Quality assurance of arsenic, lead, tin and zinc in copper alloys using axial inductively coupled plasma time-of-flight mass spectrometry (ICP-TOF-MS) Ha°kan Emteborg, Xiaodan Tian and Freddy C. Adams* Micro and Trace Analysis Center (MiTAC), Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerp, Belgium Received 25th May 1999, Accepted 12th July 1999 Results for As, Pb, Sn and Zn are reported for five copper alloys. Following dissolution in a refluxing HCl–HNO3 mixture and subsequent dilution, the elements were determined by inductively coupled plasma time-of-flight mass spectrometry (ICP-TOF-MS) using In as an internal standard.The following isotopes: 64Zn, 66Zn, 67Zn, 68Zn, 70Zn, 75As, 206Pb, 207Pb 208Pb, 115In, 116Sn, 118Sn and 120Sn were monitored and the intensity ratios to 115In as an internal standard were used. The analytical results were compared with results obtained by flame atomic absorption spectrometry (FAAS) for assessment of accuracy.The results agreed fairly well, except for Zn in some compositions, which was due to background interference eVects, and for Sn, where dissolution problems occurred. The relative standard deviations, % RSDs, for six replicate measurements of the elements in each copper alloy were in the range 2.0–6.1% for As, 3.8–11.0% for Pb, 1.4–9.4% for Sn and 3.6–12.9% for Zn. Uncertainty budgets for Sn and Zn associated with these determinations are described.The detection limits (3s criterion) in the solid materials with a 1.0 g sample intake and appropriate dilutions were 0.7 mg g-1 for Pb, 2.5 mg g-1 for Sn, 11 mg g-1 for As and 15 mg g-1 for Zn. trometer was commercialised by LECO, St. Joseph, MI, USA, Introduction and the performance of this instrument is described here and ICP-MS combines parts per trillion detection limits, a linear compared with results obtained independently by flame atomic range of 6–7 orders of magnitude, isotopic measurement absorption spectrometry (FAAS).capability and limited spectral interferences with a high sample Principle of operation of ICP-TOF-MS throughput and almost complete elemental coverage.1–4 Since the introduction of commercially available ICPs in 1983, most The ions are formed in the plasma and extracted into the mass instruments incorporate quadrupole mass filters as these com- spectrometer through the sampler and skimmer cones.7 The bine the necessary resolution with a reasonable cost.4 With ions are accelerated to similar kinetic energy before entering the introduction of high-resolution ICP-MS the sensitivity was the field-free flight-tube and the arrival time of an ion at the increased and isobaric interferences such as the overlaps detector is proportional to the square root of its m/z since from 40Ar+, 56ArO+ and 80ArAr+ on Ca, Fe and Se could be ions of diVerent mass acquire a diVerent velocity.To increase eliminated.5 resolution, a reflectron is used to diminish small variations of A less attractive feature of quadrupole ICP-MS is the the kinetic energy acquired by ions of the same mass, doubling moderate signal stability over a reasonably short time, typically also the flight path.The continuous production of ions in the 1–5% RSD.3 The origin of instability has several sources such ICP requires modulation since a new bundle of ions can only as changes in nebulization eYciency and variations in the be allowed to enter the flight tube when the heaviest ion has plasma tail where the ions are sampled.The signal stability reached the detector. The measurement over the whole mass can be increased significantly by measuring intensity ratios to range is essentially simultaneous and>20 000 full mass spectra an internal standard provided that sequential measurements per second can be acquired. The repetition rate of a TOF-MS of analyte and internal standard are performed within a short instrument is determined by the flight time of the