Microwave Digestion of Plant and Grain Reference Materials in Nitric Acid or a Mixture of Nitric Acid and Hydrogen Peroxide for the Determination of Multi-elements by Inductively Coupled Plasma Mass Spectrometry SHAOLE WU*, XINBANG FENG AND ADOLPH WITTMEIER Alberta Research Council, P.O. Bag 4000, Vegreville, Alberta, Canada T 9C 1T 4 The closed-vessel microwave digestion of four plant standard HNO3, HClO4, H2SO4, HCl, HF and H2O2. The solutions reference materials (SRMs) and two grain reference materials are then heated to near-dryness and the resulting residues are (RMs) in nitric acid or in a mixture of nitric acid and re-dissolved in an appropriate solvent (usually HNO3).These hydrogen peroxide was explored for the direct determination ashing procedures eectively decompose the organic materials of 26 elements, Al, As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Li, in biological samples and minimize the dissolution reagents in Mg, Mo, Ni, Pb, Sb, Se, Sn, Sr, Ti, Tl, Th, U, V and Zn, by the final solution. Thus, the possible matrix and spectral ICP-MS.In 2.5 or 5 ml of HNO3 with or without the presence interferences in the subsequent ICP-MS determinations are of 2 ml of H2O2, 0.5 g of grain or plant sample was pre- eliminated or reduced. However, ashing procedures are usually digested overnight at room temperature, then at an elevated slow and tedious, needing constant attention, and are subject temperature of 120–165 °C with a pressure limit of 13.8 bar to possible contamination and potential loss of some volatile (200 psi) for 20 min. The presence of H2O2 helped to maintain elements.Direct wet dissolution procedures are usually a higher temperature under the pressure limit and reduced the achieved in concentrated HNO3 or in a mixture of HNO3 and carbon content in the digestates; but its impurities hampered H2O2 at an elevated temperature and pressure in closed vessels the ICP-MS analysis for certain elements at low levels. The or bombs heated thermally or by microwave energy.Many ICP-MS system was calibrated by the method of external studies have used this dissolution approach and have reported standards prepared in reagent blank solutions with In as the good recoveries for certain elements in various biological internal standard. Interferences from the sample matrices were materials analysed not only by ICP-MS6–12,15–18 but also by eliminated, corrected or reduced by subtracting the blank GFAAS and ICP.15,19,20 Compared with other acids such as signals, selecting suitable isotopes and applying the appropriate HClO4, HCl and H2SO4, HNO3 produces the least backinterference correction equations.Using this method, nearly all ground spectral interference in ICP-MS analysis21,22 and has of the predigestion spike recoveries for the 26 elements were become the most preferred sample preservative and dissolution within 90–110%. For the grain RMs studied including Corn reagent for ICP-MS analysis.With H2O and reactive oxygen Bran and Wheat Flour, the majority of the recoveries for most being the decomposition products, H2O2 has become the elements studied were within 85–115%. For the plant SRMs preferred additional oxidizing reagent used in digestion for studied including Pine Needles, Tomato Leaves, Apple Leaves subsequent ICP-MS analysis. This dissolution approach is and Peach Leaves, the majority of the recoveries were within simple, rapid and subject to less potential contamination and 90–115% for the determination of As, B, Ba, Ca, Cd, Cu, loss of volatile elements.However, using this approach the Mg, Mn, Mo, Pb, Sr, and Zn, within 70–100% for Al, Co, digestion of the siliceous materials in the samples is incomplete. Cr, Fe, K, Sb and V, but were 40–80% for Ni, Th, Ti and U. The organic materials in the samples may not be as completely The low recoveries were caused by the siliceous materials in decomposed as in the dry and wet ashing approaches.Also, these samples which were not decomposed during digestion. the biological materials studied using this digestion approach Keywords: Closed-vessel microwave digestion; inductively were mostly various animal tissues and biofluids, only a few coupled plasma mass spectrometry; biological material; spectral being plant materials. Most digestion bomb systems used were interference; environmental either heated thermally or heated by microwave energy without temperature and pressure control.The performance characteristics of the closed-vessel microwave digestion systems used in Inductively coupled plasma mass spectrometry (ICP-MS) has these studies varied widely from simple domestic microwave become one of the most attractive detection systems for the ovens to advanced laboratory microwave systems equipped determination of trace and ultra-trace elements because of its with temperature and pressure regulation and automatic power excellent detection limits, wide linear dynamic range, multicontrol. The maximum high-pressure of the closed vessel used element capability and the ability to measure isotope ratios.in these studies also varied from the (low) high-pressure of In recent years ICP-MS analytical methods have been increas- 15.8 bar (220 psi) to the (medium) high-pressure of 30–41 bar ingly applied to the determination of trace elements in biologi- (600 psi)11,12 or the (high) high-pressure of 117 bar (1700 psi).13 cal materials.1–18 The dissolution procedures used for biological In this study, the direct ICP-MS determination of 26 materials include dry ashing,1–3 wet ashing,4–6,15 or direct wet elements in plant SRMs and grain RMs digested in concen- dissolution.6–15 The dry ashing procedures include drying the trated HNO3 or in a mixture of HNO3 and H2O2 at an sample in a mue furnace for 8 h prior to wet dissolution,2 or elevated temperature and pressure was explored.A closed- using Mg(NO3)2 as an ashing aid in a tedious dissolution vessel laboratory microwave system equipped with temperature procedure.3 In wet ashing procedures, samples are initially digested in a mixture of acids or oxidizing reagents, such as and pressure regulation with the maximum (low) high-pressure Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 (797–806) 797of 15.2 bar (220 psi) was used for the digestion under optimized sample with 5.0 ml being added to the sensor vessel (a minimum of 5 ml of solution is required to immerse the