Microwave digestion of plant and grain standard reference materials in nitric and hydrofluoric acids for multi-elemental determination by inductively coupled plasma mass spectrometry Xinbang Feng,* Shaole Wu, Angela Wharmby and Adolph Wittmeier Alberta Research Council, P.O. Bag 4000, Vegreville, Alberta, Canada T9C 1T4 Received 22nd June 1998, Accepted 29th March 1999 A microwave-assisted HNO3–HF digestion system was explored for the total dissolution of biological plant and grain materials followed by multi-elemental determination using ICP-MS, in order to improve the low recoveries of several elements observed in a previous study using a microwave-assisted nitric acid digestion system.NIST standard reference materials (SRMs), including Apple Leaves (1515), Peach Leaves (1547), Wheat Flour (1567a), Rice Flour (1568a), Tomato Leaves (1573) and Pine Needles (1575), were analyzed. Approximately 0.5 g of sample was digested in 5 ml of HNO3 and 0.1 ml of HF, with or without a subsequent digestion stage with boric acid.The matrix eVect for boron was evaluated for an ICP-MS system and signal enhancement was observed for all the elements tested. Potential spectral interferences in ICP-MS with HNO3–HF, boron and biological matrices are discussed and the spectral interferences on Co, As and Se are tabulated. The ICP-MS system was calibrated using external standards prepared in undigested reagent blanks with In as an internal standard.It was found that with a low but suYcient amount of HF in the digestion, the possible precipitation of metal fluorides in the digestate (without boric acid) was not significant. The recoveries for some silicon-bound elements, such as Al, Co, Cr, Ni, Th, U and V, were significantly improved compared with those from digestions with HNO3 alone. Using the HNO3–HF digestion procedure, the ICP-MS results for 30 elements, Al, As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sb, Se, Si, Sr, Th, Ti, Tl, U, V and Zn, agreed well with the certified values in leaf and grain SRMs.The recoveries were mostly within the range 85–115%. Hence, the use of boric acid in the digestion was not necessary, which simplified the procedure, minimized the content of the total dissolved solids in solution for ICP-MS analysis and allowed the determination of boron. The microwave-assisted nitric acid digestion of biological plant HNO3–HF alone without the addition of boric acid, has been reported for the dissolution of atmospheric aerosol samples tissue and grain materials was studied in previous work for and analysis by ICP-MS.12 trace multi-elemental determination using inductively coupled In this study, the microwave-assisted HNO3–HF digestion plasma mass spectrometry (ICP-MS).1 The method is simple system, with and without the addition of boric acid, was and convenient to use.The analytical results for most SRMs explored for the total dissolution of biological plant and grain tested were satisfactory except for some silicon-bound material, followed by the multi-elemental determination using elements, such as Al, Ni, Th and U, with recoveries in the ICP-MS.The spectral characteristics produced from the range 40–80%, and V, Co and Cr, with recoveries varying HNO3–HF digestion and the HNO3–HF–H3BO3 digestion from 70 to 100%.1 If these elements are of interest, an alternative were compared.Potential spectral interferences, especially with digestion method for plant and grain materials would be regard to Co, As and Se, are discussed. NIST standard required. Complete digestion of silicon-containing materials reference materials (SRMs) were analyzed for 30 elements. with hydrofluoric acid is well established for the dissolution The ICP-MS results were compared with the certified or of environmental, biological and geological solid samples such reference values to evaluate the accuracy and precision of as peat, leaves, oyster tissue, bovine liver, fly ash, coal, soils, the methods.sediments, ores and rocks.2–10 Studies of the use of a mixture of HNO3 and HF with microwave heating for the dissolution of some plant tissues followed by ICP-AES and ICP-MS Experimental analysis2 and HF digestion of dry-ashed orchard leaves for Standard reference materials, reagents and standard solutions the determination of Th and U using ICP-MS3 have been conducted.After HF digestion, any free HF can be eliminated The NIST SRMs used for this study were Apple Leaves either by evaporating the solution2,11 or by adding boric (1515), Peach Leaves (1547), Tomato Leaves (1573), Pine acid4,10 or water-soluble tertiary