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Electroerosion of metal in aqueous solution for sample introduction into an inductively coupled plasma mass spectrometer
Spectrochimica Acta Part B 58 (2003) 1325–1334
Electroerosion of metal in aqueous solution for sample introduction into an inductively coupled plasma mass spectrometer Douglas Goltz*, Michael Boileau, Gundars Reinfelds Department of Chemistry, University of Winnipeg, 515 Portage Avenue, Winnipeg, Man., Canada R3B 2E9 Received 24 October 2002; accepted 5 April 2003
Abstract When high current (1–10 A cmy2) is applied between two conductive samples (metals) in aqueous solution, electroerosion occurs on the surface as a result of electrolysis and possibly collisions of dissolved ions with the metal surface. The power supply for the electroerosion apparatus in this work was a modified spark source unit. Current could be varied in intervals of 2.5, 5 and 10 A in either half-wave (unipolar) or full-wave (bipolar) output. The electroeroded metal forms a colloidal suspension in aqueous solution with particle sizes of the order of 1–10 mm and possibly larger. The suspension is readily dissolved using a small amount (100 ml) of concentrated acid (HCl or HNO3) prior to analysis. Electroerosion of steel and brass in aqueous solution is described both for rapid sample dissolution and as a solid sampling approach for ICP-MS. Some of the electroerosion properties described in this paper include rates of erosion as a function of gap between the conductive samples and solution conductivity. Rates of electroerosion decreased from 120 to 30 mg sy1 as the gap was increased from 2 to 5 mm. Rates of electroerosion also increased significantly from 200 to 1000 mg sy1 as the conductivity of the electroerosion solution increased from 0.01 to 0.05 M NaCl. Interfacing the electroerosion apparatus to an ICP-MS was straight forward, as no special equipment was required. Therefore, the electroerosion apparatus can be used for rapid ‘on-line’ sample dissolution prior to introduction into an ICP. ICP-MS time profiles of selected metals in stainless steel 308L illustrate the behavior of 52Crq, 55Mnq and 60Niq during a typical electroerosion cycle. Aspiration of the colloidal suspension into the ICP did not appear to load the plasma significantly, however, all of the metals produced noisy signals ("10%). A glass concentric nebulizer was used without clogging, so it is likely that the heterogeneous nature of the colloidal suspension caused this effect. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Electroerosion; Heterogeneous nature; Colloidal suspension; Metal dissolution
1. Introduction The ability to analyze solids directly can provide many important advantages to the atomic spectroscopist including: minimal sample preparation and *Corresponding author. Tel.: q1-204-786-9730; fax: q1204-775-2114. E-mail address: [email protected] (D. Goltz).
sample contamination from solvents such as mineral acids. As a result of these advantages, instrumental techniques have been developed such as LASER and spark ablation for sample introduction into ICPs. Much of the early work on spark ablation for sample introduction into an ICP was directed toward solid sampling of ferrous alloys such as steels w1–6x and used atomic emission
0584-8547/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0584-8547Ž03.00070-3
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spectroscopy (AES) for detection of transition metals. Other applications of spark ablation—ICPAES have been reported on the analysis of arc furnace flue dusts w7x, geological materials w8,9x and even non-conducting materials w10x. Most work has been carried out in radial mode, however, some work has been done on spark ablation for axially viewed plasmas w11x. The robustness of the radio frequency ICPs (27 and 40 MHz) makes them suitable for handling the larger particle sizes (1–10 mm) of the eroded material, however, some work has also been carried out to interface a spark ablation apparatus to an Ar microwave-induced plasma w12,13x. More recently, spark ablation for sample introduction into an ICP-MS has been described by a number of workers w14–19x. Clearly, the important advantage of using MS for detection is that trace and ultratrace analysis of a large number of metals is possible. Many aspects of interfacing a spark ablation apparatus to an ICP-MS are similar to that of a LASER (LA) for LA-ICP-MS. The advantage of using a LASER is obvious in that the sampling position is easily controlled and does not require a conductive surface like the spark. The most important advantage of using a spark, however, is the significantly lower cost when compared to the LASER. A small number of papers w20–22x have compared various solid sampling techniques for sample introduction into ICPs including spark, LASER and glow discharge. The generation of a spark in an aqueous solution has been explored as an alternative approach for the rapid dissolution of metals. Both high-power (275–1100 W) w23x and low-power (100–300 W) w24x spark ablation in aqueous solution have been investigated for the dissolution of brass, steel and aluminum alloys. Other workers w25–28x have also explored the use of low-power spark ablation for dissolution or ‘electric dispersion’ of metals in aqueous solution for trace metals analysis using GFAAS. An alternative to the spark for dissolution of conductive solids in aqueous solution is electroerosion or electrolytic dispersion. Electroerosion occurs when a high current (1–10 A cmy2) is applied to two conductive samples (metals) in a conductive aqueous solution. Interaction of high
electric fields, which causes electrolysis of one or both metal surfaces and possibly collisions of highenergy ions with the metal surface, causes rapid erosion. As with a spark, a colloidal dispersion of metal forms in the aqueous solution that is easily dissolved with a small amount of concentrated HCl or HNO3. There are at least two important differences between spark ablation and electroerosion in aqueous solution: (1) a significantly larger gap is required between the conductive samples and (2) a conductive medium is required to carry the charge between the conductive samples. The incentive for using electroerosion rather than a spark is that it provides a very gentle erosion process that is easier to control than a spark. In terms of sampling, the spark generally acts on a single spot resulting in the formation of a crater on the surface of the sample. Electroerosion in aqueous solution does not sample a single spot; rather the surface area of metal that is submerged is eroded without the forming of a crater on the surface. Electroerosion has been explored for rapid on-line dissolution of alloys such as steel w29–32x. Bergamin et al. w29x dissolved alloys for the determination of Al and Mn using flow injection analysis. Flock and Ohls w30x analyzed steel samples using electrolytic dissolution with flame atomic absorption spectroscopy (FAAS). More recently, Gervaio et al. w33x used electroerosion for rapid dissolution of steel for on-line measurement with ICP-AES. Kondo et al. w31,32x have used electrolytic dissolution for rapid determination of S and transition metals in steel using ICP-AES. This paper describes some of the properties of the electroerosion process in aqueous solution for rapid sample dissolution. The usefulness of electroerosion for rapid on-line sample dissolution prior to sample introduction into an ICP-MS is also described. 2. Experimental The power supply used for this work was an Applied Research Laboratories (ARL) Model 26000 spark source unit from a spectrographic analyzer. It should be noted that while a spark source unit was used, any DC power supply that is capable of providing 1–10 A should work
