The continuous development of lithium-ion battery technology is a key step in moving away from the combustion of fossil fuels at point of use. Lithium-based batteries are the most promising alternative because they combine high capacity and good cycle stability while being moderately inexpensive. To achieve the intended performance and test for purity of the raw materials used, including cathode materials (like binary or ternary alloys containing lithium, cobalt, manganese and nickel) and electrolytes (lithium hexafluorophosphate) is highly important. For analyzing trace elements at the required levels, techniques based on inductively coupled plasmas (ICP) are the ideal choice, especially ICP–optical emission spectroscopy (ICP-OES) and also increasingly ICP–mass spectrometry (ICP-MS).

The fast market adoption of electric vehicles requires the development of analytical methods that ensure the consistent performance and safety of batteries as one of the crucial components. For production facilities, especially in the ramp-up phase, regular and rigorous quality control (QC) of all the components, including the lithium salt, anode and cathode material, and the electrolyte, of a lithium-ion battery is therefore needed. It is paramount to control impurities to avoid premature capacity loss or even failure, which could have severe consequences to the vehicle and the driver. Two typical sample types, both of high importance for the industry, are presented in this article, the analysis of brines and the analysis of electrolytes.

The large amounts of lithium required for the production of batteries is currently being explored predominantly from underground brine repositories (1). Knowing the lithium content and the amount of impurities present in the brine are important in defining the right steps to extract and purify the lithium contained. Underground brine reserves are also rich sources of lithium, in addition to mineral sources, such as spodumene ore. The abundance of lithium in brine solutions can vary significantly in the con- centration range of 10–4000 mg/L and possibly exceed that depending on the geography of the source.

A typical electrolyte that is used in current lithium-powered batteries is a mixture of different linear organic carbonates, such as diethyl carbonate (DEC) and ethyl-methyl carbonate (EMC), with lithium hexafluorophosphate (LiPF

) (2). The electrolyte plays an important role in the charging and discharging performance of the battery; hence, it needs to be checked for potential impurities. At the same time, the electrolyte is also a sample type that allows the investigation of aging processes because degradation products from all components of the battery can accumulate within it over time. Finally, once the battery is at the end of its life, all components must be thoroughly screened to ensure that potential environmental contamination and injury risks to personnel disassembling the batteries are minimized.

Both of the aforementioned sample types present specific challenges when it comes to the analysis of elemental impurities, which can be uniquely addressed when using ICP-OES. Brine solutions (typically containing percent levels of mostly sodium chloride) can cause issues with the sample introduction system, such as clogging the nebulizer or the injector tube. However, when the sample introduction system is configured accordingly, samples containing up to several percent of total dissolved solids (TDS) can be analyzed with high sensitivity and accuracy. The typical samples that need to be analyzed for composition and failure analysis of battery electrolytes would typically contain elevated amounts of organic solvents (as highlighted in this article), as well as hydrofluoric acid, derived after the partial hydrolysis of LiPF

. Here, the standard components of the sample introduction system, commonly made from high purity quartz, will need to be replaced with hydrogen fluoride (HF)-resistant replacements.

An iCAP PRO XP inductively coupled plasma–optical emission spectroscopy (ICP-OES) Duo instrument was used in this study to carry out all measurements. The instrument was operated using intelligent full range (or iFR) mode, allowing a complete screening of the ultraviolet (UV) as well as the visible part of the spectrum in one single exposure. Because of the sensitivity requirement, the plasma was observed axially for analyzing the electrolyte solutions, whereas lithium was measured using the radial viewing mode. Analyzing samples containing highly dissolved solids is often a challenge for ICP-OES instruments, and particularly so for dual-view systems, because of problems such as salting up (leading to a blocked nebulizer or torch injector) and sample deposition on optical components (leading to signal drift and the need for increased instrument maintenance). To account for the high salt loads when analyzing brine solutions, the instrument was equipped with a ceramic torch and a Burgener Mira Mist nebulizer. To prevent salting up of the torch injector, an additional flow of argon gas was introduced at a flow rate of 0.15 L/min around the sample aerosol with the help of a sheath gas adaptor.

The instrument configuration and the operating parameters used for both sample types are summarized in Table I.

