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

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 u....

Protocol

1. Sample homogenization

1. 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.

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<.......

Materials

Name	Company	Catalog Number	Comments
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.