Hammer Daniel, Andrey Daniel
Nestlé Research Centre, Quality and Safety Assurance Department, Vers-chez-les-Blanc, 1000 Lausanne 26, Switzerland.
J AOAC Int. 2008 Nov-Dec;91(6):1397-401.
The performances of 2 official methods for iodine analysis based on inductively coupled plasma-mass spectrometry (ICP-MS) and the ion-selective electrode (ISE) method were compared for milk-based products. The aim of the study was to determine the performance characteristics of both methods to check the labeled concentration of iodine. Good precision was found for both methods with highest relative standard deviation of repeatability (RSD(r)) at 2.3 and 2.7% for ISE and ICP-MS, respectively. Intermediate reproducibility (RSD(iR)), single laboratory within 6 different days, was also good with the highest values at 7.3 and 8% by ISE and ICP-MS, respectively. Measurement uncertainty was estimated based on the RSD(iR) data, and it was concluded that both methods were capable of determining iodine concentrations within an uncertainty below +/- 20%. The accuracy of the methods was determined by analyzing certified reference materials, in-house proficiency test samples, and commercial products. Both methods returned similar results when applied on freshly opened samples. In samples that had been opened and kept exposed to air during storage, ISE returned lower iodine concentrations than ICP-MS. In commercial samples, the linear regression between both methods was ISE = 0.95 x ICP-MS -0.060 for freshly opened samples and ISE = 0.85 x ICP-MS + 0.069 for samples exposed to air. The tendency of ISE to return lower results than ICP-MS is explained by the fact that ISE is sensitive to iodide but does not measure iodine that may be bound organically to the matrix. This seems to be more pronounced in samples that were stored longer. Because in most countries iodine is labeled as total iodine, acceptance of an international standard based on the ICP-MS technique which takes all forms of iodine into account, is recommended. This would help to avoid any potential dispute on the accuracy of labeled iodine concentrations in finished products.
比较了基于电感耦合等离子体质谱法（ICP-MS）和离子选择电极（ISE）法的两种官方碘分析方法在乳基产品中的性能。该研究的目的是确定这两种方法的性能特征，以检查碘的标注浓度。两种方法均具有良好的精密度，ISE和ICP-MS的重复性相对标准偏差（RSD(r)）最高分别为2.3%和2.7%。中间精密度（RSD(iR)），即单个实验室在6个不同日期的精密度也良好，ISE和ICP-MS的最高值分别为7.3%和8%。基于RSD(iR)数据估计了测量不确定度，得出结论：两种方法都能够在不确定度低于±20%的范围内测定碘浓度。通过分析有证标准物质、内部能力验证样品和商业产品来确定方法的准确性。当应用于新开封的样品时，两种方法得到的结果相似。在储存期间已开封并暴露于空气中的样品中，ISE测得的碘浓度低于ICP-MS。在商业样品中，对于新开封的样品，两种方法之间的线性回归为ISE = 0.95×ICP-MS - 0.060，对于暴露于空气中的样品，线性回归为ISE = 0.85×ICP-MS + 0.069。ISE测得的结果低于ICP-MS的趋势可以解释为ISE对碘化物敏感，但不测量可能与基质有机结合的碘。这在储存时间较长的样品中似乎更为明显。由于在大多数国家碘标注为总碘，因此建议采用考虑了所有碘形式的基于ICP-MS技术的国际标准。这将有助于避免成品中碘标注浓度准确性方面的任何潜在争议。
