Tea is a very important beverage in the world. It is rich in chemical components and is considered to be an important source of minerals and trace elements in the daily diet. However, due to the super absorption and accumulation capacity of tea tree, it can also absorb aluminum in the soil. Aluminum is not an essential trace element for human body. Once excessive aluminum is accumulated in human body, it will cause toxic reaction. Drinking tea for a long time may increase aluminum intake by more than twice. Therefore, it is necessary to detect and determine the content of aluminum in tea. In this Application Note, the main methods for the determination of aluminum in tea are reviewed, including spectrometry, spectrophotometry, ²⁷Al nuclear magnetic resonance spectroscopy and inductively coupled plasma mass spectrometry.

Detection Methods

• Spectrometry

At present, the methods that can be used to detect the content of aluminum in tea mainly include inductively coupled plasma - optical emission spectrometry (ICP-OES), graphite furnace atomic absorption spectrometry (GFAAS), flame atomic absorption spectrometry (FAAS), and tungsten coil atomic emission spectrometry (WCAES). These methods are also commonly used in laboratories and can meet the daily detection work.

Figure 1 Commonly used spectral detection methods

• Spectrophotometry

The chrome azurol S method method is the most widely used spectrophotometric method in the determination of aluminum content in food. The principle of this method is that under certain pH conditions, aluminum ions and chrome azurol S solution can react to form a blue-green chelate, and the converted absorbance value is the aluminum content.

• ²⁷Al Nuclear Magnetic Resonance Spectroscopy (NMR)

• Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Summary

Spectrophotometry is the most commonly used method for the detection of aluminum in tea, but the accuracy is not enough. The great advantage of ²⁷Al nuclear magnetic resonance spectroscopy for the determination of aluminum in tea is that the detection limit can be very low. Compared with the sample size of 2 ~ 3 cm³ needed for ICP-OES analysis, this method only needs a sample of 0.5 cm³ at most. Using FAAS can save costs, but the sensitivity is not as high as GFAAS. GFAAS has high efficiency and long detection time. WCAES has low power requirements and a very fast heating speed. Although it has low accuracy, it can be made portable and suitable for on-site use. ICP-MS has very fast sample detection speed, less interference than spectroscopy, high accuracy, and high instrument operating costs. Therefore, in the actual detection work, it is necessary to select efficient, reasonable and accurate detection methods according to the actual situation.

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References

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• Yanqin, Q,; et al. Determination and leaching at brewing of aluminum, iron and manganese in white tea. Acta Tea Sinica. 2018, 59(4): 211-214.

• Koch, K.R.; et al. Determination of aluminium levels in tea and coffee by inductively coupled plasma optical emission spectrometry and graphite furnace atomic absorption spectrometry. Analyst. 1989, 114(8): 911-913.

• Costa, L.M.; et al. Comparison of heating extraction procedures for Al, Ca, Mg, and Mn in tea samples. Analytical sciences. 2002, 18(3): 313-318.

• Santos, L.N.; et al. Evaluation of tungsten coil atomic emission spectrometry for the direct determination of alin tea infusions and iced tea. Food analytical methods. 2016, 9(3): 624-629.

• Chunbei, X.; et al. Aluminum content determination in brick-tea with chrome azurol S spectrophotometric method. China Academic Journal Electronic Publishing House. 2006(25): 703-704.

• Koch, K.R. Quantitative determination of aluminium in tea by means of aluminium-27 nuclear magnetic resonance spectroscopy. Analyst. 1990, 115(6): 823-825.