The Determination of Toxic, Essential, and Nutritional

APPLICATION NOTE
ICP – Mass Spectrometry
Authors:
Cynthia Bosnak
Senior Product Specialist
Ewa Pruszkowski
Senior Product Specialist
PerkinElmer, Inc.
Shelton, CT USA
The Determination of
Toxic, Essential, and
Nutritional Elements in
Food Matrices Using the
NexION 300/350 ICP-MS
Introduction
The elemental and dynamic range of inductively
coupled plasma-mass spectrometry (ICP-MS) makes
it ideally suited for the analysis of food materials.
The ultratrace detection limits of ICP-MS permit
the determination of low-level contaminants such
as Pb, As, Se, and Hg, while the macro-level
nutritional elements such as Ca, Mg, K, and Na can
be quantified using the extended dynamic range
capability of ICP-MS which provides 9-orders of
magnitude. However, there are still a number of challenges to overcome, which
makes the routine analysis of foods difficult unless the sample dissolution procedure is well thought out and instrumental conditions are optimized for complex
sample matrices.
For example, the wide variety of edible products available
means that a highly diverse range of matrices must be
brought into solution for ICP-MS analysis. These complex
acid-digested matrices, which are a combination of dissolved
carbohydrates, fats, and proteins, can pose major problems
for any ICP-MS because of the potential for blocking of the
interface cones and/or deposition on the quadrupole ion
deflector (QID). For this reason, if instrument design does
not account for high-matrix samples, long-term stability can
be severely compromised.
In addition to signal drift, digested food matrices can also
cause major spectral complications. The sample’s organic
components, together with macro minerals, can combine
with elements present in the digestion acid and/or the plasma
argon to form polyatomic interferences. For example, chloride
ions (at mass 35) combine with the major argon isotope
(mass 40) to produce the argon chloride interference
40
Ar35Cl+, which interferes with arsenic at mass 75. Another
example is the argon dimer (ArAr+), which forms from the
plasma gas and exists at the same masses as the major
selenium isotopes. In addition, the major isotope of
chromium at mass 52 is overlapped by 40Ar12C+, 35Cl17O+,
and 35Cl16OH+ interferences generated by the sample matrix
and the plasma gas. As a result, these kinds of spectral
interferences have made the determination of both trace
and macro elements in food samples extremely challenging.
To overcome these issues, a NexION 300X ICP-MS
(PerkinElmer, Inc., Shelton, CT) was used for the analysis
of various food substances, focusing on toxic and typical
essential and macro elements in a group of NIST®
(Gaithersburg, MD) standard reference materials (SRMs).
®
Experimental
Six different NIST® SRM food samples that represent a typical
cross-section of the types of foods for human consumption
were chosen for the evaluation. The foods included spinach
leaves (leafy vegetable), corn bran (grain), wheat flour
(grain), bovine muscle (meat), mussel tissue (shellfish), and
milk powder (dairy product). The samples were brought into
solution with a Multiwave™ 3000 microwave digestion
system. Details of the sample digestion procedure are shown
in Table 1.
Sample Preparation
Approximately 0.5-0.6 g of each SRM was digested in
duplicate with 5 mL of nitric acid (Fisher Optima HNO3)
and 2 mL of hydrogen peroxide (Fisher Optima H2O2) in
precleaned PTFE HF-100 microwave sample vessels. The filled
2
vessels were placed on a 16-position rotor with an internal
p/T sensor positioned in one of the samples to monitor the
pressure and temperature inside the sample container. In
addition, an external IR sensor provided the temperatures
for each individual sample in the tray. The digestion program
consisted of 30 min of heating and 15 min of cooling, as
shown in Table 1. All the SRM samples were completely
dissolved, resulting in clear solutions that were diluted to
a final volume of 50 mL with deionized water. No further
sample dilutions were necessary. Gold was added to all
solutions at a final concentration of 200 µg/L to stabilize
mercury. Preparation blanks, consisting of the acid mixture,
were taken through the same microwave digestion program
as the samples.
Table 1. Microwave Digestion Heating Program for All Six
NIST® Food SRMs.
Step
Power (W)
Ramp (min)
Hold (min)
1500 1
4
2 10005
5
3 14005
10
4 (cooling)
15
0
—
Instrumental Conditions
All data in this study were generated under normal operating
conditions on a NexION 300X ICP-MS using an autosampler.