The brine samples used in this study were prepared using commercially available sodium chloride salt. To simulate the typical matrix of brine solutions, a 25% (w/w) solution of sodium chloride was prepared by dissolving 25 g of salt in 100 g of distilled water. All samples were then acidified at 0.5% (v/v) using nitric acid prior to analysis. To reduce the matrix impact, matrix-matched calibration standards were prepared. A mixed stock solution of 10 mg/L of the trace level analytes was prepared in 2% (v/v) nitric acid, which was then gravimetrically diluted to yield calibration standards in the desired concentration range between 10 and 1000 μg/L. In addition, a matrix-matched calibration curve was generated for lithium, covering a range between 10 and 5000 mg/L.

In addition to the brine samples, three different electrolyte samples were measured in this exercise. These were fresh unused LiPF

 electrolyte solutions in organic solvent mixtures like EC + EMC and EC + DEC. Approximately 2.5 g (~2 mL) of electrolyte samples were accurately diluted in 50 mL of an organic diluent consisting of 5% (v/v) EMC and 20% (v/v) ethanol in 18 MΩ·cm ultrapure water. A calibration blank and a set of calibration and linearity standards in a similar concentration range as for the brine samples were prepared using single element standards of individual analytes in the same diluent as for the samples.

Table II provides an overview on the selected wavelengths for both sample types as well as the obtained correlation coefficients (

) and resulting method detection limits (MDL). As can be seen, the wavelength selection had to be modified for some elements because of potential interferences caused by the sample matrix, but overall, good linearity and MDLs in the low mg/L concentration range were generally achieved for both sample types.

The accuracy and precision of the method was assessed by monitoring the concentration recoveries of two fresh electrolyte samples spiked with 50 μg/L of the target elements. Recoveries were found to be within the accepted range of 80–120%, with most elements showing >90% (Table III). A recovery of the internal standard (5 mg/L yttrium) of approximately 90% in fresh electrolyte sample matrices demonstrated low matrix suppression, further ensuring data accuracy.

To verify whether accurate lithium measurements were possible over a wider range of concentrations, lithium was spiked into unknown brine samples at both low and high concentration levels (here, 10 mg/L, 100 mg/L, and 3000 mg/L), and the percent recovery was calculated. The accuracy of the entire analytical setup was assessed by measuring an independently prepared QC sample containing 3000 mg/L of lithium. The QC sample was analyzed every 10 samples throughout the entire analytical run of 11 h. Accurate measurement of lithium at lower concentrations in brines is as important as being able to measure high lithium concentrations because the ability to achieve lower level detection is vital for ensuring that efficient lithium extraction has been achieved after processing the brine solutions. Extraction efficiency can be easily assessed by analyzing the original brine solution after the extraction process. Table IV shows the spiked concentrations and percent recovery observed for lithium in this additional study. The recovery presented was the average value calculated from six replicate measurements of samples spiked at 10 mg/L and 100 m/L analyzed at different intervals over the duration of the analytical run. The recovery presented for 3000 mg/L of lithium-spiked concentration was the average of 18 replicate measurements. The observed average recovery for lithium in each spiked solution was found to be in the range of 100±5% with a relative standard deviation of approximately less than 5%.

To evaluate the robustness for both sample types, the response of the internal standard (yttrium) was monitored during the measurement of a longer batch containing 135 samples (for electrolytes) and more than 200 samples (for brine), taking 7 h and 11 h to complete, respectively. The response is shown in Figure 1.

ICP-OES is a perfect tool for laboratories supporting the sourcing of raw materials or finished products for the manufacturing of lithium ion batteries. Typical samples, such as brines or electrolytes, expose specific challenges to the instrument. However, with the right configuration of the sample introduction system and optimized method settings (that is, wavelength selection), accurate and precise results can be obtained even for sequences exceeding typical working days of the laboratory personnel.

(1) S. Murodjon, X. Yu, M. Li, J. Duo, and T. Deng, in 

, P. Vizureanu, Ed. (IntechOpen, London, United Kingdom, 2020), ch. 10, pp. 1–39.

Lithium Batteries — New Materials, Developments, and Perspectives

 (Elsevier Science, Amsterdam, The Netherlands, 1994).

(3) Thermo Fisher Scientific Application Note AN 000602, Determination of Lithium and Other Elements in Brine Solutions Using ICP-OES (2022).

