Signal enhancement effect of halogen matrix in electrothermal vaporization-inductively coupled plasma-mass spectrometry

SPECTROCHIMICA ACTA PART B

SpectrochimicaActa Part B 51 (1996) 1551-1565

ELSEVIER

Signal enhancement effect of halogen matrix in electrothermal vaporization-inductively coupled plasma-mass spectrometry Naoko Nonose*, Naoki Matsuda, Noriko Fudagawa, Masaaki Kubota National Institute of Materials and ChemicalResearch, 1-1, Higashi, Tsukuba, lbaraki 305, Japan

Received 24 April 1995; accepted4 March 1996

Abstract

In tungsten furnace electrothermal vaporization(ETV)-inductively coupled plasma mass spectrometry(ICP-MS), the presence of halogen matrices caused a signal enhancement for volatile elements such as Zn, Cd and Pb, whose halides melting and boiling points were relatively low. In order to clarify the mechanism of signal enhancement in ETV-ICP-MS, the effects of chemical interaction between analytes and halogen matrices on the surface of ETV furnace, the transport efficiency of vaporized analytes from the furnace into the ICP and the physical properties of the ICP itself and of the micro plasma (interface plasma) in the interface region between the sampling and the skimmer cones were investigated by atomic absorption and atomic emission spectrometry. Among the effects mentioned above, neither the chemical interaction on the surface of the ETV furnace nor the transport efficiency of vaporized analytes could be related to the analyte signal enhancements. The degree of enhancement was found to depend on the ionization potential of the coexisting halogen and was not caused by a variation in the physical properties of the ICP but rather by a variation of those of the interface plasma. These results suggest that the halogen matrices may affect the physical properties of the interface plasma, contributing to the promotion of the ionization of analytes. Keywords: Electrothermal vaporization; Halogen matrix; Interface plasma; Ionization equilibrium; Signal enhancement

1. I n t r o d u c t i o n

Inductively coupled plasma mass spectrometry (ICP-MS) has been widely applied to ultratrace analysis. The success of this technique is often related to the method of sample introduction into the ICP. Conventional nebulization (NEB) has been typically used because of it's rapidity and good stability. However, there are some negative aspects such as poor transport efficiency, requirement several milliliter of sample volume and severe spectroscopic interference due to the solvent. Increasing attention has, therefore, been devoted to * Corresponding author.

electrothermal vaporization (ETV) as an alternative sampling technique, overcoming the drawbacks encountered in NEB-ICP-MS. The effect of the ETV method on analytical performance was first described by Gray and Date[l], who demonstrated that the formation of polyatomic ion species due to the solvent and the plasma gas was considerably suppressed with the use of ETV because the solvent could be completely eliminated prior to the vaporization stage. Since the first publication by Gray and Date, many researchers have applied ETV-ICP-MS to the analyses of trace elements in geological[2,3], industrial[4,5], biological[6,7] and environmental samples[8]. Most of them have reported that the analyte signals were slightly enhanced or suppressed in the

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N. Nonose et al. / Spectrochimica Acta Part B 51 (1996) 1551-1565

presence of matrix constituents. Matrix effects in ETV-ICP-MS were first discussed by Park and Hall[9], who found that the transient signal for TI was enhanced by Pb and Cd matrices, whereas it was suppressed by the Na matrix. Gregoire[3], who measured Os isotope ratios, demonstrated that the signal for Os was enhanced 5- to 20-fold by various matrices such as Ni, Se and Te. In our previous work on the determination of metal impurities in potassium hydrogenphthalate used as a reference material for volumetric analysis, trace signals for Pb, Co and Ba were suppressed by a 104 excess of matrix [10]. Most ETV-ICP-MS researchers reporting severe matrix effects supposed that the signal enhancement resulted from less "transport losses" of analytes vaporized from the ETV surface into the ICP. The concept of "transport losses" with the ETV device was often discussed in the field of ETV-ICP-atomic emission spectrometry(AES) [11-13]. On the contrary few workers attempted to interpret the mechanism for signal enhancement in ETV-ICP-MS. A recent paper by Ediger and Beres [14] on the effect of various matrices on the transport efficiency of analytes reported that the vaporized matrix present in excess of 105 times of the analyte amount tends to condense into particles more rapidly than the analytes [15]. The vaporized matrix thus prevents the analyte vapor from condensing onto the graphite furnace in the ETV chamber and, as a result, the analyte transport efficiency is improved. This mechanism was supported by Gregoire et al. [16], who investigated the dependence of"transport loss" on the amount of Mg/ Pd modifier, though they pointed out a chemical interaction between the analytes and the ETV surface was also important in the vaporization process. Also, under the condition that solvent loading was completely reduced, matrix constituents may affect the physical properties of both the ICP and the micro plasma (interface plasma) formed at the interface region between the sampling and the skimmer cones. Thus, it is plausible that changes in the electron number density (ne) and in the ICP and/or interface plasma temperature cause an improved ionization degree of the analytes and a resulting signal enhancement. In this study, for the precise determination of metal impurities in potassium iodate (KIO3) which is used as a reference material, the matrix-dependent sensitivity

of trace analytes in ETV-ICP-MS was compared with that in NEB-ICP-MS. Matrix effects on the transport efficiency of the analytes vaporized from the ETV surface to the ICP and on their degree of ionization in both the ICP and the interface plasma were investigated by atomic emission and atomic absorption spectrometry (AAS). A possible interpretation of the mechanism of signal enhancement in ETV-ICP-MS is then attempted.