D. Goltz et al. / Spectrochimica Acta Part B 58 (2003) 1325–1334 Table 1 Instrumental operating conditions for the ARL 26000 spark source unit Power requirement (V) Current settings (A) Electrode polarity
115 (60 Hz frequency) 2.5, 5.0, 10.0 Positive or negative
Waveform Half (unipolar) Full (bipolar) Spark gap (mm)
2.5 and 5.0 A 5.0 and 10.0 A 2.5–6 mm
equally well. The electroerosion apparatus has been described previously w23x for spark ablation in aqueous solution. Important experimental parameters are summarized in Table 1. The power requirement of the electroerosion apparatus was 115 V at 50 or 60 Hz AC and 10 A. The electrodes have an output from a 14-kV transformer and two 0.5-mF capacitors (in series), which are in parallel to the high-voltage transformer. Two parallel inductors (50 mH) in series with the capacitors and a rectifier circuit control the output current. The output current of the discharge can be fullwave rectified (bipolar) at 5 and 10 A or halfwave rectified (unipolar) at 2.5 and 5 A. The spark source unit was modified by removing the graphite electrode holders and the wires attached to them from the output transformer. Longer wires were connected from the output transformer to allow easy (and removable) connection to the electroerosion vessel. The electroerosion vessel consisted of a scintillation vial with a modified cap to hold the metal samples in place. The
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modified cap was machined in-house and has three holes in the top for easy insertion of two metal samples (rods), as well as a hole for ventilation or tubing for interfacing to the ICP. A schematic diagram of the experimental setup is shown in Fig. 1. The gap between the metal samples was measured visually using a magnified eye piece equipped with scale of "0.1 mm. Precise control of the gap between the conductive samples during electroerosion was not possible. The gap will increase slightly (-0.1 mm) with time as the surfaces of the metal samples are eroded. Metal rods (2-mm diameter) used as conductive samples were cut in approximately 7–10 cm lengths and shaped as needed. The electroerosion apparatus is not limited to rods, as pins and flat surfaces have been successfully eroded. The electroerosion process is typically carried out in a 10-ml volume for 5–30 s. The electroerosion solution consisted of deionized water containing an electrolyte (0.01 M NaCl). By making the solution conductive, generation of a spark was avoided when larger gaps ()2 mm) were used. After electroerosion was complete the metal samples were removed from the solution, rinsed with deionized water and a small amount (100–200 ml) of concentrated HCl or HNO3 was added to the solution to dissolve the colloidal suspension. Typically, the sample solution was then further diluted to a final volume of approximately 20 ml prior to analysis. By carrying out the electroerosion in a known volume of water and by carefully weighing the metal samples before and after the electroerosion process, accurate quantifi-
Fig. 1. Schematic diagram of electroerosion—ICP-MS apparatus.
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cation of bulk and trace metals in the electrode is possible. Deionized water (Milli-Q) was used for preparing all solutions. Standard solutions were prepared by appropriate dilution from 1000 mg mly1 standards (SCP Science). All solutions were made with high purity AR Select䉸 Plus HNO3 (Mallinckrodt). Measurement of bulk elements was carried out using a Perkin-Elmer 601 FAAS and trace elements were measured using a PerkinElmer Sciex Elan 500 ICP-MS. Unless indicated, standard conditions were used for both instruments. Instrumental parameters for the ICP-MS are shown in Table 2.
Fig. 2. Mass of Cu and Zn removed from brass over time in 0.01 M NaCl and a sample gap of 5.0 mm.
3. Discussion When high current (1–10 A cmy2) is applied between two conductive samples (metals) in an aqueous solution, electroerosion occurs on the surface as a result of electrolysis and possibly collisions of dissolved ions with the metal surface. As long as a large enough current is used, erosion will occur at one (unipolar) or both (bipolar) metal surfaces in aqueous solution. If the gap between the conducting samples is too small, an explosive spark can form. A number of properties influence the amount of material that is eroded from the metal surface including the gap, applied current (or power) and the conductivity of the electroerosion solution. Prior to using electroerosion as a sample introduction device for ICPs, some of these parameters were examined to characterize the electroerosion apparatus.
The first property investigated was the mass of metal removed and the rate of erosion of metals in an aqueous solution for a given power setting. Fig. 2 shows the mass of Cu and Zn eroded from a sample of brass over time. For these measurements, the electroerosion process was carried out at 2.5 A cmy2 in a known volume (;10 ml) of an aqueous 0.01 M NaCl solution. After fixed periods of time, small volumes (0.5 ml) of liquid media were removed, diluted to 20 ml and analyzed using flame AAS. Since the concentration and volume were known, the mass of metal eroded was readily calculated. Fig. 2 shows that the mass of Cu and Zn in the electroerosion solution increases with time, as would be expected, but the mass of metal appears to erode at an increasing rate over time. Using the same data, the rate of metal erosion was also calculated and plotted as a func-
Table 2 Instrumental operating and data acquisition parameters of ICP-MS ICP mass spectrometer RF power Coolant Ar flow Auxiliary Ar flow Carrier Ar flow Nebulizer
1100 W 15.0 l miny1 1.4 l miny1 1.0 l miny1 Glass concentric
ICP-MS data acquisition Dwell time Scan mode Number of masses (myz) monitored Signal measurement Resolution
50 ms Peak hopping 3–8 Average counts 0.9 amu at 10% peak height
D. Goltz et al. / Spectrochimica Acta Part B 58 (2003) 1325–1334
Fig. 3. Rate of electroerosion of Cu and Zn from brass over time in 0.01 M NaCl and a sample gap of 5.0 mm.
