Sonja Farak, 35, was arrested late Saturday by state police, authorities said, after being charged with two counts of tampering with evidence and two counts of drug possession in separate cases early this month.
The charges came a month after another state crime lab chemist in Boston, Annie Dookhan, 34, was indicted on charges of falsifying drug evidence in thousands of cases. She was arrested in September.
Investigators identified some 10,000 people convicted or accused of crimes based on evidence that Dookhan handled at the Hinton lab in Jamaica Plain, and state officials said in December that hundreds of people had been released from prison pending new trials in the investigation.
While the new charges raised worries about potential effects on criminal cases containing evidence handled by Farak, authorities tried to minimize such concern, saying a preliminary investigation indicated Farak's activities did not taint any cases.
"On its face, the allegations against this chemist do not implicate the reliability of testing done or fairness to defendants," Massachusetts Attorney General Martha Coakley told a news conference.
The Amherst Drug Laboratory in western Massachusetts stores and analyzes controlled substances seized by local and state police, usually for the part of the state west of Worcester.
Staff at the Amherst lab on Friday contacted State Police to report a discrepancy in the controlled substance inventory held in evidence, and police began investigating, Coakley said.
On two days in January, Farak completed tests on samples that tested positive for drugs, then removed relatively small quantities of heroin and cocaine for personal use and replaced the drugs with other substances, authorities said.
Coakley said that all of the state's district attorneys have been notified of the Farak investigation, which is ongoing.
Farak began working as a state lab chemist at the Hinton lab in 2002 and moved to the Amherst lab in 2004, Coakley said. State police assumed oversight of the Hinton and Amherst labs in July after learning of Dookhan's activities.
The Amherst lab had three chemists and a supervisor. It has been closed temporarily and the chemists moved to another lab.
Farak, who lives in Northampton, Massachusetts, was scheduled to be arraigned on Tuesday in Eastern Hampshire County District Court, Coakley said.
(Reporting by Zach Howard; Editing by David Bailey, Ellen Wulfhorst, Mary Wisniewski and Vicki Allen)