I am currently working on biogenic sediments from different lakes and I want to determine the amount of mineral matter by removing the rest of the material. For analyses I used 0,25 to 0,5 g of dried and homogenized material. My first step was loss on ignition in 550C to remove organic matter. Then I did LOI in 950C to remove carbonates. For the removal of diatom frustules I tried 2 times with Na2CO3. I mixed the sediment with around 30 ml of 2M Na2CO3. I did a water bath and heated the samples in 70-80C for 6 hours. After that there were still frustules so I decided to try 1M NaOH. First try was with 50 ml of it. I heated the sample to around 80C and was mixing it using magnetic stirrer for 1 hour. Then I checked the samples and still there were frustules. I tried again but with 100 ml of 1M NaOH and I heated the sample to 100C and left it for 3 hours without stirring. After that I noticed that frustules were partly dissolved but still there were a lot of them. I was afraid to use stronger NaOH as it might destroy mineral matter that I need.