temperature conditions.The objectives were to investigate the potential and limitations of applying this simple method to analyse probe). Samples were pre-digested overnight (16 h) in a class 100 clean fumehood at room temperature. The grain and plant plant and grain samples collected in environmental monitoring, assessment and remediation projects, which often require a materials were digested in separate batches so that the sensor and the sample vessels contained similar matrices.A maximum high throughput of samples. The challenges of the study were the low analyte content of the samples and the relatively severe of twelve vessels including a reagent blank and a sensor were sealed and digested using the following heating program: heat spectral interferences arising from the biological matrix subjected to this direct wet dissolution procedure.to 165 °C within a ramp time of 10 min at full power (1000 W), hold for 20 min at 165 °C under the recommended maximum working pressure of 13.8 bar (200 psi). After the vessel had EXPERIMENTAL cooled, 97.5 and 95 ml of DDW were added to each vessel Standard Reference Materials and Reagents containing grain and plant samples, respectively. Sensor vessel contents were discarded. Prior to ICP-MS analysis, aliquots Four plant NIST SRMs, 1515 (Apple Leaves), 1547 (Peach of the digestates were further diluted 1.25-fold for the grain Leaves), 1573 (Tomato Leaves) and 1575 (Pine Needles), and samples and 2.5-fold for the plant samples.The overall dilution two grain RMs, 8433 (Corn Bran) and 8436 (Wheat Flour) was 250 or 500 (v/m) for the grain and plant samples, respect- were used for this study. The two grain RMs were prepared ively. The final solution for the ICP-MS analysis contained and characterized by Agriculture Canada and distributed by 2% HNO3 for both grain and plant samples.NIST with only best estimated values or estimated values The procedures used in the HNO3–H2O2 digestion were the provided. same as described above, with the addition of 2–5 ml of H2O2 High-purity concentrated HNO3 (68–71%, sub-boiling (30%) into each PFA liner either immediately after the addition double distilled in quartz, Seastar Chemicals, Sidney, British of HNO3, or after the overnight predigestion with HNO3 but Columbia, Canada) and, whenever required, certified 30% 1–2 h prior to the microwave digestion, or after the microwave H2O2 (analytical reagent grade, BDH, Poole, Dorset, UK) digestion with HNO3 followed by a second-stage digestion were used for the sample digestion.De-ionized distilled water using the same heating procedure (this will be called two-stage (DDW) was obtained from a three-column NANOpure water digestion hereafter). purification system with 18 mV cm specific resistivity Field samples spiked with standard solutions and reagent capability.blanks were digested in the same manner. Microwave Digestion System ICP-MS Measurement and Data Reporting A QWAVE-1000 microwave digestion system (Questron The ICP-MS system used was the Perkin-Elmer (Norwalk, Corporation, Mercerville, NJ, USA) equipped with tempera- CT, USA) ELAN Model 5000. This system and the operating ture and pressure regulation (through a sensor vessel) was parameters used were described in detail in previous work.23 used. This system is equipped with one sensor and up to eleven The ICP-MS system was calibrated by the method of sample digestion vessels in a 12 position rotating carousel.external standards with In as the internal standard. The reagent Each vessel consists of a PFA rupture disk, an outer pressure blank solution contained 2% of concentrated HNO3. Mixed vessel and a PFA inner liner with 100 ml capacity. Each vessel standard solutions containing 23 elements, Al, As, B, Ba, Cd, is capable of withstanding pressures up to 15.2 bar (220 psi) Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se, Sn, Sr, Th, Ti, Tl, U, and temperatures up to 200 °C.The pressure and temperature V and Zn, at the concentration levels of 1, 10 and 100 mg l-1, are measured by a transducer and a thermocouple, respectively, were prepared in reagent blank solutions. A single standard which are located in the sensor vessel. The heating program is solution containing 100 mg l-1 of Sn, 100 mg l-1 of Ca and controlled and monitored by a personal computer with QW405 177 mg l-1 of Cl was similarly prepared.This standard solution software installed, where power, ramp and dwell times, temwas used to derive the correction coecients for Ca and Cl perature or pressure reached, and the temperature and pressure interferences presented in Table 1, as well as to calibrate the limits in each of 9 heating steps can be programmed.After the system for the determination of Sn, Ca and Cl. In some cases, pre-set temperature (or pressure) in the sensor vessel is reached, a single mixed standard solution containing 50 mg l-1 of Mg the power is automatically regulated to maintain this value and K was used for the calibration of these elements using the unless the pre-set pressure (or temperature) limit is reached. ‘Omni’ range (i.e., reduced detection voltage). A 100 ml aliquot Prior to their use, the PFA inner liners were first soaked in of In solution (3.5 mg l-1) was added to 10 ml of each blank, 1+2+9 HNO3–HCl–H2O solution overnight.The following standard and sample solution. All signals were corrected by microwave cleaning procedure was carried out: a 40 ml aliquot subtraction of reagent blank signals. The calibration curves of 10% HNO3 was added to each liner; the digestion vessels were plotted linearly through zero for each isotope tested with were sealed and the temperature was raised to 165 °C in 10 min and without using the interference correction equations pre- and held at 165 °C for 10 min (the dwell time); after cooling, sented in Table 1.A unique number was assigned to each the contents of the vessels were discarded and the liners were correction equation used for the same isotope. The default thoroughly rinsed with DDW. equation number was zero, indicating that no correction was applied. Sample Digestion The signals at 43Ca and 35Cl were constantly measured to estimate the possible interferences from Ca and Cl and to Several digestion parameters were tested, including the amount of concentrated HNO3 (1, 2, 2.5 and 5 ml ), the dwell time derive the coecients used in the interference correction equations.The