the gas is transferred back and forth betwcm burcte 1 and 2 for mixing. The amount of time dcvoted to this operation is important. As much as 40 minutes was required for proper mixing and using shorter mixing times resulted in inconsistent results. This is probably the main drawback of this piece of equipment; however, it can be improved appreciably by including another bulb, of about 100-cc. capacity, helow the calibrated section of buret 1, to allow complete transfer of the gases from one buret to the other and to increase the amount of miyinp; in any given transfer. After the gases are properly blended, the mercury level in the central tube is lowered helow the mercury cutoff, and the blend is introduced into the vacuum-system manifold by opening stopcock C. From the values of pressure and volume of the gases the value of the PV product was calculated, by use of the ideal-gas law. The temperature of thc water bath was noted to ascertain the mercury-density correction
factor, but was not used to reduce the PV product to a standard temperature, because both gases are a t the same temperature and the per cent correction would be the same for both gases. The number of moles of each gas was computed from the PV product and the value of the actual molar volume of the gas. For the gases used in this workhydrogen and nitrogen-the above calculation procedure is justified, because the deviations from idcal gas behavior would introduce errors smaller than 0.01% in the final molar composition of the blend. The pressure readings were corrected for the capillarity effects of the tubes of the burets. The inside diameter of the manometer tube, 18 mm., was chosen to eliminate the need for any correction. The hollow bore of stopcocks A and B was partially filled with sealing wax, to eliminate the dead volume and the possibility of having an unmixed pocket of gas. For the same
reason the mercury level in the central tube is raised to point K . This dcsign of blending apparatus, in which the gases are measured in two independent burets, performed very satisfactorily after the effect of mixing time was ascertained and allowed for. It is believed that the average error in the composition of a blend was of thc order of 0.01'36, as evidenced by the data obtained in the calibration of an optical interferometer. LITERATURE CITED
(1) Bus~y, R. H., Barthauer,
G. L.,
Metier, A. V., IND.Esa. CHEM.,ANAL.
ED. 18,407 (1946).
(2) Cook, M. W., "Solubili;y of Hydrogen m Ron-Polar Solvents, Univ. Cali-
fornia Radiation Laboratory, Rept. UCRL-2459 (January 1054). (3) Langor, A,, Rev. Sn'. Inslr. 18, 101
Constant-Volume Fraction Collector for Column Chromatography
W. J. Wechter,' J. E. McCarty, and 5. E. Fisher, Department of Chemistry, University of California, 10s Angeles 24, Calif. XPERIENCE
with automatic fraction
E collectors has shown that a collector
which measures the eluate directly is preferable. Volumes measured by time flom (5, 6) will decrease appreciably as the resistance to flow increases with gradual packing of the column. The flow rate is also sensitive to temperature changes, and volumes measured hy time flow may vary as much as 50% owing to the decrease in density and viscosity attended by a corresponding temperature increase. Volumes measured by drop count (2, 8) are subject to three major difficulties: (1) Changes in concentration of the developing buffer will result in variations as great a6 30% ( 4 ) ; (2) temperature variations will affcct drop size and lead to inequities in the volumes collected; and (3) the column must be run slowly, so that individual drops may he counted by the photoelectric system. Fractions collccted by weight (7) will vary invcrscly as the density of the eluate. The density can change considerably during the course of a chromatogram. Although siphon-type collectors (4, 6) overcome all of the ahove difficulties, they are subject to others. As these collecton retain eluent in their siphon tubes, there is partial mixing of successive fractions. Such receivers 1
are subject to considerable losses by evaporation, except for the special siphon apparatus designed by Mader (6). Dutton and Castle ( I ) overcame these difficulties, in the design of their simple collector, which wm actuated by a photoelectrir volume device controlling a solenoid lifted iron ball valve. This equipment was designed for use with the Technicon fraction collector for volumes of from 5 to 20 ml. This automatic frytion collector collrets accurate volumes from 7 to 500 ml. (if the column is flowing at such a rate that thc eluate delivered during the time the dump valve is open is not a significant fraction of the volume of cluntc collected). As the receiver flask
Figure 1. Representative receiver flasks 135. loo-. and 500-ml.)
. .
is glass (Figure 1) and the valve impermeable to low surface tension solv e n t s i . e . , n-pentane-virtually any solvent employed in chromatography can be employed. The machine provides automatic changing of sample flasks by volume actuation, and an automatic shutoff after the last sample. This apparatus combines the virtues of loiv price (about 5180 for parts and materials exclusive of fabrication time), a wide range of adaptability with respect to fraction size, constint volume fractions, and good separation of the eluate from consecutive fractions. OPERATION
When the machine (see F i y r e 2)
~
I
Present address, The Upjohn Co.,
Kalamaaoo, Mich.
VOL. 31, NO. 1, JANUARY 1959
159
is sm%ched on, the table assembly may be rotated in either direction by means of the motor pushbutton and reversing switch on the motor control box. The motor switch takes precedence over the change sequences. The cycle is initiated by the start button, which closes the 8 volt-ampere relay, and turns on the photoccll light source and the filament of the 2050. When the receiver fills, the silvered glass float interrupts the light beam between the light source and the photoccll. When the light t,o the photocell is thus interrupted, its resistance rises and the bias level of the 2050 approachcs -1.4 volts, allowing the tube to conduct and close relay, R2. This relay in turn actuates a self-holding circuit through the selenium rrrt,ifiw, K 5 resistor, and rnicrosivitch 1. Bt the same timr, the dump solenoid and niotor time drlay relays ( R 3 and K 4 ) are actuated, allo\T-ing the rrcewer flask to emotv and dr& for 30 seconds. After i? additional srronds the motor rotates the t,ablr assembly. hficroswitchrs 1 and 2 are then reversed by the cam action of the recesses in the flnsk holdrr plate portion of thr tahlr assrmbly (Figure 3) and relay K 5 clos~s. Rela)- Z
