Analysis of a commercial surfactant for digital PCR assay
Digital PCR assay represents an example of the power of PCR and provides unprecedented opportunities for molecular genetic analysis in cancer.
The technique is to amplify a single DNA template from minimally diluted samples, therefore generating amplicons that are exclusively derived from one template and can be detected with different fluorophores or sequencing to discriminate different allele.
Digital PCR assay has been applied in quantification of mutant alleles and detection of allelic imbalance in clinical specimens, providing a promising molecular diagnostic tool for cancer detection.
In this application note we are investigating the usability of the commercially available surfactant dSurf for an exemplary digital PCR-assay.
Introduction to digital PCR
Digital droplet-based assays offer promising opportunities for the absolute quantitation of low concentration analytic species. During the last decade digital-PCR assay (dPCR) became one of the most prominent assays for this class of analytical methods. For performing the assay, the sample volume is split into multiple droplets in such a way that each droplet contains either one or none of the target DNA molecules. Due to the small droplet volume, the PCR reaction runs very efficiently even from a single molecule.
During amplification, a fluorescent dye is formed or activated. The positive droplets become fluorescent. Absolute quantitation of the number of target molecules is simplified to the count of fluorescence active droplets in the generated droplet collection. Not regarding the simplicity of the approach, its technical implementation is challenged by stabilizing the droplets collected over the complete assay avoiding unwanted droplet coalescence or crosstalk between the droplet ingredients. This has been solved by utilizing perfluorinated mineral oils as the carrier oil in combination with advanced perfluorinated surfactants, which stabilize the emulsion and avoid crosstalk and DNA exchange between the individual droplets.
Digital PCR Assay: Materials and methods
Droplets were generated at a working pressure of 240 mbar for the dSurf and 140 mbar for the PCR-Mix. The chip was connected with PEEK 1/32” tubing OD x .010” and 2x sleeves 1/16” OD x .033” ID x 1.6”, tubing length: 200 mm. Generated droplets were collected into a 0.2 ml PCR vial. Amplification was performed in a conventional thermocycler with the settings displayed on the right. For readout, the amplified droplet collection was transferred into a cell counting capillary slit chamber for subsequent brightfield and fluorescence image acquisition.
Digital PCR Assay: Partial results
The experiments have shown that the dSurf surfactant is suitable for scientific as well as routine digital PCR applications. The generated droplets were homogeneous in shape and size. Superior droplet stability of the dSurf surfactant system was observed during the amplification process. A few droplets have dissipated during the experiments, but this can be neglected. The reproducibility of the experiments was also confirmed. Droplet generation with identical parameters leads to identical droplet size and quality. Summarily, dSurf can be employed as a surfactant composition for digital droplet-based assays, and therefore, for digital PCR assay.
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