(4) Thermo Fisher Scientific Application Note AN 000243, Robust and Sensitive Measurement of Trace Element Impurities in LiPF

 Electrolyte Solutions Using ICP-OES (2022).

 are with Thermo Fisher Scientific, in Bremen, Germany. 

 is with Thermo Fisher Scientific, in Hemel Hempstead, United Kingdom. Direct correspondence to: 

Articles

Lithium-based batteries are key for moving away from the combustion of fossil fuels at the point of use. ICP-OES and ICP-MS methods can measure trace-element impurities that may affect battery performance.

Analysis of Trace Elements as Impurities in Materials Used for Lithium-Ion Battery Production

Inductively coupled plasma–mass spectrometry (ICP-MS) is a technique increasingly being used in commercial laboratories and industries that analyze samples with complex matrices, such as seawater, wastewater, saline groundwater, and other high salt samples. These laboratories often choose ICP-MS for its fast, multielement capability and high sample throughput, but time and cost pressures mean that laboratories have little time for sample preparation and method development. The time and cost pressures can lead to performance and operational issues because an ICP-MS instrument optimized for the highest sensitivity may not have the sufficient matrix tolerance to analyze high salt samples. In this article, we describe a method to optimize plasma robustness and interference control that enables a modern ICP-MS to perform simple, accurate, and routine analysis of critical trace elements in undiluted seawater.

As inductively coupled plasma mass spectrometry (ICP-MS) instruments become more widely available, less expensive, and easier to use, the technique is increasingly being used for routine trace element analysis in more challenging samples. High and variable matrix levels can be a feature of samples from many different industries and applications. Examples include food and agriculture, pharmaceuticals, chemical and petrochemical manufacturing, mineral prospecting, clinical research, and environmental analysis of groundwater, wastewater, and seawater.

Trace element levels in coastal and estuarine water are monitored to ensure the quality and safety of waters used for fishing and shellfish farming, as well as for recreational activities. Coastal seawaters contain higher concentrations of trace elements than open-ocean seawater, but priority contaminants such as toxic heavy metals must still be measured at low and sub-ppb (μg/L) concentrations, which means ICP-MS is often the preferred technique. Sample salinity in estuaries and coastal regions varies because of freshwater mixing, giving a range from seawater at approximately 3.5% total dissolved solids (TDS) to low salinity river water. As a result, routine ICP-MS methods must provide accurate analysis for varied sample matrices, a challenge that is further complicated by the fact that estuarine and coastal waters can contain high levels of dissolved minerals and organic matter.

Traditional ICP-MS methods for analyzing saline waters typically involve diluting the samples to reduce the salt matrix to below 0.2%, or 2000 ppm, TDS. This historical TDS limit has long been accepted as a requirement for routine ICP-MS analysis and has also been adopted in several standard ICP-MS methods, such as EN-ISO 17294-2 (Water Quality), US-EPA 6020B (Water and Wastes), and EN 13805 (Trace Elements in Food). However, diluting samples—either manually or using an autodilutor—degrades detection limits and risks adding contamination from the diluent and the laboratory equipment used to process the samples. The relative contribution from contaminants in the diluent rises as the dilution factor increases, so laboratories must take care to avoid errors being multiplied in the dilution-corrected concentrations reported in the original samples. Therefore, routine laboratories often prefer simple and more consistent methods where samples are analyzed directly without dilution.

Minimizing or avoiding sample dilution helps reduce contamination but results in higher matrix levels, potentially far above the accepted 0.2% TDS limit. Some modern ICP-MS systems have been engineered to tolerate high salt matrices for extended periods, offering an alternative to liquid dilution. One approach, known as aerosol dilution, dilutes the samples in the aerosol state, using argon gas as the diluent (1). Aerosol dilution avoids time-consuming sample handling steps and eliminates contamination from liquid diluents. Another benefit of aerosol dilution is that it reduces the aerosol load reaching the plasma, which results in a higher effective plasma temperature. High matrix levels require more plasma energy to decompose, so the plasma must be optimized for maximum robustness (lowest CeO

 ratio) to ensure it has sufficient energy to fully decompose the matrix. Commercial ICP-MS instruments range from a CeO/Ce ratio of approximately 0.03 (3%) for the least robust plasma to approximately 0.01 (1%) for the most robust plasma. Using aerosol dilution further improves the plasma robustness to a CeO/Ce ratio of approximately <0.005 (0.5%). Under these very robust, low CeO/Ce plasma conditions, undiluted seawater can be measured for extended periods without significant signal drift because of matrix deposition on the interface cones.

