Julian Wills is Senior Application Specialist: ICP-MS with Thermo Fisher Scientific.

Julian Wills

In this study, apple juice samples are analyzed by IC–ICP–MS to determine the concentration of six arsenic species: the two inorganic, and highly toxic, species (As (V) and As [III]) and four organic species (arsenobetaine [AsB], arsenocholine [AsC], monomethylarsonic acid [MMA], and dimethylarsinic acid [DMA]). 

An iCAP Qc ICP-MS (Thermo Fisher Scientific, Bremen, Germany) was equipped with a Peltier cooled cyclonic PFA spray chamber and a PFA-LC nebuliser (Elemental Scientific, Omaha, Nebraska). A demountable torch was equipped with a 2 mm i.d. quartz injector. 

 Chromatographic separations were carried out using a Dionex ICS-5000 ion chromatogram (Thermo Fisher Scientific, Sunnyvale, California). 

The column outlet from the ICS-5000 was directly connected to the PFA-LC nebulizer. Full bidirectional communication between the instruments was achieved using a trigger cable. All data evaluation was performed using the tQuant features of the ICP-MS Qtegra software platform. 

Sample and Calibration Solution Preparation 

Four different apple juices were bought in a local supermarket and the total arsenic concentration was first determined after dilution of 1 mL juice in 7 mL of ultrapure water/2 mL of 2% nitric acid. Samples that showed the detectable presence of total arsenic were passed for subsequent speciation analysis. 

Figure 1: Model separation of six arsenic species, each at 0.45 ng/g in dilute HNO3. 

Results from two of the four apple juices where arsenic was detected are summarized in Table I. Method detection limits (MDLs) for the arsenic species were calculated from three times the standard deviation of four repeat blank injections. Total arsenic concentrations for the two samples also are shown. 

Table I: Arsenic species concentrations, method detection limits (MDLs), and total concentrations in two of the apple juice samples analyzed. All concentrations have units of ng/g. 

The trace amounts of arsenic found in this study fit well with a typical range of arsenic concentrations in apple juices (between 2 and 6 ng/g) as determined by the United States Food and Drug Agency (US FDA) in a previous study (1). 

A new IC-ICP-MS method for the separation and quantification of six arsenic species in apple juice has been developed. The combination of ion chromatography with ICP-MS has been shown to be a powerful tool for the detection of low concentrations of arsenic species in fruit juices. 

 Although there is no legislative limit for arsenic species in fruit juices currently, the detection limits obtained by this method (~5 pg/g) far exceed those in similar sample types, for example bottled water (FDA limit set at 10 ng/g). 

http://www.fda.gov/Food/FoodSafety/FoodContaminantsAdulteration/Metals/ucm272705.htm

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Articles

In this study, apple juice samples are analyzed by IC–ICP–MS to determine the concentration of six arsenic species: the two inorganic, and highly toxic, species (As (V) and As [III]) and four organic species (arsenobetaine [AsB], arsenocholine [AsC], monomethylarsonic acid [MMA], and dimethylarsinic acid [DMA]).

IC–ICP–MS Speciation Analysis of As in Apple Juice

Inductively coupled plasma–mass spectrometry (ICP-MS) has been established as the most appropriate method for routine environmental and geological applications, owing to its extended elemental coverage, superior sensitivity, and rapid analytical capabilities. Nevertheless, the technique's association with spectroscopic interferences remains, despite the fact that in recent years the introduction of collision–reaction cell (CRC) instrumentation has solved this shortcoming. This article will discuss the ICP-MS method, its usability in environmental and geological analysis and relevant regulations, and will build the case for the CRC approach as the most efficient solution addressing the limitations of ICP-MS. An experiment illustrates the flexibility and power of collision cell technology–based ICP-MS to analyze a variety of common environmental and geological sample matrices.

