Low-level analysis of food matrices has placed a demand on manufacturers, testing laboratories, and instrumentation vendors worldwide. Stricter regulations, better analytical instrumentation, and greatly improved sample preparation (preanalytical) techniques have focused efforts to simplify and standardize these analyses. Often overlooked, the preanalytical step determines the quality of the resulting data and requires careful attention to a number of details, including sample size, digestion parameters, and the level of detection needed. This article describes the selection of critical factors used in the optimization and simplification of food and mixed-batch food microwave digestion.

	Food and contract testing laboratories are in an uphill battle in determining the best approach in processing foodstuffs. Regulatory bodies such as the Food and Drug Administration (FDA), United States Department of Agriculture (USDA), and the European Commission (EC) have each established element concentration limits on food and agricultural products. A recent addition to the methods menu from the FDA provides a guideline for the preanalytical step for microwave sample digestion followed by inductively coupled plasma–mass spectrometry (ICP-MS) analysis of trace metals in food (1). Many publications and the Association of Analytical Communities (AOAC) performance methods have validated microwave digestion as a standard approach to food analysis. In fact, Method 4.7 from the FDA is an example of an internal validation exceeding standards set forth by the AOAC. The USDA has also recently updated their chemistry laboratory guidebook for the analysis of food utilizing inductively coupled plasma–optical emission spectrometry (ICP-OES) and ICP-MS following sample microwave digestion (2). The important feature in many of these methods relates to the choice of analytical tools and sample preparation technique, which leads to the questions: What is the best way to perform sample preparation and which instrument is best for multielement analysis within a laboratory setting?

	Sample preparation is given little attention in the overall process for the analysis of trace metals, and for many sample types it turns out to be routine. Normally, scientists simply look at the detection limits for an ICP-OES or ICP-MS system and calculate back from a detection level or interferences to determine the instrument of choice. Unfortunately, with highly organic sample types and the breadth of complexity increasing, the need for a good quality digestion process is more critical. A list of preanalytical contributing factors that can affect the backend analysis includes sample size, acid digestion volume, sample fat and organic content, contamination, throughput, and post-digestion residual carbon content. Each of these will play a role in the methods chosen for processing a food sample.

	Larger samples sizes historically have been used because of the limits of analytical detection and sample inhomogeneity. The consideration on what to use for the sample preparation then becomes a choice between ashing and microwave digestion. Ashing is complicated by the loss of critical elements such as Cd, Pb, and Hg along with contamination issues (3). Microwave digestion also historically has been a concern with the lack of vessel temperature and pressure capability for complete digestion of reasonably sized sample amounts. This picture becomes even more opaque with incomplete digestions causing problems for the ICP-OES or ICP-MS analysis with a resulting high residual carbon content (4–6). While ICP-OES still dominates the testing laboratory environment, there is a consistent movement toward ICP-MS with all of the advantages and trace metal regulations it offers (7).

	Conventional closed-vessel microwave digestion has been recognized for the past two decades as the most effective technique for the digestion of the widest range of sample types in metals analysis, largely because of its ability to operate at high temperatures and pressures. Because the digestions are in a closed vessel, cross-contamination and the loss of volatile elements are eliminated (8). Additionally, microwave sample preparation techniques have overcome challenges of poor digestion quality by providing better digests with low residual acidity, lower carbon content, and lower concentrations of dissolved solids, a critical feature necessary for back-end ICP-MS and ICP-OES analysis (9). It was recently suggested that an ideal sample preparation procedure would be capable of digesting large sample amounts, provide high throughput, be compatible with multielement analysis, and be safe, green, and easy to use (9). A recent publication illustrates the importance of sample size, lower acid consumption, complete digestion with low residual carbon, and the effects of temperature and pressure with a variety of food samples in a conventional microwave and a new single reaction chamber technology (10). This research goes a long way into the basic understanding of how to think about sample preparation as a process. To further illustrate the completeness of sample preparation, we used a single reaction chamber (SRC) microwave digestion approach for processing mixed-batch and large food samples.

	SRC microwave digestion has been utilized extensively in the last several years. The operation, specification, sequence of operation, and application has been published for further reading (11,12). In short, the SRC system uses a large, prepressurized chamber where multiple independent samples or sample types are digested simultaneously up to 300 °C and 199 bar (including multiple acid combinations for determining the right digestion chemistry). By choosing different size racks to fit inside the chamber, different sample sizes and throughput can be performed depending on internal needs. Prepressurizing the chamber allows a single microwave method to be chosen for all types without the need for extensive method development.

	All digestions were developed using an UltraWAVE microwave (Milestone) based on the SRC design. The microwave delivers 1500 W of microwave power to a 1-L stainless-steel reinforced PTFE chamber that can hold sample racks of 5, 15, or 22 vessels depending on sample and acid volume. Direct temperature and pressure control provides direct control of each sample (all samples reach the same temperature and pressure). The experiments described in this article were performed using a 15-position rack with 15-mL disposable glass vials and a five-position rack with quartz vials for large sample sizes. An Agilent 7700x ICP-MS system was used for the analysis of all the elements. The operating conditions for the ICP-MS system were as follows: RF applied power to the torch, 1.55 kW; carrier gas flow, 0.95 L/min; dilution gas flow rate, 0.15 L/min; plasma mode normal, He cell gas flow, 4 mL/min. Operating in He collision mode for samples allowed for routine robust analysis.

	Samples were selected to include a variety of food types along with standardized reference materials. The primary "big four" toxic metal profiles were used to establish a microwave method and instrument baseline numbers. Following this approach, an expanded set of metals was determined using the same microwave method. The final experiment examined a set of samples with sizes greater than 2 g to illustrate the feasibility of processing larger food samples with an identical microwave method established in the previous experiments (expands the capability to use ICP-OES in the well established food industry).

 Typical food samples (0.5 g) were placed in disposable glass test tubes and placed in a 15-position rack for microwave operation. The samples were treated with a 4-mL nitric acid and 0.5-mL hydrochloric acid combination (for Cd, Pb, As, and Hg stabilization) or 5 mL of nitric acid for the expanded list of elements and placed in the microwave chamber (note: the PTFE liner contains 110 mL of water, 10 mL of hydrogen peroxide, and 3 mL of sulfuric acid as the microwave load for parameter control). Following prepressurization of the chamber with nitrogen to 40 bar, the samples were digested using the following time–temperature microwave method: Ramp to 240 °C over 30 min, hold at 240 °C for 15 min, cool to 60 °C, and depressurize the chamber for analysis; the total time was 55 min. It should be noted that pressures can reach 100 bar or greater during the course of a digestion and sample decomposition, illustrating one of the critical needs to have a system capable of higher temperatures and pressures for quality digestions. Systems or vessels venting during the course of these conditions would preclude development with the loss of volatile elements at a partial digestion point during a method. 

