APPLICATION NOTE ICP – Mass Spectrometry Authors: Cynthia Bosnak Senior Product Specialist Ewa Pruszkowski Senior Product Specialist PerkinElmer, Inc. Shelton, CT USA The Determination of Toxic, Essential, and Nutritional Elements in Food Matrices Using the NexION 300/350 ICP-MS Introduction The elemental and dynamic range of inductively coupled plasma-mass spectrometry (ICP-MS) makes it ideally suited for the analysis of food materials. The ultratrace detection limits of ICP-MS permit the determination of low-level contaminants such as Pb, As, Se, and Hg, while the macro-level nutritional elements such as Ca, Mg, K, and Na can be quantified using the extended dynamic range capability of ICP-MS which provides 9-orders of magnitude. However, there are still a number of challenges to overcome, which makes the routine analysis of foods difficult unless the sample dissolution procedure is well thought out and instrumental conditions are optimized for complex sample matrices. For example, the wide variety of edible products available means that a highly diverse range of matrices must be brought into solution for ICP-MS analysis. These complex acid-digested matrices, which are a combination of dissolved carbohydrates, fats, and proteins, can pose major problems for any ICP-MS because of the potential for blocking of the interface cones and/or deposition on the quadrupole ion deflector (QID). For this reason, if instrument design does not account for high-matrix samples, long-term stability can be severely compromised. In addition to signal drift, digested food matrices can also cause major spectral complications. The sample’s organic components, together with macro minerals, can combine with elements present in the digestion acid and/or the plasma argon to form polyatomic interferences. For example, chloride ions (at mass 35) combine with the major argon isotope (mass 40) to produce the argon chloride interference 40 Ar35Cl+, which interferes with arsenic at mass 75. Another example is the argon dimer (ArAr+), which forms from the plasma gas and exists at the same masses as the major selenium isotopes. In addition, the major isotope of chromium at mass 52 is overlapped by 40Ar12C+, 35Cl17O+, and 35Cl16OH+ interferences generated by the sample matrix and the plasma gas. As a result, these kinds of spectral interferences have made the determination of both trace and macro elements in food samples extremely challenging. To overcome these issues, a NexION 300X ICP-MS (PerkinElmer, Inc., Shelton, CT) was used for the analysis of various food substances, focusing on toxic and typical essential and macro elements in a group of NIST® (Gaithersburg, MD) standard reference materials (SRMs). ® Experimental Six different NIST® SRM food samples that represent a typical cross-section of the types of foods for human consumption were chosen for the evaluation. The foods included spinach leaves (leafy vegetable), corn bran (grain), wheat flour (grain), bovine muscle (meat), mussel tissue (shellfish), and milk powder (dairy product). The samples were brought into solution with a Multiwave™ 3000 microwave digestion system. Details of the sample digestion procedure are shown in Table 1. Sample Preparation Approximately 0.5-0.6 g of each SRM was digested in duplicate with 5 mL of nitric acid (Fisher Optima HNO3) and 2 mL of hydrogen peroxide (Fisher Optima H2O2) in precleaned PTFE HF-100 microwave sample vessels. The