TD Ameritrade Holding Corp. (Nasdaq:AMTD) announced earnings of $0.23 on Tuesday, missing analyst expectations of $0.26, according to data compiled by FactSet. Earnings were down $0.08 year-on-year and $0.03 quarter-on-quarter.

TD Ameritrade was troubled by a declining net interest margin (NIM), or the net yield, for its spread-based assets, which mostly consisted of customer margin balances and various deposits.

While its balance of the spread-based assets has grown from an average of $25.4 billion last year to $48.1 billion this year, the NIM has declined from 3.78 percent to 2.07 percent.

“Rates are driving approximately $200M of lower revenue year-over-year …approximately $0.20 of EPS impact on the quarter,” the company said in a released statement.

The company projects its growing balance in assets will position it for substantial earnings once the NIM rises.

TD Ameritrade has maintained a flat market share in customer trades per day, accounting for approximating 7.1 percent of market share, from 7.2 percent a year ago and 6.7 percent in 2007.

After reporting earnings this morning, it is trading up 3.6 percent.