Zhibin Yin

Elemental fractionation and matrix effects in laser sampling based spectrometry

The utilization of laser sampling technique in analytical spectrometry has long prevailed as it does not require sample pretreatment, large samples or cause contamination. However, it suffers from a series of defects such as elemental fractionation, matrix effect and shortage of matrix-matched reference materials for most samples of interest. To correct these undesired effects and achieve better analytical performance, it is vital to be conscious of when and how the above deflecting effects occur, to what extent the various parameters involved influence them, and what means can be applied to minimize them. The present review summarizes the research work dealing with elemental fractionation and matrix effects in laser sampling approaches. The review is arranged as follows: Various phenomena of these effects in laser sampling based spectrometry are presented in Section 2; the processes involved are discussed in Section 3; subsequently, the impact of laser parameters and ablation background gas is discussed in Section 4 and 5, respectively; several theoretical studies concerning element-/matrix-specific ablation behavior are briefly considered in Section 6; the means to ultimately minimize elemental fractionation and matrix effect are presented in Section 7; and artificial matrix matched/non-matrix matched analysis approaches are summarized in Section 8.

Pulsed microdischarge with inductively coupled plasma mass spectrometry for elemental analysis on solid metal samples.

Pulsed microdischarge employed as source for direct solid analysis was investigated in N2 environment at atmospheric pressure. Compared with direct current (DC) microdischarge, it exhibits advantages with respect to the ablation and emission of the sample. Comprehensive evidence, including voltage-current relationship, current density (j), and electron density (ne), suggests that pulsed microdischarge is in the arc regime while DC microdischarge belongs to glow. Capability in ablating metal samples demonstrates that pulsed microdischarge is a viable option for direct solid sampling because of the enhanced instantaneous energy. Using optical spectrometer, only common emission lines of N2 can be acquired in DC mode, whereas primary atomic and ionic lines of the sample are obtained in the case of pulsed mode. Calculations show a significant difference in N2 vibrational temperatures between DC and pulsed microdischarge. Combined with inductively coupled plasma mass spectrometry (ICPMS), pulsed microdischarge exhibits much better performances in calibration linearity and limits of detection (LOD) than those of DC discharge in direct analysis of samples of different matrices. To improve transmission efficiency, a mixture of Ar and N2 was employed as discharge gas as well as carrier gas in follow-up experiments, facilitating that LODs of most elements reached ng/g.

Comprehensive analysis of metalloporphyrins via high irradiance laser ionization time-of-flight mass spectrometry

High-irradiance laser ionization time-of-flight mass spectrometry (LI-TOFMS), an established technique for elemental determination, has been applied for the analysis of metalloporphyrins. Many porphyrins and their metal complexes, being organometallic compounds, are hard to dissolve in general organic solvents, hampering the wider application of traditional mass spectrometric techniques. With LI-TOFMS, an environmentally friendly analytical strategy has been demonstrated, which is capable of matrix- and solvent-free analysis of metalloporphyrins, with advantages including direct solid sampling, ease of implementation, and avoidance of sample pre-treatment. Moreover, information about elemental composition, fragments, and intact molecules can be obtained simultaneously using LI-TOFMS, hence expediting the identification of metalloporphyrins. A comparative study of LI-, laser desorption ionization (LDI-), matrix-assisted laser desorption ionization (MALDI-) and electrospray ionization (ESI-) mass spectrometry (MS) has also been conducted.

Thermal Diffusion Desorption for the Comprehensive Analysis of Organic Compounds

Comprehensive analysis of organic compounds is crucial yet challenging considering that information on elements, fragments, and molecules is unavailable simultaneously by current analytical techniques. Additionally, many compounds are insoluble or only dissolve in toxic solvents. A solvent- and matrix-free strategy has been developed which allows the organic compound analyzed in its original form. It utilizes thermal diffusion desorption with the solid analyte irradiated with high energy laser. It is capable of providing explicit elemental, fragmental, and molecular information simultaneously for a variety of organic compounds. Thermal diffusion desorption has many advantages compared to the electrospray and MALDI techniques. The protons that form the protonated molecular ions originate from the analyte itself. All the elements and fragments are also derived from the analyte itself, which provides abundant information and expedites the identification of organic compounds.

