Fast scan mode

Figure 8. STM image of the basal-plane surface of HOPG obtained in 1 M NaCl. Bias = 550 mV (tip +), it = 1 nA. Image was obtained in the fast scan mode. Dark spots, separated by 2.5 A, correspond to large tunneling currents.

To obtain IR spectra on a time scale of nanoseconds, the sample cell in conventional spectrometers is usually excited by an Nd YAG laser. Flow cells with a pathlength of at least 0.1 mm must be used for photoreactive samples and the pulse repetition frequency is then limited to 1 Hz. In step scan FTIR spectroscopy,211 the time evolution is collected at single points of the interferogram, which is then reconstructed point-by-point and subsequently transformed to time-resolved IR spectra. Alternatively, dispersive instruments equipped with a strong IR source can be used.212 The time resolution of both methods is about 50 ns. FTIR instruments provide a triggerable fast-scan mode to collect a complete spectrum within a few milliseconds.213... 

One would generally prefer to operate in a fast scan mode without a chopper, as it allows monitoring of the source variability, atmospheric variability and noise spikes, and also faster operation. However, depending on the detector time of response. 

The Instrument is set to Fast Scan mode with five prescans. The Sequence file is set up with three warm-up blanks prior to the true blank in order to assure low background and instrument warm-up. Samples are set up for blank subtraction after internal standard correc-... 

The full-scan mode is needed to achieve completely the full potential of fast GC/MS. Software programs, such as the automated mass deconvolution and identification system (AMDIS), have been developed to utilize the orthogonal nature of GC and MS separations to provide automatically chromatographic peaks with background-subtracted mass spectra despite an incomplete separation of a complex mixture. Such programs in combination with fast MS data acquisition rates have led to very fast GC/MS analyses. 

TOF High resolution and mass accuracy when operated in reflectron mode or in Q-TOF systems, elemental composition can be obtained Tandem MS available for generation of sub-structure information or quantitative analysis High sensitivity Very fast scan speed Unlimited mass range Initial energy and spatial distribution must be corrected for ions High-performance electronics needed... 

Eletrochemical detection has been used for the detection of synthetic dyes. Fogg et al. (226) described a method for the qualitative and quantitative determination of several synthetic dyes using polarographic detection. The system was a stationary mercury drop electrode operated in the differential-pulse mode. Ashkenazi et al. (131) used fast-scan square-wave voltammetry for the polarographic detection of five synthetic dyes. The voltametric mode was observed to be much faster than the differential-pulse method. Another advantage is that the experimental measurement produces, in addition to the peak current, the redox potential of the dye, which can serve to identify the analyte further. 

While an intensity profile at the detector as a function of retardation may be acquired in a step-scan mode, two major drawbacks affect this method of interferogram acquisition. First, the mirror(s) requires stabilization times with mirror inertia and time constants of the control loop determining this parameter in achieving a given optical retardation. Second, additional hardware and control mechanisms need to be incorporated into the spectrometer, thus increasing instrument cost and complexity. In certain cases, however, the utility of a step-scan instrument justifies this additional expense. Historically, the step-scan approach was favored with slow detectors. With the advent of fast detectors and electronics, step-scan interferometry became... 

Fig. 5 Statistical evaluation of LC-MS-based methods for tropane alkaloids referred in this chapter. (a) Relative frequency of ionization methods. +APCI positive atmospheric pressure chemical ionization, +ESI positive electrospray ionization, FAB fast atom bombardment, +TSP positive thermospray, (b) Relative frequency of scan modes used. MS full scan MS, MS/MS tandem mass spectrometry (product ion scan), MRM multiple reaction monitoring, SIM selected ion monitoring, (c) Relative frequency of mass analysers used. EBQtQ2 double focusing sector field mass spectrometer, IT ion trap, QqQ triple quadrupole, SQ single quadrupole. Considered publications were found by PubMed data-based search and references cited in these articles...

