Application
News
User Benefits
Easy measurement of trace heavy metals in “foods with nutrient function claims” (FNFCs) is possible by atomic absorption
spectrophotometer (AA).
Sample preparation can be conducted quickly with minimal environmental pollution by using microwave digestion.
Data can be acquired with high sensitivity at the ppb order in measurements by the furnace method.
No. A642
Measurement of Trace Heavy Metals in Food with
Nutrient Function Claims by Microwave Digestion-
Furnace Method
Y. Jiang
Atomic Absorption Spectrophotometer AA-7000F / AAC / GFA-7000A / ASC-7000
Introduction
Foods with nutrient function claims (FNFCs) are food products
which are used to supplement designated nutritional
components and display the functions of the nutritional
components concerned. Many people use FNFCs to maintain
health or to supplement nutritional components. The
components which can be shown in product labeling and their
content ranges are regulated, and include five types of minerals
(zinc, calcium, iron, copper, magnesium) and 12 types of vitamins.
In many cases, substances extracted from natural products and
food additives are used as raw materials for manufacturing FNFCs.
However, there is a possibility of contamination by toxic heavy
metals contained in the raw materials or in the manufacturing
process. Japan has set standard values for heavy metals in
unpolished rice, drinking water, and food additives, and
international organizations and various foreign countries have
also set standard values for heavy metals in many products.
This article introduces an example in which a commercially-
available vitamin product was decomposed by the microwave
digestion method, and the heavy metals lead (Pb), arsenic (As),
and cadmium (Cd) were measured by the furnace method.
The Microwave digestion method has various advantages,
including a high decomposition capacity which enables fast
decomposition treatment, small consumption of reagents, and
minimal loss of volatile metals elements.
The furnace method enables highly sensitive measurement of
numerous elements in small samples.
Instrument Configuration and Measurement
Conditions
The instrument used here was a Shimadzu AA-7000F/AAC
atomic absorption spectrophotometer. A graphite furnace
atomizer and autosampler were used in the furnace method.
The furnace method enables higher analytical sensitivity in
comparison with the flame method.
The measurement was carried out by the calibration curve
method. Table 2 shows the measurement conditions and
atomization conditions of the instrument used.
Table 2 Measurement Conditions
Sample Preparation
First, 0.5 g of the vitamin product was weighed and transferred
to a microwave digestion container which had been washed
with dilute nitric acid in advance, 6 mL of nitric acid and 2 mL of
hydrogen peroxide were added, and the container was sealed,
after which microwave digestion was carried out under the
conditions
(1)
in Table 1.
Table 1 Microwave Digestion Conditions
Step Time (min) Temperature (˚C) Output (W)
1 0 0 0
2 2 70 500
3 5 50 0
4 20 200 500
5 30 200 500
After cooling, the digestion solution was transferred to a beaker
and heated with a hot plate until immediately before
exsiccation. The dried and hardened sample was then dissolved
by adding 0.25 mL of nitric acid and a small amount of water,
and diluted in a 50 mL measuring flask by adding pure water up
to the marked line.
Element Pb As Cd
Analysis wavelength 283.3 nm 193.7 nm 228.8 nm
Slit width 0.7 nm 0.7 nm 0.7 nm
Lighting mode BGC-D2 BGC-D2 BGC-D2
Lamp current 10 mA 12 mA 8 mA
Incineration temperature 700 ˚C 900 ˚C 500 ˚C
Atomization temperature 2000 ˚C 2100 ˚C 2000 ˚C
Tube type
Pyro-coated
graphite tube
Platform tube
Pyro-coated
graphite tube
Pd 50 ppm addition None 2 μL 2 μL
Sample injection volume 10 μL
Number of repeated
measurements
2 times (maximum 3 times)
Measurement Results
Table 3 (a) to (c) show the measurement results for the standard
solutions of Pb, As, and Cd, respectively. Figs. 1 to 3 show the
corresponding calibration curves. A satisfactory correlation (r =
0.999 or higher) was obtained for all calibration curves.
Table 4 shows the measurement results. None of the target
elements Pb, As, and Cd was detected from the samples. The
limit of quantitation (LOQ) was expressed by the value of 10 σ
calculated from the standard deviation (SD) obtained by 10
repeated measurements of a blank sample. The LOQs of Pb, As,
and Cd in the solution were 0.3 ppb, 3 ppb, and 0.05 ppb,
respectively.
