Conference Agenda

Speciomics is a term adapted from the biological sciences [1], referring to the study and evaluation of chemical species within the context of omics approaches. The related term speciome arises from the synergy between chemical speciation and omics, and is defined as the complete set of chemical species analyzed using omics strategies. In simpler terms, speciomics serves as an “umbrella” concept encompassing all omics techniques—such as metabolomics, proteomics, metallomics, and genomics—that are applied to speciation analysis [2].

To illustrate this concept, we present a study involving two distinct biological systems: a biotechnological material (soybean callus), treated or untreated with nanoparticles for preservation, and a marine animal (turtle). From the perspective of speciomics, we aim to identify and characterize molecular species present in these samples, as well as to discover new ones [3].

Through a multimodal analytical platform, we have already identified species containing arsenic (As), calcium (Ca), iron (Fe), magnesium (Mg), zinc (Zn), lipids, and phosphorus-containing metabolites. This was achieved using both positive and negative ionization modes. Notably, novel molecular species have also been discovered. This comprehensive, multimodal approach demonstrates the potential of non-targeted speciomics to provide high-resolution, multi-elemental insight into complex biological systems.

[1] AltTox.org at https://alttox.org/mapp/emerging-technologies/omics-bioinformatics-computational-biology/, Accessed on January, 8^th, 2024.

[2] Arruda, M. A. Z., Jesus, J. R., Blindauer, C. A, Stewart, A. J., J. Proteomics, 12(2020)1878.

[3] Kato, L. S., Silva, V. H. C., Andrade, D. C., Cruz, G., Pedrobom, J. H., Raab, A., Feldmann, J., Arruda, M. A. Z., Anal. Chim. Acta, DOI: 10.1016/j.aca.2024.343084 R.

Gold nanoparticles (AuNPs) are utilized in a wide range of applications including cosmetics, novel medical applications and as food additives. However, there is often a lack of comprehensive knowledge regarding the prevalent particle size distribution, particularly given the potential for changes to occur during processing or storage. The quantification and sizing of AuNPs as well as speciation, with the aim of differentiating between metal ions and nanoparticles, commonly involves the combination or hyphenation of (size) separation techniques, such as field flow fractionation (FFF) with element-specific detection methods, including inductively coupled plasma-mass spectrometry (ICP-MS). However, the identification of very small NPs with diameters in the single digital nm range remains challenging especially in the presence of the ionic metal [1].

Graphite furnace atomic absorption spectrometry (GFAAS) is a technique that enables direct speciation and sizing of metal nanoparticles in liquid as well as solid samples following minimal sample preparation and without the necessity for metal species separation [2]. This approach not only simplifies and speeds up the analysis process but, more importantly, it eliminates or minimizes the risk of unintended alterations in particle size or aggregation. The method is based on the distinction in thermal energy required to atomize metal ions and NPs of different sizes, resulting in a clear correlation between the temporal shift of the transient absorbance signal and the respective size [3].

In the present study, the ability to size AuNPs in the single-digit nm range and to distinguish them from ionic gold in aqueous samples was investigated using the high-resolution continuum source GFAAS (HR-CS-GFAAS). The "time of first inflection point" (t_ip) was introduced as a newly sizing parameter, resulting in a highly reproducible calibration function within the tested working range of 2.2 nm to 10.1 nm. Using t_ip, the size of an unknown particle suspension as determined by high-resolution transmission electron microscopy (HR-TEM) was fully recovered. In addition, a minimum distinguishable size difference of 1.6 nm was experimentally demonstrated, and a theoretical size resolution of 0.3 nm has been predicted. By optimizing the graphite furnace temperature program, we were also able to significantly distinguish particles as small as 2.2 nm from gold ions. Moreover, a detection limit of 0.4 pg, equivalent to 10 ng/L in liquid samples, was achieved for gold quantification.