Depth profiling of nanometer thin layers by laser desorption and laser postionization time-of-flight mass spectrometry

A depth profiling technique has been developed and employed for ultra-thin layer analysis using a newly constructed laser desorption and laser postionization time-of-flight mass spectrometer (LD-LPI-TOFMS). This technique achieves the superiority of an extremely low average ablation rate down to ∼0.026 nm in depth per pulse for a series of Ni coated samples with varied thicknesses. Compared to high-cost SIMS apparatus, it offers an alternative strategy for nanometer thin-layer analysis. The integration of the LD-LPI source and TOFMS offers multi-element information on the constituents of each layer and substrate, as well as the trace impurities of sputtering targets. It contributes to comprehensively characterizing nanometer thin layers for the quality evaluation and process control of coatings. Additionally, an underlying capability of this method for the thickness determination of thin-layers was demonstrated. The investigations and results here indicate the potential of LD-LPI-TOFMS as a versatile tool among the available techniques in depth profiling of nanometer thin-layers, filling the gap in ultra-thin layer analysis for laser-based techniques.

Probing gas-phase interactions of peptides with “naked” metal ions

A novel strategy for the generation of metal–peptide complexes in the gas phase is proposed, which is of great value for probing the interactions of “naked” metal ions with peptides. “Naked” metal ions are generated from the metal target by laser ionization (LI) in open air, whereas gas-phase peptide ions are electrosprayed separately, facilitating the formation of gas-phase metal–peptide complexes. Compared to the conventional electrospray ionization (ESI) method, more control is offered for generating complexes, avoiding signal suppression and dilution effects induced by electrospraying solutions composed of metal salts and peptides. Higher stabilities of metal–peptide complexes can be obtained by a direct gas-phase reaction of peptides with “naked” metal ions devoid of counter-ions and surrounding solvent due to stronger noncovalent interactions, such as coulomb interactions and charge–dipole interactions. This approach leads a knowledge of the intrinsic properties of complexes and provides accurate gas-phase results in closer proximity to theoretical calculations irrespective of the solvent effect. The influence of the number and position of basic residues in peptides on the binding site of metal ions and CID fragmentation patterns of complexes is explored and discussed. Plausible mechanisms responsible for fragments remote from the initial binding sites of metal ions are proposed. Additionally, the diffusion model is introduced to expound on the high reaction yield of metal–peptide complexes and distribution evolution of metal ions, verified by both the calculated and experimental results.

Role of three-body recombination for charge reduction in MALDI process

Ions in Matrix-Assisted Laser Desorption/Ionization (MALDI) are predominantly singly charged for small analyte molecules. With the estimated high number density and low temperature of electrons, the three-body recombination mechanism is attractive and should be considered as an important cause for the charge reduction in the plume. Theoretical calculations indicate that the rate coefficient of the three-body recombination is about 50 times higher than that of the two-body recombination if the analyte molecule has insufficient degrees of freedom. Experimental results show that, for small analyte molecules, the ratio of AH2(2+)/AH(+) is close to the theoretical 5% value from the three-body recombination modeling and this ratio declines with the increasing electron and matrix molecule number density caused by greater laser irradiance. The ratio of [A + 2](+)/[A + 1](+) is higher than the theoretical isotopic value, and the excess [A + 2](+) could exclusively result from the three-body recombination collisions. Further evidence demonstrates that [A + 2](+)/[A + 1](+) increases with electron number density, which is in correspondence with the model. All of these theoretical and experimental results indicate that three-body recombination is an essential charge reduction mechanism for small molecules in the MALDI plume.