2. Experimental 2.1. A p p a r a t u s

The ICP-MS and ICP-AES instruments with an ETV device as a sample introduction system are an SPQ 8000A and an SPS 1200 (Seiko Instrument Inc., Japan), respectively. The AAS instrument used for the examination of the chemical interaction on the ETV furnace is an SAS 760 (Seiko Instrument Inc.). The ICP-MS system employs a shielded ICP to eliminate the secondary discharge at the tip of sampling cone. The shielded ICP was used through this study. Each ETV device in ICP-MS and ICP-AES consists of a tungsten ribbon furnace with a 50 #1 capacity hole, which is encased in a 20 ml Pyrex glass chamber. The high-melting point metal furnace offers several advantages over a conventional graphite furnace: longer life time (more than 500 repetitive hearings are possible); less memory effect due to chemical interaction between the analytes and the furnace material; no need to use matrix modifier for the analysis of Pb and Cd; and wider dynamic range of the calibration curve. It should be noted that the experimental results observed in this study are valid for the tungsten furnace employed. To interpret whether a similar behaviour could be observed with a graphite furnace, further study is required. The furnace is resistively heated up to 2700°C. A small amount of hydrogen gas was mixed with the argon carrier gas in order to suppress oxidation of the furnace. Typical operating conditions for the ICP and the ETV devices are listed in Table 1. Each mode of sample introduction, NEB and ETV, required different optimization settings of the ion lens. In this study, the settings for NEB were optimized for Be, Co, Ba and Pb by continuous solution nebulization,

N. Nonose et al. / Spectrochiraica Acta Part B 51 (1996) 1551-1565 Table 1 Typical operating conditions for instruments

1CP R.F. power Reflected power Cartier gas flow rate Outer gas flow rate Intermediate gas flow rate Temperature of spray chamber

1.2 kW < 5W 0.9 1 min -1 15 1 min -~ 1.3 1 min -~ 10*C

Mass spectrometer

Signal measurement

15 mm Optimized for 10 ng m1-1 Be, Co, Ba and Pb in NEB-ICP-MS and for Fig in ETV-ICP-MS 150 2.6 kV Dwell time 10 ms

Heating temperature Cooling temperature

130°C 0°C

Sampling position" Ion lens setting

Resolution/(M/A M) Voltage applied to ehanneltron

Desolvator

Electrothermal vaporizer Carrier gas flow rate Ar gas

H2 gas

0.9 i min -1 in ETV-ICP-MS and ETV-ICP-AES 5.0 I min -1 in AAS 20 ml rain -I in ETV-ICP-MS and ETV-ICP-US 1.0 1 min -1 in AAS

Heating cycle Drying Ashing Vaporizing

60 s at 100°C 30 s at 100°C 6 s at 2400°C for Zn and Pb at 21000C for Cd

the ion count. Hereafter, "signal" in ETV-ICP-MS means the integrated ion count. A system for observing the interface plasma was made with an optical fiber cable (1.5 mm in diameter, 2.0 m long, 40 fibers bundled) inserted into the interface region between the sampling and skimmer cones. This system enabled us to observe emission spectra of the H a line and alkali earth metals, which were used for the calculation of electron number density (ne) and ionization of temperature (Tio,), respectively. The details of this system were presented previously [17]. The fibre cable was connected to a spectrometer (Japan Spectroscopic Co. Ltd., CT-50C, 50 cm Czerny-Turner with a 2400 groove mm -t grating). The optical fibre was also used to obtain the same physical properties of the ICP, which were then compared with those of the interface plasma. For the measurement of the ICP properties, the ICP was moved away from the interface (sampling position 110 ram) and an observation height was fixed at 15 mm from the load coil. We note, however, that the plasma observed in this case is different from that which is sampled, but this was necessary in order to gain the maximum emission intensity for the analytes. The theory of Saha-Eggert ionization equilibrium was applied to the determination of Tio. values on the assumption that the thermal equilibrium is reached in both plasmas. The expression for the equilibrium[18] is given by:

• Distance between sampling orifice and top of load coil.

ne

and those for ETV were optimized for Hg by introducing HgI2 gas mixed with the Ar carrier gas to provide maximum signals.

2.2. Measurement procedure The details of the measurement procedure were described in a previous paper[5]. A 20 #1 aliquot of sample solution was dropped in the furnace. Samples vaporized during the vaporization cycle were carried into the ICP by a stream of the Ar-H2 carrier gas. The signal measuring sequence starts at the beginning of the vaporization cycle. Transient signal profiles were recorded as ion count versus time curves. At the end of the cycle, peak areas were calculated by integrating

1553

=

2(21rmkTion)3/2 I(I) g(H) A(H) k(I) h3 I(II) g(I)A(II) )~(II) x exp

(E(I)-E(II)-Va) kT

(1)

where (I) and (II) refer to the neutral atom and the singly charged ion species, respectively, I is the emission intensity, g the statistical weight of the emitting level, A the transition probability, X the emission wavelength, E the excitation energy, Va the ionization energy, m the electron mass, k the Boltzmann constant and h the Planck constant. In Eq. (1), ne is an experimental value derived from the Ha Stark broadening profile. Details of the measurement method were given elsewhere [18-20]. For atom/ion emission line pairs, Mg I 285.213 nm and Mg II 279.553 nm were chosen.