tion of time as shown in Fig. 3. This experiment illustrates that the rate of erosion of Cu and Zn is not constant; rather it increases significantly over time. For example, after 30 s, the rate of erosion of Cu and Zn in brass doubled the rate after 5 s. This suggests that physical or chemical properties of the electroerosion process are changing over time. During the erosion process at least two parameters change constantly with respect to time. One is the gap, which must increase slightly as metal is removed from the surface of the sample. A second parameter that must change is the chemical composition of the aqueous solution. It is possible that the eroded metal in the aqueous solution could increase the overall conductivity of the electroerosion solution, particularly between the metal samples. It is worth noting that if high concentrations of NaCl (0.05 M) are used, the contribution of ions from the eroded metal should be minimal. Another property that also changes in the electroerosion solution is temperature. The temperature of the solution changes significantly with respect to time, depending on the applied current. Usually the temperature in the solution can reach a boiling point in less than 1 min at 2.5 A cmy2. In general, for electrolyte solutions, molar conductivity increases with increasing temperature. Experiments were carried out to look at the effect of the electrode gap and the conductivity of the electroerosion media on the rate of erosion. The effect of increasing the electrode gap on the rate of erosion is illustrated in Fig. 4. An erosion time of 15 s was used at 2.5 A cmy2 in 0.01 M NaCl. Fig. 4 illustrates that increasing the gap
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Fig. 4. Effect of gap on the rate of erosion of Cu and Zn in brass and using 0.01 M NaCl.
between the conductive samples will decrease the rate of metal erosion. This suggests that changes in the gap have greater effects on rates of erosion when a small gap (1–2 mm) is used. As the change in gap is quite small (-0.1 mm) during a typical experiment, it is unlikely to have any noticeable effects on the electroerosion rate over this short period of time. This is in contrast to spark ablation where small gaps are used (-1 mm) and very small changes in the electrode gap can have profound effects on the energy of the spark and subsequent rate of ablation w23x. The effect of conductivity as shown in Fig. 5 illustrates clearly that the higher the concentration of NaCl in the electroerosion solutions, the greater the rate of erosion of metal. For the large gaps (2 mm) used in these studies, the effect of conductivity appears to be a more important consideration than the electrode gap. One reason that the rate of
Fig. 5. Effect of solution ionic strength on the rate of erosion of Cu and Zn in brass and using a sample gap of 5 mm.
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Fig. 6. Comparison of the masses of Cu to Zn eroded from brass in 0.01 M NaCl and a sample gap of 5.0 mm over time.
erosion increases at an increasing rate is because the conductivity between the metal samples increases as more ionic species are formed in the solution between them. It may be logical to minimize the role of the gap at this stage, however, it should be noted that as the gap increases, the effect of conductivity on the rate of erosion decreases significantly. It should also be noted that if the conductivity of the solution is high enough (0.05 M NaCl), the contribution of eroded metal to overall solution conductivity should be minimal. One fundamental question that requires examination is whether one metal is preferentially eroded relative to another in an alloy. If one element is removed preferentially, then erroneous analysis will result and the overall effect will be the same as an incomplete acid digestion. Perhaps, the easiest way to demonstrate whether preferential erosion of one metal is occurring in an alloy is to plot the mass of one metal with respect to another. Experiments were carried out using Cu and Zn from a brass electrode. Fig. 6 shows a plot of mass of eroded Cu with respect to eroded Zn in a sample of brass. For these experiments, a single sample of brass was eroded over time at 2.5 A cmy2 in 0.01 M NaCl, with an electrode gap of 2 mm. At fixed intervals, a small amount of liquid was removed (0.1 ml), diluted and the eroded masses of Cu and Zn were quantified. The ratio of Cu:Zn in this brass sample is approximately 3:1. Each data point in this graph represents a different time interval when the solution was sampled. The high degree of linearity (r 2s0.996) in
Fig. 6 strongly suggests that neither Cu nor Zn is removed preferentially over time for an alloy like brass. One general property that was of interest for metal dissolution was to establish whether different alloys have different rates of electroerosion. It is reasonable to hypothesize that metals and alloys with higher tensile strength have lower rates of electroerosion than metals with higher tensile strength. To test this hypothesis, a series of different metals were electroeroded under identical conditions and the mass of the metal was determined after a fixed period of time. In general, lower rates of erosion are observed with higher tensile strengths for one type of alloy (e.g. stainless steel). These results also illustrate the difficulty in trying to compare rates of erosion to tensile strength alone when looking at alloys of different bulk metal composition. For example, stainless steel appears to have a significantly lower rate of electroerosion than aluminum alloys even though it has a much higher tensile strength. It would seem logical that other factors such as the surface area of the metal and the presence of an oxide coating on the metal surface are also important considerations with regard to erosion rates. Aluminum is a good example of a metal that readily forms an oxide layer, making this type of sample less conductive. The result of this chemical property that is lower rates of erosion was observed. Solubility of the eroded metals in aqueous solution should also have a noticeable effect on the rates of erosion. This would depend, to a certain extent, on the role of dissolved oxygen or the ability to form other complexes in aqueous solution. Introduction of the solutions containing electroeroded metals into an ICP-MS was carried out using stainless steel 308L in 0.01 M NaCl. Fig. 7a–c show the mass scans of over 0–15 s. These figures illustrate, in terms of signal intensity, the length of time required for detecting trace levels of metals such as Ni and Mn in stainless steel. It should be noted that when an electrolyte such as NaCl is used, some practical considerations regarding its use are in order. For example, although the addition of the electrolyte is essential for carrying charge in the electroerosion solution, it is a possible source of metal contamination as well as a
D. Goltz et al. / Spectrochimica Acta Part B 58 (2003) 1325–1334
Fig. 7. ICP-MS mass scans showing the formation of 58Niq, 60 Niq and 55Mnq in stainless steel for (a) 0 s; (b) 5 s and (c) 15 s in the electroerosion solution.