amines4 to neutralize excess Needles (1575), Wheat Flour (1567a) and Rice Flour (1568a). fluoride ion. However, the evaporation procedure is time Both high-purity HNO3 (68–71%) and HF (45–50%) (double consuming and suVers from partial losses of some volatile sub-boiling distilled in quartz; Seastar Chemicals, Sidney, BC, elements such as Si, B, Ge and Hg.2 The addition of boric Canada) were used for sample digestion.A 5% H3BO3 solution acid, which masks excess HF by forming tetrafluoroboric acid, was prepared using analytical-reagent grade reagent (BDH, makes the determination of boron impossible and increases Poole, Dorset, UK). Distilled, de-ionized water (DDW) was the total dissolved solids (TDS) content in the sample obtained from a three-column de-ionizing system with 18 digestate.In addition, the presence of boric acid results in mV cm specific resistivity capability. more complicated spectral interferences, matrix eVects and A set of undigested standard solutions (1, 10 and 100 mg l-1) poor detection limits for certain elements due to high back- including about 30 elements were prepared in the corresponding undigested reagent blank solutions. In most cases, a 100 ml ground signals.10 A simple digestion method, utilizing J.Anal. At. Spectrom., 1999, 14, 939–946 939reagent blank or standard solution contained 2 ml of concen- with In as the internal standard. A 100 ml aliquot of In solution containing 3.5 mg l-1 of In was added into each 10 ml of trated HNO3 and 0.04 ml of HF for the HNO3–HF system and an additional 2.4 ml of 5% H3BO3 for the standard or sample solution prior to ICP-MS analysis. Each calibration curve was constructed linearly through zero after HNO3–HF–H3BO3 system. Reagent blanks were processed for background subtraction in order to subtract any contami- subtraction of the reagent blank.nation contributions especially from the use of an analyticalreagent grade reagent such as the 5% H3BO3 solution. Results and discussion Microwave digestion system and digestion procedures Evaluation of matrix eVect All sample digestions were accomplished using a QWAVE-1000 Matrix eVect problems in ICP-MS can be divided into two microwave sample preparation system (Questron, Mercerville, basic categories: matrix induced signal intensity changes and NJ, USA), equipped with temperature and pressure regulation matrix induced spectral overlaps.A high concentration of a (through a sensor vessel ) and controlled by a personal com- matrix element is known to alter the analyte sensitivity and puter. The details of the system and the procedure used for analyte signal suppression is the most commonly observed cleaning the PFA liners of the digestion vessels were described eVect in ICP-MS.10,13–22 In the HNO3–HF digestion of biologiin a previous paper.1 cal plant materials, the matrix induced signal change was Two digestion procedures were tested: digestion with minimal since the sample matrix from carbohydrates generally HNO3–HF only and digestion with HNO3–HF followed by a did not contribute, having been converted to CO2 during second-stage digestion using boric acid.Approximately 0.5 g digestion and subsequently released. The concentrations of (dry mass) of plant or grain SRM was weighed directly into major elements, such as K, Na, Ca, Mg and P, were not high the PFA inner liners, to which 5.0 ml of concentrated HNO3 enough (<10–30 mg l-1) in the analyzed solutions to produce and 0.1 ml of HF were added. Samples were pre-digested significant matrix eVects. In the two-stage HNO3–HF–H3BO3 overnight in a clean fume hood at room temperature.Twelve digestion, H3BO3 was added after the dissolution to mask the vessels including a reagent blank vessel and a sensor vessel free HF and to dissolve the fluoride precipitate in the digested were sealed and digested using the following procedures. The solution by forming tetrafluoroboric acid. Hence the matrix temperature was ramped to 165 °C within 10 min with the eVect of the boric acid and spectral interferences from the application of 1000 W power, followed by a dwell time of boron matrix cannot be avoided in the ICP-MS procedure.10 20 min at 165 °C.The temperature limit was 175 °C and the In general, lower mass elements are subject to more serious pressure limit was 15.2 bar (220 psi). After cooling, each vessel matrix eVects and heavier matrix elements cause more severe was vented. For the HNO3–HF digestion, the sample digestate suppression.14–16 was diluted to 100 ml (by mass) with DDW. For the For the boron matrix, both suppression and enhancement