Secondary absorption, fluorescence

Tbt aracteristic fluorescent radiation is absorbed throughout the trajectory before exiting the sample in the direction of the detector this is secondary absorption. 

Secondary absorption Absorption of the emitted radiation in fluorescence or phosphorescence spectrometry compare with primary absorption. 

There are three main matrix effects in XRF primary absorption, secondary absorption and secondary fluorescence. Primary absorption refers to the radiation that is absorbed on the beam s path to reach the atoms to be excited. Secondary absorption refers to absorption of fluorescent radiation from atoms that occur along its path inside the specimen to the detector. Secondary fluorescence refers to the fluorescent radiation from the atoms which are excited by the fluorescent radiation of atoms with a higher atomic number in the same specimen. This phenomenon is possible when energy of the primary fluorescent radiation from heavier atoms is sufficient to excite secondary fluorescence from lighter atoms in the specimen. The absorption effects reduce the intensity of characteristic X-ray lines in spectrum, while secondary fluorescence increases the intensity of lighter elements. The matrix factors of EDS analysis in an electron microscope (EM) are described later in Section 6.8. 

Another factor responsible lor negative departures from linearity at high concentration is secondary ah-snrpiion. Secondary absorption occurs when the wavelength of emission overlaps an absorption hand. I luo-rcsccncc is then decreased as the emission iravcrscs the solution and is reab.sorbed by other inolecule.s in solution. Secondary absorption can be absorption by the analyte species itself or absorption by other species in the solution. The effects of these phenomena arc such that a plot of fluorescence versus concentration may exhibit a maximum. Alisorplion effects are often termed inner jtlier effeas. 

Secondly, the fluoresced intensity is modified by the effects of primary and secondary absorption in the specimen. This is a major source of the so-called matrix effects. Primary absorption is defined by the term / (Eo) esc V i. It reduces the effectiveness of the x-rays from the excitation source. Secondary absorption is defined by the term (Ei) esc yjri It reduces the intensity of the desired characteristic x-rays as they leave the specimen. Note that m(Eo) and /x(Ei) for the specimen are functions of the specimen composition through the relation... 

Formulas similar in form to Eq. (2.31) can be derived for the coherent and incoherent scattered intensities. Instead of Qif(Eo), a term proportional to the scattered intensity described in Sec. 2.5 is inserted and the solid-angle effect in Eq. (2.22) is handled differently. However, the scattered x-rays suffer primary and secondary absorption effects similar to the fluoresced x-rays. An additional contribution to the fluoresced intensity resulting from secondary fluorescence will be considered in Sec. 2.11. 

Optically active polymers containing carbazole groups may be synthesised by polymerisation of intrinsically optically active carhazole-containing monomers or by copolymerisation of a variety of optically active co-monomers with nonchiral carbazole-containing monomers. Full details are given and it is concluded that the second method is most useful, not least because it permits a wider variation in polymer backbone structures. V. V. absorption fluorescence emission, NMR, and circular dickroism spectra are reported in detail and help to establish a correlation between photophysical behaviour widi both primary and secondary structural features of the polymers. 

Poly(ALethyl-2-vinylcarbazole) (formula 7a) has been prepared by free-radical polymerization, whereas poly(A -ethyl-3-vinylcarbazole) (7b) was synthesized by cationic polymerization with a boron trifluoride initiator [150], The 2-isomer is reported to exhibit higher carrier mobility than PVK, while that of the 3-isomer is lower [151], Similar polymers with optically active groups such as poly((5)-9-(2-methylbutyl)-2-vinylcarbazole) (formula 7c), poly((5)-9-(2-methylbu-tyl)-3-vinylcarbazole) and poly((5)-3-(2-methylbutyl)-9-vinylcarbazole) have been prepared by Chielini et al. [152-154], The UV-absorption, fluorescence emission, NMR, and circular dichroism spectra were reported in detail and used to establish a correlation between the photophysical behaviour and both primary and secondary structural features of the polymers,... 

NEXAFS can be applied to a large number of adsorbate/substrate combinations. If the absorption is detected via the secondary electron yield, highly insulating substrates represent a problem. The situation is, however, considerably better than in conventional photoelectron specfioscopy, since small (several eV) shifts in the kinetic energy of the secondary electrons do not affect the positions of resonances in the spectra. Charging problems are eliminated by employing the X-ray fluorescence to monitor the absorption (Fluorescence Yield Near Edge Structure or FYNES) instead of the secondary electrons. 

In the photochemistry of hydrazine induced by an ArF laser, strong two-photon-Induced fluorescence from excited radicals NH(A H) was observed and attributed to secondary absorption by the N2H3 primary photoproduct according to N2H3 + hv-> NH2+NH [22] for critical remarks, see [23, 24]. 

Ema data can be quantitated to provide elemental concentrations, but several corrections are necessary to account for matrix effects adequately. One weU-known method for matrix correction is the 2af method (7,31). This approach is based on calculated corrections for major matrix-dependent effects which alter the intensity of x-rays observed at a particular energy after being emitted from the corresponding atoms. The 2af method corrects for differences between elements in electron stopping power and backscattering (the correction), self-absorption of x-rays by the matrix (the a correction), and the excitation of x-rays from one element by x-rays emitted from a different element, or in other words, secondary fluorescence (the f correction). 

