Sweep width

Precisely controllable rf pulse generation is another essential component of the spectrometer. A short, high power radio frequency pulse, referred to as the B field, is used to simultaneously excite all nuclei at the T,arm or frequencies. The B field should ideally be uniform throughout the sample region and be on the order of 10 ]ls or less for the 90° pulse. The width, in Hertz, of the irradiated spectral window is equal to the reciprocal of the 360° pulse duration. This can be used to determine the limitations of the sweep width (SW) irradiated. For example, with a 90° hard pulse of 5 ]ls, one can observe a 50-kHz window a soft pulse of 50 ms irradiates a 5-Hz window. The primary requirements for rf transmitters are high power, fast switching, sharp pulses, variable power output, and accurate control of the phase. 

A computer-controlled bandpass filter system controls the size of the acquired spectral window. Typically, this is set to about 120% of the desired sweep width. Only frequencies within these limits are allowed to reach the ADC. Those frequencies outside the limits would only contribute to the noise in the final spectmm. The need for this system is dictated by the nonselective nature of the excitation rf pulse. 

Gas chromatographic analysis at 79° using a flame detector in conjunction with a 183 x 0.32 cm. stainless-steel column containing Dow-Corning 550 fluid on silanized support gave peaks for l-bromo-3-chloropropane (6.5 minutes) and 6-chloro-2-hexyne (9.3 minutes) whose areas were shown to be proportional to the mole fractions. The latter were determined by integration of the expanded (50 Hz sweep width)... 

All P.M.R. spectra were measured with a Varian HA 100 spectrometer operating in the frequency-sweep mode with tetramethylsilane as the reference for the internal lock. The double and triple resonance experiments were performed using a Hewlett Packard 200 CD audio-oscillator and a modified Hewlett Packard 200 AB audio-oscillator (vide infra). Spectra were measured using whichever sweep width was required to ensure adequate resolution of the multiplets under investigation, generally 250 or 100 Hz, and sweep rates were selected as necessary. Extensive use was made of the Difference 1 and Difference 2 calibration modes of the instrument, both for the decoupling experiments and for the calibration of normal spectra. 

NMR Spectroscopy. All proton-decoupled carbon-13 spectra were obtained on a General Electric GN-500 spectrometer. The vinylldene chloride isobutylene sample was run at 24 degrees centigrade. A 45 degree (3.4us) pulse was used with a Inter-pulse delay of 1.5s (prepulse delay + acquisition time). Over 2400 scans were acquired with 16k complex data points and a sweep width of +/- 5000Hz. Measured spin-lattice relaxation times (Tl) were approximately 4s for the non-protonated carbons, 3s for the methyl groups, and 0.3s for the methylene carbons. 

The PVA sample was run at 55 degrees centigrade. A 90 degree (6.8us) pulse was used with a inter-pulse delay of 2.1s. Exactly 800 scans were acquired with 16k complex data points and a sweep width of +/-2000 Hz. 

Figure 1.33 The underlying principle of the Redfield technique. Complex Fourier transformation and single-channel detection gives spectrum (a), which contains both positive and negative frequencies. These are shown separately in (b), corresponding to the positive and negative single-quantum coherences. The overlap disappears when the receiver rotates at a frequency that corresponds to half the sweep width (SW) in the rotating frame, as shown in (c). After a real Fourier transformation (involving folding about n = 0), the spectrum (d) obtained contains only the positive frequencies.

Another consequence of the large sweep width needed for 19F acquisition is that the electronics of the instrument are pushed to the limit it is difficult to generate uniform r.f. irradiation over such a large frequency range and for this reason it may be necessary to acquire spectra in different spectral ranges, depending on the expected fluorine environment. This is particularly so in the case of high-field (>400 MHz) spectrometers. 

Adiabatic pulse A type of pulse employing a frequency sweep during the pulse. This type of pulse is particularly efficient for broadband decoupling over large sweep widths. 

Proton nmr spectra of fractions A, B and C and all bottoms products were recorded on a Varian HA lOOnmr spectrometer using a solution of the sample dissolved in pyridine-d5. Spectra were run at room temperature with tetra methyl silane (TMS) as an internal standard, with a sweep width of 0 to 1000 cps from TMS. Fraction D and the whole coal were only partly soluble in pyridine and it was therefore not possible to get representative spectra from them. 

Microwave power Sweep width 1 st or 2nd harmonic Signal gain... 

Center Field, Sweep Width and Field Offset... 

Once you know, or can guess, the field limits of your spectrum, setting the center field and sweep width values is not very difficult. The center field corresponds to the middle of the spectrum and a sufficiently large sweep width chosen so that all of the spectrum is recorded. 

If you do not know the field range occupied by your spectrum in advance, the center field must be chosen by educated guess set the sweep width 2-4 x greater than the expected width. Hopefully, you will see at least a piece of your spectrum and can make appropriate adjustments to zero in on the correct settings. 

Sweep width Pulse width = Acquisition time = Data table ... 

Fig. 23 ESR spectra of Cu2Cr/aniline sulfonic acid derivatives after a thermal treatment at 200 °C for 4h in air recorded at 105 K with a sweep width of a 6500 G b 150 G. H(I) o-aminobenzenesulfonic acid H(I) m-aminobenzenesulfonic acid H(III) ...

Figure 10. Selective irradiation of linear PE (2 X 10 mol wt, 1 — K 0.5). Spectral details are 35°C 67.9 MHz sweep width 5 KHz (quadrature detection) line broadening 9.7 Hz pulse width 35 jisec (90°C = 48 /jsec) delay = 1.0 sec, 4K data points 1024 scans accumulated 10-mm sample tube. Decoupling 7W (forward), 0.4W (reflected), broad band noise modulated decoupling.

Define a new data set and read a suitable parameter file for the 1-D H experiment. Record a preliminary 1-D H spectrum with one scan and a relatively large sweep width (at least 10 ppm). 

From this preliminary spectrum, the sweep width of the 1-D H spectrum must be defined so that all anthocyanin signals are included. The anthocyanin H and l3C signals are normally found in the spectral regions 0.5 to 9.5 ppm and 10 to 180 ppm, respectively. 

Spectral width or sweep width (SW) The defined range of frequencies in which signals tire expected to be found. 

The solid-state deuterium NMR experiments were performed on surface-adsorbed material from triethoxyaminopropylsilane-rf, (DAPES) and triethoxy-aminobutylsilane-dj (DABES) prepared as described previously [9]. The samples were heated to 90°C at 10 mmHg for 12 h after adsorption. These deuterated coupling agents have been characterized by IR and NMR spectroscopy. The solid-state deuterium NMR spectroscopy was done on a Varian VXR-200 at 30.7 MHz. A quadrupole echo pulse sequence was used with a 2 s interval, 2 /is 90° pulse, 30 ps echo time, 2 MHz sweep width, and, typically, overnight accumulation. 

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