Ultrapure Water to Assess Toxic Elements in Environmental Analyses

Water Quality Requirements for the Analysis of Toxic Elements

The quality of reagent water used to measure the presence of toxic elements during environmental analyses is critical to results reliability and accuracy. This study demonstrates the suitability of freshly prepared ultrapure water, produced by Mill-Q^® water purification systems, for ICP-OES and ICP-MS trace element analyses in environmental laboratories.

Dramatic improvement in the sensitivity of analytical instruments over the last decades has changed our understanding of environmental contamination and hazardous effects of metals such as Be, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Sb, Ba, Hg, Tl, and Pb. This has resulted in a number of regulations and guidelines that establish the maximum acceptable or recommendable concentrations of toxic metals in drinking water,¹ marine water,² and wastewater.³ The requirements instituted by authorities consequently have resulted in a growing need for toxic metal monitoring in environmental laboratories where spectrometry techniques are standard instrumentation recommended for the determination of trace elements.^4,5 The preponderant role of ICP-MS and ICP-OES in the detection of traces of toxic metallic elements in environmental analyses of water and soil has led to higher quality requirements for ultrapure water, which is the most frequently used reagent in ICP-MS and ICP-OES analyses. In particular, ultrapure water is used as the reagent blank, for sample and standard preparation, and for instrument and sample container cleaning (Figure 1). Therefore, the ultrapure water must be free of metals to preserve analytical instruments from contamination and to avoid interferences with analyzed elements, in order to ensure the accuracy and precision of measurements.

Optimal Water Quality for ICP-OES and ICP-MS Analyses

To benefit fully from modern ICP-OES and ICP-MS instrumentation, high-quality ultrapure water is required. Any contamination coming from laboratory reagents will increase background equivalent concentration (BEC) and the limit of detection (LOD), resulting in poorer performance of the technique. Therefore, the suitability of reagent water used in all steps of ICP-MS or ICP-OES analyses is defined by the general rule that the measured element should not be detectable in the blank. If it is detected, its BEC should be negligible relative to the desired analytical range. In environmental analyses, elements in water samples are usually analyzed at μg/L (ppb) analytical range⁶ and in soil samples, at mg/L (ppm) range.⁷ To ensure the success of experiments in the ppb-ppm range, it is desirable that BEC values of target elements do not exceed ppt or sub-ppt range. Moreover, as LOD (Limit of Detection) is separately specified in certain analyses,¹ in addition to a negligible level of contamination, the usage of ultrapure water of consistent quality is critical.

Suitability of Milli-Q^® Ultrapure Water for Elemental Analyses

To evaluate the suitability of reagent water necessary for ICP-MS and ICP-OES environmental analyses, we measured for the presence of toxic elements in freshly produced ultrapure water from a Milli-Q^® water purification system. Table 1 presents the resulting BEC of reagent water, as well as the detection limits in ng/L level. The results show that when using Milli-Q^® ultrapure water, BEC levels for the majority of analyzed elements are in the sub-ppt or low ppt range (experiments are done under normal laboratory conditions, not in a cleanroom). In case there is a need to achieve significantly lower levels of elements, it is reasonable to perform analyses in a cleanroom or metal-free laboratory environment⁸ and to use an additional polishing step such as a a Milli-Q^® IQ Element purification unit which makes it possible to obtain BECs at sub-ppt and ppq level.⁹

ICP-MS Experimental Conditions

Tap water was purified in two steps to obtain ultrapure water:

1. Pure water was obtained from tap water thanks to the combination of intelligent reverse osmosis, Elix^® electrodeionization (EDI), and a bactericidal UV lamp, using a Milli-Q^® system similar to the Milli-Q^® IX pure water system.
2. Ultrapure water was obtained by further purifying the above pure water with a Milli-Q^® polishing system, similar to the Milli-Q^® IQ 7000 ultrapure water system, fitted with a Millipak^® final filter. Note, for the analysis of Hg, ultrapure water was obtained from the Milli-Q^® Direct system, which does not contain an Elix^® EDI module.

The ultrapure water samples were analyzed for Be, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Sb, Ba, Tl and Pb using an Agilent^® 7700s ICP-MS instrument, and for Zn and Hg with an Agilent^® 7500s ICP-MS instrument. All experiments were performed under regular laboratory conditions (not in a clean room).

Instrumental details and parameters for the Agilent^® 7700s: PFA (perfluoroalkoxy)-50 nebulizer, PFA spray chamber, sapphire inert torch, quartz 2.5 mm i.d. torch injector, platinum sample and skimmer cone, RF power 600 / 1600 W, sampling position 12 / 8 mm, carrier gas flow 0.90 L/min, makeup gas flow 0.32 / 0.51 L/min, auto detector mode, calibration through 1, 5, 10, 50 ng/L.

Instrumental details and parameters for the Agilent^® 7500s: quartz nebulizer, quartz spray chamber, quartz 2.5 mm i.d. torch injector, nickel sample and skimmer cone, RF power 1300 / 1550 W, sampling position 8 mm, carrier gas flow 0.96 L/min, makeup gas flow 0.23 L/min, auto detector mode, calibration through 1, 20, 50, 100 ng/L.

Containers were all PFA pre-cleaned with ultrapure water. All ultrapure water samples (resistivity of 18.2 MΩ·cm and TOC below 5 ppb) from the Milli-Q^® water purification systems were analyzed immediately after water collection.

Reliability of Milli-Q^® Ultrapure Water for Elemental Analyses

The importance of reagent water quality for toxic element analyses in environmental samples was discussed and low levels of elements in ultrapure water produced by a Milli-Q^® water purification system were demonstrated. Laboratories performing trace element analysis can rely on Milli-Q^® ultrapure water purification system to meet their stringent requirements for the highest purity water for their sensitive applications. Choosing water from a Milli-Q^® ultrapure water systems for trace element analyses will help to ensure the generation of accurate and high-quality data.