Base line correction method and apparatus

A liquid chromatography system for performing programmable solvent gradient scans, comprising

• a liquid chromatograph,
• a spectrophotometer connected to said chromatograph,
• and apparatus for compensating for base line shift as the proportions of solvents are changed comprising
• a digital data memory for storing a series of digital numbers representing the required values of base line shift correction signals at spaced time intervals during a solvent gradient program,
• control means for reading out said digital numbers in sequence at spaced time intervals,
• a digital-to-analog converter connected to receive said base line shift correction signal numbers and to convert each number to a corresponding base line correction signal voltage,
• a sample-and-hold circuit,
• switch means for connecting the voltage from said digital-to-analog converter to said sample-and-hold circuit at spaced time intervals as different base line shift correction signals are available at the output of said digital-to-analog converter, and
• an offset amplifier having at least two input signals,
• one input signal to said offset amplifier being received from said spectrophotometer,
• the other input signal of said offset amplifier being received from said sample-and-hold circuit for subtracting the base line shift correction signals stored within said sample-and-hold circuit from the signals received from said spectrophotometer,
• the output of said offset amplifier comprising the base line shift corrected output of said liquid chromatography system.