Simon Nelms

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.

FIGURE 1: Internal standard response during (a) the analysis of brine solutions and (b) electrolytes over 11 and 7 h, respectively.

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

Analytical techniques based on inductively coupled plasma (ICP) are commonly used for analyzing the elemental composition of a sample and are widely applied in analytical testing laboratories. Particularly for larger laboratories, analysis time and throughput are essential performance indicators, which can be severely impacted if sample analysis needs to be repeated because of insufficient stability or quality control (QC) failures. However, such failures are, in many cases, avoidable by selecting the right configuration of the sample introduction system of the instrument. This tutorial explains the most critical components of the sample introduction system of modern ICP-optical emission spectroscopy (OES) and ICP-mass spectrometry (MS) instruments, providing analysts with a guide for initial configuration settings and recommended maintenance intervals for reliable daily operation.

Inductively coupled plasma (ICP), when combined with optical emission spectroscopy (OES) and mass spectrometry (MS), is a well-established tool for elemental analysis. Continued technological development has led to significant increases in sensitivity and interference removal (for common polyatomic as well as other types of interferences). However, the technology surrounding sample introduction has remained comparable over the years.

Many of the challenges analysts face today, such as signal suppression, drift, or even failure of quality control (QC) checks can be related to the sample introduction system setup. In many cases, incorrect configuration or errors during routine maintenance are the root cause for these issues.

Unexpected Contamination: Sample Vials and Tubing

Techniques such as ICP-OES and ICP-MS provide the high sensitivity required to analyze the concentrations of toxic metals at trace and ultratrace levels. However, high sensitivity makes both techniques susceptible to problems associated with unintended contamination, such as increased background levels, elevated detection limits, or over and underestimated results. There are multiple ways in which a sample can become contaminated, including, but not limited to, sample preparation, workplace cleanliness, and the purity of reagents used (that is, acids used in digestion or dilution steps). However, in most cases, the impact of plasticware used in the laboratory is underestimated. Plasticware comprised of perfluoroalkoxy (PFA) polymers is known to be available in high purity, whereas vials that are used for sample storage are often made from polypropylene (PP) or polyethylene (PE), which have lower purity.

Before starting an analysis, it is good practice to look at the quality of all materials and reagents used in the sample preparation process. Contamination can arise from the surrounding laboratory environment (even in areas dedicated to trace metal analysis) where sporadic and unexpected contamination can occur. Usually, this contamination can be attributed to residues in the plasticware used, such as vessels or peristaltic pump tubing. Many laboratories refrain from switching suppliers of sample tubes because it can potentially introduce contamination because of differences in the manufacturing processes and materials. Although most tubes are fairly clean, experience has shown that vial caps can be a significant source for metal contamination, particularly aluminum, zinc, nickel, and copper. The extent of contamination can be significantly reduced by rinsing vials and caps with dilute nitric acid prior to use. The same considerations may apply to peristaltic pump tubing (typically made from polyvinyl chloride [PVC]). A brief cleaning or conditioning with an acidic solution (a few minutes is sufficient) can wash out any unwanted traces of metals before they can be introduced into the sample analysis.

Sample Consumption and Handling: Peristaltic Pump, Nebulizer, and Spray Chamber 

Problems with an incorrect sample delivery to the plasma often express themselves in a secretive manner. An important parameter to be checked during the daily performance check is the short-term stability of the system, or percent relative standard deviation (%RSD), of the signal determined over a period of 10 min. Some laboratories or methods require continuous monitoring of the signal stability in each run, where data showing a %RSD greater than 2–3% must be discarded. In many cases, instabilities observed either during set up of the instrument or the sample analysis are related to the sample delivery to the plasma. The most common source of instability is related to the peristaltic pump delivery speed, the worn out peristaltic pump tubing, or the uneven draining of the spray chamber. When signal variation appears to be periodic (a similar pattern repeating every few seconds), sample delivery is most often found to be the source of the problem.

If increased signal fluctuation is only observed for a subset of analytes, the root cause can sometimes be found in the preparation of the sample. Elements, such as aluminum or iron, need to be stabilized in an acidic environment. If they are not, they may form colloid species like hydroxides, which can lead to unstable signals in both ICP-OES and ICP-MS. This problem can be observed with samples having a pH value of approximately 4, where additional acid is needed to lower the pH further to improve the stability of the signal.

