2.2. Sample preparation
2.2.1. Water samples
A total of 20 water samples (including 5 samples of surface
water, tap water, reverse osmosis drinking water and bottled
drinking water) were preserved to pH 2 by the addition of2 mL of
conc. nitric acid per litre. No sample pretreatment was made
except filtering of the sample just before the analysis. Tap water
samples, surface water samples and reverse osmosis drinking
water samples were collected in clean polyethylene bottles (1 L).
Tap water samples were collected from the water supply network
at different locations in Maha Sarakham Province, Thailand.
Surface water samples were collected from the lagoons nearby
Mahasarakham University. Reverse osmosis drinking water
samples were collected from the reverse osmosis drinking water
machines located around Mahasarakham University. Five different
brands of bottled drinking water samples packed in plastic bottles
were selected and bought directly from local superstores in Maha
Sarakham Province, Thailand.
2.2.2. Beverage samples
The most frequently consumed brands of 15 beverage samples
(including carbonated beverages, vegetable/fruit juices and readyto-drink teas) packed in various packages such as aluminium cans,
glass or plastic bottles and laminated paperboard packages
(Tetrabrik) were selected and bought directly from local superstores in Maha Sarakham Province, Thailand. All the samples were
produced in Thailand. Each sample consisted of 6 bottles or cans
chosen at random which were homogenized immediately before
the use. Beverage samples were digested by dry-ashing to
completely destroy organic matter. Beverage samples of 10 mL
were pipetted into porcelain crucibles and the volume was reduced
using a hot plate. Some conc. nitric acid was added, and the
samples were evaporated until dry. Finally, the samples were
ashed in a muffle furnace for 24 h/350 8C. The white ashes were
dissolved in 1 mL of conc. nitric acid and transferred to a 25 mL
volumetric flask, where the volume was made up with deionized
water. The samples were then transferred into polyethylene
bottles and stored at 4 8C until further analysis.