Secondary-ion mass spectrometry (SIMS) is an important tool for investigating isotopic composition in the chemical and materials sciences, but its use in biology has been limited by technical considerations. Multi-isotope imaging mass spectrometry (MIMS), which combines a new generation of SIMS instrument with sophisticated ion optics, labeling with stable isotopes, and quantitative image-analysis software, was developed to study biological materials.
The elemental composition of melanin granules and other components of the hair shaft was determined by multi-isotope imaging mass spectrometry, a method with unique advantages for the visualization and quantification of stable isotopes and the elemental composition in study of the fine structure of biologic samples. We mapped and quantified the chemical composition of hair cross-sections using secondary ions generated from naturally occurring 16O, 12C14N, 32S, and 34S with a maximum lateral resolution of 35 nm. Based on these elemental maps of unprecedented resolution we obtained simultaneously the chemical fingerprints and the structural features, such as cuticle, melanin granules, the macro fibrils of the cortex, and small sulfur-rich domains in the medulla, in the hair cross-section. We found an intriguing distribution of 16O, 12C14N, and 32S in melanin granules that we interpret as a highly anisotropic pattern of oxidation.
We report the measurement of the natural isotope ratios of nitrogen and carbon in subcellular volumes of individual cells among a population of cultured cells using a multi-isotope imaging mass spectrometer (MIMS), [MIMS is the prototype of the NanoSIMS 50, Cameca, France.] We also measured the nitrogen and carbon isotope ratio in cells after they had been cultured in media enriched with the amino acid glycine labeled with either 13C or 15N. The results demonstrate that 13C/12C and 15N/14N isotope ratios can be measured directly on a subcellular scale. This opens the way for the use of stable isotopes, in particular 15N, as labels to measure the intracellular turnover of biomolecules. Such a capability should help resolve a wide range of biomedical problems.
The mechanism of long chain free fatty acid (FFA) transport across cell membranes is under active investigation. Here we describe the use of multi imaging mass spectrometry (MIMS) to monitor intracellular concentrations of FFA and provide new insight into FFA transport in cultured adipocytes. Cells were incubated with 13C-oleate:BSA and either dried directly or dried after washing with a medium deprived of 13C-oleate:BSA. Cells were analyzed with MIMS using a scanning primary Cs+ ion beam and 12C-, 13C-, 12C14N-, 13C14N-) (or 12C 15N-) were imaged simultaneously. From these quantitative images the values of the 13C/ 12C ratios were determined in the intracellular lipid droplets, in the cytoplasm and outside the 3T3F442A adipocytes. The results indicate that after incubation with 13C-oleate:BSA the droplet 13C/ 12C ratio was 15 +/- 6%. This value is about 14-fold higher than the 13C/ 12C terrestrial ratio (1.12%). After washing the 13C-oleate:BSA, the droplet 13C/ 12C ratios decreased to 1.6 +/- 0.1%, about 40% greater than the natural abundance. Results for washed cells indicate that relatively little FFA was esterified. The unwashed cell results, together with the value of the lipid water partition coefficient, reveal that intracellular unbound FFA (FFAu) concentrations were on average about 4.5-fold greater than the extracellular FFAu concentrations. These results are consistent with the possibility that FFA may be pumped into adipocytes against their electro-chemical potential. This work demonstrates that MIMS can be used to image and quantitate stable isotope labeled fatty acid in intracellular lipid droplets.
We have designed a vacuum bench to study the parameters of thermoionization sources with the ultimate goal of obtaining high spatial resolution for biomedical applications of secondary ion mass spectrometry. In the bench, the source ionizer can be directly heated with an electron gun positioned perpendicular to the axis of the ion beam and focused with an optical system including slit lenses and a magnetic sector. The source cross over diameter is measured by forming the image of the source using an Einzel lens at a 1×magnification. The ion beam current is measured in a Faraday cup placed after a movable diaphragm. The temperature of the diverse elements of the ionizer assembly is measured through a mirror with a micropyrometer. Using the vacuum bench with a cesium carbonate source, we measured a 35 μm minimum cross over size, and we calculated a 400 A/cm2/srmaximum brightness. We obtained an intense cesium ion beam when heating the ionizer with the electron gun. The vacuum bench will be used to compare the effect of the heating mode of the ionizer (i.e., indirect by filament electron emission or direct by electron beam) on the brightness of the cesium source, and to develop a thermoionization iodine negative ion source.