Analytical Approaches to Arsenic (and Selenium) Analysis in Foods Brian Jackson Trace Element Analysis Core Dartmouth College
Arsenic in food
Arsenic levels in rice and rice products FDA data WHO data 120 100 Frequency 80 60 40 20 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 20 40 60 80 100 120 140 160 180 200 220 Inorganic arsenic concentration (ppb)
FDA proposes action level for As in juice 10 µg/l inorganic arsenic
Food analysis 0.25 g sample 2.5 ml optima acid HNO 3 or 9:1 HNO 3 :HCl 15 min ramp 20 min hold @ 200C Dilute final sample to 25 ml 100-fold dilution Collision or reaction cell ICP-MS
What affects arsenic and selenium analysis by ICP-MS? Polyatomic Interferences As, m/z 75, Se, 77,78, 80, 82 ArCl, CaCl, ArAr Solved by collision and reaction cell ICP-MS Matrix effects Low molecular weight organics cause dramatic sensitivity increases for As and Se. Doubly-charged ions ICP-MS actually measures m/z ratio, but we assume that z = 1. Elements at twice the mass of the analyte can interfere if they form 2+ ions in the plasma Mass 150 = samarium, neodymium Mass 156 = gadolinium
Sensitivity enhancement for As and Se Dramatic increase in ICP-MS response for As and Se in the presence of low molecular weight organics Well known effect, used to advantage in chromatography to increase ICP-MS sensitivity by adding methanol to mobile phase. Larsen and Sturup, J. Anal. Atom. Spectrom. 1994 Add methane to carrier/makeup as optional gas
Overcoming the sensitivity effect Add a swamping amount of C to the samples or the sample introduction system. Methane as a optional gas 5% Butanol or IPA to the internal standard mixture Use an appropriate internal standard Te or Ge Matrix match standards or standard additions calibration
Analysis of diluted apple juice Internal standard External calibration Standard addition As value RSD As value RSD None 24.303 18.6% 14.589 11.8% Rh 20.807 6.5% 15.650 5.6% Te 18.740 4.9% 15.707 5.3% Ge 17.948 2.9% 16.008 4.2%
Selenium analysis in NIST 1849a Adult/infant nutritional formula
Doubly-charged ions 42 nd in crustal abundance* 7.4% abundant at m/z 150 Arsenic: 0.06 ± 0.018 ppm Selenium: 0.120 ± 0.009 ppm Neodymium: 7 ppm Samarium: 1 ppm Gadolinium:1 ppm Collected from Peach County, GA http://reefacts.com/ 33 rd in crustal abundance* 5.6% abundant at m/z 150 * Arsenic is 46 th in crustal abundance Arsenic: 0.038 ± 0.007 ppm Selenium: 0.050 ± 0.009 ppm Neodymium:17 ppm Samarium: 3 ppm Gadolinium: 3 ppm Collected from PA fruit orchards
Collision (KED) and reaction cells Kinetic energy discrimination (use He as collision gas) Polyatomics lose more energy than analyte ions and do not leave the cell. Analytes suffer some sensitivity loss at high gas flow. Reaction cells use reactive gases Reaction gas reacts with interferent to remove it from m/z of analyte Reaction gas reacts with analyte to shift it to new m/z free of interferences
Doubly charged REEs in collision cell ICP-MS Arsenic Selenium
Doubly charged REEs in H 2 reaction cell ICP-MS Arsenic Selenium
Summary statistics and quantitative analysis of apple and peach leaves by He and H2 modes Arsenic KED-He reaction-h 2 Summary statistics Certified (mg/kg) uncorrected corrected uncorrected corrected Peach 0.060 ± 0.018 0.212 ± 0.004 % doubly 0.142 charged ± 0.009 0157 ± 0.008 Detection limits 0.118 ± 0.012 Apple 0.038 ± 0.007 0.313 Nd ± 0.015 Sm 0.162 ± 0.014 Gd 0.184 As IDL ± 0.010 (ppb) Se 0.104 IDL (ppb) ± 0.012 He -4.5 ml/min 1.2 0.9 0.25 0.01 0.06 H 2-6 ml/min 0.06 0.36 0.006 0.022 0.03 Selenium Peach 0.120 ± 0.009 0.488 ± 0.039 0.296 ± 0.06 0.117 ± 0.009 n.a. Apple 0.050 ± 0.009 1.088 ± 0.09 0.656 ± 0.08 0.048 ± 0.001 n.a.
O 2 as a reaction gas using QQQ -ICP-MS 40 Ar 35 Cl + 40 Ca 35 Cl Nd ++ /Sm ++ 75 As + 91 Zr Reaction gas (O 2 ) 40 Ar 35 Cl +, 40 Ca 35 Cl +, Nd ++ /Sm ++ 75 As 16 O + 94 SeO + 91 Zr + Q1 set to m/z 75, so rejects all ions except m/z 75. 91 Zr at mass 91 is rejected 75 As 91 AsO + 78 Se 94 SeO + As + reacts with O 2 cell gas to form AsO + product ion. 40 Ar 35 Cl +, 40 Ca 35 Cl +, Nd ++ /Sm ++ don t react and stay at m/z 75 Q2 set to m/z 91, AsO + product ion mass rejects original on-mass interferences Q1 set to m/z 78, so rejects all ions except m/z 78. 94 Zr/Mo at mass 94 is rejected Se + reacts with O 2 cell gas to form SeO + product ion. 40 Ar 38 Ar +, 40 Ca 35 Cl +, Gd ++ /Dy ++ don t react and stay at m/z 78 Q2 set to m/z 94, SeO + product ion mass rejects original on-mass interferences
Summary statistics and quantitative analysis of apple and peach leaves by O 2 modes 75 -> 91 As [ O2 ] 78 -> 94 Se [ O2 ] measured % doubly charged % recovery Certified Detection limits Certified (mg/kg) measured % recovery NIST 1515, Apple 0.038± Nd/Sm0.0001 Gd 100.5 As 0.050 IDL ± (ppb) 0.009 Se 0.053± IDL (ppb) 0.007 106 leaves 0.038 ± 0.007 NIST 1547, Peach O 2-0.35 ml/min 0.066 n.a. ± 0.003 0.04 110 0.12 0.0006 ± 0.009 0.127± 0.00050.002 106 Leaves 0.06 ± 0.018
Summary Collision cell and reaction cell ICP-MS effectively reduce polyatomic interferences for As and Se analysis in foods Standard additions, additional organic modifier and use of Ge or Te can normalize sensitivity differences due to food sample matrix Collision cell ICP-MS is not effective for doubly charged ions Reaction cell technology is effective H 2 or O 2 for Se O 2 for As QQQ-ICP-MS provides low detection limits and no interferences
Acknowledgements