My service engineer told me that the sensitivity of negative ion is generally more sensitive to the cleanliness of front end of mass spec. In other words, if the front end gets dirty, the sensitivity for negative ion drops more easily than the one for positive ion. I tried to get a reference for this, but he can not find. It seems that this is widely accepted at least to these service engineers. But I still want to know more about the scientific basis. Does any one know the literature reference and give me a clue? many many thanks!!
Austin

Hi Austin,

I have asked similar questions about sensitivity in positive and negative ion-mode. The resume of the replies I took was that the sensitivity in the negative ion-mode is governed by the lower background noise in comparison with positive one. As a result the S/N will drop more easily as you pollute the MS. In the positive ion-mode they told me that when ions arrive the electromultiplier they generate more detectable species (fragments and electrons?) in relation to the negative ion-mode.

I must point out here that these are information that they gave me which I have not verified by bibliographic data so I can not garanty for their accuracy. Maybe others can clarify this point better than me.

Kostas