Sources of background

Since the minimum detection limit is controlled by the sensitivity for the element and the background contribution, it is important to understand the sources of background in order to reduce their contribution for the trace element analysis. The sources of background are described in the following sections. 

Since both coherent and incoherent scattering are involved, there is broadening of the characteristic lines, depending on the line energies and the spectrometer resolution. Frequently the X-ray tube spectrum contains imwanted characteristic lines from materials used in the anode and window construction. These lines become interfering peaks as they scatter from the specimen just like the major characteristic anode lines. 

The maximum energy of free electrons knocked on in this way is given by Tmax = + where m is the mass of the electron E, M are 

Secondary Electron Bremsstrahlung (SEB) - cause of low energy background and is produced by the secondary electrons ejected from the target atoms during irradiations. 

Projectile Bremsstrahlung (PB) of the bombarding particles slowed down in close collisions with the matrix nuclei. 

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Minimizing Spectral Interferences The most important spectral interference is a continuous source of background emission from the flame or plasma and emission bands from molecular species. This background emission is particularly severe for flames in which the temperature is insufficient to break down refractory compounds, such as oxides and hydroxides. Background corrections for flame emission are made by scanning over the emission line and drawing a baseline (Figure 10.51). Because the temperature of a plasma is... 

Cq ), where is the blended impurity concentration of impurity a Cq, the background impurity level and the multiplication constant. Possible sources of background response include instmment noise, sample system outgassing, or interference from other impurity response signals. Proper setup, purging, and operation of the instmment should reduce background levels weU below ippb. 

The accuracy with which a system can measure lifetimes depends on a number of different factors including calibration of the instrument, the number of detected photons and also the efficiency of the analysis routines. In addition, sources of background and scattered light should be eliminated. Emission filters should be chosen with great care to make sure that no scattered laser light reaches the detector. Detection of scattered excitation light results in a spurious fast component in the decay and complicates the interpretation of the data. The choice of emission filters is much more critical in FLIM than in conventional fluorescence intensity imaging methods. 

In space-resolved immunoassays, a smooth metal slide is coated with an antibody monolayer, and a parallel laser beam is used to quantitate surface bound fluorophore.1(37,38) Scattered light is low since the excitation is reflected into a different space, although scatter still remains the principal source of background. 

If the emittivity ex really is a number < 1 determined via Eq. (4), it should not be possible to see emission except when s is positive. This statement is only compatible with experiment if it is accepted that a source of background e exists in hot colourless substances. It is not excluded that various kinds of colour centers develop in white samples in particular when heated in flames containing chemically reactive molecules. However, iiex — Io — I)lIo it is very difficult to explain why different mantles of comparable composition do not readily fall in one of the two extremes, either ex close to 0 or close to 1. For instance, the absorbance = 0.3 corresponds to = 0.5 and ex changes to 0.968 or to 0.129 when modifying ec/ by a factor of five. 

A selection of the most successful CSPs, chiral particles and chiral additive techniques used for analytical and preparative enantioseparation by LC is discussed in the following sections with respect to molecular recognition and experimental application. As additional sources of background information recent books and review articles2-16, which contain numerous relevant references and examine the most important aspects of the field of liquid chromatographic enantioseparation, should be consulted. 

Other sources of "background" cancer are trace levels of chemicals found in drinking water, food, building materials, and common activities. Such exposures are not natural, but they are ubiquitous because they are associated with elements of modern life that most people deem essential. For example, some of the chlorine used to chlorinate public drinking water forms chloroform. Vapors from dry-cleaned clothes (tetra- and trichloroethylene) mix with the air in closets. Building materials contain formaldehyde. Dioxins, furans, and PCBs are found in the food chain. 

The National Institute on Alcohol Abuse and Alcoholism (NIAAA) of the U.S. National Institutes of Health at http //www.niaaa.nih.gov/ is a good source of background information and news about current clinical trials and research studies, as well as access to various databases and treatment referrals. 

Two texts are good sources of background information one is edited by A. W. Czanderna (2) and the other by P. E. Kane and G. B. Larabee (3). If the reader has a deeper interest in the subject, the primary journals in the field are Surface Science and the Journal of Vacuum Science and Technology. Adequate descriptions of the equipment are available in the introductory reviews. 

K is a (3 -emitting nuclide that is the predominant radioactive component of normal foods and human tissue. Due to the 1460-keV 7 ray that accompanies the (3 decay, it is also an important source of background radiation detected by 7-ray spectrometers. The natural concentration in the body contributes about 17 mrem/y to the whole body dose. The specific activity of 40K is approximately 855 pCi/g potassium. Despite the high specific activity of 87Rb of 2400 pCi/g, the low abundance of rubidium in nature makes its contribution to the overall radioactivity of the environment small. 

North Sea Dimethyl Sulfide Emissions as a Source of Background Sulfate over Scandinavia... 

In the atmosphere DMS is oxidised mainly in the gas phase. Oxidation in cloud-water droplets is insignificant as the low solubility of DMS mitigates the effects of its rapid aqueous oxidation by ozone ( , McElroy, W.J., Central Electricity Research Laboratories, personal communication). Gas-phase oxidation is initiated principally by reaction with OH radicals ( ) and methanesulphonic acid (MSA) is one of the products (2). MSA has a very low vapour pressure and will be rapidly scavenged by aqueous aerosols and cloud droplets wherein further oxidation to sulphate by OH may occur. Although the kinetics and mechanism of this process have yet to be unambiguously determined, it is possible that emissions of DMS could be both a significant source of "background" sulphur and, upon oxidation, of non sea-salt sulphate. 

You and your children are likely to be exposed to very low amounts of CDDs in the diet particularly when you consume meat, milk, other dairy products, and fish. This represents the major source of background exposure to CDDs in most people. Children and adults should eat a balanced diet preferably containing low to moderate amounts of animal fats including meat and dairy products, and fish that contain higher amounts of CDDs and eat larger amounts of fruits, vegetables and grains. 

Airborne pheromone and other odors are hydrophobic and tend to adsorb onto the waxy surface of the insect cuticle. Body surfaces thus can collect odors and become sources of background noise if these odors are later released. Degradation of these surface-bound odor molecules might significantly reduce such signal noise. 

We will detect the 2S-3S transition by observing fluorescence from the 3S-2P-1S decay cascade, see Fig. 7. The 30 nm radiation may be detected using a channel electron multiplier. The dominant source of background is expected to be from the spontaneous two-photon decay of the metastable state, giving the characteristic broad, symmetrical frequency spectrum centred on 60.8 nm. A thin aluminium filter will be used to suppress this background and ehminate the scattered 328 nm light. 

The application of PIXE (Proton Induced X-Ray Emission) and micro-PIXE to quantitative analysis of impurities in polyethylene are described. The equipment, sources of background which affect the sensitivity and the precision of PIXE measurements are discussed for both thick and thin targets. A number of applications of micro-PIXE to the identification and location of trace elements in the "Trees" found in the polymer insulation of HV cables are presented. 

F.Y.T. Leung, et ah. Isotopic fractionation of carbonyl sulfide in the atmosphere Implications for the source of background stratospheric sulfate aerosol, Geophys. Res. Lett. 29 (10) (2002), doi 10.1029/2001GL013955. 

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