Interference Effects of Easily Ionizable Elements in

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Interference Effects of Easily Ionizable Elements in ICPAES and Flame AAS: Characterization in Terms of the Collisional Radiative Recombination Activation Energy a

a

Mark F. Zaranyika , Albert T. Chirenje & Courtie Mahamadi a

a

Chemistry Department, University of Zimbabwe, Mount Pleasant, Harare, Zimbabwe

Available online: 14 Nov 2011

To cite this article: Mark F. Zaranyika, Albert T. Chirenje & Courtie Mahamadi (2012): Interference Effects of Easily Ionizable Elements in ICP-AES and Flame AAS: Characterization in Terms of the Collisional Radiative Recombination Activation Energy, Spectroscopy Letters, 45:1, 1-12 To link to this article: http://dx.doi.org/10.1080/00387010.2011.579679

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Spectroscopy Letters, 45:1–12, 2012 Copyright # Taylor & Francis Group, LLC ISSN: 0038-7010 print=1532-2289 online DOI: 10.1080/00387010.2011.579679

Interference Effects of Easily Ionizable Elements in ICP-AES and Flame AAS: Characterization in Terms of the Collisional Radiative Recombination Activation Energy Mark F. Zaranyika, Albert T. Chirenje, and Courtie Mahamadi Chemistry Department, University of Zimbabwe, Mount Pleasant, Harare, Zimbabwe

ABSTRACT A simplified rate model for the interference effects of easily ionized elements on analyte absorbance and emission signals in Flame atomic absorption spectroscopy, FAAS, is presented showing that when the analyte is determined in the absence and presence of the interferent, the change in the collisional radiative recombination activation energy, DEa, is zero when the systems conform to local thermal equilibrium. The proposed model also shows that DEa can be calculated from the slope of the plot of A0 =A or I 0 =I versus the interferent-to-analyte ion number density ratio, where A and I denote absorbance and emission signal intensities, respectively, and the prime denotes presence of the interferent. DEa values of
1.5168 and
1.6924 eV where obtained when Mg was determined by FAAS in the presence of excess Ca and Sr respectively, and values of
6.62,
9.83, and
13.52 eV were obtained when CaI, CaII, and MgII respectively were determined in the presence of excess Li, confirming that these systems are not in LTE. Possible causes for departure from LTE and possible collisional radiative recombination mechanisms consistent with the results are discussed. KEYWORDS activation energy, easily ionizable elements, flame AAS, ICP-AES, interference effects, radiative recombination

INTRODUCTION

Received 22 January 2011; accepted 6 April 2011. Address correspondence to Mark F. Zaranyika, Chemistry Department, University of Zimbabwe, P. O. Box MP 167, Mount Pleasant, Harare, Zimbabwe. E-mail: [email protected]

Current theory of atomic spectrometry assumes local thermal equilibrium (LTE).[1] According to this theory, all equilibriating processes within the plasma are due to collisional processes, and the contribution from radiative processes is negligible. All processes interfering with analyte excitation and ionization should be explainable in terms of the theory. In practice, it is found however that many matrix effects observed in both flame and the ICP cannot be explained easily using this theory. Several workers have therefore studied the effects of interferents on the absorbance and emission intensity of analyte atomic and ionic lines in atomic spectrometry with a 1