spectral interference due to the formation of 35 Cl16Oq, which interferes with 51Vq. A number of simple solutions can be used to overcome these types of chemical or spectral interferences such as using an alternative salt (e.g. NH4NO3). Trace levels of transition metals can also be removed easily by running the salt solution through an ion exchange resin such as 8-hydroxyquinoline. Fig. 8 shows time profiles of 52Crq, 55Mnq and 60 Niq from the electroerosion of stainless steel 308L. The composition of 308L is 19.5–22% Cr, 9–11% Ni and 1–2.5% Mn as well as other trace metals and Fe. The argon dimer 80Arq 2 was also monitored to determine qualitatively if drifting or possibly loading of the plasma occurred as the
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electroerosion solution was introduced into it. Metal in high concentration (52Crq) was monitored as shown in Fig. 8a and for clarity, the behavior of metals that were lower in concentration (55Mnq and 60Niq) is shown in Fig. 8b. In these experiments, the electroerosion solution was continuously introduced into the ICP. A small stir bar was used to keep the electroerosion solution as homogeneous as possible. At approximately 45 s and again at 160 s, current was applied to the electrodes resulting in the increased signal after approximately 25–30 s when the solution reached the nebulizer. Sample is introduced directly into the plasma and no further treatment of the solution was performed. Since the Arq 2 was not suppressed, it appears that plasma effects such as loading were minimal. Time profiles illustrate that the sample introduction of Cr was reasonably well behaved, relative to the metals of lower concentration (e.g. Mn and Ni). The %R.S.D. of the signals between 130 and 180 s was 5.04% for Cr and 10.78% for Ni and 8.38% for Mn. The high %R.S.D. is probably due to the heterogeneity of the electroerosion solution, which contains a colloidal suspension. All of the ICP-MS signals appeared to be somewhat noisy, particularly when low levels of metals (55Mnq) are monitored. The noisy signals encountered are likely a result of the heterogeneity of the colloidal suspension. At no point did the glass concentric nebulizer clog, however, some flicker of the plasma was observed. To illustrate the effects of solution heterogeneity on signal reproducibility, an experiment was carried out to compare signals of 52Crq, 60Niq and 55 Mnq before and after the addition of 100 ml of concentrated HNO3 to a 20-ml sample containing the colloidal suspension. The colloidal suspension was prepared with an applied current of 2.5 A cmy2 using stainless steel 308L in 0.01 M NaCl for different time intervals. The results of these experiments, which are summarized in Table 3, indicates that addition of 100 ml of concentrated HNO3 improved the signal reproducibility regardless of erosion time. Addition of HNO3 did not increase signal intensity but did improve the signal reproducibility for shorter erosion times (5 s) and smaller signals. The slight decrease in signals at shorter erosion times indicates that when acid was
D. Goltz et al. / Spectrochimica Acta Part B 58 (2003) 1325–1334
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Fig. 8. Continuous monitoring of (a)
52
Crq and (b)
58
Niq,
55
added some additional loading or cooling of the plasma may have occurred, especially when large concentrations of Naq and Cly are already in the solution. Addition of acid increased both signal intensity and reproducibility for a longer erosion time (40 s). Addition of HNO3 dissolved the colloidal material and improved the ionization efficiency of the ICP. For all erosion times, dissolution of the colloidal material improved the sam-
Mnq in the electroerosion solution over time using ICP-MS.
ple heterogeneity reproducibility.
and
ultimately
signal
4. Conclusions Electroerosion in aqueous solution is potentially useful as an alternative method for rapid dissolution of metal. Unlike spark ablation in aqueous media, the electroerosion process is quite gentle
Table 3 Signal reproducibility (%R.S.D.) before and after addition of concentrated HNO3 Signal (counts sy1) 55
52
3129"5.63 32320"12.8 34270"4.28 175800"5.60 240800"2.29 343500"5.52 398900"3.59
840"4.71 11070"9.55 9975"5.23 53350"4.26 76480"3.86 128700"4.83 133600"3.60
Mn
100 ppb 5 s—no acid 5 sqHNO3 20 s—no acid 20 sqHNO3 40 s—no acid 40qHNO3
Crq
60
40
163"4.34 3983"15.8 3621"9.19 20170"6.69 28580"4.43 45540"5.53 52000"4.20
87860"3.39 87140"2.44 61410"2.14 56600"3.00 61360"2.02 64990"2.00 62240"2.44
Niq
Arq 2
D. Goltz et al. / Spectrochimica Acta Part B 58 (2003) 1325–1334
and easily controlled. Sample preparation with this approach has certain advantages over acid dissolution such as the time required. Generally, 15–20 s at high current (10 A cmy2) is required to remove a significant amount of metal (e.g. 10 mg). The length of the time required to do quantitative analysis will be dictated by the concentration of the metal in the sample. Therefore, for lower concentrations of metal, longer erosion times are required. Another consideration for long erosion times ()50 s) at high power is the increase in temperature and possibly boiling of the aqueous solution. The possible advantage of rapid on-line sample preparation prior to sample introduction to ICPs also has potential drawbacks such as noisy signals, particularly for lower concentrations of metal in the sample. When longer erosion times are used ()40 s) at high current, a lot of material ()5 mg) is eroded from the sample. The colloidal suspension which has particle sizes of 1–10 mm and possibly higher, can settle out of solution after a few minutes. For ultratrace analysis where longer erosion times may be appropriate, this property is of importance. Further studies are in order to determine the effects of erosion time and current that affect the size of the colloidal particles in the suspension. Work done by Pchelkin et al. w27x using spark ablation in aqueous solution indicated that high power produces a less stable colloidal suspension. This suggests that lower current and longer ablation times may be more desirable for introduction into the ICP. Acknowledgments The authors acknowledge the financial support of the University of Winnipeg and NSERC. Dr E. Salin is gratefully acknowledged for the loan of the Elan 500 ICP-MS and Dr D. Beauchemin for providing electronic equipment for the Elan 500. References w1x A. Aziz, J.A.C. Broekaert, K. Laqua, F. Leis, A study of direct analysis of solid samples using spark ablation combined with excitation in an inductively coupled plasma, Spectrochim. Acta Part B 39 (1984) 1091–1103.
