Flame Atomic Emission Spectroscopy (FAES)

Summary

We built a low-cost flame emission spectrometer system using a nitrous oxide-acetylene flame and an Ocean Optics Maya2000 Pro spectrometer, and studied its sensitivity and linearity in the determination of metals of interest in wastewater processing.

Introduction

A wide variety of methods are available for the determination of metal content in wastewater and tap water.

Electrochemical methods (ion selective electrodes, polarography), while of limited cost, allow the determination of only a limited subset of metals and require time-consuming operations and considerable operator skills. Likewise, colorimetric methods using selective reagents typically require complex sample preparation and suffer from selectivity issues.

Flame Atomic Emission Spectroscopy (FAES) has higher instrumental cost and requires a lamp change for each analyte. Analysis can be time-consuming due to the need to stabilise each lamp and to produce a calibration curve for each element in the matrix.

Figure 2

The preferred methods for comprehensive elemental analysis – microwave generation of plasma and observation of its light emission (ICP) or its mass spectrometric analysis (ICP-MS) – allow in principle the determination of all elements at the same time in a wide concentration range. Regretfully, ICP and ICP-MS instrumentation is expensive to purchase and operate, due the high costs of microwave generators and mass spectrometers and of the inert gas (argon) usually employed.

We investigated the suitability of a low cost diode array spectrometer (an Ocean Optics Maya2000 Pro instrument) observing a high temperature flame for the determination of metals of interest in the analysis of wastewater and tap water.

Applied in a wastewater processing plant, this system allowed the timely determination of the content of different metals in the incoming streams, hence optimisation of water treatment parameters and substantial economies in the consumption of chemical additives.

Experimental

In this work we used the burner of a commercial Atomic Absorption Spectrometer (PerkinElmer AAnalyst 200), as oriented in Figure 1. The burner has inputs for the reacting gases and includes a nebulizer for the liquid being analysed. This and equivalent parts by other manufacturers are commercially available as spare parts.

The spectrometer used was an Ocean Optics Maya2000 Pro equipped with a 600 lines/mm H1 grating and a 5 micron wide entrance slit, providing a spectral range of 200-600 nm and a resolution of 0.3 nm (FWHM).

Figure 3

Light was collected with a high temperature 5 mm diameter silica lens (74-UV-HTVAC) rated to 350 °C, placed at 7 cm distance from the flame, and was Case Study Flame Atomic Emission Spectroscopy (FAES) for elemental analysis of water samples transmitted to the spectrometer with a 1 meter, 600 micron core UV grade optical fibre (QP600-1-UV-VIS).

Preliminary work identified as best operating condition flows of 8.5 l/hour nitrous oxide and 4.6 l/hour acetylene; these values were used throughout.

Spectral line assignments were performed using Visual Spec, which is free software available from http://www.astrosurf.com/vdesnoux/.

Ocean Optics spectroscopy software OceanView was used to control the spectrometer and acquire emission spectra as well as to perform calculations of element concentrations using its scripting capabilities.

Results

The emission spectra of various metals were measured.

Typical results are given for a sample of Cr (30 ppm in 0.1 N HNO3, 0.2% KCl), at integration time of 600 milliseconds (Figure 2).

The emission lines were positively assigned (to Cr in this case) using Visual Spec software.

After measuring several multielement standards, we tested an unknown water sample.

We received for treatment a water sample of unknown composition. Emission lines of interest were seen in the 300-440 nm wavelength range. The spectrum revealed presence of iron, manganese and calcium. In figure 3 below, the sample spectrum (red) is superimposed to spectra of iron (blue), manganese (green) and calcium (grey).

Quantitative analysis

Standards were prepared in 0.1 N HNO3, 0.2 % KCl containing de 25, 50, 100, 250 and 500 ppm; Fe, 2.5, 5, 10 and 20 ppm Mn; and 0.5, 1, 2, 3 and 4 ppm Ca. The calibration curves (Figures 4-6) were exceptionally linear and noise-free. The zero offset of the calcium calibration curve (in the 50 ppb range) may be due to contamination.

The results for the unknown sample in figure 3 were:

Table 1

These results allowed us to determine the origin of the sample (metal coating processes) and to apply adequate handling for manganese recovery and treatment preliminary to discharge.

“Flame Atomic Emission Spectroscopy (FAES) of water samples using nitrous oxide/acetylene flame can be performed at relatively low cost using a diode array spectrometer and a high temperature nitrous oxide.”

Conclusions

Figure 7

Flame Atomic Emission Spectroscopy (FAES) of water samples using nitrous oxide/acetylene flame can be performed at relatively low cost using a diode array spectrometer and a high temperature nitrous oxide – acetylene flame, and allows the rapid determination of main group metals in the low ppm range with good precision and accuracy.

FAES can be a cost effective analytical tool in industrial water processing applications where timely information of metal content of wastewaters is required for their adequate and environmentally safe processing.

Main Image – Figure 1