Anal. Chem. 1994,66, 3217-3222
Modification of a Commercial Electrothermal Vaporizer for Sample Introduction into an Inductively Coupled Plasma Mass Spectrometer. 2. Performance Evaluation M. M. Lamoureux,t D. C. GrBgoire,’v* C. L. Chakrabartl,t and D. M. Goltrt Ottawa-Carleton Chemical Institute, Department of Chemistty, Carleton University, Ottawa, Ontario, Canada K1S 5B6, and Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A OE8 The performance of a modified commercial graphite furnace (ETV) used as a sample introduction device for inductively coupled plasma mass spectrometry (ICPMS) was evaluated. This novel ETV system is based on the extraction of sample vapor through the dosing hole rather than one end of the graphite tube. Absolute limits of detection for Co, Cu, Mn, Nd, V, Y, Yb, and Zn were 0.075, 0.169, 0.072, 0.651, 0.236, 0.511, 0.042, and 0.617 pg, respectively. These limits of detection are comparable to those obtained using a commerciallyavailable electrothermal vaporization system. The sensitivity of 49 elementswas determined under various experimentalconditions, and in general, it was found that these improved when NASS-3 (seawater referencematerial) was used as a chemical modifier. Increasing the vaporizationtemperature had a significant effect only on refractory, carbide-forming elements and rare earth elements. Vaporization from the graphite tube rather than from a platform surface gave better sensitivity for most elements. In part 1 of this work,l we described a modified commercial graphite tube furnace for use as an electrothermal vaporization (ETV) sample introduction device in inductively coupled plasma mass spectrometry (ICPMS). The potential of the modified ETV for mechanistic studies of fundamental processes occurring in electrothermal atomization atomic absorption spectrometry (ETAAS) was considered. The modified ETV was also shown to be an efficient sample introduction device for ICPMS measurements. A review of the recent ETV-ICPMS literature was published by Carey and Carusom2 One advantage of ETV over other sample introduction devices is the high analyte transport efficiencies attainable (2040%). Analyte mass transport efficiency in ETV-ICPMS is dependent on the degree to which analyte nucleation and condensation takes place before contact is made with transport line surfaces. Kantor3 has described the process of nucleation as it relates to ETV sample introduction into plasmas. The ideal behavior for analyte vaporized from an ETV device involves rapid nucleation to form particles that are large enough to be transported efficiently by a carrier gas through a transfer line but small enough to avoid coagulation and deposition on + Carleton University.
*Geological Survey of Canada. (1) Lamoureux, M. M.; Chakrabarti, C. L.; Goltz, D. M.; GrCgoire, D. C., Anal. Chem., preceding paper in this issue. (2) Carey, J. M.; Caruso, J. A. Crit. Rev. Anal. Chem. 1992, 23, 397-439. (3) Kantor, T. Spectrochim. Acfa, Port E 1988, 43, 1299-1320.
0003-2700/94/0388-3217$04.50/0 @ 1994 American Chemical Society
transport surfaces. Thedifference between the modified tubetype ETV and other ETV systems4s is the use of the dosing hole rather than one at the graphite tube end for extraction of analyte vapor, a novel approach not investigated by others. The objectives of this work were to demonstrate the analytical capabilities of the modified tube-type ETV for sample introduction in ICPMS and to investigate a chemical modification technique. Recently, Grdgoire6v7introduced the use of NASS-3 (seawater reference material) as a mixed chemical modifier for ETV-ICPMS. The efficacy of NASS-3 as a chemical modifier is considered. The vaporization temperature and nature of the graphite surface on sensitivity is investigated.
