Sample beams

Interference effects, which arise because of the extraordinary uniformity of thickness of the film over the spectrometer sample beam, superimposed on the absorption of incident light by parylene films, can be observed. Experimentally, a sinusoidal undulation of the baseline of the spectmm is seen, particularly in the spectral regions where there is Htde absorption by the sample. These so-called "interference fringe" excursions can amount to some... 

There is supporting evidence in the literature for the validity of this method two cases in particular substantiate it. In one, tests were made on plastics heated in the pressure of air. Differential infrared spectroscopy was used to determine the chemical changes at three temperatures, in the functional groups of a TP acrylonitrile, and a variety of TS phenolic plastics. The technique uses a film of un-aged plastic in the reference beam and the aged sample in the sample beam. Thus, the difference between the reference and the aged sample is a measure of the chemical changes. 

For technical reasons the sample beam is fixed and the plate is scanned by mounting it on a movable stage controlled by... 

With your sample in the sample beam, the instrument scans the IR spectrum. Specific functional groups absorb specific energies. And because the spectrum is laid out on a piece of paper, these specific energies become specific places on the chart. 

Slide the holder and plate into the bracket on the instrument in the sample beam (closer to you, facing instrument). 

Cut and trim an index card so that it fits into the sample beam aperture (Fig. 129). 

Punch a hole in the card with a paper punch. The hole should be centered in the sample beam when the card is in the sample beam aperture. 

The sample beam. Most IRs are dual-beam instruments (Fig. 127). 

The one closest to you, if you re operating the instrument, is the sample beam. Logically, there is a sample holder for the sample beam, and your sample goes there. And there, a beam of IR radiation goes through your sample. 

Sample beam aperture. This is where you put the holder containing your sample, be it mull or KBr pellet. You slip the holder into the aperture window for analysis. 

Additionally, with the inclusion of computers as part of an instrument, mathematical manipulation of data was possible. Not only could retention times be recorded automatically in chromatograms but areas under curves could also be calculated and data deconvoluted. In addition, computers made the development of Fourier transform instrumentation, of all kinds, practical. This type of instrument acquires data in one pass of the sample beam. The data are in what is termed the time domain, and application of the Fourier transform mathematical operation converts this data into the frequency domain, producing a frequency spectrum. The value of this methodology is that because it is rapid, multiple scans can be added together to reduce noise and interference, and the data are in a form that can easily be added to reports. 

Figure 3.1 Schematic diagram of an AAS spectrometer. A is the light source (hollow cathode lamp), B is the beam chopper (see Fig. 3.2), C is the burner, D the monochromator, E the photomultiplier detector, and F the computer for data analysis. In the single beam instrument, the beam from the lamp is modulated by the beam chopper (to reduce noise) and passes directly through the flame (solid light path). In a double beam instrument the beam chopper is angled and the rear surface reflective, so that part of the beam is passed along the reference beam path (dashed line), and is then recombined with the sample beam by a half-silvered mirror.

Figure 3.2 Beam chopper in AAS. In a single beam instrument it is mounted vertically off-centre, so that it chops the beam. In a dual-beam instrument it is angled and mirrored so that it alternately allows the sample beam through and reflects the reference beam along the secondary path.

IR spectrometers have the same components as UY/visible, except the materials need to be specially selected for their transmission properties in the IR (e.g., NaCl prisms for the monochromators). The radiation source is simply an inert substance heated to about 1500 °C (e.g., the Nernst glower, which uses a cylinder composed of rare earth oxides). Detection is usually by a thermal detector, such as a simple thermocouple, or some similar device. Two-beam system instruments often work on the null principle, in which the power of the reference beam is mechanically attenuated by the gradual insertion of a wedge-shaped absorber inserted into the beam, until it matches the power in the sample beam. In a simple ( flatbed ) system with a chart recorder, the movement of the mechanical attenuator is directly linked to the chart recorder. The output spectrum is essentially a record of the degree of... 

The pair of sodium chloride plates enclosing the liquid is then placed in the path of the sample beam and spectra recorded. 

Very few organic compounds can be examined as gases. The gaseous sample is introduced into a gas cell having the walls made up of sodium chloride. This cell can be directly put in the path of sample beam. The gas cell is usually 10 cm long. 

The two beams are subsequently reflected on a rotating segmented mirror called chopper C. The chopper rotating = 10 times per second helps the sample beam and the reference beam to be reflected alternatively to the monochromator grating D. 

The light from the infrared source C is made to split into two beams one of which passes through the sample i.e., the sample beam) while the other caters as the reference beam. This sort of double-beam arrangement facilitates in measuring difference in intensities between the two beams at each wavelength,... 

Figure 5.29. PAS/IR spectra of PTFE containing 1% polyimide (wt/wt) before (a) and after (b) exposure to subthreshold laser photons. Sampling (beam) depth is about 4 pm (from Davis etal.u).

The photolyzing light pulse is produced by a dye laser and enters the sample at about 10° to the axis of the sample beam. The observation beam originates from a 75-W xenon arc lamp. The apparatus is supplied by OLIS, Athens, Georgia USA. Reproduced with permission from C. A. Sawicki and R. J. Morris, Flash Photolysis of Hemoglobin, in Methods in Enzymology (E. Antonini, L. R. Bernardi, E. Chiancone eds.), 76, 667 (1981). 

A further benefit of the low absorbtivity of most samples in the NIR is that measurements involving scattering effects (both diffuse transmission and diffuse reflectance) are possible. The penetration depth of the sampling beam in diffuse reflectance measurements of powders in the NIR can be on the scale of millimeters, despite the numerous refractive index interfaces and extended optical path involved. Thus relatively large volumes of material can be interrogated, avoiding problems of surface contamination and sample nonhomogeneity. 

Flexural strength, or cross-breaking strength, is a measure of the bending strength or stiffness of a test bar specimen used as a sample beam in accordance with ASTM D-790. The specimen is placed on supports (Figure 14.13). A load is applied to its center at a specified rate... 

The laser hres at f = 0 and causes an increase in absorbance in the sample as a consequence the intensity of light reaching the detector decreases. While laser photolysis systems are normally single-beam spectrometers, in fact they behave as dual-beam instruments. The reference beam is separated from the sample beam in time, rather than space. Thus, the reference signal is acquired before laser excitation and leads to Iq. The absorbance at time t in Figure 18.3 is given by Eq. 1 ... 

Oscilloscopic observations were made to estimate the mixing and reaction times. Tests with dyes showed that the mixing time was well within the 33-msec, chopping time of the spectrophotometer. Brief observations during the approximately 8-msec, view of the sample beam permitted each cycle allowed an estimate of 15 to 20 msec, for the mixing time. Similar observations with ferrous mercapto-acetate solutions injected into oxygen-containing buffers showed no differences from the tests with dyes. Apparently the oxidation is completed within a few milliseconds or less. 

When insufficient sample is available, micro-sampling becomes necessary. Preparation of a KBr micro-disk should take into account sample beam geometry such that all the sample is in the beam, allowing high sensitivity. An alternate to the KBr... 

---