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Trace element analysis of geological materials by direct solids insertion of a graphite cup into an inductively coupled plasma
Specmhimica Acm.Vol.42B.Nos l/2.pp.219-225,1987. Printed in &car B&m.
Q
0584-6547/87 53.00+0.00 1987Pqmon Journals Ltd.
Trace element analysis of geological materials by direct solids insertion of a graphite cup into an inductively coupled plasma I. B.
BRENNER
Geological Surveyof Israel, 30 Malkhe Israel Street,95501 Jerusalem, Israel
A.
LORBER and
Z.
GOLDBART
Negev Center for Nuclear Research,P.O. Box 9001, Beersheva,Israel (Received10 March 1986; in revisedform 9 July 1986)
Abstract-Several interference effects are documented when a graphite cup containing a silicate sample is inserted horizontally into a low-power inductively coupled plasma. Horizontal rod insertion results in a significant decrease in the Sc II-to& I intensity ratio in the normal analytical zone. At low power and in the absence of graphite, delayed volatilization, low intensity double peaks and pronounced tailing occur owing to the thermal resistance of a glassy globule formed during the volatilization process. In the presence of a graphite diluent, glass formation is prevented, and, as a result, early volatilization, maximum signal-to-background ratios, and reduced tailing occur. At higher power the temperature of the rod apparently is elevated, and a rapid decomposition of the sample occurs. Standard reference materials were analyzed with + 25 y0 accuracy.
1. INTRODUCTION THE ANALYSIS of geological materials by inductively coupled plasma-atomic emission spectrometry (ICP-AES) when using conventional pneumatic nebulization is difficult frequently owing to chemical resistance of component minerals to chemical decomposition, solution instability and limited quantity of solid material available for the preparation of large volumes of solution for analysis. Consequently direct insertion of solid samples into an ICP may overcome some of these limitations. The direct introduction of solid samples in graphite cups into a low-power ICP was reported previously [l-5]. In most cases [l-4] the rod was injected axially into the plasma through the injector tube. LORBER and GOLDBART [5] described a horizontal insertion arrangement consisting of a graphite cup which is driven into the ICP above the torch. Several interferences will be described when diverse solid geological samples are inserted using this arrangement. The sources of interferences include chemical reactions in the graphite cup and thermal disruption of the plasma as a result of gas flow perturbation. The influence of power and graphite as a diluent is described.
2.
EXPERIMENTAL
A Jobin Yvon JY48 nitrogen-flushed polychromator and a Plasma Therm generator were employed. The measuring system and the software were modified so that 100-msmeasuring periods could be used to intergrate transient emission signals and measure the peak height or peak area by integration for the appropriate period of volatilization. A Meinhard concentric nebulizer (TR-C30-2 type) operating at 25 psi with a flow rate of 0.75 ml/min and a Scott spray chamber were employed to sustain and maintain the geometry of the plasma that was formed in a conventional Fassel type quartz torch operating with 16 l/min outer gas flow rate. Intermediate gas was not used. The electrodes were prepared from
E. D. SALINand G. HORLICK,Anal. Gem. 51,2284 (1979). D. SOMMER and K. OHLS,Frezenius 2. Anal. Chm. 304,97 (1980). Z. LI-XING,G. F. KIRKBRIGHT, M. J. COPEand J. M. WATSON,Appl. Spectrosc. 37, 11 (1983). A. G. PAGE,S. V. GODBOLE, K. H. MADMSWALA, M. J. KULKARNI, V. S. MALLAPUKAR and B. D. JOSHI, Spectrochim. Acta 39B, 551 (1984). [S] A. LORBER and Z. GOLDBART, Analyst 110, 155 (1985).