slowest time span.1,3 The quadrupole mass spectrometer is only cap- (heaviest) ion.Owing to the large data flow some trimming able of such data acquisition speed for 5–7 isotopes especially of the raw data is necessary. Present systems can measure up in very short and rapid sample pulses. High-resolution mass to 65 isotopes quasi-simultaneously under default conditions. spectrometers normally do not allow this scanning speed Obviously, a very fast detector and recording system is mandaexcept for a short mass range.6 Another important limitation tory.The selected integration time and the number of replicate of scanning mass spectrometers is that the total time for a measurements performed in each sample determine the total measurement is directly proportional to the number of isotopes time of analysis. measured. Another important feature of the axial ICP-TOF-MS system There are considerable advantages in the use of a faster is the transverse rejection ion pulse (TRIP). This feature is mass spectrometer coupled to ICP, for example, time-of-flight implemented to deflect matrix ions such as O+, OH+, NO+, mass spectrometry (TOF-MS) as outlined by Hieftje et al.1,3 Ar+, ArO+ and Ar2+ which otherwise would over-saturate In this case, the plasma can be placed either orthogonal or or destroy the detector since ions of all masses theoretically axial to the time-of-flight mass spectrometer.During 1998, an enter the mass spectrometer. The deflection windows have reproducible side eVects on neighbouring isotopes and aVect axial inductively coupled plasma time-of-flight mass spec- J. Anal. At. Spectrom., 1999, 14, 1567–1572 1567roughly 5 u seen over the whole mass range. For lower m/z Prior to further dilution and analysis all samples from C, D and E were placed in an ultrasonic bath for 15 min and when (40) the narrowest TRIP setting aVects 1–3 u.The main advantage of ICP-TOF-MS is thus speed, leading these samples were to be diluted further they were shaken vigorously to suspend all of the white precipitate. to a much higher sample throughput compared with quadrupole ICP-MS. A full elemental coverage in minute volumes of Instrumentation and quantification sample and very rapid sample transients from chromatographic or laser ablation systems is also feasible using ICP-TOF-MS. An axial ICP-TOF-MS instrument (Renaissance, LECO) was Simultaneous ion extraction and high speed also enhances used for the determination of As, Zn, Sn and Pb measuring precision of isotope ratio measurements.1,3 Finally, a rapid the isotopes: 64Zn, 66Zn, 67Zn, 68Zn, 70Zn, 75As, 206Pb, 207Pb mass spectrometer allows collection of more information 208Pb, 115In, 116Sn, 118Sn and 120Sn.The signals were measured before, for example, cone maintenance is necessary when simultaneously in two diVerent detection modes, the analog nebulizing solutions with a high content of total dissolved (at high ion intensity) and the ion counting mode.The ion solids. counting mode was used for As and Zn while Pb and Sn were This paper describes the first results obtained in our labora- measured in the analog mode. Indium-115 was used as an tory using axial ICP-TOF-MS. It was decided to test the internal standard and all isotopes were also measured as ratios capability of ICP-TOF-MS by analysing five copper alloys to 115In.It is important to add the internal standard at a also independently analysed with FAAS, as this allows a direct concentration that gives a reliable signal in both detection comparison with an independent analytical technique for a modes and 20 ng ml-1 In was added to the samples and critical evaluation of its performance. The purpose was to calibrants. All samples and calibrants were acidified to 2% obtain concentrations of As, Pb, Sn and Zn in five diVerent with HNO3 (Merck suprapur grade, v/v).From the calibration homogeneous copper alloys. The copper alloys are