HNO3–HF–H3BO3 digestion, there was a post-digestion stage, of signal intensities have been observed and reported.10,17,20 i.e., 6 ml of 5% boric acid were added to each sample digestion The mechanisms of the matrix eVects depend on the changes vessel and warmed to 100 °C in the closed vessels for 10 min in analyte ionization in the ICP, in sampling processes in the by microwave heating.After cooling, each sample was diluted interface region and in ion transmission in the mass specto 100 ml with DDW.All diluted digestates were transferred trometer.22 In this study, instead of suppression, the presence to high-density polyethylene (HDPE) bottles. Prior to ICP-MS of the boron matrix enhanced the signal intensities of elements analysis, each sample solution was further diluted 2.5-fold. at both low and high m/z regions (Table 1). The extent of the Hence the overall dilution factor was 500 (v/m) and the signal enhancement was related to the concentration of the contents of the digestion reagents in the final solution were boron matrix and the m/z values.In Table 1, the signal was about 2% HNO3, 0.04% HF (45–50%) and 0.12% H3BO3. enhanced by about 16–30% for the lighter elements such as The amounts of HF used in the digestion and the amounts of H3BO3 used in the post-digestion were also tested. In one Table 1 Enhancement of signal intensities of elements in HNO3– HF–H3BO3 at diVerent concentrations of boron matrix compared digestion batch, 0.01, 0.05 and 0.1 ml of HF were used with with that in HNO3–HF 5 ml of HNO3, followed by addition of 1.2 ml of 5% H3BO3 for the post-digestion.In another batch, 0.1, 0.3 and 0.5 ml of Enhancement (%) HF were used with 5 ml of HNO3, and 6 ml of 5% H3BO3 were added for the post-digestion. The procedures for the Element m/z 210 mg l-1 boron 350 mg l-1 boron digestion, post-digestion and dilution were the same as Li 7 22.2 23.2 described above.Be 9 11.8 17.8 Al 27 25.9 33.5 ICP-MS system Sc 45 16.4 25.5 Ti 47 8.58 11.6 All measurements were carried out using a Perkin-Elmer V 51 10.7 13.5 SCIEX (Thornhill, ON, Canada) Elan Model 5000 ICP quad- Mn 55 8.95 11.0 rupole mass spectrometer. The details of the ICP-MS system Co 59 9.12 13.1 and the operation parameters were described in a previous Ni 60 9.20 12.4 paper.10 Briefly, a GemTip cross-flow nebulizer, a Ryton spray Cu 65 8.27 10.1 Se 77 6.15 12.6 chamber and an ICP torch with a quartz injector (2.0 mm id) Sr 88 8.4 10.1 were used.The sample uptake rate was set at 1 ml min-1, the Y 89 8.13 9.29 plasma forward power was 1000 W, the outer gas and inter- Mo 98 10.2 11.9 mediate gas flow rates were 15 and 0.8 l min-1 and a central Cd 114 6.33 10.2 gas flow rate of 0.9 l min-1 was used to aspirate sample In 115 9.94 9.29 solutions. The oxide ratio of 140CeO+/140Ce+ and doubly Ho 165 11.0 9.94 Pb 208 7.15 7.21 charged species ratio of 138Ba2+/138Ba+ were maintained below Bi 209 11.2 12.0 0.03 and 0.02, respectively.‘Omni’ ranges (i.e., reduced deflec- Tl 205 7.03 8.53 tor voltage) were used for the determination of several elements Th 232 9.34 7.57 such as Na, Mg, Al, Si, K and Mn. U 238 7.86 5.4 The ICP-MS system was calibrated using external standards 940 J. Anal. At. Spectrom., 1999, 14, 939–946Table 2 Spectral interferences on Co, As and Se in ICP-MS for biological samples Element m/z Chloride/fluoride Oxide/hydride Doubly charged ionsa Co 59 40Ar19F+, 40Ca19F+ 43Ca16O+, 42Ca16OH+ As 75 40Ar35Cl+, 40Ca35Cl+ 59Co16O+, 43Ca16O2+ 150Nd2+ (5.64), 150Sm2+ (7.4), 151Eu2+ (47.8) Se 77 40Ar37Cl+, 40Ca37Cl+ 60Ni16OH+, 45Sc16O2+ 154Sm2+ (22.7), 154Gd2+ (2.18), 153Eu2+ (52.2) Se 82 12C35Cl2+ 66Zn16O+, 81BrH+ 164Dy2+ (28.2), 164Er2+ (1.61), 165Ho2+ (100.0) aPercentage natural abundance in parentheses Li, Be, Al and Sc and by about 5–9% for the heavier elements and 42CaOH+ on m/z 59, while the slope of the line represents the contribution of 40CaF+ when the percentage concentration such as Tl, Th and U.Boron is a light element with a relatively high ionization potential of 8.3 eV. Usually, a matrix consisting of HF (45–50%) is unity. Derived from the linear regression equation in Fig. 1, the interference of 40CaF+ at m/z 59 in a of a light element with a low degree of ionization leads to a minimal space charge environment.15,19 As a lighter and less solution containing 100 mg l-1 Ca and 0.04% HF solution was a similar contribution to that of the sum of 43CaO+ and energetic ion, B+ ions are defocused to a greater extent than Ar+.Little or no space charge in the beam current downstream 42CaOH+. A correction equation, 