Mass analysis with higher mass analytes was also demonstrated. An infusion-ESI-MS spectrum of a small peptide, thymopentin (RKDVY, (M+H)+ m/z=681), is shown in FIG. 15. Mass analysis was performed at a pressure of 1.3 Torr in ambient air as the buffer gas and at an RF drive frequency of 7.1 MHz. Trapping and analysis of thymopentin demonstrated that the mass range of the mini-CIT could be extended to at least 681 m/z. The largest peak is the doubly protonated species, (M+2H)2+. Under the acidic experimental conditions, this is expected due to the two basic residues present in thymopentin (R and K). In addition, the signal-to-noise ratio (S/N) for thymopentin was significantly greater than the S/N observed for the amino acids. The smaller S/N observed for amino acids versus peptides could be due to less efficient capture of small molecules due to scattering before entering the trap. Despite the difference in S/N between analytes, this simple inlet interface is an effective way of introducing ions from atmospheric pressure into vacuum.
CE-ESI-MS of Peptides
After demonstrating the viability of the atmospheric interface, the miniature CIT system was assessed as a detector for CE separations and compared with a commercial system, the Waters Synapt G2. FIG. 16 shows base peak intensity (BPI) electropherograms of a standard peptide mixture (methionine enkephalin, angiotensin II, bradykinin, and thymopentin) detected with the mini-CIT system and the Synapt G2. Fluorescein was added to the mixture as a dead time marker. Migration times are different due to slightly different field strengths.
