Online broker TD Ameritrade ( AMTD) on Tuesday raised its first-quarter earnings guidance. The Omaha, Neb.-based brokerage says it now expects first-quarter earnings to exceed 28 cents a share, which had been the high end of its earlier guidance. The company says it is benefiting from "higher-than-expected trading volume'' in the first quarter. Before the announcement, which was released after the close of trading, analysts had been predicting that TD Ameritrade would earn 22 cents in the quarter, according to Thomson Financial. The broker is scheduled to report earnings on April 24. In afterhours trading, shares of TD Ameritrade were up 67 cents, or 3.5%, to $19.83. The stock fell 82 cents, or 4%, in regular trading.