Introduction
Ocean Optics has developed sol gel coating materials that encapsulate pH indicator dyes for optical pH sensing. These materials can be easily applied to a variety of substrates, including cuvettes, self-adhesive patches, probes (reflective and transmissive) and even microtiter wells. These sampling devices are easily integrated into optical setups for pH measurements in the biological range (pH 5-9).

Our optical pH sensors use a colorimetric indicator dye (specially formulated to eliminate the effects of changes in ionic strength) that is trapped in an organically modified sol-gel coating. The sol gel is applied to a substrate, which typically includes a fiber optic probe, Smart Cuvettes or patch.

An absorbance spectrometer (benchtop and handheld options are available) measures the sensor’s indicator dye as it changes color in response to the analyte solution. That color change is then related to calibration values via a ratiometric algorithm to determine the pH of the solution. Color change occurs as hydrogen ions diffuse in and out of the silica matrix and interact with the indicator dye. A secondary baseline wavelength is also taken into account in the algorithm to correct for any optical shifts caused by the optical fiber or probe or other environmental factors.

Experimental Conditions
As part of feasibility testing for a custom pH application, the bottom part of some transparent plastic microtiter wells were affixed with a thin adhesive PMMA material coated with our Smart pH sol-gel formulation. The idea is to allow for transmissive pH sensing of the analyte solutions placed into the wells. These wells are moved via a robotic positioning system to align them with the light source and recovery.

We performed a brief titration using two buffers only: pH 1 (absorbance reference) and pH 11 (observed). The pH 11 buffer produced a strong absorbance curve very similar to what is seen for the Smart pH Cuvettes, demonstrating strong evidence that the sensing material applied to the wells would also perform.

The customer’s configuration uses photodiode detectors and light filters to observe absorbance at the necessary wavelengths. We used a USB2000+ Spectrometer configured for 200-850 nm, with a 200 µm slit, although other options are available:

•    For benchtop applications, the USB4000 Spectrometer set to 350-1000 nm, with a 25 µm slit and order-sorting filter
•    For portable applications, a Jaz system set to 350-1000 nm, with a 25 µm slit and order-sorting filter, plus pH software application and battery module for field work

Our sample holder was a 10-cm pathlength cuvette holder turned vertically, with collimating lenses on each side. Light was fed to the top of the cuvette holder from an LS-1 Tungsten Halogen Light Source with a blue filter via a 200 µm UV-Vis optical fiber. The signal was then sent through a 300 µm solarization-resistant optical fiber to the spectrometer.

The sample environment will determine your approach to sampling optics. Smart Cuvettes work well for applications that don’t require in situ measurements. That’s where our transmissive and reflective probes come in. Patches are what we actually install in our probes, although they can be used for through-package or through-container measurements as well.

Results
When our customer carried out its own set of titrations, the data did not adhere to our pH algorithm. In investigating this disparity, we discovered two issues as most probable causes of the poor data: the lack of collimating lenses being used in the customer’s testing, and the use of an incorrect baseline correction wavelength. Once we adjusted the setup to the optimum baseline wavelength, we were able to accurately monitor and report the pH of the customer’s transmissive analyte solutions.

Conclusions
Results demonstrate the viability of our proprietary optical pH sensors for a variety of analytes. Modular spectroscopy and the ability to apply pH indicators to different substrates suggest a wide range of applications is possible.

More Information

• USB4000 Miniature Spectrometer
• Jaz Modular Spectrometer Suite
• Ocean Optics Sensors Division

Introduction
Irradiance is the amount of energy at each wavelength emitted from a radiant sample. Absolute irradiance is the measure of light in absolute terms. Relative irradiance is a comparison of the fraction of energy the sample emits and the energy the sampling system collects from a lamp with a blackbody energy distribution.

Measurement of absolute solar irradiance is relevant in a number of applications: monitoring the sunlight itself, perhaps in the context of its relationship to greenhouse gases in the atmosphere; investigating the effect of solar radiation on ecological systems and crops; and evaluating the effect of UV sunlight on our skin and eyes.

Solar radiation includes spectral response across the broad UV-NIR region. We offer a number of spectrometer options that will measure solar irradiance over various portions of the solar spectrum between 200-1100 nm. A spectrometer now in development will combine both UV-Vis CCD-array and NIR InGaAs-array detectors in the same unit, extending the possible measurement range considerably.

Experimental Conditions
To measure a series of strong absorption lines in the solar spectrum, we configured our Jaz spectrometer with a grating optimized to 200-850 nm, a detector collection lens to increase light collection, a 50 µm slit and a battery module for portable operation. The Jaz is a self-contained unit that includes a microprocessor and low-power OLED display in place of a PC. Also, we added SpectraSuite spectrometer operating software and our Jaz-A-IRRAD irradiance-measurement application software to complete the basic irradiance setup.

