machine is complex, and depends on the size and shape of the tube, the type of support, the speed of the vibration, and the nature and amount of liquid in the tube. Two distinct general patterns of agitation were observed. Type 1, surface splashing, in its more vigorous manifestations violently agitates the entire contents of the tube, and may cause cavitation. Type 2, rotary agitation, results in the formation of a vortex which usually reaches to the bottom of the tube. In general, type 1 agitation appears at low speeds, increases in vigor, then transforms into type 2, as the speed increases. At the highest speeds, type 2 agitation usually reverts to type 1. An attempt was made to define the parameters of these types of agitation until it was discovered that there was no difference in evaporation rates. This is not the only consideration, how-
1 agitation is more liable to incur bumping than type 2, where the centrifugal force acting on the liquid tends to prevent spray-over. In the case of very rapid, ebullient distillation, it is often impossible to maintain type 2 agitation. The strength of the plunger springs is an important factor in determining the speed at which a given type of agitation will occur. Two types of springs were investigated (Figure 4,B). The lighter springs produce a given type of agitation at a lower speed than do the heavier. With vacuum adapters the heavier springs are advantageous. Lower operating speeds are desirable to reduce the noise level and the splashing of water from the heating bath.
ever, for type
TYPE OF TUBE
In general, the round-bottomed tubes
superior to conical tubes in evaporation rate, although the diameter appears to be relatively unimportant. For example, the evaporation rate in 16-mm. tubes was about the same as that in 20-
are
mm.
tubes.
VOLUME OF SOLUTION
In a given tube, small volumes evaporate more rapidly than large volumes. The maximum safe loads are (approximately) : 20 X 150 mm. culture tubes, 10 ml.; 12-ml. conical centrifuge tubes, 3 ml. Type 2 agitation cannot always be induced in 20 X 150 mm. tubes when the volume is less than 7 ml. When methanol is being evaporated, the volumes stated may need to be reduced because of the tendency of methanol to bump.
Method for Elimination of Interference Fringes in Spectra of Thin Films Charlotte Lutinski, The Perkin-Elmer Corp., Norwalk, Conn. of interference fringes
presence The in the infrared
transmittance spectra of thin organic coatings and films affects the accuracy with which these spectra can be analyzed. Interference fringes normally arise from multiple reflections between plane parallel surfaces. This effect can be observed in Figures 1 ,A, and 2,A. Fringes may occur throughout the entire infrared region. However, the sample thickness and absorption bands will determine where the fringes will be seen. The observed interference fringe pattern is superimposed on the absorption spectrum of the sample of interest and distorts band shape, thus affecting the
accuracy of qualitative and quantitative analysis. A method has been developed which eliminates the interference fringe pattern from the absorption spectrum. When the film is sufficiently thick, the separation of the fringes is less than the resolution of the infrared spectrometer. Films cast directly onto salt plates do not exhibit fringes. Because of the intimate contact, there is no abrupt reflective index change between the second film surface and the first salt surface. By coating a thin film with Nujol and pressing it against a thick rock salt window, an effective thick sample can be obtained which has
minimum surface reflection at the interface, and, therefore, no observable interference fringe pattern. Absorption bands introduced by the Nujol are w7eak, while the rock salt window does not introduce any additional absorption over the region of interest. If the C—H region is of interest, some nonhydrocarbon liquid such as perfluorokerosine may be used in place of Nujol. Other low index of refraction crystalline windows can also be used. Figures 1 and 2 illustrate this technique, utilizing, in the first case, reflectance to measure a coating, and, in the second case, transmittance directly through a film of polystyrene.
FREQUENCY (CM')
Figure 1.
Reflectance spectra showing absorption of this organic coating A.
on
metal
Interference fringes overlap absorption bands VOL. 30, NO. 12, DECEMBER 1958
·
2071
FREQUENCY (CM')
Figure 1. 8.
Reflectance spectra showing absorption of this organic coating
on
metal
Interference fringes eliminated by single rock salt window with Nujol between window and sample surface
FREQUENCY (CM')
ABSORBANCE
I
10000
5000 4000
3000
2500
2000 1800
1600
1400
FREQUENCY (CM’) 1200
1000
950
900
850
800
750
700
650
ABSORBANCE
Figure 2. A. 8.
Transmittance spectra showing absorption of polystyrene
Interference fringes present. Interferences fringes eliminated by sandwiching sample between two rock salt windows coated with Nu|ol.
2072
·
ANALYTICAL CHEMISTRY
True absorption spectrum is revealed
