ULTRAPURE WATER FOR DETERMINATION OF TOXIC ELEMENTS IN ENVIRONMENTAL ANALYSES
In this paper the importance of reagent water quality for toxic element environmental analyses is discussed, and the suitability of fresh ultrapure water produced using Merck water purifi cation systems for ICP-OES and ICP-MS trace element analyses in environmental laboratories is demonstrated.
Introduction and Water Quality Requirements
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 and wastewater.3
marine water,2 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.
Results and Discussion
To benefi t fully from modern ICP-OES and ICP-MS instrumentation, ultrapure water of very good quality is required. Indeed, any contamination coming from laboratory reagents will increase background equivalent concentration (BEC) and detection limit, 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 defi ned by the general rule that the measured element should not be detectable in the blank, or 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 mg/L (ppm) range.7
and in soil samples, at 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 specifi ed in certain analyses,1
the usage of ultrapure water of consistent quality is critical.
To evaluate the suitability of reagent water necessary for ICP-MS and ICP-OES environmental analyses, the measurements of toxic elements in freshly produced ultrapure water from a Milli-Q® water purifi cation system have been performed. The resulting BEC of reagent water, as well as the detection limits in ng/L level, are presented in Table 1. The results from Table 1 show that when using Milli-Q®
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 signifi cantly lower levels of elements, it is reasonable to perform analyses in a cleanroom or metal-free laboratory environment8 polishing step such as a Q-POD®
Element unit, which makes it possible to obtain BECs at sub-ppt and ppq level.9,10
Experimental Ultrapure water samples from a Merck Milli-Q®
in addition to a negligible level of contamination,
Ultrapure water samples from a Merck Milli-Q® purifi cation system, equipped with a QPAK® and Millipak® an Agilent®
Direct water TIX cartridge
fi nal fi lter, were analyzed for the Hg level using 7500s ICP-MS instrument. All experiments were
performed under regular laboratory conditions (not in a cleanroom).
The instrumental details and parameters for the Agilent® 7700s:
PFA (perfl uoroalkoxy)-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 fl ow 0.90 L/min, makeup gas fl ow 0.32 / 0.51 L/ min, auto detector mode, calibration through 1, 5, 10, 50 ng/L. The instrumental details and parameters for the Agilent®
7500s: and to use an additional
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 fl ow 0.96 L/min, makeup gas fl ow 0.23 L/min, auto detector mode, calibration through 1, 20, 50, 100 ng/L.
Advantage A10
water purifi cation system, equipped with Q-Gard® TEX cartridges, Millipak®
fi nal fi lter, and fed by an Elix®
and Quantum® Essential 5
water purifi cation system, were analyzed for the levels of Be, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Sb, Ba, Tl, and Pb using an Agilent® 7700s instrument, and for the level of Zn, using an Agilent® instrument.
IET November / December 2018
www.envirotech-online.com 7500s
The calibration standards used in experiments with the Agilent® 7700s were a mixture of Agilent® the Agilent®
and SPEXCertiPrep® 7500s, ROMIL PrimAg® pre-cleaned with ultrapure water. All Milli-Q® , and with
-xtra; containers were all PFA ultrapure water
samples (resistivity of 18.2 MΩ·cm and TOC below 5 ppb) from the Merck water purifi cation system were analyzed immediately after water collection.
Figure 1. Different types of uses of ultrapure water in ICP-MS and ICP-OES analyses
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