There are a few possibilities that could be causing the high AhR activity in your negative controls:

1. Contamination in the Instant Ocean: While Instant Ocean is a high-quality synthetic sea salt mix, it is possible that it contains trace amounts of contaminants that are affecting your assay. You could try switching to a different brand of synthetic sea salt mix to see if the issue persists.

2. Contamination during sample processing: Even if you have taken precautions to clean your glassware and use high-quality water, it is possible that some contamination has occurred during sample processing. For example, if your SPE cartridges were not washed thoroughly enough, residual salts or other contaminants could be affecting your results.

3. Interference from other compounds: It is possible that other compounds in your samples are interfering with your assay, leading to false positive results. This could include compounds that are naturally present in the Instant Ocean or introduced during the crude oil treatment process.

To troubleshoot this issue, you could try the following:

1. Use a different brand of synthetic sea salt mix to prepare your ASW.

2. Repeat your sample processing procedure with additional quality control steps, such as running blank SPE cartridges and analyzing the washes for contaminants.

3. Consider using additional assays or analytical techniques to confirm the AhR activity results and identify potential interfering compounds.

4. Consult the literature or reach out to other researchers in your field to see if they have encountered similar issues with Instant Ocean or other sea salt mixes.