Determine the Charge of the lithium ion

A mass spectrometer often is used in carbon dating to verify the ratio of C-14 isotopes to C-12 isotopes in a biological sample. This ratio then is used to estimate how long ago the once-living organism died. Because a mass spectrometer is sensitive to the charge-to-mass ratio, it is possible for a contaminant particle to alter the value calculated for the C-14/C-12 ratio, and therefore, yield erroneous results.

When ionized, C-14 forms an ion with a &4 charge, and the mass of C-14 is 14 times that of a proton. Consider a contaminant lithium particle. If the most common lithium isotope has a mass that is seven times that of a proton, what must be the charge of the lithium ion required to contaminate a carbon-14 experiment?