[l] [Z] [3] [4]
I. B. BRENNER
220
ultrapure 3/16 in. spectrographic graphite rod (Ringsdorff, Cat. no. RW 002) (Fig. 1). The other instrumental details are given by LORBER and GOLDBART [S]. Standard reference silicate materials [6,7] were employed for multielement calibration. A sample weighing 100 mg was mixed with 500 mg graphite (National SP-2) in a plastic vial (l/2 x 2 in.) containing several 3/8 in. polystyrene mixing balls in a Spex Industries Mixer Mill (model No. 8000) for 15 min. For matrix behavior studies, synthetic standards were prepared from Johnson Matthey Specpure oxides and salts. Samples weighing 50 mg were inserted into the cup, pressed manually with a venting tool, and dried at 400°Cto remove moisture and prevent sample extrusion. Boiler caps were not employed. The graphite cup was mounted on an alumina rod, which was then driven horizontally into the ICP above the torch. A metal ring was attached to the alumina rod, and a magnetic stop mounted on the top of the Plasma Therm match work compartment ensured reproducible positioning of the graphite cup in the plasma. In order to evaluate the effect on the spatial distribution of ion and atom line intensities, a 10 mg/l solution of scandium was nebulized into the plasma at 1.25 and 2.0 kW with and without cup insertion.
3. RESULTS 3.1. Spatial variation of spectral
line intensity
In the ICP without the cup inserted, the SCII-to-Sc I intensity ratio varies from 0.85 to 0.5 in the normal analytical zone (7-20 mm above the top of the torch), and the height of maximum intensity is approximately 6 mm above the top of the torch for 1.25 and 2.0 kW (Fig. 2). However, when the cup is inserted into the plasma 2 mm above the top of the torch, the region of maximum intensity for both power settings occurs immediately above the plane of insertion and the Sc II-to-& I ratio decreases to 0.02-0.1. The region of maximum ion intensity occurs immediately above the electrode cup at a power of 2 kW. The marked decrease of the ion-to-atom ratio could possibly result from a sharp decrease in plasma temperature above the rod due to heat transfer to the graphite cup or due to the disturbance in gas flow and change in plasma geometry. For analysis, a suited observation zone would be located immediately above the plane of rod insertion where the SC II-to-SC I ratio is not affected significantly, and is similar to that obtained for solution nebulization. Continuum emerging from the glowing cup is insignificant in this region.
_
3.lmm
_
2.6mm
Fig. 1. Dimensions of the graphite cup for direct insertion of geological materials into an ICP.
[6] K. GOVINDARAJU, Geostmd. Netislett., Spec. Issue VIII (1984). [7] A. T. MYERS R. G. HAVENSJ. J. CONNO&N. M. CONKLIN and H. J. Rosh JR, USGS Prof. Pap. 1013.29 (1976).
221
Analysis of geological materials
‘X ‘X ‘X
‘..
x
‘%
‘\ ‘a
'X
‘\
‘a.
\
-*
‘a
2
4~ElciiT
6
-a
‘kx ‘a
10
-a_*
-*-x--X.
k-L, --h-o,*
12
14
OF OBSERVATION
‘X-_
-w_-_--16
16
__K
__ 20
22
24
ZONE (mm)
Fig. 2. Variation of the Sc II-to-&. I intensity ratio with observation height in an ICP with and without graphite cup insertion. Without graphite cup insertion: -x - 2.0 kW, t 1.25 kW. With graphite cup insertion: - - x - - 2.0 kW, - - t - - 1.25 kW.
0
IO
TIME,
SE:
30
Fig. 3. The relation between the signal-to-background ratio and time for volatilization of a silicate glass (United States Geological Survey GSD) without graphite, as a function of power. Zn: t 1.25 kW, ---t-2.0 kW, cd: -A- 2.0 kW.
3.2. Volatilization behavior The volatilization behavior of a wide range of trace elements was studied with and without admixture of graphite at 1.25 and 2.0 kW. The relation between the signal-to-background ratio and time for synthetic and standard reference silicate glass materials with and without admixture of paphite is shown in Figs 3-5. These relationships are considered to represent volatilization sequences. 3.2.1. Without graphite. The volatilization behavior for zinc and cadmium for a synthetic silicate glass (United States Geological Survey SRM GSD) without graphite is givenin Fig. 3. At 1.25 kW a 10-s delay in the volatilization of zinc followed by two well-developed relatively
1. B. BRENNER
222
10
TrME,SEZ”
30
Fig. 4. The relation between the signal-to-~ckground ratio and time for vohtiiization of a silicate glass (ANRT VSN) with graphite (1 part sample: 5 parts graphite) as a function of power. Zn: t 1.25kW,---+--2.OkW,Cu:-x1.25kW,----x----2.OkW.