named graphs it was clear that the contribution from 115Sn (natural A–E for the five materials. For each composition, six diVerent abundance 0.36%) on the internal standard was negligible and samples were supplied. Five replicate measurements of 13 samples containing considerable amounts of Sn were diluted diVerent isotopes were performed with a 10-s integration time to a degree where the contribution from 115Sn had no eVect for each sample and standard.For each set of five replicates on the ratio. Single element stock solutions were obtained approximately 50 s is consequently necessary using ICP- from Z-TEK (Amsterdam, The Netherlands) containing TOF-MS, while for quadrupole ICP-MS, more than 10 min 1000±3 mg ml-1 of each element (traceable to NIST SRM would be required. 3128–790704). The standards were compared with another set The measurements summarised here were performed on of single element stock solutions obtained from Merck also three diVerent days with freshly prepared calibration standards containing approximately 1000 mg ml-1 of the elements.at each occasion. Except for Sn, where a diVerence of 4.4% between both standards was measured, negligible diVerences, (below 1%) were observed between the commercial stock solutions. Equal Experimental volumes of the four Z-TEK single element stock solutions containing As, Pb, Sn and Zn were mixed to give a multi- Sample intake, cleaning and digestion element stock solution from which further dilutions were then Pieces of approximately 1 g were cut from each of 30 copper made.Calibration graphs were obtained by the analysis of 12 discs that were prepared from ultra-pure powders of the mixed standard solutions prepared to contain 0, 0.2, 0.5, 1.0, required elements. The powders were thoroughly mixed and 2.5, 5.0, 10, 25, 50, 100, 250 and 500 ng ml-1 of As, Sn, Zn then fused using hot isostatic pressing to obtain copper alloy and Pb.Samples and standards were analysed with a 10-s rods from which smaller discs could be taken. The samples integration time (n=5). For the final calculations only the (five compositions, six discs) were supplied by the Institute for ratios of 208Pb, 68Zn 75As and 120Sn to 115In were used. The Reference Materials and Measurements, Geel, Belgium, Joint operating parameters for ICP-TOF-MS are reported in Research Centre of the European Commission.The samples Table 1. For the FAAS measurements (Perkin-Elmer AAnalyst were decontaminated by washing in 10 ml of acetone (this and 300), external calibration was used and the operating paramall further products were of pro analysi grade, Merck, eters are listed in Table 2. Darmstadt, Germany) in an ultrasonic bath for 15 min. After washing, the samples were etched with 1 mol l-1 HCl (Merck) Results and discussion for 10 min using ultrasound.Following etching, the samples were washed with Milli-Q water and dried in an oven at Analytical results 100 °C. This treatment caused a black tint on some samples As can be seen from Table 3 the results obtained using ICP- and others became stained with white spots. The pieces were TOF-MS agree fairly well with those obtained by FAAS. weighed and placed in an Erlenmeyer flask and 5 ml of 50% Exceptions are Zn in compositions D and E.Reasons for this HCl (Merck) and 10 ml of 50% HNO3 (Merck) were added. are discussed below in more detail. A full spectral scan of The solution was refluxed for 20–30 min to allow complete copper alloy A is shown in Fig 1–3. In Fig. 1 the mass region dissolution of the solid. After the solution had cooled and the for Zn and As is displayed and in Fig. 2 and 3 the mass cooler had been rinsed with Milli-Q water, the resulting regions for Sn and Pb are shown.solution was transferred into a poly(ethylene) vessel and made up to approximately 100 g. The blanks were processed in the Uncertainty budgets—error propagation same way without sample