Determination of cerium, neodymium and samarium in biological materials at low levels by isotope dilution inductively coupled plasma mass spectrometry Bing Li,* Yali Sun and Ming Yin Institute of Rock and Mineral Analysis, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, China. E-mail: irma@mail.sparkice.cn Received 2nd July 1999, Accepted 24th September 1999 A method was developed for the simultaneous determination of Ce, Nd and Sm in biological materials at subor low ng g21 levels by isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS).The monitored isotopic pairs for sample analysis were 140Ce/142Ce, 143Nd/146Nd and 147Sm/149Sm. The isobaric interference of 142Nd on 142Ce was corrected by measuring 143Nd and calculating the percentage contribution of 142Nd to the 142Ce analyte peak. The total mass bias was determined experimentally and corrected by use of a standard solution of natural abundances. The overall performance of the procedure was checked by analysing standard solutions of natural Ce, Nd and Sm.The recoveries of Ce, Nd and Sm at the 10 ng level were 99.7, 103 and 102% with precisions of 1.4, 1.5 and 2.6% RSD, respectively. The validity of the analytical procedure was further examined by analysing a certiÆed reference material (CRM) (Bush Leaves, GBW 07603, China). The results were in good agreement with the certiÆed values of the CRMs (all results fell within the speciÆed uncertainties), with RSDs of 6.3±8.5%.The method detection limits for Ce, Nd and Sm were 0.55, 0.17 and 0.10 ng g21, respectively. The method was used to determine Ce, Nd and Sm in two Chinese CRMs of Human Hair (GBW 09101) and Wheat Flour (GBW 08503). The results obtained by ID-ICP-MS agreed well with those obtained by external calibration ICP-MS from four laboratories; the deviations of Ce, Nd and Sm for Human Hair were 0.5, 3.4 and 1.9%. and for Wheat Flour 1.4, 4.7 and 10.1%, respectively. A t-test indicated that the results determined by ID-ICP-MS showed no signiÆcant difference from those obtained by external calibration ICP-MS (Pw0.05), except for Sm in Wheat Flour (Pv0.05).The results were also compared with those of an NAA method. The deviations of Ce, Nd and Sm for Human Hair were 2.1, 19.7 and 4.1%, and for Wheat Flour 3.7, 14.0 and 18.0%, respectively. A t-test indicated that the results for Ce and Sm in Human Hair and Ce in Wheat Flour showed no signiÆcant difference between ID-ICP-MS and NAA (Pw0.05); however, for Nd in both samples and Sm in Wheat Flour there were signiÆcant differences between ID-ICP-MS and NAA (Pv0.05).A z-score assessment program was carried out following similar procedures to those used in the International ProÆciency Test for Analytical Geochemistry Laboratories. The results indicated that all zscore results for Ce, Nd and Sm for both Human Hair and Wheat Flour samples were in the range 22vzv2.Hence, the analytical results of the present work were considered to be satisfactory. In recent years the monitoring and evaluation of rare earth elements (REEs) in some biological materials has received increasing attention, from both a nutritional and a toxicological point of view, owing to the use of REEs in agrotechniques in China. The study of REE application in the agricultural and biological Æelds has achieved signiÆcant progress in recent years.1 The optimum concentration of some light REEs can promote the growth of plants signiÆcantly.2 Information about REE distribution in biological samples is basic to an understanding of their physiology and is useful in agronomy, as for instance in the design of fertilizer operations, as well as environmental pollution assessments or in studies of biochemical processes. The quality of analytical data is of course a prerequisite for such investigations. CertiÆed reference materials (CRMs) play an important role in quality assurance.At present, however, few certiÆed values for REEs in biological reference materials are available.3 The concentration of REEs in some biological materials remains very poorly characterised. For example, Human Hair (NIES CRM 5 and GBW 09101) and