Element emission spectra

Analysis by atomic (or optical) emission spectroscopy is based on the study of radiation emitted by atoms in their excited state, ionised by the effect of high temperature. All elements can be measured by this technique, in contrast to conventional flames that only allow the analysis of a limited number of elements. Emission spectra, which are obtained in an electron rich environment, are more complex than in flame emission. Therefore, the optical part of the spectrometer has to be of very high quality to resolve interferences and matrix effects.-... 

Alternating or direct current arcs and spark discharge are common methods of excitation for emission spectroscopic analysis of rare earth elements. Emission spectra of rare earth elements contain a large number of lines. The three arbitrary groups are (i) spectra of La, Eu, Yb, Lu and Y, (ii) more complicated spectra of Sm, Gd and Tm, (iii) even more complicated spectra of Ce, Nd, Pr, Tb, Dy and Er. Rare earths have been analyzed with spectrographs of high resolution and dispersion up to 2 A/mm. Some salient information is presented in Table 1.36. 

Since an atom of a given element gives rise to a definite, characteristic line spectrum, it follows that there are different excitation states associated with different elements. The consequent emission spectra involve not only transitions from excited states to the ground state, e.g. E3 to E0, E2 to E0 (indicated by the full lines in Fig. 21.2), but also transisions such as E3 to E2, E3 to 1( etc. (indicated by the broken lines). Thus it follows that the emission spectrum of a given element may be quite complex. In theory it is also possible for absorption of radiation by already excited states to occur, e.g. E, to 2, E2 to E3, etc., but in practice the ratio of excited to ground state atoms is extremely small,... 

If we pass white light through a vapor composed of the atoms of an element, we see its absorption spectrum, a series of dark lines on an otherwise continuous spectrum (Fig 1.11). The absorption lines have the same frequencies as the lines in the emission spectrum and suggest that an atom can absorb radiation only of those same frequencies. Absorption spectra are used by astronomers to identify elements in the outer layers of stars. 

Schematic representation of an apparatus that measures the emission spectrum of a gaseous element. Emission lines appear bright against a dark background. The spectmm shown is the emission spectrum for hydrogen atoms.

The atomic spectra of most elements are complex and show little regularity. However, the emission spectrum of the hydrogen atom is sufficiently simple to be described by a single formula ... 

By the early twentieth century, scientists had analyzed the spectra of most elements. They knew that each element produced a characteristic emission spectrum. Because hydrogen is the simplest atom, much of the research to understand the nature of atomic spectra centered on it. 

Different lanthanide metals also produce different emission spectrums and different intensities of luminescence at their emission maximums. Therefore, the relative sensitivity of time-resolved fluorescence also is dependent on the particular lanthanide element complexed in the chelate. The most popular metals along with the order of brightness for lanthanide chelate fluorescence are europium(III) > terbium(III) > samarium(III) > dysprosium(III). For instance, Huhtinen et al. (2005) found that lanthanide chelate nanoparticles used in the detection of human prostate antigen produced relative signals for detection using europium, terbium, samarium, and dysprosium of approximately 1.0 0.67 0.16 0.01, respectively. The emission... 

Qualitative analysis may be made by searching the emission spectrum for characteristic elemental lines. With modem high resolution optics and computer control, the emission spectrum may be readily examined for the characteristic lines of a wide range of elements (Figure 8.13). Quantitative measurements are made on the basis of line intensities which are related to the various factors expressed in equation (8.1). Under constant excitation... 

The emission spectrum of the cathode material includes a number of intense, sharp lines arising from transitions between excited states and the ground state, so-called resonance radiation. Generally, only a few resonance lines per element are suitable for quantitative work and there will be variation in the ranges of concentration over which absorbance measurements... 

Radiation is derived from a sealed quartz tube containing a few milligrams of an element or a volatile compound and neon or argon at low pressure. The discharge is produced by a microwave source via a waveguide cavity or using RF induction. The emission spectrum of the element concerned contains only the most prominent resonance lines and with intensities up to one hundred times those derived from a hollow-cathode lamp. However, the reliability of such sources has been questioned and the only ones which are currently considered successful are those for arsenic, antimony, bismuth, selenium and tellurium using RF excitation. Fortunately, these are the elements for which hollow-cathode lamps are the least successful. 

In the case of other elements, for instance Uranium, the emission spectrum normally displays thousands of narrowly spaced lines. However, the emission source possesses a fixed amount of energy which shall be spread up eventually amongst the thousands of lines thereby minimizing the sensitivity of each line. Hence, it is rather difficult to examine the less sensitive complex spectra of elements such as uranium. 

How is the atomic emission spectrum of an element related to these flame tests ... 

Explain how electrons in an element s atoms produce an emission spectrum. 

Compare the hydrogen emission spectrum to the other spectra you observed. Why do you think emission spectra are different for different elements ... 

Why do you think that scientists initially focused their attention on the hydrogen emission spectrum rather than the emission spectrum of another element ... 

Each element has a characteristic emission spectrum. Why is it only possible to observe the emission spectra of selected elements in a high school laboratory ... 

The emission spectrum of each element is characteristic of that element. In other words, it is a fingerprint that can be used to identify the element. 

The emission spectrum of an element has spectral lines at wavelengths of 620 nm and 640 nm. Sketch the absorption spectrum for this element. 

Among other examples, time-resolved luminescence has recently been applied to the detection of different trace elements (i.e., elements in very low concentrations) in minerals. Figure 1.13 shows two time-resolved emission spectra of anhydrite (CaS04). The emission spectrum just after the excitation pulse (delay 0 ms) shows an emission band peaking at 385 nm, characteristic of Eu + ions. When the emission spectmm is taken 4 ms after the pulse, the Eu + luminescence has completely disappeared, as this luminescence has a lifetime of about 10/rs. This allows us to observe the weak emission signals of the Eu + and Sm + ions present in this mineral, which in short time intervals are masked by the En + Inminescence. The trivalent ions have larger lifetimes and their luminescence still remains in the ms delay range. 

The optical emission spectrum of technetium is uniquely characteristic of the element " with a few strong lines relatively widely spaced as in the spectra of manganese, molybdenum and rhenium. Twenty-five lines are observed in the arc and spark spectra between 2200 and 9000 A. Many of these lines are free from ruthenium or rhenium interferences and are therefore useful analytically. Using the resonance lines of Tc-I at 4297.06, 4262.26, 4238.19, and 4031.63 A as little as 0.1 ng of technetium can be reliably determined. 

No two elements produce exactly the same emission spectrum. 

There are definite distinct lines in the atomic emission spectrum of hydrogen. These lines are seen in the visible part of the spectrum and there is also a series of lines in the infrared and another series in the ultraviolet part of the electromagnetic spectrum. So, although hydrogen is the simplest element with only one electron per atom, its atomic emission spectrum is fairly complicated. 

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