GSD-gloss,
wrth graphite --_
_+&__-_--Z--_
Synthetcc geologtcol Matrix 1+5G, 100 ppm trace element5
VSN -
siltcote
1+5G
I \-b-m
, \ i
30
IO
TIME,
SE
Fig. 5. Multielement volatili~tion sequences for a wide range of volatile elements in glasses and synthetic standards, with and without graphite, at two power settings (upper line 1.25 kW, lower line 2.0 kWt.
Analysis of geological materials
223
low intensity peaks is observed. The duration of analyte signal under these conditions is 30 s. At higher power (2 kW), zinc volatilization occurs immediately after cup insertion, and the signal-to-background ratios are considerably higher than those for low power. After 5 s maximum ratios are obtained which persist erratically for an additional 10 s. The volatilization curve for cadmium is almost identical to that of zinc, and cadmium probably can be used as an internal reference to compensate for variations in the volatilization behavior of zinc. A well-developed refractory glassy globule is formed after approximately 10 s. 3.2.2. With graphite diluent. The behavior of a silicate material (ANRT VSN glass) containing graphite (1 part sample: 5 parts graphite) is illustrated in Fig. 4. In the presence of graphite diluent the volatilization rates for copper and zinc increase sharply at both power settings. The previously observed silica globule is not formed, and most of the material in the cup is consumed. At low power (1.25 kW), volatilization for copper and zinc commences after approximately 3 s compared to the 10 s delay for the silicate without graphite. Intensity is still relatively low but tailing and double peaks for zinc are not observed. However, significant tailing occurs for copper. At high power, volatilization of zinc occurs during a total period of about 2 s. Copper exhibits a somewhat different behavior. Although the signal-tobackground ratios are substantially increased, an erratic maximum vaporization event for about 10 s is followed by a well pronounced tailing effect for 15-20 s. 3.2.3. Multielement uolatilizution sequences. The volatilization sequences for a wide range of volatile and involatile elements with and without graphite, at two power settings (1.25 and 2.0 kW) are summarized in Fig. 5. Some numerical data are listed in Table 1. The volatilization behavior is reminiscent of that described by BOUMANS and MAESSEN [8] for the d.c. carbon arc plasma in argon and the sharp increase of volatilization in the presence of graphite as reported by AVNIet al. [9] for the d.c. arc in air. The refractory elements, represented by MO, Cr, Ti and Zr do not appear in this illustration, since their intensities are greatiy reduced and their volatilization times greatly exceed 60 s. The existence of three groups of volatile elements Hg-Cd, Zn-Pb-Bi-Ag and Sn-Mn-Cu-Ga-Sb are indicated. In general, tailing of the first group is not pronounced, whereas it is significant for the third group* The differences in the rates of volatilization between volatile and involatile elements in synthetic silica-based standards seems to be more pronounced than for silicate materials. Table 1. Chronologic volatiliition sequence for direct solid insertion of silicate materials with and without graphite at two power levels Element Power Ag-Gls Ag-Syn + C Sb-Gls sb-syn + c Sb-Gls + C Cu-Gls cu-syn + c Cu-Gis+C Mn-Gls Mn-Syn + C Mu-Gls + C
Commencement 1.25 5 3 5 5 4 1 3 9 10 8
2.0 0 1 5 3 2 1 2 0 3 3 0
Maximum L/B 1.25 14 8 15 -
2.0 5 5 10 5
1: 20 7 12 21 12
7” 11 5 15 10 5
Double peaks 1.25 n Y n n Y n n Y n n
2.0 n n n n n n n n n n n
Tailing 1.25 21 12 18 10 Y Y Y n n n
2.0 22 n n 10 5 Y Y Y Y n n
GLs:USGS-GSD and ANRT-VSN standard reference glasses. Syn: synthetic alkali basalt matrix, 1+ 5 graphite, 100 ppm trace elements. C: graphite diluent. n: not discerned. -: not measured. y: positive. [8] P. W. J. M. BOUMANS and F. J. M. J. MAESSEN, Spectrochim.Acra 24B, 585 (1969). C93 R. A~NI, A. HARELand I. B. BRENNEFZ, Appl. Spectrosc. 26,641 (1972).