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w24x D.M. Goltz, G. Kostic, G. Reinfelds, Rapid dissolution of metal in aqueous media using a modified commercial spark source unit, Talanta 52 (2000) 1131–1138. w25x C. Bendicho, Determination of metal impurities in electrolytic iron by GFAAS using electric dispersion in liquid medium, Fresen. J. Anal. Chem. 348 (1994) 353–355. w26x B.V. L’vov, A.V. Novichikhin, Analysis of metals by graphite furnace AAS using spark ablation, At. Spectrosc. 11 (1990) 1–6. w27x A.I. Pchelkin, I.P. Kharlamov, M.N. Gusinskii, E.V. Shipova, Method for electric discharge preparation of metal specimens for atomic absorption analysis, Zh. Anal. Khim. 42 (1987) 2138–2145. w28x V.Yu. Karyakin, I.P. Kharlamov, A.I. Pchelkin, Electric spark method for the preparation of samples for the analysis of steels and alloys by atomic absorption with electrothermal atomization, Zavod. Lab. 54 (1988) 36–41. w29x H. Bergamin, F.J. Krug, E.A.G. Zagatto, E.C. Arruda, C.A. Courinho, Online electrolytic dissolution of alloys in flow-injection analysis. 1. Principles and applications in the determination of soluble aluminum in steels, Anal. Chim. Acta 190 (1986) 177–181. w30x J. Flock, K. Ohls, Online ICP emission spectrometric steel analysis combined with an electrolytical flow attack, Fresen. Z. Anal. Chem. 331 (1988) 408–412. w31x T. Tanaka, K. Shitan, H. Kondo, Rapid determination of copper in steel by electrolytic dissolutionystripping voltammetry, Bunseki Kagaku 50 (2001) 855–860. w32x H. Kondo, M. Aimoto, A. Ono, K. Chiba, Rapid determination of sulfur in steel by electrolytic dissolution—inductively coupled plasma atomic emission spectrometry, Anal. Chim. Acta 394 (1999) 293–297. w33x A.P.G. Gervaio, G. Caseri de Luca, A.A. Meneario, H.B. Filho, B. Freire dos Reis, I. Goncalves de Sousa, Quim. Nova 22 (1999) 669–673.
Electroerosion of metal in aqueous solution for sample introduction into an inductively coupled plasma mass spectrometer Douglas Goltz*, Michael Boileau, Gundars Reinfelds Department of Chemistry, University of Winnipeg, 515 Portage Avenue, Winnipeg, Man., Canada R3B 2E9 Received 24 October 2002; accepted 5 April 2003
Abstract When high current (1–10 A cmy2) is applied between two conductive samples (metals) in aqueous solution, electroerosion occurs on the surface as a result of electrolysis and possibly collisions of dissolved ions with the metal surface. The power supply for the electroerosion apparatus in this work was a modified spark source unit. Current could be varied in intervals of 2.5, 5 and 10 A in either half-wave (unipolar) or full-wave (bipolar) output. The electroeroded metal forms a colloidal suspension in aqueous solution with particle sizes of the order of 1–10 mm and possibly larger. The suspension is readily dissolved using a small amount (100 ml) of concentrated acid (HCl or HNO3) prior to analysis. Electroerosion of steel and brass in aqueous solution is described both for rapid sample dissolution and as a solid sampling approach for ICP-MS. Some of the electroerosion properties described in this paper include rates of erosion as a function of gap between the conductive samples and solution conductivity. Rates of electroerosion decreased from 120 to 30 mg sy1 as the gap was increased from 2 to 5 mm. Rates of electroerosion also increased significantly from 200 to 1000 mg sy1 as the conductivity of the electroerosion solution increased from 0.01 to 0.05 M NaCl. Interfacing the electroerosion apparatus to an ICP-MS was straight forward, as no special equipment was required. Therefore, the electroerosion apparatus can be used for rapid ‘on-line’ sample dissolution prior to introduction into an ICP. ICP-MS time profiles of selected metals in stainless steel 308L illustrate the behavior of 52Crq, 55Mnq and 60Niq during a typical electroerosion cycle. Aspiration of the colloidal suspension into the ICP did not appear to load the plasma significantly, however, all of the metals produced noisy signals ("10%). A glass concentric nebulizer was used without clogging, so it is likely that the heterogeneous nature of the colloidal suspension caused this effect. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Electroerosion; Heterogeneous nature; Colloidal suspension; Metal dissolution
1. Introduction The ability to analyze solids directly can provide many important advantages to the atomic spectroscopist including: minimal sample preparation and *Corresponding author. Tel.: q1-204-786-9730; fax: q1204-775-2114. E-mail address: [email protected] (D. Goltz).
sample contamination from solvents such as mineral acids. As a result of these advantages, instrumental techniques have been developed such as LASER and spark ablation for sample introduction into ICPs. Much of the early work on spark ablation for sample introduction into an ICP was directed toward solid sampling of ferrous alloys such as steels w1–6x and used atomic emission
0584-8547/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0584-8547Ž03.00070-3
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spectroscopy (AES) for detection of transition metals. Other applications of spark ablation—ICPAES have been reported on the analysis of arc furnace flue dusts w7x, geological materials w8,9x and even non-conducting materials w10x. Most work has been carried out in radial mode, however, some work has been done on spark ablation for axially viewed plasmas w11x. The robustness of the radio frequency ICPs (27 and 40 MHz) makes them suitable for handling the larger particle sizes (1–10 mm) of the eroded material, however, some work has also been carried out to interface a spark ablation apparatus to an Ar microwave-induced plasma w12,13x. More recently, spark ablation for sample introduction into an ICP-MS has been described by a number of workers w14–19x. Clearly, the important advantage of using MS for detection is that trace and ultratrace analysis of a large number of metals is possible. Many aspects of interfacing a spark ablation apparatus to an ICP-MS are similar to that of a LASER (LA) for LA-ICP-MS. The advantage of using a LASER is obvious in that the sampling position is easily controlled and does not require a conductive surface like the spark. The most important advantage of using a spark, however, is the significantly lower cost when compared to the LASER. A small number of papers w20–22x have compared various solid sampling techniques for sample introduction into ICPs including spark, LASER and glow discharge. The generation of a spark in an aqueous solution has been explored as an alternative approach for the rapid dissolution of metals. Both high-power (275–1100 W) w23x and low-power (100–300 W) w24x spark ablation in aqueous solution have been investigated for the dissolution of brass, steel and aluminum alloys. Other workers w25–28x have also explored the use of low-power spark ablation for dissolution or ‘electric dispersion’ of metals in aqueous solution for trace metals analysis using GFAAS. An alternative to the spark for dissolution of conductive solids in aqueous solution is electroerosion or electrolytic dispersion. Electroerosion occurs when a high current (1–10 A cmy2) is applied to two conductive samples (metals) in a conductive aqueous solution. Interaction of high
