Trace ion chromatography

Transcription

1 Trace ion chromatography The R&D Notebook 7 A publication of the Lab Water Division of EMD Millipore Stéphane Mabic, Ichiro Kano, and Daniel Darbouret Research & Development, Lab Water Division, EMD Millipore, Saint-Quentin-en-Yvelines, France. Abstract : Ion chromatography (IC) has been recognized as one of the most competitive techniques for trace analysis, based on results from continuing research and development work into different types of instrumentation. Ultratrace analysis requires the use of extremely clean reagents, including water. The ionic content of water was monitored by IC along a water purification chain, from tap water to ultrapure water. Anion and cation concentrations decrease from ppm levels, in the tap feedwater, to ppt levels, in ultrapure water. This study shows that ultrapure water from a Milli-Q system is suitable for trace analysis by IC. Introduction Due to the increased sensitivity of instrumentation, analytical detection limits can now reach ng/l (ppt) levels on a routine basis, and even pg/l (ppq) levels under certain conditions. Confidence in the data recorded for these trace analyses relies on the control of several conditions and parameters. Besides the technical performance of instruments, careful sample handling and reagent purity are the main parameters that can be controlled. 1 Water is one of the most important reagents because it is used in so many applications. Rinsing of liquid chromatography chains, dilution and preparation of blanks and samples, and final elution are just some examples where water is used. Because of this broad range of applications and the volumes required, extreme care must be taken with water quality. In particular, in trace ion analysis, constant and reproducible high purity water, free of residual ions, is necessary. Several methods are available to analyze ions with varying degrees of sensitivity: Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), 2,3 Flame Atomic Absorption Spectroscopy (FAAS), Electrical Atomic Absorption Spectroscopy (GFAAS) 4 and Ion Chromatography (IC) combined with UV-Vis spectroscopy or electrical conductimetry. 5 Ion chromatography, combined with conductivity measurement, has become a reliable and popular method for analyzing a large variety of anions and cations in dilute samples. 6,7 This method has been developed in several research areas, such as the study of Antarctic ice, 8 in the semi-conductor industry, 9 and in analytical research domains, such as environmental analyses of air, 10 soil, 11 and water. 12,13 The need for qualification and validation of drinking water, in order to meet new standards and regulations, has also encouraged more widespread use of IC, and the development of novel analytical techniques. There is a real challenge involved in developing a water purification system that can produce suitable water quality for trace analysis, and can monitor this water quality on demand, in compliance with Good Laboratory Practices. In this study, concentrations of the major ions detected in tap water were studied by IC at various stages of the water purification chain. EMD Millipore is a division of Merck KGaA, Darmstadt, Germany

2 Materials and methods Water purification chain A water purification chain used to produce water for analytical applications employs a combination of several technologies. The chain can be divided into two basic parts; pretreatment, involving an initial purification system, such as the Elix system, which produces pure water that is fed to a storage reservoir, and a final polishing unit, such as the Milli-Q system, which produces ultrapure water from the reservoir water 14 (Figure 1). The pretreatment system used here, the Elix 10 system, combines sequential water purification by filtration/adsorption, reverse osmosis (RO), and electrodeionization (EDI). The initial purification step in the Elix system employs a Progard pretreatment pack, which is located before the RO membrane, for several important reasons. Activated carbon beads in the Progard cartridge prevent damage to the RO membrane from chlorine (Cl 2 ) present in tap water, and remove some dissolved organics by adsorption. Polyphosphates, polymeric sequestering anti-scaling agents, act as an efficient chelating agent for Ca 2+ and Mg 2+ ions, 15 protecting the RO membrane and the EDI module from scaling. Finally, a 0.5 μm nominal pore size polypropylene depth filter effectively removes large colloids, silt, and particulates. This pretreated water is then processed through a spiral, thin-film, composite RO membrane, which reduces the ion content significantly. RO permeate water is directed to an EDI module, which combines ion-exchange membranes, ion-exchange resins, and a direct electrical current to remove ions from the water. 14 The current continuously regenerates the ion-exchange resins in the system avoiding the need for resin replacement. The Elix unit used in these experiments delivered ten liters per hour of purified water from tap water to a storage reservoir. This reservoir, made of specifically selected polyethylene, 16 fed the final polishing unit to remove traces of residual ions. The Milli-Q Element system was used here as a final polishing system, and delivered ultrapure water via sequential photooxidation, ion exchange, and filtration (Figure 2). 17,18 The photooxidation process employs a UV lamp ( = 185 nm and 254 nm), causing organic chelating molecules to break up, and inducing a release of chelated ions. Breaking up the organics also helps to avoid organic fouling of the resin beads. Ions are then sequentially removed by two mixed-bed resins made of low-leaching materials. A microfiltration membrane was placed at the point-of-use of the polishing system for particulate removal and to prevent reverse contamination from the external environment. The flow schematic depicted in Figure 1 shows the purification chain and the various sampling ports. Samples were collected at each purification step in polyethylene (PE) bottles 19 that were previously rinsed thoroughly with ultrapure water. Sampling ports were selected to monitor the efficiency of each purification step in the following places: from the tap water, the RO reject and permeate water, the EDI concentrate and product water, the reservoir water, and the final ultrapure water. Figure 1 : Water purification chain and sampling ports. Figure 2 : Milli Q Element Ultrapure Water System (UHMWPE filter - Ultra-High Molecular-Weight Polyethylene, PVDF final filter - Polyvinyldiene fluoride) Ion chromatography Ion analysis was performed using a Dionex DX 500 apparatus with a GP50 pump and an AS40 autosampler, connected to a CD20 conductivity meter with peaknet software. Sample volumes of 25 μl were used for a 50 to 5,000 ppb range, and 200 μl for a 5 to 1,000 ppb range. For anion chromatography, AS12A/AG12A columns with 2.7 mm Na 2 CO 3 /0.3 mm NaHCO 3 eluents, and a 1.5 ml/min flow rate, were used. An ASRS Ultra suppressor, at 100 ma, with an external water feed, was chosen. 2

