A chemist at a state crime lab in Amherst who is accused of altering drug evidence will be suspended without pay while her case is pending, authorities said. The suspension of Sonja Farak, 35, of Northampton, will take effect when she is arraigned on Tuesday, said a spokesman for the state Executive Office of Public Safety. Farak is accused of testing two samples believed to be heroin and cocaine on Jan. 2 and Jan. 8 and then replacing the drugs with counterfeit substances. She is charged with tampering with evidence, possession of a class A substance, and possession of a class B substance. She will be arraigned at Eastern Hampshire District Court.