Further Readings
8.1 Introduction, History and Principles
Analyte atoms can be excited to upper electronic states by absorption of photons from a radiation source, relaxation to lower levels results in emission of some pho- tons; this process is called atomic fluorescence (AF). The phenomenon of AF has been known since late 19th century; fluorescence signals from several atoms, such as Na, Hg and Cd, were observed in flames. In the early years of studies on AAS, a new interest was initiated on AF; Alkemade suggested its use for chemical analysis in 1956; Winefordner and Vickers realized true chemical measurements using atomic fluorescence spectrometry (AFS). Since then, and especially with the use of laser sources for excitation, AFS has been an important subject of study for the researchers in the field of atomic spectrometry. However, the acceptance of the technique in the form of commercialized instruments could not be realized to the same extent as research studies. Despite this very low level of acceptance by the users, active research continues on AFS because of its high sensitivity in elemental analysis.
There are several types of AF as shown in Figure 8.1; the resulted spectra involve lines only. When the wavelengths of excitation and emission are equal,resonance fluorescence is observed as the both types shown in Figure 8.1a. The wavelengths of excitation and emission are not the same in many occasions; for these cases of non- resonance fluorescence, the terms stokesor anti-stokesare used when the excitation wavelength is shorter or longer than the emission wavelength, respectively. Direct- line fluorescence results if the same upper level is involved in both the excitation and emission processes (Figure 8.1b). On the other hand, if different upper levels are involved in excitation and emission, the term stepwise fluorescence is used (Figure 8.1c). If the radiational excitation is followed by thermal excitation, it is called ther- mally assisted fluorescence(Figure 8.1d). When two or more photons are used in exciting the analyte atoms to an upper level, multi-photon fluorescence results (Figure 8.1e). For all the kinds of AF mentioned above, the analyte atoms to absorb radiation may also be in an excited state; in this case the term excited state fluores- cence is used so that both the upper and lower levels are in excited states. In the process of sensitized fluorescence, the photons from the light source are first
absorbed by a donor species D; the excited donor D* transfers its energy to an accep- tor A; finally the excited state A* emits fluorescence as it relaxes (Figure 8.1f).
Sensitized and multi-photon AF produce low radiational power; thus they are not analytically important. The resonance fluorescence is commonly used. The non-res- onance fluorescence techniques are also used; since the wavelengths for excitation and emission are different, scattering problems are minimized.
The relation between the power of fluorescence emission and analyte concentra- tion is given below in a simplified form. All the parameters are for a specific wave- length of measurement.
F⫽kΦP02.3εbC (8.1)
Figure 8.1 Some common types of energy transitions in AFS. Full lines involve photons; dot- ted lines are for the radiationless transitions. (a) Resonance fluorescence;
(b) direct-line fluorescence; (c) stepwise fluorescence; (d) thermally assisted fluorescence; (e) multi-photon fluorescence (for the case in the figure only two photons are involved and (f) sensitized fluorescence. D: donor; A: acceptor;
*denotes the excited states; hνexand hνemare photons for excitation and emission, respectively
166 Chapter 8 F is the power of fluorescence emission; kthe geometrical factor representing the efficiency of optical collection for both the exciting beam on atom cloud and the emission beam on the detector; Φthe quantum yield, the efficiency of the fluores- cence process as compared to absorption; P0the radiational power of the excitation source; εbCthe absorbance as given by Beer’s law, the terms referring to absorptiv- ity, pathlength and molar concentration, respectively; Φ,εand cthe intrinsic prop- erties of analyte; and k, b and P0 the instrumental parameters. For most of the experimental conditions, where P0is below the saturation level, the Equation (8.1) above describes a linear calibration plot when Fis measured against concentration of analyte. If P0 is excessively increased, the rates of absorption and deactivation becomes equal; this corresponds to a situation where the photons available for absorption are very high in number. In such a case, saturation of fluorescenceis achieved, and the fluorescence signal does not increase with P0anymore; an exam- ple is shown in Figure 8.2.
The linearity of a calibration plot for a fluorescence measurement is affected by self-absorption where the emitted radiation is absorbed on its way to detector by the ground-state analyte atoms. For high analyte concentrations, this phenomenon causes a roll-over effect, so that a single F-value corresponds to two concentration values as shown in the Figure 8.3. If the atomic fluorescence signal was recorded by using aspiration to a flame atomizer in conventional manner, this situation is easily noticed as the steady-state signal will first have a peak higher than the rest of the sig- nal. With a graphite furnace, or any sampling system producing a transient signal, it is difficult to realize whether the analyte has the lower or higher concentration value
Figure 8.2 Saturation of fluorescence for Tl in an air-H2flame. Laser excitation at 377.5 nm was used and the fluorescence emission was observed at 535.0 nm. The satura- tion takes place at 2.8⫻10⫺8W cm⫺2Hz⫺1. (Adapted from Ref.1 with permission from Elsevier)
as shown on the calibration plot; measurement of another sample aliquot diluted by a different factor can solve the question.