Validation and measurement uncertainty of the determination of 24 elements in drinking water using ICP-MS

According to the World Health Organization (WHO), the term ‘drinking water’ refers to enough water for human consumption and home use. The safety of city drinking water has long been a contentious societal issue. It has been reported that approximately 2 billion people do not have access to safe drinking water (World Health Statistics 2023). One of the most important of these problems is the problem in drinking water supply. Drinking water quality degrades until it reaches the tap from the drinking water treatment plant (Liu et al. 2019), discoloration issues arise (Vreeburg & Boxall 2007; Han et al. 2018), heavy metals accumulate in excessive concentrations, and other water quality issues can have serious consequences for human health. Rainwater has been reported to have an impact on drinking and utility water in air mass transportation (Berberler et al. 2022). When all of these transports are considered, there are 35 hazardous elements, 23 of which are heavy metals, whose presence in drinking water is questionable. These metals are found in modest concentrations in natural diets and are helpful to human health. Heavy metal concentrations in the environment, on the other hand, can infiltrate the food chain and create harmful and severe effects on the human body if they surpass normal levels (Karaouzas et al. 2021). Heavy metals in drinking water are a significant issue for these reasons because they cannot be digested by the body and instead accumulate in adipose tissue, muscles, bones, and joints, producing health complications and diseases (Vatanpour et al. 2020). In another study, it has been reported that the accumulation of Pb and Cd in the human body can lead to cancer, kidney disorders, skeletal damage, brain damage, reproductive failure, and poisoning (Shariatifar et al. 2022).

Heavy metals deplete and replace other minerals necessary for an organism's health (Liu et al. 2020). Heavy metal contamination in drinking water can cause acute and chronic diseases (Zhang et al. 2022). The uncontrolled disposal of industrial, agricultural, and domestic waste has contaminated groundwater (Yadav et al. 2012). It has been reported that the increasing chemical pollution in surface, groundwater, and coastal sea waters around the world has become an important problem today, and in addition to affecting the health of aquatic animals, especially through the pollution of groundwater and surface water, it also affects human health and the quality of life (Balaram et al. 2023). There are many studies in the literature on drinking and utility water. Groundwater pollution has been reported to cause adverse effects on aquatic habitats and human health (Chakrabarty & Sarma 2011; Mahato et al. 2016). In another study, it was reported that the main sources of trace elements in plant and animal feeds include irrigation using wastewater, irrigation with groundwater, disposal of municipal and industrial wastes, mining, smelting processes, and atmospheric pollution (Karimi et al. 2023). They reported that rapid industrialization and urbanization resulted in trace metal accumulation in water and soil (Kumar et al. 2019). Important pollutants such as arsenic (As), chromium (Cr), iron (Fe), pesticides, and other industrial chemicals are causing groundwater pollution. A serious environmental concern is needed, as water pollution leads to the overuse of groundwater (often considered clean water), which is attributed to the declining water level (Kumar et al. 2019). As a result, it is everyone's responsibility to find a safe, clean, and consistent source of drinking water to maintain our quality of life. There could be two major sources of elements in fresh and drinkable water that are suspected of being polluted. One of them is due to natural degradation and the other is due to industrial activities, which has been the subject of great concern recently (Vutukuru 2005). Furthermore, the vast range of water pipelines, long transmission and distribution, diverse pipe materials, and water quality conditions generate undulating circumstances in the pipeline's inner wall. With water flow, Fe, Mn, Pb, Zn, and other metal contaminants from raw water, water treatment agents, and pipe materials enter residential use (Tian et al. 2021). Particles will pass through the drinking water pipe wall by some physical, chemical, and biological means, adsorption, deposition, and different ways. Decomposition of heavy metals is difficult and accumulation is easy (Zhang et al. 2022). It is important to detect heavy metals suspected to be present in drinking water with method validation and good laboratory practice, reliable methods (Ismail & Afify 2022). By following the procedures and recommendations outlined in the validation guide, accurate and reliable results can be obtained. These procedures and recommendations may be adjusted or modified based on the specific type of analytical method being investigated.

