Chemistry Reference
In-Depth Information
spectrometer. This technique is known as electron probe microanalysis or microbeam X-ray fluorescence
spectrometry (
-XRF) [105]. Excitation is restricted to thin surface layers because the electron beam
penetrates to a depth of 1 or 2 mm into the specimen.
In X-ray photoelectron spectroscopy (XPS), the probe is an X-ray photon and the detected particle is the
photoelectron emitted by it [106].
The excitation of the atoms by direct bombardment of the sample with electrons is the basis of Auger
emission spectroscopy [107]. Once an atom is ionized, it relaxes by emitting an electron with an energy
characteristic of the atom. The basic advantages of this technique are its high sensitivity for chemical analysis
in the 5 to 20 Å region near to the surface, a rapid data acquisition speed, its ability to detect all elements
above helium and its capability of high-spatial resolution making Auger emission spectroscopy a really
important tool for solving problems related to surface analysis [108].
Although complex experimental equipment is required, measurement in X-ray absorption methods is
straightforward. In X-ray absorption the intensity of an X-ray beam is diminished as it passes through the
sample material.
μ
6.3.6
Other surface analysis techniques
Other commonly used surface spectroscopy techniques for analyzing the composition and chemistry of solid
surfaces are secondary-ion mass spectrometry (SIMS) and ion scattering spectroscopy (ISS).
In SIMS, secondary ion emission results from the collision of high energy particles with the sample. After
the impact, most chemical bonds are broken and atoms or combinations of atoms are released from a few
atomic layers of the sample surface [109]. In practice, the penetration depth is around a few tens of nm.
However, the major part of the emitted ions arises from the atoms present in the first 2 nm. The use of mass
analyzers; such as double focusing magnetic sector, quadrupole mass analyzer and time of flight spectrometer,
allows the recording of sample images with an appropriate spatial resolution. Two types of SIMS can be
identified: 'static'' SIMS, introduced by Benninghoven [110] and 'dynamic' SIMS [111]. In dynamic SIMS,
elemental information can be extracted from the sample. On the other hand, static SIMS measurements
provide molecular information.
Low-energy ion scattering (LEIS), also called ISS, is an unique tool in surface analysis, since it provides
the atomic composition of the outer atomic layer. In LEIS a sample is bombarded with noble gas ions (He
+
,
Ne
+
or Ar
+
) at an angle smaller than 60° respect to the surface normal and only projectiles that are backscattered
into a certain angle, typically 140°, are analyzed [112]. For quantitative surface analysis, only the signal of
scattered ions, S
, is analyzed. One of the main advantages of LEIS is that it offers the possibility to analyze
a great diversity of materials, from metals, oxides and polymers to even liquid surfaces.
+
6.4
New challenges in direct analysis
In the previous sections a literature survey of the main techniques available for direct analysis of samples
without any sample damage or with a reduced physical erosion of the sample surface has been made. In fact,
all the mentioned techniques offer fast, easy to perform and environmentally friendly alternatives to the
classical methods based on sample digestion and/or analyte extraction and, from our point of view, the main
aspects of all discussed alternatives comes from the fact that no reagents and no sample treatments are
required; thus avoiding the use of toxic reagents and the generation of dangerous wastes. However, additional
efforts are required in order to improve standardization of measurements and also chemometric efforts will
be welcome in order to obtain as much as possible information from the sample signals. So, in the next few
years we could perform many new applications of remote sensing and non-invasive methods based on the
