Twenty of the common amino acids were chosen as the model analytes for the development of the microchip to MS interface. The Infusion-ESI microchip was used in development of the interface so a constant source of ions was present. Representative Infusion-ESI-MS spectra of four amino acids (arginine, histidine, glutamic acid and proline) collected using the atmospheric interface and differential chamber setup are shown in FIG. 14. Mass analysis was performed at a pressure of 1.2 Torr with ambient air as the buffer gas at a drive frequency of 10.2 MHz. Each spectrum is an average of 1000 individual mass spectral scans. The (M+H)+ peak of each amino acid is clearly detected, which provides sufficient information for identification of these species. In the case of histidine and glutamic acid, some fragmentation is also observed. ESI is a soft ionization technique, but operation at high pressures results in increased ion-buffer gas collisions, which can impart the energy required to induce fragmentation. These fragmentation patterns may aid in the identification of chemical species, including the differentiation of isobars. Detection of the twenty common amino acids demonstrates the ability to detect a wide range of analytes varying in size, polarity, and basicity.