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view to elucidating the mechanisms responsible for analyte excitation and ionization.[2–22] Three approaches are used: active spectroscopic methods, passive spectroscopic methods, and kinetic modeling methods.[23,24] The combination of active and passive spectroscopic plasma diagnostic methods[2–22,25–40] has yielded a lot of information about the spatial variation of the composition of the plasma, spatial variation of interference effects, and the effects of varying radiofrequency power. In addition, considerable success has been achieved in the determination of the electron number density (ne), electron temperature (Te), the gas kinetic temperature (Tg), and the argon atom number density (nAr), although it has been found that values of Te and Ne in the ICP depend upon plasma configuration and measurement procedure. Another major conclusion emanating from active and passive spectroscopic studies is that the ICP is not in LTE.[38–41] The mechanisms responsible for analyte excitation and ionization can however not be explained fully on the basis of the results from active and passive plasma diagnostic methods.[41,42] Mechanisms that have been proposed include Penning ionization,[43] charge transfer,[44,45] ion–electron radiative recombination,[42,45] collisional excitation,[3,2,36] ambipolar diffusion,[2,3] and shifts in ionization equilibrium.[3] The classical collisional–radiative rate model approach[46–48] takes into account all possible electronic states of the analyte and matrix. The major problem associated with this classical approach lies in the complexity of the models such that arriving at solutions is difficult. Simplified rate models that focus on only one particular electronic level[49,50] and that were designed to simulate the interference effects of easily ionized elements (EIEs) on analyte signal in the ICP show that simulation of EIE interference effects is only possible if Ar species (atoms, ions or electrons resulting from the ionization of Ar) are not involved in the collisional processes leading to the observed interference effects. This noninvolvement of Ar species cannot be explained fully as long as all the electrons in the plasma are regarded as being equivalent in accordance with the LTE theory. Upon ionization, electrons are released with kinetic energy equal to the ionization potential of the respective atom. The LTE theory assumes that M. F. Zaranyika et al.

the electrons quickly lose this energy through thermal collisions so that at thermal equilibrium all the electrons will have kinetic energy equal to kT. If it is assumed that electronic collisions with heavy particles can occur before or after thermal equilibration, then the electron should experience different activation energies depending on whether collisions leading to the observed interference effects occurred before or after thermal equilibration. As discussed above, most studies of interference effects have focused on probing the changes in electron number densities and changes in signal enhancement=depression effects as a function of concentration of the interferent.[2–22,25–40] No studies have been carried out in order to characterize interference effects in terms of activation energies involved in the interactions. In this paper we report the results of a study we carried out to characterize (1) the interference effects of excess Ca and Sr on Mg line absorbance in the air– acetylene flame and (2) the interference effects of Li on CaI, CaII, and MgII line emission intensity in the ICP, in terms of the activation energies involved when the determination is done in the absence and presence of the interferent, assuming steady state kinetics in the plasma and the ionization limit ion– electron complex as the activated transition state complex. The study was carried out by measuring analyte-line absorbance or emission intensity ratios as a function of analyte concentration as described previously.[49–51]

MATERIALS AND METHODS ICP Equipment A Unicam 701-Emission Inductively Coupled Argon Plasma Echelle Spectrophotometer (Unicam Ltd, Cambridge, UK) with ‘‘crossed’’ dispersion was used. The spectrophotometer was fitted with an aperture plate of 90 mm by 1.5 nm slits etched at 2 mm intervals; a photomultiplier tube (PMT) detector mounted on a movable frame for radial view of the ICP; a torch with three concentric tubes for outer gas, auxiliary gas, and sample transport; a 40.68 MHz R.F. generator that supplied power up to 2 kW; a grid-type nebulizer fitted with Pt screens and Pt orifice; a 5-channel computer controlled peristaltic pump; and an automatic Ar gas flow rate optimizer. 2

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The argon (99.95%) was supplied from a pressurized tank (BOC GAS Ltd, Zimbabwe): The outer gas was supplied at 14 L min
1, power 1.0 kW, nebulizer pressure 40 psi (280 kPa), and auxiliary gas at 1 L min
1. Under these conditions, the excitation temperature is between 7000 and 9000 K.[52] Calcium line emission signals were recorded at 393.3 nm. The instrument was optimized using a 100 mg=L solution of Mn. The strong emission of Mn line was used to find the optimum zone for analysis. The maximum temperature of 9000 K was used in the calculations for maximum effect of the interferent.

Flame Equipment Flame experiments were run using a Shimadzu AA
6701 Flame Atomic Absorption=Emission Spectrometer (Shimadzu, Japan) fitted with a highresolution Czerny-Turner monochromator, automatic baseline drift correction using electrical double beam signal processing, and an air-cooled premixtype 100 mm single slot burner with a stainless steel head, Pt-Ir capillary nebulizer with Teflon orifice, glass impact bead, and polypropylene chamber. The spectrometer was coupled to an ASC-6100 Shimadzu Auto Sampler. The air was supplied by an Atlas Copco air compressor (ETS SESCA, France) at 350 KPa input pressure, while the fuel gas, acetylene, was supplied from a pressurized tank (Oxyco Zimbabwe, Harare) at 1000 KPa. The spectrophotometer was fitted with an automatic fuel-gas flow-rate optimizer for each element to be measured. The optimum air–acetylene gas flow rate for Mg was 1.8 L=min. Under these conditions, the temperature of the flame is 2300 C (Shimadzu AA
6701 user manual). Other instrumental settings employed were as follows: wavelength ¼ 285.2 nm for Mg, slit width ¼ 0.1 nm, burner height ¼ 7 mm, burner angle ¼ 0 degrees, secondary acetylene gas pressure ¼ 90 KPa, prespray time ¼ 3 s, integration time ¼ 5 s, and response time ¼ 1 s.