The inductively coupled plasma–mass spectrometry (ICP-MS) technique is based upon the coupling of an inductively coupled plasma, producing ions, and a mass spectrometer that separates and detects the ions. Argon gas (Ar) is used for the production of a plasma or gas consisting of ions, electrons, and neutral particles. Following dispersion into the stream of argon gas, samples are carried to the inductively coupled plasma, where their elements are atomized and ionized. The resulting ions are then passed through a series of apertures (cones) into a high-vacuum mass spectrometer where they are separated according to their mass-to-charge ratio (

) and counted. The isotopes of the elements are identified by their 

. The intensity of a specific peak in the mass spectrum is proportional to the amount of that isotope in the original sample.  

ICP-MS in Environmental and Geological Analysis

ICP-MS is a popular method, routinely used in both standard and high-sensitivity modes for fast and accurate measurements of trace (parts-per-million to parts-per-billion) and ultratrace (parts-per-billion to parts-per-quadrillion) elemental concentrations in a wide range of geological and environmental applications. The increasing popularity of the technique is attributed to the important benefits that it offers, including its multielemental capability compared with conventional elemental techniques, high sensitivity, superior detection limits, increased dynamic range, and isotopic ratioing capabilities. ICP-MS can be used to analyze both solid and liquid samples, requiring only small sample quantities thanks to its analytical range from the parts-per-trillion to the parts-per-million regions.  

Used in conjunction with varying nebulization and calibration techniques, ICP-MS is applicable in both liquid and solid materials. This highly sensitive and powerful technique performs reliable trace element distribution analysis of geological materials; identification of ultratrace cesium (Cs) and rubidium (Rb) in hydrothermal pore waters; and monitoring of transition, trace, and heavy metal contamination in environmental water samples. Further applications include preliminary investigation into the determination of osmium (Os) in sea water, lead isotopic ratio determination in mud cores taken from salt marshes, and analysis of uptake fluxes of trace redox-sensitive elements from sea water in sediments. Regardless of the sample type, it is important that it is free of particulate matter, as even the smallest particle can disrupt sample flow and block or clog the sample tips within the spray chamber. 

Having recognized the unparalleled sensitivity, precision, and reliability of ICP-MS, environmental protection regulatory bodies across the world have mandated the use of ICP-MS for a variety of applications. 

The U.S. Environmental Protection Agency (EPA) National Environmental Methods Index currently lists 11 approved ICP-MS methods by agencies within the United States. The most commonly used methods are those that have been developed by the EPA covering the use of ICP-MS for monitoring various elements in drinking water (Method 200.8), wastewater and solid waste (SW-846 Method 6020), and low-level trace in drinking water (Method 1638). In addition, the EPA has recognized ICP-MS as the only approved multielement method for determining arsenic and uranium in drinking water. The U.S. Department of Agriculture, Food Safety and Inspection Service, Office of Public Health Science has introduced Method CLG-TM3.01. This method specifies the use of ICP-MS for the analysis of cadmium (Cd) and lead (Pb) at parts-per-billion levels in beef, pork, and poultry muscle, liver, and kidney.  

Although very powerful and widely recognized by regulatory agencies, ICP-MS presents a few limitations, including spectroscopic interferences. 

Interferences are caused by polyatomic species that have similar mass to the target species and that stem from the sample matrix, Ar, or entrained gas. Solvents, reagents, glassware, and other sample processing hardware also can yield artifacts or interferences to sample analysis.  

There are other types of interferences including isobaric elemental interferences that are caused by isotopes of different elements forming atomic ions with the same nominal 

. Ions consisting of more than one atom or charge trigger the formation of isobaric molecular and doubly charged ion interferences, respectively. Physical interferences are associated with the sample nebulization and transport processes as well as with ion-transmission efficiencies. Memory interferences or carryover occur when there are large concentration differences between samples or standards that are analyzed sequentially.  

Interferences limit the ability of ICP-MS to determine certain elements of interest while also increasing maintenance requirements and reducing the reliability and quality of the data produced. Eliminating interferences provides numerous advantages, including significantly improved detection limits for interfered analytes, analyte confirmation by isotope ratio measurement, and superior analytical confidence in complex matrices. 

