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Solid sampling electrothermal vaporization inductively coupled plasma atomic emission spectrometry (ETV-ICP-AES): influence of some ICP operating param
Specmchrmrca Acta, Vol Printed m Great Bntam.
4SB. No
5, pp. 671+580,
t .oo
05844547/93 s6.00 Pergamon Press Ltd
1993
@1993
Solid sampling electrothermal vaporization inductively coupled plasma atomic emission spectrometry (ETV-ICP-AES): influence of some ICP operating parameters
P. VERREPT,G. GALBACS,L. MOENS and R. DMS Laboratory of Analytical Chemistry, Institute for Nuclear Sciences, University of Ghent, Proeftuinstraat 86, B-9000 Ghent, Belgium
and U. KURF~~RST University of Fulda (Fachhochschule),
MarquardstraBe
35, D-6400, F.R.G.
(Received 13 July 1992; accepted 23 December 1992) AbstractSome parameters of solid sampling electrothermal vaporization inductively coupled plasma atomic emission spectrometry (SSETV-ICP-AES) were investigated. The study concentrates on the determination of Cu, Cd and Pb. Observation height and carrier gas flow rate were optimized for different materials. Observation heights were found to be only slightly influenced by the matrix, which however, has a large impact on the optimal carrier gas flow rate. Radio frequency (rf) power has little influence on the signal, but for a higher forward power, the plasma will tolerate a higher matrix loading. The transport relative efficiencies for pneumatic nebulization, liquid-ETV (solution-ETV) and solid sample ETV (SS-ETV) for different solid materials were compared. In the case of Cd, the transport efficiency for ETV-ICP-AES was found to be about lo-30 times higher than with pneumatic nebulization whereas for Cu only, a factor of 4 and for Pb a factor of 10 was observed. Between solution-ETV and SS-ETV no significant difference was found for the transport efficiencies of Cu and Pb for different matrices, while the efficiency of the Cd transport turned out to be highly matrix-dependent.
1. INTRODUCTION and interest for sample introduction systems for direct analysis of solid samples in ICP-AES is increasing. Different approaches are being investigated including laser ablation [l, 21, direct sample introduction (DSI) [3], spark ablation [4], fluidized bed samples [5] and electrothermal vaporization (ETV-ICP) [6-201. ETV can be used to introduce both liquid and solid samples in the ICP and different approaches and methods for the introduction of the latter are being investigated [B-19]: a miniature cup technique was used by ATSUYA et al. [13], and a pelletized solid sample ETV by KARANASSIOS et al. [14]; KANTOR et al. [15, 161 used halocarbon vapour to elevate vaporization. In the case of ETV-ICP, transient signals are obtained as in ET atomic absorption spectrometry (AAS). Owing to the expansion of the carrier gas, a background change can occur [12, 141. Therefore, an accurate background correction device is needed, which can correct for the changing background during the data acquisition. In this work wavelength modulation by means of an oscillating refractor plate is used. A detailed decription of such a background correction system was published elsewhere [21, 221. One of the difficulties of direct solid sampling is finding a suitable calibration method. From the knowledge of the advantages and disadvantages of the different techniques applied in solid sample Zeeman atomic absorption spectroscopy (SS-ZAAS), the most appropriate method for SS-ETV-ICP-AES is the calibration by means of standard reference materials [13]. However, when using reference materials or any other solid calibrant, one should be aware that the matrix of the sample and the calibrant can have a different influence on the signal of the analyte; this is even more the case when standard solutions are used as calibrants for solid samples. Therefore, the effect of the matrix on different parameters was studied here. From
THE URGE
671
672
P. VERREF-~ et al. Table 1. Operating conditions for ETV-ICP-AES
ICP-AES:
PLASMA-THERM HFP-25OOD Forward power (kW) Reflected power (W) Outer gas (Urnin) Intermediate gas (Ymin) Observation height (mm) Pneumatic nebulizer Sample uptake rate (mUmin)
1.25 <5 14.2 0.5 5-19 Meinhard TR-SO-C3 0.83
MONOCHROMATOR JOBIN-YVON VHR 1000 Slits (urn) Grating (grooves/mm) Background correction Oscillation frequency GRiiN ANALYTISCHE MESS-SYSTEMF GMBH SM-30 Dimensions graphite tube (mm) Dimensions graphite boat sample holder (mm) Connection tube (glass, mm) Carrier gas (Ymin) Secondary gas (Ymin) Temperature cycle: Drying step Ashing step Vaporization Clean-out Measuring time (s) Amount of sample Solid (mg) Solution (ul) Balance Readability (mg)
70 3600 Refractor plate l-40 Hz (depending on the desired resolution)
53 x 8 (L x Diameter) 4 x 7(WX L) 500 x 4.7 (L x Diameter) 0.18-0.95 0.5 (only during loadings) 10 s; 20 s; 12 s; 3 s; 35
8% 10% 60% 90%
(120°C) (- 600°C) (= 2300°C) (- 2700°C)
OS-2 >40 Sartorius M3P 0.001
the different ICP parameters, observation height, carrier gas flow rate and rf power were chosen since they have the major influence on the obtained signals. In addition, the relative S!S-ETV transport efficiency for Cu, Cd and Pb for different matrices is determined. Therefore, the normalized (to equal analyte mass) signals obtained for SS-ETV are compared to those obtained with pneumatic nebulization. From the relative transport efficiencies, the absolute transport efficiency for the SS-ETV-ICP-AES system can be estimated.
2.
EXPERIMENTAL
2.1. Instrumentation The equipment used for this investigation and the operating conditions, are listed in Table 1. The monochromator was modified to enable the handling of transient signals and the changing background. A quartz refractor plate placed behind the entrance slit and used as a wavelength modulator allows accurate background correction. The background is carried out on each wavelength scan and the background corrected peak areas are integrated in time, resulting in a time-resolved signal (Fig. 1) [21, 221. Our ETV-system was originally designed for SS-ZAAS [23]. It is a commercially available boat-in-tube (graphite) type ETV from Griin Analytische Mess-Systeme GmbH. As can be seen from Table 1, the dimensions of the tube are larger than usual in ET-AAS to allow solid powders to be handled conveniently. Coupling this system to the ICP required some modifications. One side of the furnace was closed with an automatic valve, while the other side was connected to the ICP by a glass tube. These modifications are described in another paper [24].
