Compositional Analysis of Metal(IV) Phosphate and Phosphonate Materials

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This entry is adapted from the peer-reviewed paper 10.3390/separations10120600

Metal(IV) phosphate and phosphonates materials have increasingly found their applications in water purification, heterogeneous catalysis, drug delivery, and proton-exchange membrane fuel cells. The strong linkage between tetravalent metal cations and phosphate/phosphonate groups offers a unique bottom-up design platform, resulting in chemically stable inorganics or hybrids. Task-specific physiochemical functionalities could be deposited by modifying the phosphate/phosphonate groups before the material synthesis. The high reactivity between the metal centre and the phosphorus-containing linker, on the other hand, often leads to obtaining unordered materials (amorphous solids or coordination polymers). The chemical composition of the prepared materials is a key parameter in guiding the synthetic approach and in governing their performances.

metal phosphate metal phosphonate characterisation composition analysis

1. Analysis of Metal and Phosphorus Contents

Since any metal phosphate/phosphonate materials are essentially built by metal centres and phosphorus-containing ligands, it is important to understand how many of the building blocks in the precursor actually translate into solid materials. Furthermore, the phosphorus-to-metal atomic (molar) ratio gives an indication of the possible presence of other components. For crystalline materials, the phosphorus-to-metal ratio should correspond to the chemical stoichiometry, as validated by any structural characterisations (i.e., single crystal diffraction methods). This composition of the materials needs to be characterised irrespective of the structural characterisations, and such is generally lacking in the literature, which leaves the possibility of, for example, the aforementioned amorphous metal oxide to be present in the material, which complicates the assessment of the material’s functional results.

2. Dissolution and Digestion

As the first step in any solution-based analysis methods, the metal phosphate/phosphonates must be completely solubilised into an aqueous solution. A combination of concentrated nitric acid, perchloric acid, and hydrofluoric acid (HF) is generally adopted for this purpose, among others ^[1]^[2]. Due to the slow and low dissolution process of typical tetravalent metal compounds oxo clusters, HF is added to form soluble complexes (e.g., ZrF₆²⁻). Nitric acid and perchloric acid as strong oxidation agents are used to digest the organic components. The digestion process could be carried out on a heating plate (with plastic or Teflon beakers) or in any compatible high-pressure digestion system (microwave digester or autoclave). Literature recipes for digestions are rarely clearly mentioned and they vary greatly from one material to another. However, the overall goals for the digestion are (i) to transfer all the metal and phosphorus as soluble species into the solution; (ii) to decompose any remaining organic compounds; and (iii) to obtain a clean solution matrix that is suitable for subsequent measurement. The last goal usually requires the elimination of residual HF and perchloric acid. Depending on the material composition, different approaches need to be considered, particularly considering reprecipitation and possible polyatomic phosphorous species ^[3]^[4]. Quality control samples (e.g., stoichiometric metal oxides, non-volatile organophosphorus compounds, or highly crystalline metal phosphate phases) are highly recommended for any of these digestion processes since they may generate volatile (highly toxic) compounds.

3. Atomic and Mass Spectroscopy

Metal(IV) and phosphorus dissolved in a solution can be quantified by atomic or optical spectroscopic methods, or mass spectroscopic methods. However, it is crucial to ensure the complete dissolution of the compounds containing these elements as explained earlier. For atomic/optical spectroscopy, the analysis is based on the characteristic photon emission(s) or absorption(s) of different atoms. There are many available techniques for atomisation and excitation of the elements—chemical flame, graphite furnace heating, direct-current plasma, and inductively coupled plasma. They differ from one another mostly on the temperature of the flame/plasma, thus resulting in different atomisation and ionisation capabilities. Common techniques include flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS), microwave plasma atomic emission spectrometry (MP-AES), and inductively coupled plasma optical emission spectrometry (ICP-OES). The detection limits for common metal(IV) in solution are at the level of sub-mg/L, sometimes even possible for the level of μg/L. The more sensitive analysis method is the inductively coupled plasma mass spectrometry (ICP-MS), where the detection limits could reach ng/L or pg/L. It should be noted that phosphorus content cannot be measured by AAS techniques since the atomisation condition provided is not enough to decompose phosphate into phosphorus atoms. The main challenge of obtaining an accurate result is associated with the efficiency of the sample disintegration and dissolution process.