electric fields, which causes electrolysis of one or both metal surfaces and possibly collisions of highenergy ions with the metal surface, causes rapid erosion. As with a spark, a colloidal dispersion of metal forms in the aqueous solution that is easily dissolved with a small amount of concentrated HCl or HNO3. There are at least two important differences between spark ablation and electroerosion in aqueous solution: (1) a significantly larger gap is required between the conductive samples and (2) a conductive medium is required to carry the charge between the conductive samples. The incentive for using electroerosion rather than a spark is that it provides a very gentle erosion process that is easier to control than a spark. In terms of sampling, the spark generally acts on a single spot resulting in the formation of a crater on the surface of the sample. Electroerosion in aqueous solution does not sample a single spot; rather the surface area of metal that is submerged is eroded without the forming of a crater on the surface. Electroerosion has been explored for rapid on-line dissolution of alloys such as steel w29–32x. Bergamin et al. w29x dissolved alloys for the determination of Al and Mn using flow injection analysis. Flock and Ohls w30x analyzed steel samples using electrolytic dissolution with flame atomic absorption spectroscopy (FAAS). More recently, Gervaio et al. w33x used electroerosion for rapid dissolution of steel for on-line measurement with ICP-AES. Kondo et al. w31,32x have used electrolytic dissolution for rapid determination of S and transition metals in steel using ICP-AES. This paper describes some of the properties of the electroerosion process in aqueous solution for rapid sample dissolution. The usefulness of electroerosion for rapid on-line sample dissolution prior to sample introduction into an ICP-MS is also described. 2. Experimental The power supply used for this work was an Applied Research Laboratories (ARL) Model 26000 spark source unit from a spectrographic analyzer. It should be noted that while a spark source unit was used, any DC power supply that is capable of providing 1–10 A should work
D. Goltz et al. / Spectrochimica Acta Part B 58 (2003) 1325–1334 Table 1 Instrumental operating conditions for the ARL 26000 spark source unit Power requirement (V) Current settings (A) Electrode polarity
115 (60 Hz frequency) 2.5, 5.0, 10.0 Positive or negative
Waveform Half (unipolar) Full (bipolar) Spark gap (mm)
2.5 and 5.0 A 5.0 and 10.0 A 2.5–6 mm
equally well. The electroerosion apparatus has been described previously w23x for spark ablation in aqueous solution. Important experimental parameters are summarized in Table 1. The power requirement of the electroerosion apparatus was 115 V at 50 or 60 Hz AC and 10 A. The electrodes have an output from a 14-kV transformer and two 0.5-mF capacitors (in series), which are in parallel to the high-voltage transformer. Two parallel inductors (50 mH) in series with the capacitors and a rectifier circuit control the output current. The output current of the discharge can be fullwave rectified (bipolar) at 5 and 10 A or halfwave rectified (unipolar) at 2.5 and 5 A. The spark source unit was modified by removing the graphite electrode holders and the wires attached to them from the output transformer. Longer wires were connected from the output transformer to allow easy (and removable) connection to the electroerosion vessel. The electroerosion vessel consisted of a scintillation vial with a modified cap to hold the metal samples in place. The
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modified cap was machined in-house and has three holes in the top for easy insertion of two metal samples (rods), as well as a hole for ventilation or tubing for interfacing to the ICP. A schematic diagram of the experimental setup is shown in Fig. 1. The gap between the metal samples was measured visually using a magnified eye piece equipped with scale of "0.1 mm. Precise control of the gap between the conductive samples during electroerosion was not possible. The gap will increase slightly (-0.1 mm) with time as the surfaces of the metal samples are eroded. Metal rods (2-mm diameter) used as conductive samples were cut in approximately 7–10 cm lengths and shaped as needed. The electroerosion apparatus is not limited to rods, as pins and flat surfaces have been successfully eroded. The electroerosion process is typically carried out in a 10-ml volume for 5–30 s. The electroerosion solution consisted of deionized water containing an electrolyte (0.01 M NaCl). By making the solution conductive, generation of a spark was avoided when larger gaps ()2 mm) were used. After electroerosion was complete the metal samples were removed from the solution, rinsed with deionized water and a small amount (100–200 ml) of concentrated HCl or HNO3 was added to the solution to dissolve the colloidal suspension. Typically, the sample solution was then further diluted to a final volume of approximately 20 ml prior to analysis. By carrying out the electroerosion in a known volume of water and by carefully weighing the metal samples before and after the electroerosion process, accurate quantifi-
Fig. 1. Schematic diagram of electroerosion—ICP-MS apparatus.
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cation of bulk and trace metals in the electrode is possible. Deionized water (Milli-Q) was used for preparing all solutions. Standard solutions were prepared by appropriate dilution from 1000 mg mly1 standards (SCP Science). All solutions were made with high purity AR Select䉸 Plus HNO3 (Mallinckrodt). Measurement of bulk elements was carried out using a Perkin-Elmer 601 FAAS and trace elements were measured using a PerkinElmer Sciex Elan 500 ICP-MS. Unless indicated, standard conditions were used for both instruments. Instrumental parameters for the ICP-MS are shown in Table 2.
Fig. 2. Mass of Cu and Zn removed from brass over time in 0.01 M NaCl and a sample gap of 5.0 mm.
3. Discussion When high current (1–10 A cmy2) is applied between two conductive samples (metals) in an aqueous solution, electroerosion occurs on the surface as a result of electrolysis and possibly collisions of dissolved ions with the metal surface. As long as a large enough current is used, erosion will occur at one (unipolar) or both (bipolar) metal surfaces in aqueous solution. If the gap between the conducting samples is too small, an explosive spark can form. A number of properties influence the amount of material that is eroded from the metal surface including the gap, applied current (or power) and the conductivity of the electroerosion solution. Prior to using electroerosion as a sample introduction device for ICPs, some of these parameters were examined to characterize the electroerosion apparatus.
