Determination of metals in foods by atomic absorption spectrometry after dry ashing: NMKL Collaborative Study

A method for determination of lead, cadmium, zinc, copper, and iron in foods by atomic absorption spectrometry (AAS) after dry ashing at 450 degrees C was collaboratively studied in 16 laboratories. The study was preceded by a practice round of familiarization samples and another round in which solutions were distributed and the metals were determined directly by AAS. The study included 5 different foods (liver paste, apple sauce, minced fish, wheat bran, and milk powder) and 2 simulated diets. A single analysis was carried out with each sample. Suitable sample combinations were used as split-level combinations for determination of the repeatability standard deviation. The reproducibility relative standard deviation for each of the elements ranged from 20 to 50% for lead concentrations of 0.040-0.25 mg/kg, from 12 to 352% for cadmium concentrations of 0.001-0.51 mg/kg, from 4 to 8% for zinc concentrations of 0.7-38 mg/kg, from 7 to 45% for copper concentrations of 0.51-45 mg/kg, and from 11 to 14% for iron concentrations of 4-216 mg/kg.     

Cadmium and other metals in Swedish wheat and rye flours: longitudinal study, 1983-1997.

Wheat flour (sifted), wheat bran, and rye flour were sampled annually in several different areas in Sweden for 15 consecutive years (1983-1997) for a total of 105, 90, and 30 samples, respectively. These samples were analyzed for their content of Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn by atomic absorption spectrophotometry with background correction after dry ashing at 450 degrees C. As part of the quality control procedures, an in-house reference material was analyzed in parallel to the samples to ensure reliability of the results. In addition, a certified reference material was analyzed to monitor accuracy of the results. The Cd level in wheat and rye flours (mean 0.029 and 0.017 mg/kg dry wt, respectively) correlated significantly with time (p < 0.05) in 2 phases, tending to increase during the first half of the period and decrease during the latter half. In wheat bran, Cd (mean 0.15 mg/kg) levels did not correlate significantly with time. In wheat flour, Cu, Mn, and Zn (means 1.7, 6.2, and 7.6 mg/kg, respectively) showed a significant linear increase (p < 0.05) in concentration over time. In rye flour, the levels of Cu, Fe, Mn, and Zn (means 3.8, 36, 30, and 28 mg/kg, respectively) all decreased significantly (p < 0.05) over time. Similarly, in wheat bran Cu (mean 14 mg/kg) increased and Pb (mean 0.027 mg/kg) decreased significantly (p < 0.05) over time, whereas Ni and Zn showed a significant curved correlation with time and peaked at about the middle of the measurement period.     

Dry ashing,sources of error,and performance evaluation in AAS

Three different stages of the analytical chain will be discussed. Firstly, the merits and demerits of the traditional dry ashing, a well established technique that is sometimes viewed with unwarranted sceptisism. Secondly, control of ashing temperature, background correction systems and autosampler volumes in AAS-determinations will be discussed. Thirdly, the role of intercalibrations and the use of reference materials will be discussed in terms of their limitations in providing information on analytical performance. Two ways to obtain more complete information on a laboratory's performance will be described.     

Determination of arsenic, cadmium, mercury, and lead by inductively coupled plasma/mass spectrometry in foods after pressure digestion: NMKL interlaboratory study

Thirteen laboratories participated in an interlaboratory method performance (collaborative) study on a method for the determination of arsenic, cadmium, mercury, and lead by inductively coupled plasma/mass spectrometry (ICP/MS) after pressure digestion including the microwave heating technique. Prior to the study, the laboratories were able to practice on samples with defined element levels (pretrial test). The method was tested on a total of 7 foodstuffs: carrot puree, fish muscle, mushroom, graham flour, simulated diet, scampi, and mussel powder. The elemental concentrations in mg/kg dry matter (dm) ranged from 0.06-21.4 for As, 0.03-28.3 for Cd, 0.04-0.6 for Hg, and 0.01-2.4 for Pb. The materials used in the study were presented to the participants as blind duplicates, and the participants were asked to perform single determinations on each sample. The repeatability relative standard deviations (RSDr) for As ranged from 3.8 to 24%, for Cd from 2.6 to 6.9%, for Hg from 4.8 to 8.3%, and for Pb from 2.9 to 27%. The reproducibility relative standard deviations (RSDR) for As ranged from 9.0 to 28%, for Cd from 2.8 to 18%, for Hg from 9.9 to 24%, and for Pb from 8.0 to 50%. The HorRat values were less than 1.5 for all test samples, except for the determination of Pb in wheat flour at a level close to the limit of quantitation (0.01 mg/kg dm). The study showed that the ICP/MS method is satisfactory as a standard method for elemental determinations in foodstuffs.