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Earth Observation

 

 

Earth Observation in the frame of EO-MINERS - Earth Observation methods

Geo- and Hydrochemical

Chemical analytical methods aim at the identification and quantification of chemical elements, compounds and their species. These methods can be grouped into destructive and non-destructive methods. Their majority would be considered invasive, while some non-destructive methods can be considered non-invasive, meaning that no samples as such have to be taken. Many analytical methods are well established for many decades, but the scientific technical development continuously strives to lower detection limits as well as to improve specificity and ease of application. Many new instrument-base techniques have been developed over the past decades. Spectroscopic methods in particular have profited from the development of digital computing. Analog-digital data capturing and complex processing routine greatly facilitate and speed-up analyses. Automated sample handling contributes to repeatability and precision in many cases.

Being a well-established scientific and practical field, for most methods standard procedures have been developed and set down in national (e.g. DIN, ASTM) or international standards and norms (e.g. ISO).

 

Sampling and sample preparation - Destructive methods require the taking and preparing of samples of waters or solid geological materials. While sample preparation for water samples usually is minimal, many analytical techniques require the material to be present as solution, so that more or less evolved procedures for solid samples have to be followed. Sampling procedures are designed to minimise disturbances and the creation of procedural artefacts. One frequent concern is oxidation of waters or sediments that are brought to the surface. Another concern is the sorption of trace components from water samples to containers. While standard sampling procedures and protocols exist and are laid down in DIN, ASTM etc. norms, certain problems may require the development of specific procedures.

In natural waters, chemical elements and compounds can be either present in solution, or as suspensions. Thus, a separation of the dissolved fraction by filtration may be needed and often is the only preparation step.
Depending on the problem in hand one may be interested in the total elemental composition of a geological material, in the fraction that can be easily dissolved, in elements that are adsorbed onto minerals and so forth. Accordingly, specific sample preparation procedures exist that aim to bring only the fraction of interest into solution for analysis.

physico-chemical measurements
Taking water samples and performing physico-chemical measurements in the field (from www.eurogeosurveys.org)

 

Physico-chemical methods - A water analysis typically comprises the determination of a number of physical and physico-chemical parameters, including temperature, conductivity, pH-value, redox-potential, or the concentration of dissolved volatile gases, e.g. O2. These parameters are easily disturbed so that they have to be determined either in situ or at least on site immediately after sampling.

 

Classical wet-chemical methods - are largely superseded for practical analytical purposes by instrument-based techniques such as mass-spectroscopy and ion chromatography. One exception are (potentiometric) titration methods to determine the alkalinity (carbonate and hydrogen-carbonate ions). Wet chemical methods, for practical reasons, usually require larger sample quantities, tens of ml to litres, depending on concentrations and pre-concentration needs. This makes them more intrusive compared to the methods described below for which a few ml or less may be required.

 

Ion-chromatography - the water sample to be analysed is injected into a column containing a stationary phase (resin or gel) of the opposite charge (positive for anions, negative for cations) of the ions to be separated. Different ions appear at different times when eluted. The ions are detected in either a conductivity or UV/VIS spectroscopy cell. This technique, which entered the commercial market in the 1980s, now has virtually replaced all wet-chemical analyses for common anions (e.g. nitrate, sulfate, phosphate), except the carbonate and hydrogencarbonate ions. IC is also used to determine higher molecular weight constituents, such as humic and fulvic acids. Unless particular species are to be determined, other methods are typically used to determine (total) cation concentrations.

chromatography
The principle of (ion-)chromatography (from serc.carleton.edu)

 

High-Performance Liquid Chromatography - HPLC is the more generic form of IC and generally applied to separate and determine non-ionic, non-polar organic contaminants. A wide variety of stationary phases and eluents can be used to suit the problems. Likewise various detection methods are commonly employed. The separation can be based on chemical, physico-chemical, or physical interaction of the compound(s) of interest with the stationary phase. A detailed review of all the possible applications and variants is beyond the scope and purpose of this short review of common analytical techniques.

 

UV/VIS Photometry/Spectroscopy - While IC has largely replaced classical photometric and spectroscopic methods for the quantitative determination of anions, the latter are still used for quick, semi-quantitative, process water and field analyses. Certain anions, such as nitrite or nitrate exhibit fluorescence when exited using certain bands in the UV spectrum, which can be used for analytical purposes. In other cases complexes of cations or anions with certain ligands show concentration-dependent absorption or fluorescence that can used for analytical purposes.

spectroscopy diagram
The principle of UV/VIS spectroscopy (from www.chem.agilent.com)

A wide variety of atomic spectroscopy methods have been developed and made commercially available over the past forty years to quantitatively determine elemental concentrations in samples. They are based on absorption, emissions or fluorescence of vaporised samples.