Materials The following were used: calcium chloride dehydrate, M ¼ 147.02 g=mole, AR grade (impurities present: sulphate 0.005%, total nitrogen 0.005%, phosphorous 0.001%, lead 0.001%, iron 0.0005%, magnesium 0.01%, sodium 0.01%, and potassium 0.01%); magnesium chloride hexahydrate, 3

M ¼ 203.30 g=mol., AR grade (impurities: free acid 0.001%, magnesium oxide 0.0005%, nitrogen compounds 0.0002%, arsenic 0.0001%, barium 0.0002%, calcium 0.00055%, potassium 0.005%, sodium 0.0055%, and zinc 0.0025%); strontium chloride hexahydrate, M ¼ 266.62 g=mol., AR grade (impurities: water insoluble matter 0.005%, sulphate 0.001%, lead 0.0002%, iron 0.0001%, zinc 0.0001%, barium 0.02%, and substances not precipitated by sulfuric acid 0.0002%); lithium chloride, M ¼ 42.39 g=mol; Merck GR grade (impurities: sulphate 0.005%, total nitrogen 0.001%, heavy metals [as lead] 0.0005%, iron 0.0005%, calcium [Ca] 0.005%, magnesium [Mg] 0.005%, sodium [Na] 0.002%, and potassium [K] 0.02%) (Merck, Darmstadt, Germany); deionized water of conductivity 0.001 mSm
1.

Procedure: ICP Experiments The experimental procedure adopted was described previously.[49,50] Two sets of standard solutions containing 0–30.0 mg=L analyte (Mg or Ca) were prepared from freshly prepared solutions of their chloride salt solutions. One set was spiked with 1000 mg=L of interferent (Li) also prepared from the chloride salt. The other set was left unspiked. The interferent concentration was kept constant at a very high level (1000 mg=L) relative to that of the analyte, while the analyte concentration was varied, in order to minimize changes in the physical properties of the test solution upon the introduction of the interferent. Under these conditions any effect due to the changes in the physical properties of the test solution in going from the interferent-free solution to the interferent-spiked solution would affect the series of interferent-spiked solutions to the same extent, and this can be compensated for by taking blank readings of a solution containing the interferent salt only. The analyte-line emission intensity readings, I and I0 , where the prime denotes presence of the interferent, were made in triplicate. I0 =I ratios were calculated and corrected for the contribution from the contamination from the respective interferent. Typical I, I0 , and I0 =I results obtained for Mg as analyte and Li as interferent are shown in Table 1. The I 0 =I ratios calculated for the other systems studied are shown in Table 1. Preliminary experiments were run to determine the aspiration rate and the nebulization efficiency Interference Effects of Easily Ionizable Elements in ICP-AES

TABLE 1 I, I0 and I0 /I Values: Effect of Excess Li Interferent on MgII, CaII and CaI Lines

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[M] (mg=L) 0.1 0.2 0.4 0.6 0.8 1.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 20.0 25.0 30.0 

I(MgII)

I0 (MgII)

I0 =I(MgII)

3273  36 12294  41 22150  62 40521  77 61776  67 83303  121 100318  253 191024  189

1346  23 7552  45 15808  78 32533  76 53742  78 73857  130 91115  245 176564  197

1.621 1.302 1.245 1.15 1.082 1.074 1.074 1.069

I0 =I(CaII)

I0 =I(CaI)