filled 2 vessels were placed on a 16-position rotor with an internal p/T sensor positioned in one of the samples to monitor the pressure and temperature inside the sample container. In addition, an external IR sensor provided the temperatures for each individual sample in the tray. The digestion program consisted of 30 min of heating and 15 min of cooling, as shown in Table 1. All the SRM samples were completely dissolved, resulting in clear solutions that were diluted to a final volume of 50 mL with deionized water. No further sample dilutions were necessary. Gold was added to all solutions at a final concentration of 200 µg/L to stabilize mercury. Preparation blanks, consisting of the acid mixture, were taken through the same microwave digestion program as the samples. Table 1. Microwave Digestion Heating Program for All Six NIST® Food SRMs. Step Power (W) Ramp (min) Hold (min) 1500 1 4 2 10005 5 3 14005 10 4 (cooling) 15 0 — Instrumental Conditions All data in this study were generated under normal operating conditions on a NexION 300X ICP-MS using an autosampler. The instrumental operating conditions are shown in Table 2. Table 2. ICP-MS Instrumental Operating Conditions for this Application. Component/ParameterType/Value/Mode Nebulizer Glass concentric Spray chamber Glass cyclonic ConesNickel Plasma gas flow 18.0 L/min Auxiliary gas flow 1.2 L/min Nebulizer gas flow 0.98 L/min Sample uptake rate 300 µL/min RF power 1600 W Total integration time 0.5 (1.5 seconds for As, Se, Hg) No. of replicates per sample ™ 3 Universal Cell Technology * KED mode *PerkinElmer, Inc. Calibration • Low-level essential analytes: 0-2 ppm Multielement calibration standards, representing all the analytes covered by the six NIST® SRMs, were made up from PerkinElmer® Pure single and multielement standards and diluted into 10% HNO3. Gold was added to all solutions at a final concentration of 200 µg/L to stabilize mercury. However, it is important to mention that each food SRM was certified for a slightly different group of elements. For that reason, quantitation was only carried out on the analytes that had reference values. Calibration standard ranges were based on whether the analyte was expected to be a high-level, nutritional element like potassium (K) or sodium (Na), a low/medium-level essential element like manganese (Mn) or iron (Fe), or a trace/ultratrace contaminant such as lead (Pb) or mercury (Hg). Depending on the certificate value of the analytes, five different calibration ranges were made up to cover the complete range of elements being determined. They were: • Trace-level contaminants: 0-200 ppb • Ultratrace-level contaminants: 0-20 ppb Figure 1 shows representative calibration curves for each range. • High-level nutritional analytes: 0-300 ppm • Medium-level essential analytes: 0-20 ppm 54 Fe Correlation Coefficient = 0.99997. 23 Na Correlation Coefficient = 0.99996. 63 Cu Correlation Coefficient = 0.99999. 31 P Correlation Coefficient = 0.99999. 78 Se Correlation Coefficient = 0.99995. Figure 1. Calibration curves for 23Na (0-300 ppm), 31P (0-20 ppm), 54Fe (0-2 ppm), 63Cu (0-200 ppb) and 78Se (0-20 ppb). 