Analytical Techniques for Trace Element Determination

2.1 Atomic absorption spectrometry

FAAS is one of the most conventional techniques for the determination of trace metal ions because of the relative simplicity and inexpensiveness of equipment. In this technique, a sample is introduced into a flame where it is dissociated into constituent atoms. Electromagnetic radiation in the UV/Vis part of the spectrum is directed through the flame and partially absorbed by the atoms. Methodology for most elements is well known and allows to use the FAAS technique to determine trace elements directly in various samples’ materials [10, 11, 12]. However, in many cases the available analytical instrumentation does not have enough sensitivity for the analysis of natural samples and suffers from the matrix interferences. Several procedures have been developed for preconcentration and separation of traces of metals required prior to instrumental determination to lower the detection limits, to improve the precision and accuracy of analytical results, and to bring the analyte concentration into the dynamic range of the detector. The preconcentration methods like solvent extraction, ion exchange, adsorption, and coprecipitation were frequently used for trace analysis of lead, cadmium, copper, cobalt, chromium, nickel, tin, gold, palladium, iron, and zinc in different research materials [13, 14, 15, 16, 17, 18, 19, 20]. The coprecipitation method is useful for the preconcentration of trace metal ions and is one of the most useful ways for the preconcentration as well as separation of trace elements from the sample matrix. For the determination of Cr(III), Cu(II), Fe(III), Pb (II), Pd(II), and Zn(II) in food samples, the use of Ni(II)-α-benzoin oxime as a coprecipitation agent can be successfully applied without too much prolongation of the procedure [21]. Liquid–liquid extraction transferring analyte from the aqueous sample to a water immiscible solvent is widely used for samples’ preparation. Cloud point extraction (CPE), similar to liquid–liquid extraction, transferring analyte from the aqueous sample to a water immiscible solvent, is widely used for samples’ preparation and coupled with AAS technique. CPE is based on the property of surfactants to form micelles, which under certain conditions (temperature and concentration) separate into two phases: a surfactant-rich phase of a small volume and a large aqueous phase. Hydrophobic complexes of metallic elements present in such media are trapped in the hydrophobic micellar core and extracted in the surfactant-rich phase, which is directed to AAS detector. The small volume of the surfactant-rich phase obtained after the CPE methodology seems to be ideal for coupling with electrothermal AAS, even though there are applications of CPE coupled with FAAS [16].

Nevertheless, the above-mentioned methods are time consuming and require at least some chemical additives and complex equipment. Miniaturization of liquid extraction methods can be achieved by a drastic reduction of the extractant phase volume by single-drop microextraction, hollow fiber liquid-phase microextration, and dispersive liquid–liquid microextraction (DLLM) allowing for separation and preconcentration of organic and inorganic contaminants in a single step. Due to the need of a sample volume between 2 and 4 mL for FAAS analysis, a microsample injection system (MIS) in case of small volumes obtained after preconcentrations methods might be applied [17, 22]. Online separation and preconcentration based on the adsorption of an analyte on an appropriate material representing the solid phase (SP) and then followed by an elution step directing the solution to the FAAS detector allows the use of wide range of sorbents, chelating agents, and eluents; it is also easily implemented and controlled. The analytes can be retained in their complexed or ionic forms on sorbents or functionalized with specific ligands. The online separation or preconcentration system requires stable material packed in minicolumns placed immediately after the injection valve or its sampling loop. Due to the high enrichment factor, high recovery, low cost, low consumption of organic solvents, and the ability to combine with different detection techniques, an SP extraction (SPE) has been commonly coupled with FAAS. The analytical parameters such as selectivity, affinity, and capacity depend on the sorbent chosen for SPE. Different materials can be used as sorbents for online preconcentration: modified silica gel, modified chitosan resins, chelating resins, magnetic nanoparticles, carbon materials, cellulose, and egg shell membrane.