The instrumental operating conditions are shown in Table 2.
Table 2. ICP-MS Instrumental Operating Conditions for this
Application.
Component/ParameterType/Value/Mode
Nebulizer
Glass concentric
Spray chamber
Glass cyclonic
ConesNickel
Plasma gas flow
18.0 L/min
Auxiliary gas flow
1.2 L/min
Nebulizer gas flow
0.98 L/min
Sample uptake rate
300 µL/min
RF power
1600 W
Total integration time
0.5 (1.5 seconds for As, Se, Hg)
No. of replicates per sample
™
3
Universal Cell Technology * KED mode
*PerkinElmer, Inc.
Calibration
• Low-level essential analytes: 0-2 ppm
Multielement calibration standards, representing all the
analytes covered by the six NIST® SRMs, were made up from
PerkinElmer® Pure single and multielement standards and
diluted into 10% HNO3. Gold was added to all solutions
at a final concentration of 200 µg/L to stabilize mercury.
However, it is important to mention that each food SRM
was certified for a slightly different group of elements.
For that reason, quantitation was only carried out on the
analytes that had reference values. Calibration standard
ranges were based on whether the analyte was expected
to be a high-level, nutritional element like potassium (K)
or sodium (Na), a low/medium-level essential element like
manganese (Mn) or iron (Fe), or a trace/ultratrace contaminant
such as lead (Pb) or mercury (Hg). Depending on the certificate
value of the analytes, five different calibration ranges were
made up to cover the complete range of elements being
determined. They were:
• Trace-level contaminants: 0-200 ppb
• Ultratrace-level contaminants: 0-20 ppb
Figure 1 shows representative calibration curves for each
range.
• High-level nutritional analytes: 0-300 ppm
• Medium-level essential analytes: 0-20 ppm
54
Fe Correlation Coefficient = 0.99997.
23
Na Correlation Coefficient = 0.99996.
63
Cu Correlation Coefficient = 0.99999.
31
P Correlation Coefficient = 0.99999.
78
Se Correlation Coefficient = 0.99995.
Figure 1. Calibration curves for 23Na (0-300 ppm), 31P (0-20 ppm), 54Fe (0-2 ppm), 63Cu (0-200 ppb) and 78Se (0-20 ppb).
3
In addition to the analyte elements used for the multielement
calibration, the standards, blanks, and samples were also
spiked on-line using a mixing tee with a solution of 6Li, Sc,
Ge, In, and Tb for internal standardization across the full
mass range. Acetic acid was added to the internal standard
solution to compensate for residual carbon leftover from the
sample digestion.
Results
Quantitative results for two sample preparations of six
NIST® SRMs (Corn Bran, Bovine Muscle, Mussel Tissue, Milk
Powder, Wheat Flour, and Spinach Leaves) are shown in
Tables 3-8, respectively. All elements in every sample were
determined with kinetic energy discrimination (KED) mode
using helium as the collision gas. Figures in parentheses ( )
in the reference value column are not certified values, but
are included for information purposes only. The data show
very good agreement with the certified values, especially for
the elements that suffer from known spectral interferences.
The elements that are outside the specified limits are mostly
the ones that are well recognized as environmental contaminants, which have probably been impacted by the sample
preparation procedure.
Food samples are complex acid-digested matrices and can
create major problems for some ICP-MS systems because
of deposits on the interface cones and on the ion optics
caused from high concentrations of dissolved solids. For this
reason, long-term stability can be poor. However, the triple
cone interface and the quadrupole ion deflector design of
the NexION guarantee exceptional long-term stability. For six
hours, food samples with high concentrations of dissolved
solids were analyzed and a quality control (QC) sample was
read every 5 samples. Figure 2 shows the long-term stability
over 6 hours.
Conclusion
The ICP-MS system used in this study is well suited for the
analysis of complex digested food materials. The agreement
between experimental and certified results for six NIST® food
SRMs demonstrates that the NexION 300X ICP-MS can effectively measure various food samples. In addition to removing
interferences, the NexION 300X allows the determination of
macro-level nutritional elements in the same analysis run as
lower-level elements, without having to dilute the samples.