2.040 2.080 1.828 1.440

0.924 0.995

7.776 1.671 1.126 1.252 1.152 0.946 0.938 0.932 1.173 1.095

0.985 0.988

823121  1778

779988  1578

1.051 0.917

1.155

1445334  1567 1726821  1187

1373761  1623 1695955  1216

1.049 1.015

0.868

2300465  1589

2268479  1498

1.012

0.829

1.018 1.008 0.998

Corrected for contamination by the Li interferent. kMgII ¼ 279.6 nm; kCaII ¼ 393.6 nm; kCaI ¼ 422.67 nm.



for the type of solutions under analysis.[49,50] Mean values obtained for the aspiration rate and nebulization efficiency were 1.00  0.04 and 5.0  0.7% (n ¼ 8) respectively.

Procedure: Flame Experiments Absorbance (A) readings were made for the sets of solutions of Mg spiked with excess Ca and Sr, as well as the unspiked set, using distilled water as blank. The readings for the spiked sets of Mg solutions were then adjusted for blank readings of the solution containing the interferent only. The A0 =A ratios calculated and corrected for the contribution from the contamination from the respective interferent are shown in Table 2. Preliminary experiments were run to determine the aspiration rate and nebulization efficiency for the type of solutions under analysis. A mean aspiration rate (n ¼ 8) of 3.00  0.06 mL=min and a mean nebulization efficiency (n ¼ 8) of 6.3  1.7% were obtained.

Theoretical Calculations ICP experiments. Number densities were calculated assuming the aforementioned aspiration rate and nebulization efficiency values and 9000 K temperature for the ICP experiments. Number densities for MII assumed 93.00, 98.90, and 99.40% degree of M. F. Zaranyika et al.

ionization,[46] while those for MIII assumed 0.174, 6.43, and 19.72% degree of ionization.[53] The degree of ionization for Li and K was based on the Saha equation:[54] log

Nrþ1 3 2Urþ1 Ne ¼ vr;rþ1 H
log H þ 20:936 þ log 2 Nr Ur

where Nr and Nrþ1 are the number densities of atoms in the r and r þ 1 stages of ionization, Ne is the TABLE 2 A0 /A Values: Effect of Excess Ca and Sr Interferent on MgI Line Absorbance in the Air–Acetylene Flame [M] (mg=L) 0.2 0.4 0.6 0.8 1.0 2.0 4.0 6.0 8.0 10.0 14.0 20.0 25.0 30.0 

A0 =A(MgI=Ca)

A0 =A(MgI=Ca) (Corrected)

A0 =A(MgI=Sr)

2.220 1.847 1.560 1.554 1.544 1.382 1.152 1.068 1.044 1.034 1.032 1.031 1.022 1.027

1.480 1.478 1.337 1.381 1.404 1.316 1.124 1.050 1.031 1.024 1.025 1.026 1.018 1.024

2.350 2.190 2.142 1.844 1.839 1.599 1.444 1.265 1.130 1.083 1.006 1.002 1.013 1.009

kMgI ¼ 285.2 nm. Corrected for contamination by the Ca interferent.



4

TABLE 3 Number Densities Calculated for Ca, Mg, Li, and Ar Species in the ICP Experiments M Ca Mg Li Ar 

nMI

nMII (Saha)

nMII

nMIII

2.3419e þ 10(c) 3.8605  1010c 1.3527  1014 8.3803  1017

2.3419e þ 10(c) 3.8605  1010c 1.3527  1014 2.2596  1015

2.3161e þ 10(c) 3.5903e þ 10(c)

1.4893e þ 9(c) 6.2471e þ 7(c)

Mþ=M ¼ 98.90%(Ca) and 93.00% (Mg), experimental values.[19] M2þ=Mþ ¼ 6.43%(Ca) and 0.174%(Mg), theoretical value based on thermodynamic simulation at 9000 K.[53]

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TABLE 4 Ground State Atom Number Density (no and nmo) and Electron Number Densities (ne and Dne), Resulting from the Atomization and Ionization of the Analyte (Mg) and Interferents (Ca and Sr) in the Air–Acetylene Flame M

no (cm
3 s
1)

nmo (cm
3 s
1)

ne (cm
3 s
1)a

Dne (cm
3 s
1)a

Mg 2.5154  1012c 1.8252  109c1=2 14 Ca 2.5431  10 5.6225  1011 14 Sr 8.3084  10 2.5694  1012 a Based on Saha relationship. M ¼ element; a ¼ degree of ionization assuming 2573 K flame temperature; c ¼ analyte concentration (ppm) in the test solution.

