**1. Introduction**

According to Webster's dictionary, a neural network is defined as *"a computer architecture in which a number of processors are interconnected in a manner suggestive of the connections between neurons in a human brain and which is able to learn by a process of trial and error"*. The "*processors"* maybe

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individual computers, but they do not have to be. Typically, such "processors" reside in the same computer and most frequently are referred to as *"neurons"*. Also according to Webster's dictionary, artificial is something *"made or produced by human beings rather than occurring naturally, typically as a copy of something natural"*. In this chapter, spectral interference correction in optical emission spectrometry (OES) will be discussed, using artificial neural networks (ANNs).

**Spectral interference** (a long standing problem in optical atomic spectrometry [1–4]) arises when a spectral line emitted from an **analyte** (defined as the chemical species of interest) in an **analytical sample** (i.e., one to be used in chemical analysis) is overlapped with (or **interfered** by) a spectral line emitted from another chemical constituent that is also present in the same sample or in the sample's **matrix** (defined as whatever an analyte is in). For example, Zinc (Zn) as an analyte (**A**) in an iron-rich soil matrix. In a widely used, 6000–10,000 K hot, inductively coupled plasma (ICP), iron (Fe) can emit as many as 4000 spectral lines, whereas Zn has less than 10 sensitive (i.e., analytically useful) lines, thus making interference from an Fe line (e.g., from the soil matrix) on a Zn line likely. An additional example will be discussed in conjunction with the simulated spectral windows shown in **Figure 1**.

Assuming that all species involved (per legend of **Figure 1**) are present in a sample and that they all are at equal concentrations, only Copper (Cu, at 213.853 nm) and Nickel (Ni, at 213.856 nm) are shown to overlap with the Zn spectral line (at 213.856 nm). Per legend, and

**Figure 1.** Spectral interferences in a simulated 100 pico-meter (pm)-wide spectral window (between 213.806 and 213.906 nm) centered on the analyte zinc (Zn) 213.856 nm spectral line.

with all participating species at equal concentrations, the spectral lines of V (Vanadium), W (Tungsten) and Re (Rhenium) are not sufficiently intense to be considered (or even to be graphed). However, if the concentrations of these elements is higher (e.g., by 10–100 times) rather than equal to that of the analyte Zn, they will have to be considered. Other elements from the periodic table do not have to be considered even if they are present in the sample or its matrix, because they do not emit spectral lines in the 100 pm-wide spectral window of interest. From this, the significance of spectral interference begins to emerge.

individual computers, but they do not have to be. Typically, such "processors" reside in the same computer and most frequently are referred to as *"neurons"*. Also according to Webster's dictionary, artificial is something *"made or produced by human beings rather than occurring naturally, typically as a copy of something natural"*. In this chapter, spectral interference correction in optical emission spectrometry (OES) will be discussed, using artificial neural networks (ANNs). **Spectral interference** (a long standing problem in optical atomic spectrometry [1–4]) arises when a spectral line emitted from an **analyte** (defined as the chemical species of interest) in an **analytical sample** (i.e., one to be used in chemical analysis) is overlapped with (or **interfered** by) a spectral line emitted from another chemical constituent that is also present in the same sample or in the sample's **matrix** (defined as whatever an analyte is in). For example, Zinc (Zn) as an analyte (**A**) in an iron-rich soil matrix. In a widely used, 6000–10,000 K hot, inductively coupled plasma (ICP), iron (Fe) can emit as many as 4000 spectral lines, whereas Zn has less than 10 sensitive (i.e., analytically useful) lines, thus making interference from an Fe line (e.g., from the soil matrix) on a Zn line likely. An additional example will be discussed

Assuming that all species involved (per legend of **Figure 1**) are present in a sample and that they all are at equal concentrations, only Copper (Cu, at 213.853 nm) and Nickel (Ni, at 213.856 nm) are shown to overlap with the Zn spectral line (at 213.856 nm). Per legend, and

**Figure 1.** Spectral interferences in a simulated 100 pico-meter (pm)-wide spectral window (between 213.806 and

213.906 nm) centered on the analyte zinc (Zn) 213.856 nm spectral line.

in conjunction with the simulated spectral windows shown in **Figure 1**.