EXPERIMENTAL SECTION Apparatus. The inductively coupled plasma mass spectrometer (ICPMS) used for this work was a Perkin-Elmer Sciex ELAN 5000. For the measurement of major ions such as chloride, an offset voltage was applied to one of the ion lenses of the mass spectrometer via the “OmniRange” facility. This was done to effectively reduce the sensitivity of the ICPMS for individual m/z to a level that within the linear dynamic range (&lo6 counts s-l) of the detector. Optimization of the ICP mass spectrometer and plasma operating parameters was done using solution nebulization sample introduction and aqueous standards. Reoptimization of these parameters on switching to ETV sample introduction was unnecessary, with the exceptionof small adjustments (k50 mL min-l) to the carrier argon gas flow. For multielement determinations, compromise mass spectrometer conditions were used, giving good sensitivity over the entire mass range. All ICPMS signals were blank corrected. Modifications made to the HGA 76B (Perkin-Elmer) graphite furnace as well as a detailed description of gas flows and interfacing requirements are given in part 1 of this work.’ Standardsand Reagents. Stock solutions containing 10pg mL-l mixed analytes (Delta Scientific) were diluted with 0.2% HN03 (Ultrex) in 25.00-mL calibrated flasks to give a final concentration of 10ng mL-’ for each analyte. These standard (4) Shen, W. L.; Caruso, J. A.; Fricke, F. L.; Sauger, R. D.J . Anal. At. Spectrom. 1990, 5, 451455. (5) Carey, J. M.;Evans, E. H.; Caruso, J. A.; Shen, W. L. Spectrochim. Acta, Part E 1991, 46, 1711-1721. ( 6 ) Sturgeon, R. E.; GrCgoire, D. C.;Willie, S.N.; Zheng, J.; Kudo, A. J. Anal. At. Spectrom. 1993, 8, 1053-1058. (7) GrCgoire, D. C. Trends Appl. Spectrosc. 1993, 1 , 313-324.
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solutions wereused for the determination of limits of detection and sensitivity. The seawater reference solution NASS-3 (3.5% (mass/v) of chloride salts), was diluted 500-fold with ultrapure water prior to use as a chemical modifier. The 1000pg mL-I stock solution of Ag was prepared by dissolving 0.1579 g of AgNO3 (99.99%, Anachemia) in ultrapure water; the solution was then transferred to a 100.00-mL calibrated flask and made up to volume with 0.2% (v/v) HNO3 (Ultrex, Baker). A series of standard solutions in the range 0.2-200 ng mL-l were prepared by dilution of the 1000 pg mL-l Ag stock solution. All test solutions used contained 0.2% (v/v) HNO3 (Ultrex, Baker). Silver was determined in two geological reference materials, BHVO- 1 (Basaltic lava from Kilauea caldera, Kilauea volcano, Hawaiia ) and AGV-1 (Andesite from east wall of Guano Valley, Lake County, OR8), by use of isotope dilution calibration. The isotope dilution ICPMS experiments were done by spiking the geological reference materials in solution with Io9Ag-enriched silver (99.4% lWAgpure, US Services Inc.). A stock solution of lo9Ag-enriched silver was prepared by dissolving lo9Agenriched AgNO3 in 5 mL of 0.2% (v/v) HNOs. The concentration of the lWAg stock solution, determined by solution nebulization ICPMS, was 76 ng mL-l. The lWAg stock solution was diluted 1000-fold prior to use. Geological reference materials were obtained in solution from the Geological Survey of Canada and were prepared using a standard sample dissolution method.9 The test solution for the determination of Ag by isotope dilution was prepared by mixing 500 pL of the solution containing the geological reference material with 500 pL of the 0.076 ng mL-l lo9Ag spike solution. This mixture was allowed to equilibrate for 2 h before analysis. A 5-pL volume of the test solution was deposited into the graphite furnace with an Eppendorf microliter pipet. The pipet was inserted through the quartz spout and the graphite tube dosing hole, and the test solution was deposited on the graphite tube wall or graphite platform directly below the dosing hole. Limits of detection and the sensitivity for 49 elements were determined using the modified ETV system and NASS-3 chemical modifier. NASS-3 is a seawater reference material (deep ocean water) which is well characterized for its trace element content and is of high purity (subnanogram per gram levels of trace elements) with respect to the analytes studied here.6 NASS-3 is essentially an aqueous solution composed mainly of chloride salts of Na, K, Mg, and Ca with a nominal 3.5% dissolved salt content.
