Capillary Voltage (LC/MS)

The parameter Capillary Voltage in the Tune > Source Section sets the voltage applied to the entrance of the capillary. This voltage is relative to the nebulizer and spray chamber which are at ground potential. The capillary voltage is entered in the software as an absolute number. The actual polarity for the Capillary Voltage is always opposite the polarity of the ions being analyzed.

A voltage is also applied to the end plate, spray shield, and cylinder electrode (electrospray only) in the spray chamber. This voltage is always 500 V less than the Capillary Voltage.

Minimum	Maximum	Default
0	6000	4000

While 4000 V is a typical Capillary Voltage, some individual spray chambers work better at 2500 V. This is especially true in negative ionization, APCI mode. The optimum Capillary Voltage can also be different for different compounds. A series of flow injection analyses, each at a different Capillary Voltage, is one way to determine the optimum Capillary Voltage.

APPI ionization can be performed with or without a dopant. When no dopant is used, the optimum Capillary Voltage is much lower than with ESI or APCI. The optimum is usually between 1000 and 2000 V with the typical setting being 1500 V. Capillary Voltage settings show a strong mass dependence when using APPI without a dopant. There is no Capillary Voltage setting that will work well across the full mass range of the instrument. Fortunately, the mass dependence is weak for ions in the 100 to 1000 m/z range. For masses above 1000 m/z, strong response requires Capillary Voltage settings of 2500 V or greater. This setting will result in a very weak response at masses below 300 m/z.

Some analyses work best with a dopant. When a dopant is used, the Capillary Voltage setting and mass dependence behavior is the same as in APCI. A typical setting would be 4000 V.

	Positive mode (V)	Negative mode (V)
ESI	4000	3500
AJS ESI	2500 - 3500	2500 - 3500
APCI	4000	4000
APPI	1500	4000*
APPI (dopant)*	4000	4000
MMI-ESI+APCI MMI-ESI MMI-APCI Nano Spray	2000	2000
HPLC-Chip	1000 - 2000	1000 - 2000 (max)

In API-ES negative ion mode, excessive capillary voltage can result in corona discharge. This is visible as a blue glow at the tip of the nebulizer.

For Nanospray, the needle is positioned very close to the capillary inlet. Because of this, the needle is prone to arcing damage if the Capillary Voltage exceeds 2500 V.