The Relationship Between Atomic Absorption (AA) Spectroscopy and High Purity Water

For more than thirty years, heavy metals have been considered potential toxic environmental contaminants. It is a fact that most of today’s environmental challenges revolve around the determination of trace levels of elemental contaminants in water, sludge, soil, air and industrial hygiene samples. Atomic absorption (AA) spectroscopy is a mature technique that is used in elemental analysis. Figure 1 is a simplified schematic of a basic atomic absorption system.

Figure 1. Simplified schematic of a basic atomic absorption spectrometer

In order to analyze a sample for a specific element, a light source is required, which emits light at a specific wavelength that the atoms will absorb. When a ground state atom absorbs energy in the form of light of a specific wavelength, it is promoted to a higher energy level. The amount of energy absorbed at this wavelength is proportional to the number of atoms of a particular element.

The electrode-less discharge lamp(EDL) and the hollow cathode lamp (HCL) are two common light sources used in atomic absorption. The ground state atoms required for atomic absorption to occur are produced by the sample cell or the atomizer. This involves the application of thermal energy by a graphite furnace or a flame to break the bonds that hold atoms together.

The specific wavelength of light to be used is isolated by the monochromator and the detector measures the light accurately. The EPA specifications for environmental testing for some metals call for the use of graphite furnace AA devices (GFAA). The high sensitivity of GFAA, with routine determinations for most elements in the ppb levels, makes it ideal for environmental applications.

Advancements in instrumentation have allowed the use of GFAA to detect trace elements in complex matrices, like biological and geological samples. Trace elemental analysis requires that solvents and high purity agents are used to ensure the accuracy and precision of measurements. This work is a proof of concept that illustrates the suitability of an ultrapure water purification system for AA applications.

Method

The method is as follows:

• The concentrations of the following elements in ultrapure water freshly delivered from a Direct-Q^® 3 UV water purification system as shown in Figure 2 were measured by GFAA
• These elements include Arsenic (As), Cadmium (Cd), Chromium (Cr), Lead (Pb) and Selenium (Se).
• A PerkinElmer^® AAnalyst™ 800 instrument as shown in Figure 3 was used for the analyses.
• This instrument is equipped with a transversely heated graphite furnace (THGA), longitudinal Zeeman-effect background correction, and solid-state detector. Each injection was prepared with 20 pl of water sample + 5 pl matrix modifier.
• A matrix modifier was added to the sample to prevent volatilization of the analyte.

Table 1 provides the lamps and matrix modifiers used for each element analyzed.

Table 1. Light sources and matrix modifiers

* EDL = Electrodeless discharge lamp; HCL = Hollow cathode lamp

Figure 2. Direct-Q^®3 UV water purification system

Figure 3. The PerkinElmer AAnalyst 800 instrument

Results and Discussion

Arsenic, cadmium, chromium, lead and selenium are elements that are introduced naturally into our environment are found practically in all living organisms. They participate in a wide range of biological functions that includes being components of enzymatic systems.

However, in recent years, human activities have released a very high amount of these elements into the environment, resulting in levels that could be toxic to humans. Table 2 summarizes the maximum contamination level (MCL) in drinking water of these five elements, their potential health effects upon ingestion of contaminated water, and the sources of contamination. (Source: EPA web site)

Table 2. Maximum contamination levels of As, Cd, Cr, Pb and Se in drinking water according to the EPA.5

* This is the action level for lead. If more than 10 % of tap water samples tested for lead exceed the action level, then the public water systems must take additional treatment steps to reduce lead content to acceptable levels.

Standard solutions of the elements under investigation were prepared and injected into the PerkinElmer^® AAnalyst instrument using the same experimental parameters as the samples. Signal plotting was done against the concentrations to construct a standard calibration curve. Figure 4 is the calibration curve for Pb.

Figure 4. Standard calibration curve for Pb obtained using the PerkinElmer^® AAnalyst 800 instrument.

The concentration of the element in the sample is calculated using the standard calibration plot. Table 3 shows the calculated concentrations of As, Cd, Cr, Pb and Se in ultrapure water, as well as the detection limit for the method. Under these experimental conditions, none of these elements were detected in ultrapure water. These results indicate that ultrapure water from a Direct-Q^® 3 UV system is suitable for use in AA analyses, since the risk of inaccuracy or lack of precision that may come from the presence of trace elements in the water would be avoided.

Table 3. Concentration of As, Cd, Cr, Pb and Se in Direct-Q^® 3 UV system water

The Direct-Q^® 3 UV system used in this experiment combines reverse osmosis, UV photooxidation, activated carbon and ion-exchange resins to produce ultrapure water directly from tap water. The purification steps are illustrated in Figure 5.

Figure 5. The purification steps in a Direct-Q^® 3 UV system.

Conclusion

GFAA is a highly sensitive method suitable for the routine measurement of trace elements. Since the detection limits for most elements are well below ppb levels, it becomes necessary that reagents and solvents be free of the element under investigation because this would lead to inaccurate results. Ultrapure water freshly delivered from a water purification system such as a Direct-Q^® 3 UV system is a suitable source of water for GFAA analyses.

A proof of concept was carried out by measuring the elements As, Cd, Cr, Pb and Se in ultrapure water by GFAA. None of these elements were detected under the experimental conditions provided by the method.

About EMD Millipore - Lab Water Business Unit

Water is the most commonly used solvent in laboratories and constitutes often more than 99% of the mass of solutions used in experimentations. The quality of water used in the lab is therefore critical for the success of the tests performed.

This information has been sourced, reviewed and adapted from materials provided by EMD Millipore - Lab Water Business Unit.

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