When Problems Become Visible in ICP-OES: Torch

Have you ever noticed the central channel of your plasma turn orange when running an analysis? The orange glow is emission from excited sodium atoms and is a sign that your sample contains high levels of dissolved solids (typically dissolved salts of mainly sodium, potassium, magnesium, and calcium). In addition, the presence of organic solvents is immediately visible in an incorrectly configured system. In the latter case, the plasma exhibits a green emission followed by an orange glow from the carbon deposited at the sample cone tip, with the plasma often extinguishing after a short time. All instruments available on the market allow at least a visual observation of the plasma using a window. Certain instruments allow direct observation of the plasma directly on the control PC by means of a camera. In either case, a close look at the plasma when challenges are observed may help to identify issues with the analysis. In many cases, problems related to the analysis of difficult sample types, such as brine or organic solvents, can be overcome by selecting a different configuration of the sample introduction system configuration. Table I contains a few recommendations for analysis carried out using an ICP-OES instrument. ICP-MS is generally able to handle samples containing completely dissolved solids of up to 0.2% (m/v). Higher total dissolved solids can be handled by an ICP-MS instrument, but it would require a special configuration using argon gas from the instrument as a diluent that is introduced to the sample flow at the spray chamber. If the samples contain hydrofluoric acid (HF), then all components of the sample introduction system need to be exchanged for HF-resistant versions.

More Than Just an Interface: The Role of Cones in ICP-MS 

Although ICP-OES and ICP-MS have a lot in common when looking at the general setup of the sample introduction system, they differ significantly when it comes to the most important sample types. ICP- OES is able to analyze high matrix-containing samples such as brines and wastewater. On the other hand, ICP-MS is preferably applied for sample types that contain less problematic matrices or those that have undergone thorough cleanup procedures to remove the matrix components. The main reason for this distinction is that in ICP-MS, ions need to be transferred into the high vacuum of the mass spectrometer, whereas the emission of light can be viewed at an ambient atmosphere (residual air needs to be removed inside the optical system though to avoid emission signal absorption, particularly for wavelengths in the UV range of the spectrum). The ion extraction is accomplished in the interface with a set of two narrow orifices (less than 1 mm diameter) kept at an intermediate pressure level of less than 2 mbar. The interface cones of an ICP-MS can typically handle up to 0.2% (m/v) dissolved solids entering the plasma. Above this level, blocking of the orifices at the interface will occur, particularly on the skimmer cone where its cooler temperature makes it more prone to clogging from the buildup of accumulated solids. As mentioned previously, sample analysis with a higher matrix load by ICP-MS is possible with the introduction of argon gas as a diluent between the spray chamber and the torch. Although no manual dilution with a liquid is made, the sample flow is reduced, thus reducing the amount of matrix introduced. Lower sample flow results in a reduction in sensitivity of the system, but the impact on the method detection limits (MDL) is often not noticeable compared to a manual or automated liquid dilution. The direct analysis of samples (without prior manual dilution) is attractive as less diluents are used, reducing cost, labor, and the risk of errors or contamination. However, this approach is less flexible, with the dilution factor remaining constant for all the samples in a batch and samples containing less challenging matrix being analyzed under the same conditions (that is, reduced sample flow) as the more challenging samples. In addition, automated adjustment of the dilution factor (based on the concentration found in a sample relative to the calibrated range) is not possible as this causes a mismatch between the calibration solution signal response and the diluted (with a different dilution factor, that is not exactly calculated under argon gas dilution conditions) sample signal response.

Problems at the interface region can often be recognized by checking the response of the internal standards. Internal standards are elements that are not present (or present at very low levels) in samples. They are added, at a fixed concentration, to all blanks, calibration solutions, and samples, either on-line via a mixing tee or valve port (easy), or off-line, by manual addition (tedious). When a sample contains high concentrations of sodium, potassium, and calcium, the internal standard signals are suppressed compared to what they are in the blank or matrix-free solutions. Many regulated methods specify how much internal standard suppression is permitted in a sample, which is typically between 75–80% of the signal observed in a sample matrix-free blank (usually the first calibration blank in the analysis). In general, if internal standard recovery is less than 75–80% in any sample, dilution and re-analysis is required. The most common reason for internal standard recoveries being greater than 120% is often attributed to the ICP-MS interface not being conditioned properly. Conditioning is the process of running a dummy sample containing similar matrix to the samples to be analyzed through the instrument for approximately 30 min before you start the actual analysis, which causes the interface cones to become coated and helps to stabilize ion signals and minimize drift. It remains somewhat unclear as to why cone conditioning helps improve signal stability, but it is a well-documented fact that it does and conditioning in this way is standard practice. Cone conditioning is especially important after cleaning the interface cones or when they are replaced. In general, cleaning the interface cones should only be considered when signs of blockage or unremovable contamination are observed or if the instrument performance has significantly dropped.

