Fiber Optic Sampling by UV/Vis and UV/Vis/NIR Spectroscopy

Transcription

1 Fiber Optic Sampling by UV/Vis and UV/Vis/NIR Spectroscopy UV/VIS AND UV/VIS/NIR SPECTROSCOPY A P P L I C A T I O N N O T E Introduction The availability and applications of fiber optic sampling in UV/Vis and UV/Vis/NIR spectroscopy is expanding rapidly. Fiber optics provides a remote sampling capability where sample requirements preclude use of cuvettes, or where rapid remote sampling is required, i.e. on-line applications. Fiber optics accessories can be added to the current line of the LAMBDA spectrophotometers and are available in different lengths and wavelength ranges. All fiber optic accessories require an optrode of some type or measurement head. The type of optrode is usually dictated by the application. This application note will describe one of the fiber optic measuring options available from PerkinElmer, the dipping probe accessory. Technical specifications, performance, and example data will be presented. Fiber optic dipping probe A recent introduction to the PerkinElmer LAMBDA UV/Vis and UV/Vis/NIR series is the fiber optic dipping probe (Figure 1). This probe includes a unique design allowing a single probe body to be used with different pathlength tips. The probe is coupled to the LAMBDA instrument through a transfer optic assembly, which provides light coupling to and from the probe. Energy efficiency of the dipping probe is typically between 8% to 15% relative to an open beam. The fiber optic dipping probe provides remote analysis ( remote cuvette ) capabilities to your spectrophotometer. The probe can be used for transmission or absorption readings

2 in many liquids, including ones with suspended solids. Readings can be taken over the UV/Vis wavelength range, from 200 nm to 1100 nm. Fiber optic dipping probe features: Durable stainless steel design Multiple pathlengths with replaceable threaded tips High throughput efficiency Pathlength is two times the gap width (dual pass lightpath) High repeatability Light shield to reduce stray light The replaceable threaded tips (Figure 2) are available in standard pathlengths of 2 mm, 5 mm, and 10 mm, with special order pathlengths to 50 mm. The 10 mm pathlength tip was used for all data acquired for this application note. Fiber optic dipping probe specifications: Fiber type: Silica/silica,.22NA Fiber size: 600 microns (core) Probe body material: Stainless steel 316 Diameter: 6.35 mm (.25 ) Length of probe body: ~150 mm (6 ), depending on pathlength Pathlength: 2, 5, 10 mm replaceable tip; up to 50 mm possible Connections: SMA-905 Overall length: 1.5 m (4.5 ) Fiber optic cable sheathing: PVC monocoil Wavelength ranges: UV/Vis ( nm) Upper temperature range: 85 C The dipping probe incorporates a light shield so measurements can be conducted in normal light. The outlet and inlet fiber optics are routed directly into the sample compartment, so there is no need to lift the sample compartment cover during operation. Measurements can be taken directly on liquids in test tubes, beakers, and even large drums, making this accessory ideal for assaying the purity of incoming materials. The LAMBDA 35 running UVWinLab software can collect scan, rate reaction, wavelength programming, and concentrations data using this accessory. A close-up picture of the sample compartment transfer optic is shown in Figure 4. This allows connection of the SMA connectors of the dipping probe. The hardware shown can also accommodate other fiber optic assemblies that the user may wish to substitute. Performance The LAMBDA double beam series of spectrophotometers all have state-of-the-art performance specifications. The models include the LAMBDA 45/35/25 and the new reference grade LAMBDA 950/850/650. The high energy throughput of these units allows the dipping optrode to be added with little loss of data quality. Corrected baselines with the LAMBDA 35 can typically be acquired to ±0.002 A or better using the dipping probe accessory. The accessory provides excellent energy throughput for a 600 micron monofiber, typically in the range of 10% to 20% transmission compared to an empty compartment. This high energy allows flat baselines to be acquired even at higher scan speeds, as shown in Figure 5. For best and consistent results, the placement of the dipping probe for sample measurement should be as close as possible to where the background correction was acquired. Shown are three overlaid corrected baselines of distilled water acquired with the dipping probe accessory, scanned at 960 nm/min, with a 2 nm bandpass. The absorbance peak-to-peak RMS noise data of the dipping probe mounted in a LAMBDA 35 is shown in Table 1. The overall low noise over the entire wavelength range allows quality spectra with excellent signal-to-noise to be collected. Figure 1. Fiber optic dipping probe accessory. Figure 2. Different pathlength tips for the fiber optic dipping probe accessory. Figure 3. PerkinElmer LAMBDA 35 UV/Vis spectrophotometer fitted with dipping probe accessory. 2

