Co-adding spectra

Transmission infrared spectra of species adsorbed on the catalyst were taken with a Digilab FTS-10M Fourier-transform infrared spectrometer, using a resolution of 4 cm-l. To improve the signal-to-noise ratio, between 10 and 100 interferograms were co-added. Spectra of the catalyst taken following reduction in H2 were subtracted from spectra taken in the presence of NO to eliminate the spectrum of the support. Because of the very short optical path through the gas in the reactor and the low NO partial pressures used in these studies, the spectrum of gas-phase NO was extremely weak and did not interfere with the observation of the spectrum of adsorbed species. 

MQCs are not excited uniformly and the efficiency with which the various orders of MQC are excited depends specifically on the parameters of the spin system (dipolar couplings, scalar couplings, quadrupolar couplings, chemical shifts) in the spin system and the choice of the preparation time t. Many researchers have co-added spectra acquired with different preparation times to ensure that all transitions are observed with reasonable intensity. A number of broadband excitation techniques have been developed,13-15 where the value of t in the preparation sequence has been varied either in a pseudo-random or systematic fashion to achieve a more uniform excitation in the multiple quantum domain. An experimental search method has been used to optimise the delays in the preparation period of the MQ excitation sequence16 and Wimperis17 used average Hamiltonian theory to propose... 

The ApoE (— / —) mice in the current study were placed on a high fat/ cholesterol (1-2%) diet for 3 months until sacrifice. We employed Beamline 1.4.3 at the Advanced Light Source (ALS) synchrotron at LBNL (http // infrared.als.lbl.gov/) as our mid-IR source. The synchrotron IR beam has brightness in the range of approximately 10 to 7x10 photons/sec-mm mrad-0.1%BW when focused to a 10 pm spot. Each aortic specimen was placed luminal side up beneath an IR-transparent ZnSe window (International Crystal Labs, NJ) inside a custom-built environmental chamber where the temperature was kept at 5 °C to maintain the freshness of the specimens during reflectivity measurements. All FTIR measurements were recorded in the standard reflection mode in the mid-IR 4000 to 650 cm region, and consisted of 128 co-added spectra at a spectral resolution of 4cm ... 

Aperture/pixel size No. of co-added spectra RMS noise (%) >... 

Figure 7. 2 L-B monolayers of cadmium arachidate on Ni. P polarization. 4000 spectra co-added. Ratioed to Ni reference mirror. Resolution ca. 10 cm-1 3000 - 2800 cm-1.

Fig. 6 IR spectra of adsorbed CO on 1 ML Co deposited at room temperature on the alumina fflm as a function of CO coverage. Spectra were taken at 44 K. Black trace second up from the bottom shows the spectrum resulting from saturation coverage of a 50 50 mixture of i CO i CO. Overlaid gray trace is artificially created by adding the saturation coverage spectra CO and CO scaled by a factor 1/2. Lowest black trace shows the spectrum of CO saturation coverage on particles grown hy 1 MLCo + 0.05 MLPd. Corresponding grey trace is the pure Co spectrum for comparison. Right inset shows TPD spectrum of 1 ML Co [64]...

Probably the most basic parameter that you will be able to set is the number of spectra that will be co-added. This is normally called the number of transients or number of scans . As mentioned elsewhere in the book, the more transients, the better the signal to noise in your spectrum. Unfortunately, this is not a linear improvement and the signal to noise increase is proportional to the square root of the number of transients. As a result, in order to double your signal to noise, you need four times the number of scans. This can be shown graphically in Figure 3.1. 

Figure 15 HMBC and broadband HMBC spectra of cyclosporine in C6D6 recorded with the pulse sequence shown in Figure 14. (A) HMBC spectrum recorded with A = 65.0 ms and 32 scans. (B) HMBC spectrum where two subspectra of 16 scans each recorded with A = 65.0 ms and 120 ms, and co-added in absolute-value mode. (C) broadband HMBC spectrum where four subspectra of eight scans each were recorded with A = 96.7, 84.4, 81.8, and 80.8 ms, respectively, and co-added in absolute-value mode.

In the second, more commonly-employed, approach a spectrum, Sb, of N co-added and averaged scans (i.e. individual spectra) are collected at the base potential, Eb. The potential is then stepped to successively higher (or lower) values , and further spectra, S , taken. As above the spectra are plots of A R/R vs, v, where ... 

Due to the small size and applied bias voltage (when used in photoconductive mode) the response time of each diode element in the PDA can be very fast indeed, so total integration times can also be short, hi principle, data rates of 200 spectra per second are achievable, but in reality spectral co-adding is a more sensible objective, to improve overall SNR. 

Figure 6. CO adsorption. IR spectra of solid C (a) treated with hydrogen at 200° C, evacuated at 25° C, (b) sample A + CO at room temperature (p = 50 torr)f (c) sample B evacuated at 25° C, (d) sample C desorbed at 200°C, CO added at 25°C p = 50 torr), (e) and evacuated at 25° C...

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