3 For cation analysis, CS12A/CG12A columns, with 20 mm methanesulfonic acid eluents and 1.0 ml/ min flow rates, were used. In this case a CSRS Ultra suppressor, with a 50 ma recycle mode, was used. For trace analysis, the same columns and suppressors were used, but a preconcentration column was installed beforehand. A Dionex TAC-2 preconcentration column was used for trace anion analysis and a Dionex TCC-2 was installed for cation trace analysis. To avoid airborne contamination, samples were kept in PE bottles, inside a chamber under bar N 2 pressure. Samples of 7 ml were then forced, under pressure, to the preconcentration column at a rate of 1 ml/ min. PE bottles were rinsed three times before sample collection, and one liter of water was flushed through the system before collecting samples. Standard solutions for trace analysis were prepared by mixing and diluting standard solutions (from SpexCertiprep Inc, c.a. 1,000 ppm) for each ion. Water from the Milli-Q Element system was used for dilution. Results Pretreatment purification unit: to ppb levels As mentioned above, the initial treatment step of tap water using activated carbon beads and an antiscaling agent does not reduce the total amount of ions present in tap water, but efficiently protects the water purification chain from chlorine and scaling. Reverse osmosis was the first process employed to eliminate ions (Figure 1). Concentrations of the major ions present in tap water (Li +, Na +, K +, Mg 2+, Ca 2+, F -, Cl -, NO 3-, and SO 4 ) and the respective concentrations in the RO permeate are displayed in Table 1. Also reported is the percentage rejection of each ion, i.e. the percentage of each ion eliminated by RO. Clearly, an RO system is very efficient in removing a very large proportion of ions (87% to 99%, depending on the ion) present in tap water. The selective permeability is a function of the charge, the size, and the solvation of a particular ion. Thus, divalent ions were removed more efficiently (> 99% for Mg 2+, Ca 2+, and SO 4 ) than monovalent ions (87% to 93%). In parallel with ion concentrations, the conductivity decreased from 569 μs/cm in tap water to 12.3 μs/cm in RO permeate, corresponding to an overall rejection of 97.8%. Because ion rejection is a percentage of the incoming total ion concentration, it is clear that the ion concentration in the RO permeate is feedwater related. In a second experiment, monitoring of ion concentrations over a 20 min period (ca. 3.3 L) showed that the system reached the final concentration within five minutes, or after 0.8 L (Figures 3A and 3B). More importantly, this ion concentration remained stable for all ions studied, as long as the feedwater quality remained fairly stable. As could be anticipated, the conductivity readings paralleled these results and reached a stable value within five minutes. It can, be concluded, therefore from these results that reverse osmosis is a very efficient means of consistently removing all types of ions. Such a system is particularly suitable as the initial purification treatment in a complete water purification chain. Ions Tap water a RO permeate a % Reject Li Na K Mg Ca F Cl No SO Table 1 : Effect of reverse osmosis on ion concentration. a Ion concentrations in ppb (µg/l) Figure 3 : Reverse osmosis performance for cation (Figure 3A) and anion (Figure 3B) rejection. 3