The process of identifying an analytical need and validating that the method under consideration is compatible with what the application demands is referred to as method validation (Kılıç et al. 2015). Selectivity, limit of detection (LOD) and limit of quantitation (LOQ), working range, analytical sensitivity, accuracy, precision, and measurement uncertainty are all performance parameters that are generally evaluated during method validation (Kargarghomsheh et al. 2023; Kılıç et al. 2015). However, for quantitative analysis, the uncertainty for analytical measurements must be estimated by quality control principles and ISO/IEC 17025:2017 (Loizou et al. 2010). There are numerous analytical techniques available for detecting and quantifying elements in drinking water. However, inductively coupled plasma-mass spectrometry (ICP-MS) is regarded as a critical, dependable, and widely used analytical technique for this purpose (Komorowicz & Barałkiewicz 2016).

Heavy metals and metalloid arsenic (⁷⁵As) can be found in water and everywhere (Sahu & Saha 2020). Natural (stripping of ore deposits and bedrock) and anthropogenic (industrial and agricultural wastewater) sources of arsenic and heavy metal pollutants in water contamination have been reported in over 70 countries, threatening over 100 million people worldwide (Rasool et al. 2016). As and heavy metal contamination in drinking water increases the risk of chronic disease and cancer in humans (Ravenscroft et al. 2009; Khan et al. 2019). Therefore, elemental analysis, especially the determination of ⁷⁵As concentrations in drinking and utility water, should be based on reliable results. The combination of chlorine (³⁵Cl) in drinking water and argon gas (⁴⁰Ar) used to act like ⁷⁵As is the most likely interference. As a result of entering the mass detector with a molecular weight of (⁴⁰Ar + ³⁵Cl ⁷⁵As), it causes an increase in the amount of arsenic in the sample results and adverse effects in terms of reliability during concentration determinations. Some interferences are shown in Table 1 (Soriano et al. 2018). We discovered that the procedure we used on the samples reduced the amount of chlorine in the study we conducted with our team. Although there is no significant salt content in drinking and utility water, there may be some overloading with the high number of sample readings. These issues can have an impact on concentration. As a result, it is intended to minimize the chlorine contamination that may occur during the preliminary preparation technique used on the samples (Kilic et al. 2023).

Salt deposits on the extraction or skimmer cones, or both, reduce the efficient cone hole diameter and thus ion conduction, potentially preventing the entire sample from reaching the detector (Brenner 2010). Elements such as Na, K, Li, Ca, and Mg disrupt the normal distribution of excited ion and atom species in multi-element analysis plasma. Interference effects also occur at the interface between the ICP and the sampling cone and skimmer cone of the mass spectrometer (Brenner 2010). Although salt ions are present at low concentrations in drinking and utility water, the ICP-MS device was kept under control with the QC and approved reference material standards throughout the operation, taking into account the contaminations it may cause (Ismail & Afify 2022). About 50 samples of drinking utility water were collected from various locations around Antalya/Konyaaltı in the spring and summer of 2023. The amounts of water-soluble elements were measured using an ICP-MS. The study's main goal is to support them with method validation studies, and it aims to ascertain the concentrations of 24 elements with the isotopes of (⁷⁵As, ²⁰⁸Pb, ¹¹¹Cd, ⁵⁹Co, ⁵²Cr, ⁵¹V, ⁵⁷Fe, ⁶³Cu, ⁵⁵Mn, ⁶⁰Ni, ²⁷Al, ¹³⁸Ba, ²⁴Mg, ¹⁰⁷Ag, ²⁰⁵Tl, ⁹Be, ²⁰⁹Bi, ¹³³Cs, ⁷Li, ¹²¹Sb, ⁸⁸Sr, ⁶⁶Zn, ¹¹B, and ²⁰²Hg) suspected to be present in drinking and utility water utilized in Antalya/Konyaaltı.

