After extracting and measuring the total lipids from microalgae, the remaining lipid extract is dried using a nitrogen flow, followed by drying with magnesium sulfate at 90°C. However, it has been observed that traditional dry combustion at 500°C only decomposes approximately 50% of phospholipids (Hu et al., 2022). To achieve complete conversion of phospholipids to pyrophosphate, a temperature of around 800°C is required, but such high temperatures cannot be used with glassware. As the acid digestion method involves using only 500 µL of 0.2 M HCl, which must be placed in tightly capped glass vials to prevent concentration changes due to evaporation, combustion must be carried out using glassware instead of crucibles. It should be noted that the recovery rate of phospholipids is around 80% when combusted at 650°C, but this recovery rate is consistent, making the use of glass vials applicable. Therefore, we recommend using 650°C to combust phospholipids and using 80% to correct the final results.
The resulting ash is digested using 0.5 mL of 0.2 M HCl for 30 minutes at 90°C. After digestion, the resulting orthophosphate is detected by mixing the sample with a combination of molybdate and ascorbic acid to produce molybdenum blue, as described in Chen's work (1956).