Baseline compensation for chromatographic analyzer

Claim 1

In analysis apparatus including a chromatographic column, a sample Valve, first conduit means communicating with said valve to introduce carrier fluid, second conduit means communicating with said valve to introduce sample material, third conduit means communicating between said valve and the inlet of said column, a detector adapted to establish a first analog output signal, fourth conduit means communicating between the outlet of said column and said detector, and a recorder connected to said detector; the improvement comprising: a second recorder having first and second recording channels on a common recording medium; first means selectively connecting the output of said detector to said second recorder, said first means including first and second frequency modulation networks having substantially the same mean frequency, means applying the output of said detector to said two networks so as to establish first and second signals of frequencies which deviate from said mean frequency by like amounts, the frequency of said first signal being above said mean frequency and the frequency of said second signal being below said mean frequency, and means applying said first and second signals to said first and second channels, respectively; signal-subtracting means; first and second frequency demodulation circuits; means selectively connecting the first and second channels of said recorder to the respective inputs of said first and second circuits; circuit means to combine the outputs of said first and second circuits to establish a second analog output signal corresponding to said first analog signal; means applying said first and second analog signals to the respective inputs of said subtracting means; and means connecting the output of said subtracting means to said first-mentioned recorder.