In the spring and summer of 2023, drinking water samples were collected from about 50 different locations within the Antalya/Konyaaltı sampling area. Using a polyethylene bottle that had been acid-washed, 500 mL of drink water was collected and brought to the lab for elemental analysis. Three replicates of each sample were examined, and mean concentrations were determined. The ICP-MS device, a Perkin Elmer Elan DRC-e model, was utilized to acquire prompt and dependable results because the elements to be detected in drinking and utility water samples have concentrations that fall within the low-high dynamic range. The element concentrations in the samples were measured using the analysis method (EPA METHOD 6020 A; Kilic et al. 2023).

Perkin Elmer Inc. (Boston, USA) provided the ICP-MS (mix-3) multi-element calibration standard 3 solutions (10 mg L⁻¹, Perkin Elmer N9300233). Nitric acid (Suprapur grade, 65%), hydrochloric acid (Suprapur grade, 35%), and HF (40%) were supplied from Merck (Darmstadt, Germany). The solutions were diluted with (18.2 MΩ cm at 25°C) deionized water from a Millipore^® ultra-pure water purification system (Bedford, USA). For the examination of the trueness, the matrix reference material (Certified Reference Material (CRM); Enviro MAT-Drinking Water-Low SCP SCIENCE, Quebec, Canada) was employed. A Thermomac TM 19-D (Biotech GmbH, Germany) hot plate was utilized to perform open acid digestion (Kılıç et al. 2015).

Around 100 mL of aliquot of 500 mL samples were taken and transferred to a 250 mL beaker. The acid digestion was applied to the whole drinking water sample without any pretreatment to obtain the total concentration (not only dissolved) of elements. The acid mixture of 4 mL of HNO₃, 0.5 mL of HCl, and 0.5 mL of HF (HF was added to dissolve silicates in particles that may be present in water samples) was added to a 100 mL drink water matrix to dissolve all particulates and to remove Cl⁻ ions as gaseous HCl on the hot plate at 60°C under the hood. The sample evaporation was continued to 10–15 mL without boiling. The evaporation processes were repeated two times on the heater. After the evaporation process was completed, the volume was completed to 25 mL with deionized water. With this preliminary method, the elements found in low concentrations in the samples were easier to detect by concentrating them (Kılıç et al. 2015).

A matrix was created by combining three drinking water samples in order to validate the procedure. The necessary additions from the stock calibration standard and CRM solutions were made on the drink water matrix for the recovery, precision, and trueness studies. Following these preparations, the calibration standards were applied after the blank was examined, the matrix blank analysis, and the blank analysis. An analysis of recovery standards was done at the end. QC monitoring (5 and 25 μg L⁻¹) was carried out, while the ICP-MS device was used to determine the sample concentrations (Kılıç et al. 2015). Deviations in tune values in device performance were considered very important in terms of monitoring device performance (Berberler et al. 2022). The CRM was preferred as another QC solution and ran every 10 samples. According to the results obtained, it was used to detect problems such as poor performance, interference, and deterioration of calibration (Kilic et al. 2023). It has been emphasized that the use of CRM is very important in validating different analytical methods, analytical measurement procedures, and protocols (Balaram & Satyanarayanan 2022).