A further issue that can affect ICP-MS analysis of high salt samples is loss of analyte sensitivity because of ionization suppression. This effect occurs when free electrons from easily ionized major elements—such as sodium, potassium, and calcium—suppress the ionization of less easily ionized analyte elements, leading to lower analyte sensitivity. Ionization suppression affects poorly ionized elements to a greater degree and unfortunately several critical analytes—including arsenic, cadmium, and mercury—are poorly ionized. Using more robust plasma conditions reduces ionization suppression, increasing the signal for poorly ionized analytes and giving a more consistent analyte signal in varying levels of easily ionized matrix elements (2).

Finally, the high levels of several major elements in seawater can cause matrix-based polyatomic ion overlaps that lead to errors in the results. Avoiding, correcting, or resolving such spectral overlaps is a key part of developing a robust and reliable method for saline water samples. Some of the matrix-based polyatomic ion overlaps that can affect common analytes in saline samples, such as seawater, are shown in Table I. Mathematical interelement correction equations have historically been used in ICP-MS methods to correct the contribution from certain polyatomic ions, and such equations are still defined in some standard methods, such as EPA 200.8. However, these corrections are not always reliable in complex samples. For example, a standard correction equation for arsenic corrects for the argon chloride overlap on arsenic at mass 75. Argon chloride-75 is calculated from argon chloride-77 after subtracting the contribution from the selenium-77 signal, which is in turn calculated from the selenium-82 signal after correction for the contribution from krypton-82. However, this correction fails if significant bromine or sulfur is present in the sample because mass 82 also suffers overlaps from BrH

Most modern ICP-MS instruments now include a collision–reaction cell (CRC), which can give much more effective and reliable control of the polyatomic ions. CRCs use either reaction chemistry or collision mode, using a reactive or an inert cell gas, respectively. Reactive cell gases can be used on single quadrupole ICP-MS, but they have limited applicability as reactive gases can lead to significant new errors because of cell-formed reaction product ions. Consequently, on single quadrupole ICP-MS, collision mode using helium (He) cell gas is a much more reliable and universal approach, which is especially true for multielement analysis in high matrix samples because the inert He cell gas eliminates the possibility of new reaction product ions being formed from matrix elements. With an optimized cell configuration and operating conditions, polyatomic ions can be filtered out effectively using helium cell gas and kinetic energy discrimination (KED). KED works for all polyatomic ions, so the same cell conditions can be applied successfully to multiple interferences, multiple analytes, and for complex and varied sample types (3).

A standard Agilent 7900 ICP-MS instrument fitted with the ultrahigh matrix introduction (UHMI) aerosol dilution system was used for all measurements. UHMI utilizes the standard peristaltic pump and concentric nebulizer, but the plasma is optimized for highly robust conditions, with a longer sampling depth, a reduced carrier gas flow rate, and an additional dilution gas flow. A standard glass concentric nebulizer, a quartz-spray chamber temperature controlled to 2 °C, and a quartz torch with a 2.5-mm injector, were used together with standard nickel sampling and skimmer cones. Analyte isotopes, integration times, and cell conditions were defined by the selected preset method (EPA 6020). Plasma conditions were defined by the UHMI setting, and other instrument tune settings were optimized automatically using the ICP-MS MassHunter autotune function. No manual optimization was required.

The Agilent 7900 was used to analyze multiple elements of interest in two natural seawater samples known to contain low levels of trace elements. Samples were measured with and without a 10 μg/L multielement spike, with the results being calibrated against simple (no salt matrix), acidified, aqueous calibration standards. The standards were prepared from a mixed stock standard (multielement calibration standard 2A, Agilent part number 8500-6948) and single element standards for molybdenum, antimony, tin, thorium, and mercury (Kanto Chemical Co). The standards were diluted using ultrapure, 18 MΩ cm water (Millipore) and acidified to 1% nitric acid (HNO

) and 0.5% hydrogen chloride (HCl) (Kanto Kagaku EL grade).