	Three separate microwave digestion runs were performed with a variety of food sample types and multiple sample sizes. Each of the microwave methods formed clear digestates with lower acid volumes, which were diluted with water to 25 mL, followed by a 2-mL aliquot diluted to 40 mL, and analyzed using ICP-MS. The first sample set was analyzed for As, Cd, Hg, and Pb. The second sample set was expanded to include Mg, Mn, Cu, Fe, and Zn. Lastly, larger sample sizes were digested and analyzed for Se and Hg. For quality control (QC), an acid blank, spikes, and duplicates were analyzed as part of ongoing instrument calibration verification. Quantitative data, detection limits, and QC data are shown in Tables I, II, and III.

	Given the high-throughput nature of testing laboratories, the standardization of the SRC microwave method aligns well with the use of ICP-OES or ICP-MS analysis for all sample types. SRC microwave digestion in food, testing, and manufacturing laboratories for the analysis of trace metals provided a single optimized sample preparation method. To achieve complete digestions for multiple sample types simultaneously and larger same sizes, an optimized temperature of 240 °C was used; disposable glass vials were used to eliminate the need for vessel cleaning in subsequent digestion runs. With a 15-position rack and a 55-min digestion time start to finish, a series of multiple samples can be processed for multielement trace metals. Standardizing the SRC method provided a way to scale the process to larger sample sizes without the need for extensive method development, batching, vessel cleaning, and assembly.

 is the applications manager at Milestone, Inc., in Shelton, Connecticut. Direct correspondence to: 

	(1) P. Gray, W. Mindak, and J. Cheung in 

Elemental Analysis Manual (EAM) for Food and Related Products

 (U.S. Food and Drug Administration, U.S. Department of Health and Human Services, Silver Spring, Maryland, 2013), section 4.7. Retrieved from
	

http://www.fda.gov/downloads/Food/FoodScienceResearch/	LaboratoryMethods/UCM377005.pdf

	(2) United States Department of Agriculture (USDA), 

Determination of Metals by ICP-MS and ICP-OES (Optical Emission Spectrometry)

 (United States Department of Agriculture, Food Safety and Inspection Service, Office of Public Health Science, Washington, DC, 2013). Retrieved from
	

http://www.fsis.usda.gov/wps/wcm/connect/b9a63ea1-cae9-423b-b200-36a47079ae49/CLG-TM3.pdf?MOD=AJPERES

	(3) J.S. Barin, B. Tischer, R.S. Picoloto, F.G. Antes, F.E.B. Silva, F.R. Paula, and E.M.M. Flores, 

	(4) M. Wurfels, E. Jackwerth, and M. Stoeppler, 

	(6) G. Grindlay, L. Gras, J. Mora, and M.T.C. de Loos-Vollebregt, 

	(7) S. McSheehy, M. Hamester, and M. Godula, 

 (February/March, 2010). Retrieved from
	

http://www.foodquality.com/details/article/809781/ICP-MS_for_Detecting_Heavy_Metals_in_Foodstuffs.html

Spectroscopy supplement: Applications of ICP & ICP-MS Techniques for Today's Spectroscopists 

	(9) J.A. Nobrega, C. Pirola, L.L. Fialho, G. Rota, C.E.K.M.A. Campos Jordão, and F. Pollo, 

	(10) C.C. Mueller, A.L.H. Muller, C. Pirola, F.A. Duarte, E.M.M. Flores, and E.I. Muller, 

 Applications of ICP & ICP-MS Techniques for Today's Spectroscopists 

Articles

Low-level analysis of food matrices has placed a demand on manufacturers, testing laboratories, and instrumentation vendors worldwide. Stricter regulations, better analytical instrumentation, and greatly improved sample preparation (preanalytical) techniques have focused efforts to simplify and standardize these analyses. Often overlooked, the preanalytical step determines the quality of the resulting data and requires careful attention to a number of details, including sample size, digestion parameters, and the level of detection needed.

Simplifying Mixed-Food Microwave Sample Preparation for ICP-MS Analysis

Multiple-collector inductively coupled plasma–mass spectrometry (MC-ICP-MS) has some specific advantages over traditional ICP-MS instruments. Because of its ability to measure high-precision isotope ratios, MC-ICP-MS shows promise for applications as diverse as tracing the transport of heavy metals into bivalves (oysters and mussels), studying isotopic variations of metals in the human body, and identifying the source of volcanoes. Spectroscopy recently spoke with Dominique Weis, PhD, a Canadian Research Chair, Tier I, and Director at the Pacific Centre for Isotopic and Geochemical Research (PCIGR), in the Department of Earth, Ocean, and Atmospheric Sciences at the University of British Columbia, about her research on this technique.

How do methods using MC-ICP-MS differ from traditional inductively coupled plasma–mass spectrometry (ICP-MS) methods? What are the advantages and disadvantages of MC-ICP-MS?

 There are several main differences between the methods using MC-ICP-MS and those using traditional ICP-MS. First, MC-ICP-MS instruments are mostly used to measure high-precision isotope ratios whereas ICP-MS instruments are used to determine elemental concentrations. Second, MC-ICP-MS instruments measure within a narrow range of masses (the isotopes of the element of interest and spectral interferences) whereas ICP-MS analyses typically cover a wide to full mass range. Third, high-precision isotope analyses by MC-ICP-MS require minimum matrix load in samples, whereas concentration measurements by ICP-MS have a much higher matrix tolerance. 

The advantages of MC-ICP-MS versus conventional ICP-MS (sector field inductively coupled plasma–mass spectrometry [SF-ICP-MS] or quadrupole inductively coupled plasma–mass spectrometry [Q-ICP-MS]) are the ability to simultaneously collect masses as a result of the presence of up to 16 collectors as well as a wider mass dispersion because of larger geometry.  

