3.3 Components of an Inductively Coupled Plasma—Atomic Emission Spectrometry System (ICP-AES)
3.3.3 The Inductively Coupled Plasma Torch
The torch unit of an ICP is used to create and sustain a plasma. A plasma is an electrically conducting gaseous mixture containing enough cations and electrons (though the plasma has a neutral charge overall) to maintain the conductance. One common example of a simple plasma is a regular flame which will conduct an electrical current across it; cations and electrons are created upon ignition of the fuel and travel upward in the flame until they are cooled above the flame. Another common plasma is used in scanning electron microscopy (SEM) where a sample is coated with graphite or a metal in a vacuum chamber in order to make the surface conductive (a standard requirement for obtaining high quality images). DC arc and microwave plasmas can also be used to generate plasmas, but for purposes of metal analysis, the inductively coupled plasma system described below is most important. The purpose of the torch is to (1) evaporate the solvent (usually water) from the analyte salts, (2) atomize the atoms in the salt (break the ionic bonds and form gaseous state atoms), and (3) excite or ionize the atoms. In the case of ICP-AES, only excitation is needed as in FAES but given the extreme temperatures (up to 10 000 K) of the argon plasma used in modern ICP systems, the excitation of atoms is virtually complete for most elements. (Chapter 4 describes how the ICP-MS capitalizes on the efficient ionization of atoms in the plasma.) A modern torch system is illustrated in Animations 3.1 and 3.2. The torch described here is composed of three concentric quartz tubes. The samples and the argon gas used to aspirate it pass through the center tube. The plasma generating gas (argon) passes through the middle tube, and the argon passing through the outer tube is used to cool the quartz torch.
The plasma is sustained by a radio-frequency generator that creates an oscillating magnetic field around the torch that results in ohmic (inductive) heating of the charged gases at the end of the torch. . Ohmic heating occurs when an electrical current is passed through a conductor, that in turn, creates more heat. Three types of radio-frequency generators have been used. Older types are based on piezoelectric crystal oscillators and “free-running” generators in which the oscillation is set according to the combinations of the components of the circuit; examples include the Armstrong, Hartley, Colpitts, and tuned-anode-tuned gate oscillator electronic circuits. Most modern analytical ICP systems use solid-state semiconductor generators where the circuit consists of (1) a capacitor used to store a high electrical charge (thus requiring the 220-240 V electrical power requirements) and (2) an inductor coil to deliver the oscillating current to the torch and generate the magnetic field around the torch. The capacitor is hidden from view in ICP systems and buried in the electronics of the instrument. The inductor coil is visible and is the approximately 3-mm hollow copper coil wrapped three times around the end of the torch. The capacitor responsible for generating the plasma from argon gas, oscillates an electrical field at a rate between approximately 27 and 41 MHz (a frequency regulated by the FCC) and through induction creates a magnetic field in the plasma. The intensity of the frequency, measured in Watts, is sufficient to promote the valance electrons in some of the Ar atoms but not ionize them sufficiently to initiate or sustain a plasma. The creation of plasma occurs when a spark (from a Tesla coil; basically an automatic gas grill lighter) introduces free electrons at the end of the torch when the electrical field is being oscillated at a specific frequency by a RF generator. The seed electrons from the Tesla coil oscillate in an angular path and periodically collide with argon gas atoms and ionize them, releasing more electrons. Due to their kinetic energy and collisions with other atoms a large amount of heat is generated, enough to generate and sustain a plasma at temperatures up to 10 000 K. In terms of an electrical analogy, the term “inductively coupled” in ICP is a result of the coupling of the induction coil and the electrons. The copper induction coil serves as the “primary winding” of the radio-frequency transformer and the “secondary winding” is the oscillating electrons and cations in the plasma; the two “windings” are thus coupled together because the second winding depends on the presence of the first.