Microtrace Analysis of Rare Earth Element Residues in Femtogram Quantities by Laser Desorption and Laser Postionization Mass Spectrometry

A newly developed laser desorption and laser postionization time-of-flight mass spectrometer (LD-LPI-TOFMS) for the direct microtrace determination of rare earth elements (REEs) in residues has been presented. Benefiting from spatially and temporally separated processes between desorption and ionization, LD-LPI-TOFMS plays a dual role in alleviating the barriers of deteriorating spectral resolution at high irradiance, serious matrix effects and elemental fractionation effects at low irradiance. Compared with the conventional laser desorption/ionization (LDI) method, this technique offers unambiguous full-elemental determination of 15 REEs with more uniform relative sensitivity coefficients (RSCs) ranging from 0.5 to 2.5 for all REEs investigated, satisfying the semiquantitative analysis criteria. More importantly, a highly sensitive analysis of REEs with very little consumption was achieved by getting the utmost out of desorbed neutral atoms instead of increasing the amount of the sample, resulting in outstanding relative and absolute limits of detection (LODs and ALODs) of ∼ng/mL and ∼femtogram. The results presented here indicate that LD-LPI-TOFMS offers great potential in microtrace determination for elements in solution samples with minor sample preparation.

Direct and comprehensive analysis of dyes based on integrated molecular and structural information via laser desorption laser postionization mass spectrometry

Laser desorption laser postionization time-of-flight mass spectrometry (LDPI-TOFMS) was employed for direct analysis and determination of typical basic dyes. It was also used for the analysis and comprehensive understanding of complex materials such as blue ballpoint pen inks. Simultaneous emergences of fragmental and molecular information largely simplify and facilitate unambiguous identification of dyes via variable energy of 266nm postionization laser. More specifically, by optimizing postionization laser energy with the same energy of desorption laser, the structurally significant results show definite differences in the fragmentation patterns, which offer opportunities for discrimination of isomeric species with identical molecular weight. Moreover, relatively high spectra resolution can be acquired without the expense of sensitivity. In contrast to laser desorption/ionization mass spectrometry (LDI-MS), LDPI-MS simultaneously offers valuable molecular information about dyes in traces, solvents and additives about inks, thereby offering direct determination and comprehensive understanding of blue ballpoint inks and giving a high level of confidence to discriminate the complicated evidentiary samples. In addition, direct analysis of the inks not only allows the avoidance of the tedious sample preparation processes, significantly shortening the overall analysis time and improving throughput, but allows minimized sample consumption which is important for rare and precious samples.

Confirmatory surface analysis of equivocal documents with pigment-based gel inks via laser desorption laser postionization mass spectrometry imaging

AbstractLaser desorption laser postionization time-of-flight mass spectrometry (L2MS) was applied for unambiguous discrimination of pigment-based inks in blue, black, and red gel pens and molecular imaging of equivocal documents in a quasi-non-destructive way. In comparison to laser desorption mass spectrometry (LD-MS), additional discriminatory information on ink components is acquired uniquely, facilitating the distinct differentiation of various pigmented gel inks. More importantly, diversified images of additional characteristic ions achieved using L2MS offer reliable support to discriminate forged documents and decipher important hidden contents. Apart from minimized matrix effect and maximized ionization yield, direct and confirmatory identification of forged documents is achieved successfully without solvent or matrix involved, not only eliminating unwanted damage and contamination to the samples but significantly shortening the overall analysis time. In addition, L2MS is a minimally destructive approach with tiny analyte consumption. With these appealing qualities, L2MS imaging is poised to be a powerful tool for confirmatory surface analysis of complex pigment-based samples. Graphical AbstractWeight and see: Highly distinct and comprehensive images of counterfeit documents with blue-pigmented gel inks are achieved successfully, due to the high sensitivity and increased ion yield of laser desorption laser postionization time-of-flight mass spectrometry. The hidden important contents of the obliterated documents are visually deciphered with the help of the additional chemical information.