Calibration curves were obtained by adding standards to the matrix. For Hg calibration, calibration standards were prepared with final volumes of 0.2, 0.5, 1.5, 2.5, and 5 μg L⁻¹ in 2% HNO₃ (v/v) and 1% HCl solutions using an intermediate stock standard (1,000 μg mL⁻¹). For this reason, low concentrations of calibration points were preferred for the determination of mercury element, which has a legal limit of 1 μg L⁻¹ in drinking water. Calibration standards (2, 5, 10, 25, 50, 100, and 200 μg L⁻¹) were prepared for boron using 2% HNO₃ (v/v) by preparing intermediate stock solutions from a certified standard (1,000 μg mL⁻¹) solution (Ismail & Afify 2022). Calibration standards were prepared using 2% HNO₃ (v/v) and 1% HCl (2, 5, 10, 25, 50, 100, and 200 μg L⁻¹) from a certified 1,000 μg mL⁻¹ antimony standard. Calibration standards (2, 5, 10, 25, 50, 100, and 200 μg L⁻¹) from a standard stock solution from a certified standard (mix-3 10 μg mL⁻¹) for As, Pb, Cd, Co, Cr, V, Fe, Cu, Mn, Ni, Al, Ba, Mg, Ag, T1, Be, Bi, Cs, Li, Sr, and Zn. All samples, blanks, and standards were prepared in polypropylene bottles (Kılıç et al. 2015). Three drinking and utility water samples were mixed to obtain a matrix to validate the method. Required additions from the stock calibration standard for recovery, precision, and accuracy studies and CRM solutions were made on the matrix (Yenisoy-Karakaş 2012). A two-step digestion procedure was performed, and the final volume was made up to 25 mL with deionized water. Table 2 lists the chemicals and devices used.

All measurements were done by using the Perkin Elmer ELAN DRC-e model ICP-MS instrument to determine the concentrations of elements (As, Pb, Cd, Co, Cr, V, Fe, Cu, Mn, Ni, Al, Ba, Mg, Ag, Tl, Be, Bi, Cs, Li, Sb, Sr, B, Zn, and Hg). Table 3 lists the instrument method parameters for ICP-MS. Before beginning analysis with the ICP-MS instrument, performance calibration tests were configured according to the instrument manufacturer's instructions. Running the particular tuning solution through the instrument confirmed its stability,. the relative standard deviation of the absolute readings for all analytes is less than 5% (Environmental Protection Agency Method 2008). Calibration tests in the ICP-MS device software were renewed using their solutions (detector voltage, mass calibration, dual detector, lens voltage, auto lens, nebulizer gas flow, ICP RF Power, X–Y calibration, and daily performance) (Brenner 2010). The tune parameter values applied were determined in the desired ranges and concluded successfully. The data obtained are shown in Table 4.

The performance of the applied method was assessed in terms of linearity, LOD and LOQ, recovery, accuracy, and precision. The CRM includes 20 elements (As, Pb, Cd, Co, Cr, V, Fe, Cu, Mn, Ni, Al, Ba, Mg, Tl, Be, Li, Sb, Sr, Zn, and B). Because CRM lacks Ag, Bi, and Cs, the calibration standard containing Ag, Bi, and Cs was preferred for recovery and precision calculations at three levels of support (Balaram et al. 2023). Spiking was performed on the sample matrix at three fortification levels, representing the early (2 μg L⁻¹), middle (25 μg L⁻¹), and last (50 μg L⁻¹) parts of the linear range of the calculations plot. Recovery values, mean recovery, and percent relative standard deviation (%RSD) results were calculated from the data obtained (Yenisoy-Karakaş 2012). The data obtained are shown in Table 5. Since the concentration of mercury in drinking and utility waters is monitored at low concentrations within the legal limits, recovery studies were prepared using a certified standard substance on the matrix, with final volumes of 0.5 and 2 μg L⁻¹. The standard addition method on the matrix was made to obtain the calibration curves (Kılıç et al. 2015). All monitored isotope calibration plots had a correlation coefficient of 0.9994 or higher. The obtained data are shown in Supplementary Table S1.

LOD is the limit of detection, LOQ is the limit of quantitation (both measured in μg L⁻¹), σ is the standard deviation of 10 blank solutions (or the lowest level standard), and S is the slope (μg L⁻¹).

