A mass spectrometric imaging experiment was performed to characterize the homogeneity of a MALDI spot prepared by ES. The experiment was performed on a PE Biosystems Voyager Elite STR at Lawrence Livermore National Laboratory. Access to the instrument was provided by Dr. Sharon Shields and her cooperation is greatly appreciated.
Mass spectrometric imaging is straightforward. Spectra are acquired in a pattern defined by the user and stored individually. Figures 1a and 1b are pictures of the spot that was analyzed. The spot is 800 um in diameter and was sampled every 100 um in the x and y directions. A 9 X 9 array (red dots in Figure 1b) was produced corresponding to 81 individual spectra. Each spectrum was the sum of 25 laser shots. All spectra were acquired with exactly the same instrument parameters (e.g, laser fluence, acceleration voltage, m/z range etc…) so that they could be equally compared. Figures 2a and 2b contain one of the spectrum acquired from the spot in Figure 1. Figure 2b is an expansion of the region around m/z 1607 to illustrate the resolving power of the instrument.
The two major peaks at m/z 1272 and 1607 were used to compare signal homogeneity across the sample surface. Integration of the peak profiles for each ion were performed for all 81 spectra and the data is plotted in Figures 3 and 4. The color surface plots are nearly identical indicating that both ions are similarly distributed in the sample. That is, there are no local regions where the ratio of the ion abundances is significanlty different than that observed in Figure 2. The overall abundance of the peak at m/z 1607 is greater than the abundance for m/z 1272