Seawater contains approximately 3.5% total dissolved solids, but the matrix is mostly sodium chloride (NaCl), which is relatively easily dissociated in the ICP plasma. Therefore, undiluted seawater can be run on an Agilent 7900 ICP-MS instrument using an aerosol dilution setting of UHMI-8 (approximately 8× aerosol dilution). As is normal when analyzing high salt matrices, an argon humidifier accessory was used to reduce the buildup of salt crystals on the nebulizer and torch injector. Normal internal standard (ISTD) pump tubing with an internal diameter (i.d.) of 0.25 mm was used for the online addition of the ISTD solution. The sample solution was pumped using the standard 1.02-mm i.d. tubing, so the sample matrix was only diluted approximately 6% by the addition of the ISTD solution prior to nebulization.

The Agilent 7900 is typically optimized for robust plasma conditions, indicated by a low CeO/Ce ratio of approximately 0.012 (1.2%). UHMI conditions further improve plasma robustness to a CeO/Ce ratio of approximately 0.005 (0.5%), which ensures effective ionization of high first ionization potential (IP) elements, such as beryllium, zinc, arsenic, selenium, cadmium, and mercury, even in the presence of a high level of easily ionized matrix elements. No special sample preparation or operating conditions—such as the addition of carbon to the sample solutions and N

The EPA 6020 preset method used includes both no gas and He mode tune steps, but only the helium mode results are reported in this work. With the octopole reaction system (ORS) cell used in the Agilent 7900, helium mode conditions maintained high ion transmission while providing the high cell gas pressure required for effective interference removal by KED. Operating conditions used for the analysis of undiluted seawater are shown in Table II.

To validate the plasma conditions used for undiluted seawater analysis, samples containing a range of salinity levels were measured to assess the ionization suppression for the poorly ionized element cadmium. The saline samples were measured under several different plasma conditions covering the range typically reported for commercial ICP-MS systems, from the least to the most robust plasma conditions (CeO/Ce ratios from 0.03 (3%) to 0.005 (0.5%) with aerosol dilution). Cadmium, which has a high first ionization potential (1st IP) of 8.994 eV, was measured in various concentrations of salt matrix from 0.035% TDS (100× diluted seawater) to 3.5% TDS (undiluted seawater). Cadmium recovery was calculated for each salt matrix, calibrated using simple calibration standards (no salt matrix). The recoveries, shown in Figure 1, indicate that robust plasma conditions (CeO/Ce of 0.5%) gave consistent and accurate results for cadmium, eliminating the suppression observed under less robust (higher CeO/Ce) conditions. Robust conditions increase the amount of plasma energy available, giving higher and more consistent ionization of poorly ionized elements such as cadmium.

Spike Recovery and Stability in Seawater Reference Materials 

Using the UHMI-8 conditions (CeO/ Ce ratio of ~0.005 (0.5%)), the two different undiluted seawater samples were analyzed alternately, with and without a spike containing the analytes at 10 μg/L (mercury at 1 μg/L), over a period of 9 1⁄2 h. Trace element concentrations were quantified against simple aqueous calibration standards. The measured concentrations of one analyte, manganese, in the alternating spiked and unspiked samples are plotted in Figure 2. The mean spike recoveries for all elements are shown in Figure 3. Excellent recoveries of the spike concentrations were achieved, with most elements within +/- 5% of the spiked value, and with precision better than 5% RSD for the entire 9 1⁄2-h sequence.

The good recovery of the poorly ionized elements zinc, arsenic, selenium, cadmium, and mercury in the undiluted seawater matrix—measured against non-matrix-matched calibration standards—demonstrates the good control of ionization suppression. This is particularly important for applications where sample composition varies between samples, such as in the analysis of mixed saline environmental waters, where salt levels vary dramatically between river, estuarine, and seawater. Good recoveries for the interfered elements confirm the effective control of polyatomic ion overlaps using helium KED.

For any ICP-MS application that involves analysis of high matrix samples, it is recommended that ISTD signals are monitored throughout the analytical sequence. The ISTDs correct for the physical differences in sample transport and nebulization between low matrix (for example, simple calibration standards) and high matrix (such as undiluted seawater) solutions. The ISTD stability plot for each batch also provides an indication of the consistency of the raw signals measured by the ICP-MS, which is useful for monitoring drift and suppression. Figure 4 shows the ISTD signals from the long-term analysis of undiluted seawater samples. The robust UHMI conditions used ensured that there was minimal matrix deposition and signal drift throughout the 9 1⁄2-h analysis.

