge transfer to the excitation of CuII 224.70 nm is larger than other Cu lines and the contribution declines significantly for other quasi-resonant levels. Therefore, the emission intensity of CuII 224.70 nm line should follow closer to the emission of CuI line than other quasi-resonant CuII lines. As a result, the CuII 224.70 nm line shows a smaller depression in normalized relative intensity in the presence of Ca and Ba matrices.

Ni is similar to Cu in that charge transfer reaction plays a role in the excitation of the Ni quasi-resonant levels of total excitation energy of 14.03–14.62 eV w12x. Although the magnitude of correlation for Ni was slightly larger than Cu in Farnsworth’s correlation spectroscopy experiments, the correlation magnitude was still low (peak values0.06) w12x. Therefore, similar to Cu, the contribution of charge transfer may be minor in the overall excitation of the NiII lines used in this study. Depression in normalized relative intensity of the NiII lines is observed. The depression for NiII 228.77 nm (16.16 eV), however, is smaller (Fig. 4) than other NiII lines, probably because of more contribution from charge transfer. NiII lines of total excitation energy )16 eV were not included in the correlation spectroscopy work by Farnsworth et al. w12x. However, it is reasonable to expect that charge transfer contributes to the excitation of the NiII 228.77 nm line. There are some nearby NiII levels of the correct state (4P) that satisfies the Wigner spin rule. These lines have small energy defects (y0.30 to y0.40 eV) and have a small energy difference of 0.01 to 0.11 eV from the NiII 228.77 nm line w45x. The NiII 228.77 nm line can probably be excited indirectly by charge transfer. In contrast to Mg, Cu and Ni, the contribution of charge transfer reaction to the overall excitation of Mn and Cr was reported to be small w12x. The energy level for the emission line of MnII 270.10 nm is close to ionization potential of Ar. The normalized relative intensity of this emission line, however, is similar to other MnII emission lines within experimental uncertainties (only data from Ca matrix is available due to spectral interference from Ba matrix). This MnII line suffers from the same matrix effect of Ca as other MnII lines, despite the small energy defect. Similarly, all CrII lines, irrespective of the excitation potential, suffer from the same matrix effects of Ca and Ba. The observation agrees with the hypothesis that charge transfer reaction with Ar ions reduces matrix effect of Ca and Ba by linking of the population of the quasi-resonant states with the atomic ground state in one step. Although depressions in normalized relative intensity are observed for the PdII line in both Ca

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and Ba matrices, the depression is relative small (only 2–3%) compared to ionic spectral lines of other test elements. In previous paragraphs, ionic lines that show less depression in normalized relative intensity are those that can be at least partially excited (either directly or indirectly) by charge transfer reaction with argon ion. The smaller depression in normalized relative intensity of the PdII line suggests that charge transfer probably contribute to the excitation of the PdII lines. There is no report in the literature on the dominant excitation mechanism for PdII. However, electron collision is likely not the dominant mechanism. Boumans and De Boer w42x reported that the measured ionic-to-atomic emission intensity ratio of Pd is 1900 times larger than the theoretical ratio at LTE. Pd was the element that deviated most from LTE prediction in their study. PdII 248.89 nm is relatively overpopulated with respect to the corresponding atomic levels w42x. The overpopulation of PdII levels must be due to additional mechanisms other than electron collision. Therefore, it is possible that charge transfer contributes to excitation of the PdII line and minimizes the matrix effect of Ca and Ba on the normalized relative intensities. 4. Conclusions The effects of Na, K, Ca and Ba matrices on emission line intensities in ICP can be grouped into two categories. Na and K belong to a group that has no matrix effects on the normalized relative intensity of the spectral lines studied. Ca and Ba belong to another group of matrices that has no effects on the Boltzmann balance but effects the Saha balance and reduces the normalized relative intensity of some spectral lines. Furthermore, the dominant excitation mechanism of a spectral line determines the extent of the matrix effects. In general, atomic lines of low energy levels show no matrix effect. Atomic lines of high first ionization potential (e.g. Zn, Cd and Be) are depressed because of the hard line behavior of these lines. Ionic lines are generally depressed, with the exception of those ionic lines that are excited by charge transfer reactions with argon ions. Charge transfer reaction provides an excita-

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tion route directly from ground atomic state to high level ionic states in one step. Characteristics of ionic lines excited by this mechanism would resemble the characteristics of an atomic line. Finally, more than one excitation mechanism may contribute to the overall excitation of an emission line. The observed matrix effects reflect the sum of the effects due to individual excitation mechanisms. A common method to correct or compensate for matrix effects is the internal standard method w46– 48x. For the correction of plasma related matrix effects, the spectral line of the internal standard should have energy levels close to that of the spectral line of the analyte w46x. An implication of this study is that closeness in energy levels may be insufficient. The dominant excitation mechanisms of both spectral lines should also be the same. For example, charge transfer can reduce or eliminate matrix effects of Ca and Ba, while spectral lines of similar energy that are excited by a different mechanism of electron collision suffer from matrix effect. However, studies of charge transfer in the literature have been limited to a few elements and quantitative discussion is rare. Further information on charge transfer reactions of other elements is needed for the selection of internal standards based on excitation mechanisms. This study alone does not provide enough information to deduce the mechanism of the matrix effects of Ca and Ba at a fundamental level. However, there is no experimental evidence that Ca and Ba matrices affect the Boltzmann balance of the analyte (test element). The matrices seem to affect only the Saha balance because only ionic emission lines and atomic lines of significant contribution from ion–electron recombination were depressed. In addition, ionic lines that are excited by charge transfer has smaller or no matrix effects, suggesting that the matrix may have no significant effect on the distribution in population of the argon species (excitedyground state and atomsyions) in the plasma. Although the detailed matrix effect mechanism is not yet determined, the above deductions point to the direction of future experiments for the study.

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Acknowledgments This work was supported by the Hong Kong Research Grant Council under Grant No. HKU 7097y00P.

w13x w14x

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