Operating parameters in ETV-ICP-AES
solid sampling
673
cu I” BCR 320 RS
Pb I” BCR 038
FA
Cd I” EICR 176 IA
0
5
IO
15
Time,
2C
25
30
35
s
Fig. 1. Background-corrected time-resolved signals for 50 ng Cu in River Sediment BCR 320 (324.754 nm, observation height (O.H.) 9 mm, carrier gas flow rate (G.F.R.) 0.61 Vmin), 50 ng Cd in Incineration Ash BCR 176 (226.502 nm, O.H. 7 mm, G.F.R. 0.18 Vmin) and 50 ng Pb in Fly Ash BCR 038 (283.306 nm, O.H. 11 mm, G.F.R. 0.38 Vmin).
2.2. Solutions and samples Cu and Cd stock solutions of 1 g/l (Fluka A.G., Ion Standard Solution) were diluted with 1% sub-boiled HNOs and doubly distilled water. For solution-ETV, 10 p,l of a 5 mg/I solution was used, whereas for SS-ETV sample amounts of 0.5-2.0 mg were used and use was made of BCR Standard Reference Materials (CRM) [25-301. The latter were weighed on a micro-balance (Sartorius, M3P) with a readability of 1 pg. 2.3. Measurements For the determination of the relative transport efficiencies, at least six replicates for each point were measured. In all other experiments, at least three replicates (up to eight) were performed, and in addition the trends of the curves were confirmed by repeating the experiments under the same conditions on different days. The measured signals were mostly normalized to 50 ng of the analyte element and averaged. In the case of ETV with solutions, a typical RSD of < 5% is obtained, whereas for SS-ETV the RSD is typically around 10%.
3. RESULTS AND DISCUSSION 3.1. Optimization 3.1.1. Observation height. To compare the transport efficiencies, optimal conditions should be used for each matrix. One of the most critical parameters in ETV-ICP-AES is the observation height. For dry plasmas, the latter can be optimized regardless of the chosen carrier gas flow rate because it was found that the same optimal observation height [31] occurs for all gas flow rates and thus a univariate optimization was possible. Therefore, the observation height was optimized before the optimization of the gas flow. The optimal observation height for different elements with ETV-ICP-AES in a 0.14 M HN03 solution was determined. As can be seen in Fig. 2, the optimal observation height is approximately the same (around 10 mm above the coil) for all three elements measured, which is obviously favourable for multielement determinations. The influence of different matrices on the optimal observation heights for Cu, Cd and Pb was studied. In Fig. 3 the intensity of the emission signal at 324.754 nm produced by 50 ng of Cu is plotted against the observation height. The experiment was carried out for pneumatic nebulization, solution-ETV and SSETV of the materials listed in Table 2. The data shown in Figs 4 and 5 are the results of a similar experiment for 50 ng of Cd (226.502 nm; in solid samples the 228.802 nm line was found to be interfered with by As) and for 50 ng of Pb (283.306 nm). For SS-ETV-ICP-AES, the solid materials used are also included in Table 2. The choice of these materials is mainly restricted to the concentrations of the elements in the different materials, Since
P.
674
0
VERREIT et al.
5
IO
Observation
15
height,
200
mm
Fig. 2. Comparison of the optimal observation heights for different elements in 0.14 M HNO, solution, at a carrier gas flow rate of 0.61 I/mitt.
2ooL60
501 0
5
15
IO
Observation
height,
200
mm
Fig. 3. Comparison of the optimal observation height for 50 ng Cu in different matrices, at a carrier gas flow rate of 0.61 Umin.
Table 2. Different materials used for the different elements. Concentrations are given in &g with the 95% confidence interval Material* FA AP OL LSS SSIO IA ES RS
CU
Cd
Pb 262 r 11 63.8 t 3.2
46.6 -t 1.8 27.5 ‘- 0.6
37.8 k 1.9 71.7 “_ 2.6 470 rt 9 11.9 t 0.4
146 2 3
44.1 a 1.0
* FA: Fly Ash (BCR CRM 038 [25]); AP: Aquatic Plant (BCR CRM 060 1261); OL: Olive Leaves (BCR CRM 062 [26]); LSS: Light Sandy Soil (BCR CRM 142 [27]); SSIO: Sewage Sludge Industrial Origin (BCR CRM 146 [28]); IA: City Waste Incineration Ash (BCR CRM 176 1291); ES: Estuarine Sediment (BCR CRM 277 1301); RS: River Sediment (BCR CRM 320 f30]).
Operating parameters in ETV-ICP-AES
i
20
Pneumatic nebullzer
6
675
solid sampling
cn
5 cn 0
5
Observation
IO
15
*o”
height, mm
Fig. 4. Comparison of the optimal observation height for 50 ng Cd in different matrices, at a carrier gas flow rate of 0.61 Urnin.
0
c
IC
15
2o”
Observation height, mm
Fig. 5. Comparison of the optimal observation height for 50 ng Pb in different matrices. at a carrier gas flow rate of 0.61 Vmin.
a range of sample amount of 0.5-2 mg is used, the number of materials in which the elements have a sufficiently high concentration is limited. Moreover, the element of interest must have been certified so that the obtained signals can be recalculated to an absolute amount of 50 ng of analyte. Figures 3-5 show that for pneumatic nebulization, the optimal observation height is higher (around 13-15 mm) than for ETV-ICP-AES. VAN BERKEL et al. [31] suggested that this shift towards the load coil is assigned to the difference between wet and dry plasmas. In a wet plasma, the lower part of the plasma (in the region below 10 mm above the load coil) is cooled by the introduction of water (low solvent loading nebulizers can reduce this effect [32]), whereas the thermal conductivity due to the dissociation products of water yields the highest excitation temperature higher in the plasma. In a dry plasma, the lower part is not cooled by water and as such there are no dissociation products of the solvent to increase the thermal conductivity higher in the plasma, so that excitation already takes place at a lower spot in the plasma, i.e. close to the load coil. For Cu and Pb (Figs 3 and 5) the optimal observation heights for solution-ETV and SS-ETV are not significantly different, so the matrix is found to have hardly any influence. Earlier experiments with pneumatic nebulizers have shown that matrix effects are more pronounced when measuring “soft” (first I.P. < 8 eV) lines [33-351. Because the Cu I line (324.754 nm) and the Pb I line (283.306 nm) used are softer than the Cd I line (226.502 nm), the matrix influence was expected to be larger for Cu and Pb than for Cd. Figure 4 however, shows a more outspoken matrix effect for
676
P.
