Flame photometry, alkali metals

All the alkali metals have characteristic flame colorations due to the ready excitation of the outermost electron, and this is the basis of their analytical determination by flame photometry or atomic absorption spectroscopy. The colours and principal emission (or absorption) wavelengths, X, are given below but it should be noted that these lines do not all refer to the same transition for example, the Na D-line doublet at 589.0, 589.6 nm arises from the 3s — 3p transition in Na atoms formed by reduction of Na+ in the flame, whereas the red line for lithium is associated with the short-lived species LiOH. 

There are now two main methods used for flame emission spectroscopy. The original method, known as flame photometry, is now used mainly for the analysis of alkali metals. 

Flame emission photometry is used mainly for the determination of alkali metals and some easily excited elements (Na, K, Li, Ca, etc.). This is related to the fact that the number of excited atoms in the flame decreases exponentially with increasing excitation energy. Moreover, at variance to AAS, where the sensitivity is directly proportional to the number of atoms in the ground state, the sensitivity of AES increases with an increasing number of atoms in the excited state. 

The ratio, Nj/N0, can therefore be calculated. For the relatively easily excited alkali metal sodium, it is 9.9 x 10 6 at 2000 °K and 5.9 x 10 4 at 3000 °K this latter temperature is about the highest commonly obtained with flames used for atomic absorption or emission work. Hence, only about 1(T3 % of the sodium atoms are excited at 2000 ° and 6 x 1(F2 % at 3000°. For an element such as zinc,Nf/N0 is 5.4 x 10"10 at 3000 and so only 5 x 10"8% is excited. In spite of the small fraction excited, good sensitivities can be obtained for many elements by flame photometry if a high temperature flame is used, because the difference between zero and a small but finite number is measured. For example, seventy elements can be determined by flame photometry using the nitrous oxide-acetylene flame 1H. 

Atomic absorption spectrometry is one of the most widely used techniques for the determination of metals at trace levels in solution. Its popularity as compared with that of flame emission is due to its relative freedom from interferences by inter-element effects and its relative insensitivity to variations in flame temperature. Only for the routine determination of alkali and alkaline earth metals, is flame photometry usually preferred. Over sixty elements can be determined in almost any matrix by atomic absorption. Examples include heavy metals in body fluids, polluted waters, foodstuffs, soft drinks and beer, the analysis of metallurgical and geochemical samples and the determination of many metals in soils, crude oils, petroleum products and plastics. Detection limits generally lie in the range 100-0.1 ppb (Table 8.4) but these can be improved by chemical pre-concentration procedures involving solvent extraction or ion exchange. 

Flame atomic absorption spectrometry has achieved very wide use as a routine method for the determination of trace metals in solution. However, for alkali metals flame photometry has remained popular. Why is this ... 

Flame Photometry. An analytical method suitable for qual and quant detn of about 70 elemcats, flame photometry is based on the classical flame tests for the alkali and alkaline-earth metals (Na yellow, K purple,... 

The content of aluminum in the reaction solution was determined at 50 p.p.m., the content of sodium at 100 p.p.m. Up to 95% conversion, the metal content in solution did not change, but at the end of the esterification about 50% of the aluminum had separated as an insoluble substance, and only 10% of the original concentration of the alkali still existed in solution. After removing the remaining alcohol and brightening the plasticizer with adsorbents, the content of the two metals lies with 0.07 p.p.m. sodium and 0.1 aluminum at the limit of detectability. The sodium was determined by flame photometry the determination of aluminum is discussed later. 

Flame photometry has promise for the measurement of sodium, lead, and potassium. An application to measurement of sodium and alkali metals has been reported. The continuous measurement of sulfur-containing particles has received considerable attention. The motivation for observation of sulfur-containing particles comes from concern about the potential hazard posed by sulfate in the atmosphere. 

Emission spectroscopy with corrstant ciurent arc excitatiorr was made possible in the 1950s, allowing determinatiorr of contents of 10 % and, in special cases, even 10 % in solid samples. Such determinatiorrs could be described generally as semiquantitative however, in studies of metals and mineral objects they were of great importance. Similar limits were obtained with the use of flame photometry for alkali and alkaline elemerrts in hquid objects. 