present. Five procedural and reagent blanks were made i.e. one blank for each alloy composition. The uncertainty budget for Sn in alloy E and the uncertainty budgets for Zn in alloys B (high in Zn) and C (low in Zn) are For sample compositions C, D and E a white precipitate was formed rapidly after dilution with water.No further attempts included to illustrate the overall uncertainties associated with these measurements. The uncertainty budgets are presented in were made to dissolve chemically the precipitate, which was likely to be meta-stannic acid8 since X-ray fluorescence analy- Tables 4–6 and summarised in Table 7 and were made according to the guidelines issued by Eurachem.9 ses (XRF) of the precipitate mainly showed the presence of Sn. The loss of Sn during dissolution and subsequent dilution Because of the complexity of a multi-step analytical procedure where all sub-steps are associated with an uncertainty, of these solutions might aVect the analytical results for analytical techniques that require dissolution, as discussed below. it is necessary to simplify reality when an uncertainty budget is 1568 J. Anal.At. Spectrom., 1999, 14, 1567–1572Table 1 Operating parameters for axial ICP-TOF-MS Category Parameter Day 1a Day 2a Day 3a ICP conditions: Forward power/kW 1.29 1.30 1.31 Plasma flow/l min-1 16.1 14.5 14.5 Auxiliary flow/l min-1 1.00 0.804 0.782 Nebulizer flow/l min-1 0.730 0.731 0.751 Frequency/MHz 40.68 Ion deflection: O+, 16–19 u Defl. 1 start/ms 0.966 Defl. 1 width/ms 0.150 O2+, 32–36 u Defl. 2 start/ms 1.356 Defl. 2 width/ms 0.104 Ar+, 40 u Defl. 3 start/ms 1.504 Defl. 3 width/ms 0.100 Ar–Ar+, 80 u Defl. 4 start/ms 2.076 Defl. 4 width/ms 0.030 Ion focusing: Ion lens 1/V -412 Ion lens 2/V -294 Mass calibration (flight time): 7Li/ns 6746 24Mg/ns 12 138 59Co/ns 18 800 89Y/ns 22 996 115In/ns 26 084 138Ba/ns 28 536 140Ce/ns 28 740 208Pb/ns 34 942 209Bi/ns 35 024 Mass spectrometer: Flight tube/V -1470 Reflectron low/V 199 Reflectron high/V 1540 Detector/V -2490 Y steering/V -1640 Einzel lens 1/V -1280 Einzel lens 2/V -768 X steering/V -1470 aFor the categories Ion deflection, Ion focusing, Mass calibration and Mass spectrometer the settings were set identically between days.Table 2 Operating parameters for FAAS (Perkin-Elmer AAnalyst 300) Parameter As Pb Sn Zn Wavelength/nm 193.7 283.3 286.3 213.9 Slit-width/nm 0.7 0.7 0.7 0.7 Read time/s 5 5 5 5 Read delay/s 5 5 5 5 Read frequency 5 3 3 3 Flame type Air–C2H2 Air–C2H2 N2O–C2H2 Air–C2H2 made. It is the responsibility of the analyst to make sound judgements on which uncertainty components should be included and which can be overlooked. Simplifications are also necessary to break down or reduce mathematical expressions to simpler forms.The general mathematical expression to obtain the final concentration in the bronze components is: Fig. 1 Full spectral scan of mass range 62–76 from the analog channel for copper alloy A. Prominent peaks are 63Cu, 64Zn, 65Cu, 66Zn and Y={[(S/k)×Vinit×Df1×Df2]/w} (1) 68Zn. Minor peaks are derived from 67Zn, 70Zn and 75As. Note that where Y is the concentration in the sample, S is the ratio the ion counting mode was used for quantification of As and Zn.between the analytical signal for the analyte and the internal standard and k is the slope of the calibration graph. Vinit is the density-corrected weight (i.e. volume) of the initial dilution diVerent calibrants was first assessed separately. It was found that subsequent dilutions on the balance resulted in low of the dissolved sample. Df1 and Df2 are dilution factors used and w is the weight of the solid sample. The expression uncertainties as described for steps 3 and 4 given in Table 4.The two major sources of uncertainty in the calibrants thus contains only products or quotients and thus the uncertainties associated with each term can be summed up as squared arise from the