VERREPT
et
al.
---m-__*
In
:
:
IX-
BCR 062 OL /
1u ^ l0O 5 i 80. BCR 320 RS E 5 GO8 g
‘0
k i7,
20 -
m
/=-
,.-a---S
. 1000
E ;: 1u
- 800
6 33 D
,/-‘-_a’ *---+.__a
r,’
/’,*___.___H
/’ 7
,’
/’ ,,r
/ECR 142LSS
I’ f .i
0
I’ 6 , ,,%__A
,,;A, ,,;/
,:/P
40
,,a-•-.*
0 20 Carrier
,’
I’
’fF---*---=_ -h I’ ETV-Ltquld
- 6OC; 2 - 400 8 b -
,/
200
’
6 g, G
, 04c gas
.
,
,
060 f Law rate,
0 80
10:
I/min
Fig. 6. Comparison of the optimal carrier gas flow rate for 50 ng Cu in different matrices, at optimal observation height.
Cd. We therefore assume that in ETV-ICP-AES the dominant matrix effects are mainly due to the vaporization of the analyte element and the transport to the ICP of the aerosol formed and not to the excitation processes in the ICP. Even though in Fig. 4 the different materials were measured on different days, and therefore emission intensities should not be strictly compared, the Cd signal for Incineration Ash is significantly higher and the optimal observation height has shifted to a lower position. A very tentative explanation might be the following. It was found that sometimes, in the case of solid samples, during the rapid heating, a small “explosion” may occur. In this particular case (Incineration Ash) it is possible, considering the small particle size of the material (more than 50% of the particles are smaller than 3 pm [29]), that during such an “explosion” original particles are transported very efficiently to the ICP. Under normal conditions the analyte in the formed gas phase must condense on particles to be carried to the ICP. This condensation must happen before the vapour enters the cold transport tubing or the analyte will condense on the walls (in practice at least 20-30% will condense during transport). In the case of transport of original particles, the vaporized analyte can, in addition, easily condense on these particles, increasing the transport efficiency again [36]. 3.1.2. Currier gas jlmvs. When dealing with aerosol transport, the optimal carrier gas flow rate is obviously very important. When having a laminar flow at too low a flow rate, excessive losses of sample owing to sedimentation and condensation in the transport tubing occurs, while too high a flow can result in turbulences causing impaction of a part of the sample on the walls of the transport tubing. Therefore, a compromise carrier gas flow rate should be sought. For instance, calculations showed that the Reynolds number under normal conditions is around 2500, so that indeed both cases can be obtained. Therefore the optimal carrier gas flow rate was determined for Cu (Fig. 6), Cd (Fig. 7) and Pb (Fig. 8) in the same materials as used for the determination of the optimal observation height. For Cd and Cu, the matrix was found to have a more pronounced influence on the optimal carrier gas flow rate than on the observation height, indicating a strong influence of the matrix on the transport efficiency. For Cu the optimum flow rate for all solid samples is higher than for the ETV of liquids, indicating that the efficiency of the transport of an aerosol is strongly determined by the particle size. In the case of solids it is indeed reasonable to assume that larger particles. are formed than for liquids and thus a higher flow is needed to avoid sedimentation of the particles in the transport tubing. For Pb this shift is not so pronounced. For Cd, a very low optimal flow (0.183 Vmin) was observed and though, at high carrier gas flow rates, unusually high RSDs were found (up to 30%) for all measurements, the curves shown in Fig. 7 were confirmed by repeated measurements on different days. The low optimal flow rate might be due to the volatility of Cd. All
Operating parameters
ETV60
in ETV-ICP-AES
Lqu,d-----+-“.,. :
- BCR 176 3..
‘1 :
.D-_ -‘1 2 0’ 60
G &
‘\
-
10
677
solid sampling
: ‘\;,, \
BCR 146 SSIO 40
G
b
=x.
BCR 277
, ‘2, ‘h 4=.. ‘..,.,
*.
a... ES
.. “,‘\.,, ‘.
....
‘\
20 -
‘.>\
-~--~-=--+.~__).:Qt~~~~ *. ~----C_-__~ 0
c2
04
Carrier
06
0
08
gas flaw rate,
I/mm
Fig. 7. Comparison of the optimal carrier gas flow rate for 50 ng Cd in different matrices, at optimal observation height.
277
BCR 142 LSS (/
,_--.
ETV-
----.
L,qu,d/
BCR 060
,,I’ Y AP,, ,’ . ,’ I a’
_
ElCR033FA
0
I.
1.
0.
01
02
3.1,
03 Garner
04 gas
8,
8,
1,
1
05
06
07
08
*
1,
09
3
flow rate, I/mu7
Fig. 8. Comparison of the optimal carrier gas flow rate for 50 ng Pb in different matrices, at optimal observation height.
measurements were carried out at a compromise volatilization temperature of ? 2300°C. At this high temperature the vaporization of Cd may indeed proceed so fast that at a high carrier gas flow rate the residence time of the analyte pulse in the plasma is very short. Although under these circumstances the scanning frequency (only 4 Hz) of the refractor plate may be too low so that only a part of the analyte is detected, which could explain the high RSDs for high gas flows, this cannot be the only reason. Many groups working on ETV-ICP, involving Cd transport, have seen abnormal behaviour of this element [ll, 12, 14, 331 and the effect has up to now not fully been understood. This illustrates the need, in future research, for more information on the different processes involved during the aerosol transport. 3.1.3. Radio frequency power. It was reported before that raising the power will only slightly increase excitation temperatures in a dry plasma [31]. In the case of ETV-ICP-AES, the influence of the rf power on the signal was found to be small. In SS-ETV-ICP-AES however, increasing the rf power has the advantage that the plasma will tolerate a higher matrix loading. For instance, when analysing Light Sandy Soil, it was found that at a power of 1 kW, the plasma becomes unstable for sample masses above about 2.5 mg. When increasing the power to 1.125 kW, the amount of sample can be raised to 7 mg, and at 1.25 kW the plasma is stable up to 9 mg. The limit of determination for an analyte element (ng/g) is thus influenced by the rf power, and can strongly vary for different matrices since the maximum amount of sample that can be analysed depends on the nature of the sample.
