Chapter 18: Increasing Pathlength | CHM 214 | 156

Jacob Stewart
16 Apr 202104:32
EducationalLearning
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TLDRThe video explores innovative methods for measuring absorbance beyond traditional cuvettes and UV-Vis spectroscopy. It highlights two techniques: multi-pass absorption cells and cavity ring-down spectroscopy, which enhance detection by increasing path length. Multi-pass cells use mirrors to reflect light hundreds of times through a sample, while cavity ring-down spectroscopy employs highly reflective mirrors to measure light leakage over a much longer path. Both techniques are advantageous for detecting low concentrations or weak absorbance, with cavity ring-down offering kilometer-level path lengths.

Takeaways
  • 🌟 Absorbance measurement can extend beyond traditional cuvettes and UV-Vis spectroscopy.
  • 📈 A significant limitation in absorbance spectroscopy is that signal value depends on both concentration and path length.
  • 🔍 To measure substances with very low concentrations, increasing the path length can enhance the signal observed.
  • 🏠 Multi-pass absorption cells are a technique used to increase path length by bouncing light hundreds of times through a sample.
  • 💡 Lasers are typically used with multi-pass absorption cells due to their specific properties, unlike typical lamps or broadband light sources.
  • 🛠️ Multi-pass absorption cells require precise optical tuning and alignment of the laser for optimal performance.
  • 🎓 Cavity ring-down spectroscopy is another technique with even longer path lengths, though it can be more challenging to set up.
  • 💸 Cavity ring-down spectroscopy utilizes high reflectivity mirrors that are expensive and require careful alignment.
  • 🕒 In cavity ring-down spectroscopy, the time it takes for light to decrease in the cavity is measured, which correlates to the absorption of the sample.
  • 🌐 Cavity ring-down spectroscopy can achieve path lengths at the kilometer level, significantly more than typical absorption cells.
  • 🚀 Both multi-pass absorption cells and cavity ring-down spectroscopy are powerful tools for pushing the limits of absorbance spectroscopy.
Q & A

  • What is the main limitation of absorbance spectroscopy using typical cuvettes and UV-Vis spectroscopy?

    -The main limitation is that the signal obtained doesn't only depend on concentration but also on the path length. This makes it difficult to measure very small concentrations with a small path length, as the signal might not rise above the noise level or background level of the instrument.

  • How can the signal-to-noise ratio be improved in absorbance spectroscopy for low concentration measurements?

    -The signal-to-noise ratio can be improved by increasing the path length of the light through the sample. This allows for a stronger signal to be observed, making it easier to detect even at low concentrations.

  • What is a multi-pass absorption cell and how does it work?

    -A multi-pass absorption cell is a technique used to increase the path length of light through a sample. It involves placing mirrors on both sides of the sample and allowing the light to bounce back and forth hundreds of times before being measured. This significantly increases the path length and enhances the detection of weak absorbance or low concentrations.

  • Why are lasers typically used with multi-pass absorption cells?

    -Lasers are typically used with multi-pass absorption cells because of their specific properties. Lasers emit light that is coherent, monochromatic, and highly directional, which makes them suitable for the precise optical alignment required in this technique.

  • What is cavity ring-down spectroscopy and how does it differ from multi-pass absorption cells?

    -Cavity ring-down spectroscopy (CRDS) is a technique that uses two highly reflective mirrors to create an optical cavity. Light bounces between these mirrors and a small amount leaks out over time. The time it takes for the light to decrease is measured, which is related to the absorption of the sample. Unlike multi-pass absorption cells, CRDS can achieve even longer path lengths, up to the kilometer scale.

  • What are the challenges of setting up cavity ring-down spectroscopy?

    -Setting up cavity ring-down spectroscopy can be challenging due to the need for precise alignment of the expensive, highly reflective mirrors. Additionally, the system can be difficult to get working initially, although once operational, it provides high sensitivity measurements.

  • How does the reflectivity of the mirrors in cavity ring-down spectroscopy affect the measurement?

    -The mirrors used in cavity ring-down spectroscopy are designed to be very highly reflective at the specific wavelength being measured. A reflectivity of 99.998 percent means that only a tiny fraction of the light escapes with each bounce, which allows for very sensitive detection of absorption in the sample.

  • What is the advantage of measuring absorption indirectly in cavity ring-down spectroscopy?

    -Measuring absorption indirectly by observing the time it takes for the light to decrease in the cavity provides an advantage because it can be less affected by fluctuations in the light source and allows for highly sensitive detection of even very weak absorbance.

  • How do you differentiate between a pulse laser and a continuous laser in the context of cavity ring-down spectroscopy?

    -A pulse laser emits light in short bursts and the decrease in signal over time is naturally observed in CRDS. A continuous laser emits light constantly, but a fast switch is used to turn off the light, allowing the observation of the decrease in signal over time as it leaks out of the optical cavity.

  • How do the properties of lasers enable the use of techniques like multi-pass absorption cells and cavity ring-down spectroscopy?

    -Lasers provide coherent, monochromatic, and highly directional light, which is essential for the precise optical alignment and sensitivity required in techniques like multi-pass absorption cells and cavity ring-down spectroscopy.

  • What are the potential applications of pushing the boundaries of absorbance spectroscopy using these advanced techniques?

    -Advanced techniques like multi-pass absorption cells and cavity ring-down spectroscopy can be used to detect and analyze trace amounts of substances in environmental monitoring, chemical sensing, medical diagnostics, and fundamental research, enabling the study of processes that occur at very low concentrations.

