Frequency scan mode

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...

However, the SIM technology was only applied with minor frequency of all scan modes (17 %) (Fig. 5b). 

The t scale disappeared with the development of frequency-scan instruments and of the pulsed FT mode, which is essentially an instantaneous frequency scan. The terms upfield and downfield are now obsolete and have been replaced, respectively, by shielded (lower S, or to the right) and deshielded (higher S, or to the left). 

In a quadrupole mass analyzer, only a single mass-to-charge ratio m/z) value is transmitted to the detector for any given combination of radio frequency (RF) and direct current (DC) potentials. Typically, the RF/DC ratio is held constant and scanned to provide a mass spectrum. If, for example, a quadrupole is scanned from m/z 1 to 1000 in 1 second, then any particular m/z is transmitted to the detector for only 1 millisecond, representing a duty cycle of 0.1 %. Thus, a quadrupole mass analyzer has a low transmission duty cycle in the full-scan mode, which results in limited full-scan sensitivity. In contrast, ion-trap and TOF mass analyzers have the theoretical potential to transmit all ions that enter the mass analyzer and yield far better sensitivity across the entire mass spectrum. In reality, the pulse sequences associated with these analyzers devote significant time to functions such as ionization and detection. The actual duty cycles are generally between 10 and 25%, still far better than a scanning quadrupole mass spectrometer. 

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... 

Fig. 5.—Diagrammatic Representation of the Relationship of the Observe and Irradiate" Fields Used for Performing Various Double-resonance Experiments in the Frequency-sweep Mode. [A, For spin-decoupling, the irradiation field is positioned at the mid-point of the X-resonance B, for spin-tickling, the irradiation field is set on transition X-2 (for both of these experiments, the effect of the perturbation is monitored by scanning the observing field) and C, for an INDOR experiment, the observing field is held positioned on the transition X-2, and now it is the irradiating field that is scanned.]...

The most popular two-dimensional mass spectrometry configuration at present is the QQQ, or triple-sector quadrupole, represented schematically in Fig. 3.9. Three scan modes are possible with this configuration product ion scan, precursor ion scan, and constant neutral loss scan. Product ion scan is the most widely used, and involves using Qj to selectively transmit one precursor ion to Q2 where it is fragmented, normally by collisions with an inert gas such as helium. This type of fragmentation is referred to as collision-induced dissociation, or CID. Q2 is operated in radio frequency mode only, and thus stores ions of a broad m/z range until they are transmitted to Q3 for mass analysis of the product ions. 

Fig. 2. Low-frequency Raman spectra, showing the mode around 220 cm", with B band excitation 4545 A) of (a) HbCO (1 mAf, pH 7) flowing in a free jet through (0.3 isec interaction time) the laser beam, which is both the photolysis and the Raman source, and (b) deoxyHb (0.3 mAf, pH 7) in a recirculating capillary. Spectra were obtained in scanning mode at 5 cm increments, 1 sec increment. (From Stein et al. )...

If the system under investigation, for example a thin film, is deposited on a semiconductor insulator structure, the photocurrent decreases as shown in Fig. 25 (curve b). The difference in photocurrent is a function of the complex impedance of the layer added. Local differences in the impedance can be detected in the scanning mode. An additional variation of the modulation frequency of the light results in ac-impedance spectra with lateral resolution. 

In the continuous-scan mode, a Michelson interferometer generates modulation of the radiation at each wavenumber v with a frequency F = 2Vv, where V is the mechanical velocity of the scanning mirror in centimeters per second. The frequency F is called the Fourier frequency. A typical speed of the scanning mirror is 0.1-10 cm s so that signals within the IR spectral region fall into the 10 -10" -Hz range. Since in most measurement schemes a low-pass filter is used to separate the Fourier and modulation frequencies, the modulation frequency should satisfy the sampling theorem. Specifically, the modulation rate... 

Figure 7.46. In-phase (solid) and quadrature (dashed) potential-modulated ATR-SEIRA spectra of 4-mercaptopyridine (PySH) SAM on 20-nm-thick (80-nm-size particles) Au evaporated electrode in 0.1 M HCIO4. Modulation frequencies are shown. Amplitude of potential modulation was 400 mV, between -0.1 and 0.3 V (SCE). Spectra were recorded using Bio-Rad FTS 60A/896 FTiR spectrometer equipped with dc-coupled MCT detector and bandpass optical filter transmitting between 4000 and 1000 cm. Spectrometer was operated in step-scanning mode using setup shown in Fig. 4.56. Reprinted, by permission, from K. Ataka, Y. Hara, and M. Osawa, J. Electroanal. Chem. 473, 34 (1999), p. 39, Fig. 6. Copyright 1999 Elsevier Science S.A.

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