Acknowledgement:

We thank Chistopher Leist (SALVE Center, UUlm) for TEM and Gregor Neusser (FIB Center, UUlm) for SEM characterization of NPs. Funding was obtained from DFG in project LE 2457/12-1.

Literature:

[1] Blaimer, D. et al., Analytical methods for identification, characterization, and quantification of metal-containing nanoparticles in biological and biomedical samples, food and personal care products, Trends in Anal. Chem. 2024, 181, 118031.

[2] Leopold, K., et al., Sizing gold nanoparticles using graphite furnace atomic absorption spectrometry, J. Anal. At. Spectrom. 2017, 32 (4), 723-730.

[3] Brandt, A., et al., Investigation of the atomization mechanism of gold nanoparticles in graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 2018,150, 26–32.

Several analytically important elements such as As, Se, Pb, Sn, Sb, Bi, Te and Ge can be quantitatively converted to their coresponding hydrides. Such a derivatization results in enhanced analyte introduction efficiency and reduced risks of interferences, when compared to liquid nebulization. Hydride generation (HG) can be coupled with atomic absorption (AAS), atomic fluorescence (AFS) or optical emission spectrometry (OES). The most commonly used hydride atomizers in AAS are externally heated quartz tubes (QTA). However, new types of hydride atomizers based on various types of plasma have been used as an alternative in recent years. They are based either on a dielectric barrier discharge (DBD), or atmospheric pressure glow discharge (APGD), the latter one investigated in detail in this work.

APGD is a non-equilibrium plasma sustained between two electrodes powered by a high voltage. The discharge gap reaches typically from 1 to 5 mm and discharge current varies between 10 to100 mA. Plasma gas temperature reaches from 1500 to 3500 K with electron number density of 10¹⁴-10¹⁵ cm^-3. These features indicate the potential of APGD to be used for efficient hydride atomization in HG-AAS. A novel APGD hydride atomizer was constructed in this work with its design derived from a quartz body of a QTA. Instead of resitive heating of the optical arm of the atomizer, two electrodes were inserted into its central part to sustain the APGD discharge (0.5 kV, 30 mA) in the optical axis of the spectrometer.

Atomization conditions have been optimized individually for each element including As, Se, Sn, Sb, Pb, Bi and Te. The effects of discharge gas nature (Ar or He) and its flow rate, the delivered power as well as the role of water vapor and co-generated aerosol on analyte response were investigated. Subsequently, analytical figures of merit, including sensitivity and limits of detection (LOD), were determined under optimum atomization conditions. The sensitivity values ranged between 0.03 and 0.30 s ng^-1, while LODs from 0.1 to 1.5 ng ml^-1 were found. APGD performance, including its resistance to interferences, was compared to that of other hydride atomizers - QTA and DBD.

This research has been supported by the Czech Science Foundation under Contract 23-05974K, by the Charles University Grant Agency (project no. 614225), and by the Czech Academy of Sciences (Institutional research plan RVO:68081715).

Coupling hydride generation with atomic absorption or fluorescence spectrometry is a well-established technique for trace element and speciation analysis, enabling efficient, and matrix-free introduction of analytes into the detector. While heated quartz tube atomizers and diffusion flames remain the most widely used hydride atomizers, alternative plasma-based atomizers - particularly dielectric barrier discharges (DBD) and atmospheric-pressure discharges (APD) - have gained attention. The DBD can efficiently atomize As, Se, Sb, and Bi hydrides while reaching poor sensitivity for Pb, Sn, and Ge. In particular, Ge is detected with low sensitivity even in the most common hydride atomizers.