results for the analysed sample solution (in mg l-1) (10, 20, 30 and 40 min), and the use of H2O2 in the digestion. Unless otherwise specified, the following digestion procedure and for the sample solid (in mg g-1) obtained with and without interference corrections were simultaneously calculated and was used for the HNO3 digestion.Approximately 0.5 g (dry mass) of grain or plant sample was stored in the database. For field samples, these results were inspected prior to data reporting. When the sample solution weighed, to the nearest 0.1 mg, in the PFA inner liners including that of the sensor vessel. Then 2.5 ml (grain samples) or 5.0 ml analysed by ICP-MS contained relatively high concentrations of Ca or Cl that aected the results of relevant isotopes, the (plant samples) of concentrated HNO3 was added to each 798 Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12Table 1 Interference correction equations Element m/z Eqn. No. Correction equation Interferences Corrections for Cl interferences: V 51 1 51M-3.127 53M+0.3535 52M 35Cl16O V 51 2 51M-0.0015 35M* 35Cl16O Cr 53 1 53M-0.0005 35M* 37Cl16O As 75 1 75M-3.127 77M+2.548 82M 40Ar35Cl As 75 2 75M-0.0003 35M* 40Ar35Cl Se 77 1 77M-0.0001 35M* 40Ar37Cl Corrections for Ca interferences: Fe 57 1 57M-0.090 43M* 40CaOH Co 59 1 59M-0.00160 43M* 43CaO, 42CaOH Ni 60 1 60M-0.00600 43M* 43CaOH, 44CaO Correction for isobaric elemental interferences: Cd 114 1 114M-0.02747 118M 114Sn In 115 1 115M-0.016 118M 115Sn Pb 208 1 206M+207M+208M Isotope abundance variation * Coecient measured in each run.results calculated using the interference correction equations were selected and reported.Otherwise, the results obtained without correction were selected and reported. However, to demonstrate the interferences, results for the same element obtained using dierent isotopes or using the same isotope with dierent correction equations will be presented in this paper. RESULTS AND DISCUSSION HNO3 Digestion The minimum amount of concentrated HNO3 required for digestion was studied, in order to use a low dilution factor and to have a suciently low concentration of HNO3 (1–2%) in final solutions for ICP-MS analysis.Tests showed that undigested particles remained in the digestates when 1 or 2 ml of concentrated HNO3 for a 0.5 g of grain sample was used. While 2.5 ml of concentrated HNO3 was sucient for digesting the grain samples, 5.0 ml of concentrated HNO3 was required for the digestion of 0.5 g of SRM 1547 (Peach Leaves) to eliminate PFA inner liner damage by localized overheating. Thus, 2.5 and 5.0 ml of HNO3 were used for each 0.5 g of grain and plant samples, respectively.Fig. 1 The typical temperature and pressure profiles of samples using The microwave heating dwell time was studied using NIST the closed vessel microwave digestion: (a), digested in HNO3 alone; SRM 1515 (Apple Leaves). There are no significant dierences (b), digested in HNO3–H2O2, 2 ml H2O2 was added 16 h after the among the results obtained at dwell times of 10, 20, 30 and addition of 5 ml HNO3 but 1 h prior to the microwave digestion. 40 min, except for slightly lower Sb results at longer dwell times. Since only one type of sample, SRM 1515, was tested, which improved but did not guarantee the reproducibility of a dwell time of 20 min was chosen to minimize the possibility the temperature profiles. In addition, variations in temperature of incomplete digestion of any sample types being tested in profile can exist between digestion batches as well as between this study without a large increase in total time for microwave samples in the digestion vessels and the sensor vessel in the digestion and the cooling of the digestion vessels.same batch. All these factors could cause variations in digestion Fig. 1(a) demonstrates the variance in typical temperature eciencies for certain elements. To overcome this, a longer and pressure profiles of samples (in sensor vessels) with HNO3 predigestion time, or predigestion at an elevated temperature, digestion. After the temperature of the solution reached 100 °C, or using the HNO3–H2O2 digestion as discussed below may the pressure inside the vessel increased rapidly due to the be required.generation of CO2 and other gases from the decomposition of organic materials. When the pressure did not exceed the preset pressure limit of 13.8 bar (200 psi), the microwave power HNO3–H2O2 Digestion was regulated by the pre-set temperature of 165 °C. Otherwise, it was regulated by the pressure limit and the temperature Using HNO3 digestion, the diluted sample digestates had a light-yellow colour, indicating that the organic materials in started to decrease slowly to about 140–145 °C [Temperature 1 in Fig. 1(a)] or even 120 °C [Temperature 2 in Fig. 1(a)]. these samples were not completely decomposed. Also, as mentioned above, the digestion temperature profiles were not The digestion process controlled by pressure limit was not desirable since the temperature reached could vary greatly very reproducible.In addition, the recoveries of several elements such as Ti, Th, U, Ni, Al and V in the plant SRMs depending on sample organic matter content and result in variations in digestion eciencies. By pre-digesting samples were generally low. Thus digestion with both HNO3 and H2O2 was tested to find out if the use of H2O2 would improve the overnight, the organic materials were partially decomposed, Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 799Table 2 Elemental contaminations in 2% solution of H2O2 (30% digestion eciencies for these elements.The addition of 2–5 ml m/v) analysed with ICP-MS of H2O2 guaranteed good contact between the temperature probe and the digestion solution. Tests (as described in the Concentration/mg l-1* experimental section) showed that in the presence of H2O2, the diluted digestate was lighter in colour after one-stage H2O2, (BDH H2O2, (BDH digestion and colourless after two-stage digestion.However, Element m/z Lot 116837–48528) Lot 95885–5063) no improvement in the accuracy of the analytical results was Al 27 0.8 32 observed with the HNO3–H2O2 digestion, except that carbon B 11 0.2 0.26 interferences on 52Cr by 40Ar12C and on 82Se by 35Cl35Cl12C, Ba 137 —† 0.06 Ca 43 16 62 although still severe, were reduced. Thus the use of H2O2 Cr 52 0.13 1.3 indeed reduced the C content in the digestate as reported by Cu 65 0.18 