Flame photometry (see also p. 168) is almost exclusively used for the determination of alkali metals because of their low excitation potential (e.g. sodium 5.14eV and potassium 4.34 eV). This simplifies the instrumentation required and allows a cooler flame (air-propane, air-butane or air-natural gas) to be used in conjunction with a simpler spectrometer (interference filter). The use of an interference filter allows a large excess of light to be viewed by the detector. Thus, the expensive photomultiplier tube is not required and a cheaper detector can be used, e.g. a photodiode or photoemissive detector. The sample is introduced using a pneumatic nebulizer as described for FAAS (p. 172). Flame photometry is therefore a simple, robust and inexpensive technique for the determination of potassium (766.5 nm) or sodium (589.0nm) in clinical or environmental samples. The technique suffers from the same type of interferences as in FAAS. The operation of a flame photometer is described in Box 26.2. 

The general viability of low-temperature flame photometry depends on two factors. First, the alkali and alkaline earth metals of analytical interest (sodium, potassium, lithium, cesium, rubidium, magnesium, calcium, strontium, and barium) reach their excited states at relatively lower temperatures than do most other elements. Second, the emission wavelengths offer enough resolution such that optical filtering can be accomplished at a relatively low cost. 

With the use of fuels that produced hotter flames, earlier flame photometers became useful for analyzing elements beyond the alkali and alkaline earth metals. The development of atomic absorption spectrophotometers in the late 1960s provided the analytical chemist with a better tool for many of these applications. Later developments in high-temperature flame photometry narrowed the analytical applications of low-temperature flame photometry even further. The utility of the flame photometer to the clinical chemist, however, was not diminished until the development... 

Alkali metals PC Water Elution, flame photometry ... 

In the early years of flame photometry, only relatively cool flames were used. We shall see below that only a small fraction of atoms of most elements is excited by flames and that the fraction excited increases as the temperature is increased. Consequently, relatively few elements have been determined routinely by flame emission spectrometry, especMly j ew of those that emit line spectra (several can exist in flames as molecular species, particularly as oxides, which emit molecular band spectra). Only the easily excited alkali metals sodium, potassium, and lithium are routinely deterniined by flame emission spectrometry in the clinical laboratory. However, with flames such as oxyacetylene and nitrous oxide-acetylene, over 60 elements can now be determined by flame emission spectrometry. This is in spite of the fact that a small fraction of excited atoms is available for emission. Good sensitivity is achieved because, as with fluorescence (Chapter 16), we are, in principle, measuring the difference between zero and a small but finite signal, and so the sensitivity is limited by the response and stability of the detector and the stability (noise level) of the flame aspiration system. 

Excitation of the outer ns electron of the M atom occurs easily and emission spectra are readily observed. We have aheady described the use of the sodium D-line in the emission spectrum of atomic Na for specific rotation measurements (see Section 3.8). When the salt of an alkali metal is treated with concentrated HCl (giving a volatile metal chloride) and is heated strongly in the non-luminous Bunsen flame, a characteristic flame colour is observed (Li, crimson Na, yellow K, lilac Rb, red-violet Cs, blue) and this flame test is used in qualitative analysis to identify the M ion. In quantitative analysis, use is made of the characteristic atomic spectrum in flame photometry or atomic absorption spectroscopy. 

Although optical techniques —particularly photometry— prevailed in automatic methods of analysis for a long period, the advent of lon-selectlve electrodes (iSEs) marked the beginning of the automation of electroanalytical techniques. The variety of analysers currently available that Incorporate electro-analytical detection not only outperform those based on optical sensing (e.g. In analyses for alkali and alkaline-earth metals with ISEs as opposed to flame photometry), but also they have fostered the development of in vivo measurements, no doubt the most exciting and promising area of clinical chemistry. 

Flame photometry is applied particularly in determining alkali metals and alkali-earth metals. Using suitable fuel gases, it is possible to determine as many as 50 elements, but most of them can also be determined by... 

Allhough the alkali metals are determined daily in a huge number of samples throughout the world, most are clinical samples that are analyzed potentiometri-cally (see ( hapter 2.5), Flame photometry is currently used for only a tiny fraction of these samples. 

Qualitative analysis Elame photometry is particrflarly useful for the determination of alkali metals and alkaline-earth metals. It provides the basis for flame tests used in qualitative... 

Flame atomic emission spectroscopy, also called flame photometry, is based on the measurement of the emission spectrum produced when a solution containing metals or some nonmetals such as halides, sulfur, or phosphorus is introduced into a flame. In early experiments, the detector used was the analyst s eye. Those elements that emitted visible light could be identified qualitatively, and these flame tests were used to confirm the presence of certain elements in the sample, particularly alkali metals and alkaline-earth metals. A list of visible colors emitted by elements in a flame is given in Table 7.1. 

---