The Agilent 7900 ICP-MS instrument with UHMI aerosol dilution can tolerate high sample matrix levels, allowing stable, accurate, long-term analysis of samples such as seawater without needing either online dilution or manual sample dilution prior to analysis. UHMI aerosol dilution provides fully automated and calibrated setup of plasma conditions and aerosol dilution factors, delivering consistent analytical performance for routine laboratories.

Very robust plasma conditions (CeO/Ce ratio of 0.005 or 0.5%) reduce ionization suppression, increase sensitivity, and improve detection limits for critical poorly ionized elements such as arsenic, cadmium, and mercury. Reduced ionization suppression allows high and variable sample matrices to be measured against simple aqueous calibration standards, eliminating the need for matrix matching of calibration standards. This provides a greatly simplified workflow in laboratories that analyze samples with unknown or variable composition.

In this study, an Agilent 7900 ICP-MS with UHMI aerosol dilution was used to analyze a range of trace elements in undiluted seawater samples. The accurate spike recoveries against simple aqueous standards and the excellent long-term stability demonstrate the matrix tolerance and simple operation offered by the method.

The Open Chemical and Biomedical Methods Journal

(3) T. Kubota, “Analysis of Undiluted Seawater using ICP-MS with Ultra High Matrix Introduction and Discrete Sampling,” Agilent Publication 5994-4467 EN (2021).

 are with Agilent Technologies, Inc. Direct correspondence to: 

ICP-MS is increasingly being used to analyze complex matrices,
but an ICP-MS instrument optimized for the highest sensitivity may
not have the sufficient matrix tolerance to analyze high-salt samples. We describe a method to optimize plasma robustness and interference control for accurate, routine analysis of critical trace elements in undiluted seawater.

ICP-MS Configuration and Optimization for Successful Routine Analysis of Undiluted Seawater

Desalination has been growing rapidly globally to meet the potable water demands in areas where access to fresh water is limited. The byproduct of desalination is a brine that has a salinity approximately two times higher than seawater and is usually discharged back to the ocean, where it can have a negative environmental impact, especially if harmful elements were picked up during the desalination process. However, this brine has recently begun to be viewed as a resource where elements of economic and industrial importance can be recovered. Both these applications rely on the accurate analysis of various elements in the brine, which can be accomplished with inductively coupled plasma–optical emission spectroscopy (ICP-OES), given its high matrix tolerance and flexibility. This work demonstrates the accurate analysis of desalination discharge brines for elements of both environmental and economic importance using ICP-OES.

With climate change, access to fresh water is decreasing in many areas around the world. Combined with the continuing rise in global population, a shortage of potable water is imminent unless other sources of fresh water can be found. One way to increase amounts of potable water is through desalination of seawater, a technique that has been rapidly growing in importance. In 2019, there were approximately 16,000 desalination plants operating globally (1), and that number has increased yearly as more desalination plants come online.

The primary method of desalination is through reverse osmosis, a process that uses high pressure to force seawater through a membrane that separates the minerals from the water, resulting in both fresh potable water and a brine that contains the minerals removed from the seawater. The salinity of these brines is generally twice the salinity of seawater (≈6% dissolved solids) and is usually pumped back into the ocean, although other treatments are also used occasionally (2). Over 142 million cubic meters of desalination brine are returned to the ocean globally on a daily basis (1).

With so much desalination brine being returned to the ocean, brine must be monitored for elements that may have been picked up during the desalination process and can have a negative impact on the aquatic environment. However, there are no international standards or regulations for monitoring elements in desalination discharge brines returned to the ocean: elements and regulated levels vary widely and are set on a country or regional basis (3–6).

However, rather than being returned directly to the ocean, these discharge brines can also be used as a source of elements of economic and industrial importance (7). A variety of different elements are recovered from desalination brines where it can be more economically feasible than mining. For example, with the prevalence of battery technology, increasing amounts of lithium are required. Currently, lithium is mined and recovered from salar brines but can also be recovered from desalination brines. Rubidium and cesium are other rare elements that are difficult to mine and can be extracted from desalination brines (8).

