Apparatus and methods for monitoring, analyzing, and/or discriminating molecular species, preferably a biomolecule, within a medium using a multisensor array (MSA) and multivariate processing. Biological compounds such as nucleotides and polynucleotides can be detected and analyzed. A reaction process such as an accumulation cycle of nucleic acids can be monitored, analyzed, and controlled using a multisensor array (MSA) and multivariate processing. Monitoring a biomolecule includes interrogating the medium, and preferably its gas phase, by coupling a sensor responsive to any changes of the medium and or biomolecule and its secondary products when, for example, an amplification reaction is proceeded. It is also a scope of the present invention to use direct detection and monitoring of biomolecular reactions in real-time without radioactive or fluorescent labeling. A preferred application is real-time polymerase chain reaction (PCR) detection.