Background:

When the Dominion's intention to invade the Alpha Quadrant became clear, measures were taken to develop means of detecting the Founders, and of inhibiting their ability to change shape.

One of the first effective anti-Changeling devices was developed by the Cardassian secret service, the Obsidian Order. The device worked by emitting a quantum stasis field that prevented the Changeling from altering its biomolecular structure, and thus its form. As the Founders must return to their natural state within a given period of time, the device caused them great distress, and made it easy to detect them. However, the device had some drawbacks - in its prototype form it had a very limited range, and the suspected Changeling had to be kept in a fairly confined area. In 2371, a prototype of the Cardassian device was used on Deep Space Nine's Odo when he was being held prisoner by a combined Tal Shiar-Obsidian Order fleet. A forcefield around the bulkheads prevented escape from the confines of the room, and the prototype forced Odo to remain in his human shape. Slowly, his body appeared to become desiccated: his face cracked, his skin flaked off, and his uniform crumbled. It was clear that he was in great pain.

A less sophisticated way of detecting Changelings, and one that has been employed by the Federation, is blood screenings. A Changeling can assume any form, but any part of them that is removed from their body rapidly reverts to its natural state. Theoretically, a simple extraction of blood proves whether or not a person is a Changeling, because humanoid blood remains as blood, but a Changeling's 'blood' quickly reveals its true nature. Blood screenings are not only time-consuming and invasive, but largely ineffective, as the tests have been administered frequently to be of any real value. A slightly more effective anti-Changeling device developed by the Federation involves using low-level phaser sweeps. Phaser settings as low as 3.5 are enough to cause a Changeling pain and force a return to its gelatinous state. Unfortunately, this method can only be used to reveal a Founder who has assumed the shape of an object; to deploy phaser sweeps over a group of humanoids in order to flush out a Changeling would be too extreme a measure.

It appears that some natural substances may provide effective anti-Changeling methods. In 2370, Odo was exposed to a noxious gas that caused him to lose control over his morphing abilities. At present, the gas has only been found on the planet LS-VI in the Gamma Quadrant, making it difficult to obtain.