678
P. VERREP~et al. Table 3. Optimized conditions used to determine the relative efficiencies for Cu, Cd and Pb of different materials
Observation height (mm)
Carner gas flow rate (I/min~
solution OL RS LSS
13 9 9 9 11
1 0.61 0.55 0.86 0.77
solution ES AP LSS PA
11 11 11 11 11 11
1 0.38 0.38 0.38 0.28 0.38
solution ES SSIO IA
13 11 II 9 7
1 0.18 0.18 0.18 0.18
solution RS
10 11 9
1 0.47 0.28
Conditions
cu
with PN* with ETV in:
Pb
with PN with ETV in:
Cd
AS
with PN with ETV in:
with PN wtth ETV in:
transport
* PN = pneumatic nebuhzer.
3.2. Transport efficiency The sensitivity of a technique and thus also the limit of detection (ng) are to a large extent determined by the transport efficiency. The relative transport efficiency was measured as the ratio between normalized (to equal mass) signals for ETV-ICP-AES and nebulization in each case under optimized conditions, which are summarized in Table 3. An estimation of the absolute transport efficiency can then be made by multiplying the absolute transport efficiency for the pneumatic nebulizer (assumed to be l-2%) with the relative transport efficiency. For pneumatic nebulization, 5 mg/l solutions of Cu, Cd and Pb were taken up at a rate of 0.83 ml/min; the signal was measured during 15 s, corresponding to an uptake of 1.0375 p.g of Cu, Cd and Pb. In the case of the ETV-ICP-AES for liquids, 10 )11 of 5 mg/l Cu, Cd and Pb solutions were used. For the determination of the Cu, Cd and Pb signals with solid sampling ETV-ICP-AES, the same materials as before (Table 2) were used. Again a range of 03-2.0 mg of the reference materials were used and the obtained signal was normalized to a 1 mg sample. This is only correct if the calibration lines are linear, which is indeed the case for several orders of magnitude [24]. All signals used were also integrated peak areas as explained before in the section on instrumentation. Each sample was measured at least six times (typical KSD < 10%) and all experiments were performed during one day. For each material and each element, the normalized signals were finally averaged and normalized to an amount of 1.0375 Fg of analyte to be comparable to the results of pneumatic nebulization. As can be seen from Table 4, the relative transport efficiency for Cd is about 15-30 times higher for ETV than for pneumatic nebulization. Assuming a 2% efficiency for the latter and 20 times higher relative transport efficiency, the absolute transport efficiency of ETV can be calculated and confirms results of other researchers reporting a 40% transport efficiency for liquid-ETV of Cd [II]. For 01, on the other hand, the enhancement of the transport efficiency with respect to pneumatic nebulization amounts to a factor of only four. Probably, the vaporized Cu easily condenses on the cold ends of the graphite tube, whereas for Cd the temperature of the carrier gas at the end of the
Operating parameters in ETV-ICP-AES
679
solid sampling
Table 4. Transporr efficiencies of ETV compared to transport efficiency of a pneumatic nebulizer Relative transport efficiencies Nebulizer
Cd
1
Pb
1
cu
1
ET%-Solid
Ed-Liquid Estuarine Sediment 14.1 c 1.7 Estuarine Sediment 10.55 t 0.16 10.64 t 0.18 Light Sandy Soil 3.98 c 0.11 4.06 2 0.24
31.8
2 1.8
Sewage Sludge IO 19.5 + 2.7 Aquatic Plant 9.18 f 0.27 River Sediment 3.74 2 0.41
incineration Ash 29.9 f 2.0 Light Sandy Soil 11.46 + 0.97 Olive Leaves 3.58 2 0.20
Fly Ash 7.77 2 0.35
graphite tube is still high enough to prevent condensation. Indeed, it was found that for a very high temperature clean-out step (- 3~OC), large Cu signals were recorded, whereas for Cd no signal was measured. Probably, there are already enough condensed particles formed, at the time the Cd starts condensing, to carry it to the ICP. In addition for Pb, which has a volatility between that of Cd and Cu, the relative transport efficiency found is around 10. It must be stressed that these relative transport efficiencies were determined under conditions that were optimized to obtain maximal background-corrected signals. When detection limits were to be compared, one should of course optimize conditions for maximal signal-to-noise ratios (SNR). As shown in Table 4 for Cu and Pb, no significant difference was found between the relative transport efficiencies for the same analyte in different matrices. Thus standardization with standard solutions instead of standard reference materials becomes possible. For Cd, on the other hand, the transport efficiency strongly varies with the nature of the matrix. Similar findings were reported by other authors [ll, 141 such as MILLARD et al. [ll], who found that the transport for Cd can increase from 20 to 60% in the presence of Se. Probably, other elements can also influence the Cd vaporization and transport efficiency, forming more volatile components or components with a smaller particle size.
4. CONCLUSIONS The results obtained in this study suggest that dominant matrix effects in SS-ETV-ICP-AES occur in the processes of vaporization and transport rather than in the processes taking place in the plasma. For Cu and Pb, the transport efficiency was found not to differ significantly for different matrices, whereas for Cd the influence is higher. These results for Cu and Pb imply that calibration with other solid materials or with liquids will be possible. The value of this work is that even without a detailed knowledge of the different processes involved in the transport, analysis for certain elements is possible. Results of determinations with SS-ETV-ICP-AES of Cu and Pb were presented at the 5th Solid Sampling Colloquium in Geel, Belgium and are published elsewhere [24]. On the other hand, it was found that for other elements (e.g. Cd), difficulties arise concerning calibration. The reason for this is that there is a lack of information on the transport processes, which are in this case strongly influencing analyte transport, So it is the authors’ opinion that in future this must be a subject of detailed research. Acknowledgemen&--The authors gratefully acknowledge the instrumental support of Mr W. MORITZof GrCn Analytische Mess-Systeme GmbW, Ehringshausen, F.R.G. The first author also wishes to thank the IWONL and the fifth author (U.K.) Volkswagen-Stiftung for financial support.
