183 



cell compartment serve as on-off switches for the light source and amplifier in- 

 put. In operation, one need only place the absorption cell in its compartment, 

 close the door, and read the dial when it has come to balance. Narrow band 

 interference filters can be obtained for any +10m|i band in the spectral range in 

 which a glass-covered photocell will respond? The instrument will accommo- 

 date cylindrical end window type absorption cells up to 150 mm cell path. An 

 ASOP has been used at the Chesapeake Bay Institute for nearly two years and the 

 only maintenance other than routine cleaning has been the replacement of a paper 

 per cent transmission dial by one imbedded in plastic. 



Ford (1950) described a photoelectric filter photometer that was designed 

 and constructed especially for use at sea. The instrument has been named 

 Electric Eye Photometer or EEP after the electric eye used as a null indicator. 

 It uses a 100 watt projector bulb and an optical system to form a split light 

 beam, one half of which passes through a lucite rod and the other half through 

 the test solution. The two emergent light beams fall on a pair of matched No. 

 926 phototubes. The output difference of the phototubes goes through one stage 

 of amplification (type 38 tube) and onto a 6E5 electric eye null indicator. Bal- 

 ance is obtained by moving a logarithmic shaped aperture into the beam passing 

 through the lucite rod. A vernier adjustment used to position the aperture pro- 

 vides the balance reading when the electric eye has been brought to minimum 

 slit. The original design employs absorption cells of 26, 9, or 1.5 cm absorp- 

 tion path. The 26 and 9 cm cells are modified Nessler tubes used vertically 

 with a glass plunger immersed in the test solution to reduce meniscus effects. 



With both the 9 and 1.5 cm cells, a lucite spacer columnates the light 

 from the aperture mechanism to the end of the absorption cell. With the 1.5 

 cm cell, which is square, the instrument is used in a horizontal position. The 

 power supply includes a pair of VR105 voltage regulators, and supply variations 

 from 105 to 130 volts "produce no appreciable errors in the operation of the in- 

 strument. " The 26 cm cell path is longer than is available in any commercial 

 instrument except the Beckman model DU. The 50 cm Beckman cell, however, 

 requires special attachments and is not convenient to use intermittently with 

 shorter cells. 



CONCENTRATION TECHNIQUES 



Most of the dissolved substances in sea water are in such low concentra- 

 tion that they cannot be determined by direct analysis. They must be concen- 

 trated before an analysis can be performed. Sverdrup et al. (1946) (Table 36) 

 list forty-four elements as present in solution in sea water. The major con- 

 stituents, chlorine, sodiunm, magnesium, sulphur, calcium, and potassium 

 miake up 94.2% by weight of the dissolved solids. Each of these can be deter- 

 mined without prior concentration. Of the remaining thirty-eight, thirteen 

 are in the range 10"^ to 10"^%, seventeen in the range 10"^ to 10"^%, and eight 

 are less than 10~8%. Of these only a few can be determined without prior con- 

 centration. In addition to these inorganic constituents, in vitro experiments 

 with phytoplankton organisms are pointing to the importance of micro-quantities 

 of dissolved organic substances in the role of growth promoting and inhibiting 

 agents. Nothing is known of the range of concentration of most of these sub- 

 stances in the sea. 



The importance of many of the minor elements in geochemical and bio- 

 logical processes is of course an open question. It inay be significant, how- 

 ever, that the presence of several elements in sea water was unknown until 

 found in the rennains of marine organisms where they had been concentrated by 

 metabolic processes. Some trace constituents, however, are known to play 

 major roles in marine systems, yet, lacking information about the distribution 



