I. Preface

DHA, docosahexaenoic acid, commonly known as "brain gold", is a very important polyunsaturated fatty acid for the human body and an important member of the Omega-3 unsaturated fatty acid family. DHA is a major element for the growth and maintenance of nervous system cells and an important constituent fatty acid of the brain and retina. It accounts for up to 20% of the cerebral cortex in the human body and has the largest proportion in the retina of the eye, approximately 50%. It is crucial for the development of an infant's intelligence and visual ability. DHA algal oil is extracted from Marine microalgae and has not been passed through the food chain, making it relatively safer. Its EPA content is very low. To detect the content of various heavy metal elements in DHA algal oil, Jiahang's microwave digestion instrument was selected for pretreatment to explore the most suitable digestion parameters. This method also has the characteristics of high recovery rate and low blank, which is conducive to the rapid and accurate determination of various inorganic elements in the subsequent process.

Ii. Instruments and Reagents

2.1 Instruments

Digipol-MWD60 fully automatic intelligent microwave digestion instrument, acid remover, analytical balance (one part per 100,000), etc.

2.2 Reagents

Nitric acid (68%)

Iii. Experimental Methods

3.1 Digestion

Weigh approximately 0.2g of DHA algal oil sample (accurate to 0.1mg), add 8mL of nitric acid, let it stand for 15 minutes, assemble the digestion tank, and conduct the experiment according to the following set parameters:

The reaction pressure of the sample is too high. Terminate the experiment.

3.2 Adjust the digestion parameters

Weigh approximately 0.2g (accurate to 0.1mg) of the DHA algal oil sample again, add 8mL of nitric acid, place the digestion tank on the acid eliminator and pre-treat it at 120℃ for about 30 minutes. When the yellow smoke has completely emerged, remove it, cool it down, and then add 2mL of nitric acid. Assemble the digestion tank and conduct the experiment according to the following set parameters:

After the experiment, when the temperature drops below 60℃, take out the digestion tank and transfer it to a fume hood. Slowly open it and place it on an acid-expelling device at 150℃ to expend the acid to about 0.5mL. Then transfer it to a beaker and add water to dilute it. The digestion solution should be clear and transparent, and the sample should be completely dissolved.

Iv. Results and Discussion

The DHA algal oil sample selected for the experiment was 0.2g in quantity. To prevent the microwave digestion pressure from being too high, it was pre-treated after adding nitric acid, and then assembled for the experiment on the machine. The maximum experimental temperature was 200℃, and it was maintained for about 30 minutes before being completely dissolved.