Finding the right configuration of the sample introduction system for a specific sample type is easy once a few questions are answered up front. The most important questions are related to the sample matrix: is it aqueous or organic? What is the amount of total dissolved solids? And what is the approximate composition?

Figure 1 summarizes a few of the crucial aspects that should be considered when setting up an instrument (either ICP-OES or ICP-MS) for analyzing a new sample type.

FIGURE 1: The route to success in managing the analysis of previously unknown sample types.

If a method is defined and validated but does not perform up to expectations, it is important to know which key areas of the system configuration to focus on and to understand how the different components of the sample introduction system are working together. As a general rule, instability is usually caused by problems upstream of the sample introduction system, whereas drift and suppression are rather caused downstream (injector tube, torch, and interface cones [in the case of ICP-MS]). In some cases, effects that may be attributed to the hardware may also originate from unexpected interferences from the sample, but that is a subject for another tutorial.

 are with Thermo Fisher Scientific. Direct correspondence to: 

This tutorial explains the most critical components of the sample introduction system of modern ICP-optical emission spectroscopy (OES) and ICP-mass spectrometry (MS) instruments, providing analysts with a guide for initial configuration settings and recommended maintenance intervals for reliable daily operation.

Understanding the Fundamental Components of Sample Introduction for ICP-OES and ICP-MS

Despite the numerous inherent advantages of using inductively coupled plasma–mass spectrometry (ICP-MS) as a tool for trace-elemental analysis, the removal of spectral interferences is still a concern for some applications, especially in metallurgical, environmental, and geological studies. Isobaric interferences such as 

Pb are difficult to remove even using high-resolution instruments. The use of reactive gases inside the ICP-MS collision–reaction cell (CRC) system may help to overcome such isobaric interferences, since isotopes of different elements may react differently and hence may differ in mass after a chemical reaction. To control unwanted side reactions, an additional mass-filtering quadrupole needs to be situated ahead of the collision–reaction cell, so that ions entering the cell can be controlled and unwanted side reactions can be avoided.

	Unresolved spectral interferences may lead to biased results in inductively coupled plasma–mass spectrometry (ICP-MS). Single-quadrupole ICP-MS typically uses a comprehensive interference removal mechanism with an inert collision gas (helium) and kinetic energy discrimination (KED). This approach is based on the difference in size between an analyte and polyatomic interference and as such is effective for the removal of many common interferences. In some cases though, two elements may share isotopes with identical mass number. These isotope overlaps are commonly referred to in ICP-MS as 

. As the elemental ions are of a similar size, KED is not able to resolve these interferences. In addition, as the difference in mass between overlapping isotopes is extremely low they cannot be spatially resolved using either quadrupole-based or high-resolution ICP-MS. Often, the correction of isobaric interferences is accomplished using mathematical methods; however, this procedure may lead to increased measurement uncertainties. Isobaric interferences may be resolved using reactive gases (for example, O

), in the cases where one element forms a different product ion to the other. However, side reactions may create new unwanted interferences with other ions extracted from a sample, so that full interference removal may not be possible. The use of oxygen as a reactive gas leads to product ions that are fairly easy to interpret (mass shifts of mainly 16, but also 17 and 18 amu), whereas using ammonia as a reactive gas leads to the formation of a wide variety of product ions, because different cluster ions will be formed for each individual isotope of a given element. With ammonia, mass shifts from as low as 15 amu are observed, but also product ions with over 100 amu mass differences can be formed. Interpreting results from unknown or complex sample matrices may therefore be challenging. Using triple-quadrupole ICP-MS, a dedicated mass filtering step ahead of the collision–reaction cell (CRC) system overcomes the problem of uncontrolled side reactions, because only ions with a mass-to-charge ratio similar to that of the analyte may enter the cell and subsequently react with gas molecules. In this work, the performance of quadrupole-based ICP-MS (using single- and triple-quadrupole operation modes) is evaluated for the removal of isobaric interferences.

