Mass Spectrometer Interface
The transport of ions from the elliptical lens to the MS was investigated by comparing the ion signal recorded by the MS using the elliptical electrode to the intensities recorded by nanoESI without the electrode but with the same tip to inlet distance. When the elliptical electrode was used potentials of 3 and 4 kV were supplied to the ellipse and sprayer, respectively, while the spray tip was 22 mm from the MS inlet. For the study of intensities without the use of the ellipse, the sprayer was again positioned 22 mm from the inlet and was shielded from air currents that might disrupt the signal intensity. The potential applied to the sprayer in this case was 1 kV to match the offset potential used when the elliptical electrode
Fig. 2.5 Intensities of different ions detected by MS as a function of potential applied to elliptical electrode: a sprayer potential held 1 kV higher than ellipse potential throughout scan and b chromatograms of ion intensities using the ellipse electrode (solid lines) and without the ellipse electrode (dashed lines). Potentials of 3 and 4 kV were applied to the ellipse and sprayer, respectively. For nanoESI without the elliptical electrode, spray potential was 1 kV. In a the sprayer was 27 mm from the inlet of the LTQ. Tip to inlet distance for Fig. 2.6b was 22 mm
Fig. 2.6 a Spectrum recorded for LTQ calibration solution using the elliptical electrode with potentials of 6 and 5 kV applied to the sprayer and ellipse, respectively and b spectrum taken for LTQ calibration solution without the use of the focusing electrode at a spray potential of 1 kV. The spray tip to inlet distance was 22 and 3.3 mm in a and b, respectively
was employed. Figure 2.5b shows the result of these experiments by plotting a chromatogram of several ions characteristic of the calibration solution.
The results shown in Fig. 2.5a clearly indicate that increasing the potential of the electrode and sprayer results in an increased number of ions delivered to the mass spectrometer. With the larger ion (m/z 1322), the intensity increase is not as dramatic. As larger ions are less mobile than their smaller counterparts, they are transported to the inlet at a slower rate so that less signal is observed. The result is an increased sensitivity for smaller ions when using the elliptical electrode. Up to an 100 fold enhancement of ion signal was achieved with the use of the elliptical electrode at distances of several centimeters. It must be noted that intensities higher than those obtained with the elliptical electrode are possible through the use of nanoESI alone. This is accomplished by placing the spray tip in close proximity (25 mm) to the inlet; however, this does not always allow for sufficient evaporation of solvent and the spectra obtained are remarkably different in regards to relative ion intensity. Additionally, operation in this manner has the potential to introduce a large amount of contamination on the inlet of the mass spectrometer, manifested as carryover between experiments. One exemplary result of the use of the elliptical electrode was a 4-fold increase in the signal to noise ratio for the detection of MRFA peptide (m/z 524) over that achieved even at optimum proximity for nanoESI without the focusing electrode (Fig. 2.6).
The congested appearance of the m/z 150-300 range in Fig. 2.6a also supports the conjecture of a mobility dependent ion delivery when using the ellipsoidal electrode as these ions are the same as those seen in low abundance in Fig. 2.6b. A spray distance of several centimeters is impractical for normal nanoESI analysis, but this distance allows for the inclusion of additional sprayers at different distances when the objective of the experiment is to obtain large ion currents. Such cases include ion soft landing, surface modification using ions under ambient conditions, as well as the exploration of novel chemistry observed in ambient ion-molecule reactions in which higher yields remain an issue for practical use.