Time-resolved spectrometry

The fact that conventional continuously scanning FT-IR spectrometers can measure an interferogram in a time of 1 second or less makes them excellent tools to follow transient or time-varying processes. Let us first examine some of the instrumental aspects of FT-IR spectrometers used for time-resolved spectrometry. 

Fourier Transform Infrared Spectrometry, Second Edition, by Peter R. Griffiths and James A. de Haseth Copyright 2007 John Wiley Sons, Inc. 

In practice, whether or not the spectra can be measured beneficially at high scan speeds is often determined by the duty-cycle efficiency of the interferometer, which is defined as 

The response time of the detector should also be borne in mind when the time resolution is to be reduced well below 1 s. If the firequency of the HeNe interferogram is raised much above 10 kHz, the response of the DTGS detector is too slow and a faster detector must be used. In the mid-infrared, this is not a major problem, since MCT detectors operate optimally for modulation frequencies above 1 kHz. For near-infrared measurements, however, while InSb has a very fast response time, other quantum detectors, such as InGaAs, cannot be used at data acquisition speeds much above 5 kHz (see Section 18.2.5). 

TIME-RESOLVED MEASUREMENTS USING STEP-SCAN INTERFEROMETERS 

---

Similar to time-resolved spectrometry through which the transient molecular rection can be observed optically, the dynamics of modified or unmodified BLMs, or reconstituted bioreaction on a BLM, can be studied electrically by CV technology where the capacitance, resistance, membrane potential, and current peak are the fundamental parameters in determining the static or the dynamic change in the BLM system. Among these studies, the formation of a s-BLM is valuable in analyzing BLM mechanics and further electrochemical reactions thereon [34,35]. 

During the last two decades, there has been an enormous increase in the use of photophysical methods in supra-molecular chemistry. Until recently, photophysical methods, such as transient spectrometry and time-resolved fluorescence spectrometry, were primarily research tools in the arenas of photokinetics of small molecules, materials physics, and biophysics. This situation changed dramatically with the introduction of commercial, user-friendly electro-optical components such as charge-coupled detector (ED)-based spectrometers, solid-state pulsed lasers, and other instrumentation necessary for time-resolved measurements. As a result, time-resolved spectrometry became more available to the community of supramolecular chemists, who now reached the level of sophistication that can benefit from the new horizons offered. 

There are experiments for which rapid-scan interferometers are not well suited. These experiments include depth profiling by photoacoustic spectrometry (Section 20.3), hyperspectral imaging (Section 14.5), fast time-resolved spectrometry... 

There are several types of measurements for which standard rapid-scanning interferometers may be inappropriate. These include hyperspectral imaging (Section 14.5), high-speed time-resolved spectrometry (Section 19.2), photoacoustic spectroscopy (Section 20.3), and sample modulation spectroscopy (Chapter 21). For these measurements it is necessary to hold the optical path difference constant while a measurement is made, after which the OPD is rapidly advanced to the next sampling position and then held constant once again for the next measurement. This process is repeated until all the data needed to obtain the interferogram are acquired. Such interferometers are called step-scan interferometers. 

Most ion-molecule techniques study reactivity at pressures below 1000 Pa however, several techniques now exist for studying reactions above this pressure range. These include time-resolved, atmospheric-pressure, mass spectrometry optical spectroscopy in a pulsed discharge ion-mobility spectrometry [108] and the turbulent flow reactor [109]. 

Kamada, K., Matsunaga, K., Yoshino, A. and Ohta, K. (2003) Two-photon-absorption-induced accumulated thermal effect on femtosecond Z-scan experiments studied with time-resolved thermal-lens spectrometry and its simulation. J. Opt. Soc. Am. B, 20, 529-537. 

The application of semiconductor lasers to a broad range of areas in spectrometry has recently been reviewed by Imasaka. 67, 68) Topics covered include photoacoustic, absorption, and thermal lens, as well as steady-state and time-resolved fluorescence. Patonay et al. have reviewed the application of diode lasers to analytical chemistry.(69) The performance of several commercially available laser diodes for fluorimetry has recently been compared. 70 ... 

Instrnments combining several analyzers in sequential order are very common. This combination allows mass spectrometry and mass spectrometry experiments (MS/MS) to be carried out. Modern MS/MS includes many different experiments designed to generate substructural information or to qnantitate componnds at trace levels. A triple quadru-pole mass spectrometer allows one to obtain a daughter ion mass spec-trnm resnlting from the decomposition of a parent ion selected in the first qnadrnpole. The MS/MS experiments using an FTICR or ion trap, however, are carried ont in a time-resolved manner rather than by spatial resolntion. 

Note The acronyms used here are OSPED (optical spectroscopy in a pulsed electrical discharge), FAMS (flowing afterglow mass spectrometry), SIFT (selected ion flow tube), TRAPI (time-resolved atmospheric pressure ionization mass spectrometry), PHPMS (pulsed high-pressure ionization mass spectrometry), ICRMS (ion cyclotron resonance mass spectrometry), and ADO (averaged dipole orientation collision rate theory). 

Biomaterials. Recently, an improved experimental setup for time-resolved insource pyrolysis /py./ field ionization /f.i./ mass spectrometry /m.s./ has been described (10) and examples of its application to studying various biomaterials have been shown. There is a number of characteristic features of the py.-f.i.m.s. that are... 

Temperature-programmed vacuum pyrolysis in combination with time-resolved soft ionization mass spectrometry allows principally to distinguish between two devolatilization steps of coal which are related to the mobile and non-mobile phase, respectively. The mass spectrometric detection of almost exclusively molecular ions of the thermally extracted or degraded coal products enables one to study the change of molecular weight distribution as a function of devolatilization temperature. Moreover, major coal components can be identified which are released at distinct temperature intervals. 

Flash Photolysis with Time-Resolved Mass Spectrometry (Carr). 

The rapid formation of molecular iodine following the flash photolytic dissociation of CHgl has been observed by time-resolved mass spectrometry.60 This has been attributed to the reaction (20) rather than to slow termolecular recombination. The experimental difficulties associated with sampling by this technique have been discussed by Meyer.61 This reaction is further discussed in Section IX.D on reaction of I(52Py2) with alkyl iodides. 

Fig. 15.14 Analytical techniques for time-resolved headspace analysis. An electronic nose can be used as a low-cost process-monitoring device, where chemical information is not mandatory. Electron impact ionisation mass spectrometry (EI-MS) adds sensitivity, speed and some chemical information. Yet, owing to the hard ionisation mode, most chemical information is lost. Proton-transfer-reaction MS (PTR-MS) is a sensitive one-dimensional method, which provides characteristic headspace profiles (detailed fingerprints) and chemical information. Finally, resonance-enhanced multiphoton ionisation (REMPI) TOFMS combines selective ionisation and mass separation and hence represents a two-dimensional method. (Adapted from [190])...

Taylor, A.J., Sivasundaram, L.R., Linforth, R.S.T., Surawang, S. (2003) Time-resolved head-space analysis by proton-transfer-reaction mass-spectrometry. In Deibler, K.D., Delwiche, J. (eds) Handbook of Flavor Characterization. Sensory Analysis, Chemistry and Physiology. Dekker, New York, pp 411-422. 

EARLY DEVELOPMENT OF TIME-RESOLVED MASS SPECTROMETRY... 

---