Instrument design characteristics eliminate deposition on the
ion optics, leading to long-term stability in high-matrix samples
while permitting trace levels to be accurately measured.
Table 3. Analysis of NIST® 8433 Corn Bran using the
NexION 300 ICP-MS.
ElementMass Reference
(amu)
Value (mg/kg)
Experimental
Value (mg/kg)
B
11
2.8 ±1.2
3.2
Na
23
430 ±31
399
Mg
26
818 ±59
787
Al
27
1.01 ±0.55
1.15
P
31
171 ±11
158
S
34
860 ±150
738
K
39
566 ±75
548
Ca
44
420 ±38
434
V
51
0.005 ±0.002
0.005
Cr 52(0.11)
0.08
Fe
54
14.8 ±1.8
13.7
Mn
55
2.55 ±0.29
2.53
Co 59(0.006) 0.005
Ni
60
0.158 ±0.054
0.143
Cu
63
2.47 ±0.40
2.54
Zn
66
18.6 ±2.2
17.0
As 75(0.002) <0.006
Figure 2. % Recovery of several analytes in the QC standard during 6-hour
analysis.
4
Se
78
0.045 ±0.008
0.056
Sr
88
4.62 ±0.56
4.56
Mo
98
0.252 ±0.039
0.255
Cd
111
0.012 ±0.005
0.013
Sn
118–
0.015
Sb
121(0.004)
0.003
Ba
137
2.40 ±0.52
2.26
Hg
202
0.003 ±0.001
0.005
Pb
208
0.140 ±0.034
0.122
Tl
205–
<0.0001
Th 232–
<0.00008
U
<0.00002
238–
Table 4. Analysis of NIST® 8414 Bovine Muscle using the
NexION 300 ICP-MS.
Table 5. Analysis of NIST® 2976 Mussel Tissue using the
NexION 300 ICP-MS.
ElementMass Reference
(amu)
Value (mg/kg)
Experimental
Value (mg/kg)
ElementMass Reference
(amu)
Value (mg/kg)
Experimental
Value (mg/kg)
B
11
0.6 ±0.4
0.4
B
11–
27.5
Na
23
2100 ±80
2000
Na
23
(35000 ±1000)
35000
Mg
26
960 ±95
960
Mg
26
(5300 ±500)
4800
Al
27
1.7 ±1.4
1.6
Al
27
(134 ±34)
149
P
31
8360 ±450
7250
P
31(8300)
S
34
7950 ±410
6820
S
34(19000) 16000
K
39
15170 ±370
14180
K
39
(9700 ±500)
9700
Ca
44
145 ±20
143
Ca
44
(7600 ±300)
7400
V
51(0.005) 0.006
V
51–
0.87
Cr
52
0.071 ±0.038
0.092
Cr
52
(0.50 ±0.16)
0.50
Fe
54
71.2 ±9.2
71.2
Fe
54
171.0 ±4.9
190
Mn
55
0.37 ±0.09
0.44
Mn
55
(33 ±2)
40
Co
59
0.007 ±0.003
0.014
Co
59
(0.61 ±0.02)
0.67
Ni
60
0.05 ±0.04
0.05
Ni
60
(0.93 ±0.12)
0.87
Cu
63
2.84 ±0.45
2.81
Cu
63
4.02 ±0.33
3.91
Zn
66
142 ±14
140
Zn
66
137 ±13
145
As
75
0.009 ±0.003
0.011
As
75
13.3 ±1.8
16.4
Se
78
0.076 ±0.010
0.11
Se
78
1.80 ±0.15
2.52
Sr
88
0.052 ±0.015
0.081
Sr
88
(93 ±2)
79
Mo
98
0.08 ±0.06
0.08
Mo 98–
0.56
Cd
111
0.013 ±0.011
0.013
Cd
111
0.82 ±0.16
0.88
Sn
118–
0.14
Sn
118
(0.096 ±0.039)
0.103
Sb
121(0.01)
0.01
Sb
121–
0.011
Ba
137(0.05)
0.04
Ba
137–
0.61
Hg
202
0.005 ±0.003
0.003
Hg
202
0.061 ±0.0036
0.058
Pb
208
0.38 ±0.24
0.34
Pb
208
1.19 ±0.18
1.06
Tl
205–
0.002
Tl
205(0.0013) 0.003
Th 232–
<0.00008
Th
232
U
<0.00002
U
238–
238–
(0.011 ±0.002)
6900
0.012
0.22
5
6
Table 6. Analysis of NIST® 1549 Milk Powder using the
NexION 300 ICP-MS.