Consequently, APD-based atomizers were developed and investigated in this work to overcome the low sensitivity observed in DBD for the elements mentioned above (Pb, Sn, Ge). Four APD designs were developed and tested. The first APD construction resembled the design of the diffusion flame, using a quartz capillary nested within a stainless steel anode and an opposing tungsten rod cathode. The zone of atomization was shielded with argon flow to prevent the entrance of oxygen from the ambient atmosphere. However, the discharge was unstable with this construction. The second design, based on two opposite rod electrodes, demonstrated stable discharge and obtained signal was comparable to that of diffusion flames. Thus, this design could be a robust alternative, and it is ready for optimization using an atomic fluorescence spectrometer. The other two APD designs tested were derived from the quartz tube atomizer. In the first arrangement, analyte hydride was introduced through a quartz and stainless steel capillary in a parallel direction with the plasma. In the second arrangement, analyte hydride was introduced through an inlet arm perpendicularly to the plasma and the opposite tungsten rod electrodes. The atomization area was protected from the ambient atmosphere by the optical tube eliminating the need for additional argon. The last design was selected as the most promising, and its performance was compared to the DBD and heated quartz tube atomizer. Current-voltage characteristics were evaluated, as they are crucial parameters for discharge performance. Due to the limited atomization efficiency achieved with commercially available high-voltage and direct current power sources, custom pulsed direct current power sources were developed. Various configuration will be presented.

Moreover, the distribution and absolute concentration of hydrogen radicals/free analyte atoms in the most promising APD design were studied by two-photon/laser-induced fluorescence, and the results will be correlated with atomic absorption spectrometry experiments.

Acknowledgments

This research has been supported by the Czech Science Foundation under contract 23-05974K and by the Institute of Analytical Chemistry of the Czech Academy of Sciences (Institutional Research Plan no. RVO: 68081715).

Hydride generation (HG) is a useful sample derivatization step in trace element analysis being applicable to several analytically and toxicologically important elements including As, Se, Te, Pb, Sn Sb, Bi and Ge. It reaches almost quantitative and matrix-free analyte introduction into the atomic spectrometric detector. Hydride atomizers based on flame, heated quartz tube (QTA) or plasmas are employed in atomic absorption spectrometry (AAS). QTAs are the most common hydride atomizers in AAS offering high sensitivity universally for all hydride forming elements with the only exception of Ge, for which significantly impaired sensitivity is reached. Recently, ambient plasmas such as volume dielectric barrier discharges (DBDs) or atmospheric pressure glow discharge (APGD) have been reported to be an alternative to QTAs. Significant differences in sensitivity were found among individual hydride forming elements in ambient plasma-based hydride atomizers, especially in the DBD, even under atomization conditions optimized individually for each analyte. Since efficient hydride generation, i.e., analyte conversion to a corresponding binary hydride, has been proven for all hydride forming elements investigated in our previous studies, atomization mechanisms and the fate of free analyte atoms were investigated in this work using various advanced spectrometric techniques. Although DBD hydride atomizers offer either the same, or worse sensitivity than QTA, they allow simple and fast in-situ preconcentration of hydrides prior to AAS detection leading to significant LOD improvement.

Laser induced fluorescence (LIF) was employed as a useful diagnostic tool capable of determination of spatial distribution of free analyte atoms in the atomizers, with or without the preceding in-situ preconcentration step, as well as quantifying their absolute concentration, leading to assessment of atomization efficiency. Hydrogen radicals were detected by two-photon absorption LIF (TALIF) as important species responsible for hydride atomization in all types of atomizers. Time-resolved optical emission spectrometry revealed the basic plasma dynamics. The analyte fraction deposited by decay reactions of free atoms at inner surface of the hydride atomizers was quantified by leaching experiments with ICP-MS detection while their morphology and composition were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively.

A detailed insight into the mechanisms of hydride atomization and preconcentration in ambient plasmas has been reached. The results found by these techniques are in perfect agreement with the observations made by AAS. Owing to a comprehensive approach based on a combination of advanced spectrometric methods, further improvements in the performance of DBD and APGD hydride atomizers will be feasible.

Acknowledgements

Financial support from the Czech Science Foundation (23-05974K), Institute of Analytical Chemistry (RVO: 68081715) and MŠMT ČR (LM2023039) is gratefully acknowledged.