Co-Adding

The signal-to-noise ratio (S/N) of a spectrum is increased by repetitive scanning as the contributions of random noise are averaged out by coaddition of scans (equation (11)  

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If was nof until fhe developmenf of Fourier fransform infrared (FTIR) specfromefers (see Section 3.3.3.2) fhaf fhe possibilify of using an infrared laser routinely was opened up. The intensify advanfage of an infrared interferometer, wifh which a single specfrum can be obfained very rapidly and fhen many specfra co-added, coupled wifh fhe developmenf of more sensitive Ge and InGaAs semiconductor infrared defectors, more fhan compensate for fhe loss of scatfering intensify in fhe infrared region. 

The growth and decay of all other species (including O3) were monitored by Fourier transform infrared (FT-IR) spectroscopy at a total pathlength of 460 meters and a spectral resolution of 1 cm". At this pathlength, the intense absorptions of H2O and CO limit the usable IR spectral windows to the approximate regions 750-1300, 2000-2300, and 2400-3000 cm". Each spectrum (700-3000 cm" ) was adequately covered by the response of the Cu Ge detector. Approximately 40 seconds were required to collect the 32 interferograms co-added for each spectrum. 

Figure 6. PMMA on Cr - solution deposition. P polarization 180 A layer [16] Resolution = 4 cm-1 3000 - 1000 cm-1. Ratioed to Cr reference mirror. 4000 scans co-added.

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.

Figure 7.6 STM images (100 x 100) A2 of Pt(lll) under different catalytic conditions 7 (a) 20 mTorr H2 (b) 20mTorr H2 and 20mTorr C2H4 (c) 20mTorr H2 plus 20mTorr C2H4 and 2.5mTorr CO(g). The CO added induced the formation of a ( /l9 x /l9) R23.4° structure in (c). In (d) are shown two rotational domains of the /T9 structure. (Reproduced from Ref. 7).

Figure 7.7 STM images of Pt(lll) at 300K (a) (75 x 75) A2, 20 mTorr cyclohexene plus 20mTorr H2 no catalytic products formed (b) (50x 50)A2, 200 mTorr H2, 20 mTorr of cyclohexene, disordered surface and cyclohexane formed (c) (90 x 90) A2, CO added, no catalytic activity. (Reproduced from Ref. 11).

Twenty scans wepe co-added to produce one spectrum at a resolution of 4 cm every 30 seconds. The reaction was followed by monitoring the absorbance of isocyanate as a function of time. Film thickness changes were compensated for by normalizing the isocyanate absorbance to the 1446 cm band which remains constant in absorbance during the reaction. The infrared absorbance of the free isocyanate is converted to concentration by comparison to the absorbance observed in a similar sample which has no functional groups with which the isocyanate can react. 

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.

Figure 16 u/tro-HMBC spectrum of cyclosporine in C6D6 recorded with the pulse sequence shown in Figure 14 where four subspectra of eight scans each were recorded with A = 181.1,160.0,115.0, and 99.3 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 ... 

Fig. 3. Top panel Observed and synthetic Ha profiles of NGC 6397/SG793 (black 5480K, grey 100K). The data was retrieved from the VLT archive (observing dates 2000-06-18 and 20, one spectrum per night), reduced using REDUCE, co-added and rectified using parabolic fits to the continua in adjacent orders. An effective temperature of 5480 K is indicated, in good agreement with Gratton et al. [3],...

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