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Discover how geoscientists use Ocean Optics optical oxygen sensors for reaction kinetics studies of pyrite oxidation, which is often linked to environmental issues such as remediation monitoring in aquifers.

Measuring O2 in the Earth’s Subsurface

Although Raman spectroscopy is an excellent analytical tool, Raman signals are often weak and traditional slit spectrometers typically have poor optical throughput, limiting their effectiveness in low light level setups. Using a high throughput virtual slit design, the Apex 785 Raman spectrometer from Ocean Optics eliminates trade-offs between sensitivity and resolution.

Conventional spectrometers use a narrow entrance slit to achieve high resolution, but at the expense of throughput. The slit can reject as much as 75–95% of the light entering the optical bench. With high-throughput virtual slit (HTVS) technology licensed from Tornado Spectral Systems, Apex spectrometers overcome the compromise between throughput and spectral resolution.  

Apex uses a beam reformatting technology that eliminates the need for a physical slit to achieve high resolution. When combined with precision optical coatings, volume phase holographic (VPH) gratings, and solid state detector technology, spectrometer throughput as high as 95% is achievable. 

Also, Apex spectrometers use a large input aperture and specially configured optical components to compress, reformat, and then expand the light beam, which narrows the input aperture along the dispersion axis while preserving total flux. 

To maintain peak optical throughput, Apex uses a 1200 line/mm VPH grating blazed at 830 nm in an optical bench with an uncooled, NIR enhanced back-thinned CCD array detector. 

Raman measurements of paracetamol (acetaminophen), polystyrene, isopropanol, and toluene were made using the Apex spectrometer (APEX-RAMAN-785) in a setup with a 100 mW 785 nm laser diode, a Raman sample holder and a 785 nm Raman fiber optic probe. Integration time intervals ranged from 

195 ms for toluene to 750 ms for paracetamol. 

As shown in Figure 1, the Apex measures spectral data over a Raman shift range beyond 3800 cm

  with high spectral density, delivering Raman data with more spectral features for better library matching and sample identification. Also, the Apex spectrometer enables the acquisition of high quality Raman spectral data at very short integration times. 

Figure 1: This Raman spectrum for paracetamol was acquired at 750 ms integration time.			

Typical Raman measurements require seconds to achieve similar signal levels. This rapid measurement speed enables measurements in process flows and for samples that change over time. The low integration times required for the Apex also allow for the measurement of multiple samples in the time it takes to obtain one spectrum from a less sensitive spectrometer. This provides the opportunity for signal averaging to increase SNR, further improving measurement accuracy. 

A new spectrometer design based on HTVS optical slicer technology has been shown to provide a significant increase (a factor of 10–15×) in optical throughput without any detriment in optical resolution. In addition, the technology allows users to achieve high sensitivity without the need for long integration times, detector cooling, or high powered excitation sources; to use shorter measurement times for faster measurements; to acquire more accurate spectral signatures; and to use lower power excitation sources to minimize photodegradation. 

 (1) Tornado Spectral Systems contributed to this application note. 

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Although Raman spectroscopy is an excellent analytical tool, Raman signals are often weak and traditional slit spectrometers typically have poor optical throughput, limiting their effectiveness in low light level setups.

Eliminating the Trade-Off Between Resolution and Throughput in Raman Spectroscopy

Near-infrared spectroscopy is a common analytical technique for chemistry and process control, where typical applications include identification of species and determination of water and fat content. In applications like those, absorbance peaks are often broad and optical resolution requirements of lesser concern than performance parameters such as low noise and high sensitivity. 

Yet there also are number of NIR applications where optical resolution of <5.0 nm (FWHM) or better is critical. Characterization of laser lines — examples include solid state lasers at 1064 nm and at wavelengths from 1020–1050 nm, as well as semiconductor lasers with response in the 900–1800 nm range — often require even better optical resolution. Optical fiber characterization and determination of atomic emission lines are other high-resolution NIR applications. 

To test the optical resolution performance of our NIRQuest512-2.2 spectrometer, which has a Hamamatsu G9206-512W InGaAs-array detector and is responsive from 900–2200 nm, we measured the spectrum of a xenon source used for spectrometer wavelength calibration. The low-pressure gas-discharge source has a number of closely aligned emission lines in the region from ~820–2000 nm, making it a good choice for our experiment. 

 Figure 1: The closely aligned peaks of a xenon source used for spectrometer wavelength calibration can be distinguished with the high-resolution NIRQuest NIR spectrometer.			