Termination 1.25 32 15 23 25 30 30 28 30 30 30
2.0 28 12 20 18 18 30 30 28 30 20 30
I. B. BRENNER
224
Volatilization of the very volatile elements is faster from the synthetic matrix than from the silicate under identical ICP operating conditions. For example, Pb, Cd, Bi and Hg volatilize in l-10 sin the synthetic matrix, whereas in VSN glass these elements require longer time (for Bi 3 times, Zn twice). On the other hand, volatilizations of copper and manganese are somewhat quicker from VSN and GSD silicate matrices than from the synthetic SiOz-based standards. Furthermore, the maximum signal-to-background ratios for the volatile elements in the synthetic standard appear somewhat earlier in the volatilization sequence than in the case of VSN. However, the maximum ratios for the involatile elements appear earlier in the volatilization sequence of VSN compared to the synthetic standard (Table 1). Consequently synthetic materials cannot be used as calibration standards for the analysis of silicates. 3.2.4. Calibration. Samples of standard reference silicate materials [6] (1 part sample + 5 parts graphite) were employed for multielement calibration. An example is shown in Fig. 6 and indicates that the accuracy is approximately + 20 %. 4.
DISCUSSION AND CONCLUSIONS
Because of volatilization differences, synthetic salts and oxides cannot be used as calibration standards for the analysis of silicate materials. In a synthetic standard the volatilization rates are a function of the properties of the oxides and the salts used to prepare the synthetic standards, whereas in geological materials, the elements occur in different minerals each characterized by different thermal properties. In addition to the thermal stabilities of the constituent minerals, the formation of a glassy globule in the absence of graphite can explain the low volatilization rates of the silica matrix, low analyte intensities, double peaks, and tailing effects. This persistent globule inhibits volatilization of the involatile and refractory elements. In the presence of graphite and by increasing the power to 2 kW, the cup temperature is increased and the formation of the silica globule is inhibited. Evidently, graphite causes the formation of carbides, which for volatile and some less involatile elements are unstable at plasma temperatures and rapidly decompose. However some of the carbides of the involatile elements and most of the carbides of the refractory elements inhibit volatilization. Disruption of the flow of plasma gases, with resultant change of plasma geometry due to the horizontal configuration also plays an important role. The use of graphite as a diluent has a number of analytical benefits: intensities are enhanced earlier in the volatilization sequence, double peaks are less prominent, the volatilization sequence is terminated earlier, and tailing is minimized. Analytical calibration using this technique must take into consideration volatilization differences between the elements and their different behaviors in different matrices. The large differences between volatile and involatile elements can be reduced, and the low rates of volatilization of refractory constituents increased by the use of buffers [8] and carriers [lo] as described for the d.c. arc. Some of these effects can be compensated with the use of internal references having similar
Fig. 6. Calibrationcurve for copper using Cu I 324.7 nm. Natural SRMs wereemployed. [lo] B. STIUYZEWCSKA, Spectrochim.Acta 2?B, 227 (1972).
Analysis of geological materials
225
thermal and excitation properties [ 111. Interference effects resulting from differences in rates of volatilization also can be minimized by optimizing crater geometry [12]. The rates of volatilization of the involatile elements using this direct insertion technique are lower than those observed for the d.c. arc in air and in argon [8,9]. This limits the range of elements and reduces the multielement capability of the ICP. In ICP-AES using solution nebulization, accurate and precise simultaneous multielement analyses can be made for refractory elements (e.g. Zr and Nb), and volatile elements (e.g. Pb, Zn and Cd) using a single set of ICP operating conditions, calibration standards, and a single solution. Owing to the similarities in the performance of this technique and d.c. carbon arc emission spectrometry [ 131, there appears to be little advantage in the use of the direct insertion technique and ICPAES for multitrace element analysis of diverse solid geological materials. However, this technique does possess an advantage; it can be adapted readily to most ICP-AES instrumentation and complements analysis using conventional solution nebulization. Acknowledgements-The authors wish to express their appreciation to Prof. RAMON BARNESand the reviewers for their constructive scientific remarks and editorial assistance. I.B.B. is grateful to Prof. BARNES for financial assistance which enabled him to attend the Winter Conference in Hawaii and present this paper.
[l l] W. B. BARNEY, [12] Y. SHAO and G.