P. VERREPT et al.
680
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4SB. No
5, pp. 671+580,
t .oo
05844547/93 s6.00 Pergamon Press Ltd
1993
@1993
Solid sampling electrothermal vaporization inductively coupled plasma atomic emission spectrometry (ETV-ICP-AES): influence of some ICP operating parameters
P. VERREPT,G. GALBACS,L. MOENS and R. DMS Laboratory of Analytical Chemistry, Institute for Nuclear Sciences, University of Ghent, Proeftuinstraat 86, B-9000 Ghent, Belgium
and U. KURF~~RST University of Fulda (Fachhochschule),
MarquardstraBe
35, D-6400, F.R.G.
(Received 13 July 1992; accepted 23 December 1992) AbstractSome parameters of solid sampling electrothermal vaporization inductively coupled plasma atomic emission spectrometry (SSETV-ICP-AES) were investigated. The study concentrates on the determination of Cu, Cd and Pb. Observation height and carrier gas flow rate were optimized for different materials. Observation heights were found to be only slightly influenced by the matrix, which however, has a large impact on the optimal carrier gas flow rate. Radio frequency (rf) power has little influence on the signal, but for a higher forward power, the plasma will tolerate a higher matrix loading. The transport relative efficiencies for pneumatic nebulization, liquid-ETV (solution-ETV) and solid sample ETV (SS-ETV) for different solid materials were compared. In the case of Cd, the transport efficiency for ETV-ICP-AES was found to be about lo-30 times higher than with pneumatic nebulization whereas for Cu only, a factor of 4 and for Pb a factor of 10 was observed. Between solution-ETV and SS-ETV no significant difference was found for the transport efficiencies of Cu and Pb for different matrices, while the efficiency of the Cd transport turned out to be highly matrix-dependent.
1. INTRODUCTION and interest for sample introduction systems for direct analysis of solid samples in ICP-AES is increasing. Different approaches are being investigated including laser ablation [l, 21, direct sample introduction (DSI) [3], spark ablation [4], fluidized bed samples [5] and electrothermal vaporization (ETV-ICP) [6-201. ETV can be used to introduce both liquid and solid samples in the ICP and different approaches and methods for the introduction of the latter are being investigated [B-19]: a miniature cup technique was used by ATSUYA et al. [13], and a pelletized solid sample ETV by KARANASSIOS et al. [14]; KANTOR et al. [15, 161 used halocarbon vapour to elevate vaporization. In the case of ETV-ICP, transient signals are obtained as in ET atomic absorption spectrometry (AAS). Owing to the expansion of the carrier gas, a background change can occur [12, 141. Therefore, an accurate background correction device is needed, which can correct for the changing background during the data acquisition. In this work wavelength modulation by means of an oscillating refractor plate is used. A detailed decription of such a background correction system was published elsewhere [21, 221. One of the difficulties of direct solid sampling is finding a suitable calibration method. From the knowledge of the advantages and disadvantages of the different techniques applied in solid sample Zeeman atomic absorption spectroscopy (SS-ZAAS), the most appropriate method for SS-ETV-ICP-AES is the calibration by means of standard reference materials [13]. However, when using reference materials or any other solid calibrant, one should be aware that the matrix of the sample and the calibrant can have a different influence on the signal of the analyte; this is even more the case when standard solutions are used as calibrants for solid samples. Therefore, the effect of the matrix on different parameters was studied here. From
THE URGE
671
672
P. VERREF-~ et al. Table 1. Operating conditions for ETV-ICP-AES
ICP-AES:
PLASMA-THERM HFP-25OOD Forward power (kW) Reflected power (W) Outer gas (Urnin) Intermediate gas (Ymin) Observation height (mm) Pneumatic nebulizer Sample uptake rate (mUmin)
1.25 <5 14.2 0.5 5-19 Meinhard TR-SO-C3 0.83
MONOCHROMATOR JOBIN-YVON VHR 1000 Slits (urn) Grating (grooves/mm) Background correction Oscillation frequency GRiiN ANALYTISCHE MESS-SYSTEMF GMBH SM-30 Dimensions graphite tube (mm) Dimensions graphite boat sample holder (mm) Connection tube (glass, mm) Carrier gas (Ymin) Secondary gas (Ymin) Temperature cycle: Drying step Ashing step Vaporization Clean-out Measuring time (s) Amount of sample Solid (mg) Solution (ul) Balance Readability (mg)
70 3600 Refractor plate l-40 Hz (depending on the desired resolution)
53 x 8 (L x Diameter) 4 x 7(WX L) 500 x 4.7 (L x Diameter) 0.18-0.95 0.5 (only during loadings) 10 s; 20 s; 12 s; 3 s; 35
8% 10% 60% 90%
(120°C) (- 600°C) (= 2300°C) (- 2700°C)
OS-2 >40 Sartorius M3P 0.001
the different ICP parameters, observation height, carrier gas flow rate and rf power were chosen since they have the major influence on the obtained signals. In addition, the relative S!S-ETV transport efficiency for Cu, Cd and Pb for different matrices is determined. Therefore, the normalized (to equal analyte mass) signals obtained for SS-ETV are compared to those obtained with pneumatic nebulization. From the relative transport efficiencies, the absolute transport efficiency for the SS-ETV-ICP-AES system can be estimated.
2.
EXPERIMENTAL
2.1. Instrumentation The equipment used for this investigation and the operating conditions, are listed in Table 1. The monochromator was modified to enable the handling of transient signals and the changing background. A quartz refractor plate placed behind the entrance slit and used as a wavelength modulator allows accurate background correction. The background is carried out on each wavelength scan and the background corrected peak areas are integrated in time, resulting in a time-resolved signal (Fig. 1) [21, 221. Our ETV-system was originally designed for SS-ZAAS [23]. It is a commercially available boat-in-tube (graphite) type ETV from Griin Analytische Mess-Systeme GmbH. As can be seen from Table 1, the dimensions of the tube are larger than usual in ET-AAS to allow solid powders to be handled conveniently. Coupling this system to the ICP required some modifications. One side of the furnace was closed with an automatic valve, while the other side was connected to the ICP by a glass tube. These modifications are described in another paper [24].