N. Nonose et al. / Spectrochimica Acta Part B 51 (1996) 1551-1565

1554 18 16

1.2 ~

ETV-ICP-MS

NEB-ICP-MS

14

"~

Jz 0.8 f

10

.~ N Z

11.6

8 /

/

11.4

Z

4

11.2 tl

KIO 3 , /z g g Fig. 1. Matrix effect of

KIO 3 on

0

........

m~

KIO 3,

3.1. Comparison of matrix effects in ETV-ICP-MS and NEB-ICP-MS In order to characterize the difference between the matrix effect observed in ETV-ICP-MS with that in the case of NEB-ICP-MS, the behaviour of trace analyte signals on the KIO3 concentration was investigated for both methods. Fig. 1 compares the signal behaviour for 100 ng m1-1 of Zn, Cd and Pb in ETV-ICP-MS and NEB-ICP-MS in the presence of KIO3 as matrix constituent. The concentration of KIO3 varied in the range from 1 to 100 #g m1-1. Zinc, Cd and Pb were selected as test elements because these elements are easily vaporized from the furnace at relatively low temperatures. In ETV-ICP-MS, one of the most important parameters affecting the signal is the vaporization temperature of the furnace material [5,21]. The effect of the vaporization temperature on the signals was examined with matrix-containing and matrix-free solutions. With both solutions, the temperatures giving maximum signals for Zn, Cd and Pb were 2400, 2100 and 2400°C, respectively. In NEB-ICP-MS, the addition of the matrix in a range

b.p./°C

100

/zggl

exceeding 1000 times the amount of analyte caused no remarkable change in the signal levels. In the ETV method, however, KIO3 matrix caused a drastic enhancement of the signals, except for Pb. The different signal behaviour between Pb and the other two elements may be due to their different volatility. The melting points (m.p.) and boiling points (b.p.) of three elements and their iodides are collected in

4

100

Zn+.

2 I I

× 105 4

Cd +

-

1

x 106

Table 2 Volatility characteristic of Zn, Cd and Pb

Metal Iodide Metal Iodide

........

Zn, Cd and Pb signals in ETV-ICP-MS and NEB-ICP-MS.(e)Zn ÷, (A) Cd ÷ and ( 1 ) Pb ÷.

3. Results and discussion

m.p./°C

I 10

100

I0

!

Zn

Cd

Pb

419 446 906 625

321 385 767 713

327 402 1755 954

I 0.5

I 1.0

! .5

2.0

2.5

3.0

Time, s

Fig. 2. Signal profiles ofZn, Cd and Pb in the presence of 1, 10 and 100 g g-i KIO3 in ETV-ICP-MS.Heating temperature: 2400°C for Zn and Pb 2100°C for Cd.

N. Nonose et al. / Spectrochimica Acta Part B 51 (1996) 1551-1565

and v.p. values are considerably higher (3382 and 5527°C) than those of the elements tested. Such signal enhancements for Zn and Cd were not observed for other elements (Al, Cr, Mn, Fe, Co, Ni and Ba) whose b.p. values are relatively high (-2000°C), irrespective of the volatility characteristics of the iodide matrix.

× 1o 6

3.2. At which stage signal enhancement occurs in ETV-ICP-MS ?

× 106 8

1555

W+

T=2400

i6 ~

4 2 0.5

1.0

1.5

2.0

2.5

3.0

Time, s Fig. 3. Signal profiles o f I and W in E T V - I C P - M S .

Table 2. Although no clear difference in m.p. values is seen among these elements, a large difference in b.p. values between Pb and the other elements exists: the signal behaviour seems then to depend on the b.p. values. To illustrate more clearly the volatility characteristics of the different analytes, transient signal profiles were obtained for the analyte elements and compared with those of the matrix constituent and of the vaporized tungsten. Fig. 2 shows such profiles for 100 ng g-1 of the analytes in the presence of different concentrations of the KIO3 matrix (1, 10 and 100 /~g g-l). Signal profiles f o r / , arising from the KIO3 matrix, and for W, at heating temperatures of 2100 and 2400°C are reported in Fig. 3. Although the optimized heating temperatures differ for the different analytes, the signal profiles present almost the same peak appearance time of 1.0 s. The peak time, observed between 1.2-1.5 s, was slightly dependent on the concentration of the KIO3 matrix. On the contrary, the KIO3 matrix and the vaporized tungsten showed a quite different signal behaviour. Two peaks appeared in the iodine signal profile, the first peak appearing earlier and the second later in time with respect to the peak times of the analytes. In addition, the behaviour of the second peak was similar to that observed for W. These results, presented in Figs 2 and 3, indicate that the elements vaporizing easily from the furnace at a relatively low temperature are not affected by the vaporization of tungsten but rather by that of the matrix constituent. The delayed signal observed for Wis likely due to the fact that its the m.p.

3.2.1. Matrix effect in AAS AAS measurements with the tungsten furnace were performed in order to examine a possible relationship between the signal enhancement observed in ETVICP-MS and the vaporization (atomization) efficiency of analytes from the furnace. The furnace material and the operating conditions employed in the AAS study are almost the same as those in ETV-ICP-MS, except for the ratio of hydrogen to argon carrier gas flow rates. As shown in Table 1, the ratio of hydrogen to argon flow rates introduced into the ETV chamber in AAS was about 0.2, five times more than that in ETV-ICP-MS, because the AAS system uses an open-type ETV chamber. In Fig. 4 absorbance values for Zn are plotted against the hydrogen flow rate with and without the KIO3

1.1 ~atrix

gO

0.9

A~ t.. O

-~

0/z g g-t

Matrix 100/z g g-t 0.8

0.7

0.6

,

0.4

I

0.6

,

i

0.8

t

I

1

,

t

1.2

,

I

i

1.4

I

1.6

t

1.8

H 2 flow rate, ! rain "I Fig. 4. Dependence o f Z n atomic absorbance on the h y d r o g e n flow

rate addedto the argoncarrier gas and obtainedin the presenceof the matrix and with matrix-freesolutions.(Q)KIO3concentration 0 # g g-t(K)KIO3concentration100 tzg g-1.

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N. Nonose et al. / Spectrochimica Acta Part B 51 (1996) 1551-1565 to

I

1.z I

ICP

Furnace (A)

e" t._ @

0.8

0.6 7 t_ @

Ar + H2

0.4

~b

Z

Fig. 6. Schematic diagram of the ETV chamber with two tungsten furnaces.