V. A. FASEL and
R. N.
KNISELEY,Spectrochim. Acta
239 643 (19tXt).
HORLKK, Appl. Spectrosc. 40, 386 (1986).
Cl33 I. B. BRENNER,H. ELDAD, L. ARGOV, A. HAREL and M. Assous, Appl. Spectrosc. 29,812 (1975).
Q
0584-6547/87 53.00+0.00 1987Pqmon Journals Ltd.
Trace element analysis of geological materials by direct solids insertion of a graphite cup into an inductively coupled plasma I. B.
BRENNER
Geological Surveyof Israel, 30 Malkhe Israel Street,95501 Jerusalem, Israel
A.
LORBER and
Z.
GOLDBART
Negev Center for Nuclear Research,P.O. Box 9001, Beersheva,Israel (Received10 March 1986; in revisedform 9 July 1986)
Abstract-Several interference effects are documented when a graphite cup containing a silicate sample is inserted horizontally into a low-power inductively coupled plasma. Horizontal rod insertion results in a significant decrease in the Sc II-to& I intensity ratio in the normal analytical zone. At low power and in the absence of graphite, delayed volatilization, low intensity double peaks and pronounced tailing occur owing to the thermal resistance of a glassy globule formed during the volatilization process. In the presence of a graphite diluent, glass formation is prevented, and, as a result, early volatilization, maximum signal-to-background ratios, and reduced tailing occur. At higher power the temperature of the rod apparently is elevated, and a rapid decomposition of the sample occurs. Standard reference materials were analyzed with + 25 y0 accuracy.
1. INTRODUCTION THE ANALYSIS of geological materials by inductively coupled plasma-atomic emission spectrometry (ICP-AES) when using conventional pneumatic nebulization is difficult frequently owing to chemical resistance of component minerals to chemical decomposition, solution instability and limited quantity of solid material available for the preparation of large volumes of solution for analysis. Consequently direct insertion of solid samples into an ICP may overcome some of these limitations. The direct introduction of solid samples in graphite cups into a low-power ICP was reported previously [l-5]. In most cases [l-4] the rod was injected axially into the plasma through the injector tube. LORBER and GOLDBART [5] described a horizontal insertion arrangement consisting of a graphite cup which is driven into the ICP above the torch. Several interferences will be described when diverse solid geological samples are inserted using this arrangement. The sources of interferences include chemical reactions in the graphite cup and thermal disruption of the plasma as a result of gas flow perturbation. The influence of power and graphite as a diluent is described.
2.
EXPERIMENTAL
A Jobin Yvon JY48 nitrogen-flushed polychromator and a Plasma Therm generator were employed. The measuring system and the software were modified so that 100-msmeasuring periods could be used to intergrate transient emission signals and measure the peak height or peak area by integration for the appropriate period of volatilization. A Meinhard concentric nebulizer (TR-C30-2 type) operating at 25 psi with a flow rate of 0.75 ml/min and a Scott spray chamber were employed to sustain and maintain the geometry of the plasma that was formed in a conventional Fassel type quartz torch operating with 16 l/min outer gas flow rate. Intermediate gas was not used. The electrodes were prepared from
E. D. SALINand G. HORLICK,Anal. Gem. 51,2284 (1979). D. SOMMER and K. OHLS,Frezenius 2. Anal. Chm. 304,97 (1980). Z. LI-XING,G. F. KIRKBRIGHT, M. J. COPEand J. M. WATSON,Appl. Spectrosc. 37, 11 (1983). A. G. PAGE,S. V. GODBOLE, K. H. MADMSWALA, M. J. KULKARNI, V. S. MALLAPUKAR and B. D. JOSHI, Spectrochim. Acta 39B, 551 (1984). [S] A. LORBER and Z. GOLDBART, Analyst 110, 155 (1985).
