Application of Parallel-Detection Electron Energy Loss Spectroscopy in Biology. In: Zierold K, Hagler HK, editors. Electron Probe Microanalysis. Berlin: Springer Berlin Heidelberg; 1989. pp. 113-25. [DOI: 10.1007/978-3-642-74477-8_9]

Quantitative water mapping of cryosectioned cells by electron energy-loss spectroscopy

A direct technique based on electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has been developed to map subcellular distributions of water in frozen-hydrated biological cryosections. Previously, methods for water determination have been indirect in that they have required the cryosections to be dehydrated first. The new approach makes use of spectrum-imaging, where EELS data are collected in parallel at each pixel. Several operations are required to process the spectra including: subtraction of the detector dark current, deconvolution by the detector point-spread function, removal of plural inelastic scattering and correction for the support film. The resulting single scattering distributions are fitted to standard reference spectra at each pixel, and water content can be determined from the fitting coefficients. Although the darkfield or brightfield image from a hydrated cryosection shows minimal structure, the processed EELS image reveals strong contrast due to variations in water content. Reference spectra have been recorded from the major biomolecules (protein, lipid, carbohydrate, nucleic acid) as well as from vitrified water and crystalline ice. It has been found that quantitative results can be obtained for the majority of subcellular compartments by fitting only water and protein reference spectra, and the accuracy of the method for these compartments has been estimated as +/- 3.5%. With the present instrumentation the maximum allowed dose of 2 x 10(3) e/nm2 limits the useful spatial resolution to around 80 nm for +/- 5% precision at a single pixel. By averaging pixel intensities a value of 56.8% with a precision of +/- 2.0% has been determined for the water content of liver mitochondria. The water mapping technique may prove useful for applications to cell physiology and pathophysiology.

Characterization of biological macromolecules by combined mass mapping and electron energy-loss spectroscopy

The combination of scanning transmission electron microscopy (STEM) and parallel-detection energy-loss spectroscopy (EELS) was used to detect specific bound elements within macromolecules and macromolecular assemblies prepared by direct freezing. After cryotransferring and freeze-drying in situ, samples were re-cooled to liquid nitrogen temperature and low-dose (about 10(3) e/nm2) digital dark-field images were obtained with single-electron sensitivity using a beam energy of approximately 100 keV and a probe current of approximately 5 pA. These maps provided a means of characterizing the molecular weights of the structures at low dose. The probe current was subsequently increased to about 5 nA in order to perform elemental analysis. The 320 copper atoms in a keyhole limpet haemocyanin molecule (mol.wt = 8 MDa) were detected with a sensitivity of +/- 30 atoms in an acquisition time of 200 s. Phosphorus was detected in an approximately 10-nm length of single-stranded RNA contained in a tobacco mosaic virus particle (mol.wt = 130 kDa/nm) with a sensitivity of +/- 25 atoms. Near single-atom sensitivity was achieved for the detection of iron in one haemoglobin molecule (mol.wt = 65 kDa, containing four Fe atoms). Such detection limits are only feasible if special processing methods are employed, as is demonstrated by the use of the second-difference acquisition technique and multiple least-squares fitting of reference spectra. Moreover, an extremely high electron dose (about 10(10) e/nm2) is required resulting in mass loss that may be attributable to 'knock-on' radiation damage.

Analysis of directly frozen macromolecules and tissues in the field-emission STEM

A VG Microscopes HB501 field-emission high-resolution scanning transmission electron microscope (STEM) was used to image and analyse rapidly frozen, isolated macromolecules and small organelles in tissue cryosections. Dark-field images were obtained from frozen-hydrated microtubules demonstrating that sufficient contrast is available to reveal structural information. The samples were subsequently freeze-dried in the STEM and low-dose (approximately 10(3) e/nm2) dark-field mass maps were recorded with single electron sensitivity. Elemental analysis of individual macromolecules was achievable at high dose using parallel-detection electron energy-loss spectroscopy, albeit with some structural degradation. Detection of copper (320 atoms) in di-decameric haemocyanin molecules was easily within the limits of sensitivity. Elemental analysis of hydrated cryosections is limited by radiation damage to a resolution of approximately 1 micron2. For freeze-dried sections, however, the high probe current and stable cold stage of the HB501 STEM allow energy-dispersive X-ray (EDX) microanalysis of low elemental concentrations in highly localized subcellular volumes. EDX spectra from cryosections of cerebellar cortex show that a 100-s analysis time is sufficient to quantify the calcium content of 400-nm2 regions within Purkinje cell dendrites with an uncertainty of +/- 2 mmol/kg dry weight, equivalent to +/- 12 atoms.