initial concentration in the commercial stock solutions (i.e. 0.173% RSD) and if volumes below 0.25 ml are RSDs.9 Uncertainties associated with the concentration of the J. Anal. At. Spectrom., 1999, 14, 1567–1572 1569Table 3 Results for As, Pb, Sn and Zn in the alloys. Values are reported as % in the solid materials.Spread is given as ± one standard deviation (n=6) Copper alloy Analytical technique As Pb Sn Zn A ICP-TOF-MS 0.201±0.007 7.55±0.62 7.24±0.19 5.87±0.26 FAAS 0.184±0.003 7.93±0.59 7.26±0.09 5.96±0.14 B ICP-TOF-MS 0.106±0.006 0.397±0.015 2.10±0.04 15.09±0.54 FAAS 0.088±0.003 0.382±0.004 2.13±0.08 15.03±0.36 C ICP-TOF-MS 4.56±0.16 0.182±0.009 0.150±0.014 0.049±0.006 FAAS 4.53±0.09 0.172±0.009 0.032±0.017 0.054±0.005 D ICP-TOF-MS 0.263±0.016 9.01±1.00 8.03±0.50 0.101±0.006 FAAS 0.262±0.007 8.89±1.06 7.60±0.20 0.146±0.007 E ICP-TOF-MS 0.183±0.011 0.207±0.017 6.23±0.09 0.123±0.005 FAAS 0.181±0.007 0.196±0.003 6.30±0.40 0.153±0.003 Fig. 3 Full spectral scan of mass range 202–210 from the analog Fig. 2 Full spectral scan of mass range 110–125 from the analog channel for copper alloy A. Prominent peaks are 206Pb, 207Pb and channel for copper alloy A. Prominent peaks are 115In (internal 208Pb.Minor peak is derived from 204Pb. standard+trace amounts of 115Sn), 116Sn, 117Sn, 118Sn, 119Sn, 120Sn, 122Sn and 124Sn. Minor peaks are derived from 112Sn, 113In, 114Sn, 121Sb and 123Sb. bration graph would be the best way to account for the uncertainties in the calibrants as included in step 6. The expanded combined uncertainties and final results for used for further dilution. In the worst case, the uncertainty associated with the lowest concentration (0.2 ng ml-1) of the three examples reported in Tables 4–6 are summarised in Table 7.It can be seen that the calculated combined uncertaint- either As, Pb, Sn or Zn has been calculated to 1.02% relative uncertainty. For the 500 ng ml-1 standard solution the corre- ies are somewhat larger than the standard uncertainty from the concentrations in the six replicate samples with one excep- sponding uncertainty was only 0.201% relative. Since In was added as an internal standard to all calibrants and samples tion.This is an indication that no major source of uncertainty has been neglected and that the uncertainty budgets are likely (concentration #20 ng ml-1), the uncertainty associated with this addition to the samples must also be taken into account to be reasonably accurate. On the other hand, the lower calculated combined uncertainty for Zn in copper alloy B and is included in step 7. The addition of In is, in fact, a greater source of uncertainty alone than that associated with indicates that more sources of uncertainty possibly need to be identified and quantified.Other possible sources are (i) incom- the concentration of As, Pb, Sn and Zn in the calibrants. It was concluded that the uncertainty of the slope of the cali- plete digestion eYciency; (ii) carry-over or losses from the Table 4 Uncertainty budget for the determination of Sn in copper alloy E using ICP-TOF-MS Standard Relative Contribution to Step Description and unit Parameter Value uncertainty, u uncertainty, u (%) total uc (%) 1 Weighing of solid sample/g Ws 1.000 0.0010 0.1 2.9 2 Make up to #100 g on balancea/g D1 100.000 0.0010 0.001 0.03 3 Second dilutionb Df48 48.0 0.0010 (v1,v2) 0.1 2.9 4 Third dilutionb Df3500 50.0 0.0010 (v1,v2) 0.143 4.1 5 Measurement on ICP-TOF-MSc (ratio 120Sn/115In) MICP 1.1611 0.00191 0.16 4.7 6 Uncertainty of slope of calibration graph Uslope 0.0137 0.000225 1.64 47.8 7 Conc.of internal standard in sample/ng ml-1 SIS 20.0 0.20 1.0 29.1 8 Correction for densitya/g ml-1 r 1.0405 0.00298 0.286 8.3 aThe resulting solution following dissolution in an acid mixture was made up to approximately 100 g with Milli-Q water.The density of this solution was slightly higher than 1.00 g ml-1. In order to