Due to the high surface to volume ratio, easy derivatization procedures and unique thermal and mechanical stability carbon nanotubes (CNTs) have attracted a lot of attention. CNTs have diameters from fractions to tens of nanometers and lengths not bigger than several micrometers. The surface areas range from 150 to 1,500 m²/g, which is a basis for serving as good sorbents. To improve selectivity, CNTs can be functionalized with different organic molecules. However, CNTs need to be modified with a specific ligand to improve the performance of sorbents by increasing the sorbent capacity and selectivity [15]. The chelating resins are superior in selectivity to solvent extraction and ion exchange due to their triple function of ion exchange, chelate formation, and physical adsorption. The functional group atoms capable of forming chelate rings usually include oxygen, nitrogen, and sulfur. The properties of selectivity and sorption of these resins can be affected by different factors: chemical activity of the complexing group, the nature and kind of the metal ions, the pH of the solution, ionic strength, or polymetric matrix [23].

Among other carbon materials, carbon dots (CDs) turned out to be a selective and sensitive method to separate and determine Cr in various samples. Due to the unique physical and chemical properties, CDs functionalized can facilitate the adsorption of analytes in consequence of electrostatic interaction, anion exchange, chelate interaction, or physical structure and can be employed in SPE as separation and preconcentration material in online or offline modes. Novel water-soluble CDs capped with branched polyethyleneimine polymer with dispersed particle extraction coupled with slurry sampling technique and followed by FAAS detection were employed on Cr(VI). CDs modified with cationic surfactant promoting small droplets generation during the aspiration and nebulization processes acted as a selective sorbent for separation and preconcentration of Cr(VI) enhancing the sensitivity of its determination [19].

An interesting solution for FAAS technique for elemental analysis is modification of instrumentation and the use of thermospray flame-furnace (TS-FF) AAS improving the efficiency of sample introduction. In this case of TS-FF consisting of a nickel tube, a sample solution is nebulized via a ceramic capillary to a standard burner head of an FAAS instrument, onto which the tube is placed [24]. Comparing to a standard FAAS systems, the TS-FF introduces a complete sample to the atomizer and provides a much longer residence time of the sample in the flame. As a result, the sensitivity of measurements may increase by an order of magnitude [25].

For the determination of elements forming hydrides or volatiles species (As, Bi, Ge, Pb, Se, Sb, Sn, Te, Hg, Cd, Co, Cu, Ag, Au), a chemical vapor generation (CVG) system is applied for various samples. The direct transfer of the volatile compounds to any atomizer eliminates the need of other steps but atomization, thus improving the sensitivity. In contrast to ETAAS, the FAAS technique is compatible with the online generation systems of volatile species. Analytes are easily preconcentrated before atomization by trapping directly on the flame atomizer. Even though there are solutions of coupling CVG for both techniques: FAAS and ETAAS [26].

ETAAS differs from FAAS by the use of much higher atomization temperatures, which reach up to 3,000 K. FAAS is typically used for determining low concentrations of elements (e.g., Al, Ca, Co, Cr, As, Cd, Cu, Fe, Mn, Ni, Pb, Zn) [27, 28, 29, 30, 31, 32] and can be applied without the need of earlier preconcentration of analytes. Measurements commonly apply to only one or two elements [33, 34]. However, relatively small quantities of solid and liquid samples may be analyzed. The technique, similarly to FAAS, has various types of interferences including background absorbance, matrix effect on atomic absorbance values, and differences of the chemical form in elements. There are various tools used to eliminate or to reduce these interferences: preliminary mineralization of a sample, separation of elements to be determined from interfering components, chemical modifiers, Zeeman effect background correction, and devices with separated zone of evaporation and atomization including graphite “filter furnace” (FF) atomizer and a L’vov platform.

Graphite is the most commonly used material in ETAAS. Due to the porosity of its surface, various species in the subsurface agglomerate in tubes are present. As a consequence, several corrosion changes have been observed when tubes were exposed to different compounds. Due to the surface and in-depth corrosion of tube and platform, their total lifetime can be significantly reduced. Usually, chemical modifiers are applied to the graphite furnace by adding to the solution with or after the sample or standards. But chemical modifiers can be also applied as a metal deposit on the graphite tube surface or on the L’vov platform acting as a permanent modifier and making the pyrolysis and atomization steps possible without repeating the treatment of the tube or platform. It was demonstrated that electrodeposited noble metals could serve as permanent modifiers by intercalation and remaining near the surface region of the tube for the total tube’s lifetime. This observation developed permanent performance of various modifiers [35]. There are several permanent modifiers recommended – noble metals as well as their mixtures of less volatile metals: palladium, molybdenum, magnesium, molybdenum-iridium, molybdenum-ruthenium, vanadium, iridium, ruthenium, vanadium-iridium, vanadium-ruthenium, tungsten, tungsten-vanadium, and tungsten-magnesium [36]. In case of liquid samples, palladium and tungsten were used successfully [37, 38, 39], while the direct determination of antimony in solid samples was ineffective [40]. Problems in the determination of arsenic and antimony result from background spectral interferences and may be resolved by the selection of alternative analytical lines by the use of high-resolution-continuum source (HR-CS) ETAAS or by the application of a combination of Zeeman background correction together with the selection of an appropriate modifier.