	All measurements were accomplished using a Thermo Scientific iCAP TQ ICP-MS system (Thermo Fisher Scientific). The instrument was tuned daily using the autotune routines supplied with the operating software. All solutions were prepared gravimetrically from single-element standards. The precision of the isotope ratios was calculated from 10 main runs with 30 sweeps each. The data displayed in the following were not corrected for mass bias, so the isotope ratios may differ slightly from the true value. However, the effect of mass bias would affect all individual measurements in the same way, so that correction would not alter the fundamental findings. Generally, isotope ratios are noted together with the observed standard deviation (as an indication for the attainable precision of the measurement) throughout this work. Typically, the observed relative standard deviation of the isotope ratio was on the order of 0.1%. To compare the results obtained using conventional single-quadrupole ICP-MS analysis using kinetic energy discrimination (SQ-KED), single-quadrupole ICP-MS using reactive gases (SQ-O

), and triple-quadrupole ICP-MS with mass filtering and use of reactive gases (TQ-O

), the ICP-MS system was configured to scan all samples in the three corresponding measurement modes. An overview of the instrument's settings can be found in Table I.

	Two prominent examples for isobaric interferences are the detection of 

Sr (7.00% abundant in nature) in the presence of Rb, which also has an isotope with mass number 87 (

Hg (6.85% abundant), which is of special concern for geological applications, where the 

Pb isotope is often used as a reference isotope (because it is not affected by radioactive decay of U or Th).

	Using single-quadrupole ICP-MS, strontium is typically analyzed using kinetic energy discrimination to make sure polyatomic interferences are removed, but also use of the standard mode (no interference removal) may be feasible depending on the sample. For removal of the isobaric interference of 

Rb, oxygen can be used as a reactive gas in the CRC system. Whereas Sr undergoes an oxidation reaction and hence forms product ions with 

Rb does not react with oxygen. If the analyzing quadrupole of either a single- or triple-quadrupole ICP-MS system is set to scan 

 103 and 104 under these conditions, only Sr will be detected. However, only triple-quadrupole instruments can filter out ions potentially interfering at the new product ion mass of Sr, such as 

. Figure 1 shows how all quadrupoles need to be set to scan for this application.


	Figure 1: Schematic illustrating the analysis of strontium isotopes in presence of Rb using triple-quadrupole ICP-MS operated with oxygen as reaction gas.

	To evaluate the performance in different measurement modes for Sr, a calibration curve was acquired using SQ-KED as a reference, and using triple-quadrupole MS in combination with oxygen as a reactive gas. The resulting detection sensitivity using oxygen and a mass shift reaction was about 30% in comparison to SQ-KED (14 kcps/µg/L in comparison to 51 kcps/µg/L); however, the instrumental detection limit was not affected (0.001 µg/L for 

	In a second experiment, solutions containing 10 µg/L and varying concentrations of Rb were analyzed and the isotope ratio 

Sr was evaluated. Furthermore, solutions containing equal amounts of Sr and Rb in a matrix containing either or both 1 mg/L of Pd and 10 mg/L of Ru (to account for the lower abundance of the 

Ru isotope in comparison to monoisotopic 

Rh) were analyzed. Typically, considering the isotopic abundance of the strontium isotopes 

Sr, one would expect a theoretical isotope ratio of 11.7971. The results are displayed in Table II.

	As can be seen from Table II, the increasing amount of Rb (and hence the abundance of ions with 

 87) has no effect on the Sr isotope ratio when using the TQ-O

 mode. In contrast, the single-quadrupole approach using KED reveals a strong effect of the increasing amount of Rb in the sample solution, because the isobaric interference is not resolved. When using oxygen as a reactive gas in single-quadrupole ICP-MS analysis, the obtained isotope ratio is mainly determined by the interfering ions on the product ion mass of 

O, which are not removed and remain occupied, since neither Rh nor Ru react with oxygen. Even the observed isotope ratio for a solution containing only Sr is strongly biased by other ions of unknown identity formed in the cell.

	For Pb, similar experiments were conducted to estimate the effect of the isobaric interference caused by 

Pb. To judge how far the isobaric interference was eliminated, the isotope ratio 

Pb (reference value 0.02672) was used as a reference.

