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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