In fact, different methods of the analytical techniques in GC require to various sample preparation using variable volumes regarding to the relevant protocols (sample preparation for injecting stage). In order to better learn, you could refer to the following links.

DOI: 10.5772/intechopen.84259

https://www.glsciences.eu/optic/gc-injections-manual.pdf

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Every compound have different sensitivity, depend on your compound how much you will introduce to your instrument

Various instruments and methods have different requirements for sample volume.

In the case of classic autosamplers there is some minimal vial volume. If you choose the right vials with special inserts, the minimal sample volume would be about 10 ul.

There are some tricks to avoid this problem, e.g. elution of the sample from solid phase as in the Evosep system, but it is not very common yet.

In techniques such as MALDI ionization there is no minimal sample volume recruitments because the sample is mixing with a specialized matrix during sample preparation.

The question about sensitivity is much more complicated - different molecules will have various detection range in the same conditions. To maximize sensitivity you should carefully choose ionization conditions and try to avoid contaminations.

By our experience, in the case of MALDI, the purity of the sample often is much more important than amount of target peptide in it. Sometimes, when our colleagues use poor quality plastic and do not work properly it is impossible to obtain a good signal even in very concentrated sample. The quality of plastics and solvents is very important for mass spectrometry.

Other example comes from ESI-ionization. Addition of TFA instead of FA to the sample significantly reduces signal strength for most peptides.

Finally, do not forget about derivatization techniques. Derivatization is aimed to perform chemical modifications which significantly increase ionization efficiency of the target molecule.

https://www.sciencedirect.com/science/article/abs/pii/S016599361400096X#:~:text=Derivatization%20primarily%20aims%20to%20obtain,separation%20for%20some%20chiral%20drugs.