Noise Discrimination by Signal Inversion in Nuclear Magnetic Resonance Spectrometry A. L. Van Geet and L. D. Wechslerl Department of Chemistry, State University of New York a t Buffalo, Buffalo, N. Y. 14214
To DISTINGUISH a weak spectrum from noise, it is advantageous to display the inverse of the spectrum. An example is the N M R spectrum of a 1 % solution of ethylbenzene. Its spectrum (u-mode), taken with a Varian A60 NMR spectrometer, is shown in Figure 1, together with the inverted spectrum, l/u. Inversion has the advantage that the weak signals are amplified much more than the strong signals. If the signal is very weak, l / u becomes quite large, and the recorder runs off scale, as seen in Figure 1. Thus, very weak signals are discriminated against. In order to avoid cutting off true signals, the discrimination is adjusted by use of a potentiometer so that some noise comes through, perhaps one noise pulse every few seconds. From the Bloch equations, it follows that the inverted signal has a parabolic shape, l/c. =
(l/u>max
+
i’2tWO
- W)*/YHIMO
(1)
This shape helps to distinguish visually a signal peak from a noise spike. The inversion technique works only when the signal is stronger than the noise. It is not capable of pulling weak signals out from higher noise levels and is, therefore, not a substitute for filtering or a time-averaging computer. The inversion was accomplished using a Heath servo recorder EUW-2OA and a continuously variable voltage source, as shown in Figure 2. The five-prong plugs A , B, and C in the rear of the recorder were pulled out, and connections were made as indicated. The ground reference points were interconnected, and the sensitivity switch was set on “external.” The Heath EUW-16 was used as a variable voltage source, but a 1.5-V dry cell loaded by a 30-kQ resistor and a 2004 potentiometer in series should serve equally as well. The voltage at point C2 is f u , where u is the output voltage of the NMR, and f is a constant attenuation factor due to the voltage drop over the 120-kn resistor: f = lkn/(l20kQ IkQ. The voltage a t point B1 is pfu, where the pen position p varies from zero to one. The recorder amplifier only responds to ac signals, and the servomotor is a t balance when the points B3 and A2 of the chopper have the same potential. Thus
+
E
=
I
Figure 1. The normal (top) and inverted (bottom) NMR spectrum of ethylbenzene
(2)
pfv
where E is the output of the voltage source. When u = Eif, the pen will have its maximum deflection. For smaller signals, the recorder will run off scale. E was adjusted for optimum discrimination. If adjustment of f is desired, the 120-kQ resistor may be replaced by a variable one, such as the Heath EUW-30 decade resistance box
* Present address, Bechtel Corp., Gaithersburg.
_-
Md
The NMR spectrum was scanned a t the desired sweep width using a slow sweep time (250 or 500 seconds). For purposes of comparison, the paper speed of the servo recorder should match that of the NMR recorder. The load of the servo recorder reduces the sensitivity of the NMR recorder by a factor 2. This is compensated for by increasing the spectrum amplitude gain of the NMR accordingly. The output of the NMR is equipped with a low-pass noise filter. The cutoff frequency, Y , = 1/2aRC, of this RC filter is selected by the “filter bandwidth” switch on the front panel. The cutoff frequency is defined as the frequency which is
b Figure 2. Inversion of the NMR signal
I
I
I RED
., A
,
- c A 3 A2 .
r-
IBiACK attenuated by a factor l/& Noise components with a higher frequency are progressively removed. For optimum noise filtering, the time constant, RC, should be about equal to the time spent in traversing the resonance. At a sweep rate of 1 Hzlsecond, a peak which is inhomogeneously broadened to 0.5 Hz is traversed in 0.5 second, and the optimum setting of the bandwidth switch would be vc = 1/2n X 0.5 = 0.32 Hz. Because this setting is not available, the nearest position is selected, which is 0.4 Hz. If the cutoff frequency selected
HEATH RECORDER
EUW-20A
is too low (too much filtering), the signal is removed along with the noise. The system was satisfactory except for the rather unreliable paper drive of the servo recorder. Inversion of the signal may also be useful for ESR and optical spectra. RECEIVED for review February 3, 1967. Accepted March 27, 1967.
Improved Gas Chromatography Packings with Fluidized Drying R . F. Kruppa, R. S. Ilenly, and D. L. Smead Applied Science Luborarories, Inc., P . 0 . Box 440, State College, Pa. 16801
THEPREPARATION of column packings for gas chromatography is a critical step in the use of that analytical tool. The two ieading methods of preparation, slurry (1) and fitration (solution-coating) (2), have been frequently described, with the latter method preferred for low-loaded-i.e., 3 Z-packings. Regardless of which method is employed, the final drying of the packing is accomplished in several different ways (1,3,4). Parcher and Urone (5) described the use of a fluidized drying technique, claiming the advantages to be: “(1) a better assurance of a uniform coating. . ., (2) a distinct saving in time, (3) less fragmentalion. . .,” and “(4) expulsion of the fine particles and light impurities.” If their conclusions (l), (3), and (4) are correct, then the efficiency of a column prepared from such a packing should be better than one prepared from a packing made by the most commonly used method-namely, tray drying without stirring. I n addition, the assurance of a more uniform coating should result in greater reproducibility of packings from batch to batch. This paper reports a simple, durable fluidizer which was designed, constructed, and tested specifically for drying of
packings made by the filtration technique, although the slurry method can also be employed if the procedure of Kaiser (6) is followed. Comparisons with packings dried in a tray without stirring were made by measuring column efficiencies and packing reproducibility. EXPERIMENTAL
Fluidized Bed Drying. If a gas is passed upward through a porous plate covered with a finely divided solid, the particles are suspended and intimately mixed in the gas stream. The suspended particles exhibit fluid characteristics during this process and the phenomenon is referred to as fluidization. Among all the known drying methods, fluidization is the most rapid and effective means of drying a damp solid. T o achieve maximum drying speed and efficiency, the drying medium (in this case nitrogen) should be preheated. With this technique, optimum heat transfer (resulting in optimum drying efficiency) is achieved by convection from the fluidizing gas to the solid particles as opposed to radiation and conduction from the walls of the containing vessel. In addition, the bed temperature is uniform and closely controllable (7). Gas velocity during fluidized drying is critical if particle attrition is to be kept to a minimum (8). With excessive gas
(1) Howard Purnell, ‘‘Gat, Chromatography,’’ Wiley, New York, 1962, p. 240. (2) E. C . Horning, E. A. Moscatelli, and C. C. Sweeley, Chem. Ind. (London),1959,751.
(3) W.J. Zubyk and A. Z . Conner, ANAL.CHEM., 32,912(1960). (4) H. H. Wotiz and S. C. Chattoraj, Zbid.,36, 1467 (1964). ( 5 ) J. F. Parcher and P. Urone, J . Gas Chromatog., 2, 184 (1964).
(6) Rudolf Kaiser, “Gas Phase Chromatography,” Vol. I, Butterworths, Washington, D. C.,1963,p. 56. (7)V. Vanecek, M. Markvart, and R. Drbohlav, “Fluidized Bed Drying,” Leonard Hill, London (1966),p. 168. (8) lbid.,p. 167. VOL. 3 9 , NO. 7, JUNE 1967
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