3 In addition to the analyte elements used for the multielement calibration, the standards, blanks, and samples were also spiked on-line using a mixing tee with a solution of 6Li, Sc, Ge, In, and Tb for internal standardization across the full mass range. Acetic acid was added to the internal standard solution to compensate for residual carbon leftover from the sample digestion. Results Quantitative results for two sample preparations of six NIST® SRMs (Corn Bran, Bovine Muscle, Mussel Tissue, Milk Powder, Wheat Flour, and Spinach Leaves) are shown in Tables 3-8, respectively. All elements in every sample were determined with kinetic energy discrimination (KED) mode using helium as the collision gas. Figures in parentheses ( ) in the reference value column are not certified values, but are included for information purposes only. The data show very good agreement with the certified values, especially for the elements that suffer from known spectral interferences. The elements that are outside the specified limits are mostly the ones that are well recognized as environmental contaminants, which have probably been impacted by the sample preparation procedure. Food samples are complex acid-digested matrices and can create major problems for some ICP-MS systems because of deposits on the interface cones and on the ion optics caused from high concentrations of dissolved solids. For this reason, long-term stability can be poor. However, the triple cone interface and the quadrupole ion deflector design of the NexION guarantee exceptional long-term stability. For six hours, food samples with high concentrations of dissolved solids were analyzed and a quality control (QC) sample was read every 5 samples. Figure 2 shows the long-term stability over 6 hours. Conclusion The ICP-MS system used in this study is well suited for the analysis of complex digested food materials. The agreement between experimental and certified results for six NIST® food SRMs demonstrates that the NexION 300X ICP-MS can effectively measure various food samples. In addition to removing interferences, the NexION 300X allows the determination of macro-level nutritional elements in the same analysis run as lower-level elements, without having to dilute the samples. Instrument design characteristics eliminate deposition on the ion optics, leading to long-term stability in high-matrix samples while permitting trace levels to be accurately measured. Table 3. Analysis of NIST® 8433 Corn Bran using the NexION 300 ICP-MS. ElementMass Reference (amu) Value (mg/kg) Experimental Value (mg/kg) B 11 2.8 ±1.2 3.2 Na 23 430 ±31 399 Mg 26 818 ±59 787 Al 27 1.01 ±0.55 1.15 P 31 171 ±11 158 S 34 860 ±150 738 K 39 566 ±75 548 Ca 44 420 ±38 434 V 51 0.005 ±0.002 0.005 Cr 52(0.11) 0.08 Fe 54 14.8 ±1.8 13.7 Mn 55 2.55 ±0.29 2.53 Co 59(0.006) 0.005 Ni 60 0.158 ±0.054 0.143 Cu 63 2.47 ±0.40 2.54 Zn 66 18.6 ±2.2 17.0 As 75(0.002) <0.006 Figure 2. % Recovery of several analytes in the QC standard during 6-hour analysis. 4 Se 78 0.045 ±0.008 0.056 Sr 88 4.62 ±0.56 4.56 Mo 98 0.252 ±0.039 0.255 Cd 111 0.012 ±0.005 0.013 Sn 118– 0.015 Sb 121(0.004) 0.003 Ba 137 2.40 ±0.52 2.26 Hg 202 0.003 ±0.001 0.005 Pb 208 0.140 ±0.034 0.122 Tl 205– <0.0001 Th 232– <0.00008 U <0.00002 238– Table 4. Analysis of NIST® 8414 Bovine Muscle using the NexION 300 ICP-MS. Table 5. Analysis of NIST® 2976 Mussel Tissue using the NexION 300 ICP-MS. ElementMass Reference (amu) Value (mg/kg) Experimental Value (mg/kg) ElementMass Reference (amu) Value (mg/kg) Experimental Value (mg/kg) B 11 0.6 ±0.4 0.4 B 11– 27.5 Na 23 2100 ±80 2000 Na 23 (35000 ±1000) 35000 Mg 26 960 ±95 960 Mg 26 (5300 ±500) 4800 Al 27 1.7 ±1.4 1.6 Al 27 (134 ±34) 149 P 31 8360 ±450 7250 P 