0.47 other studies,14 but a colourless diluted digestate is not an Mn 55 —† 0.1 indicator of good analyte recovery as determined by ICP-MS.Mo 98 —† 0.99 Using two-stage HNO3–H2O2 digestion or the one-stage diges- Ni 60 0.1 1 tion in which H2O2 was added after the overnight predigestion Pb 208 0.03 0.06 Sb 121 —† 0.02 with HNO3 but 1–2 h prior to the microwave digestion, the Sn 118 0.58 1.87 analytical precision was improved, especially for the elements Sr 86 —† 0.16 with low recoveries. This is probably due to the fact that, using Th 232 —† 0.059 those digestion procedures, the microwave power is tempera- Ti 47 —† 1.0 ture limit controlled during the entire dwell time as shown in Tl 205 —† 1.0 Fig. 1(b), resulting in less variations in digestion eciencies. Zn 66 0.8 2.44 This advantage was not observed using one-stage digestion * Concentrations of As, Cd, Co, Fe, Se, U and V were below DLs. after overnight predigestion with HNO3 and H2O2. No signifi- † Below DL. cant dierence was observed when 2 or 5 ml of H2O2 was used.The major concern in the use of H2O2 for digestion is the Table 3 Basic spectral interferences in ICP-MS for elements in biological samples (percentage natural relative abundance in parentheses) Isobaric Isobaric polyatomic ions elemental Element ions H2O, HNO3 Biological matrix 24Mg (78.99) 12C2 (97.817) 25Mg (10.0) 12C2H (97.797), 12C13C (2.176), 26Mg (11.0) 12CN (99.504), 13C2 (0.012), 28Si (92.23) 14N2 (99.202) 12CO (98.702) 29Si (4.67) 14N2H (99.187) 30Si (3.10) 14NO (99.401) 35Cl (75) 34SH (4.199) 43Ca (0.135) 44Ca (2.086) 14N216O (99.503) 12CO2 (98.505) 45Sc (100.0) 14N216OH (99.988) 12CO2H (98.49), 13CO2 46Ti (8.0) 46Ca (0.004) 14NO2(99.202) 47Ti (7.3) 35Cl12C (74.966), 31P16O (99.800) 48Ti (73.8) 48Ca (0.187) 32S16O (94.81), 31P16OH (99.785) 49Ti (5.5) 37Cl12C (23.934, 35Cl14N (75.497), 32S16OH (94.796) 50Ti (5.4) 50V (0.25) 35Cl14NH 51V (99.75) 35Cl16O (75.648), 37Cl14N (24.103) 52Cr (83.789) 40Ar12C (98.504) 35Cl16OH (75.637) 53Cr (9.501) 37Cl16O (24.303) 54Fe (5.80) 54Cr (2.365) 40Ar14N (99.202) 37Cl16OH (24.148) 55Mn (100.0) 40Ar14NH (99.187) 39K16O (93.11) 56Fe (91.72) 40Ar16O (99.401) 40Ca16O (96.776) 57Fe (2.20) 40Ar16OH (99.386) 40Ca16OH (96.762) 58Ni (68.077) 58Fe (0.33) 42Ca16O (0.639) 59CO (100.0) 42Ca16OH (0.639), 43Ca16O (0.14) 60Ni (26.223) 44Ca16O (2.096), 43Ca16OH (0.14) 61Ni (1.14) 44Ca16OH (2.095) 62Ni (3.63) 46Ca16O (0.003) 63Cu (39.17) 31P16O2 (99.60),, 35Cl14N2 (75.195) 64Zn (48.6) 64Ni (0.926) 32S16O2 (94.62), 31P16O2H, 32S32S (90.25), 48Ca16O (0.18) 65Cu (30.83) 32S16O2H (94.606), 33S16O2 (0.873), 32S33S (0.76) 37Cl14N2 (24.007), 48Ca16OH (0.18) 66Zn (27.9) 34S16O2 (7.986), 32S34S2 (4.565) 67Zn (4.1) 35ClO2 (75.497) 68Zn (18.8) 40ArN2 (99.805) 35Cl16O2H (75.486), 40Ar12C16O (98.307) 75As (100.0) 36Ar38ArH (0.001) 40Ar35Cl (75.52), 37Cl2H (5.855), 40Ar34SH (4.183), 77Se (7.63) Ar2H (0.598) 40Ar37Cl (24.103), 40Ca37Cl 82Se (8.73) 40Ar2H2(99.172) 12C35Cl2 (56.824), 34S16O3(4.175), 33S16O3H (0.795) 86Sr (9.86) 12C37Cl2(5.792) 88Sr (82.58) 94Mo (9.3) 94Zr (17.38) 95Mo (15.9) 96Mo (16.7) 98Ru (5.52) 97Mo (9.6) 98Mo (24.13) 98Ru (1.99) 63Cu35Cl (52.378) 800 Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12contamination from its impurities as encountered in this study sate for multiplicative noises, but it also compensates for matrix eects and instrument drift. In most ICP-MS quantitat- (Table 2). Thus, the majority of samples were digested in HNO3 alone. However, if high purity H2O2 reagent is available, ive analyses, multi-internal standards are used to cover dierent mass regions.25 Scandium, Y and In are usually used in the then the addition of H2O2 to samples several hours after the addition of HNO3 and use of one-stage HNO3–H2O2 micro- ‘light and medium’ mass region, while heavy elements such as Tb, Ho and Bi are used in the heavy mass region.1 Since SRMs wave digestion is recommended. 1515 (Apple Leaves) and 1573 (Tomato Leaves) contain Sc, Y and Tb, the use of these elements as internal standards was Calibration of the ICP-MS System eliminated. After comparison with other internal standards, In was found to be an eective internal standard covering the It was the intention of this study to apply the simple method whole mass range. Thus, as in previous work,23 In was chosen of calibration by external standards in conjunction with as the single internal standard in this study.internal standardization, as opposed to calibration by standard additions or the isotope dilution technique, in order to improve the sample throughput rate. Tests showed that there was no Interferences Correction and the Selection of Primary Isotopes significant dierence between the analytical results of a digested and a non-digested standard solution for the elements tested The spectral interferences in ICP-MS analysis have been investigated thoroughly in many studies.21,22,25–28 The basic in this study.This indicates that none of the tested elements were lost in the closed-vessel microwave digestion procedure, spectral interferences from reagent background and biological sample matrix are listed in Table 3. The background inter- which is in agreement with our previous findings using a dierent digestion reagent system.23 Hence, undigested stan- ferences from the plasma gases, air entrainment and solvent can be corrected by subtracting the blank signals.The isobaric dards prepared in reagent blank were used for the calibration. It is recognized that in actual samples the elements are not spectral interferences originating from the polyatomic ion species involving the sample matrix elements present more present in pure ionic form but are present as oxides, organometallic complexes and various minerals and thus may not neces- challenges. They may be eliminated by selecting a suitable isotope, or may be corrected or reduced by applying inter- sarily behave the same as in the standard solution.24 The internal standard method has been used in almost all ference correction equations.Although theoretically predictable, the actual extent of the interference at a given m/z value quantitative ICP-MS determinations. Not only does it compen- Table 4 The detection limits (DLs) and method detection limits (MDLs) for the primary isotopes and associated equations DL MDL Element m/z Eqn.No. Solution*/mg l-1 Solid†/mg g-1 Solution§/mg l-1 Solid‡/mg g-1 Al 