Fast and reliable analysis of wastewater and sludges with a scanning array ICP emission spectrometer

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Courtesy of PerkinElmer, Inc.

Wastewater and sludge analyses generally require the determination of many elements and are economically performed using ICP-OES, due to the speed and robustness of this technique. Several ICP-OES methods are available for this determination including EPA method 200.7 (1). In the USA, DIN 38 406 E22 (2) in Germany, or ISO 11885 (3) on a global scale.

For this type of application, the concentrations of typical toxicologically- relevant metals to be measured are relatively high compared to the detection limits of this technique. However, the matrix may vary considerably and high matrix concentrations may be observed for some types of samples. In these cases, a matrix-tolerant sample introduction system is as important as hardware and software safeguards in diminishing the effect of unexpected interferences. Array-type ICP emission spectrometers supply the user with spectra rather than single data points (4,5). This allows the user to utilize the spectral information around an analytical wavelength, so spectral interferences can be detected, avoided by selecting an undisturbed line or corrected for. With an array detector, the rate of data acquisition is much faster, because the system does not need to scan between analyte wavelength and background correction points. In addition, simultaneous background correction yields better precision, contributing to overall accuracy.

The use of ICP-OES in the environmental laboratory ranges from the occasional sample which needs to be run by the technical production workforce to production analyses that occupy the instrument 24 hours a day. Consequently, the requirements on startup and long-term performance are different for each laboratory. Ease-of-use, instant readiness, and robustness of the overall system are some of the considerations in evaluating the instrument.

A new scanning array ICP-OES instrument has been designed, which is able to handle the various demands of environmental analysis. This paper shows the analytical performance of this instrument for wastewater and digested sludges.

Experimental Instrumental
The instrument used was an Optima 2000™ (PerkinElmer Instruments, Norwalk, USA). The spectrometer uses a double-pass Echelle optical mount with movable grating and prism in order to slew to any analytical wavelength. A linear (CCD) as a detector registers the spectrum around the analyte wavelength, which allows simultaneous background correction in order to minimize flicker noise. The detector is split into two parts, one of which is being used to measure the analyte spectrum while the other registers a neon spectrum which is travelling the same path as the light originating from the plasma. If the system registers deviations from initial settings, then the wavelength setting is actively changed to its original setting, allowing for wavelength stability in the order of 0.1 pm. An example of this wavelength stability is shown in Figure 1.

The system allows the plasma to be viewed either axially or radially. Axial viewing was used in this application to obtain the lowest detection limits.

A newly constructed solid-state RF generator supplies power up to 1500 W with good output stability. For further stabilization of the signal, the sample introduction system can be thermostatted for a constant evaporation rate in the nebulizer chamber. The operating conditions are listed in Table 1. The Mg I/II ratio was calculated to be 11.8.

The distance of the injector to the plasma can be adjusted mechanically to optimize performance in the presence of high-dissolved solids. In case of clogging, it is easy to remove the injector from the outside without having to dismount the sample introduction system.

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