59M=59M-0.002543M, should be applied for the determination of 59Co+, where the of the skimmer is expected, so the signal enhancement may be attributable to the decrease in the space charge.The matrix coeYcient should be measured for each analytical batch.1 The spectral interferences on 75As+ and 77Se+ are generally induced signal suppression or enhancement could be compensated by using internal standardization18–22 and/or matrix from the high content of chlorides in biological samples.17–20 Both the 75As+ and 77Se+ signals suVer from the isobaric matching of calibration standards.20,22 overlap of argon chlorides (40Ar35Cl+ and 40Ar37Cl+). Actually, the spectral interferences on 75As+, 77Se+ and 82Se+ Evaluation of potential spectral interferences are very complicated.In addition to the argon chlorides, there In addition to the matrix induced signal enhancement, matrix are other interfering species, such as hydrides, oxides and induced spectral interferences were observed in the ICP-MS doubly charged species of some rare earth elements in the of these biological samples. Molecular background species in biological samples. As listed in Table 2, 75As+ may also be ICP-MS are derived from components of the overall sample aVected by 59Co16O+ and/or by the doubly charged species of solution matrix.Typical spectral interferences in biological the rare earth elements such as 150Nd2+, 150Sm2+ or 151Eu2+, samples digested in HNO3 were well documented in previous and 77Se+ may be aVected by 60NiOH+, 154Sm2+, 154Gd2+ or work.1 Spectral interferences originating from major elements 153Eu2+. Also, 82Se+ may be overlapped by the peaks of such as Ca, Cl, P, K, C, Na and S were usually found in the 81BrH+, 164Dy2+, 164Er2+ or 165Ho2+.These interferences are ICP-MS of biological samples.23–26 The most serious spectral not easily corrected and may cause biased results especially interferences for the HNO3–HF digestion system involve the when they dominate the interferences. monoxide, hydroxide, fluoride and argide species of Ca, Cl With the HNO3–HF–H3BO3 digestion system, the formation and C.These species are CaO+, CaOH+, CaF+, ArF+, ClO+, of boron oxides, boron hydroxides, boron hydrates and boron ArCl+ and ArC+, which aVect the results for 57Fe+, 59Co+, argides, such as BO+, BOH+, BH2 O+, BO+2, BO2 H+, 60Ni+, 63Cu+, 51V+, 52Cr+, 75As+ and 77Se+. As shown in B(OH)+2 and BAr+,10 aVect the determination of several Table 2, Co at m/z 59 suVers from interferences from 40ArF+, elements such as 27Al+, 43Ca+, 45Sc+, 47Ti+ and 51V+. All 43CaO+, 40CaF+ and 42CaOH+.The only diVerence between the isotopes of Ti and V from m/z 46 to 51 suVer from isobaric using HF–HNO3 and HNO3 alone is the interference of overlaps from boron argide species such as 10B36Ar+, 40CaF+ and 40ArF+ on m/z 59. The interference from 40ArF+ 11B36Ar+, 10B38Ar+, 11B38Ar+, 10B40Ar+ and 11B40Ar+. was easily corrected by subtracting reagent blanks. The inter- Because of the presence of the boron argides, the formation ference from 40CaF+ was more severe than that from 40ArF+, of boron–argon oxides and hydroxides is possible.These 43CaO+ and 42CaOH+, and was directly related to the concen- molecular species (10B40ArO+ and 11B40ArO+) may aVect the tration of HF in the solution. Fig. 1 shows that the signal determination of zinc. On the other hand, boron fluoride ratios of m/z 59 to m/z 43 in solutions containing 100 mg l-1 species such as BF+, BF2 +, BF3 + and BF4+ in the plasma Ca and various amounts of HF increased linearly with increase may also exist, which overlap with other peaks such as NO+, in HF concentration.The intercept of the linear regression 11B38Ar+ and 11BAr40OH+ peaks at m/z 30, 49 and 68, curve shown in Fig. 1 represents the contribution of 43CaO+ respectively. However, the boron matrix related interferences can be simply corrected through reagent blank subtraction.10 Analytical results The analytical results are given in Tables 3–6, in which the uncertainties are shown at the 95% confidence level.The matrix induced signal enhancement was compensated by using In as an internal standard. The matrix induced background interferences were corrected using reagent blank subtraction. Spectral interferences originating from the biological sample matrix were corrected by either selecting alternative analytical isotopes or using correction equations. The related correction equations for 51V+, 53Cr+, 57Fe+, 59Co+, 60Ni+, 75As+ and 77Se+ and the assigned equation numbers were given in a previous paper.1 Briefly, the default eqn. (0) indicates that no correction was applied, while the eqn.