Institute of Analytical Chemistry and Radiochemistry, University of Innsbruck, Innrain 80/82, 6020, Innsbruck, Austria.
Institute of Analytical Chemistry and Radiochemistry, University of Innsbruck, Innrain 80/82, 6020, Innsbruck, Austria.
Talanta. 2021 Jan 1;221:121165. doi: 10.1016/j.talanta.2020.121165. Epub 2020 May 21.
We investigated caffeine and l-theanine, quality characteristics for camellia sinensis, in milled and ground black tea samples with near-infrared (NIR) spectroscopy giving a direct comparison between the performances of benchtop and handheld NIR spectrometers. The constructed partial least squares regression (PLSR) models for all spectrometers were validated by test-set-validation and according to the obtained root mean square errors of prediction (RMSEP). The performances of the spectrometers were as follows: The benchtop spectrometer NIRFlex N-500 (Büchi, Flawil, Switzerland) showed the best results for milled samples with a RMSEP of 0.14% for caffeine and 0.12% for l-theanine. For the ground samples, a RMSEP of 0.17% for caffeine and 0.12% for l-theanine was gained. While the handheld spectrometers MicroNIR 2200 (Viavi Solutions (former: JDS Uniphase Corporation), Milpitas, USA) and the microPHAZIR (Thermo Fisher Scientific, Waltham, USA) both provided good results for the prediction of caffeine in milled samples (RMSEP of 0.22% and 0.26%), only the microPHAZIR was able to satisfactorily determine the caffeine content in ground samples (RMSEP of 0.28%). The investigation of l-theanine with handheld spectrometers did not lead to convincing results, since R was 0.75 for milled samples while ground samples could not be calculated. Decisive differences were concluded in how different NIR instruments capture the chemical information on caffeine vs. l-theanine. The handheld spectrometers manifested limited applicability to l-theanine. Deeper insight was obtained through the detailed NIR band assignments of caffeine and l-theanine derived from quantum mechanical simulation. Narrow working spectral region of handhelds omits the characteristic absorption bands of l-theanine. Therefore, information on l-theanine content measured by the evaluated miniaturized spectrometers is insufficient to enable its effective quantification. In contrast, the most characteristic NIR absorption of caffeine matches the working spectral regions of the handheld NIR spectrometers, hence their performance is comparable with the benchtop device.
我们使用近红外（NIR）光谱法研究了茶叶中咖啡因和茶氨酸这两种品质特征成分，对碾磨和粉碎的红茶样品进行了直接比较，同时比较了台式和手持近红外光谱仪的性能。通过测试集验证，对所有光谱仪构建的偏最小二乘回归（PLSR）模型进行了验证，并根据预测均方根误差（RMSEP）进行了评估。结果表明：台式光谱仪 NIRFlex N-500（Büchi，Flawil，瑞士）对碾磨样品的效果最佳，咖啡因和茶氨酸的 RMSEP 分别为 0.14%和 0.12%。对于粉碎样品，咖啡因和茶氨酸的 RMSEP 分别为 0.17%和 0.12%。而手持式光谱仪 MicroNIR 2200（前身为 JDS Uniphase 公司，Viavi Solutions，Milpitas，USA）和 microPHAZIR（Thermo Fisher Scientific，Waltham，USA）均能很好地预测碾磨样品中的咖啡因（RMSEP 分别为 0.22%和 0.26%），只有 microPHAZIR 能够满意地测定粉碎样品中的咖啡因含量（RMSEP 为 0.28%）。使用手持式光谱仪对茶氨酸进行的研究并未得出令人信服的结果，因为碾磨样品的 R 为 0.75，而粉碎样品则无法计算。研究得出，不同的 NIR 仪器在捕获咖啡因和茶氨酸的化学信息方面存在显著差异。手持式光谱仪在测定茶氨酸方面适用性有限。通过量子力学模拟得出的咖啡因和茶氨酸的详细近红外波段分配，进一步深入了解了这一现象。手持式光谱仪的工作光谱范围较窄，无法涵盖茶氨酸的特征吸收带。因此，评估的小型化光谱仪所测量的茶氨酸含量信息不足以对其进行有效定量。相比之下，咖啡因最具特征性的 NIR 吸收与手持式 NIR 光谱仪的工作光谱区域相匹配，因此其性能与台式仪器相当。