The disadvantages of MC-ICP-MS versus conventional ICP-MS (SF-ICP-MS or Q-ICP-MS) are that the instruments are too slow to scan over a wide mass range, and to achieve better precision the detectors are Faraday cups, which have better linearity yet lower sensitivity than ion counters (ICs) typically used for SF-ICP-MS or Q-ICP-MS. 

What strategies are used in MC-ICP-MS to avoid analytical artifacts from spectral interferences and matrix effects in Earth science studies, and in isotope ratio measurements in particular (1,2)?

 The main strategies to reduce or remove spectral and matrix effects include 					

chemical separation of the analyte from the matrix;

using desolvating systems (reducing oxide interference with the partial removal of the water in the sample);

monitoring possible interfering masses in addition to the masses of interest and correcting off-line;

spiking the sample and the standard with a similar matrix;

different tuning settings in ion optics or subtle variations at the front end, such as torch position, nebulizer selection, and introduction of additional gases; and

using high resolution including the pseudo-high resolution option.

The most difficult interferences to deal with in MC-ICP-MS measurements are isobaric interferences. The separation of the isotope of interest in MC-ICP-MS is achieved according to the principle that ions are bent into different radius of curvature passing through the magnetic field according to their mass-to-charge ratios (

). The only way to remove isobaric interferences is to use the higher resolution feature of the instrument to either completely separate the peak of the interferent from that of the analyte (for example, Nu1700) or to partially separate the adjoining peaks to reveal a sufficient mass width of the analyte peak to be used for the measurement (for example, Finnigan Neptune, or Nu Plasmas I and II). Technically, the higher resolution is achieved by reducing the width of the slits to narrow down the ion beam, which in turn reduces the sensitivity. There is generally a trade-off to make between the sensitivity and resolution adopted.  

In your research with MC-ICP-MS on natural and anthropogenic sources of metals in bivalves (3), how important is the ionization efficiency provided by the technique in obtaining meaningful data? What other techniques have been used to look at the small variations in the isotopic composition of heavy metals in the environment?

 In our study using isotopic analyses of Cd (and Zn) in bivalves to trace the fate of these metals in the environment, the precision and the ionization of the MC-ICP-MS technique were critical, as the variations were too small to be resolved with the precision of thermal ionization instruments. 

The fate and the transport of heavy metals in the environment are a function of their speciation, a key parameter to keep in mind when interpreting isotopic results for such studies. Bulk analyses, as produced in many laboratories around the world for environmental purposes, will be progressively outdated. However, it is so far extremely challenging to analyze the species-specific isotope composition for a metal. A lot of development work remains to be done to allow coupling classical on-line speciation techniques with MC-ICP-MS as those techniques generate a lot of "noise" in terms of matrix in the plasma, as well as transient signals (few seconds), totally different than those usually measured (10 min) with traditional isotope ratio techniques. 

Tremendous developments are being made in the field of in situ analyses, with the coupling of a laser ablation system to an MC-ICP-MS system, allowing researchers to narrow down the scale of observation to micrometers or tens of micrometers. 

(2) A.E. Shiel, J. Barling, K.J. Orians, and D.Weis, 

(3) A.E. Shiel, D. Weis, and K.J. Orians, 

This interview has been edited for length and clarity. To read the full interview visit: 

Spectroscopy Spotlight

Multiple-collector inductively coupled plasma–mass spectrometry (MC-ICP-MS) has some specific advantages over traditional ICP-MS instruments.

What New Methods to Measure Isotope Ratios Can Reveal About the Environment, Humans, and the Earth

A novel approach to handling samples with very high and variable levels of sample matrix is to use aerosol dilution, whereby the amount of sample aerosol that reaches the plasma is reduced by dilution with an additional argon gas stream. This allows the inductively coupled plasma–mass spectrometry (ICP-MS) system to directly measure samples containing matrix levels of up to 25% total dissolved solids (TDS), which is more than a factor of 100 higher than the accepted maximum. Samples containing variable levels of NaCl were measured against simple aqueous standards, and accurate spike recoveries were obtained for all levels of matrix without requiring any prior knowledge of matrix levels, and without predilution of the samples.

Since inductively coupled plasma–mass spectrometry (ICP-MS) became commercially available in the 1980s, the technique has been adopted for trace-element analysis across a wide range of industries and applications. One of the barriers to wider acceptance of the technique has been its limited tolerance to the high matrix levels found in many samples encountered in environmental, clinical, food, nuclear, and geological applications. In contrast to ICP–optical emission spectrometry (ICP-OES), ICP-MS uses an interface to transfer ions into a high vacuum region for separation and detection. This interface includes plates or "cones" with small-diameter orifices to allow ions to pass through while the vacuum is maintained. When samples that contain high levels of total dissolved solids (TDS) are analyzed, the dissolved matrix can deposit on the interface cone orifices, leading to signal loss and instability. 

When the sample contains a high concentration of easily ionized elements such as Na and K, the signal for analyte elements can also be reduced because of ionization suppression. This effect occurs when the plasma is flooded with free electrons from the easily ionized matrix elements, which reduces the ionization of analyte elements. Ionization suppression particularly affects those analytes with higher ionization potentials such as the commonly measured trace elements As, Se, Cd, and Hg (1). A very high concentration of a matrix element may also cause defocusing of the extracted ion beam because of space charge effects, causing further significant loss of analyte sensitivity (2). 

A high level of matrix also changes a sample's physical parameters such as viscosity and surface tension, which affects the uptake and nebulization of the liquid; these effects can usually be corrected by using internal standardization. 

A further problem encountered in ICP-MS is that of spectral interferences, where high levels of matrix elements such as Na, S, Cl, K, and Ca give rise to polyatomic ions that overlap the major isotopes of Ti, V, Cr, Ni, Cu, Zn, As, and other elements in the mass spectrum (3). For these reasons, as well as lower capital cost and higher sample throughput, ICP-OES is often used for the analysis of high salt samples (4,5), but ICP-OES has reporting limits typically in the microgram-per-liter range, rather than the nanogram-per-liter range offered by ICP-MS. 

