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Cell Matching

What does matching mean?

Matching of cells refers to the degree to which absorption cells give similar absorbance or transmission reading when empty or filled with water. The practice was started early in the history of spectrophotometer cell manufacturing and absorption instruments were single beam (lacking the ability to automatically adjust for a blank). As high quality cells from major manufacturers like Starna have become the norm and instruments have improved the concept of matching has become less important. A poor cell can appear matched because measuring a cell with no sample does not test the accuracy of the pathlength nor the dimensional quality of the windows. The important elements of cell quality are listed below with the specifications that you can expect from Starna cells.

Window Parallelism

The windows must be parallel so that the pathlength remains static over the entire cell window.
Quality Parameter Specification
Parallelity of Windows better than 3 minutes of arc

Window Flatness

The windows must be as flat as possible so that the light is not focused, reflected or refracted.
Quality Parameter Specification
Flatness of Windows less than 4 Newton Fringes
Window Polish
The windows must be polished to a high tolerance to keep light dispersion to a minimum
Quality Parameter Specification
Window Polish 60/40 scratch/dig
High tolerance pathlength
The distance between the interior of the cell's windows (the pathlength) must be maintained to a high tolerance. The table below specifies the maximum tolerance for a Starna cell. Due to the characteristics of each material, the tolerances are different for each material.
Window Material Pathlength Range Pathlength Tolerance
Glass up to 20 mm +/- 0.1 mm
Glass 30 to 100 mm +/- 0.2 mm
Special Optical Glass up to 20mm +/- 0.01 mm
Special Optical Glass 30 to 100 mm +/- 0.02 mm
Quartz up to 0.05 mm +/- 0.001 mm
Quartz 0.1 to 0.4 mm +/- 0.005 mm
Quartz 0.5 to 100 mm +/- 0.01 mm
All of the above factors are critical to the performance of a spectrophotometer and a fluorimeter cell. When all parameters are maintained to a high degree the cell is "matched" by the fact that there is little difference in any of the cells of the same physical configuration, material and pathlength. We manufacture cells to such a high tolerance that we provide "better than matched".
Can Fluorimeter Cells be matched?
By definition matching is the testing of absorbtion directly through the pathlength of a cell and does not address any parameters used in fluorimetry. Using a high quality cell that is constructed of "background fluorescence free" quartz is much more important. Our Spectrosil quartz is an excellent material for fluorescence.

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