number density of electrons, vr, rþ1 is the ionization potential in eV from the r stage to the r þ 1 stage of ionization, H ¼ 5040 K=T (T ¼ plasma temperature), Ur and Urþ1 are the partition coefficients, and factor 2 represents the statistical weight of an electron. Number densities obtained are shown is Table 3.

atomization interference effects between Group I elements.[51] Absorbance signal enhancement is attributed to suppression of ionization, which is equivalent to collisional ion–electron radiative recombination. The following major processes affecting analyte ground state population in the flame are represented schematically in Fig. 1: thermal dissociation of the analyte salt AX2 to give the analyte metal atom Ao and counter atom X (rate constant ¼ kD); recombination of A and X (rate constant ¼ kR); excitation of analyte atom from the ground state, Ao, to the first excited state Au (rate constant ¼ kD); excitation from the first excited state ðrate constant ¼ k0D Þ through higher excited states to the ionized state; collisional recombination between Aþ and electrons to give the

Flame experiments. Ground state number densities were calculated assuming an aspiration rate of 3 mL=min and 6% nebulization efficiency as measured above and a temperature of 2573 K for the air–acetylene flame as noted above. Ionic number densities were calculated on the basis of the Saha relationship.[54] Data obtained are shown in Table 4, in the column called ‘‘no (cm
3 s
1).’’

RESULTS AND DISCUSSION Effect of Excess Ca and Sr on Mg Line Absorbance The experimental A0 =A data in Table 2 show a sharp increase in line absorbance signal enhancement with decrease in the concentration of the analyte in the test solution below about 10 mg=L. Similar results were reported previously in a study of mutual 5

FIGURE 1 Flame AAS proposed kinetic scheme: The subscripts AX2, o, u, and þ denote analyte salt, ground state, excited state, and ion respectively; M denotes interferant; kD, kD, k0D Þ denote rate constants for thermal dissociation, excitation from the ground state, and excitation from the excited state, respectively; kCT and k0CT Þ denote rate constants for collisional charge transfer involving interferant atoms and ions, respectively; kc(o) and kc(1) denote rate constants for collisional radiative recombination to the ground state and excited state, respectively; and khv and kR denote rate constants for radiative relaxation and analyte atom and counteratom recombination, respectively. Interference Effects of Easily Ionizable Elements in ICP-AES

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ground state atom (rate constant ¼ kc(o)) and the excited state (rate constant ¼ kc(1)); radiative relaxation from the first excited state with the loss of a photon (rate constant ¼ khv); and charge transfer between analyte ion and interferent atom (rate constant ¼ kCT) and between analyte atom and interferent ion ðrate constant ¼ k0CT Þ. Because of the low temperature in flame AAS, number densities of analyte ion excited states are very low and are neglected in Fig. 1. Assuming a steady state with respect to the analyte ground state and the excited state, dnA ¼ kD nAX2 þ kcðoÞ nu n0e þ kCT nþ nMo þ khn n0u dt ð1Þ 0
k/ n0o
kCT no nMþ
kR n0o nX ¼ 0 dnu ¼ k/ n0o
khn n0u
kD0 n0u ¼ 0 dt

ð2Þ

where n denotes number density, and the subscripts AX2, X, Mo, Mþ, denote analyte salt, counter atom, interferent metal ground state atom, interferent metal ion, respectively; no, nu, and nþ denote analyte metal ground state, excited state, and ion number densities respectively, and the prime denotes values in the presence of the interferent; kD, kD, k0D denote rate constants for thermal dissociation, excitation from the ground state, and excitation from the excited state respectively; kCT and k0CT denote rate constants for collisional charge transfer involving interferent atoms and ions respectively; kc(o) and kc(1) denote rate constants for collisional radiative recombination to the ground state and excited state respectively; and khv, kR and kU denote rate constants for radiative relaxation, analyte-atom and counteratom recombination, and analyte radiation absorption respectively. Rearranging Eq. (2), we have khn n0u ¼




 khn k/ n0o khn þ kD0

ð3Þ


k/0 ¼ k/ 1


khn khn þ kD0

 ð6Þ

Therefore   kD nAX2 þ kCRðoÞ nþ n0e þ kCT nþ nmo n0o ¼ no kD nAX2 þ kCRðoÞ nþ ne ! k/0 þ kR nX  0 n 0 k/0 þ kCT mþ þ kR nX

