Na2hpo4

United States Pharmacopeia (USP) reference standard omeprazole was procured from USP Convention. Authentic omeprazole standard capsules (Losec) were provided by AstraZeneca. Lansoprazole (internal standard) was from Sigma Aldrich (India). NaH₂PO₄.2H₂O, Na₂HPO₄, Na₃PO₄, KH₂PO₄ and other chemicals of reagent grade were purchased from Nacalai Tesque Inc. (Kyoto, Japan). Distilled water was used for the preparation of HPLC eluents.
The investigational samples consisted of 154 samples of hard gelatin capsules containing 20 mg of omeprazole in enteric-coated pellets and 2 tablet samples purchased from different drug stores in Cambodia in 2010 and Myanmar in 2014. In Cambodia, samples were collected from pharmacies, depots, wholesaler and outlets of Phnom Penh, Svay Rieng and Kandal provinces. In Myanmar, collected samples were from pharmacy, hospital, and wholesalers of Yangon region. These samples of different serial and batch number were imported to Cambodia and Myanmar from 53 different manufacturers in seven countries. Samples were stored below 25 °C after collecting and all the quality analysis of the samples was finished before the expiration date of the samples. After quality-testing as required by the indicated pharmacopoeia, we selected five samples for further investigation based on the gravity of their failure in the dissolution test.

Escherichia coli KY33 [pKPY514], a gift from David Tirrell^{23 (link)}, is an arginine-, lysine- and phenylalanine-auxotrophic strain^{23 (link)}. Escherichia coli KY33 was labelled with azido-phenylalanine following methods in a previous report^{23 (link)}. Worms were precultured with 5 mL of S medium³⁰ supplemented with 15 mg/mL E. coli KY33 cultured with phenylalanine at 20 °C, 250 rpm. Precultured worms were pelleted by centrifugation at 300 g for 5 min at room temperature and washed with 1 mL of S medium. This procedure was repeated three times. The C. elegans pellet was suspended in 5 mL of S medium supplemented with 15 mg/mL E. coli KY33 cultured with azido-phenylalanine and incubated at 20 °C and 250 rpm for 24 h.
Labelled nematodes were recovered using a 20 µm nylon filter (pluriStrainer 20 µm; pluriSelect, Leipzig, Germany) and washed with 5 mL of M9 buffer (0.6% w/v Na₂HPO₄ (Nacalai Tesque, Kyoto, Japan), 0.3% KH₂PO₄ (Nacalai Tesque), 0.5% NaCl (Nacalai Tesque)). Nematodes were recovered by centrifugation at 300 g for 5 min and processed in subsequent procedures.