The 20 primary phenyllhlohydantoln (PTH)-amlno acids can be detected and quantitated by tlme-of-flight (TOF) mass spectrometry using a two-step laser methodology. First a C02 laser pulse desorbs the PTH-amlno acid or a mixture thereof prepared as a thin film on the Inside wall of a rotating glass cup. The latter Is part of the first electrode of the TOF apparatus. The desorption process Is demonstrated to be essentially complete In the laser spot area. After a suitable time delay, a second UV laser pulse (266 nm) causes 1 + 1 resonance-enhanced multiphoton Ionization (REMPI) of the neutral cloud of desorbed molecules. The mass spectra obtained are dominated by the parent Ion peak In almost all cases.Knowledge of the velocity distribution permits flux measurement. The Ion signal Is linear In PTH-amlno acid concentration In the range of plcomoles to nanomoles. This Is the first demonstration of quantitative analysis of molecules by laser desorptlon/multlphoton Ionization.
Articles you may be interested inOn the mechanism of C60 thin film laserinduced desorption Discharge flowtube studies of O(3 P)+N2H4 reaction: The rate coefficient values over the temperature range 252-423 K and the OH(X 2Π) product yield at 298 K Resonant laser ionization of thorium using continuous thermal heating and pulsed laser evaporation AIP Conf.Laser desorption of aniline-d7 from a single-crystal surface ͑0001͒ of sapphire ͑Al 2 O 3 ͒ at a heating rate on the order of 10 8 K/s was studied using pulsed infrared laser radiation for desorption and resonance enhanced multiphoton ionization ͑REMPI͒ for detection of the desorbed aniline molecules. On the basis of single-vibronic-level fluorescence ͑SVLF͒ spectra we unambigiously assign the 10b transition. REMPI spectroscopy provides vibrational temperatures and therefore describes the internal energy distribution, whereas the time-of-flight ͑TOF͒ profiles provide translational temperatures. All results are consistent with a thermal mechanism for desorption, i.e., pulsed heating of the sapphire surface on the nanosecond time scale leads to thermal desorption and rapid thermalization of the escaping molecules.
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 N2+, N3+, and N4+, it is found for spectra containing only N4+ 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 N2+, N3+, and N4+, 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 the square root of 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.
Bradbury-Nielson gates for the modulation of beams of charged particles, particularly ion beams in mass spectrometry, have been produced with an adjustable wire spacing down to 0.075 mm. The gates are robust, they can be fabricated in less than 3 h, and the method of production is reproducible. In time-of-flight mass spectrometers, fine wire spacing leads to improvements in mass resolution and modulation rates. Gates that were produced using this new method have been installed in a Hadamard transform time-of-flight mass spectrometer in order to demonstrate their utility.
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