Not only nebulised aerosol could be fed into the ICP torch but also the ablated solid materials. Laser ablation-ICP (LA-ICP) is a technique that utilises a laser to evaporate or sublimate solid samples. This allows the direct analysis of a solid without dissolution. To the best of our knowledge, LA-ICP-based techniques for compositional analysis of metal phosphate/phosphonate were not found in the literature. The LA-ICP-MS method gives direct isotopic information and quantification information (including for oxygen ^[5]), making it an ideal method when suitable standards are used.

The element-specific quantification analysis by atomic or mass spectroscopic techniques is recommended for the compositional analysis of metal phosphate/phosphonate materials. These methods produce accurate results both in terms of overall content and metal-to-phosphorus ratio. The shortcoming of these methods is the long sample preparation routes (referring to digestion).

4. X-ray Spectroscopy

X-ray-based analysis is non-destructive and as such a very attractive measurement technique. There are several X-ray-based techniques which could give quantitative compositional information of the sample, and they differ mainly in the type(s) of incoming radiation and measurable radiation after the interaction with the matter.

The most common technique is the energy-dispersive X-ray spectroscopy (EDX or EDS), which is sometimes also referred to as energy-dispersive electron probe microanalysis (ED-EPMA). For this purpose, an X-ray detector is usually fitted to a scanning electron microscope (SEM). EDX is an element-specific method because of the intrinsic differences in X-ray emission lines of different elements, and qualitative element identification is easily achieved. However, there are many issues associated with the quantification of elemental compositions by the EDX method: (i) the existence of a background continuum in the EDX spectrum, which comes mainly from bremsstrahlung X-rays and Compton scatterings; (ii) intrinsically overlapping emission peaks; (iii) insufficiency of the sample thickness.

For obtaining quantitative elemental compositional results, care must be taken during sample preparation and the actual measurement. Standardless methods would most probably give erroneous results. Modern EDX softwares are equipped with complex computer algorithms for correcting the spectrum to achieve more accurate quantification, and the most used method is called the “ZAF” method. This method considers the atomic number (“Z”), absorption (“A”), and fluorescence (“F”) between different elements in the sample and in the standards ^[6]. To achieve an EDX quantitative analysis, one needs carefully selected standard samples, an even sample surface coated with a homogenous conductive layer, a stable electron beam current, and lastly, adequate counting statistics for the peak-of-interest. Even if all the above-mentioned problems are taken into consideration, the spectral interferences still cannot be avoided.

For example, for the most widely studied zirconium phosphate/phosphonate materials, the Kα1 spectral line of P is completely overlapping with the Lα1 line of Zr (Table 1), and the energy resolution (on the scale of 100 eV) of the X-ray detectors (typically silicon drifted detectors, SDD) cannot distinguish between these two peaks (for instance, see examples in ^[7]^[8]). With adequate precautions, the Zr quantification results would still be trustworthy by quantifying based on the Kα lines (at 15.8 keV), but the mathematical peak deconvolution of the P Kα lines from the Zr Lα lines would introduce too much uncertainty to the results. This uncertainty has been seen in the literature where crystalline zirconium phosphate materials, whose stoichiometry could be deduced from single-crystal XRD measurement, gave inaccurate Zr and P quantification results in EDX measurement ^[9]. A similar situation applies to energy-dispersive X-ray fluorescence (ED-XRF) analysis, where an X-ray tube produces the incident beam. However, the quantification of Zr and P contents by EDX measurements is not rare in the literature and could lead to significant errors.

Table 1. Characteristic X-ray emission lines of selected elements.

Element	Kα1 (keV)	Lα1 (keV)
P	2.014	-
Ti	4.511	0.452
Ge	9.886	1.188
Zr	15.775	2.042
Sn	25.271	3.444
Hf	55.790	7.899

Wavelength-dispersive (WD) X-ray analysis compensates the ED methods for spectral resolution and background reduction. Depending on the radiation source, both WD-EPMA and WD-XRF are essentially improving the corresponding ED methods. In the WD methods, the X-ray emission is monochromatised using crystal analysers before being counted on the detector. This could lead to a proper separation of the P Kα lines from the Zr Lα lines. WD methods with proper matrix corrections are better at achieving quantifiable results for the compositional analysis, especially for zirconium phosphate/phosphonate materials ^[10]^[11].

Another applicable type of X-ray-based analysis is X-ray photoelectron spectroscopy (XPS), traditionally named electron spectroscopy for chemical analysis (ESCA). This method differs from the ED or WD X-ray spectroscopy since it is the emitted photoelectrons that are being measured in XPS. The quantification by XPS provides accurate results for homogenous samples with adequate standards ^[12]. However, it should be noted that XPS is a surface analysis technique responsive only to a few-nm thickness layer, and the photoelectron generated deeper inside the materials will be attenuated and will not reach the detector. In addition, surface layer oxidation and contamination with adventitious carbon significantly impair the quantification results by XPS. Since both EDX and XPS would possibly result in erroneous results, it is common to obtain incoherent quantification data when comparing these two methods.

