Plasma examples

Table 1. Energy content of various subsystems of the low pressure plasma. Example Nitrogen high frequency discharge, 27 MHz, p. D. = 1 Torr cm, I/D = 1 A cm-, degree of dissociation a 0.6, Tt = 873°K...

K. The thermodynamic functions help to calcrrlate chemical and ionization compositions of thermal plasmas. Examples of such calcrrlatiorrs can be fotmd in Botrlos et al. (1994), as well as in Chapter 5 and later in this book with regards to the description of the composition and energy efficiency of plasma-chemical processes. 

Enzymes. Normal plasma contains only a few esterases in addition to ceruloplasmin and the blood clotting enzymes. However, in certain pathologic conditions enzymes from tissue can enter the blood plasma examples are glutamate-oxaloace-tate transaminase (with cardiac infarction) or lactate dehydrogenase (with several liver diseases). Hence, enzyme assays in plasma have acquired diagnostic value. 

A brief description of a low-density non-equilibrium plasma is given followed by a review of its characteristic features and of tire relevant collisionprocesses in tire plasma. Principles for tire generation of plasmas in teclmical devices are discussed and examples of important plasma chemical processes and tlieir technical applications are presented. 

As an example, we look at tire etching of silicon in a CF plasma in more detail. Flat Si wafers are typically etched using quasi-one-dimensional homogeneous capacitively or inductively coupled RF-plasmas. The important process in tire bulk plasma is tire fonnation of fluorine atoms in collisions of CF molecules witli tire plasma electrons... 

An important and well studied example is the deposition of plasma-polymerized fluorinated monomer films [35], Monomers are fluoroalkyls, fluorohydroalkyls, cyclo-fluoroalkyls, as well as unsaturated species. The actual... 

Precision For samples and standards in which the concentration of analyte exceeds the detection limit by at least a factor of 50, the relative standard deviation for both flame and plasma emission is about 1-5%. Perhaps the most important factor affecting precision is the stability of the flame s or plasma s temperature. For example, in a 2500 K flame a temperature fluctuation of +2.5 K gives a relative standard deviation of 1% in emission intensity. Significant improvements in precision may be realized when using internal standards. 

Sensitivity Sensitivity in flame atomic emission is strongly influenced by the temperature of the excitation source and the composition of the sample matrix. Normally, sensitivity is optimized by aspirating a standard solution and adjusting the flame s composition and the height from which emission is monitored until the emission intensity is maximized. Chemical interferences, when present, decrease the sensitivity of the analysis. With plasma emission, sensitivity is less influenced by the sample matrix. In some cases, for example, a plasma calibration curve prepared using standards in a matrix of distilled water can be used for samples with more complex matrices. 

The cold plasmas tend to be unstable, are sometimes difficult to maintain, and provide ion yields that are less than those of the hot plasmas. To obviate the difficulties of the interfering isobaric molecular ions from hot plasmas, it has been found highly beneficial to include a collision cell (hexapole see Chapter 22) before the mass analyzer itself. This collision cell contains a low pressure of hydrogen gas. lon/molecule collisions between the hydrogen and, for example, ArO+... 

The volatile hydride (arsine in Equation 15.1) is swept by a. stream of argon gas into the inlet of the plasma torch. The plasma flame decomposes the hydride to give elemental ions. For example, arsine gives arsenic ions at m/z 75. The other elements listed in Figure 15.2 also yield volatile hydrides, except for mercury salts which are reduced to the element (Fig), which is volatile. In the plasma flame, the arsine of Equation 15.1 is transformed into As ions. The other elements of Figure 15.2 are converted similarly into their elemental ions. 

A major advantage of this hydride approach lies in the separation of the remaining elements of the analyte solution from the element to be determined. Because the volatile hydrides are swept out of the analyte solution, the latter can be simply diverted to waste and not sent through the plasma flame Itself. Consequently potential interference from. sample-preparation constituents and by-products is reduced to very low levels. For example, a major interference for arsenic analysis arises from ions ArCE having m/z 75,77, which have the same integral m/z value as that of As+ ions themselves. Thus, any chlorides in the analyte solution (for example, from sea water) could produce serious interference in the accurate analysis of arsenic. The option of diverting the used analyte solution away from the plasma flame facilitates accurate, sensitive analysis of isotope concentrations. Inlet systems for generation of volatile hydrides can operate continuously or batchwise. 

Other vapor introduction systems are discussed in Parts B and C (Chapters 16 and 17) because, although liquids and solids are ultimately introduced to the plasma flame as vapors, these samples are usually prepared differently from naturally gaseous ones. For example, electrothermal (oven) or laser heating of solids and liquids to form vapors is used extensively to get the samples into the plasma flame. At one extreme with very volatile liquids, no heating is necessary, but, at the other extreme, very high temperatures are needed to vaporize a sample. For convenience, the electrothermal and laser devices are discussed in Part C (Chapter 17) rather than here. 

The nebulization concept has been known for many years and is commonly used in hair and paint spays and similar devices. Greater control is needed to introduce a sample to an ICP instrument. For example, if the highest sensitivities of detection are to be maintained, most of the sample solution should enter the flame and not be lost beforehand. The range of droplet sizes should be as small as possible, preferably on the order of a few micrometers in diameter. Large droplets contain a lot of solvent that, if evaporated inside the plasma itself, leads to instability in the flame, with concomitant variations in instrument sensitivity. Sometimes the flame can even be snuffed out by the amount of solvent present because of interference with the basic mechanism of flame propagation. For these reasons, nebulizers for use in ICP mass spectrometry usually combine a means of desolvating the initial spray of droplets so that they shrink to a smaller, more uniform size or sometimes even into small particles of solid matter (particulates). 

The flame can become unstable if too large an amount of sample is introduced or if the sample contains substances that can interfere with the basic operation of the plasma. For example, water vapor, air, and hydrogen all lead to instability of the plasma flame if their concentrations are too high. 

Other volatile compounds of elements can be used to transport samples into the plasma flame. For example, hydride reduction of mercury compounds gives the element (Hg), which is very volatile. Osmium can be oxidized to its volatile tetroxide (OSO4), and some elements can be measured as their volatile acetylacetonate (acac) derivatives, as with Zn(acac)2. 

Thermal ionization. Takes place when an atom or molecule interacts with a heated surface or is in a gaseous environment at high temperatures. Examples of the latter include a capillary arc plasma, a microwave plasma, or an inductively coupled plasma. 

Chemical removal of surface material is produced through standard bond-breaking reactions. Typically chlorofluorocarbons (CECs) have been used, eg, CECl, CE2CI2, CE Cl, CE4, CHE, C2C1E. Eor example, CE dissociates into E atoms and fluorinated fragments of CE in a plasma ... 

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