Advancing Water Quality: Evaluating Ion Identification Methods for Potable Water Production

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Introduction

The analysis of ions in water is an essential component in the process of turning raw water into potable water. Potable water refers to water that is safe to drink and requires pH, the presence of ions, oxygen levels and numerous other factors to be controlled in order to ensure it is not harmful to humans (Author of text, Year published). The presence of toxic ions, predominantly heavy metals that include lead, mercury, arsenic and cadmium, have detrimental effects to human health including damage to the mental and central nervous system function, effects on the blood composition of humans, and generally the cause of physical, muscular, neurological degenerative processes (International Journal of Innovative Research in Science, Engineering and Technology, 2013).

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The identification of inorganic substances in water samples is the first step to ensure the water individuals drink in potable. Once identified what the chemical substances are in a sample, further processes are done to remove the foreign substances to purify raw water, but this review purely focuses on the process of identification of ions in an aqueous solution.

The quantitative analysis of ions present in water samples can be found through various methods including the various spectroscopy methods, specifically Atomic Absorption Spectroscopy (AAS). The scope of the articles analysed are very broad because of the evolving technology and discoveries made over the past few decades, and span from 1978 to 2012. This area of study is relevant to the modern day because purification of water is an essential process to correctly execute due to the effect that poor execution has to human health [as stated above].

The purpose of this review is to analyse the effectiveness of numerous methods to specifically identify ions in water, and to examine whether Atomic Absorption Spectroscopy is the most effective spectrometric method of identifying ions.

Analysis of Atomic Absorption Spectroscopy

Flame Atomic Absorption Spectroscopy (AAS) is a quantitative method of analysis that uses the absorption of light by atoms to quantify the concentration of ions that may be present in a solution (Nelsons textbook - author, 2019). This method can identify specific ions in a sample due to atoms having a unique outer shell electron configuration, and this determines the absorption of energy (R. García etal., 2012). An atom in its ground state has its electron orbital closest to the nucleus and is lowest in energy level. The basic principle behind AAS involves measuring the absorption of light from a vapourized ground state atom. By determining a relationship between the light absorption and concentration, the concentration of an unknown solution containing ions can be determined. (AjaiPrakash Gupta etal., 2012). Before the spectrometric process occurs, treatment to a sample pre-analysis will need to occur. Articles by García etal. and Gupta etal. agree to pretreat the sample by using the processes of desolvation, vaporization and volatilization prior to analysis.

An article outlined that the actual AAS process needs to be performed on solutions, so the sample must be converted back into a liquid prior to analysis through a microwave decomposition to digest the sample (AjaiPrakash Gupta etal., 2012). Microwave decomposition is the technique used to dissolve metals in a solution pre-analysis to ensure a homogenous sample (B. Rock, 1979). The article of Gupta etal. highlights how this process is time-consuming and tedious, which is contrasted by another article stating that the microwave process reduces the time of the decomposition and avoids contamination of the sample (Izário Filho, 2012). This statement is backed up by another article stating that complete microwave decomposition recovered 85.5%-111% of the metals in the sample (D. A. Binstock, 2000). Microwave decompositions also take around ten to fifteen minutes (B. Rock, 1979), which further contrasts the previous statement stating that it is ‘time-consuming’. The article of ------ did not meantion any pre-analysis method

The general method of AAS actually refers to Flame Atomic Absorption Spectroscopy, and this process involves the emission of light of a certain wavelength that is characterised by the ion being tested, which is shone through an atomic vapour. Some of the light is absorbed by the atoms of the ion in a sample and this absorption is proportional to its concentration (García, R. etal., 2012). An atomiser removes the analyse atoms out of a solution and converts them into an atomic vapour, which is then directed in a path between the hollow cathode lamp source (source of radiation waves) and monochromator (a device that detects the light emitted) (Izário Filho, H.J. etal., 2012). Flame AAS, which is the most common method of AAS, uses a flame as an atomiser and measures the concentration of ions in a sample solution. Each article agrees that

The most common method of quantifying the absorption of light is via a calibration curve. In this method, more than three standard solutions of different concentrations of the examined ion are tested to establish a direct relationship between the absorption of light and concentration of an ion in a sample, since concentration is proportional to absorption (R. García etal., 2012). Another article described the process of the calibration curve, and how it developed over time as technological advancements have improved the method. When AAS was first developing, the signal obtained in the detector was recorded, and calculations of the relationship between concentration and absorption of light were done manually.

Now, technology is designed to automatically draw the analytical curve, relate the absorption to concentration, then measures the concentration of the sample being measured automatically through interpolation of the graph (H. J. Izário Filhoetal., 2012). Only one article went into depth about the non-linear aspect of the curve. The region of relatively low concentrations show a linear nature between concentration and absorption, but as it increases, the linearity is lost (Beaty, R.D. etal., 1978). Due to the older nature of the article that brought up the non-linearity as concentration increases, it could be posited that technological advancements have helped to alleviate this issue. García etal’s 2012 article stated that the selection of the monochromator is vital to obtain a linear calibration curve, meaning that there have been technological advancements to the different types of monochromators to ensure accuracy when calculating concentration.