[l] [Z] [3] [4]
I. B. BRENNER
220
ultrapure 3/16 in. spectrographic graphite rod (Ringsdorff, Cat. no. RW 002) (Fig. 1). The other instrumental details are given by LORBER and GOLDBART [S]. Standard reference silicate materials [6,7] were employed for multielement calibration. A sample weighing 100 mg was mixed with 500 mg graphite (National SP-2) in a plastic vial (l/2 x 2 in.) containing several 3/8 in. polystyrene mixing balls in a Spex Industries Mixer Mill (model No. 8000) for 15 min. For matrix behavior studies, synthetic standards were prepared from Johnson Matthey Specpure oxides and salts. Samples weighing 50 mg were inserted into the cup, pressed manually with a venting tool, and dried at 400°Cto remove moisture and prevent sample extrusion. Boiler caps were not employed. The graphite cup was mounted on an alumina rod, which was then driven horizontally into the ICP above the torch. A metal ring was attached to the alumina rod, and a magnetic stop mounted on the top of the Plasma Therm match work compartment ensured reproducible positioning of the graphite cup in the plasma. In order to evaluate the effect on the spatial distribution of ion and atom line intensities, a 10 mg/l solution of scandium was nebulized into the plasma at 1.25 and 2.0 kW with and without cup insertion.
3. RESULTS 3.1. Spatial variation of spectral
line intensity
In the ICP without the cup inserted, the SCII-to-Sc I intensity ratio varies from 0.85 to 0.5 in the normal analytical zone (7-20 mm above the top of the torch), and the height of maximum intensity is approximately 6 mm above the top of the torch for 1.25 and 2.0 kW (Fig. 2). However, when the cup is inserted into the plasma 2 mm above the top of the torch, the region of maximum intensity for both power settings occurs immediately above the plane of insertion and the Sc II-to-& I ratio decreases to 0.02-0.1. The region of maximum ion intensity occurs immediately above the electrode cup at a power of 2 kW. The marked decrease of the ion-to-atom ratio could possibly result from a sharp decrease in plasma temperature above the rod due to heat transfer to the graphite cup or due to the disturbance in gas flow and change in plasma geometry. For analysis, a suited observation zone would be located immediately above the plane of rod insertion where the SC II-to-SC I ratio is not affected significantly, and is similar to that obtained for solution nebulization. Continuum emerging from the glowing cup is insignificant in this region.
_
3.lmm
_
2.6mm
Fig. 1. Dimensions of the graphite cup for direct insertion of geological materials into an ICP.
[6] K. GOVINDARAJU, Geostmd. Netislett., Spec. Issue VIII (1984). [7] A. T. MYERS R. G. HAVENSJ. J. CONNO&N. M. CONKLIN and H. J. Rosh JR, USGS Prof. Pap. 1013.29 (1976).
221
Analysis of geological materials
‘X ‘X ‘X
‘..
x
‘%
‘\ ‘a
'X
‘\
‘a.
\
-*
‘a
2
4~ElciiT
6
-a
‘kx ‘a
10
-a_*
-*-x--X.
k-L, --h-o,*
12
14
OF OBSERVATION
‘X-_
-w_-_--16
16
__K
__ 20
22
24
ZONE (mm)
Fig. 2. Variation of the Sc II-to-&. I intensity ratio with observation height in an ICP with and without graphite cup insertion. Without graphite cup insertion: -x - 2.0 kW, t 1.25 kW. With graphite cup insertion: - - x - - 2.0 kW, - - t - - 1.25 kW.
0
IO
TIME,
SE:
30
Fig. 3. The relation between the signal-to-background ratio and time for volatilization of a silicate glass (United States Geological Survey GSD) without graphite, as a function of power. Zn: t 1.25 kW, ---t-2.0 kW, cd: -A- 2.0 kW.
3.2. Volatilization behavior The volatilization behavior of a wide range of trace elements was studied with and without admixture of graphite at 1.25 and 2.0 kW. The relation between the signal-to-background ratio and time for synthetic and standard reference silicate glass materials with and without admixture of paphite is shown in Figs 3-5. These relationships are considered to represent volatilization sequences. 3.2.1. Without graphite. The volatilization behavior for zinc and cadmium for a synthetic silicate glass (United States Geological Survey SRM GSD) without graphite is givenin Fig. 3. At 1.25 kW a 10-s delay in the volatilization of zinc followed by two well-developed relatively
1. B. BRENNER
222
10
TrME,SEZ”
30
Fig. 4. The relation between the signal-to-~ckground ratio and time for vohtiiization of a silicate glass (ANRT VSN) with graphite (1 part sample: 5 parts graphite) as a function of power. Zn: t 1.25kW,---+--2.OkW,Cu:-x1.25kW,----x----2.OkW.