The limited matrix tolerance of the ICP-MS interface is the main reason for the commonly accepted maximum recommended limit of 0.2% (2000 ppm) TDS in samples intended for ICP-MS analysis. The 0.2% TDS (or 2 g/L) limit is also defined in many standard methods for ICP-MS, such as EN-ISO 17294-2 ("Water Quality") (6), US-EPA 6020 ("Water and Wastes") (7), and EN 13805 ("Trace Elements in Food") (8). For many high matrix samples or sample digests, this TDS limit means that a dilution step is required before ICP-MS analysis. Conventional off-line sample dilution adds time and cost to the analysis, introduces the risk of contamination and dilution errors, and increases the volume of waste that must be disposed of. Automated on-line sample dilution simplifies the process, but the extra pumps and valves are often expensive and the additional tubing and connections are prone to leaks and blockages; on-line autodilution also carries the risk of contamination from the diluent. 

Alternative approaches to measuring high salt samples by ICP-MS include matrix removal, using a technique such as on-line chelation (9–12), solid-phase extraction (13), or co-precipitation (14,15). These techniques can be applied to the routine analysis of certain elements, but they are time-consuming and require a skilled operator. Moreover, because of their similar chemical properties, some elements such as Br, Rb, I, and Cs may be removed along with the matrix elements. 

Another option to reduce the exposure of the ICP-MS interface to high salt levels is to introduce only a small volume of sample solution, using a discrete sampling or flow injection approach (16,17). This technique injects a small volume of sample into a continuous carrier stream, so it reduces the total amount of matrix that reaches the plasma per sample, and thereby increases the number of samples that can be measured before interface maintenance is required. However, discrete sampling does not address the issues of ionization suppression and space charge effects because the undiluted sample matrix is aspirated during the short period of the measurement. The short duration of the signal from the discrete sample volume also limits the flexibility of the acquisition method, because it may impose shorter-than-optimum integration times or limit the number of different acquisition or cell gas modes that can be run. 

However, there is a need for low-level trace-element analysis of high matrix samples, so improving the matrix tolerance of ICP-MS is a primary goal for instrument manufacturers. In addition to environmental monitoring of high salt matrix samples such as open ocean and coastal seawater (18), enclosed lakes and lagoons with salt levels far in excess of oceanic seawater are also of interest. Known as 

, these bodies of water include Don Juan Pond and Lake Vanda in Antarctica, Lake Assal in Djibouti, Garabogazköl in Turkmenistan, the Dead Sea between Israel and Jordan, and Mono Lake and the Great Salt Lake in the United States. These hypersaline lakes may contain salt levels many times higher than seawater, with more than 40% salinity in the case of Don Juan Pond. 

Other high salt matrix samples such as saturated salt water (25% NaCl) used for dialysis, fish transportation, and during the development of oil fields may also require analysis.  

A new approach to high matrix analysis by ICP-MS is the use of aerosol dilution (19), which eliminates manual sample handling while retaining the standard ICP-MS sample introduction hardware and operating conditions, apart from the use of a lower nebulizer gas flow rate. The peristaltic pump delivers sample solution to the nebulizer at a normal uptake rate of ~0.25 mL/min, but the reduced nebulizer gas flow means that the volume of sample aerosol created decreases. A diluent gas flow (of argon) is then added between the ICP-MS spray chamber and torch to dilute the aerosol as it passes to the plasma. Aerosol dilution reduces the amount of sample aerosol that reaches the plasma, effectively diluting the sample matrix without requiring any additional hardware, addition of reagent or diluent solutions, or manual sample handling. Aerosol dilution reduces the total mass of sample aerosol that reaches the plasma, so plasma loading is reduced and more energy remains for matrix decomposition and analyte ionization. This means that both ionization suppression and matrix deposition on the interface cones are reduced. While aerosol dilution reduces sensitivity the same way as liquid dilution, it has the benefit of eliminating the risk of dilution errors and contamination, and it is simpler and more reliable than online liquid dilution. 

This article describes the use of a new aerosol dilution approach to improve the matrix tolerance of an ICP-MS instrument, allowing samples with matrix levels of up to 25% salt to be run routinely. The capability to measure such high matrix levels by ICP-MS is unprecedented and removes a long-standing limitation of the technique. In combination with a collision–reaction cell (CRC) operating in helium (He) mode for the removal of matrix-based polyatomic ion overlaps (20), aerosol dilution ICP-MS offers a complete solution for accurate trace-element analysis of high matrix samples. 

An Agilent 7900 ICP-MS system fitted with standard nickel sampling and skimmer cones, a standard glass nebulizer, quartz spray chamber chilled at 2 °C, and a quartz torch with a 2.5-mm injector was used. The Ultra High Matrix Introduction (UHMI) option was fitted and the argon carrier gas was humidified to reduce salt build up at the nebulizer. The ICP-MS system includes a fourth generation CRC, the ORS

, operating in He collision mode. The optional H

 cell gas was used for improved interference removal on elements with intense plasma-based polyatomic overlaps (Ca [

78]). An Agilent ASX-520 autosampler was used for sample delivery. 

Calibration Standards and Sample Preparation

Several salt matrix solutions containing levels of NaCl up to 25% were prepared by dissolving reagent-grade NaCl (Merck EMSURE) in deionized water stabilized with the addition of 0.5% HNO

 and 0.6% HCl. Calibration standards were prepared from custom mix and single-element stock solutions purchased from Inorganic Ventures, and spike mixes were prepared from mixed quality control (QC) stock solutions (CPI International) and a single-element Hg standard (Merck). A mixed internal standard solution was also prepared from single element stocks from the same suppliers and added on-line using the internal standard mixing tee-connector.  

UHMI 100 (100× aerosol dilution) was selected for maximum robustness and matrix tolerance, although NaCl is more easily decomposed than some refractory matrices such as CaCO

, and can be run successfully at a lower aerosol dilution factor. After setting the aerosol dilution factor, the other instrument settings were optimized using the autotune function run automatically during the instrument startup sequence. Tune settings for both tune modes (He mode for most elements and H

 mode for Ca, Mn, Fe, and Se) were optimized independently, but most parameters are consistent for the two gas modes. The ICP-MS instrument settings used are shown in Table I.  

Table I: Acquisition parameters used for high salt matrix analysis			

A range of trace elements of interest in environmental samples was measured in salt matrices ranging from zero added NaCl to 25% NaCl solution. All samples were measured against simple aqueous calibration standards (no NaCl matrix), prepared in the same acid mix as the samples (0.5% HNO

 and 0.6% HCl). Internal standards were added on-line to correct for physical sample transport effects and any residual matrix suppression. 

