Microwave Digestion—Vacuum Filtration-Automated Scanning Electron Microscopy as a sensitive method for forensic diatom test

Abstract

Diagnosis of drowning is one of the most difficult issues in forensic practice. A number of methods have been developed over the years to determine whether a person was drowned. Microwave Digestion-Vacuum Filtration-Automated Scanning Electron Microscopy (MD-VF-Auto SEM) method we developed is a new qualitative and quantitative method of diatom test for diagnosis of drowning. The new method is based on microwave digestion technique, vacuum filtration, and automated SEM, which would achieve a maximal recovery of diatoms and identify diatoms easily by SEM with high resolution. This study was designed to evaluate the sensitivity of this method, the recovery of diatom, and loss ratio of centrifugation, which were compared using the MD-VF-Auto SEM method and the conventional acid digestion method. Two groups of samples were designed in the study. Groups A (n = 20) and B (n = 20) were performed by MD-VF-Auto SEM method and the conventional acid digestion method, respectively. In addition, another eight water samples were centrifuged, and the diatoms in the supernatant and precipitate were counted and measured, respectively, in order to find out how many diatoms were lost after centrifugation. The difference between the two groups was statistically highly significant, and about 34 % of diatoms were lost after centrifugation at 4,000 rpm for 15 min. The results showed that the MD-VF-Auto SEM method was more sensitive and specific.

Detection of Diatoms by a Combination of Membrane Filtration and Transparentness,

“…Diatoms on the membranes can be viewed clearly without changes in diatoms morphology. According to our previous study , the recovery of the conventional diatom test method is lower than 5%. This novel method increases the recovery of diatoms a lot, which leads to the diagnosis of drowning cases more accurately.…”

“…The centrifugal force attracts diatoms by particle size or weight. Lots of diatoms were lost in the centrifugation process . This may be the reason why false‐negative results occur frequently in the laboratory with the conventional method.…”

Types of diatoms in China’s three major rivers and the possible application for an automatic forensic diatom test

“…During drowning, only small diatom species can reach the blood, liver, kidney, and bone marrow 19 . According to our experience, fewer than 10 genera of diatoms were found in the closed organs.…”

“…Samples were vacuum filtrated and observed by scanning electron microscopy (SEM) 19 . The vacuum degree was set at 200 mm Hg; 50 ml of ultrapure water was put into the filter bowl, and then the digestion solution was added into the filter bowl.…”

“…It is not necessary to use all species or genera of diatoms for the ADIAC in the forensic diatom test. In 2013, we reported a new method called the Microwave Digestion-Vacuum Filtration-Automated Scanning Electron Microscopy 19 . Diatoms can be identified based on (1) contour shape and (2) pore patterns or 'striae' of the shells, called striation or ornamentation 17 .…”