0.2 O

I

1

I

I

I

I I I I I

I

I

10

K I O 3 , /2 g g

I

I

J nl

100 -1

Fig. 5. Matrix effect of KIO3 on Zn, Cd and Pb atomic absorbances in AAS. (O) Zn, (A) CA and ( I ) Pb.

matrix, over the range 8 to 36% to argon flow rate. In both cases, absorbances gradually decreased by increasing the hydrogen flow rate. The decreasing effect may be due to fact that the hydrogen addition caused a reduction of the furnace temperature. However, the ratio of the absorbance with matrix to that without matrix remained almost constant, irrespective of the hydrogen flow rate. Under the experimental conditions given in Table 1, the effect of the KIO3 matrix on the absorbance was examined. Fig. 5 presents the behaviour of the absorbance values for Zn, Cd and Pb when the concentration of the matrix increased in a range 1 to 100 pg g-1. Absorbance values are normalized to those obtained with the matrix-free solutions. The behaviour of the normalized absorbance showed a trend opposite to the matrix effect observed in ETV-ICP-MS (Fig. 1). In fact, as seen in the figure, with increasing matrix concentration, the absorbance decreased slightly for Zn and Cd, and remarkably for Pb. Generally, the decrease in atomic absorbance has been attributed to analyte losses during the ashing stage and/or to vaporphase interference caused by a chemical reaction between the matrix and the analyte during the vaporization stage [22,23]. In this study, the ashing temperature after the drying stage was kept at 100°C to prevent any analyte losses. Thus, it can be argued that

the decrease in Pb absorbance was caused by the formation of gaseous PbI2 based on a reaction between the analyte and the matrix during the vaporization step. For Zn and Cd, the observed change in absorbance was much less significant. This result suggests that the signal enhancement seen in Fig. 1 may not result from a chemical interaction in the ETV furnace but from a vapor-phase interaction between the matrix and the analytes.

3.2.2. Vapor-phase interaction between matrix and analytes In order to ascertain that the vapor-phase interaction predominates over the chemical one in the ETV furnace, the signal enhancement of analytes was studied with an ETV chamber into which two tungsten furnaces (Furnace(A) and (B)) were inserted. The schematic diagram of this chamber is illustrated in Fig. 6. Three kinds of experiments performed are as follows; Furnace(A),(B): 100 ng m1-1 Zn, Cd and Pb in 0.1 M nitric acid. Furnace(A),(B): 100 ng ml-t Zn, CA and Pb in 100 ~g mlq KIO3. Furnace(A): 100ng m1-1 Zn, Cd and Pb in 0.1 M nitric acid, Furnace(B): 100/~g ml-I KIO3.

Experiment 1 Experiment 2 Experiment 3

Table 3 Comparison of on-furnace chemical interaction with vapor-phase interaction

Exp. 2/Exp. 1 Exp. 3/Exp. 1

Zn

Cd

Pb

2.2 2.8

13 13

1.1 1.1

N. Nonose et al. / Spectrochimica Acta Part B 51 (1996) 1551-1565 14

104

12

(A)

10

~

~ 1

0

0

14

1t)4

12

(B)

10

/t g g-t

~ 6

~ 6 Matrix 0 //gg-I

4

Matrix I00 /t g g ~ 7 "

1557

&

,f L

Matrix 0 ,tzgg "1

4

4P

2

2 ,

0 0

I

,

211

I

411

i

I

611

h

I

80

,

0

L

1110

1211

Hz t]owrate, ml rain'1

0

2il

4ll

6{I

8ll

Hz flowrate. ml min-t

Fig. 7. Dependenceof Zn signalson the flow rate of hydrogen added to the argon carriergas with and without matrix.(A):hydrogenwas added before the carder gas passed through the ETV chamber.(B):hydrogenwas added after the carrier gas passagethrough the ETV chamber.(O) KIO3concentration 0/zg g-t(A) KIO3concentration 100 I~gg-1. T h e ratio of the analyte signal obtained in Exp.2 to that obtained in Exp.1 (Exp.2/Exp.1) can be concerned with both effects of chemical and the vaporphase interactions. The ratio of the signal in Exp.3 to that in Exp. 1 (Exp.3/Exp. 1) indicates specifically only the effect of the vapor-phase interaction. The ratios obtained for three elements are presented in Table 3. For Zn and Cd, similar signal enhancements were observed both in Exp.2 and in Exp.3, although, for Zn, there was a slight difference between Exp.2/Exp.1 and Exp.3/Exp.1. This indicates that the chemical interaction on the surface of the furnace was negligibly small compared with the vapor-phase interaction. Gregoire [2], who measured the signal enhancement for Pd, Rh and Pt in the presence of Ni matrix using a graphite furnace, also evaluated chemical interaction on the surface versus vapor-phase interaction in the same way. He concluded that one half of the signal enhancement was due to the surface effect and one half was due to the vapor-phase effect. It seems that the chemical interaction on the surface of the tungsten furnace was considerably smaller than that on the surface of the graphite furnace.

interface plasma which favours the ionization of the analytes. In order to distinguish between the effect of hydrogen on the physical properties of the plasmas and that on oxidation of the furnace, the hydrogen was mixed with the carder gas not before but after this gas passed through the ETV chamber. In this way, only the effect due to the presence of hydrogen in the ICP and/or in the interface plasma can be observed. The results observed for Zn are illustrated in Fig. 7, with hydrogen being added before (A), and after (B) the ETV chamber 1. In both (A) and (B), the presence of the matrix caused signal enhancements of analytes over the range of hydrogen flow rates investigated. In addition, with the matrix solution, both the analyte signal in (A) and in (B) showed a similar increasing trend with an increase in the hydrogen flow rate. Thus, the signal enhancement due to the presence of matrix can be interpreted as resulting mainly from the promotion of analyte ionization in the ICP and/or in the interface plasma by the matrix constituent. It must be noted, however, that an improved transport efficiency due to the presence of the matrix can not be excluded.