228 Advanced Applications for Artificial Neural Networks

A more dramatic (and yet very realistic example) will be drawn from the practice of elemental analysis by optical emission spectrometry and will be discussed in conjunction with the data shown in **Figure 2**. In this example, Arsenic (As) is the **Analyte** (or simply **A**) and Cadmium (Cd) is the **Interferent** (or simply **I**). It is worth noting that water contaminated by As is a key problem worldwide affecting the health and well-being of more than 100 million inhabitants worldwide because As occurs naturally in the soil surrounding water wells used as drinking water supply in many parts of the world.

In the example shown in **Figure 2**, the intensities of the spectral lines of the As (**Analyte**, **A**) and of Cd (**Interferent**, **I**) are about equal (as indicated by the horizontal dashed line positioned on the intensity axis) crossing at 1 (the intensity axis was normalized to the maximum intensity of the As spectral line, the intensity of the Cd line was scaled accordingly). To produce a 1:1 intensity ratio of **A:I**, the concentration ratio of **A:I** was 1:0.3 (due to the different sensitivities of the spectral lines involved). Furthermore, in actual practice, an analyst would

**Figure 2.** Simulation of the spectral response obtained from a spectral scan (solid line) when using an optical emission spectrometer and a mixture containing arsenic (As) as the analyte **(A)** and cadmium (Cd) as the Interferent **(I)**. **C** = concentration (regardless of units).

only observe the "added" or measured response shown by the solid line in **Figure 2**. The individual responses obtained by the simulations (shown in using the dashed and the dotted lines in **Figure 2**) will not be observed and they have been added to facilitate discussion.

In this example, the intensity of the combined spectral response for As and Cd (i.e., one obtained by adding the individual responses for As and Cd from a sample containing both As and Cd, and shown by the solid line in **Figure 2** or by experimentally measuring the combined response), is slightly more than 1.2 (indicated by the horizontal dotted line). Clearly, if the Cd interference on As is left uncorrected, the concentration of As will be reported to be higher than it actually is, in this example, by about 20%. Errors as large as the one discussed here are unacceptable in analytical determinations due to potential legal or regulations compliance reasons or health implications (e.g., as used in clinical analysis for medical diagnostics). In analytical practice (e.g., by commercial chemical analysis laboratories), spectral interference is addressed routinely using a number of methods, as will be briefly outlined below.

Several approaches have been used to correct the adverse effects of spectral interference. These traditional methods can be roughly divided into three categories: **Chemistry-based approaches** (i.e., via a chemical separation of an analyte from its matrix), **Physics-based approaches** (e.g., through use of high resolution spectrometry) and **Mathematics-based (or statistics-based) approaches** (e.g., via use of inter element correction factors or through use of Chemometrics). Briefly:

	- Use of high resolution spectrometry [1] to resolve spectral interferences is not feasible in routine analytical laboratories because spectrometers with high resolution (e.g., those with a long focal length) are expensive and they drift.
	- Use of a non-interfered spectral line. This is not always possible, especially if an analyte has few spectral lines useful for analytical determinations and the interferent has many (per Zn and Fe example discussed in the introduction).
	- Use of inter element correction (IEC) factors [5], in which the intensity of a spectral line of an interferent is measured at a different wavelength, and a correction factor is applied. Depending on the type of spectrometer used, this is not always possible.
	- Use of *chemometrics* methods [6–15]*,* defined as those involving the *"application of mathematical or statistical methods for the treatment of chemical data"*. Among others, examples of chemometrics approaches include adaptive filtering, factor analysis, orthogonal polynomials or curve fitting techniques.

Unlike the traditional approaches outlined above, ANNs have been "*designed to find relationships in multi-variate data through learning"* [16–19] that is to *"learn by example'*. As such, they provide an attractive alternative to the traditional methods mentioned above. Due their unique capabilities and their significance in this work, a brief background on ANNs will be provided next.
