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To check for detector over- sensitivity due to a gas bubble in the cell or a cracked window: Make slight changes in the back pressure to the cell by partially restricting the outlet with a finger. If the detector signal changes, this condition probably exists. Be careful not to increase the back pressure so much that it breaks the cell windows or seals. Gas bubbles can lodge in the detector cell if mobile phases are inadequately degassed, especially when the chromatograph mixes two or more solvents; the resulting mixture has less capacity for gas than the individual solvents. If the individual solvents are not adequately degassed, bubbles can come out of solution in the detector. The formation of gas bubbles in the detector cell can be prevented by using a back-pressure regulator after the detector. Use of clean, burr-free tubing and frits also discourages bubble formation. A detector cell which contains dirt tends to be noisy. The Model 7125 interrupts the flow. The Model 7725 with its patented “Make-Before-Break” architecture does not interrupt flow. See the Blockage section comments on pages 19 and 20.
Symptom #14
The baseline is noisy or drifting (see Fig. 42):
- If switching to LOAD or INJECT causes a momentary detector baseline disturbance, instantaneous with the handle movement, see Cause A.
- If the
baseline is constantly noisy, or drifts excessively, see Cause
B.
Fig. 42. Examples of noisy baseline.
Cause A
The flow interruption that occurs each time the handle is turned causes a pressure disturbance in the detector. Interruption is normal with some Rheodyne injectors, see the Blockage section comments. Most detectors do not respond to this, but a detector which has become over sensitive to pressure changes will respond. Oversensitivity can be caused by a cracked cell window, by dirt or gas bubbles in the cell.
Solution
Test for oversensitivity (see comments). Flush out gas bubbles with degassed mobile phase. Adjust the injector, if necessary, to ensure that it is not injecting air (see Symptom #10).Cause B
The detector may contain gas, causing oversensitivity to slight flow and temperature fluctuations.
Solution
Test for oversensitivity (see comments). Flush out gas bubbles with degassed mobile phase. Adjust the injector, if necessary, to ensure that it is not injecting air (see Symptom #10).
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