FFF-ICP

By coupling flow field-flow fractionation (flow FFF) to ICP-MS it is possible to investigate trace metals bound to various size fractions of colloidal and particulate materials.55 This technique is employed for environmental applications,55-57 for example to study trace metals associated with sediments. FFF-ICP-MS is an ideal technique for obtaining information on particle size distribution and depth profiles in sediment cores in addition to the metal concentrations (e.g., of Cu, Fe, Mn, Pb, Sr, Ti and Zn with core depths ranging from 0-40 cm).55 Contaminated river sediments at various depths have been investigated by a combination of selective extraction and FFF-ICP-MS as described by Siripinyanond et al,55... 

Sedimentation FFF implies application of the centrifugal field, which is produced by placing the channel in a centrifuge basket. SdFFF instruments can be linked readily to analytical instruments to provide analysis in real time. For the first time, Beckett (1991) introduced FFF-ICP-mass spectroscopy (MS) as a powerful analytical tool for characterizing macromolecules and particles. Taylor et al. (1992) illustrated the characterization of some inorganic colloidal particles and river-borne suspended particulate matter of size range <1 pm using SdFFF and ICP-MS. 

FFF-ICP-MS The ICP-MS is a multielement analysis tool ideally suited for direct coupling with FFF. The ICP torch is capable of vaporizing and ionizing particles in the eluent up to 10 pm, and the plasma is then fed into an MS for simultaneous detection of many elements. Quadripole, mass-sector, and time of flight MSs are now available, depending on the sensitivity, mass resolution, and response time required. FFF-ICP-MS yields element-based size distributions. Other element detection systems that have been used include ICP-AESs, electrothermal atomic absorption spectrometers, and very recently laser-induced breakdown spectrometers. 

Let us now turn our attention to another technique that is being used to characterize nanoparticles—field flow fractionation (FFF) coupled with ICP-MS. ITF has been used for a number years coupled with UV/Vis detection, but is now being used with ICP-MS for elemental specificity. The FFF-ICP-MS technique is relatively mature compared to sp-ICP-MS, and as a result, is probably being used in more-routine type environments. 

One of the inherent limitations with quantitative applications of FFF-ICP-MS can be low recoveries, which are attributed to several factors. Probably the most significant problem area is the physical interaction of the analyte with the membrane by an adsorption mechanism, resulting in the particles sticking to the membrane and not being eluted. In addition, losses through the accumulation wall based on membrane cut-off values have been reported for samples containing dissolved and macromolecular components. Analyte loss can also occur in the ICP-MS nebulizer, spray chamber, and sample tubing, but these losses are relatively small compared to membrane interactions. 

Amaeasiriwaedena D., Siripinya-nond a. and Barnes R. M. (2001) Trace elemental distribution in soil and compost-derived humic add molecular fractions and colloidal organic matter in munidpal waste-water by flow field-flow fractionation-inductively coupled plasma mass spectrometry (flow FFF-ICP-MS),/. Anal. At. Spectrom. 16 978-986. 

Field flow fractionation (FFF), as a gentle size fractionation coupled to ICP-MS, offers the capability to determine trace metals bound to various size fractions of colloidial and particulate materials.112 On line coupling of FFF with ICP-MS was first proposed by Beckett in 1991 -113 Separation is achieved by the balance between the field force and macromolecular diffusion in the FFF channel. Depending on the field force used, FFF is classified into different techniques such as sedimentation, gravitational, electrical, thermal and flow FFF.112... 

In principle, all powerful element-specific methods that are able to monitor continuously the effluents of separation processes commonly in the range of a few mimin-1 and in element concentrations of some Klpg liter-1. A well-suited method is based on modern element-specific quadrupole mass spectrometry (MS) with an inductively coupled plasma (ICP) interface to the separation unit [e.g., liquid chromatography (LC) or field-flow fractionation (FFF)].Tlie ICP-MS detection can also be used for continuously characterizing the effluent of any kind of packed column (Metreveli and Frimmel, 2007). By this, the transport and elution properties of... 

Separation by flow field-flow fractionation (FFFF) allows the determination of size distribution of molecules and colloids in the submicrometer-diameter range and was first shown by Beckett et al. (1987). Since then, the method has been often used for studies on the mobility of DOM-related nanoparticles. Coupling the FFF with ICP-MS (Exner et al., 2000) leads to useful information on metal speciation. 