In comparison with a platform, graphite FF atomizer provides increase in sensitivity. Filtration of atomized sample through the porous graphite improves performance of the FF atomizer because during this process molecular species are eliminated from the atomization volume. A wider range of sample volumes, which can be introduced into the graphite FF atomizer, provides additional advantage of this atomizer compared with the platform. The use of FF with Pd-Mg chemical modifier in the determination of Pb, As, and Cd during direct ET AAS analysis in various food samples provided a nearly twofold increase in sensitivity in comparison with a conventional heated graphite furnace with a platform. Additionally, it significantly eliminated matrix effects including background absorbance [29].

The main disadvantage of ETAAS and FAAS analysis of solid samples is the sample pretreatment, which is often the most time consuming and problematic (e.g., incomplete dissolution, precipitation of insoluble analyte, loss of elements during heating, contamination) step. The total concentration of analyte can be determined after acid digestion or alkali fusion. Microwave-assisted sample dissolution has been employed commonly for sample dissolution but barring some obvious advantages it still causes some problems (cost, short lifetime of digestion vessels, explosions, losses and contamination, long time of cooling, small sample throughput, corrosion of microwave parts, constant supervision during the digestion), which are exaggerated during trace elements analysis. The ideal method for the analysis of solid samples would eliminate the sample dissolution minimizing the sample preparation and improving the analytical results. Solid sampling can be applied to materials of different physical structures, whereas slurry sampling is dependent on the size and structure of the particles to be analyzed. In both cases, the use of an appropriate modifier mixture and optimized pyrolysis and atomization temperatures eventually avoids background absorption caused by the complex matrix. Slurry or solid sampling with ETAAS method has been extensively used for analysis of biological materials, sediments, and soils slurries in order to simplify sample preparation and to avoid problems associated with sample dissolution procedures [34, 41, 42]. The ETAAS method with solid sampling into a graphite furnace ensures the rapid and reliable determination of metals in soils or precipitates providing strict control of the sampling efficiency, particle size, number of particles present in the injected volume, analyte homogeneity, suspension medium, slurry concentration, stirring and sample depth. Analyses of environmental samples by solid sampling used either two-stage atomization with the use of permanent modifiers or slurry sampling. ETAAS analysis of volatile elements (as arsenic or antimony), which evaporate as oxides in the temperature of graphite furnance higher than 400^°C, need the use of chemical modifiers (e.g., noble metals: palladium, nickel and magnesium nitrates, high melting of tungsten or zirconium carbides or mixtures of some of them) stabilizing analyte and facilitating the removal of the matrix by the increase of the temperature of atomization. Even ETAAS with the injection of sample suspensions into a graphite furnace ensures the rapid and reliable determination of metals; it is worth mentioning that it significantly shortens the life of tubes and lowers the sample throughput. The use of ultrasound-assisted extraction procedure can allow to avoid these problems because the sample matrix is not introduced into the atomizer, avoiding the buildup of carbonaceous residues or silicates on the graphite platform [43].

HR-CS AAS is an innovation that improves the performance of AAS. After the introduction of AAS technique in 1955 by Walsh, the use of spectral line instead of CS was necessary due to required spectral resolution, which was not achieved with the available monochromators at that time [44]. Together with efficient background corrector (like least-square background correction), it has been applied to the analysis of many analytes in a great variety of samples [45, 46, 47].