RESULTS AND DISCUSSION Limits of Detection and Sensitivity. The limits of detection (LOD) and the sensitivity for selected elements were determined. The experimental and operating conditions are shown in Table 1. The analytes chosen for the determination of LODs, Le., Co, Cu, Mn, Nd, V, Y , Yb, and Zn, are either important transition metals in environmental studies or representative of the rare earth element group. The LODs were obtained by use of tube wall vaporization, with the ( 8 ) Govindaraju, K. Geosrad. Newsl. 1989, 13 (special edition on reference materials). (9) Total dissolution method for rock samples. Method A-F-HCI-10, Geological Survey of Canada, Mineral Resources Division,Analytical Chemistry Section, Analytical Chemistry Laboratory, 1992; pp 1-10.
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Table 1. EN-ICPMS Operating Condltlons for the Determlnatlon of Senritlvltles and Detrctlon Llmltr
Heating Program step temp, OC ramp time, s hold time, s
drying
vaporization"
120 10 30
2500-26 50
Carrier Gas Flow Rates total, 1 min-1 internal, 1 min-1
0 10 1.o 0.4
Mass Spectrometer and Plasma Conditions and Data Acquisition Parameters sampler nickel, 1.14-mm orifice skimmer nickel, 0.89-mm orifice r.f. power, kW 1.05 auxiliary Ar flow rate, 1 min-* 0.9 15.0 coolant Ar flow rate, 1 min-l dwell timet ms per m / z 30 no. of measurements/peak 1 resolution high (0.6 u at 10% peak height) 7 no. of ions monitored/ determination a Actual vaporization temperature for individual experiments given in text. Dwell time is defined as the time which the detector spends measuring a given m / z .
Tabk 2. a "of Abrdutr Llmns Of mrctlon (pg) for Madlfied EN wkh Literature Values elements this work (% RSD) ETV-ICPMS (% RSD)7
WO 63CU 5SMn 14*Nd 51V 89Y
174Yb UZn
0.075 (12) 0.17 (10%) 0.072 (19%) 0.65 (1 1%) 0.24 (7.0%) 0.51 (19%) 0.042 (12%) 0.62 (12%)
0.140 (11%) 0.420 (9.9%) 0.120 (4.4%) 0.013 (5.5%) 0.360 (7.5%) 0.030 (7.7%) 0.004 (5.3%) 0.520 (6.5%)
addition of NASS-3 as a chemical modifier, and a vaporization temperature of 2650 OC. Sensitivities were obtained by use of standard solutions with and without NASS-3 chemical modifier withvaporization from thegraphite tubeand platform surfaces. Table 2 compares limits of detection for eight elements obtained by use of the modified HGA 76B graphite furnace with LODs determined on a commercial ETV7 system. The LOD is defined as the mass of analyte giving an integrated ion signal equal to 36blank (n = 7), where ablank is the standard deviation obtained for a blank solution. The LODs reported by Grbgoire et al.,7 using a Perkin-Elmer HGA 600MS ETV coupled to an ELAN 5000 equipped with an autosampling device, are lower for Co, Cu, and Mn; approximately equal for V and Zn; and higher for Nd, Y, and Yb. The modified ETV is as good or better than the HGA 600MS for the determination of transition metals but not as efficient for rare earth elements. The relative standard deviation (RSD) for the LODs obtained with the modified ETV are generally higher than those obtained with the Perkin-Elmer HGA 600MS ETV system. This may be linked to the poorer reproducibility of manual sampling using Eppendorf pipets1° when compared to RSDs obtained with the use of an autosampling device. (10) Slavin, W.;Manning, D. C. h o g . Anal. A f .Specrrosc. 1982, 5, 243-340.
