Gases argon

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. 

Density of argon gas (300 K) Specific heat of argon gas Flame temperature Flame dimensions (approx) Volume of 1.6 ml argon at 300K... 

Suffice it to say at this stage that the surfaces of most solids subjected to such laser heating will be heated rapidly to very high temperatures and will vaporize as a mix of gas, molten droplets, and small particulate matter. For ICP/MS, it is then only necessary to sweep the ablated aerosol into the plasma flame using a flow of argon gas this is the basis of an ablation cell. It is usual to include a TV monitor and small camera to view the sample and to help direct the laser beam to where it is needed on the surface of the sample. 

In a concentric-tube nebulizer, the sample solution is drawn through the inner capillary by the vacuum created when the argon gas stream flows over the end (nozzle) at high linear velocity. As the solution is drawn out, the edges of the liquid forming a film over the end of the inner capillary are blown away as a spray of droplets and solvent vapor. This aerosol may pass through spray and desolvation chambers before reaching the plasma flame. 

Three common types of nozzle are shown diagrammatically. Types A and K are similar, with sharp cutoffs on the ends of the outer and inner capillaries to maximize shear forces on the liquid issuing from the end of the inner tube. In types K and C, the inner capillary does not extend to the end of the outer tube, and there is a greater production of aerosol per unit time. These concentric-tube nebulizers operate at argon gas flows of about 1 1/min. 

The fast-flowing narrow liquid stream has a high relative linear velocity with respect to the slower flow of the argon gas stream. This leads to breaking up the liquid stream into fast-moving droplets, which strike the impactor bead and form much smaller droplets. 

In the cross-flow arrangement, the argon gas flows at high linear velocity across the face of an orthogonal capillary tube containing sample solution. The partial vacuum causes liquid to lift above the end of the capillary. Here, it meets the argon and is nebulized. 

Another variant (the cone spray) allows the sample solution to flow down the sides of an inverted cone and through a hole at the bottom of which flows a fast stream of argon gas. As the liquid film meets the gas, it is ripped apart into a finely dispersed aerosol (Figure 19.15). 

The aim of breaking up a thin film of liquid into an aerosol by a cross flow of gas has been developed with frits, which are essentially a means of supporting a film of liquid on a porous surface. As the liquid flows onto one surface of the frit (frequently made from glass), argon gas is forced through from the undersurface (Figure 19.16). Where the gas meets the liquid film, the latter is dispersed into an aerosol and is carried as usual toward the plasma flame. There have been several designs of frit nebulizers, but all work in a similar fashion. Mean droplet diameters are approximately 100 nm, and over 90% of the liquid sample can be transported to the flame. 

The sample solution flows onto a piece of fritted glass through which argon gas flows. The flow of argon is broken down into narrow parallel streams of high linear velocity, which meet the thin film of liquid percolating into the pores of the frit. At the interfaces, an aerosol is formed and is blown from the top of the frit. 

For a longitudinal disturbance of wavelength 12 pm, the droplets have a mean diameter of about 3-4 pm. These very fine droplets are ideal for ICP/MS and can be swept into the plasma flame by a flow of argon gas. Unlike pneumatic forms of nebulizer in which the relative velocities of the liquid and gas are most important in determining droplet size, the flow of gas in the ultrasonic nebulizer plays no part in the formation of the aerosol and serves merely as the droplet carrier. 

In one sense, the thermospray nebulizer could be considered a pneumatic device, in which a fastflowing argon gas stream is replaced by a very rapidly vaporizing flow of solvent from the sample solution. A typical arrangement of a thermospray device is shown in Figure 19.18. 

From the radioactive decay constants and measurement of the amount of argon in a rock sample, the length of time since formation of the rock can be estimated. Essentially, the dating method requires fusion of a rock sample under high vacuum to release the argon gas that has collected through radioactive decay of potassium. The amount of argon is determined mass spectrometrically,... 

There are different conditions for producing a plasma, which can be started in gases at low or high (atmospheric) pressures. In a plasma torch, a flow of argon gas is used at atmospheric pressure. 

All methods of plasma production require some electrons to be present as electric-discharge initiators. For a plasma torch, the initiating electrons are introduced from a piezoelectric spark directed into argon gas flowing in the interval between two concentric quartz tubes. 

The density of ions and electrons increases quickly in the argon gas, at the same time increasing their kinetic energies as they are pulled back and forth in the applied electromagnetic field and undergo frequent collisions with neutral gas atoms. Some recombination of ions and electrons also occurs to form neutrals. 

An approximate equilibrium is set up in the plasma, with the electrons, ions, and atoms having velocity distributions similar to those of a gas that has been heated to temperatures of 7,000 to 10,000°C. Since the plasma is ignited toward the end of the concentric tubes from which argon gas is issuing, the plasma appears as a pale-blue-to-lilac flame coming out of the end of the tube, which is why the system is referred to as a torch, as in a welding torch. 

The gas or vapor to be examined is mixed with argon gas to constitute the needed flow of gas into the plasma flame, and the sample vapor or gas is swept along with this argon makeup gas. 

The solid to be examined must be vaporized in some way. This vaporization can be done by using the heat of the plasma flame or, more usually, the solid is ablated separately and the resulting aerosol is mixed with argon gas and swept into the center of the flame. 

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