Preparation of Food Samples Using Homogenization and Microwave-Assisted Wet Acid Digestion for Multi-Element Determination with ICP-MS

Published: December 22, 2023

doi:

Matjaž Rantaša, David Majer, Matjaž Finšgar

¹Faculty of Chemistry and Chemical Engineering,University of Maribor

Summary

The presented protocol describes sample homogenization with a laboratory mixer, acid digestion of food samples using a mixture of 68 wt% HNO₃ and 30 wt% H₂O₂ via microwave-assisted wet acid digestion, and multi-element determination performed with inductively coupled plasma mass spectrometry.

Abstract

Sample preparation is crucial for elemental determination, and various techniques are available, one of which involves homogenization followed by acid digestion. Special care is required during sample handling in the preparation stage to eliminate or minimize potential contamination and analyte loss. Homogenization is a process that simultaneously reduces particle size and uniformly distributes sample components. Following homogenization, the sample undergoes acid digestion, wherein it is digested with acids and auxiliary chemicals at elevated temperatures, transforming solid samples into a liquid state. In this process, metals in the original sample react with acids to form water-soluble salts. Samples prepared through acid digestion are suitable for elemental analysis using techniques such as inductively coupled plasma mass spectrometry, inductively coupled plasma optical emission spectroscopy, atomic absorption spectroscopy, electrochemical methods, and other analytical techniques. This work details the preparation of food samples for multi-element determination using inductively coupled plasma mass spectrometry. The step-by-step procedure involves the homogenization process using a laboratory-sized mixer with ceramic blades, followed by acid digestion in closed vessels using microwave-assisted wet acid digestion. A mixture of 5.0 mL of 68 wt% HNO₃ and 1.0 mL of 30 wt% H₂O₂ serves as an auxiliary reagent. This guide provides an explanation of the processes involved in both stages.

Introduction

Elemental analysis is an analytical process for determining the elemental composition of various samples. It can be used to control the intake of metals into human bodies (especially heavy metals¹) since their high concentrations may cause unwanted health problems. Heavy metals are also one of the main environmental contaminants, therefore, control of their presence in the environment is necessary². Moreover, elemental analysis can be employed to determine the geographical origin of food products³ and to control the quality of food and water resources⁴. In addition, it is used for the determination of micro- and macronutrients in soils⁵ and to gain insights into geological processes throughout history by examining the chemical composition of minerals and sediments⁶. Studies have also been made to determine the presence of rare metals in electrical and electronic waste for further metal regeneration⁷, to evaluate the success of drug treatments⁸, and to verify the elemental composition of metal implants⁹.

Inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) are commonly used techniques for the elemental analysis of various samples¹⁰. They allow simultaneous determination of multiple elements with limits of detection (LOD) and limits of quantification (LOQ) as low as ng/L. In general, ICP-MS has lower LOD values¹¹ and a wider linear concentration range compared to ICP-OES¹². Other techniques to determine elemental composition are microwave-induced plasma optical emission spectrometry¹³ and several variants of atomic absorption spectroscopy (AAS), including flame atomic absorption spectroscopy, electrothermal atomic absorption spectroscopy², cold vapor atomic absorption spectroscopy, and hydride generation atomic absorption spectroscopy¹⁴. Furthermore, elemental determination with low LOD and LOQ is possible with different electroanalytical methods, especially with anodic stripping voltammetry¹⁵^,¹⁶. Of course, there are other methods to determine the elemental composition of samples, but they are not as frequently employed as the above-mentioned methods.

Direct elemental determination of solid samples is feasible using laser-induced breakdown spectroscopy and X-ray fluorescence¹⁷. However, for elemental determination with ICP-MS, ICP-OES, and AAS it is necessary to convert solid samples into a liquid state. For this purpose, acid digestion is performed using acids and auxiliary reagents (in most cases H₂O₂). Acid digestion is carried out at elevated temperature and pressure, converting the organic part of the sample into gaseous products and converting the metal elements into water-soluble salts, thus dissolving them in the solution¹⁸.