Table 3. Effect d NASS-3, Vaporization Temperature, and Vaporization Surface on SensHivlty (Counts pg-1 X 10')
+
STD NASS-3 STD, platform, platform, platform, tube surf. tube surf., elements 2500 O c a 2500 OCb 2650 OCC 2500 OCd 2650 OCe lo7Ag 75As Ig7Au 138Ba 9Be 2@Bi lI4Cd lace
0.16 0.020 0.19 0.042 0.021 0.68 0.0020 0.0090 0.059 59c0 0.55 l39Cs 0.067 63CU IUDy 0.040 0.030 166Er 1 5 3 E ~ 0.31 0.059 69Ga 0.0060 158Gd 0.029 74Ge 0.0060 lsoHf 0.086 202Hg 1 6 5 H ~ 0.16 0.45 193Ir 0.035 0.0050 139La 0.025 l75Lu 0.1 1 S5Mn 9 9 ~ 0 0.065 142Nd 0.014 5sNi 0.019 0.76 208Pb 0.019 lMPd 0.012 141Pr 0.059 195Pt 0.24 85Rb 0.50 Is7Re 0.041 l'J3Rh 0.011 102Ru 0.049 Iz1Sb 0.0030 4% 0.087 I%m 0.12 W n 0.18 88Sr 0.020 IJ9Tb 0.16 "We 0.39 zosTl 0.61 ls9Tm 184W 0.012 89Y 0.0040 174Yb 0.45 0.030 64Zn
0.67 0.068 0.55 0.42 0.074 2.6 0.23
1.o
0.60 0.081
1.1 0.11 1.3
3.5 0.18
3.8 0.21 0.067
0.010 0.49 2.6 1.1
3.0 0.38 0.11 0.089 0.42
3.5 0.91 0.34 0.33 0.70
0.38 0.018 0.23 0.010
0.052 0.16
0.34
1.1 3.0 1.2 0.066 0.57
0.18 2.5 0.37
3.0 1.1 0.018 0.16
0.054 0.17 0.052 0.50 2.0 0.092 0.021 1.2 0.26 0.058 0.25 0.48 1.3 0.35 1.8
0.22
0.16 0.17 0.58
0.23 0.059 0.30
0.17 0.35 0.34
0.92 0.64 0.23 0.43 0.046 0.16 0.63
2.3 0.60 0.23 0.39 0.088 0.32 0.56
0.12 0.48
0.34 0.40 3.8 1.4 0.08 1 0.050 0.66
3.0 0.93 0.055 0.039 0.52
0.25
a Standard solutions (STD) vaporized from graphite platform at 2500 OC. b Standard solutions (STD) plus 350 ng of NASS-1 vaporized from graphite platform at 2500 OC. Standard solutions (STD) plus 350 ns of NASS-1 vaporized from graphite latform at 2650 OC. *Standard solutions (STD) plus 350 ng of N A S i - l vaporized from raphite tube at2500°C. e Standardsolutions(STD)plus350ngofNAS8-1 vaporized from graphite tube at 2650 O C .
Table 3 summarizes the effect of NASS-3 on the sensitivity (integrated counts pg-1) for 49 elements and shows data obtained by use of vaporization from the graphite tube and platform surfaces and at two vaporization temperatures. In general, the addition of NASS-3 improved the sensitivity to some extent for most elements. Little or no improvement in sensitivity was observed for As, Eu, Hf, Hg, La, Mo, Re, Sm, Te, Tm, and W. Some of these elements (e.g., Hf, Mo, Re, and W) have a high boiling point" (>45'00 "C) and would not be expected to vaporize easily. A few elements such as Hf, La, Mo, and W can form refractory carbides that would
1E+W 800000
600000 400000
200000 0
0
1
2
3
4
5
6
1
7
Time/s
Figure 1. ETV-ICPMS signal pulses for vaporization from a platform at 2500 "C: (a) 50 pg of Hg, Tm, and Re plus 350 ng of NASS-3; (b) Na, Ca, and CI from the salts of 350 ng of NASS-3.