(1) or (2) relates to the results after interference correction. With eqn. (1) or (2), the Fig. 1 Signal ratios of m/z 59 to m/z 43 from solutions containing 100 mg l-1 of Ca and various concentrations of HF. results for 51V+, 53Cr+, 75As+ and 77Se+ were those after the J. Anal. At. Spectrom., 1999, 14, 939–946 941Table 3 Comparison of recoveries of elements in 0.5 g of NIST SRM 1547 Peach Leaves with HNO3 digestion and HNO3–HF–H3BO3 digestion.Recoveries of 59Co+ [eqn. (0)] are listed to demonstrate interferences from CaO+, CaOH+ and CaF+ Recovery (mean±uncertainty) (%) HNO3–HF–H3BO3 1.2 ml 5% H3BO3 6.0 ml 5% H3BO3 Eqn Certified HNO3 a Element m/z No. value/mg g-1 (no HF) 0.01 ml HF 0.05 ml HF 0.1 ml HF 0.1 ml HF 0.3 ml HF 0.5 ml HF Al 27 0 249±8 75.5±4 102±2 99.3±3 93± 6 88.9±2 90.0±3 94.5±0.3 Ba 137 0 124±4 102±8 102±1 101±1 97.4±4 101±2 101±0.5 102±2 Ca 43 0 15600±200 109±6 113±0.4 110±3 104±5 94.3±6 101±4 107±3 Cd 114 1 0.026±0.003 105±6 114±2 104±4 89± 5 95.6±17 87.8±15 86.2±11 Co 59 0 0.07b 118±10 143±5 332±18 503±15 147±11 408±9 935±75 Co 59 1 0.07b 79.2±9 112±5 102±9 105±4 115±9 107±4 109±19 Cr 53 0 1b 94.8±7 101±1 100±4 92.5±4 97.0±2 91± 3 101±3 Cu 65 0 3.7±0.4 105±3 105±2 101±2 98.1±3 93.2±1 95.9±3 101±2 Mn 55 0 98±3 97.7±5 104±1 99.6±3 93.7±4 94.5±4 97.8±1 100±3 Mo 98 0 0.06±0.008 94.3±11 88.9±2 105±9 92.3±5 90.2±5 95.4±4 90± 4 Ni 60 1 0.69±0.09 56.3±13 101±4 97± 1 94.6±11 98.5±6 89.6±3 104±9 Pb 208 1 0.87±0.03 102±7 94.5±0.7 94.9±0.4 96.9±0.8 93.2±0.5 89.8±2 90.7±2 Sb 121 0 0.02b 96.8±17 138±3 155±17 137±14 104±8 105±6 104±8 Sr 86 0 53±4 104±4 113±0.2 109±1 107±3 103±2 105±2 107±0.3 Th 232 0 0.05b 60.9±8 105±2 111±2 119±1 118±2 114±5 113±4 U 238 0 0.015b 65.1±8 87.8±9 99.5±9 105±4 97.9±14 98.6±16 104±10 V 51 0 0.37±0.03 99.0±5 113±4 114±5 104±7 99.3±5 106±7 115±2 Zn 66 0 17.9±0.4 108±4 110±2 106±3 103±2 97.8±1 99.9±3 103±3 aData from ref. 1.bNIST reference value. Table 4 Comparison of recoveries of elements in NIST SRM 1547 Peach Leaves and NIST SRM 1573 Tomato Leaves with HNO3–HF digestion and HNO3–HF–H3BO3 digestion. Results for 75As+, 59Co+, 52Cr+, 57Fe+, 60Ni+ and 51V+ [eqn. (0)] are listed to demonstrate interferences NIST SRM 1547 NIST SRM 1573 Recovery (mean±uncertainty) Recovery (mean±uncertainty) (%) (%) Eqn Certified HNO3–HF Certified HNO3–HF Element m/z No.value/mg g-1 –H3BO3 HNO3–HF value/mg g-1 –H3BO3 HNO3–HF Al 27 0 249±8 88.9±2.3 103±1.8 1200a —e 97.1±8 As 75 0 0.06±0.018 176±3 221±12 0.27±0.05 188±8 174±12 As 75 2 0.06±0.018 168±3b 190±14b 0.27±0.05 96.3±12 100±0.4 Ba 137 0 124±4 101±2, 3 100±2 57± 9d 112±0.3 116±4 Ca 43 0 15600±200 94.3±6.9 103±1, 30000±300 112±0.7 106±1 Cd 114 1 0.026±0.003 95.6±19 122±7 2.5±0.2d 104±0.4 104±0.1 Co 59 0 0.07a 147±12 189±4 0.525±0.046d 116±3 108±1 Co 59 1 0.07a 115±10b 99.1±3.5b 0.525±0.046d 105±4 92.6±1 Cr 52 0 1a 134±8 205±5 4.5±0.5 117±4 111±1 Cr 53 0 1a 97.0±2 104±4 4.5±0.5 109±6 104±2 Cr 53 1 1a 95.6±0.9 97.3±2.7 4.5±0.5 92.3±3 88.7±0.6 Cu 65 0 3.7±0.4 93.2±1.1 109±6 11±1 91.0±2 91.5±2 Fe 57 0 218±14 124±5 128±2 690±25 109±1 114±1 Fe 57 1 218±14 95.4±3 94.9±2 690±25 91.3±1 94.3±1 Mn 55 0 98±3 94.5±4 97.3±2 238±7 —e 95.8±0.7 Mo 98 0 0.06±0.008 90.2±6b 84.7±5b 0.53±0.09d 101±2 100±2 Ni 60 0 0.69±0.09 134±7 189±5 1.3±0.2d 152±3 161±1 Ni 60 1 0.69±0.09 98.5±7 95.1±6 1.3±0.2d 94.5±3 91.9±2 Pb 208 1 0.87±0.03 93.2±0.6 101±3 6.3±0.3 97.1±1 96.4±5.6 Sb 121 0 0.02a 104±9 114±4 0.036±0.007d 98.0±12 88.6±4 Se 77 0 0.12±0.009 <0.15 —c 0.054±0.006d <0.15 —c Se 82 0 0.12±0.009 <0.2 —c 0.054±0.006d <0.2 —c Sr 86 0 53±4 103±2 108±1 44.9±0.3 99.4±1 100±0.6 Th 232 0 0.05a 118±2 112±6 0.17±0.03 100±4 86.9±7 U 238 0 0.015a 97.9±16 95.3±6 0.061±0.003 94.1±9 80.0±10 V 51 0 0.37±0.03 99.3±5.7 105±3 1.2±0.2d 148±6 140±3 V 51 2 0.37±0.03 99.3±5.7 103±1 1.2±0.2d 127±2 124±1 Zn 66 0 17.9±0.4 97.8±1 97.8±4 62± 6 97.1±2 91.5±1 aNIST reference values.bBelow MDL but above DL; refer to Table 7. cBelow DL; refer to Table 7. dConsensus values.27 eOut of the linear calibration range; ‘Omni’ range or further dilution required. 