Accurate calibration of on-line Time of Flight Mass Spectrometer (TOF-MS) for high molecular weight combustion product analysis

A novel apparatus based on Time of Flight Mass Spectrometry (TOF-MS) was recently developed and successfully applied for the on-line analysis of combustion products from atmospheric pressure combustion systems. The main advantage of this apparatus is the possibility of real-time mass spectra detection of big molecules nominally up to thousands of mass units and therefore it is very promising for its application in soot precursors detection above the polycyclic aromatic hydrocarbons (PAH) masses. However, in order to detect masses in so high mass range, accurate calibration by standard molecules in similar mass ranges is necessary. In gas phase the only standards generally employed are constituted by gas noble mixtures, therefore arriving at 136 Da. In the present work the accurate calibration of TOF-MS system is presented by using water clusters formed by condensation of water vapor in the supersonic beam as it passes from the atmospheric to low pressure region of the source through an orifice. Water clusters up to 160 water molecules have been detected, therefore extending the calibration range up to more than 3000 Da, in the optimal operative conditions. More than the large detection extension, the other advantage of using water clusters for calibration is the possibility of regulating the operative conditions in order to form the clusters from combustion water. This can allow to calibrate the system for the analysis of combustion products before each set of measurements, without any necessity of changing the injection system for introducing external standard molecules.