In ED methods, the oxygen content associated with the analysis is always a stoichiometric calculation of the other elements and it should not be taken seriously. The X-ray emission of O is too low to be accurately quantified due to the interference from the background spectrum. However, in WD methods and XPS, the obtained oxygen content is the actual measurement result.

X-ray-based spectroscopic methods are a group of well-documented and non-destructive analysis techniques that are useful for the compositional analysis of any metal phosphate/phosphonate materials. Both the metal and the phosphorus contents could be accurately measured when proper precautions have been taken. WD techniques are recommended over ED techniques, and it is not possible to obtain trustworthy quantification results of Zr and P by using ED methods alone. Electron sources are more destructive compared to X-ray sources. The heterogeneity of the sample must be examined especially when using XPS but also for other techniques.

5. Neutron Activation Analysis

Neutron activation analysis (NAA) is by far the most accurate method for quantitative compositional analysis ^[13]. The NAA analysis usually does not require any chemical pre-treatment, and it utilises characteristic nuclear reactions of different isotopes when the sample is irradiated by a neutron source. The produced radioisotopes and/or their daughters are subsequently measured conveniently on a gamma detector. NAA suffers much less from matrix effects since light elements (H, C, N, O) in the samples do not produce significant interferences for the determination of heavier elements. It is a multi-element technique; therefore, quantification for many elements could be carried out simultaneously. Essentially, it is the neutron cross-section of each nuclear reaction that governs the analysis and is therefore device specific depending on the energy of the produced neutrons.

Concerning zirconium phosphate/phosphonate materials, Table 2 lists the nuclear reactions relevant for quantification with a simple comparator method by the fast neutron (14 MeV) activation ^[14]. The measurement methodologies for other elements are also found in the literature: Ti ^[15], Hf ^[16], Ge ^[17], Sn ^[18], and Pb ^[19]. NAA measures accurately the oxygen content, which is important for compositional analysis. The problematic part of NAA is the potentially radioactive sample handling and waste management that makes the facilities heavily regulated and not so easily accessible.

Table 2. Nuclear reactions used for elemental quantification by NAA for zirconium phosphate/phosphonate materials.

Element	Reaction	Half-Life	Gamma Energy (MeV)
O	¹⁶O(n,p)¹⁶N	7.13 s	6.13, 7.12
N	¹⁴N(n,2n)¹³N	9.97 min	0.511 (annihilation)
P	³¹P(n,α)²⁸Al	2.25 min	1.78
Zr	⁹⁰Zr(n,2n)^89mZr	4.18 min	0.588

6. Gravimetry and Colourimetry

With the lack of modern and highly sensitive spectroscopic methods, traditional gravimetric and colourimetric are also viable methods for tetravalent metal and phosphorus (mainly as phosphate) analysis. These methods are still useful and well accessible due to their relative simplicity and robustness of the analysis when large sample quantities are available, making the science more equal. The main drawbacks of them are the large amount of sample requirements as well as lower sensitivity compared to modern methods. For example, zirconium could be precipitated as zirconium cupferron (ammonium salt of N-nitroso-N-phenylhydroxylamine) and later calcined to form zirconium dioxide (ZrO₂) for gravimetric analysis ^[20]^[21]. This method suffers from specificity, i.e., titanium produces a positive interference. Colourimetric methods are more unique for a distinctive element, and the detection limit is much lower when utilising a UV/visible spectrophotometer (compared to gravimetry). Titanium, as an example, is quantifiable through the violet titanium-hydrogen peroxide complexes, whose maximum photon absorption is at 420 nm ^[22].

The measurement of phosphorus is almost entirely based on the phosphate species and therefore any organophosphorus would significantly impair the results. Calcination of metal phosphate with the addition of soluble magnesium salts results in the formation of insoluble (water) magnesium pyrophosphate for gravimetric analysis ^[23]. For colourimetric determination of phosphate, there are also a number of methods, and the most classic one is based on a blue-coloured phosphomolybdate complex (NH₄)₃[PO₄(MoO₃)₁₂] ^[24].

In general, the traditional methods are still valid and serve their purpose well via simple chemical reactions and processes. The methods produce good sufficiently reliable results when there are no known interferents present and when the sample quantity is adequate for such analysis.

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Risto Koivula

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