Detection limit atomic spectroscopy

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

Atomic absorption spectroscopy is more suited to samples where the number of metals is small, because it is essentially a single-element technique. The conventional air—acetylene flame is used for most metals however, elements that form refractory compounds, eg, Al, Si, V, etc, require the hotter nitrous oxide—acetylene flame. The use of a graphite furnace provides detection limits much lower than either of the flames. A cold-vapor-generation technique combined with atomic absorption is considered the most suitable method for mercury analysis (34). 

Atomic absorption spectroscopy is an alternative to the colorimetric method. Arsine is stiU generated but is purged into a heated open-end tube furnace or an argon—hydrogen flame for atomi2ation of the arsenic and measurement. Arsenic can also be measured by direct sample injection into the graphite furnace. The detection limit with the air—acetylene flame is too high to be useful for most water analysis. 

With modern detectors and electronics most Enei -Dispersive X-Ray Spectroscopy (EDS) systems can detect X rays from all the elements in the periodic table above beryllium, Z= 4, if present in sufficient quantity. The minimum detection limit (MDL) for elements with atomic numbers greater than Z = 11 is as low as 0.02% wt., if the peaks are isolated and the spectrum has a total of at least 2.5 X 10 counts. In practice, however, with EDS on an electron microscope, the MDL is about 0.1% wt. because of a high background count and broad peaks. Under conditions in which the peaks are severely overlapped, the MDL may be only 1—2% wt. For elements with Z < 10, the MDL is usually around 1—2% wt. under the best conditions, especially in electron-beam instruments. 

Atomic absorption spectroscopy of VPD solutions (VPD-AAS) and instrumental neutron activation analysis (INAA) offer similar detection limits for metallic impurities with silicon substrates. The main advantage of TXRF, compared to VPD-AAS, is its multielement capability AAS is a sequential technique that requires a specific lamp to detect each element. Furthermore, the problem of blank values is of little importance with TXRF because no handling of the analytical solution is involved. On the other hand, adequately sensitive detection of sodium is possible only by using VPD-AAS. INAA is basically a bulk analysis technique, while TXRF is sensitive only to the surface. In addition, TXRF is fast, with an typical analysis time of 1000 s turn-around times for INAA are on the order of weeks. Gallium arsenide surfaces can be analyzed neither by AAS nor by INAA. 

The material evaporated by the laser pulse is representative of the composition of the solid, however the ion signals that are actually measured by the mass spectrometer must be interpreted in the light of different ionization efficiencies. A comprehensive model for ion formation from solids under typical LIMS conditions does not exist, but we are able to estimate that under high laser irradiance conditions (>10 W/cm ) the detection limits vary from approximately 1 ppm atomic for easily ionized elements (such as the alkalis, in positive-ion spectroscopy, or the halogens, in negative-ion spectroscopy) to 100—200 ppm atomic for elements with poor ion yields (for example, Zn or As). 

Plasma sources were developed for emission spectrometric analysis in the late-1960s. Commercial inductively coupled and d.c. plasma spectrometers were introduced in the mid-1970s. By comparison with AAS, atomic plasma emission spectroscopy (APES) can achieve simultaneous multi-element measurement, while maintaining a wide dynamic measurement range and high sensitivities and selectivities over background elements. As a result of the wide variety of radiation sources, optical atomic emission spectrometry is very suitable for multi-element trace determinations. With several techniques, absolute detection limits are below the ng level. 

Raman spectroscopy has enjoyed a dramatic improvement during the last few years the interference by fluorescence of impurities is virtually eliminated. Up-to-date near-infrared Raman spectrometers now meet most demands for a modern analytical instrument concerning applicability, analytical information and convenience. In spite of its potential abilities, Raman spectroscopy has until recently not been extensively used for real-life polymer/additive-related problem solving, but does hold promise. Resonance Raman spectroscopy exhibits very high selectivity. Further improvements in spectropho-tometric measurement detection limits are also closely related to advances in laser technology. Apart from Raman spectroscopy, areas in which the laser is proving indispensable include molecular and fluorescence spectroscopy. The major use of lasers in analytical atomic... 

Mullins [ 189] has described a procedure for determining the concentrations of dissolved chromium species in seawater. Chromium (III) and chromium (VI) separated by coprecipitation with hydrated iron (III) oxide and total dissolved chromium are determined separately by conversion to chromium (VI), extraction with ammonium pyrrolidine diethyl dithiocarbamate into methyl isobutyl ketone, and determination by atomic absorption spectroscopy. The detection limit is 40 ng/1 Cr. The dissolved chromium not amenable to separation and direct extraction is calculated by difference. In the waters investigated, total concentrations were relatively high, (1-5 pg/1), with chromium (VI) the predominant species in all areas sampled with one exception, where organically bound chromium was the major species. 

