## 3 Results and Discussion
### Absolute Recoveries of Spiked Isotope Labelled Compounds and Ion Suppression
TIC spectra, generated by the PHWE method and the Espresso method, in the acid method is higher throughout the whole spectrum, reaching a maximum at 1.70 min and \(1.6\times 10^{11}\) counts, compared to the basic method where the maximum is \(1.2\times 10^{11}\) counts at 1.75 min, Figure 2a,b. From Figure 2a,b it is clear that the PHWE method extracts more sample components. Both above mentioned techniques rely upon the same shift in chemical and physical properties that water undergoes as the temperature is elevated. An increase in water temperature changes the relative permittivity from nearly \(\varepsilon\) = 80 at room temperature to \(\varepsilon\) = 27 at 250 \({}^{\circ}\)C. The organic solvents methanol and ethanol has a permittivity of \(\varepsilon\) = 33 and \(\varepsilon\) = 24 at 25 \({}^{\circ}\)C [36]. A declining degree of hydrogen bonding and decreased polarizablity can then also be observed. Thus, by increasing the water temperature  
Figure 2: TIC spectra obtained using Waters RADARTM function of full scan (MS). Panel (**a**) shows TICs obtained for a blank sample as well as sewage sludge extracts applying the _Espresso method_ and the _PHWE method_ at acid chromatographic conditions. Panel (**b**) shows TICs for the same samples types but analyzed at basic chromatographic conditions. the normal behaviour of water as a solvent will shift from polar to less-polar. [PERSON] and [PERSON] compared the retention properties of superheated water and organic-water eluents and came to the conclusion that approximately 3.5 \({}^{\circ}\)C rise in water temperature was equivalent to 1% MeOH increase in MeOH-water mixtures [37]. This may be one reason for the higher co-extraction of matrix components using PHWE, since a 50 \({}^{\circ}\)C higher water temperature in PHWE would be equivalent to a 14% increase in MeOH content. Turning to the three isotope labelled standards carbamazepine-D10, diclofenac-13C6 and sulfamethoxazole-13C6, which all were run in both the acid and the basic chromatographic methods, the effects of both the chromatographic conditions and the applied extraction method became even further accentuated, as seen in Table 1 and Figure 3a. Mean absolute recovery results for these labelled compounds in the basic method was 37% for the Espresso method and 24% for the PHWE method, while corresponding figures using the acid  
Figure 3: Absolute recovery for eight labelled standards spiked at the top of the extraction cell, containing the sewage sludge, and the instrumental standard thiazolproid-d4 spiked just prior to the analysis. The absolute recovery was calculated using Equation (1). Four analytes were analysed using two different chromatographic methods at acid and basic conditions (**a**), while four of the compounds could only be analysed at basic conditions (**b**). method were 24% and 16%, respectively. A first interpretation of the calculated mean values points in the direction of greater extraction efficiency using the Espresso method, but as discussed above, the results also reminds us of heavier ion suppression in the PHWE method. [PERSON] et al. (2017) came to the conclusion that a temperature of 100 \({}^{\circ}\)C was optimal when optimizing pressurized liquid extraction (PLE) for the determination of sulfonamide, and macrolide antimicrobials and trimethoprim in sewage sludge. Increasing the temperature above 100 \({}^{\circ}\)C led to decreased recoveries. For sulfamethoxazole, they noticed a reduction by 95%, and for the macrolides the reduction was 60-90%. They concluded that this was due to thermal degradation of the analytes; however they also noted increasingly darker extracts at higher extraction temperatures, indicating a larger extraction of soluble organic matter. This notion of lower recoveries might not necessarily be a consequence of thermal degradation as we have shown in a temperature stability study ([PERSON] et al., 2016), but rather a result of ion suppression, and which may partly be avoided as shown here by basic chromatography methods.