Wheat Flour (NIST SRM 1567 and GBW 08503) are certiÆed for only a few trace elements and values for the REEs are not available. This is mainly due to the limitations of available instrumentation historically and the very low levels of REEs present in these materials.This work is a continuation of our previous work,4 which is part of a project intended to study the risk of application of REEs in agrotechniques under the auspices of the Chinese National Natural Science Foundation. We were assigned to provide the concentrations of Ce, Nd and Sm by ID-ICP-MS for two Chinese reference materials of Human Hair and Wheat Flour. Isotope dilution is a powerful strategy for elemental analysis.The combination of ID with ICP-MS offers further suitability because it permits accurate and precise determination of elements, particularly in the certiÆcation of environmental and biological CRMs where analytical values from several analytical methods with different analytical principles are required. ID-ICP-MS has a number of distinct advantages over ICP-MS with external calibration, as with ID-ICP-MS the results are hardly affected by, e.g., signal drift or matrix effects, or partial loss of the analyte during sample preparation; additionally, it is also a more `traceable' technique.Although ID-ICP-MS has frequently been applied to the determination of a number of elements in various matrices,5±17 only a few applications have been reported for the REEs at ultratrace levels in biological materials, such as human hair and wheat Øour samples. Field and Sherrell18 and Esser et al.19 have successfully determined trace levels of REEs in natural samples and waters by using ID-ICP-MS.The purpose of the present study was to develop an ID-ICPJ. Anal. At. Spectrom., 1999, 14, 1843±1848 1843 This journal is # The Royal Society of Chemistry 1999MS method for the determination of Ce, Nd and Sm in biological materials and provide the ID-ICP-MS results for Ce, Nd and Sm in Human Hair and Wheat Flour reference materials. The validity of the analytical procedure was examined by analysing a Bush Leaves CRM (GBW 07603, China).The analytical results obtained by ID-ICP-MS were compared with those obtained by neutron active analysis (NAA) and external calibration ICP-MS methods. The problems of optimum dilution ratio and precision of IDICP- MS are discussed. The effectiveness of the correction for both mass bias and isobaric interferences is also discussed. Experimental Instrumentation The ICP-MS instrument used was a POEMS (Thermo Jarrell Ash, Franklin, MA, USA).In order to obtain the optimum instrumental conditions, the parameters inØuencing isotope ratio determinations were carefully optimized. The ion lens voltage settings and other parameters of the instrument were tuned to obtain a compromise between maximum sensitivity and minimum mass bias. Under the compromise conditions, nearly uniform count rates (about 100 000 counts s21) for Rh, In and Tb (10 ng ml21) were obtained. The accuracy and precision of isotope ratios were tuned, generally being 0.2±0.5% RSD, by tuning the instrumental parameters (e.g., ion lens voltages, number of sweeps and examination points per peak) for the analysis of NIST SRM 981 Natural Lead (about 0.5 mg ml21).The optimized instrumental parameters established for all further experiments are summarized in Table 1. Reagents and spike isotopes Standard solutions of REEs were prepared by diluting the stock standard solutions available from the National Research Center for CertiÆed Reference Materials (Beijing, China).All acids used were of ultrapure grade (Beijing Institute of Chemical Reagent Research, China). Nitric acid and water were further puriÆed in a clean room by quartz sub-boiling distillation. The enriched spike isotope 142Ce was purchased from the China Institute of Atomic Energy. The 142Ce spike stock solution was prepared by dissolving 142CeO2 powder in 1M nitric acid (concentration of Ce, 320.03 mg g21). The spike isotope solutions for 146Nd and 149Sm were obtained from the Laboratory of Isotope Geology, Chinese Academy of Geological