27 0 0.2 0.1 3 1.5 As 75 0 0.05 0.025 0.4 0.2 As 75 1 0.2 0.1 0.6 0.3 As 75 2 0.1 0.05 0.3 0.15 B 10 0 0.5 0.25 2 1 Ba 137 0 0.02 0.01 0.6 0.3 Ca 43 0 10 5 22 11 Cd 114 1 0.04 0.02 0.12 0.06 Co 59 0 0.01 0.005 0.2 0.1 Co 59 1 0.01 0.005 0.3 0.15 Cr 53 0 0.1 0.05 0.6 0.3 Cr 53 1 0.1 0.05 0.6 0.4 Cu 65 0 0.1 0.05 0.5 0.25 Fe 57 0 3 1.5 5 2 Fe 57 1 3 1.5 5 2 K 39 0 — — 100¶ 50¶ Li 7 0 0.2 0.1 0.3 0.15 Mg 25 0 — — 3¶ 1.5¶ Mn 55 0 0.02 0.01 0.05 0.025 Mo 98 0 0.02 0.01 0.2 0.1 Ni 60 0 0.08 0.04 0.8 0.4 Ni 60 1 0.08 0.04 0.8 0.4 Pb 208 1 0.02 0.01 0.08 0.04 Sb 121 0 0.01 0.005 0.016 0.008 Se 77 0 0.2 0.1 0.3 0.15 Se 77 1 0.2 0.1 0.8 0.4 Sn 118 0 0.05 0.025 0.12 0.06 Sr 86 0 0.04 0.02 0.2 0.1 Th 232 0 0.01 0.005 0.1 0.05 Ti 47 0 0.2 0.1 0.6 0.3 Tl 205 0 0.003 0.0015 0.016 0.008 U 238 0 0.005 0.0025 0.012 0.006 V 51 0 0.02 0.01 0.3 0.15 V 51 2 — — — — Zn 66 0 0.2 0.1 0.3 0.15 * Derived from three times the standard deviation of undigested reagent blank solutions, n=10. † Calculated from the DL (in solution) in mg l-1 with the overall dilution factor of 500 (v/m).‡ Derived from three times the within-run standard deviation of duplicate digestion/analysis of the reagent blanks, SRMs and field samples (n=7–44) with the overall dilution factor of 500 (v/m). § Calculated from the MDL (in solid) in mg g-1 using the overall dilution factor of 500 (v/m). ¶ ‘Omni’ range used.Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 801depends on the design and the operational conditions of the which will be simply called the ‘certified’ values hereafter. Other major isotopes suering interference from C species ICP-MS system as well as the concentrations of the sample matrix constituents. Hence, interferences vary greatly among include 44Ca(12C16O2), 45Sc(12C16O2H), 47Ti(12C35Cl) and 49Ti(12C37Cl), and the minor ones include 48Ti(36Ar12C), dierent operational conditions of the same ICP-MS system, among dierent ICP-MS systems and among dierent sample 53Cr(40Ar13C) and 60Ni(12C16O3) (Table 3).By selecting 10B, 53Cr and 77Se as the primary isotopes and applying matrices. For this reason, an interference investigation over the whole mass range was carried out prior to this study for suitable equations to correct for Cl interference on 53Cr and 77Se, as discussed below, the C interference on the determi- about 36 elements in 1% HNO3.Operational conditions were similar to the ones used in this study. The primary isotope nations of B, Cr and Se by ICP-MS was eliminated or reduced. The major Cl interferences in ICP-MS analysis include the and the correction equation were then selected (Table 4), based on the relative isotope abundance of the analyte, the calculated isotopes of 47Ti(35Cl12C), 49Ti(37Cl12C), 51V(35Cl16O), 53Cr(37Cl16O), 75As(35Cl40Ar) and 77Se(37Cl40Ar).The extent of relative abundance of the interfering species, the measured apparent analyte concentration produced by the interfering the Cl interference at m/z 51, 53, 75 and 77 is reflected in the corresponding measured interference correction coecients element at a given concentration and the matrix constituents of the biological SRMs studied, especially, C, Cl, Ca, K, Mg, listed in Table 1, which were the percentage ratios of the signals of a Cl standard at these m/z values to that at m/z 35 (35Cl).P and S. As already mentioned, the spectral interferences from poly- Without correction for the Cl interference (i.e., with equation number 0), the results of 75As and 77Se in NIST SRM 1573 atomic ion species involving C were more severe using this digestion procedure compared with sample dissolution by wet (Tomato Leaves) which contains #1% Cl were elevated (Table 6). Using the corresponding correction equation 2 given ashing procedures. This is reflected in Tables 5–7 by the elevated results for 52Cr(40Ar12C), 82Se(12C35Cl35Cl) and, to a in Table 1, the results for 51V and 75As were in good agreement with the certified values (Table 6).However, when the lesser degree, for 11B (tail interference by the strong 12C signal) compared with the certified or the best estimated values or corresponding equation 1 given in Table 1 was used for the correction, the results for 51V and 75As were all biased high compared with the reference, estimated or consensus29 values Table 5 Analytical results (mg g-1) for NIST SRM 1515 (Apple Leaves) and NIST SRM 1547 (Peach Leaves) digested in HNO3 alone; results of 75As (eqn 0 and 1), 11B, 52Cr, 57Fe (eqn. 0), 82Se and 51V (eqn. 1) are listed to demonstrate interferences NIST SRM 1515 NIST SRM 1547 Certified Found Certified Found Element m/z Eqn. No. Mean±Uncert. Mean±Uncert. Mean±Uncert. Mean±Uncert. Al 27 0 286±9 235±26 249±8 188±20 As 75 0 0.038±0.007 0.159±0.006§ 0.06±0.018 0.13±0.006§ As 75 1 0.038±0.007 0.132±0.028§ 0.06±0.018 0.18±0.02§ As 75 2 0.038±0.007 — 0.06±0.018 —‡ B 10 0 27±2 29.9±2.2 29±2 27.5±2.0 B 11 0 27±2 32±1.3 29±2 33.5±1.4 Ba 137 0 49±2 49.6±0.3 124±4 124±2.8 Ca 43 0 15260±150 16827±262 15600±200 16692±378 Cd 114 1 0.014* 0.016±0.002 0.026±0.003 0.027±0.002§ Co 59 0 0.09* 0.113±0.006§ 0.07* 0.074±0.008§ Co 59 1 0.09* 0.086±0.004§ 0.07* 0.048±0.006§ Cr 52 0 0.3* 1.56±0.22 1* 2.14±0.24 Cr 53 0 0.3* 0.38±0.06 1* 0.97±0.04 Cr 53 1 0.3* 0.311±0.010 1* 0.95±0.04 Cu 63 0 5.64±0.24 5.77±0.08 3.7±0.4 3.73±0.12 Cu 65 0 5.64±0.24 5.74±0.12 3.7±0.4 3.75±0.18 Fe 57 0 83±5 148±2 218±14 276±12 Fe 57 1 83±5 75.9±2.6 218±14 198±5 K 39 0 16100±200 14180±150 24300±300 19682±2500 Li 7 0 — —‡ — 0.113±0.02§ Mg 25 0 2710±80 2665±116 4320±80 4021±130 Mn 55 0 54±3 54.8±0.8 98±3 94.1±2.0 Mo 98 0 0.094±0.013 0.091±0.004§ 0.06±0.008 0.055±0.004§ Ni 60 0 0.91±0.12 1.37±0.014 0.69±0.09 1.0±0.06 Ni 60 1 0.91±0.12 0.782±0.026 0.69±0.09 0.37±0.04 Pb 208 1 0.47±0.024 0.443±0.010 0.87±0.03 0.847±0.024 Sb 121 0 0.013* 0.011±0.002 0.02* 0.019±0.002 Se 77 0 0.05±0.009 0.827±0.036 0.12±0.009 0.43±0.10 Se 77 1 0.05±0.009 0.793±0.034 0.12±0.009 0.51±0.10 Se 82 0 0.05±0.009 0.52±0.08 0.12±0.009 0.74±0.10 Sn 118 0 0.02* 0.050±0.028§ 0.2* 0.092±0.030§ Sr 86 0 25±2 26.1±0.2 53±4 54.9±1.2 Th 232 0 0.03* 1.37±0.014 0.05* 0.031±0.002§ Ti 47 0 — 9.02±3.6 — 7.92±1.0 Tl 205 0 — 0.018±0.008 — 0.023±0.002 U 239 0 0.0006* 0.0053±0.001 0.015* 0.0084±0.0006 V 51 0 0.26±0.03 0.236±0.018 0.37±0.03 0.321±0.022 V 51 1 0.26±0.03 0.324±0.006 0.37±0.03 0.69±0.06 Zn 66 0 12.5±0.3 12.8±0.4 17.9±0.4 17.7±1.0 * NIST reference value.