Matching of cells refers to the degree to which absorbtion cells will give a similar absorbance or transmission reading when empty or filled with water. The practice started at a time when spectrophotometer cells were not made very well and most absorbtion instruments were single beam (no ability to automatically adjust for the 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 path length 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 brand cells.Window Parallelism
The windows must be parallel so that the path length remains static over the entire cell window. Parallelity of Windows is better than 3 minutes of arcWindow Flatness
The windows must be as flat as possible so that the light is not focused, reflected or refracted. 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. Window Polish specification is 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 brand cell. Due to the characteristics of each material, the tolerances are different for each material.
|Window Material||Pathlength Range||Pathlength Tolerance|
|Glass||less than 10 mm||+/- 0.02 mm|
|Glass||10 to 20 mm||+/- 0.1 mm|
|Glass||20 to 100 mm||+/- 0.2 mm|
|Special Optical Glass||up to 20mm||+/- 0.01 mm|
|Special Optical Glass||20 to 100 mm||+/- 0.02 mm|
|Quartz||up to 0.05 mm||+/- 0.002 mm|
|Quartz||0.1 to 0.4 mm||+/- 0.005 mm|
|Quartz||0.5 to 20 mm||+/- 0.01 mm|
|Quartz||20 to 100 mm||+/- 0.02 mm|
How do specifications influence matching?
All of the above factors are critical to the performance of a spectrophotometer and a fluorometer 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 Fluorometer 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 fluorometry. 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.