3.2.3. Effect of hydrogen addition on signal enhancement T h e ETV technique using a tungsten furnace requires hydrogen addition to the argon carder gas to prevent the furnace from being oxidized. In our previous work[24], it was found that such addition causes an increase of the excitation temperature (Tex) and of the ne values of both the ICP and the

3.2.4. Transport efficiency of vaporized analytes Kantor[15] and Gregoire et al.[16], who reported a non-linear calibration curve for analyte at low analyte 1 In Fig. 7, 1.0 min-t of argon mixedwith 20 ml min-t of hydrogen was used as the carrier gas, because it was impossibleto vaporize tungsten under a hydrogen-freecondition. The flow rate of hydrogen added after the carrier ~as passed through the chamber was varied from 0 to 80 ml min-L

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N. Nonose et al. / Spectrochimica Acta Part B 51 (1996) 1551-1565

mass in the absence of matrix, suggested that the use of a chemical modifier can contribute to a reduction of transport losses of analytes, this improving the linearity of the calibration curve. Ediger and Beres[14] mentioned that the design of the ETV chamber was an important factor affecting the transport efficiency. As described in Section 2.1 the experimental section, our study employed a Pyrex glass chamber with a 20 ml enclosure volume so as to eliminate problems due to analyte condensation on its inner surface and to decrease the signal-depressing effect due to the expansion of the carrier gas occurring just after the heating of the furnace. The transport efficiency of vaporized Zn, Cd and Pb from the chamber to the ICP was measured both in the presence of the matrix and with matrix-free solutions. After 20 repetitive vaporization of a sample solution, where total

amounts of vaporized analytes and KIO3 were 40 ng and 40 #g, respectively, the deposition on the inner surface of the ETV chamber and the transport tube was completely washed out with 0.1 M nitric acid and collected into a 125 ml PTFE bottle. The final volume was adjusted to 100 ml with water. The concentration of each element in the collected solution was determined by NEB-ICP-MS. The recoveries for Cd and Pb in the presence of matrix were 4% and 14%, respectively. These results were almost the same as those obtained with matrix-free solutions. It is worth noting that, in the case of Zn, it was impossible to perform the experiment because of contamination problems during the collecting procedure. The above results also seem to indicate that the signal enhancement observed in this study was caused not by the matrix as a physical carrier of the analytes Cd

ETV-ICP-AES

16

NEB-ICP-AES

16

14

14

"~ 12 I .[ 1o

•

Zn (I)

•

Zn (II)

10

8 6 L

4

o

Z

2 10

1

i

i

i

i l l l

10

100

10

100

14

/

12

•

Cd (I)

"~ 12

/

..= 10

t,,.

i

16

14

Z

, _ _ ~ 1

100

16

B

2 0

0

N

4

.~ 10

/

8

6

6 4

Z

2 oT 1

t

,

,

,

i l l l l

i

i

10 K I O 3 , /~ g g-1

i

2 ,,

l l l l l

100

KIO 3, / z g g - 1

Fig. 8. Matrixeffectof KIO3on Zn and Cd emissionintensitiesin ETV-ICP-AESand NEB-ICP-AE~.(O)atom line and (A) ion line.

1559

N. Nonose et al. / Spectrochimica Acta Part B 51 (1996) 1551-1565

and Pb, but by the interaction between the matrix and the analytes in the plasmas. 3.2.5. Matrix effect in E T V - I C P - A E S

From the above considerations, it appears that the occurrence of some interaction between the analytes and the matrix in the plasmas can be responsible of the enhancement observed, rather than the occurrence of both the chemical interaction on the ETV furnace and the transport efficiency of analytes. A possible matrix effect on the ionization and excitation efficiencies of analytes in the ICP was therefore investigated in order to elucidate its possible mechanism. Fig. 8 compares the emission intensifies for neutral and ion lines of Zn and CA plotted against the K I O 3 concentration in ETV-ICP-AES with those in NEBICP-AES. ICP-AES measurements were performed for a commercially available ICP-AES instrument without using the fibre optical system, because the sensitivity of the system used in this study was too low to observe emission lines of analytes in the ICP at a concentration level of 100 ng g-1. Although this plasma is not exactly the same as that in ICP-MS, it is assumed here that such emission measurements can help to recognize the difference between the matrix effects in ETV and NEB. In NEB-ICP-AES, emission intensifies were less dependent on the matrix concentration, whereas the ETV method caused an increase in both atom and ion emission intensifies with increasing matrix concentration. Though the emission intensifies were not spatially resolved, the trend observed in the ETV method was similar to that observed in ICP-MS (Fig. 1). Assuming that the transport efficiency of analytes was not affected by the amount of concomitant matrix, the increase in emission intensifies represents a promoted excitation of atom and ion to upper energy levels. For all of the elements tested, however, the ratio of ion to atom emission intensifies tended to decrease gradually with increasing matrix concentration, suggesting a suppression of analyte ionization in the ICP. 3.3. Mechanism of matrix effect

occurred when potassium iodide (KI) was used as matrix. In an attempt to investigate and understand such a large difference between the signal behaviour in NEB-ICP-MS and that in ETV-ICP-MS, the effect of four kinds of halogen matrices (KF, KC1, KBr and KI) and of the solvent (water) on the signal behaviour were examined. Water loading was controlled by using a desolvator(DES), inserted between the Scotttype spray chamber and the quartz torch. The DES device consists of a heated glass tube and a modified Liebig condenser[25]. The operating conditions for the DES device are given in Table 1. 3.3.1. Effect of halogen matrix and water loading on signal enhancement

