The powder was determined later to pose no hazard.
Authorities did not identify the woman nor was it clear whether she would face criminal charges for the scare that lasted nearly two hours and delayed the votes.
A Republican House member this week revealed he had received a death threat on Sunday from someone who posted on his Facebook page, "I'm coming for you."
State Representative Steven Andersson, a Republican from Chicago's western suburbs, who relayed the threat to the Illinois State Police, said, "It's stressful to get this level of vitriol and hatred."
The State Police did not respond to a query by Reuters on Thursday about the incident.
Andersson accused a conservative think tank of spurring its followers to launch social media protests against him and other Republicans who broke ranks with Republican Governor Bruce Rauner by giving pivotal support to the Democratic budget and tax hike that went into effect.
In response, the think tank, the Illinois Policy Institute, encouraged its Facebook and Twitter followers to tone down their rhetoric and disavowed anyone making threats during the heat of political battle.
"I don't think it's OK to threaten public officials, and that behavior should be identified and called out," said a spokesman, Eric Kohn.
The events in Illinois are part of a larger national trend involving disillusioned political followers that, at its most extreme, resulted in a shooting last month at congressional Republicans as they practiced for a benefit baseball game.
"The days of just being cavalier and going into an auditorium or a setting with no regard of your security or not taking any precautions of what could happen are over," said Ken Wheatley, a former FBI agent and San Diego, California, security consultant.