The matrix level will affect the sample transport and nebulization processes even when aerosol dilution is used, so overall sensitivity may be reduced in very high matrix samples. But these physical processes affect the total amount of aerosol that is produced and transported to the plasma, so changes should be consistent for all elements. This can be observed in the plot of relative intensities for the internal standards measured in the variable matrix samples containing between 0% NaCl and 25% NaCl, shown in Figure 1. The internal standards included easily ionized elements (

Li, Sc, Rh, In) and poorly ionized elements (Ge, Ir). The sensitivity of all the internal standards was reduced by about the same amount, indicating that the signal reduction was mostly because of physical transport effects. If the sensitivity reduction was mainly because of ionization suppression from the NaCl matrix, the poorly ionized elements would have been affected much more than the easily ionized elements.  

Figure 1: Relative internal standard intensities measured in variable NaCl matrix solutions.			

To demonstrate the effectiveness of the aerosol dilution method to extend the matrix tolerance of ICP-MS, spike recoveries for a range of elements were measured in the series of different NaCl matrices. The spike recovery data are shown in Figure 2 for the "big four" toxic trace elements and Figure 3 for four elements that suffer severe overlap from NaCl-based polyatomic interferences. Some of these polyatomic interferences are well documented in ICP-MS such as ArCl

 63 (Figure 3) are not as well known because they only become a significant issue when very high levels of NaCl matrix are measured. However, the consistent spike recoveries demonstrate that He mode is effective at reducing all of these matrix-based polyatomic interferences.  

Figure 2: Spike recoveries for As, Cd, Hg, and Pb in variable NaCl matrices with up to 25% NaCl. The spike level added is shown as the first data point.			

Many of the elements measured (As, Cd, Hg) are relatively poorly ionized and would be expected to suffer severe signal loss from ionization suppression in a matrix containing such high levels of an easily ionized element. The consistent spike recoveries achieved in all levels of NaCl matrix demonstrate that aerosol dilution is effective in minimizing the impact of ionization suppression in variable samples containing salt concentrations up to 25% NaCl, a level that would not previously have been considered suitable for analysis by ICP-MS.  

Figure 3: Spike recoveries for interfered elements V, Cr, Ni, and Cu in variable NaCl matrices with up to 25% NaCl. The spike level added is shown as the first data point.			

The NaCl matrix was measured repeatedly to check the long term matrix tolerance of the method. Solutions containing 25% NaCl were prepared with and without a multielement QC spike (50 μg/L for most elements except As: 25 μg/L). The spiked and unspiked samples were measured alternately for a period of more than 4 h. Spike concentrations were calibrated against the nonmatrix matched (no NaCl) calibration. The results are presented in Table II, demonstrating accurate recovery within ±10% of the spike level, and precision well below 5% relative standard deviation (RSD) for most elements measured.  

Table II: Spike recovery and long term stability (4 h) for trace elements spiked into 25% NaCl matrix (n = 25)			

An ICP-MS system with aerosol dilution was used for the direct analysis of trace elements in salt matrix samples up to 25% NaCl, without prior sample dilution. 

Accurate recoveries and excellent long term stability were obtained for microgram-per-liter level spikes for all analytes. The elements of interest included several that suffer from severe polyatomic overlaps in the NaCl matrix, so the consistent recoveries demonstrate that polyatomic interferences were reduced effectively using He mode in the collision–reaction cell. 

is an ICP-MS product manager at Agilent Technologies LDA (UK) Ltd., in Cheshire, UK. 

is a development analyst with Eurofins Analytico BV in Barneveld, Netherlands. Direct correspondence to: 

(1) D. Beauchemin, J. W. McLaren, and S. S. Berman, 

(2) I. Rodushkin, T. Ruth, and D. Klockare, 

(3) N.M. Reed, R.O. Cairns, and R.C. Hutton, 

(4) P.F. Kehr, J.S. Jones, D.A. Fritz, D.E. Harrington, and W.R. Bramstedt, 

(5) W. Li, F. Pan, S. You, Q. He, D. Kang, and Y. Xu,

(6) International Organization for Standardization (ISO) 17294-2:2003 

Water quality — Application of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) - Part 2: Determination of 62 Elements 

(7) United States Environmental Protection Agency, 

Method 6020A Inductively Coupled Plasma-Mass Spectrometry.

http://www.epa.gov/wastes/hazard/testmethods/sw846

 (United States Environmental Protection Agency, 2007). 

(8) German Institute for Standardization, 

DIN EN 13805:2002 Foodstuffs — Determination of Trace Elements - Pressure Digestion (

German Institute for Standardization, Berlin, Germany). 

(9) L. Ebdon, A. Fisher, H. Handley, and P. Jones, 

(10) R.E. Sturgeon, S.S. Berman, A. Desaulniers, and D.S. Russell, 

(11) F.A.M Silva, C.L.P. Da Silveira, N. Miekekey, and I.L. Kuechler, 

(12) G.M. Greenway, S.M. Nelms, and D. Koller, 

(13) T.-t. Shih, W.-y. Chen, and Y.-c. Sun, 

A novel approach to handling samples with very high and variable levels of sample matrix is to use aerosol dilution, whereby the amount of sample aerosol that reaches the plasma is reduced by dilution with an additional argon gas stream.

Improving ICP-MS Analysis of Samples Containing High Levels of Total Dissolved Solids

This article compares several performance criteria such as full width at half maximum, background equivalent concentrations, limits of detection, purge requirements, and results for some substances for the "traditional" As lines at 188.979, 193.696, and 197.197 nm for the "new" line at 228.812 nm.

The elements arsenic (As) and cadmium (Cd) are regarded as two of the most toxic elements to living organisms. Consequently, these elements must be monitored to make sure they do not become harmful to the environment. For example, tolerable or permissible levels in drinking water are 10 μg/L for As and 3 μg/L (by the European Union [EU] and World Health Organization [WHO]) (1,2) or 5 μg/L (by the United States] (3) for Cd. In foods they range from 0.05 to 0.2 mg/kg for Cd (4). Daily intakes by pharmaceutical products must not exceed 1.5 μg of inorganic As or 25 μg of Cd (5). 

Since very low concentrations are involved, the determination of As has been a challenge for atomic spectrometry techniques. While atomic absorption (AA), with its subtechniques graphite furnace and hydride generation, is generally considered the best choice, both subtechniques have long analysis times, either during the measurement cycle itself, as is the case with graphite furnace AA, or tedious sample preparation, which is necessary for the hydride technique. 

