What additional step can be performed to confirm the presence of GFP after purification?

The most effective method to confirm the presence of Green Fluorescent Protein (GFP) after purification is through fluorescent microscopy. This technique specifically takes advantage of GFP's inherent fluorescence properties, allowing for the direct visualization of the protein in a sample. Observing cells or tissues under a fluorescence microscope can reveal the localized expression and distribution of GFP, affirming its presence in a manner that is highly specific to its unique fluorescent characteristics.

While other methods like Western blot analysis, mass spectrometry, and SDS-PAGE can provide valuable information about protein identity and size, they do not leverage the distinguishing feature of GFP, which is its ability to fluoresce. Mass spectrometry, for example, would require additional sample preparation and would not provide immediate visual confirmation of fluorescence. Similarly, while SDS-PAGE can separate proteins based on size and may indicate the presence of GFP through banding patterns, it requires downstream techniques like staining or transfer to confirm the identity of the protein rather than providing direct visualization.

Thus, fluorescent microscopy is the most definitive answer for confirming GFP presence post-purification due to its ability to utilize the native fluorescence of the protein itself.