Operating parameters in ETV-ICP-AES
solid sampling
673
cu I” BCR 320 RS
Pb I” BCR 038
FA
Cd I” EICR 176 IA
0
5
IO
15
Time,
2C
25
30
35
s
Fig. 1. Background-corrected time-resolved signals for 50 ng Cu in River Sediment BCR 320 (324.754 nm, observation height (O.H.) 9 mm, carrier gas flow rate (G.F.R.) 0.61 Vmin), 50 ng Cd in Incineration Ash BCR 176 (226.502 nm, O.H. 7 mm, G.F.R. 0.18 Vmin) and 50 ng Pb in Fly Ash BCR 038 (283.306 nm, O.H. 11 mm, G.F.R. 0.38 Vmin).
2.2. Solutions and samples Cu and Cd stock solutions of 1 g/l (Fluka A.G., Ion Standard Solution) were diluted with 1% sub-boiled HNOs and doubly distilled water. For solution-ETV, 10 p,l of a 5 mg/I solution was used, whereas for SS-ETV sample amounts of 0.5-2.0 mg were used and use was made of BCR Standard Reference Materials (CRM) [25-301. The latter were weighed on a micro-balance (Sartorius, M3P) with a readability of 1 pg. 2.3. Measurements For the determination of the relative transport efficiencies, at least six replicates for each point were measured. In all other experiments, at least three replicates (up to eight) were performed, and in addition the trends of the curves were confirmed by repeating the experiments under the same conditions on different days. The measured signals were mostly normalized to 50 ng of the analyte element and averaged. In the case of ETV with solutions, a typical RSD of < 5% is obtained, whereas for SS-ETV the RSD is typically around 10%.
3. RESULTS AND DISCUSSION 3.1. Optimization 3.1.1. Observation height. To compare the transport efficiencies, optimal conditions should be used for each matrix. One of the most critical parameters in ETV-ICP-AES is the observation height. For dry plasmas, the latter can be optimized regardless of the chosen carrier gas flow rate because it was found that the same optimal observation height [31] occurs for all gas flow rates and thus a univariate optimization was possible. Therefore, the observation height was optimized before the optimization of the gas flow. The optimal observation height for different elements with ETV-ICP-AES in a 0.14 M HN03 solution was determined. As can be seen in Fig. 2, the optimal observation height is approximately the same (around 10 mm above the coil) for all three elements measured, which is obviously favourable for multielement determinations. The influence of different matrices on the optimal observation heights for Cu, Cd and Pb was studied. In Fig. 3 the intensity of the emission signal at 324.754 nm produced by 50 ng of Cu is plotted against the observation height. The experiment was carried out for pneumatic nebulization, solution-ETV and SSETV of the materials listed in Table 2. The data shown in Figs 4 and 5 are the results of a similar experiment for 50 ng of Cd (226.502 nm; in solid samples the 228.802 nm line was found to be interfered with by As) and for 50 ng of Pb (283.306 nm). For SS-ETV-ICP-AES, the solid materials used are also included in Table 2. The choice of these materials is mainly restricted to the concentrations of the elements in the different materials, Since
P.
674
0
VERREIT et al.
5
IO
Observation
15
height,
200
mm
Fig. 2. Comparison of the optimal observation heights for different elements in 0.14 M HNO, solution, at a carrier gas flow rate of 0.61 I/mitt.
2ooL60
501 0
5
15
IO
Observation
height,
200
mm
Fig. 3. Comparison of the optimal observation height for 50 ng Cu in different matrices, at a carrier gas flow rate of 0.61 Umin.
Table 2. Different materials used for the different elements. Concentrations are given in &g with the 95% confidence interval Material* FA AP OL LSS SSIO IA ES RS
CU
Cd
Pb 262 r 11 63.8 t 3.2
46.6 -t 1.8 27.5 ‘- 0.6
37.8 k 1.9 71.7 “_ 2.6 470 rt 9 11.9 t 0.4
146 2 3
44.1 a 1.0
* FA: Fly Ash (BCR CRM 038 [25]); AP: Aquatic Plant (BCR CRM 060 1261); OL: Olive Leaves (BCR CRM 062 [26]); LSS: Light Sandy Soil (BCR CRM 142 [27]); SSIO: Sewage Sludge Industrial Origin (BCR CRM 146 [28]); IA: City Waste Incineration Ash (BCR CRM 176 1291); ES: Estuarine Sediment (BCR CRM 277 1301); RS: River Sediment (BCR CRM 320 f30]).
Operating parameters in ETV-ICP-AES
i
20
Pneumatic nebullzer
6
675
solid sampling
cn
5 cn 0
5
Observation
IO
15
*o”
height, mm
Fig. 4. Comparison of the optimal observation height for 50 ng Cd in different matrices, at a carrier gas flow rate of 0.61 Urnin.
0
c
IC
15
2o”
Observation height, mm
Fig. 5. Comparison of the optimal observation height for 50 ng Pb in different matrices. at a carrier gas flow rate of 0.61 Vmin.
a range of sample amount of 0.5-2 mg is used, the number of materials in which the elements have a sufficiently high concentration is limited. Moreover, the element of interest must have been certified so that the obtained signals can be recalculated to an absolute amount of 50 ng of analyte. Figures 3-5 show that for pneumatic nebulization, the optimal observation height is higher (around 13-15 mm) than for ETV-ICP-AES. VAN BERKEL et al. [31] suggested that this shift towards the load coil is assigned to the difference between wet and dry plasmas. In a wet plasma, the lower part of the plasma (in the region below 10 mm above the load coil) is cooled by the introduction of water (low solvent loading nebulizers can reduce this effect [32]), whereas the thermal conductivity due to the dissociation products of water yields the highest excitation temperature higher in the plasma. In a dry plasma, the lower part is not cooled by water and as such there are no dissociation products of the solvent to increase the thermal conductivity higher in the plasma, so that excitation already takes place at a lower spot in the plasma, i.e. close to the load coil. For Cu and Pb (Figs 3 and 5) the optimal observation heights for solution-ETV and SS-ETV are not significantly different, so the matrix is found to have hardly any influence. Earlier experiments with pneumatic nebulizers have shown that matrix effects are more pronounced when measuring “soft” (first I.P. < 8 eV) lines [33-351. Because the Cu I line (324.754 nm) and the Pb I line (283.306 nm) used are softer than the Cd I line (226.502 nm), the matrix influence was expected to be larger for Cu and Pb than for Cd. Figure 4 however, shows a more outspoken matrix effect for
676
P.