942 J. Anal. At. Spectrom., 1999, 14, 939–946Table 5 Analytical results (mg g-1) and recoveries of elements in NIST SRM 1515 Apple Leaves and 1575 Pine Needles with HNO3–HF digestion.Results for 75As+, 59Co+, 52Cr+, 57Fe+, 60Ni+ and 51V+ [eqn. (0)] are listed to demonstrate interferences NIST SRM 1515 NIST SRM 1575 Eqn Certified Found Recovery Certified Found Recovery Element m/z No. (mean±uncertainty) (mean±uncertainty) (mean±uncertainty) (%) (mean±uncertainty) (mean±uncertainty) (mean±uncertainty) (%) Al 27 0 286±9 293±3.9 102±14 545±30 576±6.8 106±1.2 As 75 0 0.038±0.007 0.129±0.011b 339±29b 0.21±0.04 0.212±0.014 101±6.7 As 75 2 0.038±0.007 0.107±0.010b 282±26b 0.21±0.04 0.202±0.012 96.2±5.7 B 10 0 27± 2 28.0±0.6 104±2.2 17±2d 16.6±0.3 97.6±1.8 Ba 137 0 49±2 50.0±0.5 102±10 7.2±0.8d 7.46±0.11 104±1.5 Ca 43 0 15260±150 16797±123 110±0.8 4100±200 4522±67 108±1.6 Cd 114 0 0.013±0.002 0.0277±0.018b 213±138b 0.22±0.06d 0.211±0.018 95.9±8.2 Cd 114 1 0.013±0.002 0.0126±0.0032b 96.9±25b 0.22±0.06d 0.2±0.017 90.9±7.7 Co 59 0 0.09a 0.117±0.014b 130±16b 0.122±0.014d 0.121±0.003b 99.2±2.5b Co 59 1 0.09a 0.0751±0.014b 83.4±16b 0.122±0.014d 0.11±0.0028b 90.2±2.3b Cr 52 0 0.3a 1.58±0.050 527±17 2.6±0.2 3.59±0.1 138±3.8 Cr 53 0 0.3a 0.495±0.028 165±9.0 2.6±0.2 2.8±0.086 108±3.3 Cr 53 1 0.3a 0.422±0.033 141±11 2.6±0.2 2.76±0.088 106±3.4 Cu 63 0 5.64±0.24 5.52±0.17 97.9±3.0 3±0.3 3.00±0.19 100±6.3 Cu 65 0 5.64±0.24 5.51±0.24 97.7±4.3 3±0.3 2.97±0.18 99.0±6.0 Fe 57 0 83±5 147±2.8 177±3.4 200±10 236±21 118±10 Fe 57 1 83±5 75.4±2.3 90.8±2.8 200±10 217±21 109±10 Hg 202 0 0.044±0.004 0.057±0.011b 130±25b 0.15±0.05 0.133±0.012 88.7±8.0 K 39 0 16100±200 17668±2132 110±13 3700±200 3086±380 83.4±10 Mg 25 0 2710±80 2924±44 108±1.6 1220±160d 1145±30 93.9±2.5 Mn 55 0 54±3 53.1±0.41 98.3±0.8 675±15 672±9 99.6±1.3 Mo 98 0 0.094±0.013 0.086±0.010b 91.5±11b 0.15±0.05d 0.115±0.010 76.7±6.7 Na 23 0 24.4±1.2 29.9±3.8 123±16 50±30d 29.6±5.3 59.2±10 Ni 60 0 0.91±0.12 1.33±0.051 146±5.6 2.5±0.3d 2.56±0.09 102±3.6 Ni 60 1 0.91±0.12 0.859±0.046 104±5.1 2.5±0.3d 2.43±0.09 97.2±3.6 P 31 0 1590±110 1806±25 114±1.6 1200±200 1332±21 111±1.8 Pb 208 0 0.47±0.024 0.462±0.001 98.3±0.2 10.8±0.5 11.3±0.38 105±3.5 Pb 208 1 0.47±0.024 0.458±0.001 97.4±0.2 10.8±0.5 11.2±0.38 104±3.5 S 34 0 1800a 2267±122 126±6.8 1320±111d 1370±173 104±13 Si 29 0 944±200 814d 1119±282 137±35 Sb 121 0 0.013a 0.0126±0.0024 96.9±18 0.197±0.017d 0.179±0.013 90.9±6.6 Se 77 0 0.05±0.009 <0.1 —c 0.047±0.005d <0.1 —c Se 77 1 0.05±0.009 <0.15 —c 0.047±0.005d <0.15 —c Se 82 0 0.05±0.009 <0.2 —c 0.047±0.005d <0.2 —c Sn 118 0 <0.2 4.38±6.1 3.35±0.32 Sr 86 0 25±2 26.4±0.4 106±1.6 4.8±0.2 4.75±0.03 99.0±0.6 Th 232 0 0.03a 0.026±0.004b 86.7±13b 0.037±0.003 0.035±0.0003 94.6±0.8 Ti 47 0 17.8±0.7 13.7d 14.7±0.8 107±5.8 Tl 205 0 0.014±0.001 0.05a 0.046±0.002 92.0±4.0 U 238 0 0.006a 0.0070±0.0012b 117±20b 0.02±0.004 0.0178±0.001 89.0±5.0 V 51 0 0.26±0.03 0.28±0.018 108±6.9 0.39±0.07d 0.419±0.01 107±2.6 V 51 2 0.26±0.03 0.258±0.019 99.2±7.3 0.39±0.07d 0.409±0.01 105±2.6 Zn 66 0 12.5±0.3 11.3±0.043 90.4±0.3 67±9d 78.1±21 116±31 aNIST reference values.bBelow MDL, but above DL; refer to Table 7. cBelow DL; refer to Table 7. dConsensus values.27 J. Anal. At. Spectrom., 1999, 14, 939–946 943Table 6 Analytical results (mg g-1) and recoveries of elements in NIST SRM 1567a Wheat Flour and 1568a Rice Flour with HNO3–HF digestion NIST SRM 1567a NIST SRM 1568a Eqn Certified Found Recovery Certified Found Recovery Element m/z No. (mean±uncertainty) (mean±uncertainty) (mean±uncertainty) (%) (mean±uncertainty) (mean±uncertainty) (mean±uncertainty) (%) Al 27 0 5.7±1.3 4.85±0.004 85.0±0.1 4.4±1 4.69±0.01 107±0.2 As 75 0 0.006a <0.01 —c 0.29±0.03 0.298±0.012 103±4 Ca 43 0 191±4 205±1 107±0.7 118±6 125±0.5 106±0.4 Cd 114 1 0.026±0.002 0.0282±0.0003 108±1 0.022±0.002 0.0241±0.002 109±9 Co 59 1 0.006a <0.015 —c 0.018a <0.015 —c Cu 65 0 2.1±0.2 2.07±0.01 98.8±0.6 2.4±0.3 2.47±0.008 103±0.3 Fe 57 1 14.1±0.5 14.1±0.6 100±4 7.4±0.9 7.77±0.43 105±6 K 39 0 1330±30 1263±3 95.0±0.2 1280±8 1228±9 95.9±0.7 Mg 25 0 400±20 403±4 101±1 560±20 572±5 102±0.9 Mn 55 0 9.4±0.9 9.57±0.08 102±0.8 20±1.6 20.5±0.12 103±0.6 Mo 98 0 0.48±0.03 0.487±0.001 102±0.1 1.46±0.08 1.50±0.03 103±2 P 31 0 1340±60 1497±3 112±0.2 1530±80 1713±14 112±0.9 Pb 208 1 <0.02a 0.014±0.001b — <0.01a 0.011±0.002b — S 34 0 1650±20 1671±46 101±3 1200±20 1312±23 109±2 Se 77 0 1.1±0.2 1.18±0.18 107±17 0.38±0.04 0.369±0.066 97.1±17 Sn 118 0 0.003a <0.005 —c 0.005a <0.005 —c U 238 0 0.0003a <0.002 —c 0.0003a <0.002 —c V 51 1 0.011a <0.05 —c 0.007a <0.05 —c Zn 66 0 11.6±0.4 11.5±0.1 98.9±0.6 19.4±0.5 18.5±0.2 95.3±1.0 aNIST reference value.bBelow MDL, but above DL; refer to Table 7. cBelow DL; refer to Table 7. 