In another approach, which was previously mentioned, the mass thickness, or depth distribution of characteristic X-ray generation and the subsequent absorption are calculated using models developed from experimental data into a < )(p2) function. Secondary fluorescence is corrected using the same i flictors as in ZAP. The (pz) formulation is very flexible and allows for multiple boundary conditions to be included easily. It has been used successfully in the study of thin films on substrates and for multilayer thin films. 

A consequence of absorption of X rays is the inner shell ionization of the absorbing atoms and the subsequent generation of characteristic X rays from the absorbing atoms, called secondary fluorescence, which raises the generated intensity over that produced by the direct action of the beam electrons. Secondary fluorescence can be induced by both characteristic and bremsstrahlung X rays. Both effects are compo-sitionally dependent. 

Campbell and Herring (1987) isolated and partially purified a red fluorescent protein from the suborbital light organs of M. niger. The absorption spectrum of this red fluorescent protein had a peak at 612 nm, a shoulder at 555 nm, and a secondary peak at 490 nm. 

XRF nowadays provides accurate concentration data at major and low trace levels for nearly all the elements in a wide variety of materials. Hardware and software advances enable on-line application of the fundamental approach in either classical or influence coefficient algorithms for the correction of absorption and enhancement effects. Vendors software packages, such as QuantAS (ARL), SSQ (Siemens), X40, IQ+ and SuperQ (Philips), are precalibrated analytical programs, allowing semiquantitative to quantitative analysis for elements in any type of (unknown) material measured on a specific X-ray spectrometer without standards or specific calibrations. The basis is the fundamental parameter method for calculation of correction coefficients for matrix elements (inter-element influences) from fundamental physical values such as absorption and secondary fluorescence. UniQuant (ODS) calibrates instrumental sensitivity factors (k values) for 79 elements with a set of standards of the pure element. In this approach to inter-element effects, it is not necessary to determine a calibration curve for each element in a matrix. Calibration of k values with pure standards may still lead to systematic errors for unknown polymer samples. UniQuant provides semiquantitative XRF analysis [242]. 

Matrix absorption, secondary fluorescence and scattering phenomena limit sensitivity and precision in many cases, especially with dense matrices. The sensitivity falls off with atomic number elements with Z < 15 are particularly difficult to analyse. Analysis is characteristic of surface layers (5-500 pm depth) only for a solid specimen. Instruments are often large, complicated and costly. 

When primary X-rays are directed on to a secondary target, i.e. the sample, a proportion of the incident rays will be absorbed. The absorption process involves the ejection of inner (K or L) electrons from the atoms of the sample. Subsequently the excited atoms relax to the ground state, and in doing so many will lose their excess energy in the form of secondary X-ray photons as electrons from the higher orbitals drop into the hole in the K or L shell. Typical transitions are summarized in Figures 8.35 and 8.36. The reemission of X-rays in this way is known as X-ray fluorescence and the associated analytical method as X-ray fluorescence spectrometry. The relation between the two principal techniques of X-ray emission spectrometry is summarized in Figure 8.37. 

In addition to absorption problems, measurements will be affected by secondary fluorescence and scattered radiations which will enter the detector and increase the general background. Detection limits under optimum conditions (a heavy element in a light matrix) may be as low as 10 ppm. Quantitative analysis is however difficult below the 20-100 ppm region if a reasonable precision (5% or better) is to be obtained. 

In bulk samples, X-ray yields need to be adjusted by the so-called "ZAF" correction. Z stands for the element number (heavier elements reduce the electron beam intensity more than lighter elements, because they are more efficient back-scatterers), A for absorption (different elements have different cross sections for X-ray absorption), and F for secondary fluorescence (the effect described above). Corrections are much less important when the sample is a film with a thickness of 1 pm or less, because secondary effects are largely reduced. The detection limit is set by the accuracy with which a signal can be distinguished from the bremsstrahlung background. In practice, this corresponds to about 100 ppm for elements heavier than Mg. 

All analytical methods that use some part of the electromagnetic spectrum have evolved into many highly specialized ways of extracting information. The interaction of X-rays with matter represents an excellent example of this diversity. In addition to straightforward X-ray absorption, diffraction, and fluorescence, there is a whole host of other techniques that are either directly X-ray-related or come about as a secondary result of X-ray interaction with matter, such as X-ray photoemission spectroscopy (XPS), surface-extended X-ray absorption fine structure (SEXAFS) spectroscopy, Auger electron spectroscopy (AES), and time-resolved X-ray diffraction techniques, to name only a few [1,2]. 

Other limitations involve both the mass absorption coefficient of soil components and secondary and tertiary excitation. The mass absorption coefficient can be calculated and used to correct fluorescence determinations if the exact composition of the material being analyzed is known. This is not possible in soil. Secondary and tertiary excitations occur when X-rays emitted by an element other than the one of interest may cause emission or fluorescence of the element of interest. These potential sources of error are possible in any soil analysis using XRF. 

Quantitative concentration data are often required from XRF analyses. In principle (for both WD and ED) the intensity of the fluorescent X-ray peak is proportional to the amount of the element present. This is complicated, however, by absorption and enhancement processes. Absorption can cause both attenuation of the input (primary) radiation and the fluorescent (secondary) radiation, as discussed above. Enhancement is the result of the observed element absorbing secondary radiation from other elements present in the sample, thus giving more fluorescent radiation than would otherwise... 

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