Electrochemical Determination Of Lead(ii) In Calcium Tablets

Husantha Jayaratna, M.B. Voelz* and C.S. Bruntlett
Bioanalytical Systems,
2701 Kent Avenue,
West Lafayette, IN 47906



Introduction
Many pharmaceutical and nutritional products contain low levels of lead. A prime concern is to minimize the exposure of consumers to lead. The effects of long time exposure to lead are neurological damage and behavioral problems, which are often times permanent. Young children are more susceptible to lead poisoning than adults.

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:

  • potentiostat/computer
  • working electrodes (mercury drop (CGME), mercury films on solid electrodes, solid electrodes (e.g., Au)
  • reference and auxiliary electrodes
  • purified inert gas such as N2 or Ar for de-oxygenating the sample
Different potential wave forms can be used in this technique. They are:
  • Linear Scan Stripping Voltammetry (LSSV)
  • Differential Pulse Stripping Voltammetry (DPSV)
  • Osteryoung Square Wave Stripping Voltammetry (OSWSV)
Basic steps involved in this procedure are following:
  • Sample preparation
  • Add the prepared sample to an electrochemical cell vial with appropriate electrodes and deaerate with purified N2 or Ar
  • Setup instrumental parameters for the chosen technique based on the analyte's redox potentials and the concentration range
  • Run the experiment
  • Unknown concentrations are determined by standard addition, calibration curve or internal standard methods.


Figure 1: Electrochemical Workstation BASi


Figure 2: Mercury Drop Electrode (BAS)

Sample Preparation
ASV:

  • grind Ca tablets using mortar and pestle
  • weigh ~0.5 g of the powder in a Teflon beaker
  • carefully add 1 mL of de-ionized water followed by 1 mL of conc. HNO3.
  • swirl and allow to react well for few minutes
  • carefully add 8 mL of de-ionized water and mix well for few minutes
  • transfer into a plastic tube and centrifuge
  • measure 8 mL of clear solution into the cell vial
GFAA:
Dry pulverized calcium supplement samples were prepared in triplicate using a microwave digester, DV-50 / MDS-2000 with temperature control by CEM Corp. (Mathews, NC).
  • Weigh 0.25 g of ground calcium supplement into DV-50 tube.
  • Add 5 mL of Trace metal grade 16 M HNO3 (Fisher Scientific).
  • Allow 45 min. for reaction to subside.
  • Place tubes into DV-50 turntable.
  • Place turntable with samples (n=36) into MDS-200 unit.
  • Program instrument as below and run program:
    Power 100%
    Pressure 20 PSI
    Time 60:00 min.
    TAP 30:00 min.
    Temperature 115 °C (temperature reached 115 °C within 4 min.)
  • After program is completed, add deionized water to the 50 mL mark on vessel.
  • Centrifuge samples at 1000 G's for 10 min and analyze.

Anodic Stripping Voltammograms for HNO3 Treated Ca Supplements


Figure 3

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
Ca1 <0.2* 0.0058 (7)
Ca2 0.8 (13,4) 0.69 (5)
Ca3 1.6 (7,3) 1.5 (5)
Ca4 <0.2* 0.0048 (n=1)
Ca5 0.8 (13,3) 0.74 (11)
Ca6 0.2* 0.14 (12)

* For these GFAA results, the precision cannot be given because the samples had the final concentration at the detection limit where some were undetectable.

Conclusion
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.

References
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.

Acknowledgment
Authors thank Mr. Christopher Gonzales of Xavier University of Louisiana, New Orleans, Louisiana, for performing the GFAA analysis on Ca supplements.