Two mL methanol were added to the spiked standards (or samples), mixed on a rotary mixer for 30 minutes and then centrifuged at 5000 rpm for 5 minutes. The supernatants were removed and the above process repeated with a second 2 mL methanol. The 2 x 2 mL extracts were combined, diluted 100-fold with methanol-water (50/50) and filtered through a 0.45 µm filter before injection.