A black 2002 Audi tooling around Seattle's streets has a vanity license plate that appears to have gotten in under the radar of the state licensing police.

The inscription, C9H13N, is a label for many chemicals including amphetamine.

The Merck index, the bible of chemical formulas, lists methamphetamine as C10H15N. But at least two sites list C9H13N as the label for methamphetamine, according to the University of Washington's chemistry department. C9H13N is the formula for hundreds of compounds, the department said, including amphetamine. Not that the general public would know that.

There's an outside chance that the plate may be all right because the chemical compound can be amphetamine, which is used in medicines.

The plate appears to be counter to state regulations that ban vanity plates making reference to alcohol or illegal substances.

The driver of the car, whose name was not released, told state officials in his application for the plate that the series of letters and numbers represented red food coloring, said Brad Benfield, spokesman for the state Department of Licensing.

But according to the UW Chemistry Department, the compound represented on the plate is not a formula for known red food coloring.

"This is a serious concern, if there is a license out there with something on it that a reasonable person would consider related to an illegal substance," said Benfield.

Revoking a license plate is not a simple task.

First the state would send the driver a letter questioning the plate. Then the case would go to the agency's Personalized Plate Review Committee, made up of officials from the licensing department, the Washington State Patrol, county auditors and vehicle-licensing agents.

Last week the state dismissed a complaint about a vanity plate imprinted with JOHN316 - a reference to a New Testament verse - because it wasn't deemed offensive.

About 83,000 vehicles in the state - 25,000 in King County alone - have vanity plates, according to Benfield. That's out of 6.5 million total vehicles registered. Owners pay $44.75 for the plate the first year and $30 in subsequent years.

According to state law, the licensing department may deny or cancel a vanity application if it falls into one of six categories:

• It's offensive to good taste and decency.

• It's potentially misleading.

• It's vulgar, profane or sexually suggestive.

• It's a racial, ethnic, lifestyle or gender slur.

• It relates to alcohol or illegal activities or substances.

• It is derogatory, slanderous or religiously blasphemous.

So how could this plate have been issued in the first place?

"It's pretty easy for something like this to slip through," said Benfield. "With a series of letters and numbers, if you're not a chemist it doesn't ring a bell."

Susan Gilmore: 206-464-2054 or sgilmore@seattletimes.com