The combination of ICP-MS and collision–reaction cell (CRC) instrumentation has demonstrated a unique capability of significantly reducing and even eliminating interferences.  

CRC instrumentation employs either passive collisional interactions in which no chemical reaction occurs, or reactive interactions in which the charge or structure of either the analyte or interferent are changed chemically. Collision-based instruments use an inert collision gas, helium (He), to reduce the kinetic energy of the polyatomic interferent, thus preventing it from entering the quadrupole analyzer. Reaction-based analyzers use a range of reactive gases, including hydrogen, methane, and ammonia, to chemically shift one member of the analyte–interferent pair to another mass.  

Reaction and collision cells share the same technology, but where they differ is the regime established inside. Reactive chemistry can be applied to polyatomic species that react with the gas and consequently are either eliminated or modified. Additionally, reactive chemistry can modify analyte ions and analyze them at masses different from their natural isotope mass.  

Collision instruments can separate all kinds of overlapping molecular and polyatomic analyte ions from monoatomic ions if they differ in kinetic energy. This process is based upon collisions using inert gases like He and is called kinetic energy discrimination (KED). Collision instruments using KED have the advantage of being universally able to reject all polyatomic interferences in any matrix and are therefore well suited to multielemental analysis in complex or unknown matrices.  

Using ICP-MS in conjunction with state-of-the-art collision–reaction instrumentation can reduce spectral interferences efficiently and to negligible levels while the analytes remain relatively unaffected. Accurate analysis at low concentration levels can be performed even in difficult matrices. There are a number of spectral interferences that can be reduced with reactive chemistry, while others are better treated under KED conditions. For highest flexibility and best detection power in sample analysis, experiments should switch between several cell regimes. 

An experiment was developed to illustrate the flexibility and power of collision cell technology (CCT)-based ICP-MS to analyze a variety of common environmental and geological sample matrices. 

A Thermo Scientific XSERIES 2 ICP-MS system equipped with third-generation CCTED CRC instrumentation (Thermo Fisher Scientific, Bremen, Germany) was configured with an SC2 FAST automated sample introduction system (Elemental Scientific Inc., Omaha, Nebraska). The schematic in Figure 1 outlines the principle of the sample introduction system, which relies on a six-port switching valve to load and inject samples.  

This system significantly reduces sample uptake and washout times and dramatically decreases matrix load that reaches the plasma. The ICP-MS instrumental parameters are shown in Table I. A universal H

 in He gas mixture was used in the collision–reaction cell for the suppression of interferences.  

The experimental methodology performed to test the high-throughput setup for environmental samples was based upon conditions outlined in the Multi-Media, Multi-Concentration, Inorganic Analytical Service for Superfund Method ILM05.4. This method was introduced by the EPA within the Contract Laboratory Program (CLP) for water and soil/sediment environmental analysis.  

To improve the determination of analytes with a high ionization potential (such as Be, As, and Se), a stream of methane was added to the nebulizer gas. This carbon-loading enhances sensitivity for these analytes (Figure 2) and improves long-term stability by the reduction of matrix deposition on the ICP-MS interface.  

A sequence of 500 samples including a calibration and integrated QC (Table II) was performed for 23 analytes.  

A sample turnaround of approximately 83 s, including uptake, analysis, and wash, provided an overall batch acquisition time of 12 h for the entire 500 samples. Five samples (an interference check solution [ICSA], the same solution with a 20-ppb spike [ICSAB], NRC SLRS-3 river water reference material with varying spike concentrations, NIST 1643c reference material, and a detection limit check solution [CRI]) were classed as unknown samples and looped continuously throughout the experiment. The mean results for periodic analysis (every 20 unknowns) of the laboratory control standards (LCS) are shown in Table III.  

The stability of the results for samples SLRS-3 + spike and NIST 1643c are shown in Figure 3.  

