Max Planck Institute of Quantum Optics, Hans-Kopfermann-Straße 1, Garching 85748, Germany.
Ludwig Maximilians University München, Am Coulombwall 1, Garching 85748, Germany.
Anal Chem. 2020 Jun 2;92(11):7508-7514. doi: 10.1021/acs.analchem.9b05744. Epub 2020 May 13.
The strong absorption of liquid water in the infrared (IR) molecular fingerprint region constitutes a challenge for applications of vibrational spectroscopy in chemistry, biology, and medicine. While high-power IR laser sources enable the penetration of ever thicker aqueous samples, thereby mitigating the detrimental effects of strong attenuation on detection sensitivity, a basic advantage of heterodyne-measurement-based methods has-to the best of our knowledge-not been harnessed in broadband IR measurements to date. Here, employing field-resolved spectroscopy (FRS), we demonstrate in theory and experiment fundamental advantages of techniques whose signal-to-noise ratio (SNR) scales linearly with the electric field over those whose SNR scales linearly with radiation intensity, including conventional Fourier-transform infrared (FTIR) and direct absorption spectroscopy. Field-scaling brings about two major improvements. First, it squares the measurement dynamic range. Second, we show that the optimum interaction length with samples for SNR-maximized measurements is twice the value usually considered to be optimum for FTIR devices. In order to take full advantage of these properties, the measurement must not be significantly affected by technical noise, such as intensity fluctuations, which are common for high-power sources. Recently, it has been shown that subcycle, nonlinear gating of the molecular fingerprint signal renders FRS robust against intensity noise. Here, we quantitatively demonstrate this advantage of FRS for thick aqueous samples. We report sub-μg/mL detection sensitivities for transmission path lengths up to 80 μm and a limit of detection in the lower μg/mL range for transmission paths as long as 200 μm.
液态水在红外（IR）分子指纹区域的强吸收对化学、生物学和医学中的振动光谱应用构成了挑战。虽然高功率红外激光源能够穿透越来越厚的水样品，从而减轻强衰减对检测灵敏度的不利影响，但异频测量方法的一个基本优势——据我们所知——尚未在宽带 IR 测量中得到利用。在这里，我们通过场分辨光谱（FRS）理论和实验证明，与那些信噪比（SNR）与辐射强度呈线性关系的技术相比，那些 SNR 与电场呈线性关系的技术具有基本优势，包括传统的傅里叶变换红外（FTIR）和直接吸收光谱。场标度带来了两个主要的改进。首先，它使测量动态范围的平方。其次，我们表明，对于 SNR 最大化测量，与样品的最佳相互作用长度是通常认为 FTIR 设备最佳的两倍。为了充分利用这些特性，测量不能受到技术噪声的显著影响，例如强度波动，这在高功率源中很常见。最近，已经表明，分子指纹信号的亚周期非线性门控使 FRS 对强度噪声具有鲁棒性。在这里，我们定量地证明了 FRS 在厚水样品中的这一优势。我们报告了对于传输路径长度高达 80 μm 的亚μg/mL 检测灵敏度，以及对于长达 200 μm 的传输路径的检测限在μg/mL 低范围。
