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 ParallelismThe
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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