Determination of Oxalate and Some Inorganic Anions in Green and Black Tea

Abstract

Oxalate concentration differs in various daily consumed food products. The role of oxalic acid in the human body is very significant, as its compounds are responsible for the stability of biological membranes. However, insoluble calcium and magnesium oxalates can be accumulated in the body in the form of kidney stones. Oxalate concentration has been measured by high performance liquid, gas after derivatization and ion chromatography (IC). The most effective method for the simultaneous determination of oxalate and inorganic anions is ion chromatography with conductometric detection. Here, we report the results of the measurement of oxalic acid in bleak and green tea samples. Separation was performed by IC on an anion-exchange column Shodex IC SI-90 with surface-layer sorbent and conductimetric detection. The main analytical features of the method were: limit of detection of oxalic acid 0.03 mg/l, linear range 0.1-20 mg/l, correlation 0.9998, relative standard deviation 1%. The method did not need specific sample treatment and was successfully applied to the analysis of black and green tea samples. Oxalic acid was determined in the ranges 16.7-84 mg/l for green tea and 63-116 mg/l for black tea. Green tea contained lower oxalate ions concentration than black tea. The IC method has a lower detection limit for oxalate ions than HPLS and GC, ten and two times less, respectively

Simultaneous determination of nitrate, nitrite and bromate by capillary zone electrophoresis in tea infusions grown in the Black Sea region of Turkey

“…On the other hand, the migration time value for nitrite ion in our study was in agreement with the work reported in literature [28,32]. Generally the nitrate content as shown in Table 4 for all the tea samples obtained in this study were found to be lower than the values reported by [22] and [34] in various tea samples studied. For the tea samples with nitrate content above the average daily intake limit recommended by WHO, it is recommended that measures should be put in place in the regions were these teas are grown in order reduce high usage of fertilizers.…”

Oxalate in Foods: Extraction Conditions, Analytical Methods, Occurrence, and Health Implications

“…Considering ion exchange columns, one of the most referenced columns is IonPac AS4A [1,59,60,[62][63][64]72]. Other authors have used different ones: propylamine anion exchange column (25 cm × 4.6 mm; 6 µm particle size), diethylaminoethyl (DEAE) anion exchange column (250 × 4.6 mm; 5 µm particle size), and Hamilton HA-X8.00 column with strong anion exchange resin in the sulfate form (255 × 5 mm; 7-10 µm particle size) [84]; Alltech All-Sep anion exchange column (100 × 4.6 mm; 7 µm particle size) [17] and Shodex IC SI-90 anion exchange column (250 × 4 mm; 9 µm particle size) filled with KanK-ASt (120 × 5 mm, 14 µm particle size) [71]. For ion exclusion, it seems that an ion exclusion column (300 × 7.8 mm) is frequently the selected one, either from Bio-Rad [32,34,40,61,70] or Rezex [65][66][67][68] brands.…”

“…These columns are chosen considering different separation modes of HPLC. Ion chromatography is an effective method for the determination of oxalate ions because oxalic acid is a strong acid, giving away its protons and becoming negatively charged [71].…”

“…For the measurement of oxalates, there is a wide variety of mobile phases. For ion exchange columns, a carbonate and sodium bicarbonate solution for a conductivity detector [17,59,60,71] and an aqueous EDTA solution for amperometric detection are frequently utilized [1,63,64,72]. Some other mobile phases have been reported like 0.15 M NaH 2 PO 4 (pH = 4.2); 0.30 M NH 4 H 2 PO 4 adjusted to pH 6.5 with concentrated ammonium hydroxide; 0.5 M (NH 4 ) 2 SO 4 adjusted to pH 7.25 with ammonium hydroxide; 0.3 M (NH 4 ) 2 SO 4 containing 10% methanol and 0.1-1.5 M MgSO 4 solutions for the gradient analysis [84].…”

“…However, there are other parameters which are also taken into account for this choice, such as analysis time. Other used detectors are diode array detectors (DADs) or photodiode array (PDA) [68], which also detect absorption in the UV to Vis region but can scan the entire range [86]; refractive index (RI) detectors [84]; conductivity detectors [17,59,60,71]; and amperometric detectors [1,63,64,72]. These instruments are chosen depending on the type of HPLC analysis, considering its analytes.…”