‡ Below DL, refer to Table 4. § Above DL, but below MDL, refer to Table 4. 802 Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12Table 6 Analytical results (mg g-1) for NIST SRMs 1573 (Tomato Leaves) and 1575 (Pine Needles) digested in HNO3 alone; results of 11B, 52Cr, 82Se and 51V (eqn. 1) and results of 75As (eqn. 0 and 1) for SRM 1573 are listed to demonstrate interferences NIST SRM 1573 NIST SRM 1575 Certified Found Certified Found Element m/z Eqn. No. Mean±Uncert. Mean±Uncert. Mean±Uncert. Mean±Uncert. Al 27 0 1200* —d 545±30 482±58 As 75 0 0.27±0.05 0.46±0.04 0.21±0.04 0.22±0.010 As 75 1 0.27±0.05 0.46±0.04 0.21±0.04 0.28±0.034§ As 75 2 0.27±0.05 0.31±0.016 0.21±0.04 0.21±0.012 B 10 0 33.4¶ 35.8±2 17±2¶ 17.1±1.0 B 11 0 33.4¶ 37.8±1.4 17±2¶ 17.9±0.8 Ba 137 0 57±9¶ 55.1±1.8 7.2±0.8¶ 6.04±0.28 Ca 43 0 30000±300 30819±680 4100±200 4457±146 Cd 114 1 3*, 2.5±0.2¶ 2.72±0.08 0.22±0.06¶ 0.183±0.008 Co 59 0 0.6* 0.510±0.024 0.1*, 0.122±0.014¶ 0.100±0.006 Co 59 1 0.525±0.046¶ 0.462±0.024 0.1*, 0.122±0.014¶ 0.093±0.006 Cr 52 0 4.5±0.5 4.60±0.12 2.6±0.2 2.97±0.26 Cr 53 0 4.5±0.5 4.34±0.18 2.6±0.2 2.31±0.09 Cr 53 1 4.5±0.5 3.80±0.04 2.6±0.2 2.30±0.08 Cu 63 0 11±1 11.3±0.8 3±0.3 2.77±0.12 Cu 65 0 11±1 10.3±0.4 3±0.3 3.09±0.18 Fe 57 0 690±25 663±36 200±10 177±20 Fe 57 1 690±25 528±38 200±10 159±24 K 39 0 44600±300 41681±3523 3700±200 3469±560 Li 7 0 — 0.44±0.06 — 0.127±0.04§ Mg 25 0 7000* 7328±1130 1220±160¶ 1077±144 Mn 55 0 238±7 216±8 675±15 —d Mo 98 0 0.53±0.09¶ 0.573±0.034 0.15±0.05¶ 0.112±0.02 Ni 60 0 1.3±0.2¶ 2.06±0.12 3.5* 2.33±0.14 Ni 60 1 1.3±0.2¶ 0.95±0.12 3.5* 2.18±0.14 Pb 208 1 6.3±0.3 6.05±0.14 10.8±0.5 10.3±0.34 Sb 121 0 0.036±0.007¶ 0.027±0.004 0.2* 0.154±0.014 Se 77 0 0.054±0.006¶ 0.74±0.19 0.047±0.005¶ 0.069±0.10§ Se 77 1 0.054±0.006¶ 0.056±0.012§ 0.047±0.005¶ 0.050±0.016§ Se 82 0 0.054±0.006¶ —‡ 0.047±0.005¶ —‡ Sn 118 0 — — — 0.26±0.08 Sr 86 0 44.9±0.3 41.1±0.6 4.8±0.2 4.41±0.068 Th 232 0 0.17±0.03 0.056±0.012 0.037±0.003 0.018±0.004§ Ti 47 0 56±39¶ 23.7±1.8 13.7¶ 6.95±0.52 Tl 205 0 0.05* 0.032±0.002 0.05* 0.046±0.002 U 238 0 0.061±0.003 0.026±0.002 0.02±0.004 0.0128±0.0026 V 51 0 1.2±0.2¶ 1.14±0.14 0.39±0.07¶ 0.335±0.064 V 51 1 1.2±0.2¶ 4.55±6.4 0.39±0.07¶ 0.57±0.12 V 51 2 1.2±0.2¶ 1.16±0.06 0.39±0.07¶ 0.32±0.02 Zn 66 0 62±6 61.9±0.2 67±9¶ 64.3±2.4 * NIST reference value.‡ Below DL. § Above DL, but below MDL, refer to Table 4. ¶ Consensus value.29 d Out of the linear calibration range, further dilution required. (Tables 5–7). This is because in both equations 1 (Table 1), and hydroxides species at m/z values 57, 59 and 60 is reflected in the corresponding interference correction coecients listed the signal at m/z 52 or 82 was involved, which suers severe C interference.Thus, although successfully applied to the in Table 1. These were the measured percentage ratios of the signals of a Ca standard at these m/z values to that at m/z 43 ICP-MS analysis for sediment and soil samples,23 correction equations 1 were not suitable for the determination of V and (43Ca). In this study, 57Fe was selected for the Fe determination over 54Fe and 56Fe which suered severe interference from As in biological samples digested with this procedure because of the relatively high C concentrations.The Cl interference on 40Ar14N or 40Ar16O (Table 3). The Ca interferences at m/z 57 were corrected using equation 1 (Table 1). Otherwise, the Fe 53Cr was relatively low and there was little dierence between the 53Cr results in SRM 1573 obtained with and without the results could be biased high, especially for SRMs 1515 and 1547 (Table 5). All Ni isotopes suer from Ca interference use of the correction.The Cl levels were much less in SRMs 1515, 1547, 1575 and in the two grain RMs (up to 0.04%) than (Table 3). Because of the isobaric interference of 58Fe on 58Ni and the much lower isotope abundance of 61Ni and 62Ni, 60Ni in SRM 1573. Consequently, correction equations 2 for the determination of 51V and 75As and equations 1 for 53Cr and was selected as the primary isotope and equation 1 listed in Table 1 was used to correct the Ca interference on its determi- 77Se were either not applied or, when applied, produced little dierence (Tables 5–7).Apparently, unknown interferences nations. The generally low recoveries for Ni obtained using 60Ni with equation 1 in Tables 5–7 might be caused by caused the biased high results for 77Se in SRMs 1515 and 1547. Because the digestion eciencies for Ti were generally low incomplete digestion. A similar equation was also applied to correct the Ca interference on 59Co (Table 1).The interference (#50%) as shown from the results at 47Ti in Table 6, no eort was made either to compare the Ti results using various Ti of Ca on 65Cu was much weaker and the correction was not applied. isotopes or to correct for the possible interferences at 47Ti. The Ca levels in biological samples may be as high as Plant samples may contain relatively high K concentrations, which, as 39K16O, may interfere in the determination of Mn at 1.5–3%(Tables 5–7). The extent of interference from Ca oxides Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 803Table 7 Analytical results (mg g-1) for NIST RMs 8433 (Corn Bran) and 8436 (Durum Wheat Flour) digested in HNO3 with the overall dilution factor of 250 (v/m); results of 11B, 52Cr, 57Fe, 82Se and 51V (eqn. 1) are listed to demonstrate inferferences NIST RM 8433 NIST RM 8436 Best estimated Found Best estimated Found Element m/z Eqn. No. Mean±Uncert. Mean±Uncert. Mean±Uncert. Mean±Uncert.Al 27 0 1.01±0.55 0.49±0.14§ 11.7±4.7 9.5±0.9 As 75 0 0.002±0.002 —‡ 0.03* 0.018±0.004§ As 75 1 0.002±0.002 —‡ 0.03* —‡ B 10 0 2.8±1.2 3.11±0.12 — 0.82±0.08 B 11 0 2.8±1.2 3.77±0.56 — 1.21±0.76 Ba 137 0 2.4±0.52 2.42±0.10 2.11±0.47 2.31±0.08 Ca 43 0 420±38 467±23 278±26 266±10 Cd 114 1 0.012±0.005 0.0135±0.0016§ 0.11±0.05 0.119±0.004 Co 59 0 0.006±0.006 0.003±0.004§ 0.008±0.004 0.0044±0.0035§ Co 59 1 0.006±0.006 —‡ 0.008±0.004 0.0039±0.0006§ Cr 52 0 0.101±0.087 0.46±0.10 0.023±0.009 0.44±0.20 Cr 53 0 0.101±0.087 0.11±0.04§ 0.023±0.009 0.11±0.11§ Cu 63 0 2.47±0.4 2.71±0.10 4.3±0.69 4.51±0.04 Cu 65 0 2.47±0.4 2.71±0.10 4.3±0.69 4.45±0.14 Fe 57 0 14.8±1.8 15.9±1.8 41.5±4 42.4±2.0 Fe 57 1 14.8±1.8 13.9±1.6 41.5±4 41.2±1.8 K 39 0 566±75 433±479 3180±140 3249±490 Li 7 0 — —‡ — 0.096±0.010 Mg 25 0 818±59 797±115 1070±80 1037±64 Mn 55 0 2.55±0.29 2.58±0.12 16±1 15.8±0.76 Mo 98 0 0.252±0.039 0.258±0.044 0.7±0.12 0.762±0.024 Ni 60 0 0.158±0.054 0.086±0.082§ 0.17±0.08 0.13±0.02§ Ni 60 1 0.158±0.054 0.069±0.082§ 0.17±0.08 0.12±0.02§ Pb 208 1 0.14±0.034 0.138±0.010 0.023±0.006 0.028±0.001§ Sb 121 0 0.0045* —‡ — —‡ Se 77 0 0.045±0.008 —‡ 1.23±0.09 1.39±0.04 Se 82 0 0.045±0.008 0.22±0.22§ 1.23±0.09 1.49±0.01 Sn 118 0 — † — —‡ Sr 86 0 4.62±0.56 4.97±0.15 1.19±0.09 1.22±0.02 Th 232 0 — 0.003±0.006§ — 1.21±0.02 Ti 47 0 — 4.89±0.08 5* 6.21±1.07 Tl 205 0 — —‡ — —‡ U 238 0 — —‡ — —‡ V 51 0 0.005±0.002 0.006±0.003§ 0.021±0.006 0.022±0.002§ V 51 1 0.005±0.002 0.114±0.028§ 0.021±0.006 0.112±0.008§ Zn 66 0 18.6±2.2 19.3±0.76 22.2±1.7 24.0±0.8 * Estimated value.