INDEX Backpressure

Very often baseline problems are related to detector problems. Many detectors are available for HPLC systems. The most common are fixed and variable wavelength ultraviolet spectrophotometers, refractive index, and conductivity detectors. Electrochemical and fluorescence detectors are less frequently used, as they are more selective. Detector problems fall into two categories electrical and mechanical/optical. The instrument manufacturer should correct electrical problems. Mechanical or optical problems can usually be traced to the flow cell however, improvements in detector cell technology have made them more durable and easier to use. Detector-related problems include leaks, air bubbles, and cell contamination. These usually produce spikes or baseline noise on the chromatograms or decreased sensitivity. Some cells, especially those used in refractive index detectors, are sensitive to flow and pressure variations. Flow rates or backpressures that exceed the manufacturer s recommendation will break the cell window. Old or defective source lamps, as well as incorrect detector rise time, gain, or attenuation settings will reduce sensitivity and peak height. Faulty or reversed cable connections can also be the source of problems. 

Viscosity is a temperature-dependent property. For the most commonly used HPLC mobile phase mixtures, the viscosity decreases as the temperature increases, thereby lowering the system operating backpressure. Constant temperature for the solvent reservoir, the connecting tubing, the injector, and the column is necessary to ensure reproducible chromatographic results. The entire system, including the sample, must be heated to the temperature of the detector when a refiactive index detector is used. 

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