An Improved Spectrophotometer Flow Cell Holder Paul E. Strandjord and Kathleen J. Clayson Department of Laboratory Medicine, College of Medical Sciences, University of Minnesota, Minneapolis, Minn. 55455 AUTOMATED CHEMICAL ANALYSES which employ spectrophotometers in continuous flow systems present a variety of problems in reference to the design of cuvettes and cuvette holders. The requirements for these cuvettes include the usual considerations of length of light path, cell volume, and optical properties. In addition, air trapping and flow characteristics must also be considered. Requirements in respect to these latter characteristics vary in accordance with the system of analysis in which the cell is being employed. For example, air infrequently enters a cell when it is used in a system designed to monitor the absorbance of a chromatographic effluent. O n the other hand, air frequently enters a cell when it is used in an analytical system which employs intermittent sampling and frequent changes of reagents ( I ) . Cuvette manufacturers provide accurate information concerning optical properties, length of light path, and cell volume. Less information is usually available concerning flow characteristics and the problem of air trapping. These latter considerations are of considerable importance in automated chemical analyses. This communication describes a cell holder for use in spectrophotometers such as the Beckman DU with cells such as the Beckman liquid micro-aperture flow cell. This cell holder and flow cell provide a system which meets all of the requirements outlined above in reference to automated spectrophotometric analyses with a continuous flow system. If a cell such as the Beckman flow cell is used in the customary vertical position, air which is admitted to the cell is trapped in the light path. The cell holder illustrated in Figure 1 eliminates this problem by positioning the cuvette at an angle. Air which is admitted to the cuvette rises to the upper portion of the fluid chamber. The cell holder is designed so that light passes through only the lower portion of the cuvette’s fluid chamber. Hence, when air is admitted to the cell in this position, the air does not affect absorbance. Systems for automated chemical analyses which employ continuous pumping of reagents usually employ air as a means of separating adjacent samples ( 2 ) . A glass T is placed immediately adjacent to the spectrophotometer cell chamber to permit air bubbles in the stream to pass out the exit arm of the T. Accordingly, these air bubbles d o not present a problem as far as air trapping is concerned. Unfortunately, such a T will not prevent air from entering the cuvette when any operation even temporarily decreases the fluid-air ratio of the system. Such a decrease frequently occurs, for example, when reagents are changed. Cuvettes have also been designed which permit fluid to enter from the bottom of the cell (3). Such cells have the (1) P. E. Strandjord and K. J. Clayson, J. Lab. Clin. Med., 67, 131 (1966). (2) AutoAnalyzer Manual, Technicon Instruments Corp., Ardsley, N. Y . , 1958. (3) N. G . Anderson, ANAL.CHEM., 33,970(1961).
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CUVETTE HOLDER Figure 1. Flow cell holder ( A ) Position of screws which hold side plate in position. Screws are countersunk. ( B ) Handle. (C) Diaphragm. (D)Cuvette slot. Outside dimensions follow those of the spectrophotometer’s standard cuvette carrier. Dimensions of cuvette slot depend on flow cell
advantage of automatic air removal on the basis of gravity. The volume of these cells is usually greater than the volume of cells such as the Beckman cell, however, and the clearance characteristics are frequently inferior. The disadvantage of air trapping in cells such as the Beckman cell can be overcome by use of the cell holder illustrated in Figure 1. Automated chemical methods which employ spectrophotometric analysis of a continuously flowing stream of reagents require the use of a flow cell with small fluid volume, efficient clearance, and effective removal of air bubbles in addition to appropriate light path and optical properties. When cells such as the Beckman liquid micro-aperture flow cell are used in conjunction with the cell holder described, they fulfill all of these requirements. This cell holder can be used with automated chemical systems such as the Technicon AutoAnalyzer which employ continuously flowing streams separated by intermittent air bubbles. RECE~VED for review March 28,1968. Accepted July 15, 1968.
