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Detecting all potential impurities in pure water is neither practical nor necessary. Different detection methods are suitable for different types of impurities. Currently, the most required methods in practical applications are rapid online monitoring of resistivity and TOC (total organic carbon).
According to the current national standard GB/T6682-2008 "Specifications and Test Methods for Water Used in Analytical Laboratories," laboratory water is classified into three levels based on purity:
1. Grade I Water: Used for analytical tests with stringent requirements, including tests with particle size requirements, such as high-performance liquid chromatography (HPLC) and inductively coupled plasma mass spectrometry (ICP-MS). Grade I water cannot be stored and must be prepared before use.
2. Grade II Water: Used for inorganic trace analysis, such as atomic absorption spectrometry. Appropriate quantities can be prepared and stored in dedicated polyethylene containers rinsed with the same grade of water.
3. Grade III Water: Used for general chemical analysis tests and can be prepared by distillation or ion exchange.
The technical requirements of this standard mainly focus on inorganic ions, and the main technical indicators for water used in the electronics industry are similar. For example, GB/T6682-2008 stipulates that conductivity measurements for Grade I and Grade II water must be performed online, requiring the use of a conductivity cell with an electrode constant of 0.01~0.1 cm⁻¹ and equipped with automatic temperature compensation to ensure measurement accuracy.
Resistivity or conductivity is a core physical indicator for measuring the purity of pure water. At 25℃, the theoretical resistivity limit of ultrapure water is 18.2 MΩ·cm, determined by the dissociation equilibrium of water molecules themselves, representing that ionic impurities have been almost completely removed.
However, resistivity measurement has the following limitations:
1. Inability to reflect nonionic compounds: Conductivity meters only provide an indicator of the total amount of ions in water and cannot indicate the presence and concentration of nonionic compounds (such as organic matter, colloids, and particulate matter).
2. Insufficient sensitivity to trace ions: Due to the influence of the self-dissociation equilibrium of hydrogen ions and hydroxide ions in water, resistivity is not sensitive to specific trace ions below the ppb level. If the experiment remains highly sensitive to such low concentrations of impurities, methods such as ion chromatography (IC) or inductively coupled plasma mass spectrometry (ICP-MS) are required to determine the concentration of individual ions.
3. Susceptible to carbon dioxide interference: Pure water absorbs CO₂ upon contact with air, forming carbonic acid, leading to increased conductivity. Therefore, results from online monitoring and offline sampling measurements often differ; for high-end applications, online monitoring is recommended.
Pure water still contains various complex organic compounds. Analyzing and detecting each impurity individually is impractical; therefore, instruments capable of monitoring total organic carbon concentration are needed. TOC detection has proven to be the most effective: organic matter in the water sample is oxidized, and the generated CO₂ is quantitatively detected, thus converting it into carbon concentration.
Special attention should be paid to the following: In ppb-level TOC monitoring, the instrument must have sufficient sensitivity and accuracy. Furthermore, the interference of system blank, tubing material, and cleanliness on the results must be considered.
Among these, microorganisms and endotoxins are particularly critical for cell culture, pharmaceutical, and life science experiments; particulate matter and silicon content are strictly controlled items in semiconductor chip manufacturing.
For Grade I and Grade II water, GB/T6682-2008 clearly requires that conductivity be measured online. Offline sampling and testing introduces CO₂ and dissolved impurities through contact between the water sample and air or the container, causing the results to deviate from the true water quality. Therefore, key experimental points should be equipped with online monitoring instruments for resistivity and TOC, and these instruments should be calibrated regularly to achieve real-time and continuous water quality control.