 

Atomic absorption spectroscopy - AAS is based on the weakening by absorption of an element-specific band of visible or UV-light. The sample is either vapourised and injected into a flame or vapourised in a heated graphite tube (mainly for volatile elements such as Hg) that are located in the light-path of the instrument. The absorption is proportional to the concentration in solution of the element. For each element a specific, monochromatic light source ('lamp') has to be employed. More sophisticated variants of these rather old methods have been developed.

Atomic absorption spectroscopy
The principle of AAS (from www.chromatography-online.org).

 

Atomic emission spectroscopy - AES is based on stimulating the elements of interest to emission. This commonly done in a high-temperature flame (flame atomic emission spectrophotometry, F-AES) or in an inductively coupled plasma (inductively coupled plasma optical emission spectrometry, ICP-OES). The plasma can also be created by microwaves: microwave plasma torch atomic emission spectrometry, MPT-AES.

Atomic emission spectroscopy
The principle of AES (from www.chromatography-online.org).

 

Mass spectrometry - MS is an analytical technique that measures the mass-to-charge ratio of charged particles. Chemical compounds are ionised, e.g. by an inductively couples plasma, to generate charged molecules or fragments of molecules. In an electromagnetic field the ions are separated according to their mass and charge. The spread-out ion-beam is detected e.g. in array of electron multipliers or Faraday collectors. ICP-MS is mainly used for analysing cations, including their isotopic composition, while for other compounds a wide variety different techniques of ionisation has been developed. While typically samples have to present in liquid form, with more recent developments, e.g. laser ablation, it is also possible to remove ions from solid surface and ionise them.

The principle of AES (from bgr.bund.de).

 

X-ray fluorescence spectroscopy - When solid samples are bombarded with high-energy X- or -rays, atoms are exited, resulting in fluorescence. The emitted atom-specific X-ray fluorescence (XRF) spectrum can be analysed qualitatively and quantitatively. This non-destructive method can be operated in energy-dispersive instruments (EDX) or wavelength-dispersive instruments (WDX). The X-ray source is pointed at an angle onto the sample surface. In EDX the florescent radiation is directed into photo-sensitive semi-conductors. The resulting voltage pulse is proportional to the energy of emitted photons. The resulting spectrum is processed and compared against standards. In WDX the fluorescent radiation is directed onto a diffraction grating monochromator that is adjusted to pick-up single wave-lengths. Wavelengths and energies are specific to each element. Due to fluorescent efficiencies, in principle the lightest element that can be analysed is Be (Z=4), but in practice it is difficult to analyse elements lighter than Na (Z=11). As this is an optical method, sample geometry and surface roughness are important. Solid samples are prepared as polished discs, while loose material, such as sand or clay is ground into a powder and pressed into tablets with the aid of a light-element binder. The analytical semi-conductors have to be cooled, either by liquid nitrogen or using peltier elements. In order to avoid absorption and scattering of the exciting radiation as well as the fluorescence the whole beam assembly, including the sample, in laboratory configuration is put into a vacuum chamber. For this reason hand-held, gun-shaped XRF devices that allow rapid qualitative and semi-quantitative determination of elemental surface concentrations are less sensitive and only suitable for heavier elements.

Hand-held XRF-probe diagram
Hand-held XRF-probe (from www.nachi.org).

 

Scanning electron microscopy with energy-dispersive X-ray spectroscopy - SEM/EDAX is an XRF method performed in a scanning electron microscope. Today various methods to excite elements can be used, X-rays, electron- or proton-beams (PIXE). The possibility to tightly focus the beams (Ø <1µm) and to modulate their energy allows very detailed elemental surface characterisation and depth penetration. Although SEM with EDAX or PIXE is a well-established system, they border at more research-oriented methods, rather than being routine analytical methods.

principle of SEM/EDX diagram
The principle of SEM/EDX (from www.vub.ac.be)

The methods described are the more routinely performed analytical techniques. In addition to these a vast array of analytical techniques and procedures has been developed particularly over the past 30 years to address specific questions and scientific problems. Their detailed review, again, is beyond the scope and purpose of this report. These specialised techniques are not likely to be employed in support of the candidate indicators.

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