An important aspect in recovering elements from brines is measuring their concentrations in the brine to determine the quantity present and whether it is economically beneficial to recover the metals. Likewise, the remaining brine after the recovery step should be analyzed to determine the recovery efficiency.

The analysis of a variety of elements in desalination brines can be accomplished by inductively coupled plasma–optical emission spectroscopy (ICP-OES) with minimal sample preparation and no dilution, given that ICP-OES has a high tolerance of dissolved solids. This work explores the analysis of both environmentally and economically important elements in desalination discharge brines.

Six desalination discharge brines were acidified to 1% nitric acid (v/v) and analyzed directly; no further sample preparation was performed. All measurements were made against external calibration standards prepared in 6% sodium chloride to mimic the dissolved solids content of the brines. The actual dissolved solids composition of the brines was not determined, but 6% sodium chloride proved to be a good match.

Two different instruments were used for these analyses: the PerkinElmer Avio 550 Max and the Avio 220 Max ICP-OES systems. Given the larger number of analytes, the Avio 550 Max was used for analyzing environmentally important elements, whereas the extended wavelength range of the Avio 220 Max allowed for the use of longer wavelengths, and increased sensitivity and flexibility, making it the more appropriate option for determining economically important elements. Tables I and II list the instrumental operating parameters and elements, along with their analytical wavelengths and plasma views, respectively. Because the matrix was the same on both systems, similar operating conditions were used. The main difference was the nebulizer flow. Because of the low ionization potential of lithium, rubidium, and cesium, a higher nebulizer flow was used on the Avio 220 Max to increase sensitivity. On both systems, a SeaSpray nebulizer was used in conjunction with an argon humidifier to maximize robustness. Because all analyses were done with a quartz torch, a plasma gas flow or 12 L/min was used to reduce devitrification of the torch. A ceramic torch could also be used, along with a lower plasma gas flow, although the higher plasma flow benefited the analyses performed on the Avio 220 Max. The higher-than-normal auxiliary flow was used to push the plasma farther above the injector, thus minimizing matrix deposition and potential clogging of the injector. A sample uptake rate of 0.5 mL/min was found to provide increased signal-to-background ratios because the plasma loading and matrix effects in the plasma were reduced.

Analysis of Environmentally Important Elements

When analyzing the six brine samples, only five elements were detected; all other elements were undetected. Figure 1 shows the concentrations of the five elements found in the six brine samples, along with their regulated levels in four different regions. Boron is above the Singapore limits in all six brines, whereas copper exceeds all the regulated levels in brine 2 and is higher than the Singapore limits in brine 6. Interestingly, copper was not detected in brine 5, and manganese was not detected in brines 5 or 6.

Not all elements are regulated in all regions; boron and manganese are only regulated in Singapore. Also, the regulated concentrations can vary widely among regions. For example, the regulated levels of silver vary by over two orders of magnitude. Given this wide range of regulated concentrations, the wide dynamic range of ICP-OES makes it the ideal technique to perform these analyses.

To determine the accuracy of the analyses, one of the brine solutions was spiked with all analytes at four different concentration ranges, with low representing the Dubai and half the Singapore limits, medium representing the Singapore limits, and high representing the Carlsbad limits.

As shown in Figure 2, all elements at all concentrations recovered within 10% of their true values, demonstrating the accuracy and breadth of the methodology.

Analysis of Economically Important Elements

Cesium, lithium, and rubidium are difficult to accurately measure by ICP-OES because they emit only as atom lines, yet have low ionization potentials (3.9, 5.3, and 4.1 eV, respectively), which means that most of the signal is lost because most of the atoms in the plasma are ionized. Therefore, they are not available to emit as atoms. In addition, these elements also suffer from the easily ionizable element (EIE) effect, which means that the same concentration of these elements produce different signal levels depending on the concentration of the matrix. There are several ways to minimize the EIE effect, with the most prevalent being matrix-matching the calibration standards to the sample and viewing the plasma in radial mode at a lower plasma viewing height before the atoms are ionized. However, radial view can also result in lower signal intensity, which may be an issue when measuring low concentrations.

