A Review of Advances in Measurement Techniques for Volatile Light Elements in Solid Samples

BRIEF REPORTAccurate in-solid quantification of volatile light elements, specifically hydrogen (H), carbon (C), nitrogen (N), oxygen (O), fluorine (F), sulfur (S), chlorine (Cl), and bromine (Br), underpins critical progress across geosciences, materials science, and biomedicine[1]. These elements exert first-order controls on planetary differentiation and volatile cycling, mineralization and alteration processes, and the mechanical degradation of advanced materials (e.g., hydrogen embrittlement). In biological systems, their distribution defines structure and function in both hard and soft tissues[2-4].　　In geological systems, small variations in light-element abundance record magma degassing histories and fluid–rock interactions. In engineered materials, trace hydrogen or oxygen at the micro-scale can catastrophically influence mechanical performance and long-term reliability. Despite their importance, reliable analysis of these elements at the micro-scale remains intrinsically difficult. This challenge arises from their low atomic numbers, high ionization energies, propensity to form molecular and cluster species, and high susceptibility to background contamination[5-7]. The evolution of analytical strategies is reviewed in this report and the innovative mechanisms of helium-assisted laser ablation ionization time-of-flight mass spectrometry (LAI-TOFMS) are highlighted.　　1. Established indirect analytical techniques　　1.1 Traditional thermal extraction methods　　Since the mid-20th century, indirect bulk analytical methods have served as the “gold standard” for accuracy. Techniques such as vacuum heating coupled with gas chromatography (GC), inert-gas fusion (IGF) with infrared or thermal conductivity detection, and Dumas combustion are widely adopted[21-22]. Inert-gas fusion, for example, remains the internationally certified reference method for determining H, O, and N in metals and alloys, governed by standards such as ASTM E1447[23-24]. While these methods offer high precision and traceability, they possess inherent limitations: they require relatively large sample masses (destroying the sample), involve complex multi-step procedures, and, most critically, provide only bulk-averaged data. They result in a complete loss of spatial information, rendering them unsuitable for studying heterogeneous materials, zoned minerals, or localized failure mechanisms.　　1.2 Wet chemical and electrochemical approaches　　Beyond thermal extraction, wet chemical techniques have been traditionally employed, particularly for halogens which are difficult to extract thermally. The ion selective electrode (ISE) method is widely used for fluorine determination in soils and geological samples[32]. While ISE can mitigate some matrix interferences found in spectral analysis, it typically requires laborious pre-treatment to solubilize the solid sample. Similarly, ion chromatography (IC) offers high precision for the simultaneous determination of anions like F− and Cl−[34]. However, studies indicate that IC applications are often limited by the complexity of the sample matrix, necessitating specialized column technologies or oxidation pre-treatments to protect the column hardware[35]. Like IGF, these methods are fundamentally destructive and incapable of in situ micro-analysis.　　2. Limitations of conventional direct analysis techniques　　2.1 Spectroscopic methods (XRF and LIBS)　　Direct spectroscopic techniques offer speed but face physical constraints for light elements. X-ray fluorescence (XRF) suffers from extremely low fluorescence yields for elements H–F due to the competitive Auger effect, and the resulting low-energy X-rays are strongly attenuated by air and detector windows[36]. Although partial least squares (PLS) models[37] and vacuum environments help, sensitivity remains poor.　　Laser-induced breakdown spectroscopy (LIBS) faces challenges with high excitation potentials. Since fluorine’s primary emission lines lie in the vacuum ultraviolet (VUV) region absorbed by air, researchers have developed indirect strategies. A notable innovation involves monitoring molecular bands rather than atomic lines. Tang et al.[45] successfully quantified fluorine in copper concentrates by detecting CaF molecular bands (529–531 nm) formed in the plasma, achieving a detection limit of 33 μg/g. Despite such advances, LIBS performance remains highly sensitive to ambient gas and surface morphology, making high-precision quantification without strictly matched standards difficult[47].　　2.2 Mass spectrometry challenges (SIMS, GDMS, LA-ICP-MS)　　Mass spectrometry theoretically offers higher sensitivity but struggles with interferences for light elements. Secondary ion mass spectrometry (SIMS) offers sub-micron resolution but is plagued by high background signals from residual vacuum gas (e.g., H2O, H2), degrading detection limits for hydrogen and oxygen[48]. Glow discharge mass spectrometry (GDMS) is effective for bulk purity but struggles with ionization efficiency for non-conductive solids.　　Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) is the standard for micro-analysis but fails significantly for fluorine. The ionization potential of fluorine (17.4 eV) is higher than that of the argon carrier gas, resulting in poor ionization. Moreover, severe spectral interferences, such as ArF+, obscure detection channels. To bypass this, researchers like Guo et al.