Jim Carlson, the owner of a head shop, says a new federal ban on the sale of five chemicals used to make synthetic marijuana won't make much difference - he'll just stock brands that use other, still-legal substances. Carlson said that with about 210 similar chemicals available, the manufacturers will try to keep one step ahead of the government. "Unfortunately he is correct," said Barbara Carreno, a DEA spokeswoman in Washington, who confirmed Tuesday that many suppliers are offering retailers products with new chemicals. "There are many of these substances and we chose five common ones because we don't have the resources to study all of them."