Comparison to other Spectroscopy Methods

Atomic Absorption Spectroscopy is one specific method of spectroscopy, but generally, spectroscopy is the method of using radiated energy to identify characteristics of matter (Izário Filho, H.J. etal., 2012). There are numerous methods of spectroscopy, with the most practised and supposedly the most effective being Atomic Absorption and Inductively Coupled Spectroscopy. Within these methods, there are further sub-methods of analysis e.g. Inductively Coupled Plasma-Mass Spectroscopy and Inductively Coupled Plasma-Atomic Emission Spectroscopy, but generally, the underlying principle of each is common (Gupta, A.P. etal., 2012).

Graphite Furnace Atomic Absorption Spectroscopy (GFAAS) follows the same principle, pre-analysis of sample and general apparatus set up as flame AAS, but the atomiser used differs. GFAAS uses an electrothermal graphite furnace in the spectrometer with the light passing through, and atomic absorption occurs once the tube is heated so much that the sample dissociates into ions (Beaty, R.D. etal., 1978). An article by R García etal. stated that the only difference between flame AAS and GFAAS is the way the sample enters the spectrometer. This statement is false since GFAAS measures the analyte mass rather than the concentration which is another difference, which is supported by Beaty, R.D. etal., and Izário Filho, H.J. etal.’s articles. R Garcia etal. would be correct in saying that the only apparatus difference between the two methods was the way the sample entered the spectrometer, but since that was not mentioned, the statement is invalid.

Atomic Emission Spectroscopy (AES) follows the same principle as AAS, but instead of ions absorbed by the light, the emissions that the atoms release in their excited state is measured. As stated previously, AAS requires the atoms to be at ground state energy, but with AES, so much heat energy is added that the sample vapourises and the atoms become in an ‘excited state’ (Gupta, A.P. etal., 2012). Since every element has a specific energy level, they all have unique, identifiable absorption of emissions and absorption (Gupta, A.P. etal., 2012). Another article went more in depth about how the process works in terms of the excited state electrons, stating (use 1978 article) not complete

Is Flame Atomic Absorption Spectroscopy superior to other methods

Each spectroscopy method has significant benefits and limitations relating to the accuracy and relevancy and to each method. An article mentioned that the AAS method was well-established, relatively quick, economically affordable and versatile in that it can identify more than 60 chemical elements samples (Izário Filho, H.J. etal., 2012). This statement would be correct in saying it is established since it has been a successful method of identification of ions since the establishment in 1955 (Izário Filho, H.J. etal., 2012). The statement is also backed up by the 1978 article (Beaty, R.D. etal.), where there is a table of comparison of different methods of spectrometry and defines flame AA as ‘well-established’. The initial statement of AAS being relatively quick

The economical affordability of AAS is a component of analysis that needs to be analysed successively, since

One article described the affordability of AAS to be ‘low-moderate’ (Beaty, R.D. etal., 1978). This gives very little information about where the affordability, but rather a very general idea on affordability. Another article compared the costs to other spectrometric methods, saying that it was comparatively inexpensive (.

There are however limitations to the method including the sensitivity compared to other methods and the effect of chemical, matrix and ionisationinterferences. The main benefit of GFAAS is its sensitivity, which means the sensitivity of flame AAS is less (Izário Filho, H.J. etal., 2012). This is supported in every article, with one article comparing the sensitivity of all three methods mentioned in this review. The sensitivity of AES is less than both AAS methods (García, R. etal.,, 2012).

Interferences

Is AAS the future of identification of ions

A newer and more advanced flame AAS system resolves issues that scientists face with the tradition AAS system, which include an increase in sample loads and the number of elements to be determined at one time. Although these issues have got nothing to do with the validity or accuracy of the method, a decrease in efficiency is seen by having to determine each element individually. The system named the ‘Agilent- 240FS AA’ poses benefits of identifying 10 elements in under 2 minutes and reduced gas consumption for the process (). A comparison between the gas consumption and time for identifying 9 elements in 24 samples between conventional AAS and the 240FS AA system is shown in figure 2. This would revolutionise the current flame AAS method, due to one of the limitations of the spectroscopy being time-consuming since only one element can be measured at one time. The reliability of the source of information regarding this system can be deemed bias since Agilent (the company) wrote an article relating to the information above, but they are also the company trying to sell the product. Do I say how it could be reliable

Conclusion

Each method of spectrometry posed individual benefits and limitations, but the flame Atomic Absorption spectroscopy seemed the most effective for identifying ions in water. With technology ever-evolving, new methods of AAS will continue to develop to minimise the hardly any limitations that come with current AAS methods. It is a successful method of ions including heavy metals, which poses many benefits and is one process further to making raw water potable water.