[61] developed reaction cell methods to convert F into BaF+ ions for indirect quantification. While innovative, these solutions add significant hardware complexity and are not universally applicable.　　3. Helium-assisted laser ablation ionization TOFMS (LAI-TOFMS)　　3.1 Innovation in ionization and ablation mechanisms　　LAI-TOFMS represents a paradigm shift by fundamentally altering the ionization environment to overcome the limitations of vacuum-based laser ionization. The technique couples high-irradiance laser ablation with an orthogonal TOF analyzer while introducing a low-pressure helium buffer gas in the ion source[64]. This helium environment performs three critical physical functions simultaneously. First, through “collisional cooling”, helium atoms collide with ablated ions, significantly reducing their initial kinetic energy dispersion and focusing the ion beam to drastically improve mass resolution[67-68]. Second, the buffer gas promotes charge-transfer reactions that efficiently reduce multiply charged ions to singly charged species, simplifying the mass spectrum[65-66]. Third, these collisions assist in dissociating polyatomic clusters, thereby cleaning up the background spectrum[69].　　Furthermore, when this ionization mechanism is coupled with femtosecond lasers, the analytical quality is further enhanced. The ultra-short pulse duration restricts laser-matter interaction to a timeframe shorter than the thermal diffusion time[70-71]. This “athermal” ablation mechanism effectively suppresses the heat-affected zone and minimizes elemental fractionation[72-73], ensuring that the composition of the ablated plume is a true representation of the solid target.　　3.2 Analytical performance and strategic applications　　The unique combination of helium cooling and femtosecond ablation allows LAI-TOFMS to achieve analytical figures of merit that bridge the gap between bulk accuracy and micro-scale resolution. Quantitative limits of detection (LOD) can reach the ng/g level for specific elements[74]. In materials science applications, such as the analysis of titanium alloys, Hong et al.[16] demonstrated that LAI-TOFMS could quantify hydrogen with an LOD of 4 μg/g. Notably, the analysis achieved a relative error of less than 5% compared to the traditional inert-gas fusion method, while reducing the analysis time from hours to minutes.　　In the realm of biological imaging, the technique excels by preserving sample integrity. Ma et al.[76] utilized a cryogenic platform (Cryo-LAI-TOFMS) to map fluorine distribution in human teeth. The cryogenic conditions prevented the diffusion of volatile elements, enabling high-resolution imaging (20 μm resolution) that revealed distinct fluorine gradients between enamel and dentin. Similarly, in geological applications, the technique has been successfully applied to meteorites and deep-sea nodules, providing simultaneous detection of H, C, N, and O without the molecular interferences that typically hamper SIMS or standard LA-ICP-MS[12,14].　　4. Standardization challenges and future perspectives　　4.1 The reference material bottleneck　　Despite the technological leaps, a persistent challenge for LAI-TOFMS—and indeed for all direct solid-state analysis techniques—is the scarcity of suitable reference materials. For volatile light elements, creating stable, homogeneous standards at the micro-scale is exceptionally difficult due to their volatility and tendency to segregate at grain boundaries[78]. Most commercially available standards are certified for bulk composition, which may not represent the micro-volume sampled by a laser beam. This “micro-heterogeneity” leads to calibration errors when bulk standards are used for micro-analysis[79]. Consequently, many laboratories rely on in-house synthesized standards, which limits inter-laboratory data comparability and traceability.　　4.2 Directions for future development　　To fully mature as a routine analytical tool, future developments in LAI-TOFMS must address several key areas. First, there is an urgent need for the development and certification of micro-homogeneous reference materials specifically designed for light elements in diverse matrices. Second, data processing workflows must evolve to incorporate machine learning algorithms capable of automated spectral deconvolution and correction for residual matrix effects in massive imaging datasets. Finally, the hybridization of LAI-TOFMS with other modalities, such as electron microscopy, will allow for the correlation of elemental maps with crystallographic features, providing a more comprehensive understanding of material properties.　　5. Conclusion　　While traditional indirect methods like inert-gas fusion and wet chemistry remain essential for bulk certification, they cannot meet the growing demand for spatially resolved analysis. Direct techniques like XRF and LIBS have made strides but struggle with the fundamental physics of light element detection, while LA-ICP-MS remains limited by plasma chemistry issues for elements like fluorine. In this landscape, LAI-TOFMS emerges as a powerful solution. By leveraging helium-assisted collisional cooling and femtosecond laser ablation, it effectively suppresses the interferences and fractionation that have historically plagued light element analysis. With detection limits approaching ng/g and resolution capable of cellular-level imaging, LAI-TOFMS is poised to become a cornerstone technology for in situ investigation of volatiles, provided the community can collectively overcome the current limitations in standardization.