Sciences (concentrations of Nd and Sm, 1.6812 and 0.8373 mg g21, respectively).The stock solutions were stored in polyethylene containers at 4 �C in a refrigerator. A mixed working spike isotope solution for 142Ce, 146Nd and 149Sm was prepared gravimetrically by gradually diluting the stock spike solutions to the target concentrations (142Ce about 75 ng, 146Nd about 15 ng, 149Sm about 8 ng).The accurate concentrations of the spike solutions were determined by the reverse ID technique. The isotopic compositions of the spikes were accurately checked by using thermal ionization mass spectrometry (TIMS). These data were supplied by the Laboratory of Isotope Geology, Chinese Academy of Geological Sciences. A listing of the enriched isotopic abundances for these materials is given in Table 2. Samples Three Chinese certiÆed reference materials were analysed: Human Hair GBW 09101, Wheat Flour GBW 08503 and Bush Leaves GBW 07603.All the CRMs were dried in an oven at 80 �C for 6 h and then stored in a desiccator. Sample dissolution A sample portion of about 2°0.1 g (for Human Hair and Wheat Flour) or 0.1°0.01 g (for Bush Leaves) was accurately weighed and placed in a glass beaker. About 1 g of mixed working spike solution was then added gravimetrically to the sample, after which 10 ml of HNO3 and 2 ml of HClO4 were added in sequence.After standing overnight at room temperature to ensure isotope equilibrium, the sample was evaporated to incipient dryness on a hot-plate. The residue was treated with about 8 ml of 5% HNO3 solution, then heated gently until the solution became clear. The Ænal dilution factor is about 4 for Human Hair and Wheat Flour samples, and about 80 for Bush Leaves. The solution was ready for analysis by ID-ICP-MS. The blank was prepared in exactly the same way as the samples.Isotope dilution The ID-ICP-MS calculation is based on the following conventional equation:20 Cs~ MspKÖBsR{AsÜ WsÖAx{BxRÜ (1) where Cs is the concentration of the determined element in the sample, Msp the mass of the spike, K the ratio of the natural atomic weight to the atomic weight of the enriched material,Ws the weight of the sample, Ax the natural abundance of the `reference' isotope, Bx the natural abundance of the `spike' isotope, As the abundance of the `reference' isotope in the enriched spike, Bs the abundance of the `spike' isotope in the enriched spike, andRthe measured reference/spike isotope ratio.It should be noted that all counts of `spike' and `reference' were corrected by a total mass bias factor and isobaric interference factor (see below) prior to concentration calculation. Results and discussion Optimum dilution ratio and precision The ID method is based on addition of a known amount of enriched isotope to a sample. After equilibration of the spike Table 1 Operating conditions for ID-ICP-MS Instrument POEMS (Thermo Jarrell Ash) Forward power 1350 W ReØected power v5W Coolant Øow rate (Ar) 15 l min21 Auxiliary Øow rate (Ar) 1.5 l min21 Carrier gas Øow rate (Ar) 0.8 l min21 Sampling cone oriÆce (Ni) 1.2 mm Skimmer cone oriÆce (Ni) 1.0 mm Resolution 0.8 u Acquisition mode Pulse Number of sweeps 100 Examination points per peak 5 Scan time per u 2 s No.of replicates 6 Table 2 Enriched isotope spike information for the elements determined in this study Abundance (%) Element Isotope Spike Natural Ce 140 7.525 88.48 142 92.202 11.07 Nd 143 0.392 12.17 146 96.837 17.22 Nd 144 0.692 23.85 146 96.837 17.22 Sm 147 0.273 14.97 149 96.498 13.83 152 0.517 26.72 149 96.498 13.83 1844 J.Anal. At. Spectrom., 1999, 14, 1843±1848isotope with the analyte in the sample, the altered isotope ratio is measured to calculate the analyte concentration. As a general rule in ID-ICP-MS,20 the amount of the spike is usually selected so that the measured ratio is near unity, to maximize the mass spectrometry analytical precision.Also, it is normal to use the most abundant isotope as the reference in order to obtain the best possible sensitivity. The existence of isobaric interferences can seriously degrade the accuracy of isotope dilution analysis, and the ideal is always