The microsampling itself is mechanical, usually involving
sampling by fine diamond tipped or coring bits. The sample is
taken from thin or thick sections, controlled by observation
through an optical microscope. There are several factors of this
technique which could be termed disadvantages: 1) laborious sample
prep (potentially higher blank, more time expenditure, more
preparation cost per sample); 2) Much larger spot size [limited by
the physical size of the drill bit balanced by a realistic mass
requirement for 
1 nano-gram (ng = 10
grams) or more
of the element of interest]; and 3) Limitations on diversity of
analysis (only a few isotope ratios or spiked isotope dilution
analyses can be derived from a given sample, compared with a whole
suite of potential trace-element concentrations from ICP-MS).
However, the technique offers the advantage of being able to
characterize the sample in advance by less destructive analysis,
such as with the electron microprobe. The major advantage of in
situ microsampling and standard TIMS over LA-ICP-MS and SIMS is the
much greater precision (it is the only way to measure isotope
ratios to better than 
0.01%).