Analysis of the fragments offers some details on the bonding motifs in the ion, but the mass spectrum often does not contain the intact molecular ion. The extensive fragmentation and the lack of the molecular ion pose a challenge to the precise measurement of the molecular weight and identity of the compounds present in the analyte. Thus, EI is a harsh technique and not preferred for the analysis of intact atmospheric particles. Softer ionization techniques, such as chemical ionization (CI) and electrospray ionization (ESI), produce molecular ions without inducing fragmentation. ESI has been used to generate atmospheric particles in a laboratory environment, while the chemical constituents of aerosols have been ionized by CI or PI after being sampled and vaporized. PI, CI and ESI are discussed below.