The influence of 10 -3 M KF, KCI, KBr and KI on 100 ng m1-1 analyte signals for Zn, Cd and Pb in NEB-,DES- and ETV-ICP-MS was investigated. The ratio of the analyte signals obtained in the presence of the matrix to those given by the matrix-free solutions was defined as "signal enhancement factor". The comparison results are presented in Fig. 9 (NEB-ICP-MS) and 10 (DESand ETV-ICP-MS). In NEB-ICP-MS, for all of the elements, the signal enhancement factor was slightly negative, i.e., each matrix caused a slight decrease in the signals of each analyte. In addition, the degree of signal suppression increased when the matrix was changed from KF to KI. The matrix suppression effect in NEB-ICP-MS may due to "space charge effects"[26-29] and/or due to a "mass effect"[30,31] occurring within the ion lens I.I NEB-ICP-MS

1.05

E 0.95

!

•

II

o.s5 I

0.8 KF

In the previous section, the dependence of analyte signals on KIO3 concentration was clearly observed with ETV-ICP-MS. A similar signal enhancement

!

I

KCI

I

KBr

KI

Fig. 9. Dependenceof the signalenhancementfactorfor Zn, CAand Pb on the halogenmatrixin NEB-ICP-MS.(I) Zn, (S) Cd and (4) Pb.

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N. Nonose et al. / SpectrochimicaActa Part B 51 (1996) 1551-1565

~5

Zn + T

DESJ

ETV

=

Ik

I

~

5

I

I

Cd +

E

2 ~t

X

I-

&

I

5

d

I

I

pb +

4

g3 2

•~

I

•

° ---~

0

...........

~

J

KF

•

...........

~

t

KCI

i

.................

t

KBr

KC1 and KBr. From the results of Fig. 10, it is fair to note that the signal enhancement factor in the case of DES and ETV operations were not very pronounced, except in the case of Zn and Cd with the KI matrix. The fact that the signal behaviour differs with the halogen matrix seem to imply that the coexistence of halogen atoms and/or ions affect the ionization of analytes in the plasmas. Apart from the case of Cd, the trend shown by the signal enhancement factors, i.e., an increase with increasing molecular weight of the halogen matrix in DES- and ETV-ICP-MS, were quite opposite to those observed with NEB-ICPMS. On the assumption that the transport efficiency of the sample solution from the nebulizer to the ICP in NEB-ICP-MS is approximately 1%, the amount of halogen matrix introduced into the ICP is estimated to be 10-6 mole min-1, four orders of magnitude smaller than that of water. Therefore the halogen matrix seems not to contribute to the ionization of analytes. Whereas the amount of water loading with the DES system employed in our laboratory was about 0.1 of that with the conventional nebulization system [25], and the use of the ETV removed completely the water loading. Thus, under such conditions, the interaction between the halogen matrix and analytes may occur in the ICP and/or the interface plasma.

KI

Fig. 10. Dependenceof the signal enhancementfactorfor Zn, Cd and Pb on the halogenrnatrix.(O) DEB-ICP-MS and (A) ETVICP-MS. optics and/or the quadrupole mass spectrometer. Generally, the larger the molecular weight and/or the lower the ionization potential of the matrix element, the larger the degree of signal suppression. In this case, the larger the mass of halogen, the lower the ionization potential, thus both effects could contribute to the signal suppression. On the contrary, in DESand ETV-ICP-MS, where the water loading to the ICP was suppressed or thoroughly eliminated, the signal enhancement factor exceeded unity and the larger the molecular weight of the halogen matrix the larger the signal enhancement factor of analyte. A notable exception is Cd, whose enhancement factors for in ETV-ICP-MS were negative in the presence of KF,

3.3.2. Effect o f halogen matrix on emission intensity in ICP

To investigate the effect of the halogen matrix on the ionization equilibrium of analytes in the ICP, the influence of 10-3 M KF, KC1, KBr and KI on atom and ion emission intensifies for Zn and Cd was investigated with ETV-ICP-AES. The signal enhancement factors and intensity ratios of ion to atom emission lines (I (II)/ I (I)) for Zn and Cd are plotted in Fig. 11. The definition of signal enhancement factor is the same as that used in Figs 9 and 10. Except for the cases for Zn(I) and Zn(II) in the presence of KF and Zn(II) in the presence of KCI, each halogen matrix yielded signal enhancements, demonstrating that the excitation of atoms and ions was promoted in the ICP, when the solution was thoroughly desolvated. In addition, the signal enhancement factors for both of the analytes were clearly dependent on the molecular weight of halogen matrix, i.e., the ionization potential of the halogen, in the same manner as the analyte signal behaviour shown in Fig. 10.

1561

N. Nonose et al. / Spectrochimica Acta Part B 51 (1996) 1551-1565 I

12 10

0.8

1.20E+15

~ 1.10E+I5

-.

8

0.~ ~

in

1.00E+I5

•

i7.

-

.~

.

4b

~ I

~

......

11.2

................................. i

'~

9.00E+ 14

c~

I

8.00E+14

t KF

0.5

20 18

I KCI

t KBr

KI

Fig. 12. Dependenceof the electronnumberdensityon the halogen matrix in the ICP.

0.4

0.3

-

s

.~

4

0.2 -~

0.1 ! ~ .......

KF

• i

KCI

......

KBr

i .....