simplify the final calculations, a density of 1.04 was used for the initial solution for all samples. The uncertainty associated with the density has been determined to 0.286% relative.bThe uncertainties for the second and third dilution factors are calculated as Df=Ó(uv1/v1)2+(uv2/v2)2. Note that all dilutions were performed on the balance and that the uncertainties introduced in these steps are relatively small. cMeasurements on the ICP-TOF-MS instrument were undertaken in the analog mode at m/z 120Sn and m/z 115In using an integration time of 10 s and n=5. The ratio of m/z 120Sn to m/z 115In was used for calibration and quantification. 1570 J. Anal. At.Spectrom., 1999, 14, 1567–1572Table 5 Uncertainty budget for the determination of Zn in copper alloy B (high in Zn) using ICP-TOF-MS Standard Relative Contribution to Step Description and unit Parameter Value uncertainty, u uncertainty, u (%) total uc (%) 1 Weighing of solid sample/g Ws 1.000 0.0010 0.1 1.8 2 Make up to # 100 g on balancea/g D1 100.000 0.0010 0.001 0.02 3 Second dilutionb Df48 48.0 0.0010 (v1,v2) 0.1 1.8 4 Third dilutionb Df6000 50.0 0.0010 (v1,v2) 0.25 4.6 5 Measurement on ICP-TOF-MSc (ratio 68Zn/115In) MICP 0.2401 0.00413 1.72 32.0 6 Uncertainty of slope of calibration graph Uslope 0.00131 2.52 E-5 1.93 35.9 7 Conc.of internal standard in sample/ng ml-1 SIS 20.0 0.20 1.0 18.6 8 Correction for densitya/g ml-1 r 1.0405 0.00298 0.286 5.3 abAs in Table 4. cMeasurements on the ICP-TOF-MS instrument were undertaken in the ion counting mode at m/z 68Zn and m/z 115In using an integration time of 10 s and n=5. The ratio of m/z 68Zn to m/z 115In was used for calibration and quantification. Table 6 Uncertainty budget for the determination of Zn in copper alloy C (low in Zn) using ICP-TOF-MS Standard Relative Contribution to Step Description and unit Parameter Value uncertainty, u uncertainty, u (%) total uc (%) 1 Weighing of solid sample/g Ws 1.000 0.0010 0.1 0.6 2 Make up to #100 g on balancea/g D1 100.000 0.0010 0.001 0.006 3 Second dilutionb Df48 48.0 0.0010 (v1,v2) 0.1 0.6 4 Third dilutionb Df2500 50.0 0.0010 (v1,v2) 0.1 0.6 5 Measurement on ICP-TOF-MSc (ratio 68Zn/115In) MICP 0.0032 0.0004 12.5! 78.1 6 Uncertainty of slope of calibration graph Uslope 0.00131 2.52 E-5 1.93 12.1 7 Conc. of internal standard in sample/ng ml-1 SIS 20.0 0.20 1.0 6.2 8 Correction for densitya/g ml-1 r 1.0405 0.00298 0.286 1.8 abAs in Table 4.cA high degree of uncertainty is associated with step 5 and comes from the large signal fluctuation due to high background caused by the close proximity of intense signals from 65Cu and 63Cu.The background was subtracted and the ratio to 115In was then calculated manually. Otherwise as stated in Table 5. Table 7 Summary of results from uncertainty budgets given in Tables 4–6 and measurements given in Table 3 Concentration (%)±expanded Calculated combined Standard uncertainty from combined uncertaintyb (Cx±U) Element/sample uncertainty, uc a measurements (Table 3) using a coverage factor of 2 Sn in E 0.020 0.014 6.33±0.27 Zn in B 0.028 0.036 14.92±0.82 Zn in C 0.127 0.127 0.047±0.012 aTotal combined uncertainty of u in Tables 4–6.bU is calculated according to ref. 9. A coverage factor of 2 gives an interval containing approximately 95% of the distribution of values. sample digestion vessels; and (iii) memory eVects in the sample signal obtained in the ion counting mode for the 0.2 ng ml-1 standard divided by the slope of the calibration graph for each introduction system. The latter sources of uncertainty are element and then multiplied by the dilution factor.In this diYcult to quantify and they represent type B uncertainties example, samples of approximately 1 g of material were diluted i.e. an educated estimation by the analyst would have to approximately 45 000 times (1 g to 100 ml and then diluted suYce.9 450 times). In most cases the samples