3 The quality of fiber optic sampling systems has improved dramatically over the last decade. Early accessories offered for UV/Vis spectrophotometers were often plagued by low signal-to-noise, and poor repeatability. Often these early design accessories were evaluated and rejected by laboratories. The dipping probe accessory uses the latest generation silica monofibers, providing for high energy throughput. This allows high precision of sample readings, approaching the repeatability seen in cuvette readings. Shown in Figure 6 is an overlay of 15 scans of a yellow food dye acquired with the dipping probe accessory. The solution was measured in a flask with the fiber optic probe being removed and reinserted between scans, as it would be used in a laboratory. The solution was scanned from 550 to 300 nm at 480 nm/min, and a 2 nm bandpass. The mean of the absorbance at the peak at nm was with a standard deviation of ± Experimental The dipping probe accessory provides for easy and rapid measurements of solutions in all types of containers. Because the accessory includes its own light shield, the containers can be clear, and open to the room light. Concentration assays are especially easy to setup and run with the dipping probe accessory. Four concentrations of Evan s Blue dye were prepared in flasks (shown in Figure 3), and using UVWinLab 5.1 were measured with the Scanning Quant application. The Scanning Quant application allows standards to be scanned, where the software automatically determines the peak position and absorbance, and a calibration curve is plotted in real-time. Prior to measuring the standards, the dipping probe was placed in a flask of distilled water, and a corrected baseline was acquired. The experimental data is presented in Figures 7, 8, and 9. The solutions were scanned on a LAMBDA 35 at 480 nm/min and a 2 nm bandpass. In this experiment, excellent linearity was achieved, with a calculated correlation coefficient of Because no cuvettes were needed, assay time was very rapid and easy. The excellent signal-to-noise of the dipping probe accessory fitted with a LAMBDA 35 allows very low Table 1. Peak-to-peak and RMS noise of the LAMBDA 35 with dipping probe installed. Measured with a 4 nm slit at 0 Abs. Wavelength P/P RMS Figure 5. Expanded scale overlay of 3 corrected baselines of distilled water collected with the dipping probe accessory. Conditions were 960 nm/min, 2 nm slit. The baselines are within ± A. Figure 4. Monofiber transfer optic stage mounted in the sample compartment of the LAMBDA 35. Figure 6. Overlay of 15 scans of a yellow dye collected with the dipping probe accessory. The fiber optic probe was removed and reinserted into the flask containing the solution between scans. Conditions used were 480 nm/min and a 2 nm slit. 3

4 concentration of analyte to be measured with precision. Low concentrations of benzene in water (25, 50, 100, and 150 ppb) were prepared in flasks and measured with the dipping probe accessory. The solutions were measured on a LAMBDA 35 from 280 nm to 230 nm, at 240 nm/min and a 2 nm bandpass. The results are shown in Figure 10. A background correction of distilled water was acquired and is shown as the 0 ppb curve. Even though the absorbances of these solutions were very low, and they were measured in clear flasks, quality spectra were obtained with little noise. Note that full-scale on the graph is 0.1 A, and the lowest concentration (25 ppb) had a measured absorbance at the peak of Again these measurements were rapid and easy, without the need for cuvettes, and without the need to open and close the sample compartment cover between measurements. Conclusion The dipping probe accessory is a new optional accessory available for the PerkinElmer line of LAMBDA UV/Vis and UV/Vis/NIR spectrophotometers. The accessory allows remote measurements outside of the sample compartment, and eliminates the need to use cuvettes. The high signal-to-noise of the LAMBDA 35 combined with the newest generation of very efficient silica monofibers, allows sample measurements that approach the precision of cuvette measurements. The ability to measure in any container, and in full room light, allows the purity of raw materials to be assayed rapidly. Figure 7. Four concentrations of Evan s Blue Dye (shown in Figure 3) measured in flasks with the Scanning Quant Application of UVWinLab 5.1 using the fiber optic dipping probe accessory. Figure 9. Standard curve for the Evan s Blue dye standards. Figure 8. Concentration results for Evan s Blue dye. Excellent photometric accuracy and linearity is achieved with the fiber optic dipping probe. Figure 10. The excellent signal-to-noise of the dipping probe accessory allows detection of benzene in water at very low concentrations measured in flasks. Note that the absorbance of these solutions is about 0.1 A or less. 4

5 PerkinElmer Life and Analytical Sciences 710 Bridgeport Avenue Shelton, CT USA Phone: (800) or (+1) For a complete listing of our global offices, visit /lasoffices 2004 PerkinElmer, Inc. All rights reserved. The PerkinElmer logo and design are registered trademarks of PerkinElmer, Inc. LAMBDA and UVWinLab are trademarks and PerkinElmer is a registered trademark of PerkinElmer, Inc. or its subsidiaries, in the United States and other countries. All other trademarks not owned by PerkinElmer, Inc. or its subsidiaries that are depicted herein are the property of their respective owners. PerkinElmer reserves the right to change this document at any time without notice and disclaims liability for editorial, pictorial or typographical errors _07 Printed in USA