to Ænd two isotopes, which are completely free from such interferences. However, in practice, it is sometimes hard to meet all these requirements because of the ultratrace levels of the elements to be determined and/or the limited number of available spike isotopes.Hence, the isotope dilution target ratio for spiked to reference isotope is usually determined as a compromise between minimizing the error magniÆcation factor and minimizing measurement uncertainties which increase as ratios deviate from 1 : 1. The problem of precision in the ID method has been described in the literature.12,18±21 The isotope dilution target ratio can be different when applying different pairs of isotopes. For example, the optimum dilution ratios for 187Re/185Re and 192Os/190Os are 0.21 and 0.16, respectively, in the study of the Re±Os geochronometry.21 The geometric mean of spiked to reference isotope ratio was 14 : 1 (R~0.07) for Nd and 11 : 1 for Yb (R~0.09).18 An expression for the relative error in the determination of the concentration can be derived from eqn.(1): dC dR ~f 0ÖRÜ (2) dC~C Bs BsR{As { Bx BxR{Ax dR (3) dC C 2 ~ R Bs BsR{As { Bx BxR{Ax 2 dR R 2 (4) dC/C is the relative error in the concentration. Thus, it is apparent that the relative error in the concentration depends on the error in measuring the ratio, dR, and on the magnitude of the ratio, R. Let R Bs BsR{As { Bx BxR{Ax 2 ~P (5) where P is the error magniÆcation factor. By plotting P against R, it is possible to examine the dependence of precision on the magnitude of the R-value.Fig. 1 shows the relationships between P and R for Ce, Nd and Sm. It is clear that for the pair of 140Ce/142Ce, it provides a wide spiking ratio range (from 0.5 to 1.3) without signiÆcantly sacriÆcing the precision of the calculated concentrations. However, for the pairs of Nd and Sm, the optimum ratio range is very limited. For example, the error magniÆcation factors increase rapidly at ratios of 143Nd/146Nd below 0.05 or above 0.2. In practice, the optimum isotope dilution target ratio can be derived from eqn. (5): R~ ÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ Rs|Rx p (6) where R is the optimum ratio for the lowest error magniÆcation factor, determined as the geometric mean of the reference to spiked isotope ratio.In this work, the optimum ratio for the lowest error magniÆcation factor is 0.808 for 140Ce/142Ce, 0.054 for 143Nd/146Nd, 0.099 for 144Nd/146Nd, 0.055 for 147Sm/149Sm and 0.102 for 152Sm/149Sm. According to these calculations, a mixed spike working solution of enriched 142Ce (about 75 ng), 146Nd (about 15 ng) and 149Sm (about 8 ng) was added gravimetrically, at which the ratio values determined for Human Hair and Wheat Flour were close to the calculated optimum ratios for the lowest error magniÆcation factor. In quadrupole ICP-MS, isotope ratio measurements are preferably carried out at very high count rates (about 105±106) to improve the precision of the results. At high count rates, a typical precision of 0.2±0.5% RSD can be obtained.In this work, because of the low concentrations in both samples, the counts of the `reference' 147Sm and 143Nd ( about 103) did not provide the best sensitivity even with a large amount of sample (2 g). Table 3 shows the typical precision of isotope ratios determined for the Human Hair and Wheat Flour CRM samples. The precision of the isotope ratios was generally less than 3% RSD. However, the precision is acceptable for the present work.An accurate concentration can also be obtained, e.g., the results for the Bush LeaCRM were in good agreement with the certiÆed values, provided that the isotope dilution ratio was suitable. However, it should be noted that for both the `spiked' and `reference' isotope, the signal intensity of Ce, Nd and Sm for both the Human Hair and Wheat Flour samples signiÆcantly exceeded the blank levels (at least 100 times) in the present work. Proposed pairs of isotopes and correction of isobaric overlap For determination of Nd and Sm, 143Nd/146Nd and 147Sm/149Sm were selected. Both isotope pairs are free from isobaric interferences.For