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

Our research aims to determine the elemental composition of different food samples using different spectroscopic devices to, for example, control the quality of food samples. Besides ICP-MS, our laboratory can also perform elemental analysis of the digested food samples with ICP-OS, atomic absorption spectroscopy, and ion chromatography. However, we can also perform elemental analysis with X-ray photo spectroscopy and time of flight secondary ionmostometry.

Current experimental challenges that we face are mostly in the sample preparation process. Our goal is to improve the sample throughput and to optimize the cleaning procedure to minimize any carryover of the sample. The information in the article is about sample homogenization, acid digestion, and elemental analysis to better understand critical procedures for SAPA preparation and subsequent analysis as we highlight important steps, suggestions, and limitations of the entire process, We'll focus on the optimization of the entire processes for samples such as vegetables, wine, ice creams, illiquids, and in the future, even inorganic samples, which require different sample preparation techniques.

To begin, use a clean ceramic knife to manually cut food samples into smaller pieces. Transfer the samples to 250 milliliter glass beakers. Then place the samples in a dryer at 105 degrees Celsius until there is no change in weight.

Next, transfer the dried sample into a mixer beaker. Grind the sample until it turns into a fine powder or a homogenous paste. With a clean, plastic spatula, transfer the homogenized sample into a 50 milliliter glass beaker.

Next, weigh 250 milligrams of the sample into an open reaction vessel on an analytical balance. Once the weighing is complete, cover the reaction vessel with its lid, then transfer it to a fume hood. Now open the cover lids of the reaction vessels.

Then pipette five milliliters of 68%nitric acid and one milliliter of 30%hydrogen peroxide into each reaction vessel. With a 200 microliter pipet, add 37.5 microliters of ICP multi-element standard solution into reaction vessels for spiked recovery test, ensure that each unspiked and spiked sample is prepared in triplicate. Place the cover lids on the reaction vessels and screw the thread cover on the reaction vessels to tighten the cover lids.

Allow the samples to react with the acids for two to three minutes. To perform microwave-assisted wet acid digestion, distribute the closed reaction vessels symmetrically in the microwave rack. Insert the rack into the microwave chamber and mount it on a holder.

Then close the microwave door. Digest the samples using the selected digestion program while monitoring the change in reaction conditions on the screen. Once the digestion is complete, remove the rack from the oven chamber.

Close the door of the instrument and switch it off. Now slowly open the lids of the reaction vessels to release any gases formed during digestion. Turn the reaction vessels towards the fume hood.

With a glass funnel, quantitatively transfer the digested sample into a clean, 25 milliliter glass volumetric flask. Dilute the sample with ultrapure water up to the mark on the flask. Then close the mouth of the flask with a stopper and mix the contents well.

Next, connect a 20 milliliter plastic syringe to a polyamide syringe filter. Fill the syringe with the diluted sample, then filter its contents into a 50 milliliter plastic centrifuge tube. Homogenization resulted in fine sample particulates that were uniform in size and evenly distributed.

Acid digestion caused a steady increase in temperature with time. To begin, pipette 2.5 milliliters of the filtered, digested food samples into 25 milliliter glass volumetric flasks. Make up the volume of the flask with ultrapure water.

Transfer the diluted samples into 15 milliliter plastic tubes and place them in appropriate positions in the autosampler. Next, switch on the ventilation and the chiller of the ICP-MS instrument. Program the software to allow for continuous flow of the rinsing solution from the autosampler to the instrument without pulsating.

Open the argon and helium gas cylinders to supply the instrument. Then start up the instrument and calibrate it with the tuning solution. After determining the approximate element concentrations with a semi-quantitative analysis, create a method in the software for quantitative elemental analysis.

Choose the elements to be analyzed and decide on the number and concentrations of the solutions of standard needed to construct a calibration curve. To prepare the solutions of standard with automatic pipettes, add the required volumes of multi-element standard solution into a 20 milliliter glass volumetric flask. Top off each flask with 1%nitric acid.

Prepare a calibration blank with only 1%nitric acid solution. Perform the quantitative analysis of the selected elements in samples based on the calibration curve methodology. Once the measurements are complete, switch off the plasma, close the argon and helium gas supplies.

Then switch off the ICP-MS chiller and the ventilation system. The linear concentration ranges for all measured elements were in the range from 1 to 50 micrograms per liter. The recovery percentage for all analytes measured in all four food samples was in the range of 80%to 120%indicative of the accuracy of the analytical method.