SCH 2304 Analytical Chemistry 1 Notes
Continuation
Cells and cell compartment
Cells or curvettes are of different sizes, shapes and window material. Either single piece or fused /
cemented. The single piece is preferred, but the fused cells are cheaper and therefore most
common. Be aware of solvent – cement interaction which may cause leakage of cell.
Cell shapes.
- Rectangular, cylindrical or test-tube shaped
- Test tube type though used in inexpensive equipment cause difficulty since they alter the
focus of incident radiation.
- Rectangular – two polished slides with the remaining sides and bottom made from frosted
glass and an open top. The windows should be optically flat and exhibit precise parallelism
relative to one another
- Pathlength ranges from 1.0 to 100mm. most common is 1.0 cm, volume approx. 3ml. small
cell volumes upto 10µl are available for specific applications.
Materials
- Glass of pyrex or borosilicate, transparent region is 350 – 1000 nm
- Quartz (fused silica or crystal (natural)) gives transparency in 180 -2500 nm region
- Inexpensive plastic curvettes used in the visible region
- Other materials used for specific applications are fluorite, lithium fluoride, sapphire and
diamond
Flow cells featuring long cell paths and extremely small volumes are used in LC detectors.
Gas cells have gas tight seals, valves, pressure control devices.
Handling of cells
Cells are cleaned before and after use. Then left to dry in air (never in oven or over a flame because
heat will alter the pathlength!). Do not handle the polished sides of the cell since any greases or
finger prints will alter the transmittance of the light.
Cell compartment
It is designed to permit reproducible placement (kinematic design) of cell in the compartment with
incident beam perpendicular to cell windows.
-flow cells are not removed. Need for a vacuum system to fill the cell and remove the sample, also
allow for cell cleaning cycle.
-some instruments have ability to thermostat cell compartment by circulation of water from a water
bath
-Also facility to purge the cell compartment with inert gas if working below 200nm
-cell compartment may be before or after the monochromator. After the monochromator is
preferred for photosensitive samples eg proteins.
Continuation
Cells and cell compartment
Cells or curvettes are of different sizes, shapes and window material. Either single piece or fused /
cemented. The single piece is preferred, but the fused cells are cheaper and therefore most
common. Be aware of solvent – cement interaction which may cause leakage of cell.
Cell shapes.
- Rectangular, cylindrical or test-tube shaped
- Test tube type though used in inexpensive equipment cause difficulty since they alter the
focus of incident radiation.
- Rectangular – two polished slides with the remaining sides and bottom made from frosted
glass and an open top. The windows should be optically flat and exhibit precise parallelism
relative to one another
- Pathlength ranges from 1.0 to 100mm. most common is 1.0 cm, volume approx. 3ml. small
cell volumes upto 10µl are available for specific applications.
Materials
- Glass of pyrex or borosilicate, transparent region is 350 – 1000 nm
- Quartz (fused silica or crystal (natural)) gives transparency in 180 -2500 nm region
- Inexpensive plastic curvettes used in the visible region
- Other materials used for specific applications are fluorite, lithium fluoride, sapphire and
diamond
Flow cells featuring long cell paths and extremely small volumes are used in LC detectors.
Gas cells have gas tight seals, valves, pressure control devices.
Handling of cells
Cells are cleaned before and after use. Then left to dry in air (never in oven or over a flame because
heat will alter the pathlength!). Do not handle the polished sides of the cell since any greases or
finger prints will alter the transmittance of the light.
Cell compartment
It is designed to permit reproducible placement (kinematic design) of cell in the compartment with
incident beam perpendicular to cell windows.
-flow cells are not removed. Need for a vacuum system to fill the cell and remove the sample, also
allow for cell cleaning cycle.
-some instruments have ability to thermostat cell compartment by circulation of water from a water
bath
-Also facility to purge the cell compartment with inert gas if working below 200nm
-cell compartment may be before or after the monochromator. After the monochromator is
preferred for photosensitive samples eg proteins.