The NIRQuest512-2.2 was configured with a 100 lines/mm grating set to 900–2050 nm, with a 25 µm slit and gold-coated collimating and focusing mirrors for enhanced reflectivity. This is the standard configuration for the NIRQuest product line, which includes models for 900–1700 nm, 900–2050 nm, 900–2200 nm, and 900–2500 nm. Additional grating options and slit sizes are available for custom setups.  

We used a 50 µm VIS-NIR optical fiber to collect signal from the xenon source. (For optimum results, we recommend the use of a 50 µm diameter or smaller optical fiber with all of our spectrometer wavelength calibration sources.) NIRQuest512-2.2 integration time was set to 350 ms and spectral averaging set to 5. 

The spectrum from the xenon calibration source illustrates that optical resolution of ~4.6 nm (FWHM) is possible with the NIRQuest512-2.2 in its standard configuration (Figure 1). What's more, even better optical resolution is possible in a NIRQuest512-2.2 configured with a grating that has a narrower spectral bandwidth. For example, a NIRQuest512-2.2 with a 600 l/mm grating set over a 100-nanometer bandwidth and configured with a 25 µm slit would yield optical resolution of <0.5 nm (FWHM). Resolution would improve even more with a 10 µm slit, but at the expense of throughput. For most laser applications, that's likely to be an acceptable trade-off. 

New NIR detectors and optical bench options allow researchers to experience high optical resolution performance in the region from 900–2500 nm. This elevates the versatility of smaller footprint, more modular NIR spectrometers for applications previously thought to be out of reach. 

New detector and optical bench options make it possible to configure near-infrared spectrometer setups for high-resolution applications such as laser and optical fiber characterization.

High-Resolution NIR Analysis

Spectral measurements to 153 nm can be achieved easily and economically with the high-sensitivity Maya2000 Pro with Extra-Deep-UV Option. Nitrogen purging of the spectrometer helps to mitigate water and oxygen absorption in the vacuum ultraviolet.

A variety of materials have spectral signatures in the vacuum ultraviolet (VUV) spectral region, typically described as the region from 10 nm–200 nm (1,2). VUV spectroscopy is useful for applications ranging from biomedicine to semiconductor testing. Because the spectral response of standard silicon charge-coupled device (CCD) detectors drops off rapidly at wavelengths as long as 400 nm, and because oxygen and water absorb in the VUV spectral band, maintaining sufficient signal outside a vacuum is not possible. Designing vacuum-compatible spectrometers to address these issues can be difficult and expensive. As a result, most commercially available VUV spectrometers require custom configuration and are too expensive and unwieldy for many common applications.  

 (3) with Extra-Deep UV Option is poised to change that. The spectrometer features a 101.6-mm focal length optical bench with a compact crossed Czerny-Turner design and a back-thinned CCD detector that offers superior UV and VUV performance. To minimize signal attenuation inside the optical path, the spectrometer bench is purged with nitrogen, achieving a robust signal for VUV experiments at wavelengths as low as 153 nm. 

To test performance, we started with a Maya2000 Pro configured with a high resolution, UV-enhanced 2400 g/mm holographic diffraction grating and a 5 µm slit. The back-thinned detector (Hamamatsu S10420-1106) features a peak quantum efficiency of 75%, with UV quantum efficiencies as high as 50%. A custom magnesium fluoride (MgF

) glass window is placed over the detector for improved transmission over VUV wavelengths. 

The test sample for the experiment was a VUV deuterium lamp with a VUV-grade optical window, coupled directly to the spectrometer. We enclosed the source and spectrometer and purged the region with standard laboratory-grade nitrogen.    

Integration time for the measurement was 50 ms. The resultant emission spectrum shows sharply defined spectral peaks at wavelengths as low as 153 nm, with a strong central peak around 161 nm (see Figure 1). The system operated with a spectral resolution of 0.1 nm and a signal-to-noise ratio of 450:1. 

We have also used the system to measure a solution containing carbon, hydrogen/deuterium, nitrogen, oxygen, sulfur, chlorine, bromine, silicon and fluorine chromatographically separated into a column and placed in a vacuum chamber with a helium plasma mixture. VUV spectral features were clearly apparent in the spectrum, suggesting a wide variety of short-wavelength new applications is achievable. 

The results demonstrate that the reliable and economical Maya2000 Pro can be easily extended to VUV applications. If you have spectroscopy experiments or applications in the VUV, look no further than the Maya2000 Pro as your short-wavelength, affordable, robust solution. 

 Website covers VUV under larger section on ultraviolet light

CRC Handbook of Organic Photochemistry and Photobiology

http://www.oceanoptics.com/Products/maya.asp

 with full specifications on Maya2000 Pro

Phone: +1-727-733-2447; Fax: +1-727-733-3962

Author | Ocean Optics, Inc.

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