Lasers plasma tube

A number of points are clear. First, in all cases the major expense of laser photons is the hardware, not the energy (even at Austin prices). Secondly, the intrinsically greater efficiency of the lower-energy lasers, especially the economic attractiveness of the CO2 laser, is evident. One can easily understand why laser chemistry schemes based upon multiphoton infrared absorption attract so much effort. Thirdly, on a per-unit-time basis the ion laser is more than twice as expensive to operate than even the rather exotic excimer laser. This is because of the inherent energetic inefficiency of the rare-gas plasma as a gain medium and because of the extrinsic, and hideous, expense of ion laser plasma tubes (and their poor reliability). 

Output powers in the range 1-10 W on the strongest lines at 4880 A and 5145 X is standard on commercially available argon ion lasers. A typical argon laser plasma tube would have a diameter of 3 mm, gas pressure of 0-4 Torr and... 

FIGURE 4. Side view of laser plasma tube. 

These ion lasers are very inefficient, partly because energy is required first to ionize the atom and then to produce the population inversion. This inefficiency leads to a serious problem of heat dissipation, which is partly solved by using a plasma tube, in which a low-voltage high-current discharge is created in the Ar or Kr gas, made from beryllium oxide, BeO, which is an efficient heat conductor. Water cooling of the tube is also necessary. 

Most Ar and Kr lasers are CW. A gas pressure of about 0.5 Torr is used in a plasma tube of 2-3 mm bore. Powers of up to 40 W distributed among various laser wavelengths can be obtained. 

Control Laser systems have Ar+ and Kr+ plasma tubes which can be interchanged rapidly. For other laser systems, the interchange may also be carried out. 

A large number of nonlasing plasma lines emitted from the discharge plasma tube often interfere in the recorded Raman spectra. Loader (40) listed tables of plasma lines when using the argon ion and argon/krypton ion lasers as Raman sources. 

The structure of a CW gas laser has been shown in Section 2.2. In the He-Ne laser, He (1 mm Hg) and Ne (0.1 mm Hg) gases are mixed in the plasma tube. As shown in Fig. 1, the He atoms are excited to the 5 and 3S states by electrical discharge to create population inversion. Collisions of these excited state He atoms with Ne atoms produce excited-state Ne atoms, which produce stimulated emission at 632.8 and 1,152.3 nm. The latter is eliminated by using a prism. In an Ar-ion laser, the population inversion is created by collision with energetic electrons, and the excited-state Ar ion emits a series of lines, including those of 488.0 and 514.5 nm. 

A - Fiber optic B - Collecting lens C - Laser focusing lens D - Laser plasma E - Mulllte tube F - Liquid metal... 

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

The main detectors used in AES today are photomultiplier tubes (PMTs), photodiode arrays (PDAs), charge-coupled devices (CCDs), and vidicons, image dissectors, and charge-injection detectors (CIDs). An innovative CCD detector for AES has been described [147]. New developments are the array detector AES. With modem multichannel echelle spectral analysers it is possible to analyse any luminous event (flash, spark, laser-induced plasma, discharge) instantly. Considering the complexity of emission spectra, the importance of spectral resolution cannot be overemphasised. Table 8.25 shows some typical spectral emission lines of some common elements. Atomic plasma emission sources can act as chromatographic detectors, e.g. GC-AED (see Chapter 4). 

---