During method development, it was found that calibration in 6% sodium chloride minimized the EIE effect. Because of the low intensity of cesium and rubidium, these elements had to be measured in axial mode, while lithium, which has much greater sensitivity, was measured in radial mode. As shown in Figure 3, both lithium and rubidium were detected in all the brines between 0.2–0.4 ppm (lithium) and 0.1–0.2 ppm (rubidium). However, cesium was not detected in any of the samples.

Detection limits were determined in brine 5 as three times the standard deviation of ten measurements made against the calibration curve. As shown in Figure 3, the lithium and rubidium detection limits are approximately an order of magnitude lower than the concentrations measured in the brines. The measured cesium detection limit is 0.0005 ppm, signifying that if cesium is present in the brines, it is below this concentration.

The accuracy of the methodology was determined by spiking one of the brine solutions with 0.04 ppb of cesium and 0.2 ppm of lithium and rubidium and measuring the recoveries three times. Although lithium and rubidium could be read at lower concentrations, they were spiked at 0.2 ppm because of their presence in the brine. As shown in Figure 4, all recoveries are within 10% of their true values, demonstrating the accuracy of the methodology.

This work has demonstrated the ability of ICP-OES to accurately measure elements in desalination discharge brines of both environmental and economic importance. Environmentally important elements are not globally regulated in desalination discharge brines and vary widely from region to region, yet ICP-OES provides accurate measurements over a wide concentration range. Likewise, these discharge brines can also be an economically viable source of a variety of elements, and this work has demonstrated ICP-OES can accurately and reliably measure cesium, lithium, and rubidium, three of the more challenging elements to measure. With the growing importance and capacity of desalination globally, ICP-OES can serve as the analytical technique to both help protect the oceans and turn a byproduct into a resource.

UN Warns of Rising Levels of Toxic Brine as Desalination Plants Meet Growing Water Needs

https://unu.edu/media-relations/releases/un-warns-of-rising-levels-of-toxic-brine.html

(2) R. Ketai, T.Y. Shen, I. Jafari, S. Masudy-Panah, and M.H.D.A. Farahani, in 

Desalination Challenges and Opportunities

, M.H.D.A. Farahani, V. Vatanpour, and A.H. Taheri, Eds. (IntechOpen, London, United Kingdom, 2020). DOI: 

(3) Singapore National Environment Agency, Allowable Limits for Trade Effluent Discharge to Watercourse or Controlled Watercourse (2022). 

https://www.nea.gov.sg/our-services/pollution-control/water-quality/allowable-limits-for-trade-effluent-discharge-to-watercourse-or-controlled-watercourse

(4) Dubai Electricity and Water Authority, Discharge Limits to Marine Environment (2021). 

https://www.trakhees.ae/en/ehs/env/Documents/Guidelines/Guideline%20ID-EN-G02,%20Water%20Environment,%20Rev.%2000,%20Jan18.pdf

(5) “Waste Discharge Requirements for Poseidon Resources (Surfside) L.L.C. Huntington Beach Desalination Facility Orange County”, Order R8-2021-0011, NPDSE No. CA8000403, California Regional Water Quality Control Board, Santa Ana Region.

(6) “Waste Discharge Requirements for the Poseidon Resources (Channelside) LP Claude “Bud” Lewis Carlsbad Desalination Plant Discharge to the Pacific Ocean”, Order R9-2019-0003, NPDSE No. CA 019223, California Regional Water Quality Control Board, San Diego Region.

(7) D. Frank, Mining for the Future (2022). 

https://www.aweimagazine.com/article/mining-for-the-future/

(8) W.S. Chen, C.H. Lee, H.W. Tien, Y.J. Chen, and Y.A. Chen, 

 is with PerkinElmer, Inc. Direct correspondence to: 

Brines from desalination plants are increasingly being seen as a source of valuable elements, such as lithium, rubidium, and cesium. We show how ICP-OES can be used to measure the concentrations of such elements in brines to assess the economic feasibility of their recovery.

ICP-OES and ICP-MS Techniques for Today's Spectroscopists

Analysis of Trace Elements as Impurities in Materials Used for Lithium-Ion Battery Production

ICP-MS Configuration and Optimization for Successful Routine Analysis of Undiluted Seawater

Analysis of Desalination Discharge Brines for Elements of Environmental and Economic Importance with ICP-OES