Agilent Determination of Critical Elements Application Note

Application Note

Foods

Determination of Critical Elements
in Foods in Accordance with US FDA
EAM 4.7 ICP-MS Method

Extending the scope of routine food analysis using
IntelliQuant data analysis

Authors

Jenny Nelson
Elaine Hasty2
Leanne Anderson
Macy Harris

1

Agilent Technologies, Inc.

2

CEM Corporation, USA

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2

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Introduction

Consumers expect that the food they buy will be safe to eat, so manufacturers take
steps to ensure the levels of harmful chemicals and pathogens are strictly controlled.
In addition, governments and regulatory bodies in many countries have a legal
obligation to protect public health in relation to food. The chemicals that are controlled
in foodstuffs include organic contaminants such as pesticide residues, and inorganic
contaminants such as heavy metals. In the United States (US), the Food and Drug
Administration (FDA) regulates a wide range of foods. The US FDA also publishes
details of the analytical methods that laboratories should use to help ensure food
safety. For example, FDA Elemental Analysis Manual (EAM) 4.7 is a comprehensive
method that describes how to determine 12 elements in food by ICP-MS following
microwave assisted acid decomposition. EAM 4.7 also outlines a series of quality
control (QC) tests to ensure instrument performance and data accuracy (1).

It is now easier than ever for food-testing laboratories using
the EAM 4.7 method to carry out the analysis with Agilent
ICP-MS instruments. Agilent single quadrupole ICP-MS use
an Octopole Reaction System (ORS4) cell operating in helium
(He) collision cell mode with Kinetic Energy Discrimination
(KED). This combination provides the optimum configuration
to control common polyatomic interferences, leading to
more accurate results. To improve detection of analytes
with intense background overlaps, such as Se-78, P-31, and
Si-28, the ORS4 cell can use an enhanced, high energy He
mode. Enhanced He mode provides low detection limits for
these interfered analytes, avoiding the need for a reactive cell
gas such as H2, O2, or NH3. Avoiding reactive gases ensures
that no new molecular interferences are formed in the cell,
improving the quality of the data and streamlining the
method (2). Agilent ICP-MS instruments have a wide (10 or
11 orders) linear dynamic range, so major and trace analytes
in food samples can be measured in a single run. The wide
dynamic range simplifies method setup by removing the need
for custom tuning conditions for major elements, while also
ensuring that fewer reruns occur due to over-range results.

Agilent High Matrix Introduction (HMI) technology further
improves the already exceptional plasma robustness,
enabling the ICP-MS to handle samples with total dissolved
solids (TDS) levels up to 3% (and up to 25% with Ultra (U)
HMI). HMI (and UHMI) uses aerosol dilution to handle
high-matrix samples, reducing sample preparation time
and minimizing the risk of introducing sampling errors or
contamination from conventional liquid dilution (3, 4). A
further benefit of HMI/UHMI is that it practically eliminates
matrix suppression, so varied, high matrix sample digests
can be run against a simple, synthetic calibration, without
requiring matrix-matching.

Agilent ICP-MS MassHunter IntelliQuant Quick Scan function
simplifies data review by collecting full mass spectrum data
for every sample in only a few seconds. The IntelliQuant
results provide semiquantitative concentrations for up to
78 elements, together with identification and confirmation
of unexpected elements by comparison with isotopic
abundance templates. The periodic table “heat map”
view of the results provides a quick and simple overview
of the concentration of all elements within the sample.
IntelliQuant can also calculate and display the total matrix
solids (TMS) content of each sample. TMS data provides a
useful indication of the total matrix load and any variation in
dissolved solids level through the batch (5).

This study describes the use of the Agilent 7800 ICP-MS and
Agilent SPS 4 autosampler for the analysis of 20 elements
in different food samples using a single He cell gas method.

The list of elements included the 12 elements that are
specified in EAM 4.7: arsenic, cadmium, chromium, copper,
lead, manganese, mercury, molybdenum, nickel, selenium,
thallium, and zinc. The data quality obtained for these
elements was assessed through the measurement of three
food standard reference materials (SRMs), a fortified method
blank (FMB), and two fortified analytical portions (FAPs). FAP
refers to samples that are spiked before sample preparation.

Experimental

Calibration standards

The calibration standards were prepared in 3% nitric acid
(HNO3) and 0.5% hydrochloric acid (HCl). HCl is routinely
added to samples for analysis using Agilent ICP-MS systems,
as it ensures that chemically unstable elements such as Hg
are retained in solution. Any Cl-based polyatomic overlaps
formed are easily controlled using the standard He cell mode.
Calibration standards were prepared from Agilent standard
solutions including environmental calibration standard, p/n
5183-4688, multi-element calibration standard-1, p/n
8500-6944, and 1000 µg/mL single calibration standard for
Hg, p/n 5190-8485. Most elements were calibrated from

0.1 to 25 ppb. Cu, Zn, and Mn were calibrated up to 250 ppb.
Hg was calibrated from 0.01 to 2.5 ppb. Continuing calibration
verification (CCV) standards were prepared at 1 ppb (2 ppb
for Hg), and/or 10 ppb.

