Pulsed laser spectrometers

Bilski, P., Chignell, C.F. Optimization of a pulse laser spectrometer Iot the measurement of the kinetics of singlet oxygen O2 (A g) decay in solution. J. Biochcan. Biophys. Methods 33, 73-80 (1996)... 

O Keefe A and Deacon DAG 1988 Cavity ring-down optical spectrometer for absorption-measurements using pulsed laser sources Rev. Sol. Instrum. 59 2544-51... 

Isotope shifts for most elements are small in comparison with the bandwidth of the pulsed lasers used in resonance ionization experiments, and thus all the isotopes of the analyte will be essentially resonant with the laser. In this case, isotopic analysis is achieved with a mass spectrometer. Time-of flight mass spectrometers are especially well-suited for isotopic analysis of ions produced by pulsed resonance ionization lasers, because all the ions are detected on each pulse. 

When a pulsed laser is used, ions are only produced for the duration of the pulse, i.e. they are not produced continuously and the mass spectrometer used must be capable of producing a mass spectrum from these pulses of ions. As discussed below in Section 3.3.4, the time-of-flight (ToF) mass analyser is the most appropriate for this purpose and has the added advantage of being able to measure very high m/z ratios. Indeed, the recent dramatic developments in the performance of the ToF mass analyser have largely been occasioned by the requirement to produce useful spectra from MALDI. 

Figure 4.16. Experimental setting of the combined femtosecond pulsed laser and step scan IR spectrometer (left) and modifications of the infrared interferogram after the laser pulse (right) [187].

O Keefe, A., and D. A. G. Deacon, Cavity Ring-Down Optical Spectrometer for Absorption Measurements Using Pulsed Laser Sources, Rev. Sci. Instrum, 59, 2544-2551 (1988). 

Figure 19.1. Schematic diagram of a general pump-probe-detect laser spectrometer suitable for picosecond electronic absorption, infrared (IR) absorption, Raman, optical calorimetry, and dichroism measurements. For picosecond fluorescence—a pump-detect method, no probe pulse needs to be generated.

The MALDI procedure has been used recently in several variations to determine the molecular weight of large protein molecules—up to several hundred kDa. The combination of a pulsed laser beam and a time-of-flight mass spectrometer (Section 2.2.5.) is particularly effective. 

Experimentally, information about the adsorption and desorption rates is obtained with the help of programmed desorption. One procedure is flash desorption A surface is instantaneously heated up (normally in vacuum) and we measure the temporal desorption of material, for instance with a mass spectrometer. Heating is usually done with a laser pulse (PLID, pulsed laser induced thermal desorption). 

Another interface commonly used for connecting HPLC to a mass spectrometer is not a true in-line interface. It is a robotically controlled spotter plate system for collecting samples from the HPLC to be injected into the MALDI time-of-flight laser ionization mass spectrometer for analyzing proteins and large peptides. The effluent sample dropped in the plate well is mixed with an ionization matrix already present, solvent and volatile reagents are evaporated, and the plate is then placed into the injector target and blasted with a pulsed laser to volatilize and ionize sample into the atmosphere of the interface where it can be drawn into the mass spectrometer. 

Fig. 7. Block diagram of laser-induced breakdown spectroscopy experimental setup (A) pulsed laser, (B) focusing optics, (C) microplasma, (D) collection optics, (E) spectrometer, and (F) data analyzer.

Laser Desorption Ionization. A pulsed laser beam can be used to ionize samples for mass spectrometry. Because this method of ionization is pulsed, it must be used with either a time of flight or a Fourier transform mass spectrometer (Section 1.4.5). Two types of lasers have found widespread use A COz laser, which emits radiation in the far infrared region, and a frequency-quadrupled neodymium/yttriumaluminum-garnet (Nd/YAG) laser, which emits radiation in the UV region at 266 nm. Without matrix assistance, the method is limited to low molecular weight molecules (<2 kDa). 

Evaporation of amorphous, metallic, or monoclinic Se by heating with a pulsed laser (107 W/cm2, 800 p.sec) also produces ions of type Se + (n = 1-9), Se5+ being the most abundant one. The analysis was carried out, using a time-of-flight mass spectrometer (58, 67). 

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