Effect of sequence length, sequence frequency, and data acquisition rate on the performance of a Hadamard transform time-of-flight mass spectrometer

Various factors influencing the performance of a Hadamard transform time-of-flight mass spectrometer (HT-TOFMS) have been investigated. Using a nitrogen corona discharge to produce an ion stream of N ₂ ⁺ , N ₃ ⁺ , and N ₄ ⁺ , it is found for spectra containing only N ₄ ⁺ that the signal-to-noise ratio (SNR) closely approaches the value calculated from the ion background by assuming that the ion background follows a Poisson distribution. In contrast, for a more intense beam containing N ₂ ⁺ , N ₃ ⁺ , and N ₄ ⁺ , the SNR is less than its theoretical value because of the appearance of discrete spikes in the mass spectrum caused by deviations in the actual modulation sequence from the ideal one. These spikes can be reduced, however, by decreasing the modulation voltage. Under these optimized conditions, the pseudo-random sequence length is varied to understand how it alters SNR, mass resolution, and scan speed. When the length of the pseudo-random sequence is doubled, the SNR increases by √2 while the time necessary to record a mass spectrum also doubles. Mass resolution can be varied between 500 and 1200 at m/z = 609 as the sequence length, modulation speed (10 MHz, 25 MHz), and acquisition rate (up to 50 MHz) are changed. Scan speeds of 6000 passes per s can be obtained using a sequence containing 4095 elements modulated at 25 MHz. The capability to tailor the HT-TOFMS to increase the scan speed and resolution with a constant 50% duty cycle makes the technique extremely appealing as a mass analyzer for measuring rapid changes in the composition of an ion stream.     

Remote,real-time,on-line monitoring of high-temperature samples by noninvasive open-path laser plasma spectrometry

A remote detection system based on optical emission spectrometry of laser-induced plasmas has been developed to record spectra in the visible region from samples placed at remote distances from the excitation source. Unlike from fiber-optic-based systems, light collection is performed remotely as well. Laboratory-scale experiments have shown the possibility of performing real-time analysis of samples placed remotely. The application in the noninvasive analysis of hot samples (at 1,200 degrees C) has been demonstrated as well, allowing the dynamic monitoring of selective elemental migration.     

Microsecond relaxation processes in shear and extensional flows of weakly elastic polymer solutions

In this paper, we introduce an experimental protocol to reliably determine extensional relaxation times from capillary thinning experiments of weakly elastic dilute polymer solutions. Relaxation times for polystyrene in diethyl phthalate solutions as low as 80?μ s are reported: the lowest relaxation times in uniaxial extensional flows that have been assessed so far. These data are compared to the linear viscoelastic relaxation times that are obtained from fitting the Zimm spectrum to high frequency oscillatory squeeze flow data measured with a piezo-axial vibrator (PAV). This comparison demonstrates that the extensional relaxation time reduced by the Zimm time, λ _ext/λ _z, is not solely a function of the reduced concentration c/c*, as is commonly stated in the literature: an additional dependence on the molecular weight is observed.     

Potassium Poly(Heptazine Imide): Transition Metal‐Free Solid‐State Triplet Sensitizer in Cascade Energy Transfer and [3+2]‐cycloadditions

Polymeric carbon nitride materials have been used in numerous light‐to‐energy conversion applications ranging from photocatalysis to optoelectronics. For a new application and modelling, we first refined the crystal structure of potassium poly(heptazine imide) (K‐PHI)—a benchmark carbon nitride material in photocatalysis—by means of X‐ray powder diffraction and transmission electron microscopy. Using the crystal structure of K‐PHI, periodic DFT calculations were performed to calculate the density‐of‐states (DOS) and localize intra band states (IBS). IBS were found to be responsible for the enhanced K‐PHI absorption in the near IR region, to serve as electron traps, and to be useful in energy transfer reactions. Once excited with visible light, carbon nitrides, in addition to the direct recombination, can also undergo singlet–triplet intersystem crossing. We utilized the K‐PHI centered triplet excited states to trigger a cascade of energy transfer reactions and, in turn, to sensitize, for example, singlet oxygen (¹O₂) as a starting point to synthesis up to 25 different N‐rich heterocycles.