Testing for a dirty ion source
A dirty ion source is one of the more common causes of poor sensitivity and other performance problems. All ion sources eventually become dirty. How quickly the ion source in your MSD will become dirty depends on several factors:
Number of samples analyzed - more samples dirty the ion source more quickly
Type of samples and sample matrix - highly concentrated samples in complex matrices dirty the ion source more quickly than trace samples in clean matrices.
Operating conditions (primarily ion source temperature) - higher ion source temperatures generally keep the ion source cleaner, longer.
A dirty ion source does not always look dirty on visual inspection. Clues to the ion source's actual condition can be determined from tune reports.
If, after following this procedure, you determine that the ion source is dirty, its performance can be restored by performing the cleaning procedure in the Maintenance chapter of the MSD maintenance documentation.
Select View Tunes from the Qualify menu in the Instrument Control view. The following trends may indicate the ion source is becoming dirty:
Steadily increasing ion focus voltage
Steadily increasing repeller voltage
Steadily increasing electron multiplier voltage (can also indicate an old electron multiplier horn)
Steadily poorer low mass resolution
Steadily decreasing relative abundance of m/z 502
Perform an Autotune. Signs of a dirty ion source include:
Relative abundance of m/z 502 less than 3%
Relative abundance of m/z 219 less than 30%
Repeller voltage is at maximum (35 volts for Autotune)
In manual tune, ramp the repeller voltage. A clean ion source will generate a response curve with a single, well-defined peak between 15 and 35 volts. As an ion source becomes dirty, the peak will shift higher and higher until it occurs at the maximum repeller (42.7 volts). A dirty ion source can also show little or no response at lower repeller voltages with a sudden step and a steep response curve after the step. With this type of response, the peaks is still at the maximum repeller voltage.