also be relatively involatile. Mo and W are also known to form chloride salts, which can be strongly intercalated within the basal layers of graphite and thus remain trapped within the graphite layers.1° For this reason, the addition of NASS-3 was not expected to improve the vaporization of Mo and W. Figure 1 shows ETV-ICPMS signal pulses for Hg, Tm, and Re (Figure la) and Na, Ca, and C1 from NASS-3 (Figure lb) vaporized from a platform at 2500 OC. A comparison of these signal pulses demonstrates one of the shortcoming of using NASS-3 as a chemical modifier for some elements. We observed that K, Na, and C1 from NASS-3 overlapped (in time) only with Hg, which may form a volatile chloride in the gas and/or condensed phase. Thulium also forms a chloride having a relatively low boiling point" (bp 1440 "C), but only in the condensed phase since the bulk of NASS-3 is vaporized (Figure lb, 0.6-2.6 s) when Tm itself vaporizes (Figure la, 2.8-7 s). This means that elements such as these may leave the graphite tube in advance of the chemical modifier and hence cannot benefit from its action. High boiling point and refractory elements, such as Re, would also not benefit from the addition of NASS-3 since its salts would completely vaporize at a time before these elements reached the gasphase. Except for a few refractory elements, the addition of NASS-3 improved the sensitivity of the other elements investigated. Table 3 shows that an improvementof 1-2 orders of magnitude was obtained for Ag, Au, Ba, Cd, Co, Er, Ho, Lu, Ni, Pd, Pr, T1, and Y. Figure 2 shows the effect of the addition of NASS-3 on the ion intensity and the temporal distribution of Ni, Ga, and Co. Without chemical modifier (Figure 2a), the signal profiles for Co, Ga, and Ni are low, flat, and show significant tailing. The addition of NASS-3 (Figure 2b) improved the signal profiles, producing sharper peaks although tailing of signal pulses persisted. In addition, the Ni signal was shifted to an earlier appearance time, whereas no significant shift in the appearance time was observed for ( 1 1) Weast, R. C., Ed.Handbook of Chemistry and Physics, S3rd ed.;CRC Press: Cleveland, OH, 1973.
Ana&ticalChemistry, Vol. 66,No. 19, October 1, 1994
3219
I
m
2500 20000 1500
loo00 500
0 0
1
2
3
4 5 Timels
6
7
Figure 3. ETV-ICPMS slgnal pulses for Ag, Cu, and NI vaporlzed in a graphite tube with 350 ng of NASS3: (a) 2500 OC; (b) 2650 OC.
0
1
2
3 4 Time18
5
6
7
Flgure 2. ETV-ICPMS slgnal pulses for vaporization from a platform at 2500 OC: (a) 50 pg of Ni, Ga,and Co without NASS-3; (b) 50 pg of NI, Ga,and Co plus 350 ng of NASS-3; (c) Na, Ca, and CI (broken line) from 350 ng of NASSB.
Ga and Co. The shift toward a lower temperature (appearance time) for Ni could not be explained on the basis that nickel formed a low boiling point chloride which would vaporize at a lower temperature than the nickel oxide formed without NASS-3. Sturgeon and Chakrabartilzhave shown that nickel had the same appearance time (or appearance temperature) whether it was vaporized as the chloride or as the nitrate and proposed that nickel atoms were formed from the dissociation of nickel dimer in the gas phase and from direct vaporization of the condensed-phase metal. However, Welz et al.13 suggested that large amounts of chloride salts could stabilize the nickel as chloride and allow covolatilization of the nickel chloride with other chlorides. The ETV-ICPMS temporal distribution of Co, Ga, and Ni (Figure 2b) and Na, Ca, and C1 (Figure 2c) shows that the Ni signal overlapped with the signal profile of Ca (and also Mg, not shown) and to lesser extent with Na (and K) and C1. This may suggest a covolatilization process13 as proposed by Byrne et al.14 The presence of hydrolyzable salts such as CaClz and MgCl2 can result in the expulsionof Ni along with HCl gas. Therefore, the shift in the appearance time could be associated with the covolatilization of Ni with the chloride salts of NASS-3. The increased sensitivitycould be associated with the condensation of Ni on NASS-3 particles allowing improved transport efficiency for Ni. This phenomenon may ~~
~
~~
~
(12) Sturgeon, R. E.; Chakrabarti,C. L. Prog. Anal. At. Spectrosc. 1978,1,5-199. (13) Welz,B.;Akman,S.;Schlemmer,G.J. Anal. At. Specrrom. 1987,2,793-801. (14) Byme, J. P.; Chakrabarti, C. L.; Grbgoire, D. C.; Lamoureux, M.; Ly, T. J. Anal. AI. Spectrom. 1992, 7 , 371-381.
