The present invention relates to an ionization device for ionizing a liquid sample, the ionization device disposed between a liquid chromatograph and an atmospheric pressure ionization mass spectrometer.
An ionization device for ionizing a liquid sample is generally disposed as an interface between the liquid chromatograph and the atmospheric pressure ionization mass spectrometer.
Such an arrangement of a liquid chromatograph and an atmospheric pressure ionization mass spectrometer having an ionization device disposed therebetween is disclosed in the following references.
1) Analytical Chemistry, Vol. 62, No. 13, Jul. 1, 1990, pp 713A-725A.
2) Analytical Chemistry, Vol. 60, No. 8, Apr. 15, 1988, pp 774-780.
3) J. Am. Soc. Mass Spectrom, Vol. 2, 1991, pp 497-505
4) U.S. Pat. No. 4,209,696
5) U.S. Pat. No. 4,977,785
6) U.S. Pat. No. 4,963,736
The ionization device includes a nebulizing nozzle to which a high voltage is applied nebulizes an effluent from a separation column of the liquid chromatograph by a nebulizing gas supplied into the nebulizing nozzle. The ionization device further ionizes the nebulized effluent by applying the high voltage so as to provide ionized particles of the effluent into the mass spectrometer.
When the high voltage applied to the nebulizing nozzle is higher than 3 kV, a Corona discharge is apt to be generated from the nozzle. Therefore, an auxiliary gas is mixed in the nebulizing gas in order to avoid the generation of the Corona discharge.
For example, a predetermined amount of the auxiliary gas such as halogen, oxygen or air which has a high electro-negativity is mixed in the nebulizing gas in order to prevent the solvent-vapor such as methanol or acetonitrile of the liquid chromatograph from Corona discharge.
Further, it has been proposed to mix several percent of SF.sub.6 gas as the auxiliary gas in the nebulizing gas instead of the Oxygen.
The flow amount of the nebulizing gas is controlled according to the flow rate and composition of the effluent from the liquid chromatograph and the flow rate of the auxiliary gas is controlled according to a flow rate of the nebulizing gas. Such control of the flow rate of the nebulizing gas and the auxiliary gas is so delicate that it is very difficult to keep a certain constant ratio between the nebulizing gas and the auxiliary gas.