0 KI

Fig. 11. Dependencesof the signal enhancementfactor and of the ion to atom emissionintensityratio for Zn and Cd on the halogen matrix in the ICP.(O)atom line, (A) ion line and (El)ratio of ion to atom emissionintensities. However, I (II)/I (I) values were roughly decreased with increasing ionization potential of halogen for both elements. The change in the ion/atom intensity ratios can be related to a change in physical properties of the ICP such as ne and Tion values. The effect of halogen on the electron number density, ne, in the ICP is presented in Fig. 12. The n~ values were obtained experimentally from the H a Stark broadening profile. The DES technique was employed in this experiment because the measurement of n~ requires a continuous introduction of the matrix solution into the ICP. Although, the Stark broadening profiles measured in this study were not spatially resolved, it should be noted that n~ decreased from (1.07 _+0.018) x 10 t5 cm -3 to (9.6 _+0.16) × 1014 cm -3 with increasing molecular weight of halogen matrix, presenting a tendency similar to the ion/ atom intensity ratios shown in Fig. 11. The uncertainty in the ne values, evaluated as the relative standard deviation of five replicate measurements, was in the range 1.6 to 2.0%. As mentioned in the Section 2.2 experimental section, the relationship between

physical properties of the ICP and the ion/atom ratio is expressed by the theory of Saha-Eggert ionization equilibrium (Eq. (1)). When Eq. (1) is rearranged, we obtain. I(II) 2(2rmkTion) 3/2 g(II)a(I1) h(I) I(I) ne -h3 g ( l ) A ( I I ) h(II)

/Eft)

× exp~

- E(II)_~ -

V~.) ~

(2)

The decreasing trend observed in the ion/atom ratios and in the electron density with the molecular weight of matrix in the ICP, implies a drop in the value of Tio. in the ICP, resulting in a corresponding reduction of the ionization degree of the analytes. These results are, however, in conflict with the tendency of signal enhancement of analytes when the matrix in the ETV-CP-MS is changed from KF to KI. Thus, a measurement of the physical properties of the interface plasma is required to interpret the mechanism of the signal enhancement. 3.3.3. Effect of halogen matrix on the emission intensity in the interface plasma The interaction between the matrix and the analytes in the interface plasma was investigated by using the fibre optic system inserted into the interface region. Although it would have been clearly more pertinent to use the same elements, an atom/ion spectral line pair, Mg 1 285.213 nm and Mg II 279.553 nm was chosen, because the emission intensifies of these lines were sufficiently strong even in the interface plasma and also because many transition probability data, required to calculate Tio, are available[32-34]. The emission spectra measured in this study may not

1562

N. Nonose et al. I Spectrochimica Acta Part B 51 (1996) 1551-1565 10

(A) 9

1.5 •

E

~'~-- . . . . . . . .

I .--'" i

•

~I

i

I

8

"a

7

.ig

"~ 0.5

.l

6

0

I

KF

I

KCI

I

KBr

5

KI

6.02E+13

(B) ,.?

6.00E+ 13

5.80E+13

I.

5.60E+13

5.40E+13

5.20E+!3

5.00E+13

i

KF

I

KCI

I

KBr

KI

Fig. 13. Dependence of the signal enhancement factor of the ion to atom emission intensity rations for Mg(A), and of the electron number density(B) on the halogen matrix in the interface plasma.(ll) Mg(l), (1) Mg(II) and (O) Mg(II)/Mg(I).

necessarily reflect the emission from a supersonic jet, because the fibre optic system may observe mainly the barrel shock on the outside of the supersonic expansion. Further study would be required to clarify precisely the relationship between the emission obtained using this fibre system and the physical properties of the interface plasma consisting of the supersonic jet and the barrel shock. Even so, the emission measurements performed here seem to be useful for the observation of the halogen matrix effect in the interface plasma in comparison with that in the ICP. The results obtained are shown in Fig. 13, which presents the influence of 10-3 M KF, KG, KBr and KI on the signal enhancement of Mg atom and ion emission lines and the Mg(II)/Mg(I) ratio, (Fig. 13(A)), and on the n¢ value, (Fig. 13(B)). The DES technique was employed in this experiment, too. Although the signal enhancement was less dependent on the molecular weight of the halogen matrix, the signal enhancement factor for each halogen matrix exceeded

unity, demonstrating promoted excitation of Mg atoms and ions by halogen addition. The enhancement factor, however, was not very significant. The ne value gradually increased from (5.36 _+0.091) x 1013cm -3 to (5.94 _+0.033) x 1013 cm -3 with increasing molecular weight of the halogen matrix. Both the Mg(II)/Mg(I) ratio and the ne value, which are closely correlated to /'ion as described in eqn (2), increased with increasing molecular weight of the halogen matrix. This shows a quite opposite tendency to the behaviour of Zn(II)/ Zn(I) and Cd(II)/Cd(I) ratios in the ICP presented in Fig. 11. To demonstrate more clearly the difference between the tendency in the ICP and that in the interface plasma, the effect of halogen matrices on the Mg(II)/Mg(I) ratio in the ICP was also investigated with the DES system. The Mg(II)/Mg(I) ratio in the ICP decreased gradually from 16.0 to 15.1, showing the same trend as Zn(II)/Zn(I) and Cd(II)/Cd(I) ratios, although the decreasing trend observed for Mg was not so remarkable. There is no clear interpretation of the difference between the effect of halogen on the ionization of the analyte in the ICP and that in the interface plasma. Considering that the 10-5 mol min -1 of halogen matrix was introduced into the ICP when the DES technique was used as the sample introduction method, the number density of halogen matrix in the ICP was assumed to be 1012 cm -3. Since all of the halogen matrices are completely dissociated into potassium and halogen in the ICP because of their low dissociation energies (3.31-5.10 eV), it can be presumed that the ne value may be dependent on the ionization degree of halogen. The theory of the Saha ionization equilibrium was applied to compare the ionization degree of the halogen in the ICP with that in the interface plasma, on the assumption that thermal equilibrium is reached in both plasmas. The expression for the ionization equilibrium (ni/na) is given by:

ni__l_ 2(2rmkTion)3/2 __Ziexp [ _ Va~ na

ne

h3

Za

t, kr)