GSD-gloss,
wrth graphite --_
_+&__-_--Z--_
Synthetcc geologtcol Matrix 1+5G, 100 ppm trace element5
VSN -
siltcote
1+5G
I \-b-m
, \ i
30
IO
TIME,
SE
Fig. 5. Multielement volatili~tion sequences for a wide range of volatile elements in glasses and synthetic standards, with and without graphite, at two power settings (upper line 1.25 kW, lower line 2.0 kWt.
Analysis of geological materials
223
low intensity peaks is observed. The duration of analyte signal under these conditions is 30 s. At higher power (2 kW), zinc volatilization occurs immediately after cup insertion, and the signal-to-background ratios are considerably higher than those for low power. After 5 s maximum ratios are obtained which persist erratically for an additional 10 s. The volatilization curve for cadmium is almost identical to that of zinc, and cadmium probably can be used as an internal reference to compensate for variations in the volatilization behavior of zinc. A well-developed refractory glassy globule is formed after approximately 10 s. 3.2.2. With graphite diluent. The behavior of a silicate material (ANRT VSN glass) containing graphite (1 part sample: 5 parts graphite) is illustrated in Fig. 4. In the presence of graphite diluent the volatilization rates for copper and zinc increase sharply at both power settings. The previously observed silica globule is not formed, and most of the material in the cup is consumed. At low power (1.25 kW), volatilization for copper and zinc commences after approximately 3 s compared to the 10 s delay for the silicate without graphite. Intensity is still relatively low but tailing and double peaks for zinc are not observed. However, significant tailing occurs for copper. At high power, volatilization of zinc occurs during a total period of about 2 s. Copper exhibits a somewhat different behavior. Although the signal-tobackground ratios are substantially increased, an erratic maximum vaporization event for about 10 s is followed by a well pronounced tailing effect for 15-20 s. 3.2.3. Multielement uolatilizution sequences. The volatilization sequences for a wide range of volatile and involatile elements with and without graphite, at two power settings (1.25 and 2.0 kW) are summarized in Fig. 5. Some numerical data are listed in Table 1. The volatilization behavior is reminiscent of that described by BOUMANS and MAESSEN [8] for the d.c. carbon arc plasma in argon and the sharp increase of volatilization in the presence of graphite as reported by AVNIet al. [9] for the d.c. arc in air. The refractory elements, represented by MO, Cr, Ti and Zr do not appear in this illustration, since their intensities are greatiy reduced and their volatilization times greatly exceed 60 s. The existence of three groups of volatile elements Hg-Cd, Zn-Pb-Bi-Ag and Sn-Mn-Cu-Ga-Sb are indicated. In general, tailing of the first group is not pronounced, whereas it is significant for the third group* The differences in the rates of volatilization between volatile and involatile elements in synthetic silica-based standards seems to be more pronounced than for silicate materials. Table 1. Chronologic volatiliition sequence for direct solid insertion of silicate materials with and without graphite at two power levels Element Power Ag-Gls Ag-Syn + C Sb-Gls sb-syn + c Sb-Gls + C Cu-Gls cu-syn + c Cu-Gis+C Mn-Gls Mn-Syn + C Mu-Gls + C
Commencement 1.25 5 3 5 5 4 1 3 9 10 8
2.0 0 1 5 3 2 1 2 0 3 3 0
Maximum L/B 1.25 14 8 15 -
2.0 5 5 10 5
1: 20 7 12 21 12
7” 11 5 15 10 5
Double peaks 1.25 n Y n n Y n n Y n n
2.0 n n n n n n n n n n n
Tailing 1.25 21 12 18 10 Y Y Y n n n
2.0 22 n n 10 5 Y Y Y Y n n
GLs:USGS-GSD and ANRT-VSN standard reference glasses. Syn: synthetic alkali basalt matrix, 1+ 5 graphite, 100 ppm trace elements. C: graphite diluent. n: not discerned. -: not measured. y: positive. [8] P. W. J. M. BOUMANS and F. J. M. J. MAESSEN, Spectrochim.Acra 24B, 585 (1969). C93 R. A~NI, A. HARELand I. B. BRENNEFZ, Appl. Spectrosc. 26,641 (1972).
