The elemental content of food products is extremely important in relation to both nutritional and toxic elements.
Nutritional elements can be either native to the food substance or can be added to boost the health benefits. Toxic elements can penetrate food either through processing during production or via environmental factors.
It is hoped that the presence of toxic elements will be at extremely low levels, while nutritional elements will be present at abundant levels. However, if too high, they may also be toxic; if too low, the food will not provide adequate nutrition.
Therefore, the elemental analysis of food necessitates the capacity to both measure and trace high levels.
The elemental capabilities and dynamic range of inductively coupled plasma mass spectrometry (ICP-MS) make it well-suited for evaluating food materials.
The ultra-trace detection limits of ICP-MS facilitate the identification of low-level contaminants, such as Pb, As, Se and Hg. In contrast, the macro-level nutritional elements, such as Ca, Mg, K and Na, can be determined using the extended dynamic range capacity of ICP-MS, which offers the capacity to measure concentrations across nine orders of magnitude.
However, there are still various challenges to overcome, such as interferences, complex sample matrices and high levels of dissolved solids.
With the appropriate ICP-MS instrumental conditions and design, each challenge can be tackled, enabling the successful analysis of food samples, as described elsewhere.1 This article will focus specifically on the analysis of meat and seafood; foods such as high protein content are crucial for body growth and repair.
Experimental
Sample Preparation
NIST® 8414 Bovine Muscle and NIST® 2976 Mussel Tissue were utilized for this study. Around 0.5-0.6 g were digested in duplicate with 5 mL of nitric acid (Fisher Scientific™, Optima grade) and 2 mL of hydrogen peroxide (Fisher Scientific™, Optima grade) in PTFE microwave sample vessels that were pre-cleaned.
The structure of the digestion program included 30 minutes of heating and 15 minutes of cooling, as exhibited in Table 1. All samples were dissolved entirely, resulting in clear solutions diluted to a final volume of 50 mL with deionized water: no additional sample dilutions were needed.
Table 1. Microwave Digestion Program. Source: PerkinElmer Food Safety and Quality
| Step |
Power
(W) |
Ramp
(min) |
Hold
(min) |
| 1 |
500 |
1 |
4 |
| 2 |
1000 |
5 |
5 |
| 3 |
1400 |
5 |
10 |
| 4 (cooling) |
0 |
— |
15 |
To stabilize mercury, gold was introduced to all solutions at a final concentration of 200 µg/L. Preparation blanks containing the acid mixture were passed through the same microwave digestion program as the samples.
Instrumental Conditions
A PerkinElmer NexION® 300/350X ICP-MS using an autosampler was employed to generate all data in this study under normal operating conditions. The instrumental operating conditions are displayed in Table 2.
Table 2. ICP-MS Instrumental Operating Conditions for this Application. Source: PerkinElmer Food Safety and Quality
| Parameter |
Value |
| Nebulizer |
Glass concentric |
| Spray chamber |
Glass cyclonic |
| Cones |
Nickel |
| 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) |
| Replicates per sample |
3 |
| Universal Cell Technology™* |
Collision mode |
*PerkinElmer, Inc.
Calibration
Multielement calibration standards, representative of all the analytes in the SRMs, were fabricated by PerkinElmer Pure single and multielement standards and diluted into 10% HNO3.
To stabilize mercury, gold was introduced to all solutions at a final concentration of 200 µg/L.
Calibration standard ranges were predicated based on whether the analyte would contain trace/ ultra-trace contaminant such as lead (Pb) or mercury (Hg), low/medium-level essential element like manganese (Mn) or iron (Fe), or a high-level nutritional element like potassium (K) or sodium (Na).
In line with the certified value of the analytes, five different calibration ranges were composed to cover the entire range of elements being determined:
- High-level nutritional analytes: 0-300 ppm
- Medium-level essential analytes: 0-20 ppm
- Low-level essential analytes: 0-2 ppm
- Trace-level contaminants: 0-200 ppb
- Ultratrace-level contaminants: 0-20 ppb
Figures 1 to 5 display the calibration curves for each range.
Figure 1. Calibration curves for 54Fe (0-2 ppm). Image Credit: PerkinElmer Food Safety and Quality
Figure 2. Calibration curve for 23Na (0-300 ppm). Image Credit: PerkinElmer Food Safety and Quality
Figure 3. Calibration curve for 63Cu (0-200 ppb). Image Credit: PerkinElmer Food Safety and Quality
Figure 4. Calibration curve for 31P (0-100 ppm). Image Credit: PerkinElmer Food Safety and Quality
Figure 5. Calibration curve for 78Se (0-20 ppb). Image Credit: PerkinElmer Food Safety and Quality
As well as using the analyte elements for the multielement calibration, the standards, blanks and samples were also spiked on-line with a mixing tee and solution of 6 Li, Sc, Ge, In and Tb for internal standardization across the total mass range. Acetic acid was introduced to the internal standard solution to offset any residual carbon left over from the sample digestion.
Results
Tables 3 & 4 exhibit the quantitative results for two sample preparations of the NIST® 8414 Bovine Muscle and NIST® 2976 Mussel Tissue reference materials.
