Emerging Technology Trends in Atomic Spectroscopy

42 Spectroscopy 29(3)

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Emerging Technology Trends in Atomic Spectroscopy Are Solving Real-World Application Problems This article looks at recent developments in inductively coupled plasma–mass spectrometry (ICP–MS), microwave-induced plasma–optical emission spectrometry (MIP-OES), X-ray spectroscopy, and laser-induced breakdown spectroscopy (LIBS) by exemplifying the diverse range of sample types they are analyzing and the unique application problems they are being asked to solve. Robert Thomas

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tomic spectroscopic techniques are commonly used to carry out elemental determinations in a wide range of sample matrices from sub-parts-per-trillion concentrations up to high percentage levels. Inductively coupled plasma–mass spectrometry (ICP-MS) is universally recognized as the most sensitive ultratrace multielement technique with detection limits in the low parts-per-trillion range for the majority of elements, whereas X-ray fluorescence (XRF) spectrometry has traditionally been the technique of choice for carrying out high precision analysis of samples with high parts-per-million or low percentage concentrations. However, there are many application areas that are less demanding and do not require such stringent analytical requirements as these two techniques. For example, inductively coupled plasma– optical emission spectrometry (ICP-OES) has similar sample throughput characteristics as ICP-MS, but depending on the analyte, its detection limits are approximately three orders of magnitude higher (worse). More recently, a microwave-induced plasma (MIP)-OES system has become commercially available that offers the throughput of ICP-OES, but is more aligned to flame atomic absorption (FAA) in its detection capability. Additionally, laser-induced breakdown spectroscopy (LIBS), a relatively new commercial technique, is showing a great deal of promise for the analysis of materials with challenging sampling requirements. However, until recently LIBS was struggling to demonstrate that it could differentiate itself from the other more mature solid-sampling techniques like XRF or arc/spark emission. The strength and weakness of any analytical technique are based on its ability to successfully address real-world application segments. With that in mind, let’s take a more detailed

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look at some of these techniques by exemplifying the diverse range of sample types they are analyzing, with particular emphasis on the unique application problems they are being asked to solve.

Inductively Coupled Plasma–Mass Spectrometry Even 31 years after the commercialization of ICP-MS, the flexibility of the technique is still one of its greatest attributes. Not only is it being used to carry out routine, highthroughput multielement analysis, but it is also ideally suited for more demanding applications such as ultratrace element speciation studies coupled with high performance liquid chromatography (HPLC). Every year, as more and more laboratories invest in the technique, the list of applications is becoming larger, extremely diverse, and, in some cases, more challenging. There have been many creative ways to address the difficult nature of complex sample matrices including the use of high-resolution, double focusing magnetic sector technology, automated sample introduction, and pretreatment techniques to preconcentrate the analytes of interest and remove the matrix before it enters the mass spectrometer. Recent developments have also seen the use of aerosol dilution sampling accessories to minimize the impact of the matrix on the interface cones and to reduce the solvent loading on the plasma to analyze samples with higher dissolved solids. However, in my opinion, the most significant breakthrough in quadrupole-based ICP-MS has been the use of collision– reaction cells and interfaces to minimize, and, in some cases, totally remove problematic polyatomic spectral interferences created by the combination of ions from the matrix or solvent with ions from the argon plasma. The fact that a user now has

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the choice of either using a collision/reaction interface, a multipole-based collision cell with kinetic energy discrimination (KED), a low-mass cut-off design, or a true reaction cell with bandpass filtering, makes the technology a very powerful method development tool. This approach has been further refined by the commercialization of the triplequadrupole ICP-MS system, in which one quadrupole is used as a mass filter or ion guide before the collision cell (actually an octapole and not a quadrupole) and a second analyzer quadrupole is placed after the collision cell, tuned to the analytes of interest. These advancements have opened up the technique to a multitude of new application areas for quadrupole-based ICP-MS, and, in many cases, is a viable alternative to magnetic-sector technology for spectrally complex samples. Some of these application areas include the direct analysis of seawater samples (1), the determination of trace metals in power plant flue gas desulfurization waters (FGDW) (2), the measurement of ultratrace levels of metals in human biological samples (3), and the analysis of pharmaceutical and nutraceutical materials according to the new United States Pharmacopeia (USP) Chapters 232, 233, and 2232 (4). However, there is one application area that is proving to be particularly challenging, not because it’s difficult to introduce the sample into the ICP-MS system or handle problematic interferences, but because it involves the optimization of the measurement protocol to enable the measurement of metallic nanoparticles at environmentally significant levels. The identification and characterization of extremely small nanoparticles in soils and groundwaters using ICP-MS is a rapidly emerging application that is in the process of being fine-tuned and refined by early researchers and is expected to be ready for routine use in the near future. Let’s take a closer look at this challenging application.

