How to avoid "false positive" of luminol blood test

Details:

Luminol  is a common method for preliminary identification of blood. Although there are other types of blood identification presumptive tests, luminol is still a commonly used method worldwide. This is partly because it can be used to screen for the presence of large areas of blood. However, despite its popularity, luminol has well-known shortcomings, such as the lack of specificity, which may lead to false positive indications of the presence of blood and the requirement to test in complete darkness for the best results. So is there any way to avoid this false positive.

The intensity and duration of chemiluminescence may depend on the substance reacted by luminol, i.e. blood or bleach, and the amount of reactants, which means that the change of concentration may affect the change of chemiluminescence intensity. Therefore, increasing the time and intensity of chemiluminescence is still the goal of researchers, because it will enable crime scene investigators to see the positive results more clearly.

The purpose of this study is to study the factors that may affect the luminol luminous intensity and time, in order to improve the blood detection efficiency of luminol.

Experiment 1

First, a 5mm diameter absorbent fiber disc was used for 20 minutes μ L human blood immersion. Then, the blood on the disc was diluted to 1:1 to 1:100000. Place all discs in the holes of the 96 hole plate. 40 prepared according to Weber method μ L luminol working solution was added to each disc. Chemiluminescence measurement was carried out by enzyme labeling instrument and chemiluminescence meter, which measured the light intensity 17 seconds after adding the working solution.

Experiment 2

For some discs, the blood soaked discs were pretreated with 8M urea solution for 20 minutes before adding the working solution. Urea is used to partially denature or decompose hemoglobin to enhance the chemical properties of the reaction. The authors speculate that if the chemical properties of hemoglobin can be enhanced, the intensity and time of chemiluminescence will be greater. Prepare another disc on which 30% sodium hypochlorite solution is applied to the blood soaked disc before pretreatment with urea.

experimental result

1. Monochlorotriazine with concentration of 15 mm- β- Cyclodextrin (MCT)- β- CD) can be used to enhance the chemiluminescence intensity.

2. Pretreatment with 8M urea may increase the time and intensity of chemiluminescence.

3. Pretreatment with 8M urea can eliminate the false positive caused by sodium hypochlorite (i.e. bleach).

4. Denaturation or decomposition of hemoglobin seems to contribute to enhanced chemiluminescence.

Luminol is a commonly used blood identification method, but it is not without shortcomings and can be improved. Although luminol is a common tool for on-site blood identification, bleach is a popular household product, so the test is prone to false positives. Improvements can be made to improve the practicability and efficiency of luminol and reduce the impact of false positives. Urea and some cyclodextrins, such as MCT- β- CD will enhance chemiluminescence, making luminol a more useful tool.