INTRODUCTION AND PRINCIPLES

Frequently, analytical problems are not considered to be solved until both the identity and the quantity of a substance have been determined. In the areas of drugs and environmental science, quantitative analysis is particularly important. For example, the threat posed by a particular pollutant depends on the amount of it that is in the environment, and the quantity of a drug reaching the active site will determine the extent of the biological effect. Given that some substances exert an effect on their surroundings at very low concentrations, methods are often required to be highly sensitive as well as quantitative. The technique of mass spectrometry, known for its great analytical sensitivity, is clearly a candidate for quantitative analysis.

Estimation of quantities of substances by mass spectrometry is not straightforward because mass spectrometric measurements are not exactly reproducible (section 1.2). The response to a sample at the detector depends on several parameters that are difficult or impossible to control, including the condition, temperature and pressure of the ion source and the condition of the detector. It is important to recognize that equimolar amounts of different compounds do not give an equal response because only a proportion of the total number of molecules is ionized and this proportion (the ionization efficiency) depends partly on molecular structure.