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lic domain (5,6). However, the unique properties of engineered nanomaterials have also created intense interest about the environmental behavior of these materials. Because of the increased use of nanotechnology-based products, nanoparticles are more likely to enter the environment. Different engineered nanoparticles will have different properties and, therefore, will behave very differently when they enter the environment. So, to ensure the continued development of nanotechnology prod-

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ucts, there is clearly a need to evaluate the risks posed by these engineered nanoparticles, which will require proper tools to carry out exposure assessment studies to better understand how they interact with soil, sediment, and water systems. Current analytical approaches to assess the impact of nanoparticles on the environment include a combination of computer modeling to predict life cycles and direct analytical measurement techniques. Prediction of environmental

Characterization of Engineered Nanoparticles The use and benefits of engineered nanomaterials will not be discussed in this article because there is a great deal written about them in the pub-

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Figure 1: Principles of characterizing nanoparticles using single-particle ICP-MS analysis. (Figure courtesy of Colorado School of Mines.)

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Figure 2: Time-resolved analysis of 30-nm and 60-nm gold particles by single-particle ICP-MS. Adapted from reference 9.

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Time (s) Figure 3: Time-resolved analysis of a gold nanoparticle using single-particle ICP-MS measurement protocol showing the pulse is fully characterized in less than 1 ms. Adapted from reference 9.

concentrations of engineered nanoparticles through modeling is based on

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knowledge of how they are emitted into the environment, together with their

eventual fate and behavior, which requires validation through the measurement of actual environmental concentrations. For engineered nanoparticles that have only recently been introduced into the environment, extremely sensitive methods are required. Although the direct measurement approach is not hampered by the underlying assumptions of exposure modeling, it is very important to ensure that direct observations are representative in time and space for the regional setting in which the observation was made. Many analytical techniques are available for nanometrology, only some of which can be successfully applied to environmental health and safety of nanotechnology (nano-EHS) studies. Traditional methods for assessing particle concentration and particle size distributions include electron microscopy, chromatography, field-flow fractionation (FFF), centrifugation, laser light scattering, ultrafiltration, and UV spectroscopy. Difficulties generally arise because of a lack of sensitivity for characterizing and quantifying particles at environmentally relevant concentrations (low micrograms per liter). Furthermore, the lack of specificity of these techniques is problematic for complex environmental matrices that may contain natural nanoparticles with polydisperse particle distributions as well as heterogeneous compositions. The Role of ICP-MS Because of its multielement capability and extremely low detection limits, ICP-MS is ideally suited to the characterization of engineered nanoparticles that contain elements such as Ag, Au, Ti, and Fe, which have been integrated into larger products such as consumer goods, foods, pesticides, pharmaceuticals, and personal care products. The ubiquitous use of goods containing these nanomaterials will inevitably lead to environmental releases, which may be studied and quantified using state-of-the-art ICP-MS technology. Current areas of research include the coupling of ICP-MS with FFF, producing a powerful tool for sizing and separating engineered nanoparticles with extremely high sensitivity and selectivity.

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Recently, a great deal has been published about the benefits of this hyphenated multielement technique, so it will not be discussed in this article (7). Another very exciting area of research is single-particle ICP-MS, which is a novel technique for detecting and sizing metallic nanoparticles at environmentally relevant concentrations (8). Although this method is still in the development stage, it has shown a great deal of promise in several applications, including determining concentrations of silver nanoparticles in complex matrices such as wastewater effluent. The method involves introducing nanoparticle-containing samples, at very dilute concentration, into an ICP-MS system and collecting time-resolved data. Because of the dilution factor, very high sensitivity and short integration times are necessary to ensure the detection of individual particles as pulses of ions after they are ionized by the plasma. The observed pulse number is related to the nanoparticle concentration by the nebulization efficiency and the total number of nanoparticles in the sample, whereas the mass and, thus, the size of the nanoparticle is related to the pulse intensity. The principles of characterizing nanoparticles using single-particle ICP-MS are shown in Figure 1. In this example, nano-Ag imbedded in athletic socks, which is used as a bactericide, has been shown to release during simulated wash cycles (step 1). By collecting and analyzing a simple aqueous solution with the ICP-MS system (step 2), and collecting data using the single-particle ICP-MS technique (step 3) the size, concentration, and associated dissolved material can be quantified — all of which are important parameters in environmental and biological modeling. After the raw data is collected, the dissolved content is at low signal intensity, with nanoparticles creating pulses above this background where the height of the pulse relates to the mass of analyte and the number of pulses correlates to the concentration of nanoparticles in the samples. The size distribution of particles in the sample can be calculated using well-understood single-particle ICP-MS theory (step 4). A histogram of nanoparticle

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diameter versus the number of events (nanoparticle number) can then be created to visualize the nanoparticle distribution in the sample, in addition to calculating the concentration of both the nanoparticles and dissolved fractions of the nanoparticle released from the products (step 5). The theory of size distribution will not be discussed here, but with this technique, scientists can have a better understanding of how nanomaterials will behave in the environment at realistic concentrations. Optimized Measurement Protocol However, for this approach to work effectively at low concentrations, the speed of data acquisition and the response time of the ICP-MS detector must be fast enough to capture the time-resolved nanoparticle pulses, which typically last less than 1 ms. If the electronics are not fast enough, two or three pulses can easily pass through and be erroneously detected as a single pulse. Figure 2 shows a real-world example of the time-resolved ICP-MS analysis of a mixture of 30-nm and 60-nm gold particles together with 5 ppb gold in solution (9). Each peak represents the instrumental response for each integration point. If one of these time-resolved peaks is examined closely, as shown in Figure 3, it can be seen that the gold nanoparticle has been generated in