determination of Ce, 140Ce/142Ce was selected. For the purpose of comparison, the isotope pairs 144Nd/146Nd and 152Sm/149Sm, in which 144Nd and 152Sm are overlapped by 144Sm and 152Gd, respectively, were also monitored. The reference 140Ce is the most abundant isotope; however, a more serious isobaric overlap problem exists from 142Nd (11.1% relative natural abundance) on the spike isotope 142Ce.Fig. 1 Error magniÆcation factor (P) against the ratio for reference to spiked isotope (R). J. Anal. At. Spectrom., 1999, 14, 1843±1848 1845Thus, a correction is necessary to m/z 142 to account for the contribution of 142Nd. This was performed by measuring 143Nd (12.7% relative natural abundance) and calculating the percentage contribution of 142Nd to the 142Ce analyte peak. It is noteworthy that the 142Ce spike was found not to contain Nd in sufÆcient amount to necessitate correction of 142Nd interference on 142Ce.Since a certain degree of error is always introduced into the measurement when corrections are applied, the effectiveness of the correction for the isobaric overlap, and also for mass bias correction in ID-ICP-MS, was assessed. The isobaric interference corrections are based on the following equations: 142Ce~142integral{R142Nd=143Nd|143Nd ~142integral{Ö2:227|143NdÜ (7) 144Nd~144integral{R144Sm=147Sm|147Sm ~144integral{Ö0:2064|147SmÜ (8) 152Sm~152integral{R155Gd=152Gd|155Gd ~152integral{Ö0:0136|155GdÜ (9) Isotope ratio measurements and mass bias correction It is well known that, in ICP-MS, the space charge effect and a nozzle separation effect always cause mass discrimination problems which inØuence the accuracy of isotope ratio determinations.From the equation of ID-ICP-MS, the only parameter that requires measurement is the isotope ratio, R; thus, precise and accurate isotope ratio measurements are very important.The value of R is inØuenced by a number of factors, such as the sensitivity of the instrument, the mass bias of the ICP-MS and the dead time of the detector.22,23 However, the total mass bias in a mass spectrometer can be determined experimentally by the mass discrimination factor. It is common practice to compensate for the mass bias effect by bracketing the analysis of the spiked sample with a pure reference standard of known isotopic composition.In this work, the total mass bias was determined experimentally and corrected daily by use of a standard solution of natural abundances for all counts determined prior to calculation of concentration. The mass bias factor was obtained as follows: ÖA=BÜa~ÖA=BÜtÖ1zanÜ (10) where (A/B)a is the observed ratio of isotope A to isotope B, (A/B)t the known isotope ratio of the reference standard, a the bias per mass unit and n the mass difference between the measured isotopes.Table 3 shows the typical isotope ratios and precisions for the samples. Table 4 shows the typical total mass bias determined on different days. It is clear that the mass bias is not constant across the mass range, but in contrast is strongly dependent on the analyte mass number. In addition, the mass bias factor is also not constant on different days of operation. Hence, in this work the mass bias factor used is the actual isotope ratio of interest itself and is corrected daily.It can also be seen that the mass bias for the 140Ce/142Ce isotope ratio was sometimes higher than the corresponding theoretical value. This is contrary to what is expected. There should be no Nd contamination problem, because the counts were corrected by means of eqn. (7) prior to calculation of the mass bias factor. This might be due to measurement error and isobaric interference correction error.Further investigation is required to clarify the reason. However, this is not likely to be a problem for the concentration of Ce in this work, as can be demonstrated from Table 5, in which the results for Ce with and without mass bias correction show no statistical difference. Heumann et al.22 have reported that, for ID-ICP-MS analysis, mass discrimination of the different isotopic terms compensates for each of them so that an excellent approximation