Beckett described inductively coupled plasma mass spectrometry (ICP-MS) as an off-line detector for FFF which could be applied to collected fractions [ 149]. This detector is so sensitive that even trace elements can be detected making it very useful for the analysis of environmental samples where the particle size distribution can be determined together with the amount of different ele-ments/pollutants, etc. in the various fractions. In case of copolymers, ICP-MS detection coupled to Th-FFF was suggested to yield the ratio of the different monomers as a function of the molar mass. In several works, the ICP-MS detector was coupled on-line to FFF [150,151]. This on-line coupling proved very useful for detecting changes in the chemical composition of mixtures, in the described case of the clay minerals kaolinite and illite as natural suspended colloidal matter. 

The second major environmental application of FFF has been the use of an element-specific detector, usually in series with a UV detector, to provide elemental composition data along with the PSD. Graphite-furnace atomic absorption spectrometry has been used off-line on fractions collected from the FFF run. However, the multi-element detection, low detection limits and capability to function as an online detector have made inductively coupled plasma mass spectrometry (ICP-MS) the ideal detector for FFE85-86 The sample introduction system of the ICP-MS is able to efficiently transport micron-sized particles into the high-temperature plasma,... 

Fl-FFF has also been coupled to ICP-MS for the determination of element size distributions of 28 elements, including C, in natural waters.91 Hassellov et al.91 further developed methodology for on-channel preconcentration that enables up to 50 ml of sample to be introduced onto the channel. This significantly enhances the effective detection limits of the technique, which can otherwise be problematic due to the low concentration of trace elements in natural waters, the dilution inherent in FFF analysis, and the small injection volume, typically 10 to 50 pi. 

FIGURE 11.6 UV and ICP-MS-based Sd-FFF fractogram of a colloidal (0.2 to 0.8 Urn) fraction detector responses versus time (a) particle size distribution (UV response) and element-based size distribution for Mn and U (b) and element ratios for Mn/Si and U/Si (c). 

Much more sensitive and less time-consuming techniques such as mass spectrometry, atomic emission, and atomic absorption are needed for the analysis of pollutants. Detectors such as graphite furnace-atomic absorption spectrometer (GF-AAS), inductively coupled plasma-mass spectrometer (ICP-MS), or inductively coupled plasma-atomic emission spectrometer (ICP-AES) seem to be ideal candidates for the analysis of trace metals because of their very low detection limits. The high temperatures used avoid the need for tedious digestions in many samples. FFF-gas chromatography-mass spectrometry could perhaps be used in the analysis of particular organic molecules. 

Contado et al. [4] coupled sedimentation FFF indirectly to GF-AAS as well as directly to ICP-MS to produce element composition data across the size distribution. The high levels of Cu, Pb, Cr, and Cd found were associated with colloidal particles taken from a river situated in a highly industrialized site. The two methods give comparable results, with on-line coupling of ICP-MS having a higher resolution, but ICP-AES yields data for some elements (such as potassium and calcium) where ICP-MS produces interferences. 

The particle concentration of the eluent is normally measured by means of infrared or ultraviolet photometers. Additionally, fluorescence photometer, interferometric measurements (for the refractive index), or mass-spectroscopic methods (e.g. induced coupled plasma mass spectroscopy—ICP-MS, Plathe et al. 2010) are employed. The combination of different detection systems offers an opportunity for a detailed characterisation of multi-component particle systems. Note that the classification by FFF is not ideal and the relevant material properties are not always known moreover, the calibration of FFF is rather difficult. The attribution of particle size to residence time, thus, bears some degree of uncertainty. Recent developments of FFF instrumentation, therefore, include a particle-sizing technique additional to the flow channel and the quantity measurement (usually static and dynamic light scattering, Wyatt 1998 Cho and Hackley 2010). 

Chen, B., F. Shanks, and R. Beckett. 1999. Determination of phosphorous distributions in environmental colloids using SdFFF-ICP-HR-MS. Abstract, FFF 1999, Paris, France. Sept. 5-8. 

FIGURE 20.12 A typical instrumental set-up for coupling an FFF system to an ICP mass spectrometer. The nondestructive detectors can include light-scattering, UVA is, RI, or FI techniques. 

ICP-MS is relatively straightforward to couple to FFF because the sample flow rate of the ICP-MS sample introduction system is similar to the outlet flow rate of the FFF system ( 0.5-2.0 mL/min). However, some challenges stiU have to be overcome to quantify metal concentrations in fractionated samples, because some nanoparticles tend to stick to the internal membrane of the FFF system. Figure 20.12 shows a typical instrumental set-up for coupling an FFF systan to an ICP mass spectrometer. 

FIGURE 20.13 FFF fractogram of the separation of silver and gold nanoparticles, nsing both UV absorbance (inset graph) and ICP-MS detection. 

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