Termination 1.25 32 15 23 25 30 30 28 30 30 30
2.0 28 12 20 18 18 30 30 28 30 20 30
I. B. BRENNER
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Volatilization of the very volatile elements is faster from the synthetic matrix than from the silicate under identical ICP operating conditions. For example, Pb, Cd, Bi and Hg volatilize in l-10 sin the synthetic matrix, whereas in VSN glass these elements require longer time (for Bi 3 times, Zn twice). On the other hand, volatilizations of copper and manganese are somewhat quicker from VSN and GSD silicate matrices than from the synthetic SiOz-based standards. Furthermore, the maximum signal-to-background ratios for the volatile elements in the synthetic standard appear somewhat earlier in the volatilization sequence than in the case of VSN. However, the maximum ratios for the involatile elements appear earlier in the volatilization sequence of VSN compared to the synthetic standard (Table 1). Consequently synthetic materials cannot be used as calibration standards for the analysis of silicates. 3.2.4. Calibration. Samples of standard reference silicate materials [6] (1 part sample + 5 parts graphite) were employed for multielement calibration. An example is shown in Fig. 6 and indicates that the accuracy is approximately + 20 %. 4.
DISCUSSION AND CONCLUSIONS
Because of volatilization differences, synthetic salts and oxides cannot be used as calibration standards for the analysis of silicate materials. In a synthetic standard the volatilization rates are a function of the properties of the oxides and the salts used to prepare the synthetic standards, whereas in geological materials, the elements occur in different minerals each characterized by different thermal properties. In addition to the thermal stabilities of the constituent minerals, the formation of a glassy globule in the absence of graphite can explain the low volatilization rates of the silica matrix, low analyte intensities, double peaks, and tailing effects. This persistent globule inhibits volatilization of the involatile and refractory elements. In the presence of graphite and by increasing the power to 2 kW, the cup temperature is increased and the formation of the silica globule is inhibited. Evidently, graphite causes the formation of carbides, which for volatile and some less involatile elements are unstable at plasma temperatures and rapidly decompose. However some of the carbides of the involatile elements and most of the carbides of the refractory elements inhibit volatilization. Disruption of the flow of plasma gases, with resultant change of plasma geometry due to the horizontal configuration also plays an important role. The use of graphite as a diluent has a number of analytical benefits: intensities are enhanced earlier in the volatilization sequence, double peaks are less prominent, the volatilization sequence is terminated earlier, and tailing is minimized. Analytical calibration using this technique must take into consideration volatilization differences between the elements and their different behaviors in different matrices. The large differences between volatile and involatile elements can be reduced, and the low rates of volatilization of refractory constituents increased by the use of buffers [8] and carriers [lo] as described for the d.c. arc. Some of these effects can be compensated with the use of internal references having similar
Fig. 6. Calibrationcurve for copper using Cu I 324.7 nm. Natural SRMs wereemployed. [lo] B. STIUYZEWCSKA, Spectrochim.Acta 2?B, 227 (1972).
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thermal and excitation properties [ 111. Interference effects resulting from differences in rates of volatilization also can be minimized by optimizing crater geometry [12]. The rates of volatilization of the involatile elements using this direct insertion technique are lower than those observed for the d.c. arc in air and in argon [8,9]. This limits the range of elements and reduces the multielement capability of the ICP. In ICP-AES using solution nebulization, accurate and precise simultaneous multielement analyses can be made for refractory elements (e.g. Zr and Nb), and volatile elements (e.g. Pb, Zn and Cd) using a single set of ICP operating conditions, calibration standards, and a single solution. Owing to the similarities in the performance of this technique and d.c. carbon arc emission spectrometry [ 131, there appears to be little advantage in the use of the direct insertion technique and ICPAES for multitrace element analysis of diverse solid geological materials. However, this technique does possess an advantage; it can be adapted readily to most ICP-AES instrumentation and complements analysis using conventional solution nebulization. Acknowledgements-The authors wish to express their appreciation to Prof. RAMON BARNESand the reviewers for their constructive scientific remarks and editorial assistance. I.B.B. is grateful to Prof. BARNES for financial assistance which enabled him to attend the Winter Conference in Hawaii and present this paper.
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