‡ Below DL, DLs are half of those listed in Table 4 due to a 2-fold dilution factor dierence (250 v/m versus 500 v/m). § Above DL, but below MDL, MDLs are half of those listed in Table 4 due to a 2-fold dilution factor dierence (250 v/m versus 500 v/m). m/z 55. The certified K concentrations in the plant SRMs using this ICP-MS system. For the isotopes at higher m/z values, where spectral interferences are generally less, the range between 0.4–4.4%, which is equivalent to 8–88 mg l-1 of K in the diluted sample solutions.Using this ICP-MS isotopes with higher abundance were usually preferred. To be consistent with the isotopes used for water and sediment system, these concentrations would produce apparent Mn concentrations of below 0.3 mg l-1 (or 0.15 mg g-1 in sample analysis,23 the isotopes 47Ti, 114Cd, 121Sb, 137Ba, 205Tl and 238U were selected, although 111Cd, or 123Sb, or 135Ba and 138Ba, or solids) at m/z 55.Because the SRMs and RMs studied contain relatively high Mn concentrations (2.5–675 mg g-1), the K 203Tl can also be used for accurate determinations of these elements in biological samples. interference on 55Mn was negligible (Tables 5–7). The main considerations in the selection of the other primary isotopes listed in Table 4 are given below briefly. The plant Accuracy and Precision and grain samples contain relatively high Mg concentrations.Thus, an isotope with low relative abundance is preferred and The method was applied to the analyses of field samples from environmental monitoring, assessment and remediation pro- 25Mg was selected instead of 26Mg, which may be subject to interference from the tail of a strong Al signal at m/z 27. For jects, in which plant SRMs and grain RMs were analysed for quality control (QC) purposes. The data presented in Tables the Cu determinations, the results measured at both m/z 63 and 65 show good agreement with the certified values 5–7 represent all these QC results performed over a six month period.The accuracy of the method (for the primary isotopes (Table 5–7). To be consistent with this laboratory’s analysis of water samples, with potentially high Na concentrations, 65Cu and within the analytical range of the method) can be judged from the spike recoveries presented in Table 8 and the results was selected to avoid 23Na40Ar interference on 63Cu.For Zn determination, 66Zn was selected because the interferences at for SRMs and RMs presented in Tables 5–7. It is realized that for a given isotope, a low digestion eciency in conjunction m/z 66 are much less severe than those at m/z 64, 67 and 68 (Table 3). For Mo determination, the interferences of with the existence of an interference may produce an acceptable recovery. The precision of the method can be judged from the 40Ar39K16O on 95Mo and of 40Ar41K16O on 97Mo reported in another study26 were negligible for the plant and grain samples uncertainties (at the 95% confidence level) of the results presented in Tables 5–7, and the detection limits (DLs) and tested using this ICP-MS system. The isotope 98Mo was selected because it has the highest relative abundance among method detection limits (MDLs) listed in Table 4.The MDLs are 2–10 times greater than the corresponding DLs. The DLs Mo isotopes.The interference from Ru at 98Mo was negligible 804 Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12Table 8 Predigestion spike recoveries for field plant samples Field Samples Spike* % Recovery Element m/z Eqn. No. Conc. range/mg l-1 Conc./mg l-1 Mean±Uncert. Al 27 0 95–238 50 99.5±3.2 As 75 0 0.4–1.4 5 98.6±4.3 As 75 0 0–0.7 5 98.5±10 B 11 0 117–787 50 99.8±4.4 Ba 137 0 81–139 50 99.5±3.2 Ca 43 0 10000–22000 5000 101±3 Cd 114 1 0.5–5 5 93.6±2.2 Co 59 0 2–10 5 101±4 Co 59 1 2–10 5 101±4 Cr 52 0 3–6 5 100.4±4.7 Cr 53 0 0.8–2 5 99.7±5.3 Cu 63 0 7–17 5 97.0±4.8 Cu 65 0 7–17 5 96.9±4.3 Fe 57 0 200–450 50 108±5 Fe 57 1 200–400 5 98.0±6 K 39 0 4000–20000 1000 95.0±15 Li 7 0 9–30 50 103.3±1.9 Mg 25 0 5000–8000 1000 97.7±3.2 Mn 55 0 300–1500 50 96.2±3.1 Mo 98 0 2–30 5 102±2 Ni 60 0 9–45 5 100±5 Ni 60 1 9–45 5 98.8±5.3 Pb 208 1 0.4–1.2 5 96.2±3.4 Sb 121 0 0.015–0.04 5 95.1±3.8 Se 77 0 0.1–1 5 95.4±6 Se 82 0 -25–-2 5 106±20 Sn 118 0 0.03–0.3 5 106.5±4.8 Sr 86 0 100–170 50 101.1±2.5 Th 232 0 0.01–0.08 5 97.1±7.5 Ti 47 0 6–25 5 108±5 Tl 205 0 0.01–0.04 5 96.4±2.1 U 238 0 0.008–0.03 5 102.1±2.5 V 51 0 0.8–2.3 5 103.6±5.1 V 51 1 1.6–3.7 5 103.5±5.2 Zn 66 0 110–260 5 99.2±3.0 * Spike recovery is calculated as (conc.of spiked sample)/(conc. of sample+spike). obtained with the HNO3–H2O2 digestion (H2O2, BDH Lot samples as discussed previously, which produced nonreproducible digestion eciencies for elements that are dicult 116837–48528) were similar to those listed in Table 4, except the DL for Sn was poorer (0.2 mg l-1).to digest. In contrast to the plant SRMs, most of the mean recoveries Based on Tables 5 and 6, the elements tested for the four plant SRMs may be loosely grouped as follows. The first group for the majority of the elements tested in the grain RMs were within 85–115%. However, the MDLs for the determination contains 12 elements, As, B, Ba, Ca, Cd, Cu, Mg, Mn, Mo, Pb, Sr and Zn, with the majority of mean recoveries within of As, Co, Cr, Ni and Se were not suciently low.Although the results for several isotopes/equations in these 90–115%. The 7 elements, Al, Co, Cr, Fe, K, Sb and V, with most mean recoveries varying from 70–100% make up the SRMs and RMs are biased high because of interference or are biased low because of poor digestion eciencies, the pre- second group. The third group consists of 4 elements, U, Th, Ti and Ni with mean recoveries mainly within 40–80%.The digestion spike recoveries shown