Laser sample pulsing

Both LIBS and LA-ICP-MS offer spatial and depth resolution. A single laser shot provides an instantaneous measurement of approximately several nanograms to micrograms of sample. For spatial analysis, the laser is focused to several tens of microns and scanned across a surface. For depth analysis (including inclusions), the laser is pulsed repetitively at a single location to drill a channel into the sample (Fig. 5). 

The initiation system consists of a nitrogen laser and the necessary optics to lead the beam to the sample cell. The laser emits pulses at 337.1 nm with 800 ps duration, with a typical repetition rate of less than 5 Hz. The optical components, aligned between the laser and the calorimetric cell, consist of an iris (I), a support for neutral density filters (F), and a collimating lens (L). The iris is used to cut out most of the laser output and allow only a thin cylinder of light to pass through its aperture, set to 2 mm. The laser energy that reaches the cell is further... 

This work demonstrated a number of new results and opportunities for ultrafast XAS 1) it is possible to work with highly dilute solutions in transmission mode without dramatic loss of signal-to-noise ratio. This is very promising as one can envision the study of samples, for which large concentrations are impossible to reach. Biologically relevant samples are usually investigated in solutions with up to 1 mmol/1 concentration, and we therefore can envision such studies on the ultrafast time scales in the near future. 2) It is possible to scan the time delay between the laser pump pulse and the x-ray probe pulse, and therefore follow the evolution of the system from the start. 3) It also demonstrated the operation of an optical-x-ray cross-correlator (Fig. 6.b). The time resolution is not a limiting factor and the experiments are feasible with sources of shorter x-ray pulses, provided the flux is not too low. 

Figure 8.1 Schematic diagram of a spectrographic ps flash photolysis apparatus. If laser flash pulse C, sample cell S, continuum pulse generator g, diffraction grating P, photographic plate or diode array...

Figure 2. Transient Difference Spectrum Observed from a Sample of TMP at Two Delays Following the Laser Excitation Pulse.

A laser beam serves as the source of desorption and ionization. Many different types of laser light have been studied for MALDI-TOF-MS. The most used lasers include pulsed N2 laser with a wavelength of 337 nm and Nd-YAG solid-state laser with a wavelength of 355 nm. The ideal laser should deliver an efficient and controllable quantity of energy to the samples, and in order to avoid thermal decomposition this energy must be transferred quickly. The samples have absorption of the laser energy radiation and ionization. 

The speed of MALDI analysis depends on the laser pulse rate. With the recent introduction of 200-Hz lasers, samples can be analyzed ten times faster than before. This development is especially advantageous for offline liquid chromatography (LC)/MALDI applications (Ericson et al., 2003). MALDI mass analysis is performed considerably faster than LC separation, allowing for chromatographic... 

Transient spectroscopy experiments were performed with a pump-probe spectrometer [7] based on a home-made original femtosecond Ti saphire pulsed oscillator and a regenerative amplifier system operated at 10 Hz repetition rate. The Tirsaphire master oscillator was synchronously pumped with doubled output of feedback controlled mode-locked picosecond pulsed Nd YAG laser. The pulse width and energy of Ti saphire system after the amplifier were ca. 150 fs and 0.5 mJ, respectively, tunable over the spectral range of 760-820 nm. The fundamental output of the Ti saphire system (790 nm output wavelength was set for present study) splitted into two beams in the ratio 1 4. The more intense beam passed through a controlled delay line and was utilized for sample... 

The lack of homogeneity of the sample is a major source of variance. The laser samples an average area of 1 mm% which is not sampled uniformly as different parts are exposed to different temperatures. A routine measurement typically involves about 100 laser pulses, so the examined area is in the region of 1 cm-. Inhomogeneities influence laser absorption, the mass ablated and the plasma conditions. On the other hand, there can also be substantial inhomogeneities in the standard reference samples that will pose problems with analyses. These problems become even more severe if the laser is focused more tightly in order to sample smaller amounts of material. 

Figure 3. Laser-desorption time-of-flight mass spectra of molecular carbon samples, (a, left) Pure Cm and Cw samples. The ions are produced by SO mJ pulses (over a I-mm area) of 266 nm radiation. Small peaks at lower masses are due to fragmentation occurring during desorption, (b, center) Enriched samples of the larger fullerenes. (c, right) Effect of desorbing laser pulse fluence on the mass spectrum of pure samples (pulse energies indicated in pJ per pulse).

The heat conductance through the sample and in the plasma is responsible for the fact that with the Nd YAG lasers available today, the crater diameters are still much wider than the values determined by the diffraction limitations. When using conventional lasers with pulses in the ns and ps range the plasma shields the radiation, whereas with the femtosecond lasers that are now available a free expanding plasma is obtained, where the heating of the plasma appears to be less supplemented by the laser radiation. This leads to less fractionated volatilization of the solid sample and differences in crater shape, which need to be investigated further [229]. 

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