## 3 Results and Discussion  
### Doping of PFBHA  
The selected fiber, 65 \(\mathrm{\SIUnitSymbolMicro}\)/DVB, was chosen because of its high affinity to PFBHA. The benzene rings both in DVB phase in SPME coating and PFBHA/oxime derivatives are expected to form a strong conjugated bond. The PDMS/DVB coating doped with PFBHA was found to retain a large amount of dopant after exposed in ambient air [22]. Main factors influencing the uptake of carbonyls include: (1) availability of reaction (derivatization) sites on SPME fiber coated/doped with PFBHA; (2) the kinetics of derivatization reactions; and (3) mass transport/diffusion of analytes from liquid phase to headspace, and through the boundary layer around the SPME fiber coating. On-fiber derivatization mass extracted _vs._ time profiles for 10 carbonyls were generated based on different PFBHA doping concentrations (0.5, 3.75 and 17 mg/mL). The concentration of each carbonyl compound in the standard solution was about 5 \(\mathrm{\SIUnitSymbolMicro}\)/L. Figure 1 shows the effects of PFBHA dopant concentration. Maximumcarbonyl recovery was clearly limited for the lower PFBHA dopant concentrations of 0.5 and 3.75 mg/mL. Specifically, the limited amount of PFBHA was limiting the derivatization reaction when extractions were carried out for longer than 15 min. Very similar derivatization rates (factor #2), were observed for different PFBHA dopant concentrations within the first 15 min of derivatization, since there was no limitation to the dopant availability. Figure 1: Comparison of the effects of different PFBHA doping concentrations on derivatization-time reaction profiles. Part A: PFBHA concentration at 0.5 ng/mL. Part B: PFBHA concentration at 3.75 ng/mL. Part C: PFBHA concentration at 17 mg/mL. Aldehyde extraction within the first 15 min of derivatization increased steadily regardless of PFBHA dopant concentration. The effect of PFBHA dopant concentration on aldehydes uptake within the first 15 min are shown in Table 2 and Figure 2. The mass of derivatized carbonyl taken up reflects the overall derivatization rate over a certain derivatization time. No significant differences resulting from different PFBHA dopant concentrations were observed for several aldehydes at the same derivatization time (Table 2). Derivatization rates did not proportionally increase against PFBHA dopant concentration for the other aldehydes either (Figure 2). No proportional relationship of derivatization rate against PFBHA dopant concentration was observed for the first 15 min of derivatization. This observation shows that reaction sites were not saturated with aldehydes within 15 min derivatization at the lower and lowest PFBHA doping  
However, a significant decrease in recovery after 15 min derivatization was observed for 0.5 mg/mL dopant concentration. Such a decrease was also observed after 30 min with 3.75 mg/mL dopant concentration. This indicated that the number of available PFBHA sites for reaction with headspace carbonyls (factor \(\#\)1) became the limiting factor at lower PFBHA doping concentrations. However, no apparent limitation of dopant availability was observed for up to 60 min derivatization and the highest, 17 mg/mL PFBHA concentration. This was expected since higher PFBHA concentration and on-fiber loading was proportional to the higher PFBHA doping concentration. To achieve maximum carbonyl derivatization rates, the highest dopant solution concentration of 17 mg/mL was selected. This PFBHA concentration was selected by default in earlier studies [23, 26, 32, 34]. Doping time, _i.e._, loading of PFBHA onto the fiber, was 15 min. Since no significant reduction of recovery was observed for lower PFBHA dopant concentration within 15 min derivatization time, use of lower PFBHA concentrations is feasible in future to lower analytical expenses, especially if shorter extraction/derivatization times are used and aldehyde concentrations are very low. Figure 2: PFBHA dopant concentration _versus_ aldehydes uptake profile at specific derivatization times.