Rock samples weighing 0.5 g were digested using an Aqua Regia mix (12.5 mL 30%) and diluted an additional 10 times before analysis. A sequence of 478 samples, including a calibration and integrated QC, was performed for 30 analytes. A sample turnaround, including uptake, analysis, and wash, of approximately 80 s resulted in an overall acquisition time of 11 h for the 478 samples. A standard operating procedure similar to the CLP method was followed, and a well-characterized rock was used as an LCS in the QC protocol along with a continuing calibration verification (CCV) and a continuing calibration blank (CCB). Five samples (an ICSA, the same solution with a 20-ppb spike, a rock sample [GEOMS], GEOMS spiked with varying analyte concentrations, and a CRI) were classed as unknown samples and looped continuously throughout the experiment. Results for the determination of analytes in the unknown samples over the 11-h period are shown in Figure 4.  

When used with a universal gas mixture for all analytes, CCT-based ICP-MS has been demonstrated to be a multielemental technique with high-throughput capabilities. With methane addition, the analysis can benefit from an increase in sensitivity for analytes with a higher ionization potential, such as Be and Se, which are often at lower concentrations in environmental samples, as well as benefiting from an improvement in long-term stability. Throughput and stability are further improved and necessary maintenance is reduced thanks to the configured sample introduction system, which cuts uptake and washout time and introduces less matrix into the plasma over time. 

CCT-based ICP-MS is a proven multielemental technique, capable of performing fast and accurate analyses of an extended range of common environmental and geological sample matrices. Through the use of a segmented flow sample introduction system, matrix deposition on the ICP-MS interface is reduced significantly, leading to increased first-time pass QC analyses and longer periods between necessary maintenance. Spectral interferences associated with the traditional ICP-MS method are reduced considerably, and sample throughput is increased. 

Julian Wills, Shona McSheehy, Tomoko Oki,

 are with Thermo Fisher Scientific, Bremen, Germany. 

 is with Thermo Fisher Scientific, Winsford, UK. 

The authors discuss the ICP-MS method, its usability in environmental and geological analysis and relevant regulations, and how to address its limitations.

Complete Multielemental Analysis of Environmental and Geological Samples Using CCT-Based ICP-MS

In the U.S., two major areas of legislation act as drivers for water quality measurements: drinking water and environmental water quality. This legislation forces measurements including the determination of trace and major elements.  

In the case of drinking water, the Safe Drinking Water Act (SDWA) of 1996 gave the U.S. Environmental Protection Agency (EPA) the power to set and regulate national standards for the quality of supplied drinking water as well as drinking water sources. This is regulated by the EPA's Office of Ground Water and Drinking Water (OGWDW) under regulation 40 CFR part 141. Supplied waters must comply with the maximum contaminant level (MCL) specified in the National Primary Drinking Water Regulations (NPDWR). Further contaminants are given guideline values in the National Secondary Drinking Water Regulations (NSDWR), 40 CFR part 143. 

In the case of environmental water quality, the Clean Water Act (CWA) of 1972 gave the EPA power to set and regulate permits for the discharge of polluting waste into bodies of water. Under the CWA, the National Pollutant Discharge Elimination System (NPDES) is regulated by authorized states and overseen by the EPA's Office of Water (OW). Water samples taken at point discharge sources (such as pipes and ditches) must comply with the permitted contaminant values. 

These regulations, and others, have created a large demand for the analysis of various kinds of water samples. These tests are performed at the laboratories of organizations such as the EPA, state environmental protection departments, water and utility companies, municipal environmental protection departments and water works, and commercial contract laboratories across the U.S. 

Inductively coupled plasma (ICP)-based analytical techniques have been used routinely in regulated water analysis since the early 1980s. The multielement analysis capabilities of both ICP-optical emission spectrometry (ICP-OES) and mass spectrometry (ICP-MS) techniques make them ideal tools for processing multiple analytes in large numbers of samples quickly and efficiently. The combination of low detection limits and wide analytical linear range makes these techniques particularly well suited to the analysis of the low to moderate concentrations of the many elements of interest in water samples. The inherent suitability of these techniques is what has triggered their success in this field. 