The first property investigated was the mass of metal removed and the rate of erosion of metals in an aqueous solution for a given power setting. Fig. 2 shows the mass of Cu and Zn eroded from a sample of brass over time. For these measurements, the electroerosion process was carried out at 2.5 A cmy2 in a known volume (;10 ml) of an aqueous 0.01 M NaCl solution. After fixed periods of time, small volumes (0.5 ml) of liquid media were removed, diluted to 20 ml and analyzed using flame AAS. Since the concentration and volume were known, the mass of metal eroded was readily calculated. Fig. 2 shows that the mass of Cu and Zn in the electroerosion solution increases with time, as would be expected, but the mass of metal appears to erode at an increasing rate over time. Using the same data, the rate of metal erosion was also calculated and plotted as a func-
Table 2 Instrumental operating and data acquisition parameters of ICP-MS ICP mass spectrometer RF power Coolant Ar flow Auxiliary Ar flow Carrier Ar flow Nebulizer
1100 W 15.0 l miny1 1.4 l miny1 1.0 l miny1 Glass concentric
ICP-MS data acquisition Dwell time Scan mode Number of masses (myz) monitored Signal measurement Resolution
50 ms Peak hopping 3–8 Average counts 0.9 amu at 10% peak height
D. Goltz et al. / Spectrochimica Acta Part B 58 (2003) 1325–1334
Fig. 3. Rate of electroerosion of Cu and Zn from brass over time in 0.01 M NaCl and a sample gap of 5.0 mm.
tion of time as shown in Fig. 3. This experiment illustrates that the rate of erosion of Cu and Zn is not constant; rather it increases significantly over time. For example, after 30 s, the rate of erosion of Cu and Zn in brass doubled the rate after 5 s. This suggests that physical or chemical properties of the electroerosion process are changing over time. During the erosion process at least two parameters change constantly with respect to time. One is the gap, which must increase slightly as metal is removed from the surface of the sample. A second parameter that must change is the chemical composition of the aqueous solution. It is possible that the eroded metal in the aqueous solution could increase the overall conductivity of the electroerosion solution, particularly between the metal samples. It is worth noting that if high concentrations of NaCl (0.05 M) are used, the contribution of ions from the eroded metal should be minimal. Another property that also changes in the electroerosion solution is temperature. The temperature of the solution changes significantly with respect to time, depending on the applied current. Usually the temperature in the solution can reach a boiling point in less than 1 min at 2.5 A cmy2. In general, for electrolyte solutions, molar conductivity increases with increasing temperature. Experiments were carried out to look at the effect of the electrode gap and the conductivity of the electroerosion media on the rate of erosion. The effect of increasing the electrode gap on the rate of erosion is illustrated in Fig. 4. An erosion time of 15 s was used at 2.5 A cmy2 in 0.01 M NaCl. Fig. 4 illustrates that increasing the gap
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Fig. 4. Effect of gap on the rate of erosion of Cu and Zn in brass and using 0.01 M NaCl.
between the conductive samples will decrease the rate of metal erosion. This suggests that changes in the gap have greater effects on rates of erosion when a small gap (1–2 mm) is used. As the change in gap is quite small (-0.1 mm) during a typical experiment, it is unlikely to have any noticeable effects on the electroerosion rate over this short period of time. This is in contrast to spark ablation where small gaps are used (-1 mm) and very small changes in the electrode gap can have profound effects on the energy of the spark and subsequent rate of ablation w23x. The effect of conductivity as shown in Fig. 5 illustrates clearly that the higher the concentration of NaCl in the electroerosion solutions, the greater the rate of erosion of metal. For the large gaps (2 mm) used in these studies, the effect of conductivity appears to be a more important consideration than the electrode gap. One reason that the rate of
Fig. 5. Effect of solution ionic strength on the rate of erosion of Cu and Zn in brass and using a sample gap of 5 mm.
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Fig. 6. Comparison of the masses of Cu to Zn eroded from brass in 0.01 M NaCl and a sample gap of 5.0 mm over time.
erosion increases at an increasing rate is because the conductivity between the metal samples increases as more ionic species are formed in the solution between them. It may be logical to minimize the role of the gap at this stage, however, it should be noted that as the gap increases, the effect of conductivity on the rate of erosion decreases significantly. It should also be noted that if the conductivity of the solution is high enough (0.05 M NaCl), the contribution of eroded metal to overall solution conductivity should be minimal. One fundamental question that requires examination is whether one metal is preferentially eroded relative to another in an alloy. If one element is removed preferentially, then erroneous analysis will result and the overall effect will be the same as an incomplete acid digestion. Perhaps, the easiest way to demonstrate whether preferential erosion of one metal is occurring in an alloy is to plot the mass of one metal with respect to another. Experiments were carried out using Cu and Zn from a brass electrode. Fig. 6 shows a plot of mass of eroded Cu with respect to eroded Zn in a sample of brass. For these experiments, a single sample of brass was eroded over time at 2.5 A cmy2 in 0.01 M NaCl, with an electrode gap of 2 mm. At fixed intervals, a small amount of liquid was removed (0.1 ml), diluted and the eroded masses of Cu and Zn were quantified. The ratio of Cu:Zn in this brass sample is approximately 3:1. Each data point in this graph represents a different time interval when the solution was sampled. The high degree of linearity (r 2s0.996) in
Fig. 6 strongly suggests that neither Cu nor Zn is removed preferentially over time for an alloy like brass. One general property that was of interest for metal dissolution was to establish whether different alloys have different rates of electroerosion. It is reasonable to hypothesize that metals and alloys with higher tensile strength have lower rates of electroerosion than metals with higher tensile strength. To test this hypothesis, a series of different metals were electroeroded under identical conditions and the mass of the metal was determined after a fixed period of time. In general, lower rates of erosion are observed with higher tensile strengths for one type of alloy (e.g. stainless steel). These results also illustrate the difficulty in trying to compare rates of erosion to tensile strength alone when looking at alloys of different bulk metal composition. For example, stainless steel appears to have a significantly lower rate of electroerosion than aluminum alloys even though it has a much higher tensile strength. It would seem logical that other factors such as the surface area of the metal and the presence of an oxide coating on the metal surface are also important considerations with regard to erosion rates. Aluminum is a good example of a metal that readily forms an oxide layer, making this type of sample less conductive. The result of this chemical property that is lower rates of erosion was observed. Solubility of the eroded metals in aqueous solution should also have a noticeable effect on the rates of erosion. This would depend, to a certain extent, on the role of dissolved oxygen or the ability to form other complexes in aqueous solution. Introduction of the solutions containing electroeroded metals into an ICP-MS was carried out using stainless steel 308L in 0.01 M NaCl. Fig. 7a–c show the mass scans of over 0–15 s. These figures illustrate, in terms of signal intensity, the length of time required for detecting trace levels of metals such as Ni and Mn in stainless steel. It should be noted that when an electrolyte such as NaCl is used, some practical considerations regarding its use are in order. For example, although the addition of the electrolyte is essential for carrying charge in the electroerosion solution, it is a possible source of metal contamination as well as a
D. Goltz et al. / Spectrochimica Acta Part B 58 (2003) 1325–1334
Fig. 7. ICP-MS mass scans showing the formation of 58Niq, 60 Niq and 55Mnq in stainless steel for (a) 0 s; (b) 5 s and (c) 15 s in the electroerosion solution.
