by Dr E Rosland and Dr K Harper
Brine, i.e. a concentrated aqueous solution of sodium chloride, is not only found naturally in the environment as a result of the dissolution of salt deposits, but is widely used in various applications in the food, metallurgical and chemical industries. Many production processes that use brine have evolved in recent years from employing mercury cell electrolysis to a cleaner and more environmentally friendly membrane cell technology that involves the electrolysis of brine in a membrane cell. The presence of trace metals can have a considerable impact on the life and performance of such ion exchange membrane cells so the quality of the brine used in such processes is of extreme importance. Prior to use in the membrane cells, the brine is usually assayed by measuring the levels of contaminating metals. Analytically, the estimation of low, trace levels of metals in such an aggresive solvent as brine can be quite a challenge. This article describes the evaluation of an analytical method using inductively coupled plasma mass spectrometry (ICP-MS) and its performance in a real-life industrial setting where brine is used in the production of wood-based chemicals.
The analysis of high salt matrices by optical Inductively Coupled Plasma Mass Spectrometry ICP-MS can be difficult since a dedicated radial MS system is best suited for the matrix itself, but frequent trace level impurities require the sensitivity of an axially mounted system. Common problems with the analysis of brine on an axial system include matrix-matching issues, since it is difficult and expensive to source high purity sodium chloride (NaCl) for calibration standards. Another issue occurs in the analysis of raw and purified brines when aqueous standards are used to calibrate the system since there are considerable differences in viscosity which cause differential sample transport and nebulisation efficiencies. In addition, the nebuliser and centre tube in the mass sepctrometer are prone to salt deposition from continuous aspiration of concentrated brine solutions. Dilution is often used to reduce these physical effects, but this results in a reduction of sensitivity and the degradation of the detection limits in an already challenging matrix.These challenges can be overcome with the use of modern ICP instruments which enable internal standard addition, compensating for transport effects, and the use of high-solids sample introduction kits and argon humidifiers to reduce the clogging of the system during analyses.
Comparison of methods
Two methods for the analysis of brine were compared; one a fast screening method (Method 1), and the other a standard integration method (Method 2). In both methods a Thermo Scientific iCAP 6500 Duo ICP system was used. A high solids kit and an argon humidifier were fitted to help with the handling of the high dissolved salt content of the samples. Axial view was chosen for all elements due to the trace levels to be analysed. The system parameters were optimised with a 10% NaCl solution spiked at 500 ppb, using the Optimize Source function on the system, which automatically optimises pump speed, nebuliser gas flow, auxiliary gas flow, coolant gas flow and RF power for either the best signal, the best signal to background ratio (SBR) or the best detection limits (DL), which was chosen for this method.
Sample and calibration preparation
Two sources of brine were used for studies, which were procured from European brine manufacturers - Brine 1 was 10% brine from a food production factory, and Brine 2 was a 30% solution from an industrial chemical producer. Brine 2 was diluted 3X with deionised water to a 10% mixture. In the absence of a brine certified reference material (CRM), Brine 1 was spiked with 50 ppb of all elements to be tested. These were: Al; Ba; Ca; Cu; Fe; Mg; Mn; Ni; P; Pb; Si; Sr; Ti; V; W; Zn. Calibration solutions were prepared in deionised water at 0, 50 and 250 ppb for all elements analysed. An yttrium internal standard was used with a final concentration value of 1 ppm at the plasma.
Analyses and results
The first method was a fast screening method whereas the second method used a longer integration time to improve detection limits, test the robustness of the sample introduction system over long periods of analysis and verify analytical performance in terms of sample recoveries. The screening method used a 3 replicate 10 second analysis (5 seconds for UV/VIS) and analysed 20 samples with two calibrations. The entire run was completed in less than one hour. The second method acquired data in the format of three replicates of 30 seconds (15 for UV and 15 for VIS) and was applied to 100 samples. Samples were analysed over two days with the largest run consisting of three calibrations and 30 samples – this run took two hours and 30 minutes to complete. Recoveries were totally acceptable. The results for Brines 1 and 2 were quite different, which is indicative of the differences in local procedures and plant treatments used at the different sources and also their intended final use in different industries.
Short/long term precision and accuracy
The spiked samples were analysed periodically using both methods in order to determine the stability of the instrument over an extended period, namely 60 minutes for Method 1 and 150 minutes for Method 2. The short term precision averaged <2 % RSD for Method 1 and <1 % RSD for Method 2 for all elements. It is worth noting that the integration time was not a significant factor in recoveries as both methods produced comparable data in the first hour. Method 1 had excellent accuracy (+/- 5%) for all elements except Al (which was still within +/- 15%). With Method 2, no significant degradation in analyte signals was observed after two hours and 30 minutes of analysis, showing the outstanding long-term stability and robust sample introduction of the instrumentation. All elements had accuracies of +/- 15% with most (except Al, Mn and Mo) within +/-10%.
Case study
The latest advances in trace contaminant analysis in brine using ICP instrumentation are being applied by Borregaard, a Norwegian company located in the southeastern town of Sarpsborg and one of the world’s leading suppliers of wood-based chemicals. The company’s core business is based on a biorefinery that manufactures products from the different components in wood. The company’s old mercury cell electrolysis plant has been replaced by a modern, environmentally friendly membrane-based cell electrolysis plant.Borregaard sought an ICP instrument that would be sufficiently fast and powerful to allow quick and rapid screening of brine. The company needed a rugged, reliable ICP system that could cope with the online analysis of the NaCl produced on site, day in, day out, and that would thus help to ensure the purity of the feed brine.
One of the critical parameters of the caustic soda and chlorine production processes used by Borregaard is the purity of the feed brine, particularly with respect to the concentrations of calcium (Ca) and magnesium (Mg). The sum of the concentrations of these two elements must be less than 20 µg/L in 30 % NaCl. To achieve pure brine, the raw brine is purified over two ion exchange columns and an analysis is carried out daily to measure the impurities before, between and after these columns. The routine analysis comprises approximately ten solutions including calibration standards, samples and control samples. Using a high solids kit and argon humidifier, the samples are analysed from the factory line, so a fast turnaround is required. Consequently, the method chosen uses a five second integration time as this enables rapid screening and feedback to the factory. It was found that the results for the critical elements calcium and magnesium, which cannot have a combined concentration value in excess of 20 ppb, were both well within range for both the in-house and the external brine sample detection limit, thus allowing for their easy analysis below these required levels.
Conclusions
The analysis of challenging sample matrices such as brine can be achieved easily using modern ICP instrumentation such as the Thermo Scientific iCAP 6000 Series. Of course significant benefits can be obtained in terms of detection limits by using longer integration times, but this must be balanced against the requirement for fast analysis in a production environment. Similarly, the use of an internal standard offers enhanced stability to long-term analyses by correcting for any dynamic drift on the system. However, as seen in the Borregaard case study, for small batches of samples, excellent detection limits can be achieved without the use of internal standardization, thus enabling fast, accurate analyses.
The authors
Dr Eivind Rosland,
Research Scientist, Borregaard,
Sarpsborg, Norway
& Dr Karen Harper,
Applications Group Leader,
Thermo Fisher Scientific, Cambridge, UK
Full experimental details and results can be obtained from the authors.
