Paper Strip Holder for Absorption Spectroscopy Robert M. Horowitz and Lawrence F. Atkinson, Fruit and Vegetable Chemistry Laboratory, Western Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture, Pasadena, Calif.
RADFICLD
and Flood ( J . Chem. SOC.
1952, 4740) recently published a
method for measuring the ultraviolet absorption spectra of substances directly on paper chromatograms. A holder for 'paper strips, designed specifically for use in a Model DU spectrophotometer, is extremely convenient for routine use. The area of paper containing the spot can be centered accurately a t the position of maximum concentration. The paper is held firmly in a fixed position while the spectrum is heing measured; this is essential if reproducihle readings are to be obtained. The paper may be removed after the initial determination of the spectrum, sprayed with a reagent n-hich effects a characteristic change in the spectrum, and replaced in the holder in almost emctly the same position that it occupied originally. This device is used in measuring the entire spectrum of a particular spot on a chroniatogram and is not suitable for scanning an entire chromatogram a t one m-aye length. Aluminum was used in construction, although other materials would serve equally nell. The holder consists of tn-o halves hinged a t the bottom which,
when closed and fastened with a clasp, become a unit having the same over-all dimensions as the cell holder furnished with the Model DU instrument. With the unit held in the closed position, four openings are cut n-hich have the shape, size, and spacing of the regular cell holder. In practice, the holder is opened and one side is placed flat on a table. An area of paper, about 1 X 2.5 cm., encompassing the spot in question, is
centered over the secoiiil, third, or fourth space. (The first space is reserved for a blank strip of paper.) The holder is then closed and the spectrum is determined in the usual manner. For readings below -255 nip the instrument is operated nith the selector switch set a t 0.1. The precise location of spots on the chromatogram may be determined by examining it in ultraviolet light or by spraying a parallel guide strip nith an appropriate reagent.
Polarographic Cell for Continuous Monitoring of Ion Exchange Effluents Charles
K. Mann,
Department of Chemistry, University of Texas, Austin 12, Tex.
w
HLS the elution characteristics of ion exchange columns are studied much labor can be saved if the effluent solution can be analyzed continuously and automatically. The polarographic cell described permits continuous analysis when the ion being studied is subject t o polarographic determination.
The cell, shown in Figure 1, coiisists of a 1-mm. thick-walled borosilicate glass tube connected to the column by a standard-taper joint. The tube is fitted with a port to admit the dropping mercury capillary and, directly opposite, a side arm to hold the mercury pool anode. -4glass flange around the electrode opening acts as a seat for a rubber gasket used to obtain a temporary water-tight seal. The body of the cell
Figure 1.
Polarographic cell
must be reamed out to accommodate the end of the capillary. Polarographic equipment and techniques are essentially the same as for amperometric titration. A potential from the plateau beyond the polarographic wave of the ion in question is
imposed across the ccdl electrodes. Elution of reducible material from the column is indicated by a n increase in current flow. Figure 2 shows the curve obtained for the elution of 0.010 mmole of lead with 1.05N nitric acid from a column of D o w r 50 resin operated a t a flow rate of S.3 cm. per minute. This flow rate corrcsponds to 1.00 ml. per minute. A Sargent Model XYI pen recording polarograph was used to record the curve. The polarograpli was adjusted to furnish a const,ant potential of -0.92 volt, us. the mercury pool and current iyas measured a t a sensitivity of 0.06 Fa. per inin. With a constant flow through the column, the volume of effluent can he estimated directly from the chart. When the reduction current at a dropping mercury electrode is nieasured VOL. 2 9 , NO. 9, SEPTEMBER 1957
1385
Table
1.
Effect of Flow Rate on Reduction Current
(5 X 10-4M CdC12in 0.2M KC1)
Flow Rate,
Wave Height, Mm.
0 0.43
199 199 200 199 200 201 198 208
Cm./Min. 1.77 1.1
1.7 2.4 12 18
in a moving solution, current flow is higher than for a stationary solution.
This is expect,ed, as movement of the solution would augment the normal diffusion process. This effect becomes appreciable only a t fairly high flow rates. (Table I). I n this application, it is not necessary to measure the absolute vaIue of current; only a distinct change in current flow is needed to indicate breakthrough from the column. A uniformly enhanced current flow due to movement of solution through the cell will have no deleterious effect. If the flow rate is substantially increased above the values shown in Table I-Le., to 33 cm. per minute, corresponding to 4 ml. per minute-current flow becomes very irregular. This imposes a restriction upon the size of column with which a cell could be used. This difficulty could be circumvented by using a cell
T------l K
0
1
2
3
4
TIME, MINUTES
Figure
2.
Elution curve
for
lead
0.010 mmole Pb eluted with 1.05N HNOJ from Dowex 50; flow rate, 8.3 cm./min.; polarographic sensitivity, 0.06 p . / m r n .
of larger inside diameter in conjunction with larger ion exchange columns.
Thermal Conductivity Cell for Gas Chromatography S. A. Ryce, Paul Kebarle, and W. A. Bryce, Department of Chemistry, University of British Columbia, Vancouver, B. C. used for detecting eluted substances in the carrier gas stream in gas chromatography (3, 5, 8) usually measure the change in some physical property of thegas stream caused by the presence of the eluate in the carrier g.as. Properties of use in this connection include thermal conductivity, density, viscosity, infrared absorption, and heat of adsorption. Measurement of change in thermal conductivity has proved to be a reliable and sensitive method, involving standard electrical equipment which will operate with great stability over long periods of time. Various types of thermal conductivity cells have been used in gas chromatography (1-9). Several models are available commercially. The cell described in the present report is inexpensive to construct and has performed satisfactorily for 18 months for both partition-elution and adsorption-elution chromatography . DEVICES
The construction details of the cell are shown in Figure 1. The body is a brass block through which channels containing the detecting and reference filaments are drilled. Excellent temperature compensation was achieved with the metal cell. The use of brass as the cell material did not interfere with the determination of even such chemically active compounds as hydrogen sulfide and mercaptans (9). The filaments are mounted on brass plugs which are sealed to the body of the cell with rubber O-rings. Gaskets with greater heat resistance can be used a t elevated temperatures. The filaments are helices of 0.3-cm. diameter containipg 14 cm. of 0.005-cm. platinum wire. They are silver-soldered to the ends of Kovar terminals through 1386
ANALYTICAL CHEMISTRY
which the external electrical connections are made. The operating teniperature of the filaments was approximately 100" C. a t a current of 200 ma. This temperature is sufficiently low to eliminate pyrolysis of thermolabile compounds on the filament. The entire stream from the column can, therefore. pass through the detecting channel without the use of a bypass system (4) if collection of the separated sample components is desired. The reference and detecting channels are symmetrical with both filaments located in the center of the gas stream. I n certain commercial models the compensating filament is situated in a diffusion cavity, an arrangement which offers practically no flow-rate compensa-
tion ( 7 ) . This latter factor is of great importance in operations in which a rising column temperature is used. In experiments described elsewhere (9) with the present cell a flow-rate change from 50 to 30 ml. per minute over 20 minutes had no effect on the base-line setting of the recorder. The minimal amount of material detectable with the cell was approximately 10-8 mole. The noise level was insignificant at maximum sensitivity. Compensations for changes in ambient temperature are sufficient t o eliminate the need for thermostating the cell, unless a high degree of reproducibility is desired over an extended period of time.
1215 B R A S S
.-
'AMENT
Figure 1. a.
b. C.
LOCATING PIN
Thermal conductivity cell
Frontal section Side section showing detail of one channel only Detail of filament support
