Answer 15.4

a) Imaging mass spectrometry, now popular with MALDI-MS, has originally been deleveloped in inorganic MS. Which method has been used in the very beginning of mass spectral imaging?

Secondary ion mass spectrometry has first been used to create images of element distributions on surfaces that can then be related to macroscopic structure of a sample, e.g., composition of grains of rock, structure of electronic circuitry, homogeneity of alloys. Today, SIMS can achieve lateral resolution down about ten micrometers.

b) Lasers can be used to ablate material from rather well defined spots on surfaces. Which ionization method is generally combined with that process for element imaging purposes?

Although laser ablation ICP-MS, LA-ICP-MS, does not offer the high lateral resolution of SIMS, LA-ICP-MS still reveals sufficient detail to correlate microscopic images of tissue slices with element distributions along physiological structures.

c) What is the typical size of MS images in terms of numbers of pixels along each coordinate? Explain reasons for that.

The “chemical map” created by mass spectral imaging is normally just 256 x 256 pixels wide. Compare this to digital photographic images, e.g., a common 10 MP picture measures 3888 x 2592 pixels. However, each of the 65,535 pixels of the mass spectral image is related to a full mass spectrum. Thus, assuming 0.1 MB of data per spectrum requires a total of 6.5 GB of disk space while a JPEG, for example, just requires 3 bytes per pixel (1 byte per channel for RGB color).