Time-tag mode

The time resolution of time-tag reeording is limited by the resolution of the counter that delivers the event times. Extremely high time resolution can be obtained from a TDC (time-to-digital eonverter) ehip. These ehips are combinations of a counter and an interpolation eireuit based on gate delays (see Sect. 4.2.2, page 55). A multiscaler based on a TDC actually comes elose to a TDC-based TCSPC device working in the time-tag mode. 

Either the TCSPC channels can be operated in the time-tag mode (see Seet. 3.6, page 43) or the ADC result of the second and third TCSPC channel ean be used as a routing signal for the first one. 

The structure in the time-tag mode is shown in Fig. 3.15. It contains the channel register, the time-measurement block, a macro time" clock, and the FIFO buffer for a large number of photons. It has some similarity to the multidimensional TCSPC described in the paragraphs above. In fact, many advanced TCSPC modules have both the photon distribution and the time-tag mode implemented, and the configuration can be changed by a software command [25]. The sequencer then turns into the macrotime clock, and the memory turns into the FIFO buffer. 

The output of the FIFO is continuously read by the eomputer. Consequently, the time-tag mode delivers a continuous and virtually unlimited stream of photon data. It is, of eourse, imperative that the computer read the photon data at a rate higher than the average photon count rate. However, modem operation systems are multitask systems, and it is unlikely that the computer reads the FIFO eontinu-ously. Moreover, in typieal applications bursts of photons appear on a baekground... 

The macro time clock can be started by an external experiment trigger or by a start-measurement command from the operating software. In some TCSPC modules the clock signal source of the macro time clock can be selected. The macro time clock can be an internal quartz oscillator, an external clock source, or the reference signal from the laser. Triggering and external clock synchronisation are absolute requirements for multimodule operation in the time-tag mode, see Sect. 5.11.3, page 189. 

The time-tag mode in conjunction with multidetector capability and MFIz counting capability was introduced in 1996 with the SPC-431 and SPC-432 modules of Becker Flickl. Its large potential in single molecule spectroscopy began to attract attention when sufficiently fast computers with large memories and hard discs became available. 

Spectroscopy of single molecules is based on fluorescence correlation, photoncounting histograms, or burst-integrated-lifetime techniques. Each case requires recording not only the times of the photons in the laser period, but also their absolute time. Modem time-resolved single molecule techniques therefore use almost exclusively the FIFO (time-tag) mode of TCSPC. The FIFO mode records all information about each individual photon, i.e. the time in the laser pulse sequence (micro time), the time from the start of the experiment (macro time), and the number of the detector that detected the photon (see Sect. 3.6, page 43). 

A mueh higher burst resolution ean be obtained by reeording the photons in the FIFO or time-tag mode. The time-tag mode is deseribed under Sect. 3.6, page 43. From the time-tag data, BIFL results with a burst resolution down to the laser pulse period ean be obtained. MCS traees are available, and FCS and PCHs can be ealeulated. Beeause the full information about all photons is recorded time-tag data are extremely flexible. Conformational dynamics, rotational relaxation, and intersystem erossing ean be investigated at almost any time scale [108, 154, 155, 295, 419, 500]. However, time-tag data are also voluminous. For each photon four or six bytes are reeorded, and file sizes of a gigabyte per measurement are not unusual. 

The afterpulsing probability can be measured by illuminating the detector by a source of continuous classic light, such as an incandescent lamp or an LED, and recording the detector pulses in the FIFO (time-tag) mode of a TCSPC module. 

A variation of the TCSPC technique does not build up photon distributions but stores information about each individual photon. This is called time tag , time stamp , list , or FIFO mode. The memory is configured as a FIFO buffer. For each photon, this method stores the time in the signal period ( micro time ), the time from the start of the experiment ( macro time ), and the data word at the channel input. During the measurement, the FIFO is continuously read, and the photon data are transferred into the main memory or to the hard disc of a computer. Advanced TCSPC devices often can be configured either to build up a multidimensional photon distribution or to store the individual photons. The FIFO mode is described in Sect. 3.6, page 43. 

At first glanee eombining a pieoseeond eorrelation experiment with PCS looks simple. An antibunehing experiment eould be run in the PIPO mode, and the an-tibunehing and the PCS eurves be obtained from the miero times and macro times, respeetively. Unfortunately this approaeh has a flaw. Most of the photons emitted by the sample eause either a start without a stop, or a stop without a start however, the TCSPC module reeords only eomplete start-stop events. Therefore the system records only a tiny fraction of the photons reaehing the detectors. The low effieiency makes the obtained time-tag data praetieally useless for PCS. 

The long reaction times under the action of classic heating were reduced to only a few minutes with single-mode heating at a power of 50-70 W. One example of the use of CH2CH2C6F13 (denoted F-13) -tagged organostannanes is presented in Eq. (11.30) [46],... 

Typical NP conditions involve mixtures of n-hexane or -heptane with alcohols (EtOH and 2-propanol). In many cases, the addition of small amounts (<0.1%) of acid and/or base is necessary to improve peak efficiency and selectivity. Usually, the concentration of alcohols tunes the retention and selectivity the highest values are reached when the mobile phase consists mainly of the nonpolar component (i.e., n-hexane). Consequently, optimization in NP mode simply consists of finding the ratio n-hexane/alcohol that gives an adequate separation with the shortest possible analysis time [30]. Normally, 20% EtOH gives a reasonable retention factor for most analytes on vancomycin and TE CSPs, while 40% is more appropriate for ristocetin A-based CSPs. Ethanol normally gives the best efficiency and resolution with reasonable backpressures. Other combinations of organic solvents (ACN, dioxane, methyl tert-butyl ether) have successfully been used in the separation of chiral sulfoxides on five differenf glycopepfide CSPs, namely, ristocetin A, teicoplanin, TAG, vancomycin, and VAG CSPs [46]. 

MCT can be best viewed as a synthesis of two formidable theoretical approaches, namely the renormalized kinetic theory [5-9] and the extended hydrodynamic theory [10]. While the former provides the method to treat both the very short and the very long time responses, it often becomes intractable in the intermediate times. This is best seen in the calculation of the velocity time correlation function of a tagged atom or a molecule. The extended hydrodynamic theory provides the simplicity in terms of the wavenumber-dependent hydrodynamic modes. The decay of these modes are expressed in terms of the wavenumber- and frequency-dependent transport coefficients. This hydrodynamic description is often valid from intermediate to long times, although it breaks down both at very short and at very long times, for different reasons. None of these two approaches provides a self-consistent description. The self-consistency enters in the determination of the time correlation functions of the hydrodynamic modes in terms of the... 

Figure 2. A pictorial representation of the mode coupling theory scheme for the calculation of the time-dependent friction (f) on a tagged molecule at time t. The rest of the notation is as follows Fs(q,t), self-scattering function F(q,t), intermediate scattering function D, self-diffusion coefficient t]s(t), time-dependnet shear viscosity Cu(q,t), longitudinal current correlation function C q,t), longitudinal current correlation functioa...

Many service companies offer monitoring and communications software to provide automatic adjustment to chemical dosing pumps or bleed valves, based on receiving inputs from inhibitor tracing and tagging systems that may operate in real-time, semicontinuously, or after-the-fact modes. 

Time-correlated single photon counting A technique for the measurement of the time histogram of a sequence of photons with respect to a periodic event, e.g. a flash from a repetitive nanosecond lamp or a CW operated laser mode-locked laser). The essential part is a time-to-amplitude-converter (TAG) which transforms the arrival time between a start and a stop pulse into a voltage. Sometimes called single photon timing. 

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