The EPA first published the ICP-OES water analysis method, 200.7, in 1982 (1). This has since been followed by improvements in both instrument technology and analytical methodologies, leading to the publication of several revisions of the method, including the current version, Revision 4.4 (2). ICP-MS was first commercialized in 1983, and it was not long before the EPA introduced a standard method for water analysis that took advantage of the lower detection limits the technique offers. ICP-MS method 200.8 for water analysis was first published in 1990 and has had several major revisions, the most recent of which is Revision 5.4 (3,4). 

In January 2001, the EPA set a new Public Health Goal (PHG) for arsenic in drinking water of zero and a new MCL for the element of 0.01 mg/L (10 ppb) (5). On February 22, 2002, the "Arsenic in Drinking Water Rule" became effective and water supply systems had to comply with the new 10-ppb standard by January 23, 2006. The EPA subsequently withdrew approval of ICP-OES methods 200.7 and SM3120B for the measurement of arsenic in drinking water because the detection limit of the technique was not considered routinely low enough to measure compliance with this new MCL. Thus, labs must now use either method 200.8 (ICP-MS) or 200.9 (GF-AAS) for this purpose. Because GF-AAS is a single-element technique, ICP-MS is now the only multielement method available for this analysis. Many labs performing regulated drinking water measurements have therefore upgraded from ICP-OES to ICP-MS in order to retain the convenience and efficiency of a versatile multielement technique. 

ICP-MS is an approved analytical technique for the analysis of arsenic in drinking water; however, it does suffer from a polyatomic interference generated by the combination of the argon plasma gas and chloride ions that are present naturally in samples. The argon chloride polyatomic (

) has the same nominal mass-to-charge ratio (

), and samples containing a high level of chloride can exhibit positively biased results for arsenic (6). Collision–reaction cell (CRC) technologies have been shown to successfully remove the argon chloride interference that hampers arsenic measurements in the presence of such chloride matrices (7–10). This technological solution to the interference problem can be used to extend the applicability of ICP-MS for arsenic in water measurements, as well as for other analytes affected by polyatomic interferences, without the need for interference correction equations. 

Unfortunately, the EPA has announced that ICP-MS systems fitted with CRC technology must be operated only in standard mode when used with method 200.8 for regulated drinking water analysis because it has reservations about whether the technology falls within the scope of the 200.8 method (which was written before the introduction of CRC technology). This does not affect the use of CRCs in unregulated analyses, or for other programs such as NPDES or Resource Conservation and Recovery Act (RCRA)-related analyses. 

The case of approved methods for environmental water quality is somewhat different. For such measurements applied to NPDES regulations, the ICP-OES methods are still fully approved, and it is actually the ICP-MS methods that are not formally approved in this case. General approval (with preparative method 200.2) was proposed in Federal Register Notice (Vol 69, No 66) April 2004, and this might signal further pressure on states to allow the use of 200.8 for this analysis (11)*.  

The status of method approval for the various types of regulatory water analysis is therefore somewhat confused. Laboratories that perform analyses catering to legislation in both the drinking water and water quality arenas must use multiple methods and instrumental techniques to meet the requirements of method approval. Table I illustrates the various approved analytical options available.  

Table I: Technique and method selection choices for the two water analysis areas 

Laboratories performing these analyses would prefer a single applicable method and technique that is capable of, and approved for, the analysis of all target elements in a single run. This would improve laboratory efficiency greatly and potentially could reduce the need for multiple pieces of capital equipment. 

This article describes in detail the practical work performed to re-evaluate the performance of the latest generation of ICP-OES and ICP-MS instruments for the purpose of producing a single universally applicable analytical method for water analyses. 