Outlines
00:00
🌟 Innovations in Absorbance Measurement Techniques

This paragraph introduces the topic by discussing limitations in traditional absorbance spectroscopy methods, specifically the dependency on concentration and path length. It highlights the need for alternative methods to measure very low concentrations of substances. The speaker introduces two advanced techniques: multi-pass absorption cells and cavity ring down spectroscopy, which are used to increase the path length and enhance the detection of weak absorbance signals. These methods are particularly useful when dealing with lasers due to their unique properties, unlike typical lamps or broadband light sources.

Mindmap
Keywords
💡Absorbance
Absorbance is a measure of how much light is absorbed by a substance when light passes through it. In the context of the video, it is a key parameter in spectroscopy that depends on the concentration of the substance and the path length of light through the sample. The video discusses techniques to measure absorbance for substances with very low concentrations, where traditional methods may not provide a strong enough signal to be detected above background noise.
💡Spectroscopy
Spectroscopy is an analytical technique used to identify substances through the analysis of the spectrum of light they absorb or emit. In the video, the focus is on absorption spectroscopy, which measures how much light is absorbed at different wavelengths. The speaker discusses limitations of traditional spectroscopy methods and introduces alternative techniques to overcome these limitations.
💡Path Length
Path length refers to the distance that light travels through a substance when being measured for absorbance. A longer path length results in more absorption of light, which can improve the detection of substances at low concentrations. The video emphasizes the importance of increasing path length to enhance the signal-to-noise ratio in absorbance measurements.
💡Cuvettes
Cuvettes are small, usually rectangular, containers made of transparent material, such as glass or quartz, used to hold samples for spectroscopic analysis. They are a common tool in UV-Vis spectroscopy, where light is passed through the cuvette to measure the absorbance of the sample inside. The video mentions cuvettes as a typical tool but then contrasts them with alternative methods for measuring absorbance.
💡UV-Vis Spectroscopy
UV-Vis Spectroscopy is a type of absorption spectroscopy that measures the absorption of light in the ultraviolet and visible ranges of the electromagnetic spectrum. It is widely used in analytical chemistry to identify and quantify chemical compounds. The video discusses the limitations of this method, particularly when dealing with substances at very low concentrations.
💡Multi-Pass Absorption Cells
Multi-Pass Absorption Cells are specialized devices used to increase the effective path length of light through a sample in spectroscopy. By using mirrors to reflect the light beam multiple times through the sample, the cell significantly amplifies the light-sample interaction, allowing for the detection of very low concentrations of substances. This technique is particularly useful when working with lasers due to their directional and intense light properties.
💡Lasers
Lasers are devices that emit light through a process of optical amplification, producing a concentrated and intense beam of light. They are used in various spectroscopic techniques due to their high brightness and narrow spectral width. In the context of the video, lasers are preferred for techniques like multi-pass absorption cells and cavity ring down spectroscopy because of their ability to maintain a consistent, focused beam over a long path.
💡Cavity Ring Down Spectroscopy
Cavity Ring Down Spectroscopy (CRDS) is a highly sensitive spectroscopic technique that measures the decay of light intensity in an optical cavity formed by two highly reflective mirrors. The technique allows for the measurement of very low concentrations of absorbing species by monitoring the rate at which light decreases in intensity as it bounces between the mirrors. The longer the path length in the cavity, the more sensitive the detection.
💡Optical Cavity
An optical cavity is a configuration formed by two or more mirrors that reflects light back and forth between them, creating a confined space in which light can circulate. In spectroscopy, it is used to increase the effective path length that light travels through, which enhances the sensitivity of absorption measurements. The term 'cavity' in 'cavity ring down spectroscopy' refers to this optical setup.
💡Reflectivity
Reflectivity is the fraction of the incident light that is reflected by a surface. In the context of the video, it is crucial for cavity ring down spectroscopy, where mirrors with very high reflectivity are used to maximize the amount of light that bounces back and forth within the optical cavity, leading to more precise absorption measurements.
💡Signal-to-Noise Ratio
Signal-to-Noise Ratio (SNR) is a measure of the clarity of a signal in relation to the background noise. A higher SNR indicates a clearer, more reliable signal. In spectroscopy, improving the SNR is essential for detecting weak signals from substances at low concentrations, as it allows the signal from the sample to be distinguished from the random fluctuations of the equipment.
Highlights

The video discusses alternative methods for measuring absorbance beyond typical cuvettes and UV-Vis spectroscopy.

A major limitation of absorbance spectroscopy is the dependency of signal on both concentration and path length.

For measuring very small concentrations, increasing the path length can enhance the signal observed.

Multi-pass absorption cells are introduced as a technique for increasing path length.

Multi-pass absorption cells are typically used with lasers due to their unique properties.

The concept of multi-pass absorption cells involves light bouncing back and forth hundreds of times through the sample.

The absorption cell mentioned can achieve up to 76 meters of path length, and some can even exceed 100 meters.

Cavity ring-down spectroscopy is another technique discussed, known for its even longer path lengths.

Cavity ring-down spectroscopy requires precise optical tuning and alignment, and can be challenging to set up.

This technique uses highly reflective mirrors in an optical cavity where light bounces back and forth.

The amount of light that leaks out over time in cavity ring-down spectroscopy is measured for analysis.

Cavity ring-down spectroscopy allows for the measurement of absorption without directly measuring it.

The technique provides a way to measure the time it takes for light to decrease, with absorption being directly related.

Cavity ring-down spectroscopy can increase the path length to a kilometer level, significantly more than typical cells.

These techniques leverage the properties of lasers to push the boundaries of absorption spectroscopy.

The video provides insights into innovative methods for measuring very small concentrations or weak absorbance.

Transcripts

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SpectroscopyMulti-Pass CellsCavity Ring-DownLaser TechnologyAbsorbance MeasurementScientific ResearchOptical AnalysisHigh PrecisionLab TechniquesInnovative Methods