WASHINGTON (AFP) - Can the Chemical Weapons Convention be used against a woman who tried to poison her rival in a love triangle?

The US Supreme Court on Tuesday suggested that the answer is no.

"It seems unimaginable that you would bring this prosecution," Justice Anthony Kennedy told the government prosecutor during oral arguments in the case.

Justice Samuel Alito asked jokingly if he would be at risk of prosecution after handing out Halloween candy bars since "chocolate is poison to dogs."

At issue is the case of Carol Bond. The Pennsylvania microbiologist put arsenic and potassium dichromate on the mailbox and car controls of her friend who had an adulterous fling with Bond's husband and got pregnant.

Bond was arrested in the failed attempt to kill her friend, pleading guilty in 2007 to two counts of the federal crime of having used using a chemical weapon.

She was sentenced to six years in prison and released last year.

But Bond now has appealed to the US Supreme Court, saying the law was supposed to stop terrorists from using chemical arms, not to prosecute individuals.

"If the statute (is used) for any malicious use of chemicals then it clearly exceeds Congress's powers," her attorney Paul Clement argued.

That amounts to "police power," and as such would be a responsibility of the individual US states, Mr Clement argued.

One of the justices fretted that the list of potentially harmful chemicals would be "a thousand miles long," although the three women on the nine-member court seemed favourably disposed toward the government's case.

But legal expert Lyle Denniston suggested on the ScotusBlog website that the best the government could hope for in the case is a small defeat, rather than a massive beatdown.

"It appeared that the government might just have to hope that it loses the case on narrow grounds, because it might lose it in a sweeping way," Denniston wrote.

A decision is expected by June.