were diluted further (2500–6000 times) and the detection limits under these con- Calibration, detection limits and precision ditions are obviously somewhat higher.For the five procedural Equations for the calibration graphs and their correlation blanks, the four elements were below the detection limits in coeYcients are given in Table 8 along with detection limits for the solution obtained following the second dilution. The As, Pb, Sn and Zn in the solid materials. The detection limits detection limits without including the dilution factors were: 15 ng l-1 for Pb, 55 ng l-1 for Sn, 280 ng l-1 for As and were calculated as three times the standard deviation of the Table 8 Figures of merit for the ICP-TOF-MS system for determination of As, Pb, Sn and Zn in copper alloys Element Equationa (slope) r2 Detection limitb/mg g-1 Comments As 0.0020 0.9995 11 Ion counting mode used for analytical measurements Pb 0.0336 0.9998 0.7 Analog mode used for analytical measurements Sn 0.0137 0.9963 2.5 Analog mode used for analytical measurements Zn 0.0013 0.9995 15 Ion counting mode used for analytical measurements aCalibration equations for ratio of given element to 115In, with a forced zero intercept.bNote that the ion counting mode was used to calculate detection limits. J. Anal. At. Spectrom., 1999, 14, 1567–1572 1571320 ng l-1 for Zn. The detection limits reported above can be Consequences of precipitation of tin in compositions C, D and E improved to between 4 and 50 ng l-1 for these elements by For discs C, D and E a white precipitate was formed following further optimisation of several parameters.These detection dissolution and dilution of the solid material. As mentioned limits are roughly a factor of 10 worse than for quadrupole previously, Sn is a major component of the precipitates as ICP-MS. This diVerence can be explained by the modulation qualitatively assessed by XRF. For compositions A and B no of the ion source, leading to a lower sensitivity which results precipitate was observed and the results are consequently of a in higher detection limits.The RSDs for six replicate measurehigher quality. The reason for the observed diVerence between ments of the four elements in each copper alloy were in the the alloys following dissolution could be the high amount of range 2.0–6.1% for As, 3.8–11.0% for Pb, 1.4–9.4% for Sn Zn present in compositions A and B which is more easily and 3.6 – 12.9% for Zn. oxidised than Sn when partially using nitric acid for dissolution. The low content of Zn in C, D and E thus possibly Isobaric interference facilitates formation of meta-stannic acid, which is notoriously The axial ICP-TOF-MS instrument is diVerent from a conven- diYcult to dissolve as described in ref. 8. Since both ICPtional quadrupole ICP-MS instrument as all ions theoretically TOF-MS and FAAS require dissolved samples, the results reach the detector, measured intentionally or not. The obtained for Sn in C, D and E should be interpreted with implementation of the TRIP is necessary in order to avoid some caution. One major discrepancy is also obvious from the detector overload.This is achieved by removing matrix ions result for Sn in composition C as shown in Table 3. from the plasma or the sample by pulsing a high voltage perpendicular to the ion beam at regular intervals. The TRIP Conclusions settings in this particular case are displayed in Table 1. Since copper is the matrix element in the samples (see Fig. 1 and 4), Axial ICP-TOF-MS gives reasonable precision and accuracy Zn is measured close to over-saturating copper signals.The for As, Pb, Sn and Zn in most copper alloys, quantitative data background level in the ion counting mode is therefore chang- becoming available in a short time. Following initial screening, ing rapidly for masses 64, 66, 67, 68 and 71 as shown in Fig. 4. considerable saving of time can be expected