Table 7. Analysis of NIST® 8436 Wheat Flour using the
NexION 300 ICP-MS.
ElementMass Reference
(amu)
Value (mg/kg)
Experimental
Value (mg/kg)
ElementMass Reference
(amu)
Value (mg/kg)
Experimental
Value (mg/kg)
B
11–
2.1
B
11–
0.62
Na
23
4970 ±100
4700
Na
23
16.0 ±6.1
17.0
Mg
26
1200 ±30
1170
Mg
26
1070 ±80
1030
Al 27(2)
0.7
Al
27
11.7 ±4.7
11.8
P
31
10600 ±200
10500
P
31
2900 ±220
2330
S
34
3510 ±50
3290
S
34
1930 ±280
1460
K
39
16900 ±300
16500
K
39
3180 ±140
2950
Ca
44
13000 ±500
12800
Ca
44
278 ±26
262
V
51–
0.003
V
51
0.021 ±0.006
0.026
Cr
52
0.0026 ±0.0007
<0.003
Cr
52
0.023 ±0.009
0.053
Fe
54
1.78 ±0.10
1.98
Fe
54
41.5 ±4.0
41.4
Mn
55
0.26 ±0.06
0.26
Mn
55
16.0 ±1.0
15.1
Co 59(0.0041) 0.005
Co
59
0.008 ±0.004
0.007
Ni 60–
0.013
Ni
60
0.17 ±0.08
0.17
Cu
63
0.7 ±0.1
0.6
Cu
63
4.30 ±0.69
4.18
Zn
66
46.1 ±2.2
46.7
Zn
66
22.2 ±1.7
20.6
As 75(0.0019) <0.006
As 75(0.03)
0.01
Se
0.17
Se
78
1.23 ±0.09
1.22
Sr 88–
3.7
Sr
88
1.19 ±0.09
1.19
Mo 98(0.34)
0.37
Mo
98
0.70 ±0.12
0.72
Cd
111
<0.002
Cd
111
0.11 ±0.05
0.11
Sn
118–
<0.002
Sn
118–
0.032
Sb
121(0.00027) <0.001
Sb
121–
0.002
Ba
137–
0.83
Ba
137
2.11 ±0.47
2.04
Hg
202
0.0003 ±0.0002
<0.0007
Hg
202
0.0004 ±0.0002
<0.0007
Pb
208
0.019 ±0.003
0.019
Pb
208
0.023 ±0.006
0.35
Tl
205–
<0.0001
Tl
205–
Th 232–
<0.00008
Th 232–
0.001
U
<0.00002
U
0.001
78
0.11 ±0.01
0.0005 ±0.0002
238–
238–
<0.0001
Table 8. Analysis of NIST® 1570a Spinach using the NexION
300 ICP-MS.
ElementMass Reference
(amu)
Value (mg/kg)
Experimental
Value (mg/kg)
B
11
37.6 ±1.0
37.3
Na
23
18180 ±430
17350
Mg 26(8900)
8600
Al
27
310 ±11
200
P
31
5180 ±110
4810
S
34(4600)
4400
K
39
29030 ±520
26600
Ca
44
15270 ±410
15040
V
51
0.57 ±0.03
0.58
Cr 52–
1.63
Fe 54–
265
Mn
55
75.9 ±1.9
77.9
Co
59
0.39 ±0.05
0.37
Ni
60
2.14 ±0.10
1.97
Cu
63
12.2 ±0.6
11.6
Zn
66
82 ±3
80
As
75
0.068 ±0.012
0.081
Se
78
0.117 ±0.009
0.21
Sr
88
55.6 ±0.8
58.1
Mo 98–
0.39
Cd
111
2.83
Sn
118–
0.027
Sb
121–
0.007
Ba
137–
5.8
Hg
202
0.028
Pb
208(0.20)
0.16
Tl
205–
0.018
Th
232
0.048 ±0.003
0.045
U
238
(0.155 ±0.023)
0.154
2.89 ±0.07
0.030 ±0.003
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