During the last few decades, inductively coupled plasma–optical emission spectrometry (ICP-OES) has taken the major load in the modern analytical laboratory, partly because of its speed advantage. Also, with its extremely high excitation temperatures, a lot of interferences known in AA vanished. At the same time, because of the high temperatures in the plasma, ICP spectra are more line rich and prone to spectral interferences. These have been a major concern with respect to accuracy since the introduction of ICP-OES. As a consequence, improving the spectral resolution has been a driving force in the development of optics for ICP instruments.  

When the first commercial ICP spectrometers were introduced in the 1970s, the typical resolution of the instruments was of the order of 50 pm, at that time still quoted as 0.05 nm. Although these numbers have significance to experienced users, the information on the separating power of the optics was often conveyed using spectra to show the degree of overlap of two adjacent lines. The line pair of Cd at 228.802 nm and As at 228.812 nm was commonly chosen to demonstrate the practical spectral resolution. The degree of overlap and, thus, the freedom from erroneous results were shown for Cd as an analyte and As as an interferent.  

This case has a practical background because Cd only has three lines that are sensitive enough for trace determinations, two of which are interfered by the common matrix element Fe, and the third line shows some overlap by As, as displayed in Figure 1. Thus, As has always been regarded as the problem element in this situation.  

Figure 1: Examples of the interferences of the three most sensitive Cd lines in a contaminated soil sample. The blue line shows a spectrum of the sample that includes matrix elements, and the red line displays a single-element spectrum of Cd. The spectra were taken with a spectrometer of medium resolution.			

Typically, three lines below 200 nm (at 188, 193, and 197 nm) are used for the determination of As by ICP-OES. These are affected by absorption because of oxygen, which is seriously aggravated in the wavelength region below 190 nm, where the absorption coefficient of oxygen increases dramatically. Dealing with this absorption adds to the complexity of the analysis. Typically, a prepurge time is needed to get rid of oxygen in the transfer and dispersing optics. Neglecting this prepurge step will give rise to drift in the analytical results (6). 

Thus, the analytical problem in ICP-OES for these elements is twofold: an interference-free determination of Cd and a straightforward determination of As.  

The instrument used was a PlasmaQuant 9000 (Analytik Jena AG). The spectrometer consists of a prism monochromator followed by an echelle grating monochromator. The latter uses a grating ruled at 75 lines/mm and blazed at approximately 76°. The reciprocal linear dispersion is 0.055 nm/mm at 190 nm in the 135th order. In the prism monochromator, a variable slit and a 60° quartz prism are used to selectively transmit only the grating order of interest for each measurement wavelength. Thus, an unambiguous, highly dispersed spectrum is obtained in every case. The radiation in the region of each spectral line used in an analysis is detected by means of a backside illuminated charge-coupled device (CCD) (Hamamatsu Photonics KK). The effective pixel dimensions are 18 μm × 2016 μm. Typically, 200 pixels are used for the measurement, corresponding to wavelength ranges between circa 0.2–1.0 nm.  

Table I: Sensitivities and background equivalent concentrations for prominent lines of As, arranged in ascending order for axially viewed BECs			

The plasma orientation is vertical with the option of both viewing directions: radial and axial. Since traces need to be determined, the choice for plasma viewing was set to axial because with this viewing direction, lower concentrations can be measured. Standard operating conditions (RF power: 1200 W, nebulizer gas flow 0.6 L/min, plasma gas flow: 12 L/min, auxiliary gas flow: 0.5 L/min) were used for all measurements. The instrument was equipped with a concentric nebulizer and cyclonic nebulizer chamber (Conikal nebulizer and Tracey nebulizer chamber, Glass Expansion). The pump rate was set to 1 mL/min. Three replicates were read for standards and samples and 11 replicates for detection limit measurements. The measurement time was 10 s. All peaks were processed in three pixel area modes. Automatic background correction was employed using a linear fit.  

Table II: Sensitivities and background equivalent concentrations for prominent lines of Cd, arranged in ascending order for axially viewed BECs			

Standard and blank solutions were acidified to a concentration of 2% nitric acid (30 mL nitric acid, 65%, suprapur, Merck, in 1 L of 18-MΩ water). Stock solutions (Merck) containing 1 g/L were diluted accordingly with micropipettes.  

Table III: Limits of detection (3 s) for selected As and Cd lines			

Solid materials were digested with microwave-oven assistance (Topwave, Analytik Jena AG). Approximately 0.2–0.5 g of sample was treated with 2 mL of nitric acid (65%, suprapur, Merck) and 1 mL of hydrochloric acid (35%, suprapur, Fluka-Sigma-Aldrich) using a five-step heating program with a maximum temperature of 200 °C. The solutions were filled to 25 mL directly into 50-mL autosampler containers (Greiner GmbH) to avoid contamination.  

Figure 2: Calibration plot for As at 228 nm. 			

For the goal of trace analysis, low limits of detection (LODs) are necessary. Good signal-to-background ratios are a prerequisite for low LODs. In ICP-OES, an inverse of the signal-to-background ratio is normalized to concentration units; the background equivalent concentration (BEC) is used to quantify this analytical figure of merit. If the background is very low, then using the BEC for optimization may not generate the best results. In this case, the sensitivity may be another important figure of merit to take into consideration. For evaluation, the sensitivity and BECs for the most sensitive lines were measured. The data are compiled in Tables I and II. The least sensitive lines were then excluded while the more sensitive lines were used to measure the 3σ detection limits using the blank method. The results are shown in Table III. It should be noted that there is no great difference between the detection limits for the As lines, the range being within a factor of 2 for the three wavelengths normally selected for As, while the LOD for the As line at 228 nm is in between the other LODs.  

Figure 3: Spectrum of the line pair Cd and As at 228.8 nm of the water sample (blue line, NIST 1643e) and aqua regia digest of sludge sample (black line, BCR144). The cursor is positioned on the As peak, while the Cd peak is to the left of the analyte line. The light green line shows the automatically calculated background used for correction of the water sample.			

For the determination of As and Cd in a selection of samples, the instrument was calibrated with standard solutions. Figure 2 shows an example of a calibration function. The calibration function is linear throughout the anticipated concentration range in the samples. Using these calibration functions, a variety of samples were analyzed. Figure 3 displays the spectrum of a sludge sample, which contains As and Cd. The peaks are clearly resolved, so the results for each element are not influenced by the other. The data are presented in Table IV. Four kinds of pharmaceuticals (aspirin [Bayer], Grippostad [Stada], Ibuflam [Zentiva], and Nisylen [Deutsche Homöopatische Union]) were also analyzed and As or Cd could not be detected in any of the samples, which means that the concentrations are below the permissible values. Although there are small deviations between the measured and the certified values, an excellent agreement is found between the individual results for different wavelengths of the same element.  

