Ultrahigh frequency resolution

The introduction of the SWIFT technique (10,14,21,22) makes possible FT/ICR frequency-domain excitation with the same mass resolution as has already been demonstrated for FT/ICR detect ion, provided only that sufficient computer memory is available to store a sufficiently long time-domain waveform. When ejection must be performed with ultrahigh mass resolution over a wide mass range, a simple solution is to use two successive SWIFT waveforms first, a broad-band low-resolution excitation designed to eject ions except over (say) a 1 amu mass range and then a second SWIFT waveform, heterodyned to put 2 8K data points spanning a mass range of 1-2 amu. 

Stored Waveform inverse Fourier Transform (SWIFT) excitation for FT/ICR is a newly implemented technique which includes all other excitation waveforms as subsets. Compared to prior excitation waveforms (e.g., frequency-sweep), SWIFT offers flatter power with greater mass resolution and the possibility of magnitude steps (without additional delays or switching transients) in the excitation spectrum. Briefly, SWIFT increases the mass resolution for FT/ICR excitation to the ultrahigh mass resolution already demonstrated for FT/ICR detect ion. 

FT-ICR mass spectrometry offers ultrahigh resolution. This feature is a result of the large numbers of cyclotron orbits during detection and the fact that cyclotron frequency is independent of ion velocity. Performance is not limited by the initial position, direction or speed of the ions, unlike mass spectrometers such as time-of-flight or sector instruments. 

For Lamb-dip spectroscopy with ultrahigh resolution, the output beam of the powerful laser is expanded before it is sent through the sample cell in order to minimize transit-time broadening (Vol. 1, Sect. 3.4). A retroreflector provides the coun-terpropagating probe wave for Lamb-dip spectroscopy. The real experimental setup is somewhat more complicated. A third laser is used to eliminate the troublesome region near the zero-offset frequency. Furthermore, optical decoupling elements have to be inserted to avoid optical feedback between the three lasers. A detailed description of the whole system can be found in [222]. 

For some tasks in ultrahigh-resolution spectroscopy, the residual finite linewidth AyL, which may be small but nonzero, still plays an important role and must therefore be known. Furthermore, the question why there is an ultimate lower limit for the linewidth of a laser is of fundamental interest, since this leads to basic problems of the nature of electromagnetic waves. Any fluctuation of amplitude, phase, or frequency of our monochromatic wave results in a finite linewidth, as can be seen from a Fourier analysis of such a wave (see the analogous discussion in Sects. 3.1,3.2). Besides the technical noise caused by fluctuations of the product nd, there are essentially three noise sources of a fundamental nature, which cannot be eliminated, even by an ideal stabilization system. These noise sources are, to a different degree, responsible for the residual linewidth of a single-mode laser. 

J.J. Snyder An ultrahigh resolution frequency meter . Proc. 35th Ann. Freq. Control USAERADCOM May 1981. Appl. Opt. 19, 1223 (1980)... 

Figure 3. W-Band (94.9 GHz) EPR spectra of human Hb(NO>4at sample temperatures ranging from 7 to 200 K. The spectra were obtained in CW mode with fieid moduiation of iO G amplitude at a frequency of 100 kHz. Hb(NO)4 was prepared by reaction of a phosphate-buffered (pH 7.4) saline solution of Hb A (deoxygenated with ultrahigh-puiity argon) with a deoxygenated aqueous solution of sodium nitrite (preceded by addition of sodium dithio-nite) excess reagents were removed by G-25 chromatography. For certain spectral features, highlighted with the shaded lines, the W-band spectra show a notable increase in resolution of temperature-dependent lineshape changes as compared to X- and Q-band spectra. EPR spectra exhibit both axial and rhombic spectral components in equilibrium that favors the axial components with increasing temperature.

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