VERREPT
et
al.
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In
:
:
IX-
BCR 062 OL /
1u ^ l0O 5 i 80. BCR 320 RS E 5 GO8 g
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/=-
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,’
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’fF---*---=_ -h I’ ETV-Ltquld
- 6OC; 2 - 400 8 b -
,/
200
’
6 g, G
, 04c gas
.
,
,
060 f Law rate,
0 80
10:
I/min
Fig. 6. Comparison of the optimal carrier gas flow rate for 50 ng Cu in different matrices, at optimal observation height.
Cd. We therefore assume that in ETV-ICP-AES the dominant matrix effects are mainly due to the vaporization of the analyte element and the transport to the ICP of the aerosol formed and not to the excitation processes in the ICP. Even though in Fig. 4 the different materials were measured on different days, and therefore emission intensities should not be strictly compared, the Cd signal for Incineration Ash is significantly higher and the optimal observation height has shifted to a lower position. A very tentative explanation might be the following. It was found that sometimes, in the case of solid samples, during the rapid heating, a small “explosion” may occur. In this particular case (Incineration Ash) it is possible, considering the small particle size of the material (more than 50% of the particles are smaller than 3 pm [29]), that during such an “explosion” original particles are transported very efficiently to the ICP. Under normal conditions the analyte in the formed gas phase must condense on particles to be carried to the ICP. This condensation must happen before the vapour enters the cold transport tubing or the analyte will condense on the walls (in practice at least 20-30% will condense during transport). In the case of transport of original particles, the vaporized analyte can, in addition, easily condense on these particles, increasing the transport efficiency again [36]. 3.1.2. Currier gas jlmvs. When dealing with aerosol transport, the optimal carrier gas flow rate is obviously very important. When having a laminar flow at too low a flow rate, excessive losses of sample owing to sedimentation and condensation in the transport tubing occurs, while too high a flow can result in turbulences causing impaction of a part of the sample on the walls of the transport tubing. Therefore, a compromise carrier gas flow rate should be sought. For instance, calculations showed that the Reynolds number under normal conditions is around 2500, so that indeed both cases can be obtained. Therefore the optimal carrier gas flow rate was determined for Cu (Fig. 6), Cd (Fig. 7) and Pb (Fig. 8) in the same materials as used for the determination of the optimal observation height. For Cd and Cu, the matrix was found to have a more pronounced influence on the optimal carrier gas flow rate than on the observation height, indicating a strong influence of the matrix on the transport efficiency. For Cu the optimum flow rate for all solid samples is higher than for the ETV of liquids, indicating that the efficiency of the transport of an aerosol is strongly determined by the particle size. In the case of solids it is indeed reasonable to assume that larger particles. are formed than for liquids and thus a higher flow is needed to avoid sedimentation of the particles in the transport tubing. For Pb this shift is not so pronounced. For Cd, a very low optimal flow (0.183 Vmin) was observed and though, at high carrier gas flow rates, unusually high RSDs were found (up to 30%) for all measurements, the curves shown in Fig. 7 were confirmed by repeated measurements on different days. The low optimal flow rate might be due to the volatility of Cd. All
Operating parameters
ETV60
in ETV-ICP-AES
Lqu,d-----+-“.,. :
- BCR 176 3..
‘1 :
.D-_ -‘1 2 0’ 60
G &
‘\
-
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677
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: ‘\;,, \
BCR 146 SSIO 40
G
b
=x.
BCR 277
, ‘2, ‘h 4=.. ‘..,.,
*.
a... ES
.. “,‘\.,, ‘.
....
‘\
20 -
‘.>\
-~--~-=--+.~__).:Qt~~~~ *. ~----C_-__~ 0
c2
04
Carrier
06
0
08
gas flaw rate,
I/mm
Fig. 7. Comparison of the optimal carrier gas flow rate for 50 ng Cd in different matrices, at optimal observation height.
277
BCR 142 LSS (/
,_--.
ETV-
----.
L,qu,d/
BCR 060
,,I’ Y AP,, ,’ . ,’ I a’
_
ElCR033FA
0
I.
1.
0.
01
02
3.1,
03 Garner
04 gas
8,
8,
1,
1
05
06
07
08
*
1,
09
3
flow rate, I/mu7
Fig. 8. Comparison of the optimal carrier gas flow rate for 50 ng Pb in different matrices, at optimal observation height.
measurements were carried out at a compromise volatilization temperature of ? 2300°C. At this high temperature the vaporization of Cd may indeed proceed so fast that at a high carrier gas flow rate the residence time of the analyte pulse in the plasma is very short. Although under these circumstances the scanning frequency (only 4 Hz) of the refractor plate may be too low so that only a part of the analyte is detected, which could explain the high RSDs for high gas flows, this cannot be the only reason. Many groups working on ETV-ICP, involving Cd transport, have seen abnormal behaviour of this element [ll, 12, 14, 331 and the effect has up to now not fully been understood. This illustrates the need, in future research, for more information on the different processes involved during the aerosol transport. 3.1.3. Radio frequency power. It was reported before that raising the power will only slightly increase excitation temperatures in a dry plasma [31]. In the case of ETV-ICP-AES, the influence of the rf power on the signal was found to be small. In SS-ETV-ICP-AES however, increasing the rf power has the advantage that the plasma will tolerate a higher matrix loading. For instance, when analysing Light Sandy Soil, it was found that at a power of 1 kW, the plasma becomes unstable for sample masses above about 2.5 mg. When increasing the power to 1.125 kW, the amount of sample can be raised to 7 mg, and at 1.25 kW the plasma is stable up to 9 mg. The limit of determination for an analyte element (ng/g) is thus influenced by the rf power, and can strongly vary for different matrices since the maximum amount of sample that can be analysed depends on the nature of the sample.
