Background
UltraPath was developed by WPI under a collaborative agreement with NASA (Stennis Space Center) for the spectroscopic determination of colored dissolved organic matter (CDOM) in seawater and fresh water environments, and was designed for use in the laboratory and in the field (i.e., at sea). CDOM concentrations vary significantly between open ocean samples with low CDOM (e.g., 0.007 m-1 at 380 nm), and high CDOM freshwater environments (e.g., 10-20 m-1 at 380 nm). To address these problems the design requirements of UltraPath mandated the development of a rugged portable system capable of high sensitivity measurements across a wide dynamic range.
The UltraPath system meets these stringent design criteria and enables reliable measurement of CDOM in the range of 0.002 m-1 to 200 m-1 (380 to 730 nm). Fig 1. illustrates two typical absorption spectra measured using UltraPath. The sample labeled “High” is a highly concentrated CDOM sample with low salinity collected from a marsh drainage area of an urban lake (Lake Pontchartrain, Louisiana). The CDOM sample labeled “Low” is from oligotrophic, low productive waters with high salinity collected off the West coast of Puerto Rico in the Mayagüez Bay. Special attention should be drawn to the insert of Fig. 1, which reveals the exceptional sensitivity of UltraPath enabling detection of CDOM absorption below 0.03 m-1. Based on customer request, WPI has designed an UltraPath system for UV and VIS applications, the UPUV.
Applications
The design of UltraPath is such that it will have an important use in any applications requiring precise and highly sensitive spectroscopic determination of analytes across a wide dynamic absorption range (i.e., 5 µAU/cm to 1 AU/cm), either in the lab or in the field. To further illustrate the unique performance of UltraPath, samples containing ultra-low concentrations of Phenol Red were measured. Fig.2 illustrates clearly the detectable absorption spectra obtained between 0.9 nM and 4.6 nM Phenol Red. The key design feature of UltraPath is that the optical pathlength can be easily changed to cover a dynamic range not possible using a standard single pathlength sampling system.
Design
UltraPath has four optical pathlengths contained within a single sample cell (i.e., 2 cm, 10 cm, 50 cm and 200 cm). The pathlengths are user-selectable, offering a very high sensitivity and an extended dynamic range for VIS absorbance measurements. A user-controlled switch changes the optical pathlength of the sample cell allowing absorbance measurements over a broad dynamic range. The fluid path of the sample cell is optimized to produce a laminar flow that is virtually free of interference from trapped air bubbles and adherence of dissolved substances to the cell wall. In particular, the design greatly minimizes the problems commonly found with flow cells of long optical pathlengths: the risk of trapping dust particles, fibers or particulate matter inside the cell.
The UltraPath system includes a photodiode array-based spectrometer module (TIDAS I) to measure the absorbance at the selected pathlength. Absorption of light is measured between 370 nm and 730 nm wavelength (UPVIS) and 250 nm to 730 nm (UPUIV) with a resolution of 5 nm (FWHM) and a noise level below 0.2 mAU. A fiber optic light source delivers light via optical fibers to the multiple pathlength sample cell. A peristaltic pump (PeriStar) is utilized to evenly draw sample up into the sample injector and through the UltraPath sample cell.
A standard PC or laptop (not included) is connected to the detection system via a RS232 interface. The Spectralys™ software (included) displays and performs data collection and processing. High precision absorbance or transmittance spectra can be obtained within seconds. Key features of the Spectralys software package include ease-of-use, simple spectra acquisition, extensive online help, as well as tools for sample qualification, quantification, validation and quality control. The container-based approach of taking samples within an experiment leads the user to “good laboratory practice” and documentation. Extensive calculation features allow convenient data processing (derivatives, smoothing, log, normalized, baseline and peak analysis). A very useful feature of the software is its extensive kinetics functions, tailored for flow applications. Moreover, using Spectralys’s programming features (Active X), customized and automated detection systems can be user-designed. External instruments, such as pumps or light sources can be controlled with the 5 digital inputs and 5 digital outputs of the Tidas I.
Mobility
The system is designed for mobility. The components of the UltraPath system are designed to function over a broad range of laboratory and field environments. The electrical components are designed for low power consumption. A laptop running Windows 98, Windows 2000 or Windows NT 4.0 can be used to run the spectrometer as well as analyze data output.