of the accurate result can usually also be obtained without correcting Table 3 Precision of isotope ratios determined for Human Hair and Wheat Flour Isotope ratio determined (R) 140Ce/142Ce 143Nd/146Nd 147Sm/149Sm Replicate Human Hair Wheat Flour Human Hair Wheat Flour Human Hair Wheat Flour 1 0.5882 0.8190 0.1110 0.1800 0.0536 0.0764 2 0.5742 0.7979 0.1056 0.1760 0.0531 0.0831 3 0.5753 0.8004 0.1057 0.1776 0.0531 0.0791 4 0.5853 0.7994 0.1060 0.1793 0.0548 0.0787 5 0.5855 0.8325 0.1058 0.1710 0.0539 0.0820 6 0.5871 0.7937 0.1056 0.1772 0.0513 0.0804 Mean (n~6) 0.5826 0.8073 0.1066 0.1769 0.0533 0.080 s 0.0062 0.0153 0.0022 0.0032 0.0012 0.0024 RSD (%) 1.1 1.9 2.0 1.8 2.3 3.0 Table 4 Mass bias of isotope ratios and precision for a 50 ng ml21 standard solution Standard solution 140Ce/142Ce 143Nd/146Nd 147Sm/149Sm Date Theoretical ratio 7.9730 0.7093 1.0870 26/11/98 Determined ratio 7.9412 0.6949 1.0805 Mass bias factor 20.0020 20.0068 20.0030 30/11/98 Determined ratio 7.9134 0.6914 1.0824 Mass bias factor 20.0037 20.0084 20.0021 2/12/98 Determined ratio 7.9575 0.6935 0.0777 Mass bias factor 20.0010 20.0074 20.0043 8/12/98 Determined ratio 8.0480 0.6933 1.0745 Mass bias factor 0.0047 20.0075 20.0057 10/12/98 Determined ratio 8.0547 0.6983 1.0807 Mass bias factor 0.0051 20.0052 20.0029 1846 J.Anal. At. Spectrom., 1999, 14, 1843±1848any data. A comparison was made between the results with and without mass bias correction in this work. As can be seen from the results listed in Table 5, the results for Ce with and without mass bias correction are indeed identical.However, the fact that all data for Nd and Sm were systematically biased suggests mass-dependent variations in the mass bias correction. Of course, for accurate isotope ratio determination, mass discrimination has to be appropriately corrected for. Validation of the proposed method and sample analysis To examine the reliability of the proposed method, a series of solutions containing Ce, Nd and Sm at 10, 20 and 30 ng were prepared and analysed.The recoveries are listed in Table 6. The recoveries of Ce, Nd and Sm at the 10 ng level were 99.7, 103 and 102% with precisions of 1.4, 1.5 and 2.6%, respectively. The validity of the analytical procedure was further examined by analysing a Bush Leaves CRM (GBW 07603). In order to match the concentration levels with those of the Human Hair and Wheat Flour samples, about 0.1 g of Bush Leaves was weighed for analysis.As can be seen from Table 7, all results for the three elements were in good agreement with the certiÆed values (all results fell within the speciÆed uncertainties). Hence, the present method is satisfactory for the determination of Ce, Nd and Sm in biological materials at such low concentration levels. The means of the analytical results for Human Hair and Wheat Flour reference materials prepared and analysed independently (n~7) were compared with those obtained by NAA and external calibration ICP-MS methods.4 As shown in Table 7, the values found by ID-ICPMS agree well with those obtained by external calibration ICPMS from four laboratories; the deviations of Ce, Nd and Sm for Human Hair were 0.5, 3.4 and 1.9%, and for Wheat Flour 1.4, 4.7 and 10.1%, respectively.A t-test indicated that the results determined by ID-ICP-MS showed no signiÆcant difference from those obtained by ICP-MS (Pw0.05), except for Sm in Wheat Flour (Pv0.05).The results were also compared with those of the NAA method. The deviations of Ce, Nd and Sm for Human Hair were 2.1, 19.7 and 4.1%, and for Wheat Flour 3.7, 14.0 and 18.0%, respectively. A t-test indicated that the results for Ce and Sm in Human Hair and Ce in Wheat Flour showed no signiÆcant difference between IDICP- MS and NAA (Pw0.05); however, for Nd in both samples and Sm in Wheat Flour, there were signiÆcant differences between ID-ICP-MS and NAA (Pv0.05).In order to assess further the reliability of the results obtained by the present IDICP- MS method, a z-score assessment was carried out following similar procedures to those used in the International Table 5 Comparison of results for Human