178
P
oitevin
: J
ournalof
AOAC I
nternational
V
ol
. 95, N
o
. 1, 2012
Robustness study
.—Analysis of spiking elements in eight
food-grade salts with and without ion buffer, and through
repeatability and reproducibility tests on six referencematerials
with different ICP-OES equipments (axial and radial grating
configurations), and with open- and closed-vessel microwave-
based digestions.
Ring trial
.—With nine independent and experienced food
industry laboratories on five in-house and certified reference
materials using thismethod.
Statistic results of performedSLV tests show that themethod
developed for the determination of nine nutritional minerals by
ICP-OES is fit-for-purpose in terms of specificity, sensitivity,
linearity, trueness,andprecisionaccording toISO17025.AOAC
acceptability criteria (recovery, precision, and HorRat values)
are also fulfilled for all foodmatrixes covering the nine sectors
of the AOAC food triangle. Improvements to AOACMethod
984.27
(e.g., microwave digestion, internal standardization,
ion buffering, and extension to all food matrixes) provide
global satisfactory ruggedness results for all matrixes tested.
Robustness and efficiency of themethodwas proved through a
successful ring trial.
This full validation, using well-characterized reference
materials, demonstrated that the method is a cost-efficient
(multielemental), time-saving (high-throughput sample
digestion), accurate, and fit-for-purpose analytical method for
all laboratories in the food industry, and iswell established and
harmonizedwithin25Nestlé laboratories.
AOAC
Official Method
2011.14
Calcium, Copper, Iron, Magnesium, Manganese,
Potassium, Phosphorus, Sodium, andZinc in
FortifiedFoodProducts
MicrowaveDigestion and InductivelyCoupled
Plasma-Optical EmissionSpectrometry
FirstAction2011
(Applicable to analysis of calcium, copper, iron, potassium,
magnesium, manganese, phosphorus, sodium, and zinc in
fortified food products.)
Upper limits (mg/kg): Ca (24000); Cu (210); Fe (850);
K (32000); Mg (7500); Mn (20); Na (16000); P (16000); and
Zn (320). LOQ (mg/kg):Ca (150);Cu (2); Fe (10);K (200);Mg
(50);Mn (0.05);Na (100); P (100); andZn (5).
See
Tables
2011.14A
—
I
for results of the interlaboratory
study supporting acceptance of the method. These results are
slightly different from those shown previously in
J. AOAC Int.
(1)due to theapplicationofclassical rather than robust statistics.
Digestion
A. Principle
Test portion is heated at 200°C either with nitric acid in a
closed-vessel microwave digestion system (MDC) or with a
combinationofhydrogenperoxide, nitricacid, andhydrochloric
acid in an open-vesselmicrowave digestion system (MDO).
B. Apparatus
Microwave
.—Commercial MDC or MDO designed for
laboratory use at 200 ± 20°C, up to 600 psi and controlled
temperature or pressure ramping capability. It is recommended
that vessel design be selected
that will withstand themaximum
possible pressure (600 psi), since organic residues of rich-fat
or rich-carbohydrate samples, if not given sufficient time to
predigest, will generate significant pressure during digestion.
C. Reagents
(
a
)
High-gradewater
,
H
2
O(18MΩ)
.—Forslurrypreparation
and/or dilution.
(
b
)
Nitric acid (HNO
3
), 65% (w/v)
.—Trace metal grade
throughout.
(
c
)
Hydrochloricacid (HCl), 37%
(
w
/
v)
.—Tracemetal grade
throughout.
(
d
)
Hydrogen peroxide (H
2
O
2
), 97% (w/v)
.—Trace metal
grade throughout.
D. Determination
Caution
: Before using chemicals, refer to the supplier guide
for chemical safety and/or other adequate manuals or safety
data sheets approved by local authorities. Use fume hood, and
wear full personal laboratory protective clothing, gloves, and
appropriate eye protection (safetyglasses)whenusingglassware
andpreparing standards or test portionswith acid solutions.
(
a
)
Sample preparation
.—(
1
)
Test sample preparation
.—
Homogenize a representative sample by grinding as finely
as possible and/or by preparing a slurry with H
2
O. For infant
cereals and fortified milk powders,
preheat water at 50°C.
Prepare slurry byweighing 10.0 ± 0.1 g test sample, and place
intoa100mLErlenmeyerflask; add90.0±0.1gH
2
O.Mixwell
with stopper.
(
2
)
Test
portion
preparation
.—Accurately
weigh
0.50 ± 0.01 g test portion or sample mass on a dry weight
basis in the prepared slurry to MDC vessel (1.00 ± 0.01 g
into a 100 mL volumetric flask for MDO).
Note
: An optimal
analytical test portionmass of 0.5 g (1.0 g for MDO) is based
on an empirical maximum energy release by the food of 3 kcal
and 90–110% recovery.
Line the MDC vessel walls or Pasteur pipet with weighing
paper during sample transfer to keep sample from adhering to
sides of vessel, or use aPasteur pipet to transfer liquid samples.
Weigh fluid samples or test portion from slurry test sample
directly after mixing.
Note:
Remove weighing paper from
sample prior to placingMDC vessels in oven.
Carefully add 5.0 ± 0.1 mLHNO
3
intoMDC/MDO vessel,
then 5 mL H
2
O
2
only into MDO vessel. Loosely cap MDC
vessel without sealing.
Predigest for at least 10 min at room
temperature or until vigorous foaming subsides.
Close MDC
vessels and distribute onto microwave carousel to ensure
uniformmicrowave power application on all samples.
(
3
)
Food-grade
salt
sample
preparation
.—Weigh
0.20±0.01g food-grade salt (aminimumdilution factor of 500
is recommended) intoa100mLvolumetricflask.Adddeionized
water and10mLHNO
3
.Dissolve salt anddilute tovolumewith
deionizedwater.
(
b
)
Test portion digestion
.—
Caution
: Application of MDS
involves hot pressurized acid solutions and concentrated acids.
Use fume hood and wear full personal laboratory protective
clothing, gloves, and appropriate eyeprotection (safetyglasses)
MTE-01
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