The compositional problems suggest to me that there was no second proto-planet Theia, only an exotic comet impact which stayed with the core of earth but whose energy ejected surface matter into orbit.
[quote]There are a number of compositional inconsistencies that need to be addressed.
The ratios of the Moon's volatile elements are not explained by the giant impact hypothesis. If the giant impact hypothesis is correct, they must be due to some other cause.
The presence of volatiles such as water trapped in lunar basalts is more difficult to explain if the Moon was caused by an impact which would entail a catastrophic heating event.
The iron oxide (FeO) content (13%) of the Moon, which is intermediate between Mars (18%) and the terrestrial mantle (8%), rules out most of the source of the proto-lunar material from the Earth's mantle.
If the bulk of the proto-lunar material had come from the impactor, the Moon should be enriched in siderophilic elements, when, in fact, it is deficient in those.
The Moon's oxygen isotopic ratios are essentially identical to those of Earth. Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each solar system body. If Theia had been a separate proto-planet, it probably would have had a different oxygen isotopic signature than Earth, as would the ejected mixed material.
The Moon's titanium isotope ratio (50Ti/47Ti) appears so close to the Earth's (within 4 ppm), that little if any of the colliding body's mass could likely have been part of the Moon.[end quote]
The last titanium isotope analysis is greatly in favour of the exotic comet hypothesis instead of Theia imo.