An Agilent internal standard (ISTD) solution (part number
5188-6525), containing 2 ppm Sc, Ge, Rh, In, Tb, Lu, and Bi,
was prepared in 1% HNO3, 0.5% HCl, and 10% isopropanol
(IPA). Per the 4.7 method; IPA was added to the ISTD to
help equalize As and Se sensitivities due to residual carbon
post microwave digestion. The ISTD solution was added
automatically online at a flow rate approximately 16 times
lower than the sample flow.

Reference materials and samples

Three varied food matrix SRMs from National Institute of
Standards and Technology (NIST, Gaithersburg, US) were
used to validate the method. The SRMs used were NIST
1546a Meat Homogenate, NIST 1549 Non-Fat Milk Powder,
and NIST 2385 Slurried Spinach.

In addition to the SRMs, a diverse set of food samples with
different % composition of fats, proteins, and carbohydrates
were analyzed in this study. The following products were
bought in a supermarket in North Carolina, USA: beef jerky,
fortified nutritional shake, gouda cheese, gummy bears,
powdered donuts, and dark chocolate. Other samples
consisting of pepperoni, rice noodles, frozen dinner, and
frozen pizza were also digested and analyzed in the same
batch and the results are reported elsewhere (6).

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Standard and sample preparation

All the food samples included in this study were digested
“as received”, except for the donut, which was crushed in
a bag before sampling. The samples were then prepared
for analysis according to the digestion procedure outlined
in the EAM 4.7 method using a MARS 6 closed-vessel
microwave digestion system located at CEM Corporation,
USA. Approximately 0.5 g of each food sample or SRM was
accurately weighed into a 75 mL PFA-lined MARS Xpress
vessel. 8 mL of HNO3 and 1 mL of H2O2 was added to each
vessel. Duplicates of the samples, SRMs, and spiked samples
(FAPs) were prepared and digested in a single batch, using
the heating program shown in Table 1. Each digestion batch
can accommodate up to 40 vessels containing a variety of
food sample matrices, with a single program being used for
all sample types. Finally, 0.5 mL concentrated HCl was added
to the digests, followed by de-ionized water to a final weight
of 100 g.

Table 1. Microwave digestion parameters.

Parameter Setting

Power (W) 1800

Ramp Time (min) 25

Hold Time (min) 15

Temperature (°C) 200

The analytical sequence of calibration standards, samples,
and QC solutions is shown in Figure 1. The sample block was
analyzed repeatedly with automatic insertion of the periodic
QC block after every 10 samples.

Instrumentation

An Agilent 7800 ICP-MS, which includes the ORS4 collision cell
and HMI aerosol dilution system, was used for the analysis.
The 7800 has been superseded by the Agilent 7850 ICP-MS,
but the configuration and analytical settings reported here
apply to both models. Sampling was performed using an
Agilent SPS 4 autosampler. The ICP-MS was configured with
the standard sample introduction system, consisting of a
MicroMist glass concentric nebulizer, temperature-controlled
quartz spray chamber, and quartz torch with 2.5 mm id
injector. The interface consisted of a nickel-plated copper
sampling cone and a nickel skimmer cone.

Figure 1. Analytical sequence.
Key: Instrument detection limit (IDL), initial calibration verification (ICV),
method blank (MBK), reference material (RM), fortified method blanks (FMB),
fortified analytical portion (FAP), continuing calibration verification (CCV),
continuing calibration blank (CCB).

The Rare Earth Elements (REEs) have relatively low second
ionization potentials, so readily form doubly charged ions
(M2+) in the plasma. If REEs such as Nd, Sm, Gd, and Dy are
present in a sample at a high enough concentration, M2+
interferences can affect the accuracy of the measurement of
As and Se. Therefore, the EAM 4.7 method recommends that

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analysts monitor the following isotopes:

163

Dy to assess the potential for M2+ overlaps. The Agilent

Nd,

Sm,

155

Gd,

ICP-MS MassHunter software includes an easy-to-use,
automated function to correct for the contribution that REE

2+

ions make to the signals measured for As and Se.
Unknown samples can also be easily screened for REEs (and

the entire periodic table) using the IntelliQuant function in the
ICP-MS MassHunter software (version 4.6 and later). IntelliQuant
works by performing a full mass-spectrum scan with only two
seconds measurement time. The IntelliQuant results provide
valuable information about the food samples, including:

• The elemental composition of each sample, including
REEs. The results can be displayed in a table or as a
periodic table heat map.

• Confirmation of an unexpected element using isotope
templates.

• An estimation of the Total Matrix Solids (TMS) level for
each sample or based on the analysis of a typical sample.

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