General function and description:
The sample (serum, blood, urine or any other aqueous solution) is filled in a plastic sample tube and cooled via a Peltier element. These are semiconductor devices, which become cold on one side and warm on the other when electrical current flows through. The cold side cools the sample whilst the heat from the warm side dissipates into the air by convection.
The Löser-Osmometer and Cryometers transports the warmth via cooling surfaces into the atmosphere, an extra water supply is not necessary. Operation of the instrument requires only an electrical socket. The temperature on the cold side is kept electronically constant. During the measuring process the temperature of the sample is measured by a thermistor (a temperature dependent resistor). This is the part of the measuring head onto which the tube is placed. The measuring head is attached to guide rods which protect it from accidental damage. At a defined super cooling the freezing process is started by lowering a needle with ice crystals into the sample tube. The freezing point of the sample will be reached. The method of initiating the freezing process is important for the reproducibility of measurements. Dipping a needle with ice crystals into the sample gives more exact results than stirring with a wire which is constantly dipping into the sample. Because of linear correlation between osmolality and freezing point the measurement of freezing point is
a determination of osmolality. The results are displayed in mosm / kg H2O.
The performance of an Osmometer can be adversely affected by several factors. The principle of its operation should be fully understood and likely problems can be avoided if the Osmometer is cleaned and calibrated regularly. The Thermistor also needs careful handling as any cracks that may occur will adversely affect the readings obtained. If the thermistor is cracked it can be replaced by the user. To order a new thermistor please contact firstname.lastname@example.org or call 01954233110.
Sample residues at the glass thermistor will affect the next sample measurement adversely. After every measurement you must clean the thermistor carefully with a soft lint-free tissue without any fluff or fibres remaining on the thermistor.
Usually a little residual sample material remains on the thermistor when you have switched off the Osmometer and this residual sample dries on until the next time you come to use it. Subsequent measurements will have a higher values of the same samples. Because of great sensitivity and resolution of the instrument the smallest amount of contamination can cause a deviation of a few mosm even though the contamination could not be seen with naked eye. After a long time of no usage general contaminants in the atmosphere will affect the readings. In this case you should do the first two measurements with pure water as dummy samples to clean the thermistor and needle
2. Calibration problems
Contaminants if not cleaned off properly from the thermistor can affect the calibration values. A deviation of+/- 1 mosm (0.5%) of the nominated value is acceptable. This is equivalent to +/- 2 mmol (1.0%). If you get a larger deviation than this, try measuring a new aliquot of sample before calibrating the instrument again.
3. Cleaning of the freezing needle and the needle receptacle
The menu on the Osmometer Type 15 has a cleaning protocol built into it. Select the correct menu for cleaning and follow the instructions displayed on the LCD screen. Fill the needle receptacle with cleaning fluid very carefully using a small funnel, do not use anything that might damage the needle receptacle. Don’t use forceps. Cleaning fluid should only consist of a small quantity of water and glasswasher fluid like Camclean. The end of the cleaning cycle will be marked by a short buzzer. The needle and needle tip can be wiped with a soft lint free tissue or water/alcohol mix.
Cleaning is required daily as a minimum, and is dependant on the number of samples passed through the Osmometer. Organic samples will require the Osmometer to be cleaned more frequently than non-organic samples. The organic matter i.e.protein may build-up, making the freezing needle repellent to ice and not enough ice will stop the freezing initiation process.
4. Problems with Freezing Initiations
The Type 6 Osmometer requires that the needle is dipped in pure water before switching the unit on to get the ice crystals on the needle to form. Without ice on the needle, freezing initiation and measurement will not be possible.
5. Freezing too early
The following are checks to see if the freezing is occurring too early:
- The thermistor has a little crack or dirt on it
- Clean the thermistor after each measurement carefully with a lint free cloth
- Dust or dirt in the sample cup will cause premature freezing- Sample cups (microcentrifuge tubes) must be new and clean.
- The sample is unable to supercool to -7°C. For example, a bezene sample will not work because it has a freezing point of +5.5°C
- There should be no air bubbles in the sample
- The sample should be at room temperature before measurement
- The Thermistor should never touch the sample cup
If the osmometer still has problems or error codes please call Camlab for help and advice.