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ICP‐MS uses inductively coupled plasma to ionize the atoms [3, 4]. It is used to atomize the sample, creating polyatomic ions, which are then detected. The ions from the plasma are separated through a series of cones into a mass spectrometer, usually a quadrupole. The extraction of ions is based on their mass‐to‐charge ratio. An ionic signal is received by a detector which is correlated to the concentrations of atoms in the sample. These concentrations are obtained by drawing the calibration curves using some certified reference materials (CRMs).

It has the capability to investigate several metals as well as non‐metals in the liquefied samples at milligram to nanogram levels per liter. It is also used for isotopic analysis of the elements. This technique has been recognized as one of the most important analytical techniques in a variety of industries for the monitoring of impurities in semiconductor manufacturing, environmental monitoring, geochemical analysis, mining and metallurgy, pharmaceutical industries, and biomedical analysis. ICP‐MS has a good precision, sensitivity, and greater speed. The main disadvantage of ICP‐MS is that it produces many interfering species like component gases of air, argon from the plasma, and contamination from glassware and cones used.

Table 1.1 Overview of some analytical techniques including AAS, ICP, LIBS, XRF, μ‐XRF, SR‐XRF, and TOF‐SIMS [3,8–11].

Techniques	Excitation source	Elements detected	Resolution	Detection limit	Imaging	Instrument efforts	Scan size/area analysis (mm)	Specific remarks
Spatial	Depth
XRF	X‐rays	B‐U (WDXRF); Na‐U (EDXRF)	20 mm	10 nm	1–100 ppm for most elements	No	Medium	~30 μm (EDXRF) and ~500 μm (WDXRF)	Non‐destructive
μ‐XRF	X‐rays	Multi‐element (Al‐U)	20–500 μm	10 nm	20–50 ppm	Yes	Medium	Upto 190 × 160	Relatively slow, risk for radiation damage
μ‐SRXRF	X‐rays	Multi‐element	>10 nm	1 nm	5 ppm	Yes	Very high
TXRF	X‐rays	Na‐U		>3 nm		Yes (Optional)	High		Polished surface required for best detection limits, Can analyze many substrates, e.g. Si, SiC, GaAs, InP, sapphire, glass
X‐ray fluorescence microscopy (XFM)	X‐rays	Multi‐element (Al‐U but poor 2nd row Z > 42	0.05–1 μm	>100 μm	<0.1 ppm	Yes	Medium	Upto 150 × 100	Ability for spectroscopy (XAS) to determine chemical speciation
SEM/TEM‐EDS	Electrons	Multi‐element (O‐U)	<0.5 μm	<0.5 μm	1000 ppm	Yes	Medium	7 × 7	Resolution depends on element investigated
PIXE	Protons (for biological applications)	Multi‐element (Na‐U)	2 μm	10–100 μm	1–10 ppm	Limited	Very high	4 × 4	Quantitative measurements of heavier elements that can't be resolved by RBS alone
XPS/ESCA	X‐rays	Li‐U (Chemical bonding information)	0.5 nm	3 nm	100 ppm	Yes	High	Smallest analytical area ~10 μm	Limited specific organic information and sample compatibility with UHV environment.
Auger (AES)	Electrons	Li‐U	0.2 μm	3 nm	100 ppm	Yes	High	Small area analysis (~20 nm minimum)	Analysis of insulators can be difficult and samples must be vacuum compatible.
SIMS	Ions	H‐U including isotopes	5 μm	0.1 nm	1 ppb	Yes	Medium	Small‐area analysis (1–10 μm)	Destructive,no chemical bonding information, and sample must be solid and vacuum compatible.
TOF‐SIMS	Ions	Full periodic table coverage, plus molecular species	<0.1 μm	1 nm	1 ppm	Yes	High		Samples must be vacuum compatible
RBS	He2+ Ions (alpha particles)	B‐U	2 mm	10–20 nm	1–1000 ppm	No	Medium	Large analysis area (~2 mm)	Non‐destructive, Conductor and insulator analysis
ICP‐OES	Argon plasma	Li‐U (except gases, halogens, low quantity of P and S)	No	Bulk chemical analysis technique	<1 ppb	No	Medium	No	C, H, N, O and halogens cannot be determined.
ICP‐MS	Plasma source	Most of the elements	No	No	Typically ng/ L	No	Medium	No	Polyatomic mass interferences, atmospherics and light halogens
LA‐ICP‐MS	Photos (Laser)	Multi‐element (Al‐U)	100 nm	0.1–1 μm	<1 ppb	Yes (Limited: ablated surface)	Medium	20 × 20	Elemental, stable isotope distribution analysis and mapping
LIBS	Photos (Laser)	All elements detectable		>0.1 μm	1 ppm	Yes (Limited)	Medium		Strong matrix‐effects on emission spectra
Atom Probe Tomography (APT)	Laser or voltage pulse	H‐U	<1 nm	0.3 nm	~10 ppm	Yes	High	50 × 50 nm2	Ability to identify isotopes, Cluster analysis for nanoscale precipitates
STEM	Electrons	B‐U (EDS)	3 nm	3 nm	Typically ppm	Yes	High	5 μm x 5 μm	Strong contrast between crystalline vs amorphous materials without chemical staining
AAS	Radiation Sources (Hollow Cathode Lamps HCL)	Most of the elements except some lighter elements			Typically μg/l	No	Medium	No	Destructive, time consuming, sequential analysis of 1 to 6 elements, separate method of optimization required for each type of sample.

