Several calibrations were conducted with PO43− in acetone standards. The calibration points, concentration ranges and linearity of four calibration curves are listed in Table 1, and a sample of the calibration curves is shown in Fig. 1(b). The ions in Milli-Q water and acetone were first determined, and found to be non-interfering with the target ion. Good linearity (R2 > 98.5%) was achieved for all the calibration curves in the range of 0.13–2 mg L−1, and the lower detection limit was estimated to be approximately 0.1 mg L−1. Higher upper limit (e.g. 4 mg L−1) might be obtained, provided that measurement is immediately carried out after standard preparation, as it was observed that the peak of higher concentration PO43− could decline fast with storage in acetone.