There are two main types of acid digestion, open vessel digestion and closed vessel digestion. Open vessel digestion is cost-effective¹⁴ but has limitations, such as the maximum digestion temperature, which coincides with the boiling temperature of acids at atmospheric pressure. The sample can be heated on hot plates, heating blocks, water baths, sands baths², and by microwaves¹⁹. By heating the sample in that manner, much of the generated heat is lost to the surroundings²⁰, which extends the digestion time¹⁴. Other disadvantages of open vessel digestion include greater chemical consumption, the greater possibility of contamination from the surrounding environment, and possible loss of analytes due to the formation of volatile components and their evaporation from the reaction mixture²¹.

Closed vessel systems are more convenient for the digestion of organic and inorganic samples compared to open vessel systems. Closed vessel systems utilize a variety of energy sources to heat the samples, such as conduction heating and microwaves²². Digestion methods which use microwaves include microwave-induced combustion²³, single reaction chamber systems²⁴, and commonly used microwave-assisted wet acid digestion (MAWD)²⁵^,²⁶. MAWD allows digestion at higher operating temperatures, ranging between 220 °C and 260 °C and maximum pressures up to 200 bar, depending on the instrument's working conditions²⁷.

The efficiency and rate of MAWD depend on several factors, including the chemical composition of the samples, the maximum temperature, the temperature gradient, the pressure in the reaction vessel, the amount of acids added, and the concentration of acids used²⁸. In MAWD, complete acid digestion can be achieved in a few minutes due to the elevated reaction conditions compared to longer-lasting digestions in open vessel systems. Lower volumes and concentrations of acids are required in MAWD, which is in line with current green chemistry guidelines²⁹. In MAWD, a smaller amount of sample compared to open vessel digestion is needed to perform acid digestion, usually up to 500 mg of sample is sufficient³⁰^,³¹^,³². Larger sample quantities may be digested, but they require a larger amount of chemicals.

Since the instrument for MAWD automatically controls the reaction conditions and the person does not come in direct contact with the chemicals during heating, MAWD is safer to operate than open vessel digestions. However, the person should always proceed with caution when adding chemicals to the reaction vessels to prevent them from coming into contact with the body and causing harm. Reaction vessels also need to be opened slowly as the pressure is built up inside them during acid digestion.

Although acid digestion is a useful technique for preparing samples for elemental determination, the person performing it should be aware of its possible limitations. Acid digestion may not be suitable for all samples, especially those with complex matrices and those that are highly reactive or could react explosively. Therefore, sample composition should always be evaluated to select the appropriate chemicals and reaction conditions for complete digestion that dissolves all desired elements in the solution. Other concerns that the user must consider, and address are impurities and loss of analytes at every step of sample preparation. Acid digestion must always be performed according to specific rules or using protocols.

The protocol described below provides instructions for the homogenization of food samples in a laboratory-sized mixer, a procedure for cleaning the mixer's components, properly weighing the sample, adding chemicals, performing acid digestion by MAWD, cleaning the reaction vessels after the digestion is complete, preparing the samples for elemental determination, and performing a quantitative multi-element determination with ICP-MS. By following the instructions given below, one should be able to prepare a sample suitable for elemental determination and perform the measurements of digested samples.

Protocol

1. Sample homogenization Using a clean ceramic knife, manually cut the food samples (broccoli, mushrooms, sausages, and noodles) into smaller pieces to speed up the drying process. Prepare enough samples for a minimum of 6 replicates of the acid digestion (ensure that the minimum mass of the dried samples is 1500 mg). NOTE: Increasing the surface area of the sample exposes a larger portion of the sample to the heated surrounding air, increasing the rate of evaporation of the water.</li…

Representative Results

Homogenization All samples were dried to a constant mass with the laboratory dryer to eliminate any moisture. Transferring the sample to a desiccator allowed it to cool to room temperature without binding moisture from the surrounding environment. The food samples were then homogenized using the laboratory mixer to obtain a fine powder. The resulting homogenized particles were uniform in size and evenly distributed, ensuring that subsamples (samples drawn from a larger sample) used for acid digesti…

Discussion

Homogenization
To ensure reproducible results in elemental determination, it is necessary to homogenize samples before acid digestion due to their complex and inhomogeneous structure and composition. Homogenization aims to eliminate constitutional and distributional heterogeneity. Mixing the sample minimizes distributional heterogeneity by evenly redistributing components throughout the sample. Similarly, by bringing the particle size down to a uniform size, constitutional heterogeneity is reduced<…

Divulgazioni

The authors have nothing to disclose.