HR-CS AAS is a valuable tool due to the visibility of the spectral environment of the analytical line at high resolution. With the use of line source ETAAS with Zeeman effect background correction, a high signal indicating the presence of spectral interference for lead in fertilizer samples is observed. Due to this interference, there is no possibility in the determination of lead at 217.0 nm because of the presence of phosphorus monoxide (PO) under the magnetic field and splitting of the rotational lines of the molecular spectrum. The background absorption without and with magnetic field is not the same, resulting in background correction errors. HR-CS ETAAS is a tool that allows for investigation of these interferences due to the visibility of the spectral environment of the analytical line at high resolution. With the use of HR-CS ETAAS, it is possible to detect the presence of spectral interferences (S and N) and storage of the spectra of diatomic molecules with rotational fine structure that coincide temporally and spectrally with the analyte absorption [46]. To eliminate the fine structured background using least-squares background correction, it is mandatory to identify the molecule that is responsible for the spectral interference. Then, the reference spectrum of the interfering molecule is recorded and subtracted from the sample spectrum and a spectrum of pure analyte is obtained. The fine structured background depends on the chemical composition of each sample. In case of arsenic, the spectral interferences were corrected using a reference spectrum obtained from NaCl and PO, while in the case of selenium NO and PO reference spectra were used to correct the interferences [48]. In both cases, it was possible to obtain accurate results. Sulfur and nitrogen-containing molecules were responsible for the fine-structured background, which was completely corrected in case of lead determination. Using the most sensitive line at 217.001 nm, some unknown spectral interferences were observed. The use of HR-CS ETAAS made it possible to verify that the digestion of samples did not avoid the presence of spectral interferences since digested samples presented fine-structured background similar to the samples prepared as slurry. Comparing HR-CS ETAAS and line source ETAAS with Zeeman effect background correction, the results can be considered similar, indicating that the latter technique was able to correct the herein found spectral interferences to a reasonable extent [46].

2.2 Inductively coupled plasma atomic emission spectrometry

ICPAES, also referred to as inductively coupled plasma optical emission spectrometry (ICPOES), provides an excellent scope for the determination of trace elements with high sensitivity. This is due to very high temperatures (up to 8,000 K) of plasma used for atomization of analyte present in a sample. The ICP is created by argon gas, which is ionized in the intense electromagnetic field and flows in a particular rotationally symmetrical pattern toward the magnetic field of the radio-frequency coil. A stable plasma is generated as the result of the collisions between the neutral argon atoms and the charged particles. While the sample is introduced to the plasma, it immediately collides with the electrons and charged ions and is broken down into charged ions. Various molecules break up into their respective atoms, which then lose electrons and recombine repeatedly in the plasma. Excited atoms emit electromagnetic radiation at wavelengths characteristic of a particular element. The intensity of this emission is indicative of the concentration of the element within the sample and detected by a photomultiplier or a semiconductor detector.

Even ICPAES has similar limits of detection to FAAS; it can detect many elements simultaneously and has a much larger dynamic range. On the other hand, an ICPAES suffers from a lot of interferences and is much more expensive than the AAS techniques. Since the condition of ICP is changed by matrix elements from sample solution, signal intensities derived from analytical elements might be influenced. In analysis of high-viscosity solutions, injection volume of sample might be not constant, which may result in not reproducible analytical results or higher limit of detection. Therefore, the investigation of the influence of matrix elements and high viscosity of sample must be indispensable for accurate, sensitive, and reproducible determination by ICPAES. One of the solutions to reduce the influence of some matrix elements might be vaporization of Cl⁻ and CO32− ions, decreasing of the viscosity of a sample, and lowering the pH of samples or decomposing organic compounds present in a sample. This could be obtained by the use of H₂SO₄-fume treatment [49].

Not without significance is the level of limit of detections that makes this technique not suitable for direct analysis of extremely low element levels. Therefore, prior to detection with ICPAES, an effective preconcentration step is required, similarly like in FAAS analysis described earlier [50, 51]. Various factors affecting the preconcentration process such as sample volume, concentration of the eluent, sample and eluent flow rates, as well as the accuracy of the method have to be always optimized and investigated in this case.