ð7Þ

Two major limiting cases can be defined for Eq. (7): Limiting Case I (LC I). kD nAX2  kCRðoÞ nþ n0e þ kCT nþ nmo k/0 þ kR nX n0o ¼ 0 0 n 0 no k/ þ kCT mþ þ kR nX

ð8Þ

Since kR n0X  kR nX ; n0o =no will be less than unity, that is, a depression of absorbance signal is expected in this case, contrary to the experimental results in Table 2. Although the present work reports analyteline absorption-signal enhancement, analyte-line absorption-signal depression has been reported by several workers.[55] Analyte-line absorption-signal depression was also reported previously at low flame temperature when 0.2–1.0 mg=L K solutions were determined in the presence of 1000 mg=L Na in the secondary reaction zones of the air–acetylene flame.[51] Limiting Case II (LC II). kD nAX2  kCRðoÞ nþ ne   kCRð0Þ nþ n0e þ kCT nþ nmo n0o ¼ no kCRðoÞ nþ ne ! k/0 þ kR nX  0 n 0 k/0 þ kCT mþ þ kR nX

ð9Þ

Two further limiting cases can be defined for Eq. (9): 0 Limiting Case IIA (LC IIA). k/0  kCT nmþ þ kR n0X

Substituting into Eq. (1) and rearranging, n0o ¼

where

kD nAX2 þ kCRðoÞ nþ n0e þ kCT nþ nmo   khn 0 n 0 þk kCT þ k n 1
mþ R X / khn þk0

ð4Þ

n0o kCRð0Þ nþ n0e þ kCT nþ nmo ¼ no kCRðoÞ nþ ne

ð5Þ

Since n0e ¼ ne þ Dne ¼ nþ þ anmo , where Dne represents the change in the electron number density in the presence of the interferent, and a is the degree

ð10Þ

D

kD nAX2 þ kCRðoÞ nþ n0e þ kCT nþ nmo n0o ¼ 0 n 0 0 kCT mþ þ kR nX þ k/ M. F. Zaranyika et al.

6

of ionization of the interferent, Eq. (10) becomes
 akCRðoÞ þ kCT nmo n0o ¼1þ ð11Þ no kCRðoÞ nþ Or

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n0o knmo ¼1þ no nþ

ð12Þ

that is, absorbance signal enhancement is directly proportional to interferent number density and inversely proportional to analyte number density. In addition, if we assume that A0 =A ¼ n0o =no , a plot of A0 =A versus nmo=nþ should be linear with an intercept of unity. At constant interferent concentration in the test solution, a plot of A0 =A versus 1=nþ should also be linear with an intercept of unity. Table 5 shows the slope, intercept, and R2 values for the regression plots of A0 =A versus nmo=nþ obtained when the enhancement factor A0 =A for Mg is determined in the presence and absence of Ca and Sr as interferents. It is apparent from the data in Table 5 that the obtained intercept values of 1.09 and 1.22 are close to unity, in close agreement with Eq. (12). Limiting Case IIB (LC IIB). k/0  kR nX It can be shown that

that is, although the absorbance signal enhancement is still dependent on interferent number density and inversely proportional to analyte number density, the enhancement observed will be reduced by a factor of at least nX =n0X . In addition, if we assume that A0 =A ¼ n0o =no , a plot of A0 =A versus nmo=nþ should give an intercept that is less than unity and equal to at least nX =n0X , contrary to experimental data in Table 5. These data suggest that the signal enhance-

TABLE 5 Regression Data: A0 /A vs. nmo /nþ for Mg

7

Slope

Intercept

R2

2.34  10
4 3.38  10
3

1.08714 1.21649

0.909 0.732

ð14Þ

where Dne is the change in electron number density upon the addition of the interferent, that is, Dne ¼ nmþ. If the experiment is designed so that ne