Flames sampling

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. 

Comparison between flame-sampled PIE curves for (a) m/z = 90 (C H ) and (b) m/z = 92 (C Hg) with the PIE spectra simulated based on a Franck-Condon factor analysis and the cold-flow PIE spectrum of toluene. Calculated ionization energies of some isomers are indicated. (From Hansen, N. et al., /. Phys. Chem. A, 2007. With permission.)... 

The optical path for flame AA is arranged in this order light source, flame (sample container), monochromator, and detector. Compared to UV-VIS molecular spectrometry, the sample container and monochromator are switched. The reason for this is that the flame is, of necessity, positioned in an open area of the instrument surrounded by room light. Hence, the light from the room can leak to the detector and therefore must be eliminated. In addition, flame emissions must be eliminated. Placing the monochromator between the flame and the detector accomplishes both. However, flame emissions that are the... 

Instrumental methods have become more sophisticated to face these challenges. In particular, Westmoreland and Cool have developed a flame-sampling mass spectrometer that has provided several revelations in terms of relevant molecular intermediates in combustion. " Their setup couples a laminar flat-flame burner to a mass spectrometer. This burner can be moved along the axis of the molecular beam to obtain spatial and temporal profiles of common flame intermediates. By using a highly tunable synchrotron radiation source, isomeric information on selected mass peaks can be obtained. This experiment represents a huge step forward in the utility of MS in combustion studies lack of isomer characterization had previously prevented a full accounting of the reaction species and pathways. 

Most flame spectrometers use a premix burner, such as that in Figure 21-5, in which fuel, oxidant, and sample are mixed before introduction into the flame. Sample solution is drawn into the pneumatic nebulizer by the rapid flow of oxidant (usually air) past the tip of the sample capillary. Liquid breaks into a fine mist as it leaves the capillary. The spray is directed against a glass bead, upon which the droplets break into smaller particles. The formation of small droplets is termed nebulization. A fine suspension of liquid (or solid) particles in a gas is called an aerosol. The nebulizer creates an aerosol from the liquid sample. The mist, oxi-... 

An uncooled 1/4-in. O.D. quartz probe with a 70 micron I.D. tip was used to withdraw samples from the flames. The pressure differential between the flame and the inside of the probe was approximately 70 1. The flame samples were analyzed on a Finnigan Model 1015 quadrupole mass spectrometer. Argon was substituted for nitrogen as a diluent in the flames to allow better CO analysis with the mass spectrometer. 

Pyrolysis reactions in fuel-rich flame zones may lead, however, to emissions of polycyclic aromatic compounds (PACs) and soot. The close correlation between the concentrations of PACs and the bioactivity of flame samples is indicative of some of the health hazards involved in the emissions of PACs (Fig. 3). Fluxes of PAC species determined in fuel-rich, natural gas turbulent diffusion flames show the build up of hydrocarbons of increasing molecular weight along flames (Fig. 4). It is postulated that PACs are formed by the successive addition of C2 through C5 hydrocarbons to aromatic compounds (Fig. 5). 

The atomic absorption process can be summarized as follows radiant energy is emitted from a hollow cathode lamp and passed through a flame. The flame sampling system produces ground-state atoms from the sample. The intensity of radiation before and after sample atomization is measured and the result shown on a meter readout or digitally. 

Dual-laser ionization (DLI) draws from both LEI and RIS 14). DLI utilizes the flame sample reservoir common to LEI and photoexcitation schemes which are similar to RIS. DLI may be viewed as an extension of LEI or it could be referred to as RIS in flames. 

Atomization devices fall into two classes continuous atomizers and discrete atomizers. With continuous atomizers, such as plasmas and flames, samples are introduced in a continuous manner. With discrete atomizers, samples are introduced in a discrete manner with a device such as a syringe or an autosampler. The most common discrete atomizer is the electrothermal atomizer. 

Figure 1. Molecular beam mass spectrometer flame sampling...

Figure k. 13 9" ion profiles using flame sampling mass spectrometer technique in C2H2/O2 flame (2.1 kPa, 50 cm s" unburned velocity). Critical threshold, 2.5. 

F2. Feldman, F. J., Non-flame sampling technique for atomic absorption with automatic background corrections. Pittsburgh Conf. Anal. Chem, Appl. Spectrosc., 21st. 1970. 

Agglomeration of mixed feed particles may also influence the volatilization rate of the lead oxide in the high-temperature zone of the flame, because PbO reacts with the associated Si02 particles at temperatures higher than 800°C, forming less volatile lead silicates. Special examination of the flame samples provides evidence that similar interactions occur in the flame. 

Walsh has described a system of isolation and detection of radiation by a resonance technique. The system as used for atomic absorption is shown in Figure 6-13. Radiation from a hollow cathode source is passed through the flame sample cell into a resonance detector. The resonance detector contains an atomic vapor of the specific element under analysis. The atomic vapor in the resonance detector may be produced by cathodic sputtering or thermally. The atomic vapor in the resonance detector absorbs a portion of the... 

Reactions in a flame sample cell are similar to those observed in atomic absorption and can cause problems in the preparation of standard analytical curves. 

In the first step, the combustion gases are to be generated very carefully. The composition pattern is highly dependent on the manner of the combustion of the sample. It is influenced by the ambient temperature, by the mass of the specimen, and by the amount of available oxygen. At low temperatures, when flameless decomposition occurs, the primary decomposition products dominate. Products from flaming samples, however, are rich in secondary components (carbon monoxide and dioxide) as was demonstrated quantitatively by Ettre and Varadi (Table 4.5). 

The hydrogen enters the combustion chamber and mixes with the air sample from the air inlet to form the hydrogen-air flame. Sample air may come from the GC column if components are separated before the sample enters the detector. The flame decomposes chemicals contained in the sample. In the case of CWAs in a sample, the released and excited sulfur and/or phosphorus emit photons of a characteristic respective light spectrum for detection through the use of special light filters. 

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