The Application of Ultrafast Laser Pulses to Laser Desorption Mass Spectrometry

Ultrashort femtosecond laser pulses display exceptional performance for the selective ablation of materials, includes metals, semiconductors, and biological tissues. They do not damage the remaining unablated portion of a sample, which permits the possibility of depth profiling by repeat sampling at the same location. With sufficiently micro-focused fs laser pulse length beam, high lateral resolution mass spectrometry imaging is possible, while sample damage may degrade ultimate lateral resolution in some other methods. Combining imaging and depth profiling could ultimately leads to tomographical mass spectrometry or 3D imaging MS. Laser postionization, a “soft” ionization method, was combined with ultrafast laser desorption for enhanced molecular analysis. A customized femtosecond laser desorption/ablation postionization time-of-flight mass spectrometer was designed and built. The construction and performance of both phases including the VUV source are detailed. Instrument control software was written to operate this instrument, and many automated experiments were successfully demonstrated by this software. Elemental and molecular analysis was carried out on the instrument and demonstrated exceptional performance for fs laser pulse sampling of small areas. Studies demonstrated the imaging and depth profiling capability of fs-LDPI on metals, semiconductors and intact biofilm tissues. Attempts were made to reach the limit of lateral resolution of imaging by fs-LDPI-MS. The results showed similar lateral resolution of <2 μm for both fs 800 nm and 400 nm desorption beams. To improve the repetition rate for high speed imaging application, an alternative LDPI scheme was designed and constructed. The fs 800 beam was tripled to 267 nm and delivered into the ion source as an ionization laser, while a ns 349 nm pulse laser was used for desorption. Preliminary data showed certain intact molecular ions can be detected. Fragmentation tendency was measured against various ionization laser pulse energies and photoionization time delays

Biplots of a canonical correlation analysis (CCorA) of the predicted abundances (<i>left</i>) and phi coefficients (<i>right</i>) of abundant soil fauna.

<p>Codes for habitats: 1 –ASa; 2 –ASb; 3 –BSa; 4 –BSb; 5 –CSc; 6 –CSd; 7 –DSc; 8 –DSd.</p

Phi coefficient of association between a species and a group of sites.

<p>Phi coefficient of association between a species and a group of sites.</p

Relationships between predicted abundances and phi coefficients of soil fauna.

<p>Codes for habitats: 1 –ASa; 2 –ASb; 3 –BSa; 4 –BSb; 5 –CSc; 6 –CSd; 7 –DSc; 8 –DSd.</p

The partial RDA diagram showing the relationships between soil fauna and soil chemical variables.

<p>The partial RDA diagram showing the relationships between soil fauna and soil chemical variables.</p

Relationships between phi coefficients and soil chemical variables.

<p>Codes for habitats: 1 –ASa; 2 –ASb; 3 –BSa; 4 –BSb; 5 –CSc; 6 –CSd; 7 –DSc; 8 –DSd.</p

Experimental design sketch for this study.

<p>Sa, soils originating from forest A; Sb, soils originating from forest B; Sc, soils originating from forest C; Sd, soils originating from forest D.</p

Internal Energy of Thermometer Ions Formed by Femtosecond Laser Desorption: Implications for Mass Spectrometric Imaging

Mass spectrometry (MS) imaging of biological samples would greatly benefit from improved lateral resolution and depth profiling which may be possible with femtosecond (fs), near-IR laser desorption microprobes to form ions directly in laser desorption ionization (fs-LDI) or via vacuum ultraviolet postionization (fs-LDPI). The use of fs-LDI-MS or fs-LDPI-MS for imaging, however, requires a determination of the magnitude of internal energy imparted by the ultrashort desorption pulse because the amount of internal energy determines the extent of ion fragmentation, and extensive fragmentation degrades MS imaging by complicating the mass spectra. This paper estimates internal energies imparted to 4-chlorobenzylpyridinium (CBP) thermometer ions desorbed from both simulated bacterial biofilms and bovine eye lens tissue samples by ∼75 fs, 800 nm laser pulses. Both direct ions and photoions formed by 10.5 eV single photon ionization of desorbed neutrals are analyzed by time-of-flight MS. Survival yields (SYs) of CBP varied from 0.2 to 0.8 and depended upon desorption laser fluence, overlap between desorption laser pulses, and whether direct ions or photoions were detected. SYs for photoions additionally depended on time delay between desorption and photoionization laser pulses, with the highest SYs seen at longer delay times. CBP internal energies were estimated using previously published computational results and compared with those from several other common MS imaging ion sources. The results are discussed in terms of their implications for MS imaging by fs-LDI and fs-LDPI

Transportation and Transformation of Legacy Pesticides, Currently Used Pesticides, and Degradation Products: From Corn Planting to Corn Flour Processing

Pesticide residues in food are a critical issue affecting food safety. The pesticide contaminants in food include currently used, legacy pesticides, and degradation products. Thus, this study analyzed the effects of planting and processing on the transfer and degradation of pesticide residues in corn. Specifically, we studied the transportation and transformation of 26 organochlorine pesticides (OCPs), 6 currently used pesticides, and 2 degradation products throughout corn planting and flour processing. For the currently used pesticide, diquat applied in this study did not significantly affect its concentration in soils. Different from this, λ-cyhalothrin application increased its concentration in soils. Therein, λ-cyhalothrin degraded to 3-PBA in a short time, and 3-PBA degraded faster than λ-cyhalothrin. The concentrations of legacy, currently used pesticides, and degradation products were higher in bran than in corn flour, indicating that the outer portions of corn kernels accumulated more pesticides. However, the results for λ-cyhalothrin were the opposite, indicating that the surrounding of bran is more favorable for degrading λ-cyhalothrin. The short- and long-term risks of consumer exposure to these pesticide residues via corn consumption are relatively insignificant based on the implementation time and dose in this study

Facile Synthesis of H₂O₂‑Cleavable Poly(ester-amide)s by Passerini Multicomponent Polymerization

We report the straightforward synthesis of two types of H₂O₂-cleavable poly­(ester-amide)­s (<b>P1</b> and <b>P2</b>) via the Passerini multicomponent polymerization (P-MCP) of 4-formylbenzeneboronic acid pinacol ester with 1,6-diisocyanohexane and 1,6-hexanedioic acid or a polyethylene glycol (PEG) dicarboxylic acid. The H₂O₂-cleavable phenylboronic acid ester was integrated into the polymer backbone by the in situ formed benzyl ester bond. GPC and ¹H NMR confirmed the complete H₂O₂-triggered degradation of these polymers in aqueous medium by a mechanism of sequential oxidation of phenylboronic acid ester and self-immolative elimination. Compared with the hydrophobic polymer <b>P1</b>, the PEG-based water-soluble polymer <b>P2</b> degraded much faster even at a lower H₂O₂ concentration. Cytocompatible nanoparticles of polymer <b>P1</b> loaded with fluorescent Nile red were fabricated, and controlled release of Nile red in response to H₂O₂ was achieved, thus, demonstrating the utility of these polymers as potential H₂O₂-responsive delivery vehicles