tained. The arrows in Figure 1indicate wave lengths which might be so selected.
Maximum Value. At the same wave lengths, determinations were made of t h e absorbance of t h e solute, log loil,in which lo is the intensity of t h e beam when t h e cell is filled with solvent and I is t h e corresponding intensity with t h e cell filled with sample or calibrating solution. At each wave length, y B H C was estimated by referring the absorbance of the sample to a corresponding y-BHC Calibration curve, with no correction for overlapping bands. The lowest of these estimations is the "maximum value," in this instance 101.2% of actual. This maximum value is not merely an uncorrected value, or approximation. It is also a value which, within experimental error of the absorbances, will equal or exceed the actual value, except for the very unusual situation (in the 7.5- to 15.0-micron
Figure 1. Determination of T-BHC in BHC isomers b y baseline-density methods
range) where the analytical mode is weakened by molecular complexing of substances not present in the calibrating solutions. The maximum value technique has been applied with good results to the
determination of dieldrin and aldrin (insecticides) in emulsifiable concentrates. It may be of substantial value in regulatory and control work which requires only evidence that the amount present is below a stated value.
Improved Method of Heating Chromatographic Columns L. R. Bunney, Randall Phillips, and E. C. Freiling, U. S. Naval Radiological Defense Laboratory, San Francisco 24, Calif. N INVESTIGATIOKS involving ion ex-
1 change column
chromatography it \vas desired to operate columns above room temperature to increase the rate a t which ion exchange equilibrium is approached. The range of 80" to 90" C. includes most usual operating temperatures, but some experience has been obtained a t higher temperatures. Three methods of heating hare been used b y numerous investigators: refluxing vapors of a suitable liquid, such as trichloroethylene (boiling point 87 ' C.),
nN,/
ION FXCHANGE
I
I I I
II
circulating rvater from a constant temperature bath, and flexible heating tapes wrapped around a liquid-filled jacket. These methods all have drawbacks.
I
TOP CONTACT
ELUENT [ZONE PWHEATING (-
(
ION EXCHANGE COLUMN
A 48-inch length of tubing 30 mm. in outside diameter, divided into three sections wired in parallel, is used as a heating jacket for an ion exchange column, 13 mm. in outside diameter, as shown in Figure 1. Water is used as a heat transfer agent between the two columns for temperatures below 90' C. For operating temperatures in the range of 90' to 220' C., Dow Corning 550 fluid has given the most satisfactory performance. The temperature is controlled by a Variac. The maximum service temperature of the tubing is 350' C.
7' VARIAC
U Figure 1. Schematic diagram of chromatographic column jacket
968
ANALYTICAL CHEMISTRY
A method that has been in use in this laboratory for over a year utilizes a column heating jacket of electrically conducting Pyrex tubing (Corning Glass Co., Corning, N. Y., Catalog Xo. 9340). The electrically conductive film, which acts as a resistance element, is on the outside surface of the tubing. This film is chemically inert, very hard, and transparent. Using this technique reduces the complexity of the equipment necessary for heated column chromatography and increases the reliability of the experimental setup. The shock hazard from the equipment is small, if care is taken with the polarity of the Variacs.
TEMPERATURE 1%)
Figure 2. Temperature variation in column jacket at 85', 173", and 216" C.
The temperature variation along the length of the column under steadystate conditions is shown in Figure 2 for operating temperatures of 85" I-t Z', 173' f 3". and 216" ==I 6" C.