678
P. VERREP~et al. Table 3. Optimized conditions used to determine the relative efficiencies for Cu, Cd and Pb of different materials
Observation height (mm)
Carner gas flow rate (I/min~
solution OL RS LSS
13 9 9 9 11
1 0.61 0.55 0.86 0.77
solution ES AP LSS PA
11 11 11 11 11 11
1 0.38 0.38 0.38 0.28 0.38
solution ES SSIO IA
13 11 II 9 7
1 0.18 0.18 0.18 0.18
solution RS
10 11 9
1 0.47 0.28
Conditions
cu
with PN* with ETV in:
Pb
with PN with ETV in:
Cd
AS
with PN with ETV in:
with PN wtth ETV in:
transport
* PN = pneumatic nebuhzer.
3.2. Transport efficiency The sensitivity of a technique and thus also the limit of detection (ng) are to a large extent determined by the transport efficiency. The relative transport efficiency was measured as the ratio between normalized (to equal mass) signals for ETV-ICP-AES and nebulization in each case under optimized conditions, which are summarized in Table 3. An estimation of the absolute transport efficiency can then be made by multiplying the absolute transport efficiency for the pneumatic nebulizer (assumed to be l-2%) with the relative transport efficiency. For pneumatic nebulization, 5 mg/l solutions of Cu, Cd and Pb were taken up at a rate of 0.83 ml/min; the signal was measured during 15 s, corresponding to an uptake of 1.0375 p.g of Cu, Cd and Pb. In the case of the ETV-ICP-AES for liquids, 10 )11 of 5 mg/l Cu, Cd and Pb solutions were used. For the determination of the Cu, Cd and Pb signals with solid sampling ETV-ICP-AES, the same materials as before (Table 2) were used. Again a range of 03-2.0 mg of the reference materials were used and the obtained signal was normalized to a 1 mg sample. This is only correct if the calibration lines are linear, which is indeed the case for several orders of magnitude [24]. All signals used were also integrated peak areas as explained before in the section on instrumentation. Each sample was measured at least six times (typical KSD < 10%) and all experiments were performed during one day. For each material and each element, the normalized signals were finally averaged and normalized to an amount of 1.0375 Fg of analyte to be comparable to the results of pneumatic nebulization. As can be seen from Table 4, the relative transport efficiency for Cd is about 15-30 times higher for ETV than for pneumatic nebulization. Assuming a 2% efficiency for the latter and 20 times higher relative transport efficiency, the absolute transport efficiency of ETV can be calculated and confirms results of other researchers reporting a 40% transport efficiency for liquid-ETV of Cd [II]. For 01, on the other hand, the enhancement of the transport efficiency with respect to pneumatic nebulization amounts to a factor of only four. Probably, the vaporized Cu easily condenses on the cold ends of the graphite tube, whereas for Cd the temperature of the carrier gas at the end of the
Operating parameters in ETV-ICP-AES
679
solid sampling
Table 4. Transporr efficiencies of ETV compared to transport efficiency of a pneumatic nebulizer Relative transport efficiencies Nebulizer
Cd
1
Pb
1
cu
1
ET%-Solid
Ed-Liquid Estuarine Sediment 14.1 c 1.7 Estuarine Sediment 10.55 t 0.16 10.64 t 0.18 Light Sandy Soil 3.98 c 0.11 4.06 2 0.24
31.8
2 1.8
Sewage Sludge IO 19.5 + 2.7 Aquatic Plant 9.18 f 0.27 River Sediment 3.74 2 0.41
incineration Ash 29.9 f 2.0 Light Sandy Soil 11.46 + 0.97 Olive Leaves 3.58 2 0.20
Fly Ash 7.77 2 0.35
graphite tube is still high enough to prevent condensation. Indeed, it was found that for a very high temperature clean-out step (- 3~OC), large Cu signals were recorded, whereas for Cd no signal was measured. Probably, there are already enough condensed particles formed, at the time the Cd starts condensing, to carry it to the ICP. In addition for Pb, which has a volatility between that of Cd and Cu, the relative transport efficiency found is around 10. It must be stressed that these relative transport efficiencies were determined under conditions that were optimized to obtain maximal background-corrected signals. When detection limits were to be compared, one should of course optimize conditions for maximal signal-to-noise ratios (SNR). As shown in Table 4 for Cu and Pb, no significant difference was found between the relative transport efficiencies for the same analyte in different matrices. Thus standardization with standard solutions instead of standard reference materials becomes possible. For Cd, on the other hand, the transport efficiency strongly varies with the nature of the matrix. Similar findings were reported by other authors [ll, 141 such as MILLARD et al. [ll], who found that the transport for Cd can increase from 20 to 60% in the presence of Se. Probably, other elements can also influence the Cd vaporization and transport efficiency, forming more volatile components or components with a smaller particle size.
4. CONCLUSIONS The results obtained in this study suggest that dominant matrix effects in SS-ETV-ICP-AES occur in the processes of vaporization and transport rather than in the processes taking place in the plasma. For Cu and Pb, the transport efficiency was found not to differ significantly for different matrices, whereas for Cd the influence is higher. These results for Cu and Pb imply that calibration with other solid materials or with liquids will be possible. The value of this work is that even without a detailed knowledge of the different processes involved in the transport, analysis for certain elements is possible. Results of determinations with SS-ETV-ICP-AES of Cu and Pb were presented at the 5th Solid Sampling Colloquium in Geel, Belgium and are published elsewhere [24]. On the other hand, it was found that for other elements (e.g. Cd), difficulties arise concerning calibration. The reason for this is that there is a lack of information on the transport processes, which are in this case strongly influencing analyte transport, So it is the authors’ opinion that in future this must be a subject of detailed research. Acknowledgemen&--The authors gratefully acknowledge the instrumental support of Mr W. MORITZof GrCn Analytische Mess-Systeme GmbW, Ehringshausen, F.R.G. The first author also wishes to thank the IWONL and the fifth author (U.K.) Volkswagen-Stiftung for financial support.
P. VERREPT et al.
680
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