in Table 8 are all excellent. This is a perfect example illustrating that additive spectral fourth group includes elements such as Li, Sn and Tl that do not have available certified data. The fifth group consists of interferences and low digestion eciencies cannot be detected from spike recoveries. Therefore, acceptable spike recoveries Se and Cl. The recoveries for Se determined at m/z 77 with correction equation 1 varied so greatly that the accuracy for alone may not be sucient to prove the accuracy of a method.Se analysis of field samples is unknown. The Cl was not a target analyte in this study and its results were lacking in both CONCLUSION accuracy and precision. This was caused primarily by the variable HCl residues in the digestion PFA liners and sample The closed-vessel microwave digestion of plant and grain materials in concentrated HNO3 alone or in a mixture of tubes which were soaked in 1+2+9 HNO3–HCl–H2O solution for cleaning.It seems that the plant leaves contain siliceous HNO3 and H2O2 is favorable for trace element analysis by ICP-MS. The digestion procedure is simple. With 3 sets of 12 material to which the elements listed in the second and third groups are partially bound. The siliceous materials were not digestion vessels and 3 additional sets of 12 inner liners, 6 batches of samples were processed by overnight predigestion decomposed by the HNO3 digestion, resulting in lower recoveries.The precision for the determination of the elements listed with microwave digestion the following day. None of the elements tested were lost in the digestion. With the overall in the second and third group is generally poorer than for those listed in the first group. This is probably caused by the dilution factor of 500 (v/m) for the plant SRMs and of 250 (v/m) for the grain RMs, the samples could be analysed by a variations in the actual digestion temperature profiles of the Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 8053 Buckley, W. T., and Ihnat, M., Fresenius’ J. Anal. Chem., 1993, simple ICP-MS method in which undigested external standards 345, 217. prepared in reagent blanks were used for calibration with In 4 Munro, S., Ebdon, L., and McWeeny, D. J., J. Anal. At. Spectrom., as the internal standard. The background interferences were 1986, 1, 211.corrected by subtracting the blank signals. The isobaric elemen- 5 Goossens, J., De Smaele, T., Moens, L., and Dams, R., Fresenius’ tal and polyatomic ionic interferences, especially those from J. Anal. Chem., 1993, 347, 119. 6 Beauchemin, D., McLaren, J. W., and Berman, S. S., J. Anal. At. C, Ca and Cl, were eliminated, corrected, or reduced by Spectrom., 1988, 3, 775. selecting the suitable primary isotopes and applying the appro- 7 Lyon, T. D. B., Fell, G. S., McKay, K., and Scott, R.D., J. Anal. priate interference correction equations. Although this diges- At. Spectrom., 1991, 6, 559. tion procedure did not decompose the siliceous material or 8 Gu� nther, K., von Bohlen, A., Paprott, G., and Klockenka�mper, minimize the C concentration in the samples as the wet ashing R., Fresenius’ Z. Anal. Chem., 1992, 342, 444. 9 Ebdon, L., Fisher, A. S., Worsfold, P. J., Crews, H., and Baxter, procedure does, the majority of recoveries for the 22 elements, M., J.Anal. At. Spectrom., 1993, 8, 691. Al, As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mn, Mg, Mo, 10 Evans, S., and Kra�henbu� hl, U., Fresenius’ Z. Anal. Chem., 1994, Pb, Sb, Sn, Sr, Tl, V and Zn in the pre-digested spikes and/or 349, 454. in the four plant SRMs and two grain RMs were within 11 Krachler, M., Radner, H., and Irgolic, K. J., Fresenius’ Z. Anal. 70–115%. Chem., 1996, 355, 120. The limitations of the method include the low recoveries for 12 Liu, H., Montaser, A., Dolan, S.P., and Schwartz, R. S., J. Anal. At. Spectrom., 1996 11, 307. the determination of U, Th, Ti and Ni in the plant SRMs. 13 Amarasiriwardena, D., Krushevska, A., Argentine, M., and Barnes, Also, if the Ca and Cl concentrations in the solutions are so R. M., Analyst, 1994, 119, 1017. high that the ICP-MS signals for 43Ca and 35Cl are saturated 14 Krushevska, A., La� sztity, A., Kotrebai, M., and Barnes, R. M., or the corresponding correction equations are no longer valid, J.Anal. At. Spectrom., 1996, 11, 343. then the results for the determination of 57Fe, 60Ni and 59Co, 15 Subramanian, K., Spectrochim. Acta, Part B, 1996, 51, 291. 16 Beary, E. S., and Paulsen, P. J., Anal. Chem., 1993, 65, 1602. or for the determination of 51V, 75As, 77Se and 53Cr will be 17 Pepelnik, R., Prange, A., and Niedergesa�ß, R., J. Anal. At. biased. In addition, the method detection limits are not suc- Spectrom., 1994, 9, 1071. iently low for the determination of Se in all SRMs and of As, 18 Evans, S., and Kra�henbu� hl, U., J. Anal. At. Spectrom., 1994, 9, 1249. Co, Cr and Ni in some plant SRMs and grain RMs tested. 19 Matusiewicz, H., Sturgeon, R. E., and Berman, S. S., J. Anal. At. The detection limit can be further improved by reducing the Spectrom., 1989, 4, 323. 20 De Bra�tter, V. E. N., Bra�tter, P., Reincke, A., Schulze, G., Alvarez, overall dilution factor, as long as the increased concentrations W. O. L., and Alvarez, N., J. Anal. At. Spectrom., 1995, 10, 487. of the interfering elements in the solution do not hamper the 21 Vaughan, M. A., and Horlick, G., Appl. Spectrosc. 1986, 40, 434. accuracy of the determination. Furthermore, the maximum 22 Tan, S. H., and Horlick, G., Appl. Spectrosc., 1986, 40, 445. amount of the biological sample to be eciently digested is 23 Wu, S., Zhao, Y., Feng, X., and Wittmeier, A., J. Anal. At. limited by the maximum working pressure of 13.8 bar (or 200 Spectrom., 1996, 11, 287. psi) allowed in the closed-vessel microwave digestion system 24 Nadkarni, R. A., Anal. Chem., 1984, 56, 2233. 25 Tothill, P., Matheson, L. M., Smyth, J. F., and McKay K., J. Anal. used in this study. At. Spectrom., 1990, 5, 619. Overall, the explored method is simple, rapid, and suitable 26 Vanhoe, H., Goossens, J., Moens, L., and Dams, R., J. Anal. At. for the analysis of at least 22 elements in a variety of plant Spectrom., 1994, 9, 177. and grain samples for environmental monitoring and assess- 27 Vaughan, M. A., and Templeton, D. M., Appl. Spectros., 1990, ment projects. 44, 1685. 28 Shao, Y., and Horlick, G., Appl. 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