Fig. 7.

Chemical structures and fragmentation patterns of flucloxacillin (FLX), ¹³C₄ FLX (SIL-IS) and two examples of FLX isotopes of the same mass as SIL-IS, as a possible cause of cross-signal contribution. A) FLX, MW = 453 g/mol (m/z 454), with the fragmentation pattern, b) ¹³C₄ FLX, MW = 457 g/mol (m/z 458), with the fragmentation pattern, c) and d) FLX isotopes, MW = 457 g/mol, containing various stable isotopes distributed across the molecules. FLX isotope in 7c, although it is constituted of different stable isotopes (¹³C,¹⁵N and ³⁷Cl) than ¹³C₄ FLX in 7b, it has the same molecular weight as ¹³C₄ FLX and produces the same ion fragment (m/z 160) when the ring cleaves off. This will, therefore, contribute to the signal of ¹³C₄ FLX used as a SIL-IS. The FLX isotope in 7d, containing ³⁴S and ³⁷Cl, also has the same molecular weight as ¹³C₄ FLX, but the stable isotopes are located on each side of the molecule where the bond cleavage will occur, producing a fragment ion of m/z 162 and, therefore, will not contribute to the ¹³C₄ FLX signal. The cross-signal contribution from naturally occurring isotopes of the analyte to the SIL-IS is, therefore, dependent on the product ion selected, which is always less than the isotopic distribution of a precursor ion.