Jakubek Ryan S, Bhartia Rohit, Steele Andrew, Asher Sanford A, Abbey William, Bykov Sergei V, Czaja Andrew D, Fries Marc D, Lee Carina, McCubbin Francis M, Minitti Michelle, Sharma Sunanda, Uckert Kyle
Jacobs, NASA Johnson Space Center, Houston, TX, USA.
Photon Systems Incorporated, Covina CA, USA.
Anal Chim Acta. 2025 Jul 1;1357:344072. doi: 10.1016/j.aca.2025.344072. Epub 2025 Apr 15.
The Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument on NASA's Perseverance rover is a deep ultraviolet Raman and fluorescence instrument used for organic and mineral mapping of the Martian surface. Flight instrument design and operational constraints lead to observations with low signal Raman features that must be statistically differentiated from measurement noise. There are many methods to calculate signal-to-noise ratios (SNR) in the Raman literature and we find that these are not equivalent. Thus, there is a need to 1) understand the differences and assumptions within SNR calculations, 2) aid in the comparison of SNR values across literature, and 3) identify SNR calculation methods that optimize limit-of-detection (LOD) based on the instrument characteristics.
We quantitatively compare methods in which signal-to-noise ratios (SNR) are calculated in Raman spectroscopy. Methods are separated into two broad categories, multi-pixel methods that utilize information across the full Raman bandwidth and single-pixel methods that use only the center pixel in the Raman band. We compare three SNR calculation methods on a standardized SHERLOC data set and show that multi-pixel methods detect spectral bands prior to single-pixel methods. This occurs because single-pixel methods only include signal from one pixel, ignoring the remaining signal across the bandwidth. While the manuscript focuses on Raman spectroscopy for application to SHERLOC data, the SNR calculation methodology can be utilized by any technique that reports spectral data.
This work is the first to report significant differences between methods of calculating spectroscopic SNR. This manuscript is of broad application with at least 3 points of major impact: 1) different SNR calculation methods are not equivalent and cannot be compared across literature, 2) multi-pixel SNR methods include signal from across the entire Raman bandwidth, improving the assessment of spectral features compared to single-pixel methods, and 3) we confirm a SHERLOC signal previously interpreted as the first Raman detection of organic carbon on the martian surface.
美国国家航空航天局“毅力号”火星车搭载的“利用拉曼光谱和发光技术扫描宜居环境中的有机物和化学物质”（SHERLOC）仪器是一种深紫外拉曼光谱和荧光光谱仪器，用于绘制火星表面的有机物和矿物质分布图。飞行仪器的设计和操作限制导致观测到的拉曼信号特征较弱，必须从测量噪声中进行统计区分。拉曼光谱文献中有许多计算信噪比（SNR）的方法，我们发现这些方法并不等效。因此，有必要：1）了解信噪比计算中的差异和假设；2）帮助比较不同文献中的信噪比数值；3）根据仪器特性确定优化检测限（LOD）的信噪比计算方法。
我们对拉曼光谱中计算信噪比的方法进行了定量比较。这些方法分为两大类：利用整个拉曼带宽信息的多像素方法和仅使用拉曼波段中心像素的单像素方法。我们在一个标准化的SHERLOC数据集上比较了三种信噪比计算方法，结果表明多像素方法比单像素方法能更早地检测到光谱带。这是因为单像素方法只包含一个像素的信号，忽略了整个带宽内的其余信号。虽然本文主要关注应用于SHERLOC数据的拉曼光谱，但任何报告光谱数据的技术都可以使用这种信噪比计算方法。
这项工作首次报告了光谱信噪比计算方法之间的显著差异。本文具有广泛的应用价值，至少有三个主要影响点：1）不同的信噪比计算方法并不等效，不能在不同文献之间进行比较；2）与单像素方法相比，多像素信噪比方法包含了整个拉曼带宽的信号，改善了对光谱特征的评估；3）我们证实了之前被解释为火星表面首次拉曼检测到有机碳的SHERLOC信号。