Modern QqQ instruments equipped with high-end fast electronics, and accompanying optimized software allow to follow several hundreds of analytes in one run by MRM thus making this scan mode the method of choice for qualitative and quantitative analysis. Therefore, MRM was commonly applied to TTA and QTA analysis for quantification in PK (Table 5), distribution (Table 6), and biotransformation studies (Table 7) as well as for toxicological screening and evidence of drug abuse (Table 8). Specificities and remarkable characteristics of these fields of application are addressed in the following sections. 

The information that can be obtained with electrochemical detectors is not restricted to quantification. Instead of the conventional use of electrochemical detectors in amperometric mode at fixed potential, electrode arrays with each electrode held at different values of fixed potential can be used, in order to build up chronovoltammograms, three-dimensional current-voltage-time profiles. A 32-microband electrode array has been described for this purpose and applied to phenolic compounds [17] and which permits studying the electrode reaction mechanism at the same time as identification and quantification are carried out. Alternatively, fast voltammetric techniques such as fast-scan cyclic voltammetry or square wave voltammetry can be used to create chronovoltammograms of the eluted components. 

Another approach is to use focused X-ray beams in the scanning mode. This technique will require specialized focusing optics and a fast monochromator. Such a nanoprobe beam line is currently being developed at the APS and is expected to have the spatial resolution of several tens of nanometers. At the proposed NSLS-II synchrotron, a beam line with a spatial resolution of ten nanometers is planned. With the development of these beam lines, it is expected that spatial imaging will be available for characterization of catalysts, and of course the hope is to do this with catalysts in reactive atmospheres. 

The TV-mode or fast scan , uses the normal TV-scan rate of 50 pictures per second. Thus charges are accumulated over a period of 20 msec only and then sequentially digitized. In this mode, the probabihty that saturation occurs within a period of 20 msec is negligible, even with synchrotron radiation sources. 

In modem diffractometers both scanning modes result in nearly identical quality of experimental data. A step scan is usually considered as the one with less significant positioning errors, which could be important in experiments where the maximum lattice parameter precision is essential. Continuous scans are used most often for fast experiments, whereas step scans are usually employed in overnight or weekend experiments. 

Fast atom bombardment mass spectrometry. Fast atom bom-bardment/mass spectrometry (FAB/MS) analyses were performed on a VG ZAB-HF mass spectrometer equipped with an Ion Tech fast atom gun. Xenon gas was activated to 8 kv and 1.5 mA ion current for the fast atom generation. An accelerating voltage of 8 kV was applied to the FAB source. The mass spectrometer was scanned from 800 to 80 amu using an exponential down scan mode at 5 seconds per decade with a 1 second interscan time. The data were recorded with a PDP 11/24 computer and were processed with VG 11/250 software. 

Fig. 3.38 Contact mode AFM height image of egg PC vesicles adsorbed on glass captured with an imaging force of 30 pN left) and 50 pN right). The halo in the fast scan direction right to left) indicates that the tip can no longer track the surface features accurately, when imaging force and noise of the deflection signal become comparable ( 30 pN in this case). When the imaging forces are increased to 50 pN, the surface is tracked better. The asymmetry of the features can be explained by tip convolution effects (asymmetry of the probe tip) [87]...

We use a standard tapping/intermittent contact mode set-up, which is assembled as described in Sect. 3.2.1 (see also hands-on example 31). The operation frequency and amplitude are adjusted to v at 0.85 A0 and 100 nm, respectively. The surface is tracked most accurately by utilizing a high setpoint amplitude ratio (close to 0.9). Care must be taken that the tip does not lose track of the surface when traversing elevated features, therefore the phase (and the amplitude) signal is carefully monitored. Halos in the fast scan direction that are devoid of contrast (i.e. show a constant phase shift or amplitude response), are indicative of absent tip-sample interaction. In this case the setpoint can be lowered slightly. 

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