Husantha Jayaratna, M.B. Voelz* and C.S. Bruntlett
2701 Kent Avenue,
West Lafayette, IN 47906
Recently calcium supplements have drawn attention for their lead content. The regulatory agencies have lowered the acceptable exposure limits to 0.5µg/day (1). However, the calcium sources available on the market generally contain higher levels of lead. In conjunction with the lowering of acceptable levels, sensitive techniques for quantitating lead in nutritional supplements are needed. The USP 23 determines lead by wet chemical methods. Graphite furnace atomic absorption (GFAA) and inductively coupled plasma mass spectroscopy (IPC-MS) have also been accepted techniques for determining lead levels. However the accuracy of the results is sometimes questionable due to the interference from the ingredients such as certain fillers, coatings, and additives found in some supplements (1). In this work, we have performed preliminary work to compare the GFAA technique to the electrochemical technique of anodic stripping voltammetry (ASV) for the determination of lead(II) in calcium supplements.
The results indicate that ASV produces comparable results to GFAA. In addition, the advantages of using ASV instead of GFAA should be noted. The technique is simple, does not require an acid digestion or extraction, costs less, is more sensitive, and can quantitate other elements simultaneously.
Anodic Stripping Voltammetry (ASV)
Anodic stripping voltammetry is an electrochemical technique widely used for trace metal determination. The interest in ASV has been due to its sensitivity and the ability to simultaneously provide quantitative and qualitative information on several metals at very low concentrations (in ppb to sub ppb) often without complex sample preparation procedures such as acid digestion. Basic instrumentation required is as follows:
|Temperature||115 °C (temperature reached 115 °C within 4 min.)|
Anodic stripping voltammograms for the sample Ca3 and two standard additions of 50 ppb Pb(II). Technique = OSWSV; deposition potential = -600 mV; deposition time = 1 min.; quiet time = 10 sec. S.W. frequency = 15 Hz; step potential = 4 mV; S.W. amplitude = 25 mV. These data exhibit good linearity with a correlation coefficient of 0.999.
Comparison of Results for Lead(II) in Ca Supplements From GFAA and ASV
|.||Lead (II)µg/1000mg of Suppliment|
|Sample||GFAA||ASV (RSD %), n=3|
|Ca2||0.8 (13,4)||0.69 (5)|
|Ca3||1.6 (7,3)||1.5 (5)|
|Ca5||0.8 (13,3)||0.74 (11)|
* For these GFAA results, the precision cannot be given because the samples had the final concentration at the detection limit where some were undetectable.
Comparative data on the determination of lead(II) in Ca supplements are presented here. As seen from the electrochemical response, no interfering signals found in the potential range utilized for the procedure, whereas GFAA and ICP-MS would find interference from the additives in the supplement according to the published data (1-3). Overall, the electrochemical data are in close agreement with GFAA data.
The electrochemical method does not require sample digestion using concentrated acids and heat as GFAA and ICP-MS usually do, and hence reduces the time of analysis and the cost. The analysis can be completed in less than 10 minutes/sample including the sample preparation time. The required instrumentation can be purchased for less than $20,000. As a bench top apparatus, this does not carry high installation and maintenance cost. In addition, it can be used to determine other metals such as Cu(II), Cd(II) and Zn(II) simultaneously with Pb(II) without additional expenditure. ASV is quite suitable for clinical and manufacturing environments.
1. Paul Holder, Southern Analyst, vol. 6 (Winter 97) 3
2. Paul Siitonen and Harold Thompson, Jr.; J. AOAC International, vol. 77 (1994) 1299.
3. Stephen Capar and John Gould; J. AOAC., vol. 62 (1979) 1054.
Authors thank Mr. Christopher Gonzales of Xavier University of Louisiana, New Orleans, Louisiana, for performing the GFAA analysis on Ca supplements.