In contrast, the coupling of electrochemical and spectroscopic techniques, e.g., electrodeposition of a metal followed by detection by atomic absorption spectrometry, has received limited attention. Wire filaments, graphite rods, pyrolytic graphite tubes, and hanging drop mercury electrodes have been tested [383-394] for electrochemical preconcentration of the analyte to be determined by atomic absorption spectroscopy. However, these ex situ preconcentration methods are often characterised by unavoidable irreproducibility, contaminations arising from handling of the support, and detection limits unsuitable for lead detection at sub-ppb levels. 

Petit [563] has described a method for the determination of tellurium in seawater at picomolar concentrations. Tellurium (VI) was reduced to tellurium (IV) by boiling in 3 M hydrochloric acid. After preconcentration by coprecipitation with magnesium hydroxide, tellurium was reduced to the hydride by sodium borohydrate at 300 °C for 120 seconds, then 257 °C for 12 seconds. The hydride was then measured by atomic absorption spectroscopy. Recovery was 90 - 95% and the detection limit was 0.5 pmol/1. 

Agemian and Chau [55] have described an automated method for the determination of total dissolved mercury in fresh and saline waters by ultraviolet digestion and cold vapour atomic absorption spectroscopy. A flow-through ultraviolet digester is used to carry out photo-oxidation in the automated cold vapour atomic absorption spectrometric system. This removes the chloride interference. Work was carried out to check the ability of the technique to degrade seven particular organomercury compounds. The precision of the method at levels of 0.07 pg/1, 0.28 pg/1, and 0.55 pg/1 Hg was 6.0%, 3.8%, and 1.00%, respectively. The detection limit of the system is 0.02 pg/1. 

Table 8.4 Atomic spectroscopy detection limits (micrograms/litre) (from Perkin Elmer, Guide to Techniques and Applications of Atomic Spectroscopy, 1988)...

The selection of a technique to determine the concentration of a given element is often based on the availability of the instrumentation and the personal preferences of the analytical chemist. As a general rule, AAS is preferred when quantifications of only a few elements are required since it is easy to operate and is relatively inexpensive. A comparison of the detection limits that can be obtained by atomic spectroscopy with various atom reservoirs is contained in Table 8.1. These data show the advantages of individual techniques and also the improvements in detection limits that can be obtained with different atom reservoirs. 

Limit of detection The method you choose must be able to detect the analyte at a concentration relevant to the problem. If the Co level of interest to the Bulging Drums was between 1 and 10 parts per trillion, would flame atomic absorption spectroscopy be the best method to use As you consider methods and published detection limits (LOD), remember that the LOD definition is the analyte concentration producing a signal that is three times the noise level of the blank, i.e., a S/N of 3. For real-world analysis, you will need to be at a level well above the LOD. Keep in mind that the LOD for the overall analytical method is often very different than the LOD for the instrumental analysis. 

The major anions and cations in seawater have a significant influence on most analytical protocols used to determine trace metals at low concentrations, so production of reference materials in seawater is absolutely essential. The major ions interfere strongly with metal analysis using graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma mass spectroscopy (ICP-MS) and must be eliminated. Consequently, preconcentration techniques used to lower detection limits must also exclude these elements. Techniques based on solvent extraction of hydrophobic chelates and column preconcentration using Chelex 100 achieve these objectives and have been widely used with GFAAS. 

In general, gas chromatography will undoubtedly continue to be the method of choice for characterization of light hydrocarbon materials. New and improved detection devices and techniques, such as chemiluminescence, atomic emission, and mass spectroscopy, will enhance selectivity, detection limits, and analytical productivity. Laboratory automation through autosampling, computer control, and data handling will provide improved precision and productivity, as well as simplified method operation. 

In 1C, the election-detection mode is the one based on conductivity measurements of solutions in which the ionic load of the eluent is low, either due to the use of eluents of low specific conductivity, or due to the chemical suppression of the eluent conductivity achieved by proper devices (see further). Nevertheless, there are applications in which this kind of detection is not applicable, e.g., for species with low specific conductivity or for species (metals) that can precipitate during the classical detection with suppression. Among the techniques that can be used as an alternative to conductometric detection, spectrophotometry, amperometry, and spectroscopy (atomic absorption, AA, atomic emission, AE) or spectrometry (inductively coupled plasma-mass spectrometry, ICP-MS, and MS) are those most widely used. Hence, the wide number of techniques available, together with the improvement of stationary phase technology, makes it possible to widen the spectrum of substances analyzable by 1C and to achieve extremely low detection limits. 

Analysis. The colorimetric method for In is capable of a detection limit of 20 ppb. Indium or an In compound in the flame gives an indigo blue color (451.1 nm). This photon line allows for the spectrophotometric determination ofinby AAS (atomic absorption flame spectroscopy). The method is sensitive to about 300 ppb. With ETAAS, this limit drops to 10 ppb, as it does with ICPAES. ICPMS drops the limit to 0.01 ppb. Alizarin detects In, as well as Al, but the reaction with Al can be masked by addition of F to a spot test. The limit of detection is about 1 ppm. 

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