Spectrometer double beam

Characterization of the reaction intermediate is facilitated by studies in a flow system in which the sample cell and a reference cell are mounted in series in a double beam spectrometer (IS). Not only can we observe the intermediate bands under rigorous steady state conditions, but we can monitor the conversion by sampling the effluent. In addition, the reference cell assures the spectrum we see is that of surface species. Primitive analysis of the kinetics reveals the intermediate is favored by relatively high ethylene pressures hence, use of a reference cell to cancel contributions of the gas phase is an important factor. 

The IR spectra of this new resist films on silicon substrates were measured with a Shimadzu FTIR-4000 Fourier transform spectrometer. The UV spectra of 4,4 -diazidodiphenyl methane in a quartz cell and the films of poly(styrene-co-maleic acid half ester) and the new resist on quartz substrates were measured with a Shimadzu UV-265FS double-beam spectrometer. 

The systems so far described have all been single-beam spectrometers. As in molecular spectrometry, a double-beam spectrometer can be designed. This is shown diagrammatically in Fig. 2.13. The light from the source is split into two beams, usually by means of a rotating half-silvered mirror or by a beam splitter (a 50%-transmitting mirror). The second reference beam passes behind the flame and, at a point after the flame, the two beams are recombined. Their ratio is then electronically compared. 

In double-beam spectrometers, the correction is made before the spectrum is recorded by dividing ... 

Figure 1.6 Configurations of instruments for atomic absorption spectrometry, (a) Single-beam spectrometer (b) double-beam spectrometer.

The initial single beam dispersive spectrometers that did not, at the time, produce digitised spectra (this would have allowed for baseline correction) were soon replaced by double beam spectrometers. This more complex arrangement can directly yield the spectrum corrected for background absorption. The use of two distinct but similar optical paths, one as a reference and the other for measurement, allows the alternate measurement of the transmitted intensity ratios at each wavelength. 

Figure 10.10—Double beam spectrometer with a rotating mirror. Model 1R 435 (1986, reproduced by permission of Shimadzu).

To obtain a sample spectrum equivalent to the one obtained on a double beam spectrometer, two spectra of transmitted intensities are recorded the first without sample (absorption background), and the second with sample. The conventional %Tspectrum can be obtained from the two preceding measurements (Fig. 10.12). 

N, adsorption for determination of pore size distribution(BJH method) using a Micrometries ASAP 2000 automatic analyzer. FTIR spectra were recorded on a PerkinElmer 221 spectrometer and UV-vis spectroscopic measurments were carried out using Varian CARY 3E double beam spectrometer in the range of 190 - 820 nm. 

A true double beam spectrometer requires separate optical paths for sample and reference, and involves simultaneous comparison of the two. It is possible to construct a pseudo-double beam spectrometer, employing only one detector and optical path, taking advantage of the capabilities of a computer system attached to the detector. 

A shutter and pure solvent in the sample compartment of this pseudo-double beam spectrometer permits acquisition of both a dark current (0% T) and an emmisivity/responsivity (100% T) data array. These consist of digitized responses from each of the pixels in the array under conditions where the integration time and the speed of read-out is identical to that planned for the measurement of the spectrum of a sample. The dark-current data will provide information as to the shot-noise and other inherent... 

This pseudo-double beam spectrometer requires that the source be stable over the measurement of the three required arrays. It also consumes a large amount of computational time forbidding the very rapid acquisition of spectra. 

Reference and sample measurements are performed consecutively, and the resultant (sample) spectrum is obtained as the ratio of the two photon fluxes onto the detector. In a single-beam spectrometer, there are no other options in a double-beam spectrometer, the photon fluxes of the sample and reference beam path are compared. When an integrating sphere is used with two ports and a white standard in the reference position, the photon fluxes are comparable to each other, and no problems occur. Note that the ports are part of the sphere and that any material change in the reference or sample position will change the average sphere reflectance pave. The reference measurement should be conducted with exactly the same components (windows) as the sample measurement otherwise, "substitution errors" may occur. 

When mirror or fiber optics are used with a double-beam spectrometer, the difference in throughput between the sample and reference beam... 

Figure 4.6-12 Schematic representation of the experiment for recording the optical rotation of the sample S P vector of polarizer, i.c. direction of the electric vector of transmitted radiation, A vector of analyzer the sample rotates the electric vector of the radiation through an angle g leading (as indicated by the broken lines) to different intensities in the two experimental configurations to be realized readily with a double-beam spectrometer or sequentially with a single-beam in.strument.

A typical double-beam spectrometer is shown in Fig. 3. Radiation from the source is split into the reference beam and the sample beam. Each beam passes through a comb-shaped attenuator that regulates the beam intensity. The two beams are alternately sent through the slit into the monochromator by the... 

Zhurkov, Novak and Vettegran have recently reported studies where IR was used to determine the formation of CH and C C bonds during deformation of polyethylene and polypropylene at room temperature and atmospheric conditions (11). In these studies, specimens of equal thickness, one unstrained, the other fractured, were Interposed In the balanced light beams of double-beam spectrometer. In this mode the spectrometer reportedly records the difference in absorption AD, of the undeformed and fractured specimen. Strong absorption bonds were noted at 910, 965, 1379 and 1735, cm. These were attributed respectively to (RCft CH ), (RCH=CHR ), (R-CH ), and (RCHO) groups. 

Using a mode-locked Nd + YAG laser system to generate picosecond sample excitation pulses and picosecond probing continuum pulses in their double beam spectrometer, Spalink et. al. (30) were able to measure difference absorption spectra of irradiated samples of 11-cis-rhodopsin and 9-cis-rhodopsin at selected times after excitation by means of a PAR OMA-2 optical multichannel detection system. The difference absorption spectral data were obtained over the entire spectral range from 410 nm to 650 nm at one time with an OMCD as opposed to the... 

Bearing in mind a resolution factor imposed by the method of calculation, the classical representation of the spectrum is obtained, 7 = f (A) or 7 = f (i ). According to Nyquist s theory, at least two points per period are required in order to find, by calculation, a given wavelength of the spectrum. To obtain a sample spectrum equivalent to the one obtained with a double beam spectrometer, two spectra of transmitted intensities are recorded the first, without sample (absorption background) and the second with sample. The conventional spectrum, in percentage r, is obtained from these two spectra (Figure 10.9). 

Figure 11.9 Schematic of a Shimadzu F-4500 spectrofluorometer. A fraction of the incident beam, reflected by a semi-transparent mbror, reaches a reference PMT. A comparison of the signals from the two PMTs leads to the elimination of any drifting of the source. This procedure, for single beam instruments, gives approximately the same stability as with a double beam spectrometer. However, the spectrum of a given solution will often present minor differences when recorded upon different instruments (reproduced courtesy of Shimadzu).

Double-beam spectrometers can automatically scan the wavelength and record the spectrum. 

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