Analysis of low levels of metals in drinking water with a scanning array ICP emission spectrometer and ultrasonic nebulization
Drinking water is an important factor in exposure to environmental contaminants and is used in preparing foods and beverages for human consumption. Therefore, great care is used to verify that it is free of or contains acceptable limits of toxic substances. With respect to heavy metals and other toxicologically relevant elements, atomic spectrometric techniques are frequently used to determine their concentration levels (1). While atomic absorption spectroscopy has been the main technique employed for this application, inductively coupled plasma optical emission spectrometry (ICPOES) has become more important during the past years. National and international agencies set guidelines and methodology for the use of ICPOES for water analysis (e.g., EPA 200.7 (2), ISO 11885 (3), the latter has been adopted by the European Community and its participating states).
ICP emission spectrometers equipped with an array detector show a significant advantage over conventional technology with respect to speed, sensitivity and stability(4). which has produced an ever-growing market for this type of instrumentation. The established analytical advantages of array detectors were transferred to a new scanning arraytype instrument in order to retain the analytical advantages at a reduced instrument cost. The price of the analysis is composed of many different factors. These include the instrument and maintenance costs, the speed of analysis, and system usability. Great care was taken to develop a system that can analyze at a high speed with minimum requirements for maintenance and education.
An ultrasonic nebulizer produces a very fine aerosol, so the sample delivered into the plasma is much greater (about a factor 10) than with a pneumatic nebulizer. The enhanced sensitivity can be utilized for the improvement of detection limits by about the same factor, because the signal stability is comparable to that of pneumatic nebulizers.
This work demonstrates that the new scanning array ICP emission spectrometer in combination with an ultrasonic nebulizer is capable of determining extremely low concentrations of elements in drinking water according to the requirements of the strictest national and international laws and regulations. Major elements, such as Ca, Mg, Na, and K, can be measured in the same analysis run, but was not within the scope of this work. Mercury is often combined in a multielement analysis using ICP-MS, but was not considered within the scope of this work.
The regulatory aspects are twofold: One aspect are the permissible concentrations in drinking water, the other is the regulation that guides the analyst to use a certain technique and the methodology for an accurate analysis. The issue of setting permissible values is usually a compromise between toxicological and economical issues and is strictly tied to national laws and ordinances. Table 1 lists tolerable concentrations for drinking water in several regions of the world (5-7).
The concentrations given by the World Health Organization (WHO) are recommended values. Similarly, the European Community (EC) suggests concentrations, which are adopted into national laws by the members of the EC. In the EC regulations, there are minimum requirements on the quality of the analytical data generated to verify the quality of the drinking water (7). The limits for accuracy and precision may not exceed 10%, which means that the systematic and statistical deviation may not be higher than 10% at or above the tolerance level. In addition, the detection limit must be less than 10% of the tolerance level. While normally the detection limits are calculated using 3σ, the EC suggests 5σ when using the blank method.