Sample Surface Height Research Articles

Automatic z-Axis Correction for IR-MALDESI Mass Spectrometry Imaging of Uneven Surfaces.

Two-dimensional mass spectrometry imaging (2D MSI) experiments mainly involve samples with a flat surface and constant thickness, but some samples are challenging to section due to the texture and topography. Herein, we present an MSI method that automatically corrects for discernible height differences across surfaces during imaging experiments. A chromatic confocal sensor was incorporated into the infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) system to measure the sample surface height at the location of each analytical scan. The height profile is subsequently used for adjusting the z-axis position of the sample during MSI data acquisition. We evaluated this method using a tilted mouse liver section and an unsectioned Prilosec tablet due to their exterior quasi-homogeneity and height differences of approximately ∼250 μm. MSI with automatic z-axis correction showed consistent ablated spot sizes and shapes, revealing the measured ion spatial distribution across a mouse liver section and a Prilosec tablet. Conversely, irregular spots and reduced signals with large variability were observed when no z-axis correction was applied.

Study on a Rapid Imaging Method for Scanning Ion Conductance Microscopy Using a Double-Barreled Theta Pipette.

Scanning ion conductance microscopy (SICM) is a new noncontact, high-resolution scanning probe microscopy technique, which has become increasingly popular in recent years. The hopping mode-currently the most widely used scanning mode-can be used for imaging samples with complicated surface topographies. However, its slow scanning rate seriously restricts its broader application. This paper proposes a fast imaging control mode using a double-barreled theta pipette as the probe, which effectively increases the imaging rate. In this mode, sample surface height information is obtained when the double-barreled theta pipette approaches the sample in a two-step downward process. The ion current sum of two barrels and ion current of one barrel are used as feedback signals to approach the sample until the feedback signals decrease to the set threshold, respectively, thereby obtaining the height of the imaging point. First, this work used COMSOL to establish an SICM model and perform simulation analysis. The simulation results verified the proposed method's feasibility. Second, a scanning time mathematical model was established. The results revealed that the new method is superior to the traditional method in terms of imaging rate. Finally, experiments were performed on poly(dimethylsiloxane) (PDMS) samples using the two imaging modes described above. The results demonstrated that the new scanning mode could significantly improve the imaging rate of SICM without a loss in imaging quality and stability.

Feasibility of Quantitative Analysis of Magnesium and Calcium in Edible Salts Using a Simple Laser-Induced Breakdown Spectroscopy Device.

Feasibility of a simple laser-induced breakdown spectroscopy (LIBS) device has been investigated for the analysis of Mg and Ca in edible salts. The LIBS spectrometer was assembled with a compact low-power diode-pumped solid-state laser (DPSSL) and a non-gated low-resolution handheld spectrometer. A simple sampling process was employed for on-site application. A piece of filter paper was dipped in the aqueous solution of a sample salt and dried for analysis using LIBS. Maintaining the sample surface height at the optimum position was critical to generate plasmas persistently due to the low pulse energy of the DPSSL. The varying height of the filter paper surface was monitored and compensated, while the sample stage was translated to collect spectra from different positions. The variation of line intensities of Mg and Ca could be attributed to the inhomogeneous distribution of dry residues. To correct this, the peak that consists of the Na(I) and C(II) lines at 568 nm was employed as a reference signal for intensity normalization of the analyte Mg(II) and Ca(II) lines. For edible salt products, the normalized Mg(II) and Ca(II) line intensities could be well correlated with the concentrations of Mg and Ca determined using inductively coupled plasma optical emission spectroscopy. Our results indicate that a simple LIBS device in combination with the simple sampling method is promising as an on-site salt quality assessment methodology.

Influence of optical aberrations on the peak extraction in confocal microscopy

The peak localization of axial response signal (ARS) is essential for height extraction in confocal microscopy. In previous evaluations about peak localization, an aberration-free confocal system was generally analyzed by taking the comprehensive effects of random noise and sample surface height into consideration. In this paper, the combined influence of optical aberration, random noise and sample surface height on peak extraction are evaluated using Monte Carlo simulations. At first, a certain amount of optical aberration is added into the ARS model. Then the systematic peak extraction error and standard deviation are analyzed. In the end, comparisons are provided between an aberration-free and an aberrated confocal system. Our simulations demonstrate that optical aberration is a critical influencing factor in peak extraction computation in terms of both accuracy and precision. More specifically, aberration-induced asymmetry of ARS that exceeds certain limits will result in a tremendous increase in systematic peak extraction error and standard deviation. Our developed metric “degree of asymmetry” can offer a reference for determining the aberration tolerance in confocal metrology and acting as guides for further performance enhancement of confocal microscopy.

Influence of sample surface height for evaluation of peak extraction algorithms in confocal microscopy.

The axial resolution of confocal microscopy is not only dependent on optical characteristics but also on the utilized peak extraction algorithms. In previous evaluations of peak extraction algorithms, sample surface height is generally assumed to be zero, and only sampling-noise-induced peak extraction uncertainty was analyzed. Here we propose a sample surface-height-dependent (SHD) evaluation model that takes the combined considerations of sample surface height and noise for comparisons of algorithms' performances. Monte Carlo simulations were first conducted on the centroid algorithm and several nonlinear fitting algorithms such as the parabola fitting algorithm, Gaussian fitting algorithm, and sinc2 fitting algorithm. Subsequently, the evaluation indicators, including mean peak extraction error and mean uncertainty were suggested for the algorithms' performance ranking. Finally, experimental verifications of the SHD model were carried out using a fiber-based chromatic confocal system. From our simulations and experiments, we demonstrate that sample surface height is a critical influencing factor in peak extraction computation in terms of both the accuracy and standard deviations. Compared to the conventional standard uncertainty evaluation model, our SHD model can provide a more comprehensive characterization of peak extraction algorithms' performance and offer a more flexible and consistent reference for algorithm selection.