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Despite their enormous utility and diffusion, atmospheric pressure ionization mass spectrometry techniques are subjected to relevant drawbacks called matrix effects (ME). These effects could be summarized in matrix-dependent signal suppression or enhancement that could lead to erroneous quantitative results. The most important method parameters as well as linearity, precision, and accuracy could be modified due to interfering compounds present in the matrix. No validation methods could be accepted without a thorough evaluation of ME and possible strategies to minimize or to correct their influence should be addressed. In this article, we investigate mechanisms that lead to ME and discuss calibration techniques and other strategies to limit these phenomena. Significant examples from different fields of application are discussed as well. Sample preparation procedures, together with instrumental improvements and alternative calibration techniques used by many authors, are reported.

The crystal structures of the azamacrocycle 1,7–bis(2–hydroxy-5–nitrobenzyl)-4,10–dimethyl-1, 4,7,10–tetraazadodecane (L) (zwitterionic form) (L-0.5CH3OH-3.5H2O) and the monocharged derivative (HL+) (HL-Br-2C2H5OH) are reported. The crystals of L-0.5CH3 OH-3.5H2O are rhombohedral (hexagonal axes), space group R3, a = b = 42.472(7), c = 9.415(9) A, Z = 18, R = 0,034, whereas the crystals of HL-Br-2C2H5OH are monoclinic, space group C2/c, a = 20.418(5), b = 13.814 (4), c = 11.785(3) A, β = 99.00(2)°, Z = 4, R = 0.067. The differences in their conformation are discussed. Molecular mechanics calculations were carried out and indicate that the energy difference between the two species is very small. The stepwise enthalpies of protonation in solution were determined by direct microcalorimetry: ΔH° = −23.8(4) kJ mol−1 for H−1L− + H+ = L; ΔH°≅O for reactions L + H+ = HL+ and HL+ + H+ = H2L2+. The addition of the third proton is also athermic and fully entropy-controlled.