When calculating the recovery values, the reagent blanks were subtracted from the sample values. The obtained values are shown in Supplementary Table S2. The calculated percent relative error values vary between 0.04 and 1.94%. The method's specificity/selectivity, precision, and recoveries of the standards within the described calibration range are all determined in Table 5. It shows the mean recovery and precision (%RSD) data. Studies on recovery that used CRM had acceptable ranges (98.8 and 101%) in Supplementary Table S2. Spiking was performed as low (2 μg L⁻¹), medium (25 μg L⁻¹), and the end of the linear range (50 μg L⁻¹). As a result, the method's specificity/selectivity was deemed acceptable for all elements between 98.8 and 101% (Millour et al. 2011). The average %RSD values obtained with a three-level spike for elements ranged between 0.13 and 2.31.

The (matrix) spike approach was used for recovery experiments using CRM. The analytical methods used in this investigation were created following the recommendations of Eurachem. Supplementary Table S1 summarizes the linearity and LOD–LOQ values. Studies using the spike method in drinking water were conducted for elemental analysis to represent low, medium, and high concentrations. Compared with previously published data, the LOD and LOQ values of the analytical method are close (Alrimawi et al. 2008; Kilic 2019; Ismail & Afify 2022).

C is the concentration of element in a water sample (μg L⁻¹); CA is the element concentration in the sample obtained from the calibration (μg L⁻¹); V_diluted is the final diluted volume after digestion (0.025 L); and V_original is the filtered sample volume (0.1 L).

U_std is the intermediate stock solution standard uncertainty ( 1 mg L⁻¹); C_std is the intermediate stock solution concentration (1 mg L⁻¹); V is the remaining stock solution volume (10 mL); U_v is the 10 mL volumetric flask standard uncertainty; C_element is the concentration of element in Mix-3 stock solution (10 mg L⁻¹); and U_element is the standard uncertainty of its concentration in 10 mg L⁻¹ stock solution

S stands for the residual standard deviation; B₁ represents the slope; p is the number of measurements to be determined; n is the number of calibration measurements; C_O is the determined analyte concentration in water; is the mean value of the several calibration standards (n measurements); i is the number of measurements required to obtain the calibration curve; and c_i is the calibration equation's specific calibration standard value.

The standard uncertainty from the volumetric flask (u(flask)) was calculated using formulas 6 and 7.

Acid purity, digesting procedure losses, glassware and pipette calibration precision, instrument stability, and other factors contribute to repeatability uncertainty. The complete method was applied to 30 subsamples that were prepared and analyzed on the same day at three fortification levels. The repeatability standard uncertainty determined by averaging the RSD values of all three levels. The standard uncertainty was determined by dividing the mean RSD of these samples by the sample's square root, n = 50, (RSD/⁠) (Kılıç et al. 2015). The errors coming from temperature change and the calibration of volumetric flasks determine the dilution's uncertainty. Using Equation (7), where u_temp represents the standard uncertainty of the temperature effect, V is the measured volume (e.g. 2 mL), and Q is the coefficient of volume expansion of the water (Q_water = 0.000124 °C⁻¹). The volumetric calibration standard uncertainty, u_pipcal, for a 5 mL automatic pipette is computed based on the manufacturer's reported CV value of 0.14 (i.e. u_pipcal = (0.14/100) × 2 = 0.028 mL). In Equation (6), these numbers are added together to calculate the uncertainty associated with ingesting 100 mL of water. Using the same procedures, the standard uncertainty of a 25 mL capacity volumetric flask was computed. The uncertainty stated in the certificate of the volumetric flask with a capacity of 25 mL was ±0.040 mL (Kılıç et al. 2015). The standard uncertainty of repeatability is not provided here since the repeatability for all conceivable sources is collected separately under the repeatability term.

represents spiked samples, the average recovery value obtained (C_obs./C_spike); n is the number of spiked samples (n = 14); C_obs is the spiked samples' average concentration value; S_obs is the standard deviation of spiked sample concentration; C_spike is the theoretical value of the spiked concentration; and u(C_spike) is the standard uncertainty of spiked concentration ⁠.