944 J. Anal. At. Spectrom., 1999, 14, 939–946Table 7 Comparison of ICP-MS detection limits (mg g-1, directly in correction for ArCl+ and ClO+ interferences, while the results dry solid) using diVerent digestion procedures for the primary isotopes for 57Fe+, 59Co+ and 60Ni+ were those after the correction selected in this study for CaF+ and/or CaO+ and CaOH+ interferences.1 The HNO3–HF–H3BO3 digestion system was tested for HNO3–HF–H3BO3 HNO3–HF 0.5 g of NIST SRM 1547 Peach Leaves, using 0.01, 0.05 and Eqn Element m/z No.DLa MDLb DLa MDLb 0.1 ml of HF, each with 1.2 ml of 5% H3BO3, or using 0.1, 0.3 and 0.5 ml of HF, each with 6 ml of 5% H3BO3.As shown Al 27 0 0.4 1.5 0.1 2c in Table 3, the use of 0.1 ml of HF (45–50%) per gram of dry As 75 0 0.025 0.2 0.01 0.05 sample was found to be suYcient to improve the recoveries As 75 1 0.1 0.5 0.1 0.2 (within 85–115% in most cases) for the silicon-bound elements B 10 0 0.4 1 0.4 2 Al, Co, Ni, Th and U, compared with those obtained using Ba 137 0 0.02 0.3 0.01 0.5 Be 9 0 0.3 3 0.04 0.1 the HNO3 digestion system.1 The use of 0.25–0.5 ml HF per Ca 43 0 8 15 5 12 gram dry mass has been reported to give a complete recovery Cd 114 1 0.025 0.06 0.01 0.04 of Si in food samples.4 The silicon content in most of the Cl 35 0 5 450 80 360 plant and grain samples is about 0.1–0.3%.9 The stoichiometric Co 59 0 0.005 0.1 0.015 0.2 amount of HF required for 0.3% of Si is about 0.03 ml of 50% Co 59 1 0.05 0.5 0.015 0.3 HF per gram of sample.To ensure complete digestion, 0.2 ml Cr 53 0 0.05 0.4 0.1 0.2 Cu 65 0 0.1 0.25 0.05 0.25 of HF (45–50%) per gram of dry sample was selected for Fe 57 0 1.5 4 1.5 4 subsequent use in this study, being about 3–10-fold in excess Fe 57 1 1.5 4 1.5 4 of the amount required. The amount of HF could be increased Hg 202 0 0.02 0.05 0.02 0.05 to a maximum of 1 ml if the Si content in plant materials is K 39 0 50 100c 50 100c considerably higher.In Table 3, the recoveries of 59Co+ with Li 9 0 0.05 0.15 0.05 0.3 eqn. (0) (i.e., without interference correction) are listed to Mg 25 0 0.5 1.5c 0.2 1c Mn 55 0 0.015 0.025 0.015 1c demonstrate the interferences from CaO+, CaOH+ and especi- Mo 98 0 0.01 0.1 0.01 0.1 ally CaF+.They were biased high and increased with increase Na 23 0 2 4c 2 4c in the amount of HF used. The recovery of 59Co+ with eqn. Ni 60 0 0.04 0.4 0.1 0.2 (1) was significantly improved after the interference coeYcient Ni 60 1 0.04 0.4 0.1 0.2 from CaF+, CaO+ and CaOH+ was derived and applied in P 31 0 — — 1.5 10 the correction equation, as was done in previous work.1 Pb 208 1 0.01 0.04 0.02 0.08 S 34 0 — — 50 100 As demonstrated in Table 4 for NIST SRM 1547 and 1573, Sb 121 0 0.005 0.008 0.0025 0.008 there were no significant diVerences between the ICP-MS Se 77 0 0.1 0.15 0.15 0.2 results using the HNO3–HF or the HNO3–HF–H3BO3 diges- Se 82 0 0.2 0.5 0.2 0.4 tion systems.With the HNO3–HF digestion system, the Si 29 0 — — 12 800c recoveries for Al, As, B, Ba, Ca, Co, Cd, Cr, Cu, Fe, Hg, Mg, Sn 118 0 0.025 0.06 0.01 0.15 Mn, Mo, Ni, Pb, Sb, Se, Sr, Th, Ti, Tl, U, V and Zn in NIST Sr 86 0 0.025 0.1 0.05 0.15 Th 232 0 0.0025 0.05 0.003 0.03 SRM 1547, 1573, 1515, 1575, 1567a and 1568a were mostly in Ti 47 0 0.5 1 0.15 0.3 the range 85–115% (Tables 4–6).By applying the ‘Omni’ Tl 205 0 0.0035 0.008 0.002 0.005 range, this method can also be used to determine major U 238 0 0.0025 0.006 0.002 0.008 elements such as K, Na, Mg, P, S and Si in plant and grain V 51 0 0.15 0.5 0.01 0.15 materials (Tables 5 and 6).V 51 1 0.15 0.5 0.05 0.15 Similarly to the HNO3 digestion, carbon residue still Zn 66 0 0.3 0.15 0.1 0.3 remained in solution after microwave digestion with aDetection limits derived from three times the standard deviation for HNO3–HF. The problem of the spectral interferences from blank solutions containing 2% HNO3 and 0.04% HF (45–50%) or polyatomic species involving carbon remained the same, as containing 2% of HNO3, 0.04% of HF (45–50%) and 0.12% of H3BO3 (n=10), assuming an overall dilution factor of 500 (v/m).bMethod seen from the biased high recovery of 52Cr+ arising from the detection limits derived from within-run standard deviations of dupli- interferences from 40Ar12C+. The biased high recovery of cate digestion and analysis of SRMs and digestion blanks (n7) with 82Se+ mainly came from the interferences of 81BrH+, whose an overall dilution factor of 500 (v/m). c‘Omni’ range used. contributions could be significant, but were not corrected. After correcting for ArCl+ interference, the recovery for 75As+ remained high for SRM 1515 and 1547 (Tables 4 and 5), because of the low As content and the non-corrected inter- pared with the HNO3–HF digestion system, mainly owing to the spectral interferences from the boron matrix.ferences (Table 2) which might dominate the signal at m/z 75. Satisfactory results for Se were obtained for grain