Hair (ng g21) with and without mass bias correction Ce Nd Sm Date Uncorrected Corrected Uncorrected Corrected Uncorrected Corrected 26/11/98 19.4 19.2 7.97 8.15 1.33 1.43 30/11/98 19.2 19.1 8.15 8.31 1.34 1.43 2/12/98 20.1 19.8 8.51 8.68 1.43 1.52 8/12/98 20.9 20.6 8.46 8.63 1.46 1.56 Table 6 Comparison of recoveries and precisions for standard additions at different concentrations.n~6 in all instances 140Ce/142Ce 143Nd/146Nd 144Nd/146Nd 147Sm/149Sm 152Sm/149Sm Added/ng Found/ng Recovery (%) Found/ng Recovery (%) Found/ng Recovery (%) Found/ng Recovery (%) Found/ng Recovery (%) 10 9.97 99.7 10.3 103 10.4 104 10.2 102 10.4 104 Ra 0.2302 0.0741 0.1443 0.1585 0.2888 RSD (%) 1.41 1.50 1.97 2.62 0.93 20 23.4 117 20.8 104 21.2 106 20.5 103 21.1 106 R 0.3991 0.1311 0.2588 0.2767 0.5032 RSD (%) 0.82 1.63 1.19 1.04 1.52 30 29.9 99.7 31.4 105 31.9 106 31.1 104 32.0 107 R 0.4783 0.1782 0.3523 0.3690 0.6692 RSD (%) 0.84 0.74 1.41 1.38 1.04 aR: Measured isotope ratio. Table 7 Comparison of results (mean°standard deviation, n~7) for CRMs by different methods Sample Method Ce Nd Sm Human Hair, GBW 09101/ng g21 ID-ICP-MS 19.9°0.65 8.59°0.29 1.53°0.08 ICP-MSa 20.0°0.80 8.31°0.56 1.56°0.04b t-test Pw0.05 Pw0.05 Pw0.05 NAAc 19.5°0.9 10.7°0.3 1.47°0.08 t-test Pw0.05 Pv0.05 Pw0.05 Wheat Flour, GBW 08503/ng g21 ID-ICP-MS 27.8°1.67 15.5°0.92 2.22°0.16 ID-ICP-MSa 28.2°1.2 14.8°0.80 2.47°0.11b t-test Pw0.05 Pw0.05 Pv0.05 NAAc 25.8°0.6 13.6°0.6 1.88°0.06 t-test Pw0.05 Pv0.05 Pv0.05 Bush leaves, GBW 07603/mg g21 ID-ICP-MS 2.27°0.19 1.03°0.09 0.19°0.01 ICP-MSa 2.13°0.11 0.99°0.08 0.20°0.01b CertiÆed 2.2°0.1 1.0°0.1 0.19°0.02 aExternal calibration ICP-MS data calculated as mean and standard deviation of 28 replicates from four laboratories.4 bData calculated as mean and standard deviation of seven replicates by external calibration with internal standardization ICP-MS from present work.4 cData provided by Dr.Ni Bangfa of China Institute of Atomic Energy. J. Anal. At. Spectrom., 1999, 14, 1843±1848 1847ProÆciency Test for Analytical Geochemistry Laboratories.24 The proÆciency testing program has now become well established as the standard procedure for contributing to the quality control assessment of data from analytical geochemistry laboratories.By evaluating the magnitude of the z-score, participating laboratories can decide whether the quality of their analytical data is satisfactory. A similar procedure24 was followed here. `Method consensus values', being robust estimates of the mean composition of the sample, were derived from the contributed data by different laboratories and methods, using a statistical procedure that accommodates outliers.The resulting method consensus values were used as the assigned value for elemental compositions [X(a)]. The target precision was calculated for each assigned value using a modiÆed form of the Horwitz equation: h(a)~kX(a)0.8495, where X(a) is the assigned concentration and k is a constant equal to 0.02 for applied geochemistry laboratories. A z-score was calculated from z~[x2X(a)]/h(a). z-Scores in the range 22vzv2 were considered to be satisfactory.The results of zscore calculations are listed in Table 8. It is clear that all z-score results for Ce, Nd and Sm for both Human Hair and Wheat Flour samples are in the range 22vzv2. Hence, the analytical results of the present work are considered to be satisfactory. Detection limits The detection limits and the corresponding procedural blank concentrations are listed in Table 9. The blank concentrations were measured by ID analysis of blank solutions, which went through the same acid digestion procedure as the sample.The Ce blank value shown in Table 9 is signiÆcantly higher than those of the other analytes. This is mainly because of the reagent blank from the HClO4 used in the sample digestion procedure, which was not puriÆed further. However, this is not a serious problem in the determination of Ce, because of the higher concentration of Ce in both the samples analysed compared with Nd and Sm. The detection limits were determined by ID analysis of six procedural blank solutions