An apparatus and method for noninvasive determination of analyte properties of human tissue by quantitative infrared spectroscopy to clinically relevant levels of precision and accuracy. The system includes subsystems optimized to contend with the complexities of the tissue spectrum, high signal-to-noise ratio and photometric accuracy requirements, tissue sampling errors, calibration maintenance problems, and calibration transfer problems. The subsystems can include an illumination/modulation subsystem, a tissue sampling subsystem, a data acquisition subsystem, a computing subsystem, and a calibration subsystem. The invention can provide analyte property determination and identity determination or verification from the same spectroscopic information, making unauthorized use or misleading results less likely than in systems that use separate analyte and identity determinations. The invention can be used to control and monitor individuals accessing controlled environments.