Table IV: Results for environmental samples analyzed for As and Cd			

When using an instrument with a spectral resolution of about 3.5 pm (and thus complete resolution of the line pair Cd-As at 228 nm), both lines, As and Cd, can be used at this wavelength, without the need to deal with potential interferences of one on the other. 

An additional benefit of improved resolution is the increased power of detection, which lowers the detection limits for these elements to a range that makes it attractive for use in a number of applications.  

Since the wavelength at 228 nm is not affected by absorption because of ambient oxygen, the analysis is greatly simplified; there are no purge requirements like there are for the other As lines below 200 nm. 

Therefore, it is time for a paradigm change: Instead of continuing to regard As as an interferent at 228 nm, one can consider this line as useful for the quantification of As. Thus, both elements can be successfully determined at the 228 nm wavelength.  

 is with AnalytikSupport in Stein am Rhein, Switzerland. 

 is with Analytik Jena in Überlingen, Germany. Direct correspondence to: 

(1) European Community, Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption. 

Guidelines for Drinking-Water Quality, 4rt Ed.

(3) Environmental Protection Agency (EPA), 

National Primary Drinking Water Regulations,

(4) Europäische Union, Verordnung (EG) Nr. 1881/2006 der Kommission vom 19. Dezember 2006 zur Festsetzung der Höchstgehalte für bestimmte Kontaminanten in Lebensmitteln. 

(5) United States Pharmacopeia General Chapter <232> "Elemental Impurities—Limits" (United States Pharmacopeial Convention, Rockville, Maryland). Available at: 

http://www.usp.org/usp-nf/key-issues/elemental-impurities

Fehlerfrei durch die ICP Emissionsspektrometrie

 (Wiley-VCH, Weinheim, Germany, 2012), p. 156. 

ICP Emission Spectrometry: Arsenic at 228 nm — Interferent or Analyte?

Rare earth elements (REEs) have become indispensable in many electronic, optical, magnetic, and catalytic applications because of their specific properties such as magnetism and phosphorescence, as well as their ability to both donate and accept electrons. Production of high-purity REEs is now required for advanced technology applications. Therefore, it is important to be able to determine impurity levels. In the REE mining process, the extracted REE contains many other REEs and because of the line-rich spectra emitted by these elements in inductively coupled plasma–optical emission spectrometry (ICP-OES), the quality of the final result may be compromised. The quality of the results will also be more important when the application involves NdFeB magnetic materials. Results obtained on analysis of REE impurities at the parts-per-million level in cerium oxide and gadolinium oxide matrices and on NdFeB magnetic materials are shown and discussed here. 

Rare earth elements (REEs) are being used progressively in advanced technologies because of their unique properties. One example where REEs impact our daily lives is smart phones. REEs exhibit not only magnetic properties, but also fluorescent properties. They are widely used for wind-power generators, computer disk drives, hybrid electric vehicles, and fluorescent lamps. REEs include 17 elements, all from the lanthanides group, and also yttrium. They are mainly extracted from the Earth's crust. Extracted ore usually contains 14 REEs combined together. Then there is a need to refine the ore to produce high-purity materials. Recycling is an alternative source for REEs, mainly from fluorescent lamps. In both cases, the refining process consists of several reduction and complexation reactions and, thus, there is a need for quality control of the refined high purity product. The analysis of traces of REEs in an REE matrix is then mandatory to control the quality of the refining process and the final product. 

The NdFeB magnets are widely used for applications involving magnetic properties of REEs. The quality control of final NdFeB products is required to ensure that the expected magnetic properties are obtained. Analysis requires the capability to determine the major elements Nd, Fe, and B, and also traces of multiple elements with a high iron content. 

Several techniques are available for elemental analysis, but inductively coupled plasma–optical emission spectrometry (ICP-OES) seems to be the most adequate. Thanks to its tolerance of the high salt concentration and its capability to determine trace and major elements in a single run because of the numerous wavelengths emitted by REEs or iron all over the spectrum, high-resolution ICP-OES seems to have definitive advantages for such applications. 

This article describes the use of high-resolution ICP-OES for the analysis of refined REE materials, CeO

 were supplied and prepared by a customer who specialized in the REE recycling industry. No detail was given for the sample preparation. 

 was prepared at 500 g/L and diluted to 20 g/L for analysis of La

as impurity and diluted to 100 g/L for the analysis of Ca, Na, and K at low concentrations. 

 was prepared at 100 g/L and diluted to 20 g/L for analysis of Tm

NdFeB magnets were prepared using acid digestion at 10 g/L and analyzed directly with the ICP-OES system. Acid digestion was performed using nitric acid (5 mL HNO

For both high-purity REE and NdFeB magnet analysis, it was decided to use the standard addition technique. External calibration was not used to avoid sample effects and to ensure accuracy for sample analysis. 

Standard addition was chosen because it allowed us to work directly in the high-purity sample; external calibration would have required the use of reagents that may affect accuracy because of potential impurities they may contain. 

For standard addition preparation, Spex CertiPrep single-element standard solutions were used. 

Because REEs, and also iron at high concentrations, emit numerous wavelengths over the spectral range of the ICP-OES, the choice of analytical wavelengths should be conducted with great care. For these applications, the wavelength selection was done using tools that were provided with the ICP-OES instrument and also by taking advantage of the know-how of application laboratories on such applications.  

Table I: Wavelengths used for analysis of CeO2 and Gd2O3 matrices			

The wavelengths used for the analysis of CeO

 are shown in Table I. Table II displays the wavelengths used for NdFeB magnets analysis.  

Table II: Wavelengths used for analysis of NdFeB magnets			

A Horiba Scientific Ultima Expert High Resolution ICP-OES system was used for the analysis. This instrument uses a 1-m focal length optic and is equipped with an optional high-density dual-grating system that uses a 4320-g/mm grating at 160–450 nm with resolution of <5 pm and a 2400-g/mm grating at 450–800 nm with a resolution of <11 pm. This dual-grating system is specially designed for REE-based applications, because most of these elements emit multiple wavelengths and require high resolution in the range of 300–450 nm. 