	For elimination of the isobaric interference from Hg on Pb, a reaction with ammonia can be used. Hg undergoes a chemical reaction with ammonia molecules and forms product ions that have a different 

 value. At the same time, Pb does not react with ammonia, so it can be detected free from interferences when the analyzing quadrupole (Q3) is set to the same mass as the first quadrupole (that is, mass 204 in this case). It is therefore important to avoid other reactions in the cell that could lead to the formation of a product ion with 

 204, so lower mass ions that could generate additional interferences need to be removed in the first quadrupole. Again, the objective was to investigate how far traditional single-quadrupole approaches in comparison to triple-quadrupole operation may be suitable for this application. For the single-quadrupole mode tests, ammonia was also used as the reactive gas, but with Q1 acting as an ion guide only, so that other ions also originating from the sample could enter the CRC system.

	Table III clearly shows that, again, KED does not enable the removal of isobaric interferences, as the measured isotope ratio changes with increasing concentration of Hg added to the solution. In contrast, the use of NH

 as a reactive gas in single-quadrupole mode is capable of eliminating the isobaric 

Pb. Because no other ions are present in the ion beam when analyzing a solution containing only Hg and Pb, the use of an additional mass filter or of Q1 as an ion guide is unnecessary. However, for real sample analysis this situation is almost never the case. If for example rare-earth elements are present in the sample solution (which is likely in geological samples such as soils or rock digests), other cluster ions may be formed with NH

204 is again interfered. To test this phenomenon, 1 mg/L of ytterbium was added to the measured solution to introduce 

Pb isotope ratio is observed, clearly showing that there is an effect from 

 mode. To visualize this newly formed polyatomic interference, a full mass spectrum was acquired using NH

	It can be seen that there are numerous cluster molecules formed from Yb and NH

. Although unreacted Yb still shows the highest abundance in the mass spectrum between 

 168 and 176 (indicating that the reaction is not quantitative), the characteristic isotope pattern for Yb can also be repeatedly found at higher mass, with an equal distance of 17 amu between corresponding isotopes. This mass difference reveals the successive addition of one molecule of NH

 per precursor ion and is characteristic for reactions of this gas. The main interference now found on 

, but since all seven Yb isotopes react in the same way, the other Pb isotopes would also be similarly interfered. In SQ-NH

mode, these newly formed polyatomic ions are transmitted with the isotope mass of interest and therefore introduce a new interference on the target isotope. In contrast, when switching to a triple quadrupole–based measurement mode (TQ-NH

), all other ions with lower mass (such as 

Yb) are removed from the ion beam before they enter the CRC system and are not able to generate these new interferences. The side reaction of Yb with NH

 therefore does not lead to the formation of new interferences on Pb, and in turn, the 

Pb measured isotope ratio is not affected at all through the presence of either Hg or Yb. Furthermore, other rare-earth elements that may lead to the formation of new interferences on 

Pb because of their reactivity toward ammonia-for example, cerium (

, 52.20% abundant)-are also removed by Q1 in triple-quadrupole mode.


	Figure 2: Full mass spectrum illustrating the formation of Yb-derived product ions acquired using single-quadrupole operation and ammonia as reaction gas.

	The removal of isobaric interferences using reactive gases has been demonstrated in two examples, 

Pb. Leveraging different reactivity toward oxygen and ammonia, respectively, allows a chemical separation of the isobars inside the CRC system. Although single-quadrupole instruments are also capable of using reactive gases, unwanted side reactions with other elements present in the sample may lead to the formation of new interferences, as has been shown in the case of Yb and Pb. The importance of controlling the ions entering the CRC is a key factor with respect to obtaining analytical accuracy. Triple-quadrupole ICP-MS using a dedicated mass filter in front of the CRC system is the only definitive way of achieving this required ion transmission control.

Daniel Kutscher and Shona McSheehy Ducos 

 are with Thermo Fisher Scientific in Bremen, Germany. 

are with Thermo Fisher Scientific in Stafford House, Hemel Hempstead, UK. Direct correspondence to: 

Unresolved interferences can lead to biased results in ICP-MS analyses. Here we describe an approach for removing those interferences using reactive gases.


Simon Nelms

Articles

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

Understanding the Fundamental Components of Sample Introduction for ICP-OES and ICP-MS

Effective Removal of Isobaric Interferences on Strontium and Lead Using Triple-Quadrupole ICP-MS