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FAQ: What is the detection limit of time-gated Raman spectroscopy?

FAQ series: This series of publications explores some of the most frequently asked questions regarding Timegated® spectrometers and measurements without having you read page after page of text. 

There isn’t a single universally accepted method of determining limit of detection, but these methods aim to describe the smallest concentration of an analyte that can be reliable measured by an analytical procedure. A common approach is to determine a noise level and then defining at which point a signal can be reliably distinguished from said noise i.e. signals with high signal to noise ratios (SNR) can be more reliably detected and used for quantitative purposes.  

 Excitation fluorescence wavelength

Time-gated Raman spectroscopy may provide an advantage compared with conventional Raman by reducing noise and signal “swamping” caused by fluorescence or thermal interference (see more on fluorescence rejection here [link] and on thermal rejection here [link]). Time-gating may also enable the use of shorter wavelength excitation by reducing fluorescence interference which usually leads to higher Raman intensities (see figure below). These factors together may provide higher SNR which in turn may provide lower limits of detection (LOD).  

With all Raman techniques, the observed Raman signal intensity depends on several factors including the analyte’s molecular structure and the optical properties of the sample. Estimates of the intensities can be produced mathematically but often empirical testing is more practical and feasible. This is why, for more robust LOD estimates representative sample measurements are advisable for each specific sample type and no single universally valid LOD value can be given. The signal intensities of different analytes can have major differences from not producing any Raman responses to saturating the detector.  

There are also other practical matters in LOD determination that underline the importance of empirical testing. The sample matrix might include components that produce Raman responses that overlap the analyte signals which might severely affect the analyte quantification. The sample might also be sensitive to the excitation laser which might require the reduction of laser power which in turn results in lowered Raman signal intensities. The measurement length is also an important factor. Longer measurements usually result in lower noise levels which improve the SNR and LOD estimates.