2.3 Inductively coupled plasma mass spectrometry

ICPMS is widely used in routine multielemental determination at the trace and ultratrace level in liquid samples with different matrix composition. The use of separation and enrichment techniques for analytes improves its limits of detection from the level of sub-μ g/L even to sub-pg/L. Due to the excellent sensitivity, low detection limits, possibility of isotopic determination, and small sample volume ICPMS, wide dynamic range is widely used in clinical and biological [52, 53, 54], food [55, 56] environmental, geological, industrial analysis [57, 58, 59], and in a variety of different tasks [60]. Most of the elemental analysis with the use of ICPMS described in the literature concerns easy available materials [61, 62, 63], but a great number belongs also to the limited samples [64].

In the ICPMS technique, the sample is ionized in the same type of argon plasma as in the ICPAES technique. At the first stage of the process, the liquid sample is nebulized with an effective nebulizer transforming it into a fine aerosol, which is then transported with argon to ICP torch. In the plasma, nebulized water matrix and chemical compounds are evaporated, molecules dissociated into atomic constituents, and then ionized into positively single-charged ions. Ions are extracted from the argon plasma into mass analyzers: quadrupole (QMS), double focusing sector field (SFMS), and time of flight (TOFMS). In mass analyzer, ions are separated according to their mass-to-charge ratio or energy-to-charge ratio in double focusing SF instruments. The separated ion beams are detected by photomultiplier or Faraday cups.

Among a large variety of sample introduction systems developed for ICPMS, the most common and most economical is liquid solution nebulization. Therefore, there is a need of previous digestion of solid samples that is a very important stage for the whole analytical procedure. There are a lot of different liquid sample introduction systems developed [65, 66, 67]. The most frequently used for mineralized samples is pneumatic nebulizer (concentric, cross-flow, V-groove, sonic spray, or multi-microspray) combined with spray chamber (double pass, single pass, and cyclonic) with a solution uptake rate of 0.5–2 mL/min and very low transport efficiency (1–20 %) [68]. Higher sample introduction efficiency is provided by high-efficiency nebulizers like ultrasonic nebulizers [69, 70] or electrothermal vaporization, which allows for in situ sample preparation and preconcentration [71, 72]. Due to the trace element determination in micro- or nanoliters of sample, there are also micronebulizers available with solution uptake rate of 0.1 μ L/min, which is a great advantage because of reduction of contamination problems (memory effects, deposition, clogging) or some interferences caused by solvents or possibility of coupling with techniques like electrophoresis requiring low sample consumption.

Besides the very high expenses, the ICPMS technique has a lot of advantages: sensitivity, very low limit of detections, throughput, multielemental analysis, and isotopic information, even though it suffers from atomic and molecular isobaric and multielemental interferences [73, 74]. This can be overcome simply by the choose of noninterfered isotope in case of multi-isotopic elements, by the subtraction of blanks, appropriate sample preparation [75], the use of mathematical correction [76], cold plasma conditions [77], by the use of collision or reaction cell technology [78, 79, 80, 81], or by the use of high-resolution mass spectrometers that resolve elements and interferences. In order to overcome some physical interferences, an internal standard, standard addition method, the choice of sample introduction system, or simply dilution of the sample is frequently used [62, 82, 83, 84]. Extremely important for the achieved detection limits (LODs) as well as precision (RSD) of measurements is the kind of mass analyzer used in the ICPMS system. Samples with complexed matrix are the source of many interferences. Due to the low resolution commonly used, quadrupole analyzers have a lot of limitations compared to the systems with high-resolution mass analyzers [85, 86, 87, 88].

Some of the most repeatable and accurate analytical measurements achieved today are thanks to the conjunction of an ICPMS with an isotopic dilution quantification methodology (Table 1).

2.4 Laser ablation ICPMS

Direct solid sampling is possible with an ICPMS system due to the laser ablation (LA) application. This technique is based on surface ablation of sample material by the use of focus laser beam. First, the sample is placed in a special ablation cell isolating it from the ambient environment. Then, the material is ablated and the formed dry aerosol is introduced to the plasma with the aid of a gas stream allowing for surface analysis or depth analysis of studied materials. Different ablation cells with different internal volumes and geometries mainly depending on the sample sizes influence the overall transport efficiency and signal profile. Volume of ablation cell affects mainly the dispersion of the signal [94].