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be common to other stable chloride forming salts such as Cd, Cu, Mn, Pb, Sn, and T1. It is not clear why the addition of NASS-3 improved the sensitivity of Ga and Co. There was no overlap of their temporal distribution with components of NASS-3. Gallium chloridell isnot stableat thetemperatureat whichGaappeared in the ETV-ICPMS. Cobalt forms a stable chloride with a boiling point of 1049 OC,l but cobalt is thought to be atomized via a mechanism identical to Ni.12 A possible explanation involves the alteration of vaporization surfaces by NASS-3 such that analytes are not directly in contact with the graphite surface but rather with a thin layer of metal salts. Figure 2c shows that some Na and Ca (to a lesser extent) remained in the furnace at the appearance time of Ga and Co. The quantity of N a and Ca remaining was greater than appeared from the magnitude of their signal pulses because for these elements the reduced sensitivity (OmniRange) mode was used, attenuating signal intensity by approximately 25-70%,15 depending on the analyte. Hence, a small signal in fact corresponded to a relatively large amount of the analyte being detected. Na and Ca could undergo a smooth distillation from the graphitefurnace at times greater than 3 s. Therefore, Ga and Co could have been vaporized from a modified surface composed of graphite and metal oxides (and perhaps hydroxides), but probably not chlorides, since no C1 was detected after 3.5 s. The vaporization of Ga and Co would then be more efficient than it would be from an unaltered graphite surface. Ba and Sr showed the same characteristics. Increasing the vaporization temperature either produced a large enhancement in the integrated ion intensity or had very little effect. Figure 3 shows the temporal behavior of Ag, Cu, and Ni vaporized from the graphite tube surface at 2500 (Figure 3a) and 2650 OC (Figure 3b). Figure 3bshows that the signal pulses for Ag, Cu, and Ni are sharper and are approximately 65% greater in peak height intensity and peak area. This could be due to an increase in the rate of (15) Perkin-Elmer Sciex. Aguideto the Perkln-ElmerSciex EIan5000inductiuely coupled plasma-mass spectrometer, 1992; p 7 .
0 1
1.. - . 0 1
I
. . . . . . - - . . . . . . . ..... 2 3 4 5 6 Timels
.
7
J 8
Flgure 4. ETV-ICPMS signal pulses for Pt (solid line) and Te (broken line) vaporlzed from a platform with 350 ng of NASS-3: (a) 2500 OC: (b) 2650 "C.
I
0
1
2
3
4 5 Timels
6
7
8
Flgure 5. ETV-ICPMS signal pulses for Eu (solid line) and Yb (broken line) Vaporized at a temperature of 2650 O C from a platform with 350 ng of NASS-3: (a) vaporization from the platform surface: (b) vaporization from the tube surface.
vaporization as the temperature increases, causing more analyte to condense onto NASS-3 particles. Increasing the ambient tube temperature by vaporizing analyte from a platform produced similar results; Le., the signal of some elements either improved or remained essentially the same. In addition to Ag, Cu, and Ni, Au, Ir, Pd, Rh, and Ru benefited the most from an increase in the vaporization temperature. These elements have high boiling point" (>2200 "C) and were expected to show an improvement in the integrated ion intensity with an increase in the vaporization temperature. Figure 4 shows the temporal distribution of Te and Pt vaporized from the platform surface at 2500 (Figure 4a) and 2650 "C (Figure 4b). The integrated ion intensity remained virtually the same for both Te and Pt with increasing temperature. Sharper signal pulses were observed at higher temperature, but at the expense of shorter graphite tube lifetime. Two trends were observed for elements that did not benefit greatly from an increase in the vaporization temperature. First, for elements with low boilingpoints11(
