Flame method molecule

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

Ionisation detectors. An important characteristic of the common carrier gases is that they behave as perfect insulators at normal temperatures and pressures. The increased conductivity due to the presence of a few charged molecules in the effluent from the column thus provides the high sensitivity which is a feature of the ionisation based detectors. Ionisation detectors in current use include the flame ionisation detector (FID), thermionic ionisation detector (TID), photoionisation detector (PID) and electron capture detector (ECD) each, of course, employing a different method to generate an ion current. The two most widely used ionisation detectors are, however, the FID and ECD and these are described below. 

Origin of Ions in Hydrocarbon Flames. Many ions, both positive and negative, are observed in hydrocarbon flames studied by mass spectrometric methods (9, 14, 26). Most of these are produced by ion-molecule reactions following the formation of primary ions from neutral species. 

Dependence of Flame Species Concentrations upon Additive Concentrations. A method of determining the dependence of various ionic, neutral molecule, and excited species concentrations on the concentration of hydrocarbon added to a hydrogen/oxygen or hydrogen/air flame (based on a principle similar to that of flame ionization detectors... 

In 1979, it was stated that poiybrominated aromatic ethers have received little attention (ref. 1). That statement is still applicable. Analyses to characterize this class of commercial flame retardants have been performed using UV (refs. 1-2), GC (refs. 1-6), and GC-MS (refs. 1-4). The bromine content of observed peaks was measured by GC-MS, but no identification could be made. The composition of poiybrominated (PB) diphenyl ether (DPE) was predicted from the expected relationship with polyhalogenated biphenyl, a class which has received extensive attention. NMR (refs. 3-6) was successfully used to identify relatively pure material which had six, or fewer, bromine atoms per molecule. A high performance liquid chromatography (HPLC) method described (ref. 1) was not as successful as GC. A reversed phase (RP) HPLC method was mentioned, but no further work was published. 

There are a number of interferences that can occur in atomic absorption and other flame spectroscopic methods. Anything that decreases the number of neutral atoms in the flame will decrease the absorption signal. Chemical interference is the most commonly encountered example of depression of the absorption signal. Here, the element of interest reacts with an anion in solution or with a gas in the flame to produce a stable compound in the flame. For example, calcium, in the presence of phosphate, will form the stable pyrophosphate molecule. Refractory elements will combine with 0 or OH radicals in the flame to produce stable monoxides and hydroxides. Fortunately, most of these chemical interferences can be avoided by adding an appropriate reagent or by using a hotter flame. The phosphate interferences, for example, can be eliminated by adding 1 % strontium chloride or lanthanum chloride to the solution. The strontium or lanthanum preferentially combines with the phosphate to prevent its reaction with the calcium. Or, EDTA can be added to complex the calcium and prevent its combination with the phosphate. 

A number of instrumental analytical techniques can be used to measure the total phosphorus content of organophosphorus compounds, regardless of the chemical bonding of phosphorus within the molecules, as opposed to the determination of phosphate in mineralized samples. If the substances are soluble, there is no need for their destruction and for the conversion of phosphorus into phosphate, a considerable advantage over chemical procedures. The most important methods are flame photometry and inductively coupled plasma atomic emission spectrometry the previously described atomic absorption spectrometry is sometimes useful. 

This method is based on the emission of light by atoms returning from an electronically excited to the ground state. As in atomic absorption spectrometry, the technique involves introduction of the sample into a hot flame, where at least part of the molecules or atoms are thermally stimulated. The radiation emitted when the excited species returns to the ground state is passed through a monochromator. The emission lines characteristic of the element to be determined can be isolated and their intensities quantitatively correlated with the concentration of the solution. 

Harpoon reactions of alkaline metal atoms with halogen molecules in the gas phase seem to be the first instance of the observation of chemical electron transfer reactions at distances somewhat exceeding gas-kinetic diameters. Actually, as far back as 1932, Polanyi, while studying diffusion flames found for these reactions cross-sections of nR2, somewhat exceeding the gas-kinetic cross-sections [69]. Subsequently, more precise measurements which were carried out in the 1950s and 1960s with the help of the molecular beam method, confirmed the validity of this conclusion [70],... 

Schematic energy level diagrams for the most widely used probe methods are shown in Fig. 1. In each case, light of a characteristic frequency is scattered, emitted, and/or absorbed by the molecule, so that a measurement of that frequency serves to identify the molecule probed. The intensity of scattered or emitted radiation can be related to the concentration of the molecule responsible. From measurements on different internal quantum states (vibrational and/or rotational) of the system, a population distribution can be obtained. If that degree of freedom is in thermal equilibrium within the flame, a temperature can be deduced if not, the population distribution itself is then of direct interest.

Laser-based spectroscopic probes promise a wealth of detailed data--concentrations and temperatures of specific individual molecules under high spatial resolution--necessary to understand the chemistry of combustion. Of the probe techniques, the methods of spontaneous and coherent Raman scattering for major species, and laser-induced fluorescence for minor species, form attractive complements. Computational developments now permit realistic and detailed simulation models of combustion systems advances in combustion will result from a combination of these laser probes and computer models. Finally, the close coupling between current research in other areas of physical chemistry and the development of laser diagnostics is illustrated by recent LIF experiments on OH in flames. 

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