ICP-MS for the Determination of Heavy Metals in Dietary Supplements, Part 2

See Introduction here.

Results

Analytical evaluations usually consist of 4 areas: detection limits, spike recoveries, long term stability, and actual analyte concentrations in mg/kg. Instrument detection limits are determined using a calibration blank, which is compared with a low-level standard and a regular sample. Spike recoveries were measured by adding a known concentration of an element, such as Pb, and comparing the spiked and non-spiked samples. To determine long-term stability, a signal is monitored in a sample over a period of 4-8 hours without re-calibrating the instrumentation.

The following tables show measured heavy metal concentrations (mg/kg) in several commercially available dietary supplements, as well as instrument detection limits (5).

Comparison to other techniques
ICP-MS offers much lower detection limits than other common metal detection methods, including atomic absorption spectroscopy and inductively coupled plasma optical emission spectrometry (iCPOES) (2). Most elements can be detected at part per trillion levels, while some can be measured down to the part per quadrillion range. The standard working range is nine orders of magnitude, from nanograms per liter to hundreds of milligrams per liter, and this can be further finessed. The method also offers unsurpassed efficiency, as all detectable elements are analyzed at a single time, unlike atomic absorption spectroscopy. In addition, isotopic analysis is difficult or prohibitive with these other methods.

One major limit to the method is that only the elements from lithium to uranium can be detected, excluding elements of masses outside the range 7 to 250 amu. In addition certain masses are prohibited due to the use of argon gas; for example, iron analysis is hindered because the mass of 56 amu is share by argon-oxygen species produced by the plasma. Still, reaction chambers can be used to eliminate this problem.