An HPLC detector is used to analyze the components of the eluted mixture that is collected after being run through an HPLC column. Differences in detector type are based on what is being detected. Detection can be done by measuring absorbance, changes in reflex indices, molecular weight, and fluorescence among others.
When selecting a detector, it is important to consider the sample type being analyzed. Some sample types will not work or will have lower sensitivity on particular detectors. Also of consideration is the type of information that is being sought by the analysis and what method will be used when running the column, for example, some detectors will work with the gradient method while others will not.