**3. Atomic spectroscopy: instrumentation**

Formation of the atomic vapor i.e. atomization is the major principle of emission, absorption, and fluorescence techniques. The most critical component of instruments used in atomic spectroscopy is the atomization sources and sample introduction devices with an associated spectrometer for wavelength selection and detection of light. Atomization involves the several key (the basic) steps: solvent removal, separation from anion and other elements of the matrix, and reduction of ions to the ground state atom. The design of an AFS instrument is similar to those for AAS and AES except that the light source and the detector are located at a right angle (**Figure 2**).

*A light source* which emits the sharp atomic lines of the element to be determined is selected. There are two types of light sources used in these instruments: continuous sources and line sources. A continuous source, also called to as a broad-band source, emits radiation over a broad range of wavelengths. A line source, on the other hand, emits radiation at specific wavelengths, but this source of radiation is not as pure as radiation from a laser. **Table 1** provides a list of most common kinds of lamps considered to be light sources.

*The atomizer* is any device which will produce ground state atoms as a vapor into the light path. Many atomizers utilized for AFS are similar to those used for AAS and AES. The atomizers most commonly used in these techniques are flames and electrothermal atomizers [10]. The flame provides for easy and fast measurements with few interferences and is preferred at any *appropriate concentration* for the analyte. Flame atomizers contain a pneumatic nebulizer, an expansion chamber, and an air-acetylene laminar flame with a 10 cm path length. The typical pneumatic nebulizer for sample introduction is insufficient, and although elements such as Na and K can be determined in biological samples by flame AES, flame atomization is more suitable for AAS and AFS. AAS measurements can detect concentrations at approximately 1 μg/ml (ppm) or more. Devices are being developed to overcome these limitations of the typical nebulizer. Atomization can be reached to 100% and the devices can also generate the sample as a pulse flow rather than the continuous flow. Most systems use a graphite tube which is heated electrical energy, a technique called graphite furnace atomization, although other materials are sometimes employed. A programmed sequence of the furnace temperature is used in electrically heated graphite tube. With this atomizer, 10–50 μl of test solution is dried, organic material is destroyed, and the analyte ions dissociated from anions for reduction to ground state atoms. This atomizer also produces *temperatures* up to 3000 K which allows to form an atomic vapor of refractory elements such as aluminum and chromium. Since the analyte is atomized and retained within a small volume furnace, this procures a dense atom population. The technique is extremely sensitive as it allows one to detect a few μg/ml concentrations of the analyte. Although the technique is widely used for AAS, electrothermal atomization will

**Figure 2.** *Schematic diagram of an AAS, AES, and AFS instrument.*


#### **Table 1.**

*The most common kinds of light sources.*

provide a better performance for both AES and sample introduction into an inductively coupled plasma. Traditional sources usually include arcs and sparks but modern instruments use argon or some other inert gas to create plasma. The plasma may be produced when gas atoms are ionized, Ar + e<sup>−</sup> → Ar1 + 2e<sup>−</sup>—a process generated by seeding ions from a high-voltage spark—and is sustained from a radio frequency generator in the area of the induction coil. This is known as *inductively coupled plasma* (ICP). Plasma exists at temperatures of up to 10,000 K and the instrument prevents the torch from melting. XRF requires that sample should be irradiated by high energy photons. In most instruments, the source is the polychromatic primary beam from *X-Ray tubes*. Of interest to biological applications, however, it is the use of radioactive isotopes such as 244Cm, 241 Am, 55Fe, and 109Cd [11, 12].

An ideal *wavelength selector* has a high throughput of radiation and a narrow effective bandwidth. There are two major types of wavelength selectors —**filters and monochromators**. A simple example of an absorption filter is a piece of colored glass. Absorption filters provide effective bandwidths of 30–250 nm, although the throughput can be only 10% of the source's emission intensity at the low end of this range. Interference filters constructed of a several optical layers deposited on a glass or transparent material. Typically, effective bandwidth is 10–20 nm, with maximum throughputs of at least 40% [11]. *A monochromator* is used to convert a polychromatic source of radiation at the entrance slit to a monochromatic source of restricted effective bandwidth at the exit slit. These devices are classified as either fixed-wavelength or scanning. The wavelength selects by manually rotating the grating in a fixed-wavelength monochromator. A scanning monochromator includes a drive mechanism that continuously rotates the grating, allowing sequential wavelengths to exit from the monochromator (**Figure 3**) [11].

*Detectors* use a sensitive **transducer** that converts a signal comes from light energy into electrons An ideal detector produces signal, *S*, is a linear function of the electromagnetic radiation's power, *P*,

S = kP + D

where *k* is the detector's sensitivity and *D* is the detector's **dark current**, or the background current when no radiation of source reached to the detector.

Phototubes and photomultipliers include a photosensitive surface that absorbs radiation in the *UV-visible*, or near-IR, generating an electrical current proportional to the number of photons reaching the transducer (**Figure 4**). Other *photon* 

**5**

**Table 2.**

*Examples of detectors for spectroscopy.*

*Atomic Spectroscopy*

**Figure 3.**

**Figure 4.**

*DOI: http://dx.doi.org/10.5772/intechopen.89269*

*detectors* use a semiconductor compound as the photosensitive surface. One advantage of the Si photodiode manufactured utilizing semiconductor process is that it is easy to miniaturize. Infrared photons do not have enough heat to generate a measur-

*Schematic diagram of wavelength selectors: (a) filters and (b) a diffraction grating monochromator.*

A transducer's electrical signal is sent to a **signal processor** where it is displayed in a form that is more convenient *to* explain. The analog meters, digital meters, recorders, and computers equipped with data acquisition boards are good examples of signal processors. A signal processor is used in calibrating the detector's

able current with a photon transducer [11].

*Diagram of a phototube and a photomultiplier tube.*
