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Let ${\rm Z}(G)$ and ${\rm gp}(G)$ be the zero forcing number and the general position number of a graph $G$, respectively. Known results imply that ${\rm gp}(T)\ge {\rm Z}(T) + 1$ holds for every nontrivial tree $T$. It is proved that the result extends to block graphs. For connected, unicyclic graphs $G$ it is proved that ${\rm gp}(G) \ge {\rm Z}(G)$. The result extends neither to bicyclic graphs nor to quasi-trees. Nevertheless, a large class of quasi-trees is found for which ${\rm gp}(G) \ge {\rm Z}(G)$ holds.

An ultra performance liquid chromatography-quadrupole/electrostatic field orbitrap high resolution mass spectrometry (UPLC-Q Orbitrap HRMS) method was developed for the rapid screening of 25 drug residues in aquatic products by the application of a novel enhanced matrix removal of lipids (EMR-Lipid). The samples were extracted with acetonitrile, cleaned-up with EMR-Lipid, and salted out with 3.0 g sodium chloride and 3.0 g sodium sulfate. The separation of the 25 drug residues was performed on an ACQUITY UPLC BEH C18 column (100 mm×2.1 mm, 1.7 μm) with gradient elution. Acetonitrile containing 0.1% (v/v) formic acid and 0.1% (v/v) formic acid aqueous solution were used as mobile phases. The compounds were detected by Full scan/date dependent MS/MS (Full MS/dd-MS2) via heated electrospray ionization (HESI) source. The calibration curves were linear with correlation coefficients (r) greater than 0.997. The limits of detection (LODs) ranged from 0.1 μg/kg to 1.0 μg/kg. The average spiked recoveries of the 25 target compounds were between 70.1% and 108.9% with relative standard derivations (RSDs) from 2.1% to 13.8%. Compared with the previous methods, this method has characteristics of simpler sample preparation and higher sensitivity.

An ultra performance liquid chromatography-quadrupole/electrostatic field orbitrap high resolution mass spectrometry (UPLC-Q Orbitrap HRMS) method was developed for the rapid determination of 48 contaminant residues in food plastic packaging materials. The samples were extracted with methanol using microwave-assisted extraction. The separation of the 48 contaminants was performed on a CAPCELL PAK MG Ⅱ C18 column (150 mm×2.0 mm, 5 μm) with gradient elution. Methanol (containing 0.04% (v/v) formic acid) and water (containing 0.04% (v/v) formic acid and 20 mmol/L ammonium acetate) were used as mobile phases. The compounds were detected under the full scan/date dependent-MS/MS (Full MS/dd-MS2) mode with a heated electrospray ionization (HESI) source. The calibration curves were linear with correlation coefficients no less than 0.9950. The limits of detection (LODs) ranged from 0.1 to 1.0 μg/kg. The average spiked recoveries of the 48 target compounds were between 71.2% and 108.8%, with relative standard derivations (RSDs) ranging from 2.2% to 11.8% (n=6). Compared to previous methods, this method has advantages of simpler sample preparation and higher sensitivity.