The performance of commercially available ICP-OES and ICP-MS instruments for routine water analysis was compared by assessing the results of quality control and reference sample analyses. In addition to standard ICP-MS operation, a CRC approach also was evaluated to assess its detection limit improvements for some elements. Three analytical runs were performed for each technique, each including calibration, a method detection limit study, quality control standards, the analysis of reference samples, and the analysis of unknown water samples of various types. Each analytical run consisted of more than 150 samples. 

An iCAP 6500 Duo ICP-OES spectrometer (Thermo Fisher Scientific, Cambridge, UK) was used for ICP-OES measurements. An XSERIES 2 system equipped with third generation CCT

  CRC technology (Thermo Fisher Scientific, Bremen, Germany) was used for ICP-MS measurements. The ICP-MS system was operated in CRC mode for the elements for which it was found to have detection limit benefits over standard mode operation. The CRC mode measurements were made in separate analytical runs to the standard mode measurements for comparative purposes since it is possible to operate the instrument in a single-cell mode for all analytes. Both instruments were used in conjunction with ASX-520 autosamplers (CETAC, Omaha, Nebraska). Table II shows the wavelengths and plasma views used for the ICP-OES measurements and the m/z used for ICP-MS measurements.  

Table II: Wavelengths and plasma views used for ICP-OES measurements and masses used for ICP-MS measurements 

 values (ICP-MS) of analytes were selected based upon the suggested values given in methods 200.7 and 200.8. Standard mode ICP-MS measurements used the interference corrections specified in the 200.8 method, while CRC mode used no interference corrections. The ICP-OES views were selected automatically by the instrument software. See Table II for details. 

Concentrated nitric acid (69%) and hydrochloric acid (36%) (Trace Metals Grade, Fisher Chemical, Thermo Fisher Scientific, Loughborough, UK) were used to acidify all solutions. Solutions for ICP-OES analysis were fixed with an acid matrix of 2% (v/v) hydrochloric acid and 2% (v/v) nitric acid, while ICP-MS solutions were prepared in 0.8% (v/v) nitric acid and 0.4% (v/v) hydrochloric acid. The hydrochloric acid is required in order to stabilize the solubility of silver (Ag). 

All calibration solutions and quality control solutions were prepared from single element standards or multielement solutions (SPEX CertiPrep, Metuchen New Jersey, or VHG Labs, Manchester, New Hampshire). Internal standards were added on-line using an additional channel on the peristaltic pumps of the instruments and a mixing tee prior to the nebulizer. The internal standard for ICP-OES measurements was 5-ppm yttrium (Y). For ICP-MS, a mixed solution containing isotopically enriched lithium-6 (

Li), scandium (Sc), indium (In), terbium (Tb), and bismuth (Bi) at approximately 50 ppb was used. Mathematical correction for the presence of natural lithium in samples was required. All ICP-MS solutions, including the wash solution, were spiked with gold at 100 ppb to stabilize mercury. The combination of gold and chloride stabilizes mercury due to redox chemistry. 

The ICP-OES was calibrated with a blank and a single standard, while a four-point calibration was used for ICP-MS to check linearity. The calibration concentrations are given in Table III. Calibrations were performed at the beginning of each run.  

Table III: Calibration concentrations (ppb) 

Quality control checks for both instruments were performed in a similar manner, following the protocols described in methods 200.7 and 200.8. 

An initial check of calibration accuracy was performed with an independently prepared quality control standard (QCS) for both instruments. A solution at 50% of the concentration of the highest calibration standard was used for this. The results of these measurements were deemed acceptable if the measured value was within 5% of the true concentration for ICP-OES and within 10% of the true value for ICP-MS. Ongoing instrument accuracy was checked by analyzing a quality control standard at an interval of once every 10 samples. This is known as an instrument performance check (IPC) in method 200.7, or a continuing calibration verification (CCV) in method 200.8. These solutions were prepared at the same concentration as the calibration standard for ICP-OES and at the same concentration as the mid-standard (Std-2) for ICP-MS. 