The RF generator also contains a feed back loop where the interaction of the electronic/magnetic field in the capacitor/induction coil is monitored. If the flow rate of argon gas is too low or if small amounts of O2 or high amounts of water vapor are present in the torch during the initiation of the plasma, the RF generator will sense their presence by a change in the feedback between the oscillation and the RF generator. When this happens, the RF generator shuts down to protect the electronics in the system from overheating. Thus, atmospheric O2 cannot be present in the Ar and the flow rate of nebulizer sample (water) must also be controlled to successfully light the torch. The process of lighting the plasma is visualized in Animation 3-1. In this animation the argon is turned on and the pressures are allowed to equilibrate without the introduction of any sample or blank solutions to the nebulizer. The radiofrequency generator then ramps the wattage through a series of cycles. A typical ramp cycle begins by turning the RF generator to 200 Watts. Subsequently the electronics go through a preliminary check that adjusts the electronics and minimizes the resistance between the RF generator and the induction coil. Next the RF wattage is ramped to 900 W where excitation of valence electrons in some Ar atoms occurs and the Tesla coil initiates the excitation and ionization of Ar atoms and the plasma is lit. The circulation of electrons and Ar cations in close association results in ohmic heating that generates more electrons, collisions between Ar atoms, and an increase in temperature. The wattage is then increased to 1200 and the resistance is minimized again via electronic adjustments. Finally the wattage is adjusted to the suggested energy level for the elements of interest; this is usually between 1250 and 1550 W. Please review the animation at this time.
Animation 3.1. Animation of the Gas Flow, RF Wattage Adjustment, and Lighting of the Plasma Torch Commonly used in ICP Systems.
If the region of ohmic heating is not controlled, the plasma would continue to heat until it melted the quartz torch (and the other components of the ICP spectrometer). The torch is cooled in two ways, the first is by the tangential introduction of relatively large volumes of argon gas through the outer tube of the torch. This argon flow spirals around the middle tube resulting in uniform cooling. Furthermore cooled water flows through the copper induction coil of the RF generator that is wrapped around the end of the torch. These combined cooling systems promote an equilibrium maximum temperature of approximately 10 000 K in the hottest portion of the plasma. The portion of the plasma that ICP-AES measurements are concerned with is about 5000-6000 K and is located in cone-shaped region outside the quartz torch.
After successful ignition of the torch, samples are introduced into the system through the nebulizer (Animation 3.2). Upon entry into the plasma, the solvent evaporate and salts form. Then, these compounds decompose as they move farther into the hotter portions of the plasma. Next, the valence electrons on the analyte atoms are excited (for ICP-AES measurements) or completely removed (ionized for ICP-MS measurements). The intense heat that sample molecules encounter in the plasma is sufficient to decompose most refractory compounds, thus only atoms or atomic ions are present in the plasma (Section 3.4). This heat also causes complete excitation of atoms and leads to higher detection limits, compared to FAES where the extent of excitation is element- and flame temperature-dependent. In ICP-AES, as the atoms exit the plasma and cool to approximately 6000 K, they relax and emit a characteristic photon that enters one of the monochromator/detector systems described in the next section. In ICP-MS (Chapter 4), the plasma enters a vacuum chamber where the cations are separated by a mass filter (mass analyzer) and detected by a specialized photomultiplier tube. An illustration of the drying, atomization, and excitation and ionization is shown in Animation 3.2.
Animation 3.2. Animation of Sample Introduction and the Subsequent Reactions in the Torch of an ICP System. Note that this animation is a simplistic illustration of a complex process. Charge transfer to the cations actually occurs due to Rayleigh explosion.
The consumption rate of Ar gas is an important issue and accounts for most of the operational costs of an ICP. Older systems can use up to 20 L/minute, while modern systems have reduced the flow to below 10 L/minute. Systems are being designed that are portable and use far smaller flow rates of Ar gas. Many laboratories use cryogenic sources of Ar to reduce operational costs. Current prices for liquid Ar (equivalent to approximately 5000 cubic feet at STP) are approximately $325, whereas the cost of three T-size cylinders of gaseous Ar (equivalent to approximately 334 cubic feet at STP) is approximately $420. Thus, use of a cryogenic tank is far more economical than individual gas cylinders. Gases generated from a cryogenic tank are often more pure than those obtained from a standard T-sized gas cylinder.
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