Final answer:
A photomultiplier tube is present in a fluorometer but missing in a spectrophotometer; it is used for detecting low levels of fluorescent light and differentiates these two spectroscopic instruments.
Step-by-step explanation:
The component present in a fluorometer that is missing in a spectrophotometer, differentiating the two instruments in the field of analytical chemistry or spectroscopy, is a photomultiplier tube. Fluorometers often use photomultiplier tubes for the detection of fluorescent light, which is emitted at right angles to the excitation light to minimize background interference. This detection method is particularly sensitive and is ideal for measuring the low levels of fluorescent light typically emitted from samples.
Unlike spectrophotometers which generally measure the intensity of light absorbed by a sample, fluorometers are designed to measure the intensity of light emitted from a sample after being excited by a certain wavelength of light. Therefore, the presence of a photomultiplier tube is a key distinguishing feature between these two types of spectroscopic instruments.