The water comes off a t a low temperature and oxalate decomposition occurs a t a much lower temperature. The final decomposition is alniost instantaneous. The evolution of carbon monoxide and dioxide does not appreciably inhibit the approach of oxygen to the sample, so the ox4dation of the cobalt occurs very rapidly, providing the energy for the decomposition of the oxalate. The significance of this and the previous work is that me have shown that the most common method of supporting a sample is not a t all satisfactory. The wall of the traditional crucible restricts the product gases sufficiently t o affect the partial prewire over the
sample. Either of two techniques wi11 provide a substantial improvement in weight-loss curves for reversible and irreversible decompositions and without recourse to controlled-atmosphere or vacuum thermobalances. Significant improvement is obtained even on pyrolysis of organic materials. The shallow vessel will provide partial pressures that approach the ambient pressures, but these partial pressures will remain nearly constant, so that the weight loss will occur at essentially constant pressure. The closed vessel also provides a constant pressure, but in this case the partial pressure of the product gas is 1 atm. In addition, the restriction of the product gas leads to the interesting and useful condition
A Semiautomatic Plotting Attachment
in which the atmosphere during a decomposition is always that which is pertinent to the reaction. The importance of sample holder design in thermogravimetry is closely related to the problems of reproducibility (between laboratories) of differential thermal analysis curves and correlation cf thermogravimetric and differential thermal analysis data. Both problems have plagued workers in thermal analysis. We expect to develop differential thermal analysis techniques permitting closer correlation of the data with thermogravimetric data and enabling the virtual duplication of differential thermal analysis data in apparatus dissimilar except for the sample holder.
for the Beckman DU Spectrophotometer
George N. Bowers, Jr.l, and Samuel Raymond, The William Pepper Laboratory of Clinical Pathology, University of Pennsylvania Philadelphia 4, Pa.
s MANY APPLICATIONS of spectrophotometry, it is necessary to plot absorbance readings as a function of time. S n example is the determination of transaminase activity by measurement of the rate of change of reduced diphosphopyridine nucleotide absorbancy in a suitable enzyme-substrate mixture. To obtain valid results, the enzyme concentration must be estimated from the zero-order slope of a plot of absorbancy against time. The attachment (Figure 1) eliminates recording and manual plotting of such Present address, Hartford Hospital, Hartford, Conn.
Figure 1.
readings. A steel tape on the circumference of the cam, A , which is attached t o the transmission-density knob of the Beckman DU spectrophotometer, positions the typewriter carriage according to the absorbance. The synchronous motor, B, moves chart paper a t a uniform rate past the printing area. When the spectrophotometer is balanced, a manually operated momentary-contact switch, C, activates a solenoid, D, on the period key of the typewriter, thus imprinting a dot. The only modification of the spectrophotometer is a 3/32-inch centering hole drilled into the transmission-density knob. The plastic cam is centered on the knob by
a pin and held in place with two clamps. The instrument is not impaired for other uses and, if need be, the steel tape and the cam are easily removed. Any typewriter may be easily adapted for holding and moving the chart paper. To make this adaptation, release the mechanisms controlling movement of the carriage and rotation of the paper roller. Attach a constant speed drive motor (1/5 r.p.m., from Haydon Motors, Torrington, Conn.) to the shaft of the paper roller to drive the paper a t a speed of approximately 1 inch per minute. The transverse motion of the carriage
Cam-typewriter arrangement for semiautomatic plotting attachment VOL. 32, NO. 13, DECEMBER 1960
e
1901
J iI Figure 2.
Construction details of cam
O'
1
2
3JiluIEi
4
5
6
Figure 3. Absorbance vs. time plots of three concurrent measurements illustrating typical data obtained in practice A.
Initial low absorption Zero-order kinetics over entire measured time interval Deviation from zero-order kinetics terminally because of high enzymatic activity Serum glutaric-oxalacetic-transaminase, substrate-enzyme mixture tested (A absorbance of diphosphopyridine nucleotide at 37' C.)
B.
C.
is controlled by a steel tape (l/&ch wide measuring tape from local hardware store) connecting the carriage to the transmission-density cam and acting against the return spring of the carriage. The tape should be long, to minimize angular error due to changing radius of the c x n . On our first model a length of 3 feet was adequate. The cam and tape are adjusted so that the carriage is at the ex%remeright when the absorbance is set a t zero. As the density knob and cam are rotated, the tape unwinds from the cam and pays out, allowing the carriage to be moved to the left by the carriage return spring.
Table 1.
Cam Measurements
(Material: Plexiglas, Points at Intervals of 22.5 Degrees
to 6/&nch)
Radius, Inches
m
The angular displacement of the density knob is proportional t o the optical transmission. The, transverse motion of the carriage is, however, required to be proportional to the absorbance-i.e, to the log l/8,where 8 is the angular displacement. The design of the cam must be such, there1902
e
ANALYTICAL CHEMISTRY
~
fore, that the length of the arc along the cam (equal to the length of tape payed out) is proportional to log 1/8. The dimensions of the cam are given in Figure 2 and Table I. A cam of the shape and dimensions shown gives a linear movement of the paper carriage amounting to 2.6 cm. for a change of 0.100 absorbance and is accurate to within 2% in the range of 0.0 to 0.5 absorbance. The cam was made of l/r-inch Plexiglas by E.C. Apparatus Co., Swarthmore, Pa. This degree of accuracy is sufficient for most purposes. The cam is centered on the axis of the transmission-density knob by a small pin. The angular position relative to the knob i s such that the tape leads off parallel to the 5-inch dimension when the density knob is set at zero. It is clamped in this position by two bolts 1 '/4 inches from the center, holding a nut and washer against the underside of the knob. The tape is attached to the cam a t the pointed end and is wound counterclockwise around the edge. The third component of this attachment is a mechanism for imprinting a dot on the paper. The period key and type bar of the typewriter are used for this purpose. The period key can be activated manually when the balance is reached. However, because the typewriter is some distance from the transmission knob, it i s convenient to use a solenoid-activated plunger to depress the period key, recording a dot a t the correct absorbance and time when the instrument is balanced. A momentary contact switch (Birnbach 6225) is used to activate the solenoid (Guardian Type 4), both of which are obtainable a t radio supply stores. The printing
mechanism remains stationary while the paper is moved transversely past it. This is contrary to the usual practice in recorders which generally move the printing device or recording pen. In using this attachment, blanks, standard solutions, and unknowns are placed in the cell compartment. Wave length, dark current, and other adjustments are made as usual. A sheet of chart paper is inserted into the paper carriage, and with the density knob manually set a t zero, a dot is made on the paper by manually activating the printing mechanisms. The spectrophotometer is brought into balance manually and another dot is made. The transverse displacement of the two dots is then a measure of the absorbance indicated. When the time reactions are being measured, with the motor drive turned on, the spectrophotometer can be balanced at any time interval imprinting a dot a t each balance point. It is not necessary to note the time separately in this operation as the longitudinal displacement of the dot on the chart measures the elapsed time. Three reactions can be followed simultaneously, as shown in Figure 3, with frequent zeroing of the instrument on the blank to correct for the inherent electrical instability which occurs over periods of several minutes. It is preferable to return the density knob to zero a t intervals establishing the base line of zero absorbance on the chart. For fast reactions it is advisable to adjust the balance needle a little below zero, then imprint the dot when the change due to the reaction returns the needle to zero. Changes of 0.400 absorbance within 30 seconds to a minute have been easily followed.