A vertical radially viewed torch was used with the "Total Plasma View" feature and the original sheath gas device. The sheath gas device allows for the introduction of a high concentration of salts into the plasma, up to 300 g/L NaCl, without sacrificing performance. The "Total Plasma View" feature allows for the measurement of the whole normal analytical zone of the plasma where all atoms and ions emit their wavelengths. 

Plasma parameters were optimized to obtain the best compromise between sensitivity and robustness. For the optimization of the sensitivity, the signal-to-background ratio (SBR) was optimized; the detection limit is inversely proportional to the SBR value (1,2). The robustness was monitored using the magnesium ratio because it is known as an effective method to evaluate robustness for ICP-OES (3,4).  

All parameters and details for the sample introduction system are given in Tables III and IV.  

Evaluation of the Need for High-Resolution ICP-OES

To evaluate the need for high resolution, profiles of samples were acquired using a low-resolution ICP-OES system with resolution at <8 pm on the spectral range. 

Profiles obtained for La at 333.749 nm in the cerium matrix sample, using low-resolution and high-resolution ICP-OES, are shown in Figure 1. The profiles clearly show that low-resolution ICP-OES will not allow for accurate measurement because it is not possible to separate the wavelength of lanthanum, the impurity, from the wavelengths of cerium, the matrix. Using high-resolution ICP-OES solves the issue, and separation of the wavelength of lanthanum from the doublet of cerium is possible. Because an overlap is still observed, particular attention should be paid to accuracy and stability of the measurement for the high-purity REE sample analysis.  

Figure 1: Profiles of La 333.749 nm with low resolution (red) and high resolution (blue).			

Because of this overlap of the lanthanum wavelength with the cerium doublet, profiles were obtained for both the sample and the standard additions to evaluate the capability of the ICP-OES to accurately measure each addition. The obtained profiles (Figure 2) show a good separation for all analyzed solutions and no shift for the central wavelength of the lanthanum. This analysis confirms that the separation of the wavelengths is accurate enough and that intensities measured will not be affected by the overlap. It also confirms the ability of high-resolution ICP-OES to perform measurements of concentration in the expected range.  

Figure 2: Profiles of La 333.749 nm for matrix 1 (green), standard addition 1 (20 ppm, blue), and standard addition 2 (40 ppm, red).			

Profiles obtained for the sample and the standard addition for the analysis of lutetium at 261.542 nm in the gadolinium matrix are given in Figure 3. The obtained profiles clearly show that high resolution is again mandatory to ensure accuracy of the measurements and the final result. Even with a slightly lower resolution, the separation of the two wavelengths will not be effective, and then analysis will not be possible. The profiles obtained for the sample and the standard additions made for the calibration show that measurement is possible in the expected concentration range and that the overlap doesn't affect the shape of the peak and, consequently, the accuracy of the measurement.  

Figure 3: Profiles of Lu 261.542 nm for matrix 2 (red), standard addition 1 (5 ppm, blue), and standard addition 2 (10 ppm, green).			

The capability of high-resolution ICP-OES to accurately determine concentrations in different samples was evaluated. For this purpose, the analysis was performed on unspiked and spiked samples for each matrix. The sample used for the spike came from a different batch than the one analyzed without the spike; this possibly caused some variations in the original content even if both were expected to be quite similar. Results obtained for both spiked and unspiked samples are given in Tables V–VII.  

Table V: Results obtained for the analysis of La2O3 in CeO2 matrix 			

All samples analyzed were in accordance with the specification, as shown by the results obtained on the unspiked sample. The results obtained show that a high sensitivity can be reached; some elements were measured at the sub-part-per-million level, corresponding to a few micrograms per liter of the element in the solution measured. The results obtained proved the capability of high-resolution ICP-OES to determine low concentrations of both REEs and other elements in a complicated matrix, down to a few parts per million in the original sample.  

Table VI: Results obtained for the analysis of Ca, Na, and K in CeO2 matrix			

The spikes were consistent with the high sensitivity required for these samples. Typically, spikes of 1 ppm were done for both the CeO

 matrices, corresponding to concentrations down to 20 μg/L in samples. The results obtained on the spiked samples exhibit excellent accuracy, proving the capability to provide sensitive and accurate analysis for high-purity REEs.  

Table VII: Results obtained for the analysis of Lu2O3, Tm2O3, and Yb2O3 in Gd2O3 matrix			

Accuracy is not sufficient for the analysis of such matrices that can be done on batches of multiple samples, it is also necessary to evaluate the stability of the instrument over time. Thus, analyses of La in CeO

 nitrate were conducted over 2 h and 90 min, respectively. The relative intensity, compared to the average value over the stability test was plotted as a function of time and the relative standard deviation was calculated. Results obtained (Figures 4 and 5) show perfect stability over time for both the CeO

 matrix for all elements. The relative standard deviation (RSD) obtained was <1% for all elements in both matrices, which is similar to what can be expected in water with extreme values not varying more than 2% from the average value. When looking at Figures 4 and 5, absolutely no drift can be identified, proving the stability of the instrument and its capability to handle many samples with perfect accuracy.  

Figure 4: Stability test on La 333.749 nm in CeO2 20g/L.			

Rare earth elements (REEs) have become indispensable in many electronic, optical, magnetic, and catalytic applications because of their specific properties such as magnetism and phosphorescence, as well as their ability to both donate and accept electrons.

High-Resolution ICP-OES for the Determination of Trace Elements in a Rare Earth Element Matrix and in NdFeB Magnetic Materials

Issue PDF

Click the title above to open the Spectroscopy November 2014 ICP & ICP-MS Supplement, Vol 29 No s11, in an interactive PDF format.

Special Issues-11-01-2014

Simplifying Mixed-Food Microwave Sample Preparation for ICP-MS Analysis

What New Methods to Measure Isotope Ratios Can Reveal About the Environment, Humans, and the Earth

Improving ICP-MS Analysis of Samples Containing High Levels of Total Dissolved Solids

ICP Emission Spectrometry: Arsenic at 228 nm — Interferent or Analyte?

High-Resolution ICP-OES for the Determination of Trace Elements in a Rare Earth Element Matrix and in NdFeB Magnetic Materials

Vol 29 No s11 Spectroscopy November 2014 ICP & ICP-MS Supplement PDF