Plasma generation-roles of argon flow, Rf coil, igniter pin

[mleipold]

I have a few general questions regarding the steps leading up to the formation of the plasma.

If I understand things from the manual and from the routing of the tubing I see inside the interface+torch area, from the time the argon enters the machine from the dewar/cylinder, it travels through the hollow interior of the copper Rf coil. It then gets fed into the torch body. The igniter pin sparks to help initiate the formation of the plasma.

1. What is the purpose of the trip through the hollow interior of the Rf coil? To cool the Rf coil? To begin charging the argon?

2. Does the argon that travels through the hollow interior of the Rf coil then enter the Plasma Gas port of the Torch Body (Fig 1.10 of CyTOF2manual_v4)?

3. Page 15 of the manual states "First, argon plasma gas flows tangentially from the outer chamber of the torch body. RF power supplied to the load coil produces an oscillating current (40 MHz), creating a strong electromagnetic field precisely at the point the plasma gas exits the outer chamber. A high voltage spark strips away free electrons from the exiting argon atoms. These free electrons accelerate dramatically in the electromagnetic field and collide with sufficient energy to ionize the argon gas into plasma."

Is the argon already charged by the Rf coil immediately prior to the "pop" of the plasma? Or does it need the spark from the igniter pin in order to get charge that's then amplified and maintained by the copper RF coil in the final formation of the plasma?

4. Is the igniter pin "spark" is only needed at the initial point of formation of the plasma (ie, the plasma is self-sustaining with the RF coil once formed), or does it keep delivering charge to the gas after plasma is formed?

5. Does the plasma sit inside the torch body, or just *after* the end of the torch body? I would assume just after, to avoid melting the torch body, but I wasn't sure from the text.



Thanks,
Mike

[anitamkant]

1. Argon is passed through the coil in order to cool it and not for charging the Argon. Nothing inside a charged tube can be charged: the field inside is zero.
2. Yes, Argon does travel through the hollow interior of the Rf coil and then enters the Plasma Gas port of the Torch Body
3. Argon gas is neutral and does not feel the electromagnetic field. The spark from the igniter pin creates first electrons which get energized by the electromagnetic field produced by the RF voltage applied to the load coil, these electrons then strike argon atoms and generate ions and electrons which form plasma.
4. The plasma sits inside the outer tube of the torch body and expands outside towards the sampler. The highest field and hence the hottest plasma is inside the load coil. The closest to the sampler load coil end is ~5 mm away from the torch end. Melting of the outer tube is prevented by the tangential flow of the Plasma gas flowing at ~ 17 L/min The auxiliary gas and the injector gas flows shift plasma slightly away from the inner tube and the injector tube of the torch assembly, preventing them from melting.
5. For more detailed description of the plasma generation one could refer to the book on Inductively Coupled Plasma Mass Spectrometry (Ed. Akbar Montaser, 1998).
(Thanks to Dmitry for patiently explaining the theory behind plasma generation once again)