Cleaning a Contaminated CellCleaning a Contaminated Cell
Baseline anomalies are often the sign of a contaminated cell. DSC and PDSC cells must be cleaned properly to maintain satisfactory operation. Scraping the contamination off is not recommended.
If your baseline appears to show sample contamination, try the following recommended cleaning procedure. The procedure varies slightly depending upon the type of cell (QDSC Standard Cell or QDSC Pressure Cell) and the type of cooler that is installed.
Begin by removing any pans from the cell. See also: Loading/Unloading a Sample
Choose from one of the following procedures:
Determine that the anomalies seen in the baseline are actually caused by contamination, and not a faulty Tzero™ calibration. Contamination is normally manifested in very sharp, distinct, and sometimes noisy peaks in an empty baseline (no pans) run. If the baseline run shows shifts or step changes in the baseline, large startup events, or slope, these are more likely caused by a faulty Tzero calibration and no amount of cleaning will correct the problem.
If you have determined that the anomalies in the baseline are in fact caused by contamination, select a solvent that is most appropriate for dissolving the contaminant. Note that in most cases an organic solvent such as acetone or methanol may be necessary, but an aqueous solvent might also work for some contaminates.
Dampen the end of a small cotton swab with the chosen solvent. Avoid saturating the swab to prevent the solvent from dripping into the Tzero thermocouple tube.
Gently dissolve the contaminants with the swab, starting at the outside of the silver housing within the sample chamber and working inwards toward the Tzero thermocouple. Repeat the process until the contaminants have been removed. Do not rub hard with the swab, especially at the Tzero thermocouple.
Dry the interior of the cell with a clean dry swab.
Heat the cell to 200˚C in Nitrogen at approximately 50 ml/min, and hold for 5 minutes to remove all traces of solvent.
Verify by running an empty baseline run. Always store all available Diagnostic Signals when running verification baseline runs. If the anomalies are gone, but the baseline shape or slope is not acceptable, then Tzero calibration may be necessary. If the anomalies are still there, but are less evident, repeat the above steps and re-verify the baseline.
If you are unable to remove the anomalies using the above procedure, then you will need to perform additional cell cleaning by heating the cell to a high temperature in an oxidizing atmosphere. This procedure should only be done as a last resort.
Begin by connecting the air purge. See also: Connecting Purge Gas Lines, and then follow the steps below depending on which cooler is installed.
NOTE: Cleaning of the cell is performed with the lids off to prevent the lid gas exit tube from becoming plugged by condensing off-gases. If your instrument is equipped with an Autolid mechanism you will need to open the cell manually at the start of the method. You can open the cell by selecting Lid on the Control Menu Touch Screen or by selecting Lids/Open from the Control menu.
Begin cleaning by heating the cell with an air purge to 50°C above the highest operating temperature or 600°C, whichever is lower, without pans. Use a heating rate of 20°C per minute. The Ramp test can be used for this method. Volatile contaminants can be removed at lower temperatures; decomposed contaminants must be oxidized above 550°C.
After cool-down, lightly brush out the cell with a small fiberglass brush (included in the DSC accessory kit). Then remove the residue from the cell using clean compressed air.
Run a baseline (empty cell). If there is an improvement relative to the baseline before “burn off” but the baseline is still unacceptable, repeat the procedure above.
Begin cleaning by heating the cell with an air purge to 50°C above the highest operating temperature or 550°C, whichever is lower, without pans with the cooling accessory ON. Use a heating rate of 20°C per minute. The Ramp test can be used for this method. Volatile contaminants can be removed at lower temperatures; decomposed contaminants must be oxidized above 550°C.
WARNING: Do not exceed 100°C with the RCS cooling head installed and the RCS power off. Serious damage and/or injury could occur.
CAUTION: We recommend that you do not use the RCS when running isothermal experiments above 400°C. Damage to the unit can occur if used at high temperatures for extended periods.
After cool-down, lightly brush out the cell with a small fiberglass brush (included in the DSC accessory kit). Then remove the residue from the cell using clean compressed air.
Run a baseline (empty cell). If there is an improvement relative to the baseline before “burn off” but the baseline is still unacceptable, repeat the procedure above. If you need to hold longer at the higher temperature to remove the contamination, you will need to remove the RCS or LNCS heat exchanger before conducting this experiment. You will also need to temporarily change the selected cooler type to FACS on the Tools/Instrument Preferences/Cooler Page. (Don’t forget to reselect the correct cooler type after performing this experiment.)
NOTE: When removing the RCS/LNCS heat exchanger from the cell, it is recommended that the RCS or LNCS be allowed to warm to room temperature after turning it "off" before removing the heat exchanger. That will minimize the chance for moisture condensation/contamination and will avoid the possibility of damaging the connector hose by moving while cold (and brittle).
Removing the heat exchanger requires that the DSC-RCS/LNCS system be reconditioned and recalibrated after reassembled on the cell.
Minor baseline anomalies due to sample contamination that remain after cleaning may be compensated for by a new Tzero calibration. They cannot be compensated for by a Q20/Q10 baseline calibration.
If the baseline problem remains, it is probably not due to contamination; the cell may need to be replaced (contact your TA Instruments service representative).
Once the baseline is acceptable, return to normal operation.
A poor baseline is often the sign of a contaminated cell. PDSC cells must be cleaned properly to maintain satisfactory operation. Scraping the contamination off the cell's constantan disc is not recommended because the disc is very thin (about 0.1 mm, or 0.004 inches), and if the disc deforms, the baseline may be affected.
If your baseline appears to show sample contamination, try the following recommended cleaning procedure:
Remove any pans from the cell.
Connect the air purge.
Lightly brush out the cell with a small fiberglass eraser (included in the DSC accessory kit).
Clean the disk with air.
NOTE: Be sure to wear safety glasses or goggles when cleaning the cell with air.
Begin cleaning by heating the cell with an air purge to 50 °C above the highest operating temperature or 600 °C, whichever is lower. Use a heating rate of 20 °C/min. Ramp to 600 °C, then hold isothermally for 15 minutes.
After the cell has cooled down, repeat steps 3 and 4.
Run the experiment again and compare the baselines. If there is a marked improvement but the baseline is still unacceptable, the contaminant probably oxidized and reduced to an inert ash. Run the experiment again and check for further improvement.
Once the baseline is acceptable, return to normal operation.
If the constantan disc looks clean and is not bent or cracked, but the baseline problem remains, it is probably not due to contamination; the cell may need to be replaced (contact your TA Instruments service representative).