materials SRM 1567a and 1568a in Table 6. The Se levels in the plant Conclusions materials in Tables 4 and 5 are all below the detection limits. The instrument detection limits (DLs) and the method For plant tissue and grain materials in which the Si content is usually low, the closed-vessel microwave-assisted HNO3–HF detection limits (MDLs) for both digestion systems are presented in Table 7.The MDLs were derived from the within- digestion is favorable for elemental analysis by ICP-MS. The amount of HF used in the digestion is so low (0.1 ml per 0.5 g run standard deviation of duplicate digestions of standard reference materials and digestion blanks10 (n7), and were of sample) that the coprecipitation of metal fluorides in the solution is negligible and clogging of the ICP-MS nebulizer or 2–10-fold greater than the corresponding DLs.The ‘Omni’ ranges that were applied reduced the sensitivities for Al, K, the sampling cone will not occur. Hence the use of the boric acid in the post-digestion stage was not necessary.The sample Mg, Mn and Na and resulted in poorer MDLs for these elements. With the exception of Co, which suVered from the dissolution was more complete with the HNO3–HF digestion system than that with the HNO3 digestion system. Using the high background signal from ArF+, the DLs with the HNO3–HF digestion system were similar to those reported HNO3–HF digestion system, the previously reported low recoveries for silicon-bound elements such as Al, Co, Ni, Th, with the HNO3 digestion system.1 The DLs for 9Be+, 27Al+, 43Ca+, 65Cu+, 47Ti+, 51V+ and 66Zn+ with the U and V in SRMs digested with HNO3 were significantly improved.In only one instance, that of U for SRM 1573, was HNO3–HF–H3BO3 digestion system were relatively poor com- J. Anal. At. Spectrom., 1999, 14, 939–946 94512 L. M. Jalkanen and E. K. Ha�sa�nen, J. Anal. At. Spectrom., 1996, the recovery low (80%) for the HNO3–HF digestion, whereas 11, 365–369. it was adequate (94%) with the HNO3–HF–H3BO3 digestion 13 C.Vandecasteele, H. Vanhoe and R. Dams, J. Anal. At. Spectrom., (Table 4). Biased high recoveries of V (148%) and Cr (141%) 1993, 8, 781. in Tables 4 and 5 may be due to the results being compared 14 G. Horlick, Spectroscopy, 1992, 7, 22. with a reference value and a consensus value.27 Similarly to 15 S. H. Tan and G. Horlick, J. Anal. At. Spectrom., 1987, 2, 745. 16 M. A. Vaughan and G. Horlick, J. Anal. At. Spectrom., 1989, the HNO3 digestion procedure, the HNO3–HF digestion pro- 4, 45. cedure is simple and rapid, and allows the determination of 17 D. Beauchemin, J. W. McLaren and S. S. Berman, Spectrochim B, long known to be an essential element for plant growth.28 Acta, Part B, 1987, 42, 467. 18 J. J. Thompson and R. S. Houk, Appl. Spectrosc., 1987, 41, 801. 19 G. R. Gillson, D. J. Douglas, J. E. Fulford, K. W. Halligan and S. D. Tanner, Anal. Chem., 1988, 60, 1472. References 20 S. J. Stotesbury, J. M. Pickering and M. A. GriVerty, J. Anal. At. Spectrom., 1989, 4, 457. 1 S. Wu, X. Feng and A. Wittmeier, J. Anal. At. Spectrom., 1997, 21 X. Feng and G. Horlick, J. Anal. At. Spectrom., 1994, 9, 823. 12, 797. 22 I. Rodushkin, T. Ruth and D. Klockare, J. Anal. At. Spectrom., 2 B. Madeddu and A. Rivoldini, At. Spectrosc., 1996, 17, 148. 1998, 13, 159. 3 K. Shiraishi, Y. Takacu, K. Yoshimizu, Y. Igarashi, K. Masuda, 23 M. A. Vaughan and G. Horlick, Appl. Spectrosc., 1986, 40, 434. J. F. McInroy and G. Tanaka, J. Anal. At. Spectrom., 1991, 6, 335. 24 H. Vanhoe, J. Goossens, L. Moens and R. Dams, J. Anal. At. 4 A. Krushevska, A. La�sztity, M. Kotrebai and R. M. Barnes, Spectrom., 1994, 9, 177. J. Anal. At. Spectrom., 1996, 11, 343. 25 Y. Shao and G. Horlick, Appl. Spectrosc., 1991, 45, 143. 5 L. Xu and W. Shen, Fresenius’ J. Anal. Chem., 1989, 333, 108. A. Vaughan and D. M. Templeton, Appl. Spectrosc., 1990, 6 C. S. E. Papp and L. B. Fischer, Analyst, 1987, 112, 337. 44, 1685. 7 L. B. Fischer, Anal. Chem., 1986, 58, 261. 27 Compilation of Elemental Concentration Data for NBS Clinical, 8 P. J. Lamothe, T. L. Fries and J. J. Consul, Anal. Chem., 1986, Biological, Geological, and Environmental Standard Reference 58, 1881. Materials, National Bureau of Standards, Gaithersburg, MD, 1987. 9 R. A. Nadkarni, Anal. Chem. 1984, 56, 2233. 28 S. Evans and U. Kra�henbu� hl, J. Anal. At. Spectrom., 1994, 9, 10 S.Wu, Y. Zhao, X. Feng and A. Wittmeier, J. Anal. At. Spectrom., 1249. 1996, 11, 287. 11 C. F. Wang, W. H. Chen, M. H. Yang and P. C. Chiang, Analyst, 1995, 120, 1681. Paper 8/04683B 946 J. Anal. At. Spectrom., 1999, 14, 939&ndash