compared with To avoid an over-estimation of the Zn content, the raw signals quadrupole ICP-MS.It should be noted that the four elements were treated separately in an Excel spreadsheet and the determined have diVerent masses (64–208 u) and ionization excessive background was subtracted. Nevertheless, the final energies, (As 9.81, Zn 9.39, Pb 7.41 and Sn 7.34 eV). This result for Zn in copper alloys D and E is low compared with demonstrates the wide elemental coverage, which is associated FAAS as can be seen in Table 3. The mean values obtained with ICP-MS in general and ICP-TOF-MS in particular.The are well outside±two standard deviations of the mean of the nature of the spectral interference must, however, be attributed other method of analysis. A possible reason may be that the to an inherent ICP-TOF-MS feature, namely, that all ions detector to a certain degree is blinded following the intense reach the detector unless they have been intentionally deflected. signals on m/z 63 and m/z 65. In this mass range the signals In this particular case, the very high concentration of copper at m/z 63–64, 65–66 and 65–68 are only 150 and 450 ns apart, caused interference in the determination of low concentrations respectively, and the detector does not seem to recover com- of Zn.Use of alternative plasma gases and/or the implemenpletely. This results in apparent concentrations that are too tation of a collision cell might be a way to reduce such low. Note that the observed eVect is not likely to be due to problems in the future.low abundance sensitivities as the manufacturer reports an abundance sensitivity of 105 for m/z 24 (m/m-1). It is also Acknowledgements feasible to deflect masses 63 and 65. This would aVect the Zn isotopes in a reproducible way. It was nevertheless decided A visiting post-doctoral fellowship from ‘Fonds voor not to use any deflection of the copper ions because the Wetenschappelijk Onderzoek-Vlaanderen, FWO,’ is gratefully intensities of the Zn signals are already low and adding a acknowledged by H.E. The authors are also indebted to Mr. deflection would not increase the S/N. The narrowest deflection W. Van Mol, University of Antwerp, UIA, for the FAAS window aVects roughly 5 u in this mass region. Normally, the measurements. Dr. M. Adriaens, UIA, is acknowledged for background level was low and stable i.e. around 2–5 counts the information concerning the preparation of the copper for As, Pb and Sn. alloys and Mr. P. Lemberge, UIA, is acknowledged for the XRF analyses. References 1 G. M. Hieftje, D. P. Myers, G. Li, P. P. Mahoney, S. J. Burgoyne, S. J. Ray and J. P. Guzowski, J. Anal. At. Spectrom., 1997, 12, 287. 2 Inductively Coupled PlasmaMass Spectrometry, ed. A. R. Date and A. L. Gray, Blackie, London, 1989. 3 P. P.Mahoney, S. J. Ray and G. M. Hieftje, Appl. Spectrosc., 1997, 51, 16A. 4 R. S. Houk, Anal. Chem., 1986, 58, 97A. 5 L. Moens, F. Vanhaecke, J. Riondato and R. Dams, J. Anal. At. Spectrom., 1995, 10, 267. 6 F. Vanhaecke, L. Moens, R. Dams and P. Taylor, Anal. Chem., 1996, 68, 567. 7 H. Niu and R. S. Houk, Spectrochim. Acta, Part B, 1996, 51, 779. 8 Merck Index, Merck, Rahway, NJ, 100th edn., 1989, entry 9376, Fig. 4 Zn measured in bronze component E. Note that the signals for tin. the copper isotopes (63Cu and 65Cu) over-saturate the ion-counting 9 Quantifying Uncertainty in Analytical Measurement, Eurachem detector resulting in a high and rapidly changing background. Since Guide, Laboratory of the Government Chemist, Teddington, UK, this mode had to be used for quantification, the increased background 1995. will lead to an overestimation of Zn unless it is corrected for as described in the text. The detector is, however, partially blinded, which results in an apparent concentration that is too low in the final Paper 9/04208C corrected result. 1572 J. Anal. At. Spectrom., 1999, 14, 1567–1572