Table 3. Analysis of NIST® 8414 Bovine Muscle using the NexION 300/350 ICP-MS. Source: PerkinElmer Food Safety and Quality
| Element |
Mass
(amu) |
Reference Value
(mg/kg) |
Experimental Value
(mg/kg) |
| B |
11 |
0.6 ±0.4 |
0.4 |
| Na |
23 |
2100 ±80 |
2000 |
| Mg |
26 |
960 ±95 |
960 |
| Al |
27 |
1.7 ±1.4 |
1.6 |
| P |
31 |
8360 ±450 |
7250 |
| S |
34 |
7950 ±410 |
6820 |
| K |
39 |
15170 ±370 |
14180 |
| Ca |
44 |
145 ±20 |
143 |
| V |
51 |
(0.005) |
0.006 |
| Cr |
52 |
0.071 ±0.038 |
0.092 |
| Fe |
54 |
71.2 ±9.2 |
71.2 |
| Mn |
55 |
0.37 ±0.09 |
0.44 |
| Co |
59 |
0.007 ±0.003 |
0.014 |
| Ni |
60 |
0.05 ±0.04 |
0.05 |
| Cu |
63 |
2.84 ±0.45 |
2.81 |
| Zn |
66 |
142 ±14 |
140 |
| As |
75 |
0.009 ±0.003 |
0.011 |
| Se |
78 |
0.076 ±0.010 |
0.11 |
| Sr |
88 |
0.052 ±0.015 |
0.081 |
| Mo |
98 |
0.08 ±0.06 |
0.08 |
| Cd |
111 |
0.013 ±0.011 |
0.013 |
| Sn |
118 |
– |
0.14 |
| Sb |
121 |
(0.01) |
0.01 |
| Ba |
137 |
(0.05) |
0.04 |
| Hg |
202 |
0.005 ±0.003 |
0.003 |
| Pb |
208 |
0.38 ±0.24 |
0.34 |
| Tl |
205 |
– |
0.002 |
| Th |
232 |
– |
<0.00008 |
| U |
238 |
– |
<0.00002 |
Table 4. Analysis of NIST® 2976 Mussel Tissue using the NexION 300/350 ICP-MS. Source: PerkinElmer Food Safety and Quality
| Element |
Mass
(amu) |
Reference Value
(mg/kg) |
Experimental Value
(mg/kg) |
| B |
11 |
– |
27.5 |
| Na |
23 |
(35000 ±1000) |
35000 |
| Mg |
26 |
(5300 ±500) |
4800 |
| Al |
27 |
(134 ±34) |
149 |
| P |
31 |
(8300) |
6900 |
| S |
34 |
(19000) |
16000 |
| K |
39 |
(9700 ±500) |
9700 |
| Ca |
44 |
(7600 ±300) |
7400 |
| V |
51 |
– |
0.87 |
| Cr |
52 |
(0.50 ±0.16) |
0.50 |
| Fe |
54 |
171.0 ±4.9 |
190 |
| Mn |
55 |
(33 ±2) |
40 |
| Co |
59 |
(0.61 ±0.02) |
0.67 |
| Ni |
60 |
(0.93 ±0.12) |
0.87 |
| Cu |
63 |
4.02 ±0.33 |
3.91 |
| Zn |
66 |
137 ±13 |
145 |
| As |
75 |
13.3±1.8 |
16.4 |
| Se |
78 |
1.80 ±0.15 |
2.52 |
| Sr |
88 |
(93 ±2) |
79 |
| Mo |
98 |
– |
0.56 |
| Cd |
111 |
0.82 ±0.16 |
0.88 |
| Sn |
118 |
(0.096 ±0.039) |
0.103 |
| Sb |
121 |
– |
0.011 |
| Ba |
137 |
– |
0.61 |
| Hg |
202 |
0.061 ±0.0036 |
0.058 |
| Pb |
208 |
1.19 ±0.18 |
1.06 |
| Tl |
205 |
(0.0013) |
0.003 |
| Th |
232 |
(0.011 ±0.002) |
0.012 |
| U |
238 |
– |
0.22 |
Each of the elements in all samples was identified with Universal Cell operating in Collision mode utilizing helium as the cell gas.
Figures in parentheses ( ) in the Reference Value column cannot be considered certified values, but have been included for information purposes only. The data demonstrates an excellent agreement with the certified values, particularly for the elements that experience known spectral interferences.
The elements placed outside the designated limits are the ones that are generally considered to be environmental contaminants, which have most likely been influenced by the preparation procedure of the sample.
Conclusion
This work has shown how PerkinElmer’s NexION 300/350X ICP-MS can effectively quantify macro-level nutritional elements in the same analysis run as lower-level elements without being obliged to dilute the samples.
The agreement between experimental and certified results for NIST® 8414 Bovine Muscle and NIST® 2976 Mussel Tissue shows the precision of the analysis.
Instrument design characteristics eradicate deposition on the ion optics, resulting in lasting stability in high-matrix samples while also facilitating accurate measurement of trace levels.
References
- “The Determination of Toxic, Essential, and Nutritional Elements in Food Matrices Using the NexION 300/350 ICP-MS”, PerkinElmer Application Note.
This information has been sourced, reviewed and adapted from materials provided by PerkinElmer Food Safety and Quality.
For more information on this source, please visit PerkinElmer Food Safety and Quality.