INSTRUMENTATION
Electrophoretic Mass-Transport Analyzer
Uses various methods to measure the electrophoretic mobility of particles in concentrated suspensions.
• A research instrument for the colloid and surface sci entists involved in studies of particle interactions and re search in electrokinetic phe nomena • A quality control instrument for analyzing a wide range of industrial dispersions in cluding heavy slurries hav ing up to 70% of solids without dilution
and the detector output will be zero. This will also happen for all displace ments which are odd multiples of w/4— i.e., for retardations of ±w/2, ±3w/2, ± 5w/2, and so on—where the plus and minus signs denote displacements on either side of the zero position. Simi larly, constructive interference will oc cur at displacements of even multiples of w/4, with partial destructive inter ference occurring at in-between dis placements. If Μτ were moved incre mentally, the signal produced by the detector would fluctuate rhythmically and would, in fact, be a simple cosine wave. Suppose that, instead of being dis placed step-wise, the mirror is moved smoothly with a velocity V. As the cosine wave produced by the detector goes through one cycle if Mt is dis placed by a distance ω/2, the frequency of the detector signal is then, j=V/(w/2)=2Vv Or, for a constant mirror velocity, there is a linear relation between the fre quency ν (wave numbers) of the in coming monochromatic radiation and the frequency of the detector signal. For example, with a mirror velocity of 0.5 mm/sec, monochromatic radiation of 10 micron wavelength (1000 cm-1, frequency 3 χ 1014 Hz) will produce a detector signal of 50 Hz; for 5 micron radiation, /=100 Hz. The amplitude of the low frequency signal is propor tional to the intensity of the incoming monochromatic radiation. The exten sion to a radiation mixture follows from
this: each frequency component of polychromatic radiation is made to undergo such a transformation in fre quency and produces a detector wave of unique frequency. The signal or interferogram produced by the detector is then the summation of all such waves and is a complex signal like that shown in Figure 2. The center peak in the interferogram occurs when M t and M2 are equidistant from the beamsplitter, so that all components reaching the de tector have the same phase. The peak amplitude of the interferogram is pro portional to the total energy in the incident beam. The peaks of smaller amplitude along each side of the central spike carry intensity and frequency information. Data Reduction
Such an interferogram does not at all resemble a spectrum. It is, in fact, not a spectrum, but the Fourier Transform of a spectrum, and as such carries the desired intensity-frequency informa tion within it. In principle, all the spectral information may be extracted by performing the appropriate inverse Fourier transformation. In practice, re ducing an interferogram has not been easily accomplished until recently. The computations involved in data reduc tion are so complex, lengthy, and tedi ous that manual computation is out of the question. This difficulty, as well as the absence of suitable analog conver sion devices, was one of the factors which had kept Fourier Transform spectroscopy in obscurity. However, digital computer techniques and analog analysis are now readily available. The interferogram can be recorded on magnetic tape, punched cards, pa per tape, or directly in the core mem-
INTERFEROGRAM
• Zeta potential may be calcu lated from mobility data • Useful in studying chemical processes involving coagu lation,flocculation, deflocculation and flotation Price $2,420
Numinco®
NUMEC" InstrumentsandControlsCorporation 106 North Plaza, Apollo, PA. 15613 Phone: 412 · 478-1131
Circle No. 96 on Readers' Service Card 98 A
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ANALYTICAL CHEMISTRY
TIME BASE Figure 2. Interferogram of polychromatic source The central spike corresponds to the position where the two mirrors are equidistant from the beamsplitter. The abscissa denotes time and, if the mirror velocity is constant, the mirror position. An electronically-generated time base is produced at the same time as the interferogram. The single sharp spike on the extreme left of the lower trace marks the start of the interferogram; the rest of the regular signal is used as time base. With fringe-referenced systems (see text), the time base is produced by the interferometer itself
