New instrumental developments in cluster ToF-SIMS

In this work, we report on the addition of a 20 keV C60 + primary ion source to a commercial MALDI/ESI mass spectrometer by Applied Biosystems/MDS Analytical Technologies, the QStar XL mass spectrometer. This instrument is equipped with an orthogonal ToF which allows the primary ion source to operate in a continuous fashion as opposed to short, widely spaced pulses typical of traditional instrumentation. This mode of operation allows the delivery of several orders of magnitude more primary ion fluence. In addition, the QStar XL is a multiple quadrupole mass spectrometer which enables tandem mass spectrometry experiments (MS/MS) to aid in ion identification through collision induced dissociation fragmentation analysis.Aside from the differing geometry of the ToF mass analyzer, the most unusual aspect of the QStar XL is the fact that it operates at significantly higher pressures, at least surrounding the sample, than typical high vacuum mass spectrometers. Two different gas inlets result in different pressures at the sample. In collisional cooling mode, the pressure at the sample is &sim1 Torr. This high pressure is required to reduce the fragmentation of electrospray and MALDI generated secondary ions. Unfortunately, utilizing this gas inlet resulted in a significant loss of SIMS generated secondary ions, likely due to scattering, and so proved to be unfeasible. Using the alternate gas inlet at Q0, N2 pressure at the sample is &sim5 x 10 -3 Torr. The vast majority of the experiments described in this thesis are using the Q0 gas configuration which results in collisional focusing, but not collisional cooling. Chapter two deals with the experimental considerations of operating a high voltage, high velocity, primary ion source on an instrument that requires high pressures for operation. Efforts to shield the primary ion beam from high gas include incorporating a differential pumping sleeve and a small aperture nose cone that not only restricts the amount of gas leaking in to the beam column, but also reduces the distance with which the primary ion beam needs to travel in high pressure before reaching the sample. These efforts result in a beam column pressure of less than 1x 10 -6 Torr while the sample region is at normal operating condition of 5 x 10-3 Torr. Beam spot size vs. pressure was measured with an atomic force profilometer on a thin-film of the peptide gramicidin S, and no significant broadening of the beam profile was seen for pressures ranging from < 5 x 10-4 to 8 x 10-3 Torr N2. Additionally, spectra obtained from this prototype instrument do not differ substantially from the those obtained from the high vacuum pulsed beam instrument with regards to degree of sample fragmentation.In chapter 3, comparisons to traditional SIMS instrumentation as well as to MALDI generated data from the QStar XL are made. Orthogonal ToF designs introduce a secondary ion loss that is not present in axial ToF designs, and so secondary ion efficiencies (secondary ions detected/primary ion impact) were of great interest. It was found that the secondary ion efficiencies from SIMS generated data from the QStar XL were similar to the high vacuum pulsed SIMS instrument in this lab. Secondary ion efficiencies for both instruments for indium and gramicidin S are listed in table 3-1.There are some limits to this prototype instrument, primarily in the area of beam size. In chapter 4, it is shown how this instrument can be used in tandem with a high spatial resolution pulsed beam mass spectrometer to improve sub-cellular imaging. MS/MS data of cholesterol taken with the QStar XL led to the discovery of a more prominent cholesterol fragment than is typically used to map cholesterol. The efficacy of using this fragment as an indicator of cholesterol was tested by doping one set of cells with cholesterol and comparing the fragment m/z+ 147, among others, to an undoped cell population. This fragment showed an increase that followed the pseudomolecular ion, m/z + 369 which is commonly chosen as a cholesterol indicator, but at higher intensities.Chapter 5 reports rat brain imaging, single cell sea snail neuron imaging, as well as a demonstration of the ability of MS/MS to differentiate the structural isomers leucine and isoleucine. Large samples such as rat brain sections present some unique challenges for mass spectral imaging. MALDI instruments, such as the QStar XL, have the means to image large samples, but usually only at poor spatial resolution. In this experiment, the unique abilities of a primary ion source coupled to a MALDI mass spectrometer are demonstrated by imaging a rat brain at successively smaller fields of view from 10 mm to 0.5 mm. (Abstract shortened by UMI.)...