Equipment - Atomic Emission Spectroscopy

Equipment

Instrumentation for atomic emission spectroscopy is similar in design to that used for atomic absorption. In fact, most flame atomic absorption spectrometers are eas- ily adapted for use as flame atomic emission spectrometers by turning off the hol- low cathode lamp and monitoring the difference between the intensity of radiation emitted when aspirating the sample and that emitted when aspirating a blank. Many atomic emission spectrometers, however, are dedicated instruments designed to take advantage of features unique to atomic emission, including the use of plasmas, arcs, sparks, and lasers, as atomization and excitation sources and have an enhanced capability for multielemental analysis.

Atomization and Excitation 

Atomic emission requires a means for converting an analyte in solid, liquid, or solution form to a free gaseous atom. The same source of thermal energy usually serves as the excitation source. The most common methods are flames and plasmas, both of which are useful for liquid or solution samples. Solid samples may be analyzed by dissolving in solution and using a flame or plasma atomizer.

Flame Sources 

Atomization and excitation in flame atomic emission is accom- plished using the same nebulization and spray chamber assembly used in atomic absorption (see Figure 10.38). The burner head consists of single or multiple slots or a Meker-style burner. Older atomic emission instruments often used a total consumption burner in which the sample is drawn through a capillary tube and in- jected directly into the flame.

Plasma Sources 

A plasma consists of a hot, partially ionized gas, containing an abundant concentration of cations and electrons that make the plasma a conductor. The plasmas used in atomic emission are formed by ionizing a flowing stream of argon, producing argon ions and electrons. The high temperatures in a plasma re- sult from resistive heating that develops due to the movement of the electrons and argon ions. Because plasmas operate at much higher temperatures than flames, they provide better atomization and more highly populated excited states. Besides neu- tral atoms, the higher temperatures of a plasma also produce ions of the analyte.

A schematic diagram of the inductively coupled plasma (ICP) torch is shown in Figure 10.49. The ICP torch consists of three concentric quartz tubes, sur- rounded at the top by a radio-frequency induction coil. The sample is mixed with a stream of Ar using a spray chamber nebulizer similar to that used for flame emission and is carried to the plasma through the torch’s central tube. Plasma formation is initiated by a spark from a Tesla coil. An alternating radio- frequency current in the induction coils creates a fluctuating magnetic field that induces the argon ions and electrons to move in a circular path. 

The resulting collisions with the abundant unionized gas give rise to resistive heating, providing temperatures as high as 10,000 K at the base of the plasma, and between 6000 and 8000 K at a height of 15–20 mm above the coil, where emission is usually measured. At these high temperatures the outer quartz tube must be thermally isolated from the plasma. This is accomplished by the tangential flow of argon shown in the schematic diagram.

Multielemental Analysis 

Atomic emission spectroscopy is ideally suited for multi- elemental analysis because all analytes in a sample are excited simultaneously. A scanning monochromator can be programmed to move rapidly to an analyte’s de- sired wavelength, pausing to record its emission intensity before moving to the next analyte’s wavelength. Proceeding in this fashion, it is possible to analyze three or four analytes per minute.

Another approach to multielemental analysis is to use a multichannel instru- ment that allows for the simultaneous monitoring of many analytes. A simple de- sign for a multichannel spectrometer consists of a standard diffraction grating and 48–60 separate exit slits and detectors positioned in a semicircular array around the diffraction grating at positions corresponding to the desired wave- lengths (Figure 10.50).