31(8300) S 34 7950 ±410 6820 S 34(19000) 16000 K 39 15170 ±370 14180 K 39 (9700 ±500) 9700 Ca 44 145 ±20 143 Ca 44 (7600 ±300) 7400 V 51(0.005) 0.006 V 51– 0.87 Cr 52 0.071 ±0.038 0.092 Cr 52 (0.50 ±0.16) 0.50 Fe 54 71.2 ±9.2 71.2 Fe 54 171.0 ±4.9 190 Mn 55 0.37 ±0.09 0.44 Mn 55 (33 ±2) 40 Co 59 0.007 ±0.003 0.014 Co 59 (0.61 ±0.02) 0.67 Ni 60 0.05 ±0.04 0.05 Ni 60 (0.93 ±0.12) 0.87 Cu 63 2.84 ±0.45 2.81 Cu 63 4.02 ±0.33 3.91 Zn 66 142 ±14 140 Zn 66 137 ±13 145 As 75 0.009 ±0.003 0.011 As 75 13.3 ±1.8 16.4 Se 78 0.076 ±0.010 0.11 Se 78 1.80 ±0.15 2.52 Sr 88 0.052 ±0.015 0.081 Sr 88 (93 ±2) 79 Mo 98 0.08 ±0.06 0.08 Mo 98– 0.56 Cd 111 0.013 ±0.011 0.013 Cd 111 0.82 ±0.16 0.88 Sn 118– 0.14 Sn 118 (0.096 ±0.039) 0.103 Sb 121(0.01) 0.01 Sb 121– 0.011 Ba 137(0.05) 0.04 Ba 137– 0.61 Hg 202 0.005 ±0.003 0.003 Hg 202 0.061 ±0.0036 0.058 Pb 208 0.38 ±0.24 0.34 Pb 208 1.19 ±0.18 1.06 Tl 205– 0.002 Tl 205(0.0013) 0.003 Th 232– <0.00008 Th 232 U <0.00002 U 238– 238– (0.011 ±0.002) 6900 0.012 0.22 5 6 Table 6. Analysis of NIST® 1549 Milk Powder using the NexION 300 ICP-MS. Table 7. Analysis of NIST® 8436 Wheat Flour using the NexION 300 ICP-MS. ElementMass Reference (amu) Value (mg/kg) Experimental Value (mg/kg) ElementMass Reference (amu) Value (mg/kg) Experimental Value (mg/kg) B 11– 2.1 B 11– 0.62 Na 23 4970 ±100 4700 Na 23 16.0 ±6.1 17.0 Mg 26 1200 ±30 1170 Mg 26 1070 ±80 1030 Al 27(2) 0.7 Al 27 11.7 ±4.7 11.8 P 31 10600 ±200 10500 P 31 2900 ±220 2330 S 34 3510 ±50 3290 S 34 1930 ±280 1460 K 39 16900 ±300 16500 K 39 3180 ±140 2950 Ca 44 13000 ±500 12800 Ca 44 278 ±26 262 V 51– 0.003 V 51 0.021 ±0.006 0.026 Cr 52 0.0026 ±0.0007 <0.003 Cr 52 0.023 ±0.009 0.053 Fe 54 1.78 ±0.10 1.98 Fe 54 41.5 ±4.0 41.4 Mn 55 0.26 ±0.06 0.26 Mn 55 16.0 ±1.0 15.1 Co 59(0.0041) 0.005 Co 59 0.008 ±0.004 0.007 Ni 60– 0.013 Ni 60 0.17 ±0.08 0.17 Cu 63 0.7 ±0.1 0.6 Cu 63 4.30 ±0.69 4.18 Zn 66 46.1 ±2.2 46.7 Zn 66 22.2 ±1.7 20.6 As 75(0.0019) <0.006 As 75(0.03) 0.01 Se 0.17 Se 78 1.23 ±0.09 1.22 Sr 88– 3.7 Sr 88 1.19 ±0.09 1.19 Mo 98(0.34) 0.37 Mo 98 0.70 ±0.12 0.72 Cd 111 <0.002 Cd 111 0.11 ±0.05 0.11 Sn 118– <0.002 Sn 118– 0.032 Sb 121(0.00027) <0.001 Sb 121– 0.002 Ba 137– 0.83 Ba 137 2.11 ±0.47 2.04 Hg 202 0.0003 ±0.0002 <0.0007 Hg 202 0.0004 ±0.0002 <0.0007 Pb 208 0.019 ±0.003 0.019 Pb 208 0.023 ±0.006 0.35 Tl 205– <0.0001 Tl 205– Th 232– <0.00008 Th 232– 0.001 U <0.00002 U 0.001 78 0.11 ±0.01 0.0005 ±0.0002 238– 238– <0.0001 Table 8. Analysis of NIST® 1570a Spinach using the NexION 300 ICP-MS. ElementMass Reference (amu) Value (mg/kg) Experimental Value (mg/kg) B 11 37.6 ±1.0 37.3 Na 23 18180 ±430 17350 Mg 26(8900) 8600 Al 27 310 ±11 200 P 31 5180 ±110 4810 S 34(4600) 4400 K 39 29030 ±520 26600 Ca 44 15270 ±410 15040 V 51 0.57 ±0.03 0.58 Cr 52– 1.63 Fe 54– 265 Mn 55 75.9 ±1.9 77.9 Co 59 0.39 ±0.05 0.37 Ni 60 2.14 ±0.10 1.97 Cu 63 12.2 ±0.6 11.6 Zn 66 82 ±3 80 As 75 0.068 ±0.012 0.081 Se 78 0.117 ±0.009 0.21 Sr 88 55.6 ±0.8 58.1 Mo 98– 0.39 Cd 111 2.83 Sn 118– 0.027 Sb 121– 0.007 Ba 137– 5.8 Hg 202 0.028 Pb 208(0.20) 0.16 Tl 205– 0.018 Th 232 0.048 ±0.003 0.045 U 238 (0.155 ±0.023) 0.154 2.89 ±0.07 0.030 ±0.003 PerkinElmer, Inc. 940 Winter Street Waltham, MA 02451 USA P: (800) 762-4000 or (+1) 203-925-4602 www.perkinelmer.com For a complete listing of our global offices, visit www.perkinelmer.com/ContactUs Copyright ©2010-2014, PerkinElmer, Inc. All rights reserved. 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