For absolute irradiance measurements, we added a radiometrically calibrated light source (LS-1-CAL) and cosine corrector (several options are available), which also is our sampling optic. The cosine corrector collects sunlight from 180º and attaches directly to the SMA 905 connector on the Jaz. Accessory options such as a fixture for mounting Jaz in various positions and a shoulder holster for carrying Jaz are also available.

The Jaz spectrometer is radiometrically calibrated against the NIST-traceable LS-1-CAL Light Source, with the calibration file stored on an SD card. This SD card comes with the Jaz and fits into a slot in the battery module. We used this calibration file in software to determine the absolute irradiance values of the solar spectrum.

Results
Integration time for the measurement was 4 milliseconds, with spectral averaging set at 10 and boxcar smoothing at 2. The resulting spectrum showed a number of strong absorption lines from atmospheric elements in the range from 300-900 nm, including hydrogen, helium and sodium.

Similar results can be achieved using different spectrometer models with comparable optical bench configurations, including a USB2000+ Spectrometer that has our extended-range (200-1050 nm) grating or our USB2000+RAD, an application-ready setup with all the components needed for absolute solar irradiance measurements.

Conclusions
Because of its form-factor and battery operation, Jaz is a useful option for solar irradiance measurements of all types. The system is easily portable and extremely robust as well, having traveled to the top of Mt. Everest and successfully making solar radiation measurements for researchers studying ozone depletion.

Additional Resources
http://www.oceanoptics.com/Products/jazulm.asp
http://www.oceanoptics.com/Products/usb2000rad_compare.asp
http://www.oceanoptics.com/applications/samplesetups_upwellingdownwelling.asp
http://www.oceanoptics.com/Products/jaz_el350_200.asp
http://www.oceanoptics.com/Products/usb2000+rad.asp

Method:
LIBS

Introduction:
Following the promising results obtained for the ulexite/clay pellets, additional pellets prepared from XPI-263 grab samples were prepared with a narrow boron concentration range. The pellets were prepared to assess the sensitivity of LIBS to boron concentration.

Experimental Conditions:
Ulexite pellets were prepared with closely spaced boron concentrations. The pellets were prepared with a new grinding technique. Unlike the previous set of pellets, each of the new pellets was received intact. Each pellet was placed in the LIBS sampling chamber. Ten single shot spectra were acquired at different locations on each pellet.

Data analysis involved calculating the ratio of the boron (249.5422 nm) and silicon (251.40066 nm) lines for each single shot spectrum and then calculating average ratio and standard deviation values for each sample.

Hardware Used:
Single channel LIBS spectrometer (HR2000CG-UV-NIR with LIBS upgrade serial # HR2B338)
BIF600-UV/VIS with LIBS focusing lens at the tip
Big Sky 50 mJ Nd:YAG Laser
LIBS SC Sampling Chamber with 125 mm focal length lens
Radiometrically calibrated spectrometer

Experimental Parameters:
Q-Switch Delay (delay between laser firing and data acquisition): -1.5 microseconds
Single laser shot at a fresh spot on the pellet (10 single shot spectra averaged together for standard curve calculations)

Measurement Mode:
LIBS

Results:
The complete data set acquired previously was reassessed to determine the optimal measurement parameters. Based on the standard curve generated and the standard deviation of the values, the best results were obtained with a radiometrically calibrated spectrometer for results averaged from 10 single shot spectra. Due to changes in the LIBS system configuration (probe was removed and replaced) since the last set of pellet measurements, a new standard curve was generated using the previous set of pellet samples. The new standard curve is shown in Figure 1 below. It is similar to the standard curve reported previously. As shown in Figure 1, when the ratio of these lines was taken at each ulexite concentration, the intensity trend increased with increasing boron concentration.

In Figure 2, the ratio of boron and silicon lines calculated for the new set of XPI-263 pellets is shown. Note that these pellets cover a much narrower range of concentrations (10.15 to 11.70% boron). As anticipated, based on the error associated with the wet analysis technique, the trend is noisy especially in regions where the boron concentrations were very closely spaced (e.g. 10.29 to 10.36% boron). Overall, an increasing trend in intensity is observed with increasing boron concentration. The standard deviations of the values are higher than observed for the previous set of pellets.

Conclusions:
It is difficult to determine the sensitivity of the LIBS technique from this data based on the error associated with the wet assay technique. Sensitivity at the 0.5% boron concentration may be possible based the results shown above.

Figure 1: Ratio of the average intensities (n=10) for boron and silicon lines as a function of boron concentration: Original set of ulexite pellets measured previously

Figure 2: Ratio of the average intensities (n=10) for boron and silicon lines as a function of boron concentration: New set of XPI-263 pellets