 Method detection limits (MDLs) were estimated using the procedure specified in 40 CFR part 136, Appendix B (12). 

The accuracy of the methodologies for the analysis of real samples was assessed by analyzing known certified reference materials (CRMs) including river water, hard drinking water, and soft drinking water. Spike recoveries also were performed on unknown samples, although the results of these are omitted here for the sake of brevity. 

The long-term stability of the instruments was assessed by plotting the results of the ongoing calibration accuracy QC standard over the course of long analytical runs of more than 150 samples in total. 

The MDLs determined for each technique are given in Table IV. The table also shows typical levels of interest (LOIs), based upon the levels specified in references 13–15. Typically, it is desirable to have MDLs that are an order of magnitude lower than the LOI for each element. Clearly, ICP-OES is a borderline technique for determining Ag, As, Be, and Hg when applying these criteria. If ICP-OES is used for regulatory drinking water analysis, it would probably be necessary to run these elements by alternative techniques to achieve the required detection limits with the degree of confidence required. This could be done by graphite-furnace atomic absorption spectrophotometry (GF-AAS), or by vapor generation AA or atomic fluorescence in the cases of As and Hg. ICP-MS, however, has significant detection overhead for all elements of interest. In addition, the use of a CRC provides further detection overhead for several key elements, notably As, Fe, Se, and V. This is because this mode of operation removes interferences that typically increase the apparent background levels for these elements: ArCl, ArO, ArAr, and ClO, respectively. Note that the sample matrix required for stable silver data adds chloride, which in turn increases the background and MDL for both arsenic and vanadium.  

Table IV: MDLs for ICP-OES and ICP-MS. MDLs for ICP-MS run in collisionâreaction cell mode are also shown where significant advantage is obtained over standard ICP-MS mode 

The pooled results of successive measurements of quality control standards are shown in Figures 1 and 2, for ICP-OES and ICP-MS, respectively. The figures show the mean percentage recovery for each analyte and the spread of recoveries at 1 standard deviation from the mean. Data are pooled from 45 measurements during three analytical runs for each technique. The figures show that both techniques produce accurate results that are well within the acceptable range of 90–110% recovery specified in methods 200.7 and 200.8. The overall accuracies for both techniques are similar, but the precision of ICP-MS appears slightly better than that of ICP-OES.  

Figure 1: Graph showing mean IPC standard recovery and spread at 1 standard deviation for ICP-OES measurements. 

The vast majority of elements show excellent agreement with the expected certified reference material values for both techniques as shown by the average recoveries given in Figure 3 for a number of reference waters. However, a few elements showed small discrepancies. For the NIST 1640 river water reference material, results were somewhat low for Cd, Fe, and Sb by ICP-OES. Similarly, for the hard drinking water, CA010A, Al and Sb were low by ICP-OES and Cr was low by ICP-MS. For the soft drinking water, CA021A, Al, As, and Cd were low by ICP-OES. Many of these low results were at concentrations close to the MDL of ICP-OES, and the accuracy is likely to degrade near the MDL. In general, ICP-MS tends to give better accuracy for the majority of elements at these typical concentrations in water samples.  

Figure 2: Graph showing mean CCV standard recovery and spread at 1 standard deviation for ICP-MS measurements. 

The spectrometric techniques of inductively coupled plasma–optical emission spectrometry (ICP-OES) and inductively coupled plasma–mass spectrometry (ICP-MS) are compared for their applicability to regulatory water analyses, bearing in mind recent method approval changes. ICP-OES is found to be at its limit for confident detection of several elements for drinking water analysis, but is still suitable for many environmental water quality measurements. ICP-MS is the closest there is to a universally applicable technique for water analysis.

Julian Wills

Articles

IC–ICP–MS Speciation Analysis of As in Apple Juice

Complete Multielemental Analysis of Environmental and Geological Samples Using CCT-Based ICP-MS

ICP Source Spectrometry Techniques in Regulated Water Analysis