Figure 1.1 A concise visual reference of most of the ring analytical techniques to compare the detection limits and analytical resolutions for materials characterization.

Source: Reproduced from Ref. [3] with the kind permission and copyright of © Eurofins Scientific (www.eurofinseag.com).

In contrast to traditional AAS that can detect only one element at a time, ICP‐MS instruments have ability to measure all the elements present in the sample material even at once. However, advanced AAS systems (AnalyticJena) are also available, which is of the scan variety (not independent hollow cathode lamp) and can measure the elements sequentially (http://www.analytik‐jena.com). ICP‐MS is widely used in forensic and biomedical science, in particular toxicology [3]. Depending on the specific parameters in the patient, the collection of samples taken for the analysis process can vary from blood, serum, plasma, urine, to even packed red blood cells. This instrument is also used in the environmental field. The applications include testing of water samples in the soil for municipalities water and for industrial purposes.

The ICP‐MS instrument should be free of obstruction. Even the smallest obstruction can disturb the flow of the sample, which can clog the sample tips within the spray chamber. Also, high concentrations of NaCl in samples such as sea or ocean water can lead to obstruction. These blockages can be overcome by dilution of the samples wherever a high concentration of salt has been observed and compensated for. This process comes at the cost of detection limits. ICP‐MS has been used for glass analysis in forensic applications [3, 4]. It is capable of tracing the elements on the glass. The elements detected on the glass can be utilized in order to match the sample materials observed at the crime scene.

Laser ablation ICP‐MS (LA‐ICP‐MS) uses a high‐power pulsed laser beam (typically ns) to ablate a small amount of material (picograms to femtograms) from the surface of the sample [3]. A plume of atomic particles and ions are generated which are then carried to an ICP‐MS detector with the help of a constant flow of argon (Ar) or helium (He) gas. The sample is subsequently ionized in an IC plasma, and its atomic species are transported in the form of ions, which are further separated and analyzed using their mass/charge ratio. It is used to measure major and trace elemental composition of samples at the level of parts‐per‐billion (ppb). It is considered a versatile technique due to its high analytical performance for various kinds of unprepared solid samples. A very small amount of the sample (solid and liquid) quantities (picograms to femtograms) is sufficient to produce highly sensitive results up to the ppb level, depending on the measurement system. The laser beam can be focused up to 5–200 μm range and thus allows a single spot analysis and line scanning over the surface of the samples. It is recognized as a good analytical technique that can be used for the analysis of a variety of sample materials detected in forensic applications [3]. It has already proven its potential in the forensic analysis of bone, tooth, car paint, printing ink, metals, glasses, trace fingerprints, soil, and paper fields [3].

A comparative study of LA‐ICP‐MS and micro‐XRF by Gholap et al. [12] was performed in order to compare their detection limits and spatial resolution. The experiment for elemental imaging was performed on Daphnia magna, which is typically used as an indicator of aquatic ecosystem health and is ascribed as a model organism in ecotoxicology. The authors used sections of the freshwater crustacean D. magna (typical thickness of 10‐20 μm) for the analysis and obtained the elemental localization of elements in particular Ca, P, S, and Zn which allowed elemental correlation with the tissues. The authors plotted the RGB maps (as shown in Figure 1.2) to conceptualize the simultaneous presence of metallic elements in the sample. Figure 1.2 shows the RGB representation of the distribution of Ca, Fe, P using μ‐XRF and Ca, P, Zn using LA‐ICP‐MS in the sagittal and dorsoventral parts of D. magna. The results reveal the concomitant presence of Ca/P in thoracic appendages, P/Zn in the gut and Ca/Zn in the exoskeleton. The co‐existence of Ca/P and Zn/P is ascribed to the formation of intracellular and membrane‐bound phosphate granules, which can be a reason for the storage of metallic ions Ca²⁺ and Zn²⁺ in living tissues [12, 13]. Both the techniques provide comparable limits of detection (LOD) for Ca and P which validated the imaging results. LA‐ICP‐MS was found to be sensitive in determining Zn (LOD 20 ppm, 15 μm spot size) in D. magna, but the detection power of μ‐XRF was found inadequate. On the other hand, LA‐ICP‐MS was found inadequate for the distribution analysis of S, which could be better examined and visualized using μ‐XRF (LOD 160 ppm, five seconds life time).

Figure 1.2 RGB representation of Ca, Fe, P (micro‐XRF) and Ca, P, Zn (LA‐ICP‐MS) distribution in sagittal and dorsoventral sections of Daphnia magna. The sagittal sections originate from different depths of the organism. A: thoracic appendages; B: eggs; C: carapax; D: gut epithelium.

Source: Reproduced from Gholap et al. [12] with permission from Elsevier.

Finally, they were able to conclude that the use of a super‐cell significantly reduced the volume of ablation chamber, which significantly improved the lateral resolution. The spatial resolution of LA‐ICP‐MS was found to be better than that of μ‐XRF, however wash‐out effects and spikes marginally disturbed the quality of image. μ‐XRF provided the elemental distribution for S and LA‐ICP‐MS gave the elemental distribution of Zn and thus both the techniques can be used in a complementary manner. Synchrotron radiation in μ‐XRF can be used to obtain better detection power comparable to or higher than LA‐ICP‐MS. It can also be useful in order to obtain better spatial resolution. Further, the application of LA‐ICP‐MS could be expanded to obtain 3D‐elemental distribution of elements as well as isotopes within biological tissues [14].