Question

Describe at least one analytical technique that you could use that is more sensitive than Raman spectroscopy, FTIR spectroscopy, and energy-dispersive X-ray spectroscopy (EDS) analyses, but that might be destructive in its analysis of the material. Describe the benefits and other limitations of this technique.

Answer 1

1)Raman spectroscopy relies upon inelastic scattering of photons, known as Raman scattering. A source of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range is used, although X-rays can also be used. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. Infrared spectroscopy typically yields similar, complementary, information.

2) The Energy Dispersive X-ray (EDX) microanalysis is a technique of elemental analysis associated to electron microscopy based on the generation of characteristic Xrays that reveals the presence of elements present in the specimens. The EDX microanalysis is used in different biomedical fields by many researchers and clinicians. Nevertheless, most of the scientific community is not fully aware of its possible applications.

A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the intensity of the detected signal to produce an image. In the most common SEM mode, secondary electrons emitted by atoms excited by the electron beam are detected using a secondary electron detector (Everhart-Thornley detector). The number of secondary electrons that can be detected, and thus the signal intensity, depends, among other things, on specimen topography. SEM can achieve resolution better than 1 nanometer.

The signals used by a scanning electron microscope to produce an image result from interactions of the electron beam with atoms at various depths within the sample. Various types of signals are produced including secondary electrons (SE), reflected or back-scattered electrons (BSE), characteristic X-rays and light (cathodoluminescence) (CL), absorbed current (specimen current) and transmitted electrons. Secondary electron detectors are standard equipment in all SEMs, but it is rare for a single machine to have detectors for all other possible signals.

Secondary electrons have very low energies on the order of 50 eV, which limits their mean free path in solid matter. Consequently, SEs can only escape from the top few nanometers of the surface of a sample. The signal from secondary electrons tends to be highly localized at the point of impact of the primary electron beam, making it possible to collect images of the sample surface with a resolution of below 1 nm. Back-scattered electrons (BSE) are beam electrons that are reflected from the sample by elastic scattering. They emerge from deeper locations within the specimen and, consequently, the resolution of BSE images is less than SE images. However, BSE are often used in analytical SEM, along with the spectra made from the characteristic X-rays, because the intensity of the BSE signal is strongly related to the atomic number (Z) of the specimen. BSE images can provide information about the distribution, but not the identity, of different elements in the sample. In samples predominantly composed of light elements, such as biological specimens, BSE imaging can image colloidal gold immuno-labels of 5 or 10 nm diameter, which would otherwise be difficult or impossible to detect in secondary electron images.

The advantages of a scanning electron
microscope include its wide-array of
applications, the detailed three-dimensional
and topographical imaging and the versatile
information garnered from different detectors.
SEMs are also easy to operate with the proper
training and advances in computer technology
and associated software make operation user-
friendly. This instrument works fast, often
completing SEI, BSE and EDS analyses in
less than five minutes. In addition, the
technological advances in modern SEMs
allow for the generation of data in digital
form. Although all samples must be prepared
before placed in the vacuum chamber, most
SEM samples require minimal preparation
actions.

Electron Microscope Disadvantages:
The main disadvantages are cost, size, maintenance, researcher training and image artifacts resulting from specimen preparation. This type of microscope is a large, cumbersome, expensive piece of equipment, extremely sensitive to vibration and external magnetic fields.