LA ICPMS is not completely accepted for quantitative analyses mainly due to the fractionation effects and the persistent lack of adequate reference materials for the wide variety of samples [95, 96]. Laser wavelength, pulse duration, power, and spot size influence fractionation during the LA process. The size of the aerosol determinates the particle size distribution of the generated aerosol because of its chemical composition, transport efficiency and decomposition in the ICP. The thermal character of the LA process might lead to the formation of agglomerates and molten spherical particles of different sizes in dependence of the laser wavelength.

Matrix effects may be induced in the ablation process, during aerosol transport to plasma, or in the ionization process in the plasma. Therefore, the calibration of LA ICPMS has to provide the capabilities to compensate these differences among samples and standards in order to obtain quantitative data. There are several calibration strategies for LA ICPMS; among them are internal standard method, external calibration, standard addition, isotope dilution, or matrix matched standards [96, 97, 98]. Additionally, the use of isotope dilution mass spectrometry in combination with LA ICPMS allows an accurate, precise, and time-effective quantitative analysis of trace elements in powdered samples using different isotope dilution calibration strategies.

LA ICPMS is becoming one of the most important direct analytical techniques for the rapid and sensitive determination of stable and radioactive isotopes on solid surfaces [99]. LA ICPMS avoids wet decomposition of a sample as well as the risk of contamination during sample preparation. In fact, it needs little or no sample preparation and offers a very good sample throughput and reduced spectral interferences. A significant feature of LA is the high spatial resolution between 10 and 100 μ m for nanosecond lasers and below 1 μ m for femtosecond lasers, with very low sample uptake of picograms. This microdestructive character of the technique is important in case of unique samples [100, 101, 102, 103]. LA ICPMS has been used to produce images of element distribution in various materials, mainly in clinical and biological samples [104]. The challenging task of LA ICPMS for future applications is the analysis of a single cell or particle with interesting applications in life and material science.

4.1 Techniques uses X-ray

Due to its very low limit of detections and matrix insensitivity, X-ray methods have been widely used for trace analysis in geological materials, steels, cements, archaeological samples, forensic samples, and environmental samples such as airborne particulate matter [122]. A commonly used technique is X-ray fluorescence (XRF) used to analyze for all elements. The sensitivity of XRF depends on the energy of the incident radiation, the geometry of the instrument used, and the efficiency of the detector. The precision of XRF measurements is limited by the statistics of the detected photons. The LODs depend on the sensitivity of the instrument and the background level of the sample matrix. Typical XRF precision is better than 0.1 % and typical LODs are few g/cm² for particulate material for a wide range of elements.

In total reflection XRF (TXRF), the incoming radiation is incident on the sample at less than the critical angle and is totally reflected. The low background levels result in improved LODs and even 2 pg may be detected for a variety of elements with a counting time of 1,000 s. Using synchrotron radiation sources (SR), TXRF with polychromatic beam for sample excitation allows for lowering detection limits to even 0.03 pg/m³ and the use of more intense photon fluxes will lower them by several orders of magnitude [123].

For these techniques, samples need to be in a form of thin film and that is a reason for low matrix effects conversely to the above-mentioned techniques. Quantification is performed by the addition of a single element, not present in the sample itself, as internal standard and a calibration curve valid in all matrices. The lack of robustness in these calibration methods leads to large systematic errors that are significant compared to the very low amounts being analyzed.

X-ray emission may be induced by heavy charged particles (particle-induced X-ray emission). Calibration is often by means of thin-film standards or using fundamental physical parameters in conjunction with an experimentally determined efficiency curve. LODs of ng/cm² have been claimed for particulate material on ambient air filters for a range of elements.

4.2 Activation analysis

Activation analysis refers to the identification and quantitative determination by use of radionuclides produced from a target element. It requires very little or no sample preparation, is nondestructive in nature and largely unaffected by the sample matrix, and allows for multielemental determination.

Neutron activation analysis (NAA) is the most common variant in which neutrons are used to irradiate and to activate the sample. When the measurement is carried out without prior chemical separation, the method is called instrumental NAA (INAA). As a result of a nuclear reaction between the neutron and the isotope of the element of interest, radionuclides with characteristic half-lives may be produced emitting radiation of varying energies that may be measured by a suitable detector and are characteristic of the element form that they were produced. In INAA, the decomposition of the radioactive sample is not necessary; the method is also nondestructive, which in case of unique objects is a valuable feature.