and calculation of the analyte concentration that yielded a signal three times the standard deviation of the blank signal. Conclusion In this study, a method was developed to determine Ce, Nd and Sm in human hair and wheat Øour samples simultaneously with ID-ICP-MS.The isotope dilution target ratio for spiked to reference isotope was determined as a compromise between minimizing the error magniÆcation factor and minimizing measurement uncertainties.The isobaric interference of 142Nd on 142Ce was corrected by measuring 143Nd and calculating the percentage contribution of 142Nd to the 142Ce analyte peak. The method precision is in the range 1.1±3.0% RSD. For accurate isotope ratio determination, mass discrimination has to be appropriately corrected for. The validity of the analytical procedure was examined by analysing a Bush Leaves CRM. All results for the three elements were in good agreement with the certiÆed values, with RSDs of 6.3±8.5% (all results fell within the speciÆed uncertainties).Comparison of the results of IDICP- MS for human hair and wheat Øour with those of external calibration ICP-MS and a z-score proÆciency test indicated that the present method is satisfactory for the determination of Ce, Nd and Sm in biological materials at low or sub-ng g21 concentration levels. Financial Support of the Chinese National Natural Science Foundation and Thermo Jarrell Ash Corporation is gratefully acknowledged.The comments and suggestions made by the referees and the helpful discussion of the precision problem with Professor Du Andao, He Hongliao and Luo Daihong are also greatly appreciated. References 1 B. Guo, Chin. Rare Earths, (in Chinese), 1999, 20(1), 64. 2 W. Chen, Y. Gu and G. Zhao, Chin. Rare Earths, (in Chinese), 1999, 20(1), 58. 3 Laboratory of the Government Chemist (LGC), CertiÆed Reference Materials Catalogue, 1996, Issue No. 3, pp. 15±85. 4 M. Yin and B. Li, Spectrochim. Acta, Part B, 1998, 53, 1447. 5 J. W. McLaren, D. Beauchemin and S. S. Berman, Anal. Chem., 1987, 59, 610. 6 J. R. Garbarino and H. E. Taylor, Anal. Chem., 1987, 59, 1568. 7 T. G. B. Ting and M. Janghorbani, Anal. Chem., 1986, 58, 1334. 8 J. R. Dean and L. Ebdon, J. Anal. At. Spectrom., 1987, 2, 369. 9 D. Beauchemin, J. W. McLaren, A. P. Mykytiuk and S. S. Berman, Anal. Chem., 1987, 59, 778. 10 G. E. M. Hall, C. J. Park and J. C. Pelchat, J. Anal. At. Spectrom., 1987, 2, 189. 11 J. D. Fassett and P. J. Paulsen, Anal. Chem., 1989, 61, 643A. 12 Y. Sun, N. Yin and X. Yuan, Rock Miner. Anal., (in Chinese), 1995, 14(1), 15. 13 J. P. Chang and K. S. Jung, J. Anal. At. Spectrom., 1997, 12, 573. 14 J. Yoshinaga and M. Morita, J. Anal. At. Spectrom., 1997, 12, 417. 15 U. Ornemark, P. D. P. Taylor and P. de Bievre, J. Anal. At. Spectrom., 1997, 12, 567. 16 F. Vanhaecke, S. Boonen, L. Moens and R. Dams, J. Anal. At. Spectrom., 1997, 12, 125. 17 M. Nonose and M. Kubota, J. Anal. At. Spectrom., 1998, 13, 151. 18 M. P. Field and R. M. Sherrell, Anal. Chem., 1998, 70, 4480. 19 B. K. Esser, A. Volpe, J. M. Kenneally and D. K. Smith, Anal. Chem., 1994, 66, 1736. 20 K. E. Jarvis, A.L. Gray and R. S. Houk, Handbook of ICP-MS, Blackie, Glasgow, 1992. 21 A. Du, H. He, N. Yin, X. Zou, Y. Sun, D. Sun, S. Chen and W. Qu, Acta Geol. Sin. (in Chinese), 1994, 68(4), 339. 22 K. G. Heumann, S. M. Gallus, G. Radlinger and J. Vogl, J. Anal. At. Spectrom., 1998, 13, 1001. 23 I. S. Begley and B. L. Sharp, J. Anal. At. Spectrom., 1997, 12, 395. 24 M. Thompson, P. J. Potts, J. S. Kane, P. C. Webb and J. S. Watson, Geostand. Newsl., 1998, 22, 127. Paper 9/05346H Table 8 z-Score proÆciency test Sample Element Consensus value X(a) Uncertainty (2 s) Target precision h(a) z-score (ID-ICP-MS) Human Hair (GBW 09101) Ce 19.8 2.0 0.455 0.22 Nd 8.5 1.40 0.222 0.41 Sm 1.48 0.36 0.050 1.0 Wheat Flour (GBW 08503) Ce 27.9 3.6 0.609 20.16 Nd 14.8 2.0 0.356 1.97 Sm 2.34 0.64 0.074 21.62 Table 9 Detection limits for Ce, Nd and Sm by ID-ICP-MS Element Concentration of procedural blank/ng g21 Detection limit (3s)/ng g21 Ce 1.08 0.55 Nd 0.35 0.17 Sm 0.18 0.10 1848 J. Anal. At. Spectrom., 1999, 14, 1843±1848