Acknowledgements

The authors acknowledge the financial support of the Slovenian Research Agency (Grant Nos. P2-0414, P2-0118, J1-2470, NK-0001, and J1-4416).

Materials

Ar gas	Messer	7440-37-1	Ar 5.0 gas (purity 99.999%).
AS-10 Autosampler system	Shimadzu		Autosampler connected to the ICP-MS, containing 68 ports for samples.
Automatic pipettes	Sartorius		200 µL, 1 mL, and 5 mL automatic pipettes.
Balance XSE104	Mettler Toledo, Columbus, Ohio, USA		Analytical balance scale with a maximum weighing mass of 120 g.
Ceramic knife			Ceramic knife used for cutting fresh food samples.
Dessicator			Glass desiccator with lumps of silica gel.
ETHOS LEAN	Milestone, Sorisole, Italy		Microwave system for wet acid digestion in closed 100 mL vessels made of TFM-PTFE.
Fume hood			Laboratory fume hood with adjustable air flow.
Glass beakers RASOTHERM	CarlRoth GmbH + Co.KG		50 mL, 250 mL glass beakers
Glass funnels			Small glass funnels fitting into the neck of volumetric flasks.
He gas	Messer	7440-59-7	He 5.0 gas (purity 99.999%).
Hydrogen peroxide	ThermoFisher Scientific	7722-84-1	Hxdrogen peroxide 100 volumes 30 wt.% solution. Laboratory reagent grade.
ICP multi-element standard solution VIII	Supelco	109492	100 mg/L ICP multi-element standard solution containing 24 elements (Al, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, K, Li, Mg, Mn, Na, Ni, Pb, Se, Sr, Te, Tl, Zn) in 2 % dilute nitric acid.
ICPMS 2030	Shimadzu		Inductively coupled plasma mass spectrometry system for multi-element analysis of digested samples.
ICP-MS Tuning Solution A	CarlRoth GmbH + Co.KG		250 mL tuning solution containing 6 elements (Be, Bi, Ce, Co, In, Mn) in 1 % nitric acid.
KIMTECH Purple Nitrile gloves	Kimberly-Clark GmbH		Disposable Purple Nitrile gloves (S, M or L).
Laboratory coat	Any available supplier		/
Mixer B-400	BÜCHI Labortechnik AG, Flawil, Switzerland		Laboratory mixer with ceramic blades.
Nitric acid	ThermoFisher Scientific	7697-37-2	Nitric acid, trace analysis grade, 68 wt%, density 1.42, Primar Plus, For Trace Metal Analysis.
Plastic centrifuge tubes	Isolab		50 mL plastic centrifuge tubes with screw caps, single use.
Plastic syringes Injekt	B. Braun		2 pice, single use 20 mL syringes.
Plastic tubes for autosampler	Shimadzu	046-00147-04	Plastic tubes for autosampler, 15 mL capacity, 16 mm diameter, 100 mm length.
Plastic waste containers			Plastic containers for the removal of chemicals after the cleaning procedure of reaction vessels.
Protective googles			/
Samples (broccoli, sausage, noodles, zucchini, mushrooms)			Fresh samples, which were dried to a constant weight and homogenized during the procedure. The samples were purchased from a local shop.
Spatula			Plastic spatula.
Sterilizator Instrumentaria ST 01/02	Instrumentaria		Dryer with adjustable temperature.
Syringe filters	CHROMAFIL Xtra	729212	Syringe filters with polypropylene housing and polyamide hydrophilic membrane. Membrane diameter 25 mm, membrane pore size 0.2 µm.
Ultrapure water	ELGA Labwater, Veolia Water Technologies.		Ultrapure water with a resistivity of 18.2 MΩcm, obtained with laboratory water purification system.
Volumetric flasks			25 mL glass volumetric flasks.

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Rantaša, M., Majer, D., Finšgar, M. Preparation of Food Samples Using Homogenization and Microwave-Assisted Wet Acid Digestion for Multi-Element Determination with ICP-MS. J. Vis. Exp. (202), e65624, doi:10.3791/65624 (2023).