Prolonged transport of water between water and pipes during transport from production facilities to consumers can often cause some elements to accumulate in corrosion scales (Peng et al. 2012). These deposited metals can be re-released into the water from sediments as a result of changes in water quality, stagnation time, and other factors (Sun et al. 2017). It even causes water discoloration. Although the use of metal pipes has decreased, they are largely used as indoor water supply pipes in old buildings, especially in many areas (e.g. Poland: 53%; USA: 56.6%) (Veschetti et al. 2010). In addition, due to the high costs of replacing the existing infrastructure, this situation is not expected to be replaced with new pipe structures for a long time (Li et al. 2016). As a result, existing galvanized steel pipes with extensive deposits of deposits may potentially release significant amounts of accumulated Fe, Mn, Zn, Pb, and other metal elements. According to various reports, high metal element contents in drinking water can affect turbidity and harm the human body. Large amounts of Fe and Mn elements, for example, will cause noticeable changes in turbidity, causing consumers to complain about water quality (Husband et al. 2008). In this sense, the method applied for the preliminary study to determine the concentrations of the elements suspected to be present in drinking utility water has been successful. Acceptance criteria for the recovery percentage should be between 80 and 120%. However, because permissible recovery is determined by concentration and analysis purpose, less rigorous recovery limits can be applied during recovery trials. For example, an acceptable recovery limit is recommended between 60 and 120% for concentrations ≥1 and <10 μg kg⁻¹, and 80–110% for concentrations ≥100 μg kg⁻¹. In this context, the preliminary preparation process, which is acidified twice on the magnetic heater and exposed to evaporation, can prevent the problem of undesired contamination to be seen in the samples because of the chlorination process applied to the drinking water by the local authorities. For this reason, the above-mentioned method was applied in the sample studies, since it was found to be particularly successful in contaminations affecting the arsenic concentration. After the evaporation process of the samples in the magnetic heater was completed, the final volume was completed with distilled water to be 25 mL. pH < 2 was verified before processing.

With this method, it is shown in Table 6 that the effects of undesirable contamination in the samples are reduced. Successful results have been obtained with this preliminary preparation method. It has been observed that it is more successful for elements that are difficult to detect due to their low concentration value. The concentration value of an element with a concentration value of 0.5 μg L⁻¹ in the sample was increased fourfold, making its detection easier. All samples were repeated three times and mean values were calculated. ⁵⁹Co, ⁵¹V, ⁵⁵Mn, ⁶⁰Ni, ¹⁰⁷Ag, ²⁰⁵Tl, ⁹Be, ²⁰⁹Bi, ¹³³Cs, ⁷Li, ¹¹¹B, ¹²¹Sb, and ²⁰²Hg elements could not be detected in the samples (<LOD).

A quick, easy, and concurrent analytical technique has been developed and validated for the quantitative determination of 24 elements in drinking water in terms of selectivity, sensitivity, LOD and LOQ, linearity, working range, trueness, precision, and measurement uncertainty. The removal of chlorine from the samples was realized by adding acid solutions on the hot plate at 60°C. Such a preliminary study was performed to minimize salt deposition in matrix-derived interface cones and to reduce or eliminate interference with the matrix. For recovery, precision, and accuracy studies, necessary additions from stock mix-3 calibration standard and CRM solutions were made on the drinking water matrix by the spiking method. The concentrations of 24 elements in drinking water samples were reported with traceability and estimated uncertainty.Therefore, the method was shown to be reliable and internationally comparable. After this validation, the selected performance criteria (linearity, LOD, LOQ, specificity, accuracy, repeatability) showed that both open-vessel digestion and the use of ICP-MS for detection allowed an accurate determination. Since the quality of drinking water and sources of pollution may vary from year to year, it is recommended that ongoing monitoring be maintained.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.