spectral interference due to the formation of 35 Cl16Oq, which interferes with 51Vq. A number of simple solutions can be used to overcome these types of chemical or spectral interferences such as using an alternative salt (e.g. NH4NO3). Trace levels of transition metals can also be removed easily by running the salt solution through an ion exchange resin such as 8-hydroxyquinoline. Fig. 8 shows time profiles of 52Crq, 55Mnq and 60 Niq from the electroerosion of stainless steel 308L. The composition of 308L is 19.5–22% Cr, 9–11% Ni and 1–2.5% Mn as well as other trace metals and Fe. The argon dimer 80Arq 2 was also monitored to determine qualitatively if drifting or possibly loading of the plasma occurred as the
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electroerosion solution was introduced into it. Metal in high concentration (52Crq) was monitored as shown in Fig. 8a and for clarity, the behavior of metals that were lower in concentration (55Mnq and 60Niq) is shown in Fig. 8b. In these experiments, the electroerosion solution was continuously introduced into the ICP. A small stir bar was used to keep the electroerosion solution as homogeneous as possible. At approximately 45 s and again at 160 s, current was applied to the electrodes resulting in the increased signal after approximately 25–30 s when the solution reached the nebulizer. Sample is introduced directly into the plasma and no further treatment of the solution was performed. Since the Arq 2 was not suppressed, it appears that plasma effects such as loading were minimal. Time profiles illustrate that the sample introduction of Cr was reasonably well behaved, relative to the metals of lower concentration (e.g. Mn and Ni). The %R.S.D. of the signals between 130 and 180 s was 5.04% for Cr and 10.78% for Ni and 8.38% for Mn. The high %R.S.D. is probably due to the heterogeneity of the electroerosion solution, which contains a colloidal suspension. All of the ICP-MS signals appeared to be somewhat noisy, particularly when low levels of metals (55Mnq) are monitored. The noisy signals encountered are likely a result of the heterogeneity of the colloidal suspension. At no point did the glass concentric nebulizer clog, however, some flicker of the plasma was observed. To illustrate the effects of solution heterogeneity on signal reproducibility, an experiment was carried out to compare signals of 52Crq, 60Niq and 55 Mnq before and after the addition of 100 ml of concentrated HNO3 to a 20-ml sample containing the colloidal suspension. The colloidal suspension was prepared with an applied current of 2.5 A cmy2 using stainless steel 308L in 0.01 M NaCl for different time intervals. The results of these experiments, which are summarized in Table 3, indicates that addition of 100 ml of concentrated HNO3 improved the signal reproducibility regardless of erosion time. Addition of HNO3 did not increase signal intensity but did improve the signal reproducibility for shorter erosion times (5 s) and smaller signals. The slight decrease in signals at shorter erosion times indicates that when acid was
D. Goltz et al. / Spectrochimica Acta Part B 58 (2003) 1325–1334
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Fig. 8. Continuous monitoring of (a)
52
Crq and (b)
58
Niq,
55
added some additional loading or cooling of the plasma may have occurred, especially when large concentrations of Naq and Cly are already in the solution. Addition of acid increased both signal intensity and reproducibility for a longer erosion time (40 s). Addition of HNO3 dissolved the colloidal material and improved the ionization efficiency of the ICP. For all erosion times, dissolution of the colloidal material improved the sam-
Mnq in the electroerosion solution over time using ICP-MS.
ple heterogeneity reproducibility.
and
ultimately
signal
4. Conclusions Electroerosion in aqueous solution is potentially useful as an alternative method for rapid dissolution of metal. Unlike spark ablation in aqueous media, the electroerosion process is quite gentle
Table 3 Signal reproducibility (%R.S.D.) before and after addition of concentrated HNO3 Signal (counts sy1) 55
52
3129"5.63 32320"12.8 34270"4.28 175800"5.60 240800"2.29 343500"5.52 398900"3.59
840"4.71 11070"9.55 9975"5.23 53350"4.26 76480"3.86 128700"4.83 133600"3.60
Mn
100 ppb 5 s—no acid 5 sqHNO3 20 s—no acid 20 sqHNO3 40 s—no acid 40qHNO3
Crq
60
40
163"4.34 3983"15.8 3621"9.19 20170"6.69 28580"4.43 45540"5.53 52000"4.20
87860"3.39 87140"2.44 61410"2.14 56600"3.00 61360"2.02 64990"2.00 62240"2.44
Niq
Arq 2
D. Goltz et al. / Spectrochimica Acta Part B 58 (2003) 1325–1334
and easily controlled. Sample preparation with this approach has certain advantages over acid dissolution such as the time required. Generally, 15–20 s at high current (10 A cmy2) is required to remove a significant amount of metal (e.g. 10 mg). The length of the time required to do quantitative analysis will be dictated by the concentration of the metal in the sample. Therefore, for lower concentrations of metal, longer erosion times are required. Another consideration for long erosion times ()50 s) at high power is the increase in temperature and possibly boiling of the aqueous solution. The possible advantage of rapid on-line sample preparation prior to sample introduction to ICPs also has potential drawbacks such as noisy signals, particularly for lower concentrations of metal in the sample. When longer erosion times are used ()40 s) at high current, a lot of material ()5 mg) is eroded from the sample. The colloidal suspension which has particle sizes of 1–10 mm and possibly higher, can settle out of solution after a few minutes. For ultratrace analysis where longer erosion times may be appropriate, this property is of importance. Further studies are in order to determine the effects of erosion time and current that affect the size of the colloidal particles in the suspension. Work done by Pchelkin et al. w27x using spark ablation in aqueous solution indicated that high power produces a less stable colloidal suspension. This suggests that lower current and longer ablation times may be more desirable for introduction into the ICP. Acknowledgments The authors acknowledge the financial support of the University of Winnipeg and NSERC. Dr E. Salin is gratefully acknowledged for the loan of the Elan 500 ICP-MS and Dr D. Beauchemin for providing electronic equipment for the Elan 500. References w1x A. Aziz, J.A.C. Broekaert, K. Laqua, F. Leis, A study of direct analysis of solid samples using spark ablation combined with excitation in an inductively coupled plasma, Spectrochim. Acta Part B 39 (1984) 1091–1103.
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