4 The second ion removal step occurred via electrodeionization (EDI). 14,18 Results displayed in Table 2 show a clear decrease in all ion concentrations from the RO permeate water to the EDI product water. Ions are concentrated and then rejected in the concentrating channel. Concentrations obtained in the product water were below one ppb (1 μg/l) for every ion studied, and the resulting conductivity was below 1 μs/cm. In contrast to RO purification, removal of ions by EDI is much less dependent on the nature of the ions. Due to the electrical current applied in the module, migration of a particular ion is not dependent on its size. The electric current also reduces ion solvation, so that both monovalent and divalent ions respond well to the current. The ion content of the product water is therefore not directly related to the ion content of the feedwater. Typical chromatograms (Figures 4A and 4B) show the decrease in ion concentrations from tap water to RO water to Elix water. Under the experimental conditions used here, no more ions were detected at the ppb level. Estimates of the remaining ionic contamination was obtained in a separate study using ICP-MS analysis. 18 Ions RO permeate a EDI concentrate a EDI product water Li < 1 Na < 1 K < 1 Mg < 1 Ca < 1 F < 1 Cl < 1 No < 1 So < 1 Table 2 : Effect of electrodeionization on ion concentration. a Ion concentrations in ppb (μg/l) Figure 4 : Typical ion chromatograms showing the concentration of cations (Figure 4A) and anions (Figure 4B) in tap water, RO permeate, and EDI dilute. Polishing system: to ppt levels Whilst an EDI system fed by RO permeate water delivers water with ionic concentrations at the ppb level, a higher water purity is needed for ion chromatography. To reach sub-ppb purification levels, a specific water polishing system is necessary. A Milli-Q Element system, employing a range of purification technologies, 18 specifically designed for ultratrace analysis, was set up for this experiment. The IC system was fitted with an ion-exchange preconcentration column, as described above. Elix (RO-EDI)-fed reservoir water (Figure 1) and Milli-Q water were compared to standard solutions. Solutions of cations, Li +, Na +, K +, Mg 2+ and Ca 2+, and anions, F -, Cl -, Br -, NO, NO 3-, SO 4 and PO 4, were prepared, with concentrations of 1,000 ppt for each ion. Results are reported in Figure 5 (cations) and Figure 6 (anions). For comparison purposes, chromatograms were superimposed, and tracings were arbitrarily set at 0, 0.1, 0.2, 0.3 and 0.4 μs/cm. Comparison of the IC traces indicated a reduction of ion concentrations from Elix water to Milli-Q water. Integrated peak values are reported in Table 3. Anion concentrations in Milli-Q water were all below detection limits (< 1 ppt). Experiments are currently in progress to improve detection limits for early eluting ions, using KOH as an eluent, and then an eluent generator to suppress the water deep. Cation concentrations were also very low, with the exception of Ca 2+ and NH 4+. Whilst the presence of NH 4+, most likely arose from airborne NH 3 contamination, the Ca 2+ contamination probably originated from the tubing and the PE bottle used to collect the samples. Based on these results, Milli-Q water, used in standard IC conditions, is suitable for dilution and preparation of blanks. Specific ion concentrations can be improved by using specifically designed purification resins, by carrying out particular rinses of the tubing (such as HNO 3 for cation analysis), or by special chromatographic configurations and columns. 4

5 Ions Reservoir water Milli-Q water Li + < 1 < 1 Na K + 50 < 2 Mg Ca Figure 5 : Ion chromatography of Elix (RO-EDI)-fed reservoir water, Milli-Q water and a standard (1,000 ppt) cation solution. F - < 1 < 1 Cl < 1 NO < 1 SO < 1 Table 3: Concentrations (ppt) of ions in Elix -fed reservoir water and Milli-Q Element water Figure 6 : Ion chromatography of Elix (RO-EDI)-fed reservoir water, Milli-Q water and a standard (1,000 ppt) anion solution. Conclusion Water delivered by an ultrapure water system, such as the Milli-Q, is suitable for IC analysis. However, it is important to use good quality feedwater for the polishing system, by employing a pretreatment system in the water purification chain, such as an Elix system, to ensure optimum efficiency of the polishing packs. As sampling techniques and instrumentation constantly improve, detection limits are being pushed ever lower. High quality ultrapure water, delivered by a Milli-Q system, is necessary to meet all the requirements of ultratrace analysis, as required for critical cleaning and the preparation of blanks and standard solutions. 5

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