(3)

where n, and ni are the number density of neutral atoms and of the singly charged ion species, respectively, and Z is the partition function. The ionization degree is defined as: ni na + ni

(4)

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N. Nonose et al. / Spectrochimica Acta Part B 51 (1996) 1551-1565 Table 4 Physical properties and Ionization degree of halogen in ICP and interface plasma n J c m -3 ICP

F (17.42)" Ci (12.97) Br (11.81) I (10.45)

(1.071 ± 0.018) (1.056 ± 0.021) (9.90 ± 0.16) x (9.66 ± 0.16) x

ni/na

TiJK

× 1015 × 1015 10 TM 1014

Interface plasma

ICP

Interface plasma

ICP

(5.36 (5.36 (5.59 (5.94

7607 7581 7566 7516

5871 5905 5962 5966

1.26 1.08 6.20 4.25

± ± ± ±

0.091) x 1013 0.070) x 1013 0.10) x 1013 0.083) x 1013

Interface plasma x x x x

10 -5 10 -2 10 -2 10 -1

6.60 5.07 5.59 7.06

x x x x

10 -8 10-4 10 -3 10-2

' Va (Ionization potential) in eV.

Except in the case of Iodine (see Table 4), for the other halogens the following inequality holds: na >> ni, and therefore the ionization degree of the halogen is replaced by its ionization equilibrium expressed as the Saha equation (Eq. (3)). ni

. ni =

-

(5)

n a + n i • na

In Eq. (3), ne is the experimental value attained in Figs 12 and 13(B). Tionfor the halogen was assumed to be that calculated from the Mg(II)/Mg(I) ratio, using Eq. (2). For the partition function, the data presented by de Galan et a1.[35] were used. The excitation temperatures in the ICP and in the interface plasma required for the calculation of the partition function were assumed to be 5000 K and 3500 K, respectively[17]. Table 4 lists ne, Tion and ni/na ratio for halogen matrices in the ICP and the interface plasma obtained by the DES technique. In the ICP, the effect of halogen matrix on the n e value seems to be negligible because the n~ value due to the ionization of the halogen was significantly lower than the ne value (-1015 cm -3) due to the ionization of Ar. Thus, the decreasing trend shown by ne in the ICP presented in Fig. 12 is probably due to the mass effect of the halogen. On the other hand, in the interface plasma, the ionization degree of the halogen was considerably suppressed compared with that in the ICP because of the lower plasma temperature. However, both ne and the ionization degree of the halogen showed a similar increasing trend with decreasing the Va value of the halogen added as matrix. This result suggests that the ionization of halogen may contribute to the enhancement of ne value, and to the subsequent ionization of the analytes.

In this study, only the ionization efficiency of analytes in the plasmas was taken into consideration to interpret the signal enhancement. However, to clarify the mechanism completely, further study on the electrostatic interaction between the analyte and the halogen ion species occurring in the ion lens optics and the quadrupole mass spectrometer would also be required. 4. Conslusions

In ETV-ICP-MS using a tungsten furnace, a remarkable signal enhancement was observed for easily volatile elements such as Zn and Cd in the presence of KIO3 matrix or potassium halides such as KF, KC1, KBr and KI. The signal enhancement, which was found to be dependent on both of the concentration and the ionization potential of halogen matrices, would be due to a promoted ionization efficiency in the plasmas rather than to a chemical interaction between the analytes and matrix constituents on the surface of the tungsten furnace. Most workers using ETV-ICP-MS have related the signal enhancement to the reduction of "transport loss" of analytes by the matrix. However, in this study, the transport efficiency was not found to be affected by the presence of matrix. Furthermore, the signal enhancement was also observed when the DES technique was used as a sample introduction method. These results point to a change in the physical properties of the ICP and/or the interface plasma caused by the addition of the halogen matrix. Such change would then contribute to the signal enhancement in conditions where solvent loading is completely or nearly eliminated.

1564

N. Nonose et al. / Spectrochimica Acta Part B 51 (1996) 1551-1565

Under such a dry plasma condition, the ne and Tio, values decreased in the ICP, and increased in the interface plasma with decreasing ionization potential of the halogen atom resulting from the dissociation of the halogen matrices. The signal enhancement seems therefore to result from a promoted ionization efficiency of the halogen not in the ICP but in the interface plasma. It is clear that no claim can be made here that the explanation provided for the mechanism of enhancement is effectively correct. The phenomena occurring in the various phases of the signal formation are in fact too complex: in addition, it is not always simple to devise an experiment aimed at unequivocally providing one particular mechanism. For example, it is possible that the effects observed when the matrix changes from KF to KI could be due to the change in the ionization energy of the matrix, rather than to the change of its molecular weight, as stressed in our previous discussion. Probably, the weakest point of our experimental approach, as alluded to earlier in Section 3, lies in the difficulty of demonstrating conclusively whether the results observed are typical of the gas passing through the skimmer or those of the emission excited in the barrel shock. Unfortunately, too little is known, yet, of the interface plasma and of the relation existing between the optical emission observed with a free ICP to that sampled for the mass spectrometer. It is hoped, nevertheless, that our measurements constitute a valid and sound starting point in order to gain a deeper insight into these matters, and it is our hope that this paper will stimulate further research and provide additional data for other elements and matrices.

Acknowledgements The E T V - I C P - A E S analyses reported here were undertaken with an SPS 1200 situated at Tsukuba Research Laboratory, Sumitomo Chemical Co. Ltd., Japan. The use of this facility is gratefully acknowledged. The authors also acknowledge Mr. M. Minobe and Mr. T. Yamaguchi for their useful suggestion and expert advice, and Dr. S. Houk for his constructive criticism.

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