ISO 6636-2:1981

Overview

ISO 6636-2:1981 - "Fruits, vegetables and derived products - Determination of zinc content - Part 2: Atomic absorption spectrometric method" specifies an atomic absorption spectrometry (AAS) procedure for measuring zinc (Zn2+) in fruits, vegetables and derived food products. The method covers sample preparation (dry ashing or wet digestion), instrument measurement (air–acetylene flame AAS at 213.8 nm) and reporting requirements. For some clear liquid samples (wines, clear juices) direct measurement without prior decomposition is permitted.

Key topics and technical requirements

• Scope: Determination of zinc content in fruits, vegetables and derived products using AAS.
• Sample preparation
  • Dry method: ashing (muffle furnace) followed by dissolution of ash in hydrochloric/nitric acid.
  • Wet method: acid digestion with nitric and sulfuric acids in Kjeldahl flasks; recommended sample masses: 5–10 g for pasty/solid products; 10 mL for liquid samples.
  • For viscous/heterogeneous samples, appropriate homogenization and representative test portions are specified.
• Reagents and standards
  • All reagents must be of analytical purity and free of zinc; use double-distilled or equivalent water.
  • Zinc standard solution: a 1 g Zn per litre stock is prepared and used to make calibration standards.
  • Specific acid solutions (HNO3, H2SO4, HCl) and a dilute HCl solution for dilutions are detailed.
• Apparatus
  • AAS with air–acetylene burner, set for optimum aspiration and wavelength 213.8 nm.
  • Muffle furnace controllable at 525 ± 25 °C; electrically heated oven; centrifuge; volumetric flasks (50 mL); platinum or quartz dishes or Kjeldahl flasks; analytical balance.
• Calibration and measurement
  • Calibration curves produce standards in the range 0.25–1.5 mg Zn/L (prepared differently depending on decomposition method).
  • Blank test required; blank absorbance must be ≤ 0.002.
  • If sample absorbance exceeds the highest calibration point, dilute appropriately and apply dilution factor.
• Performance
  • Repeatability: difference between two determinations by the same analyst on the same sample should not exceed 10% (relative).
• Reporting
  • Report method used, result and expression (per litre, per kg, or dry basis), and any deviations or incidents.

Applications and users

• Laboratories performing nutritional analysis, food safety testing, regulatory compliance and quality control for:
  • Fresh and processed fruits and vegetables
  • Fruit juices, wines and other derived products
  • Dried, pasty and viscous products
• Stakeholders: food testing labs, producers, regulatory agencies, research institutions and accreditation bodies.

Related standards

• ISO 5515 - Decomposition of organic matter prior to analysis - Wet method
• ISO 5516 - Decomposition of organic matter prior to analysis - Ashing (dry) method

Keywords: ISO 6636-2:1981, zinc determination, atomic absorption spectrometric method, fruits and vegetables, AAS 213.8 nm, sample digestion, food analysis, calibration graph.