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

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Introduction

The availability and applications of fiber optic sampling in UV/Vis and UV/Vis/NIR spectroscopy is expanding rapidly. Fiber optics provide a remote sampling capa-bility where sample requirements preclude use of cuvettes, or where remote sampling is required, i.e. on-line applications. Fiber optics accessories can be added to the cur-rent line of the LAMBDA™ spec-trophotometers and are available in different lengths and wave-length ranges. All fiber optic ac-cessories require an optrode or

measurement head, as 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

The fiber optic dipping probe (Figure 1) is a recent introduc-tion to the PerkinElmer LAMBDA range of UV/Vis instruments and accessories, providing remote sampling capabilities. This probe

Fiber Optic Sampling

by UV/Vis and UV/Vis/NIR

Spectroscopy

APPLICA

TION NOTE

UV/VIS AND UV/VIS/NIR SPECTROSCOPY

includes a unique design allow-ing a sallow-ingle 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 probe can be used for transmission or absorption studies in many liquids, includ-ing ones with suspended solids. Measurements can be taken over the UV/Vis wavelength range, from 200 nm to 1100 nm.

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three overlaid corrected baselines of distilled water acquired with the dipping probe accessory, scanned at 960 nm/min, with a 2 nm bandpass. Typical noise data of the dipping probe and LAMBDA 35 system is shown in Table 2. The overall low noise over the entire wave-length range allows quality spectra with a high signal-to-noise ratio to be collected.

The quality of fiber optic sampling systems has improved dramatically over the last decade. Early acces-sories offered for UV/Vis spectro-photometers were often plagued by low signal-to-noise and poor Fiber optic dipping probe features:

• Durable stainless steel design • Multiple pathlengths with

replaceable threaded tips • High throughput

• 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 path-lengths to 50 mm. The 10 mm pathlength tip was used for all data acquired for this application note. 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 com-partment, 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 concentration 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.

Performance

The LAMBDA double beam series of spectrophotometers all have state-of-the-art performance spe-cifications. Models include the LAMBDA 25/35/45 and the new re-search grade LAMBDA 650/850/950. 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 meas-urement should be as close as pos-sible to where the background correction was acquired. Shown are

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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.

Table 1. 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 (200-1100 nm)

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repeatability. Often these early design accessories were evaluated and re-jected by laboratories. The dipping probe accessory uses the latest gen-eration silica monofibers, providing for high energy throughput. This allows high precision of sample readings, approaching the repeat-ability 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 meas-ured in a flask and the fiber optic probe was removed and reinserted between scans as in laboratory use. 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 426.3 nm was 0.43596 with a standard deviation of ± 0.0012.

Experimental

The dipping probe accessory pro-vides for easy and rapid measure-ments of solutions in all types of containers.

Concentration assays are especially easy to set up 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. This application allows standards to be scanned; 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 ac-quired. 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 coeffi-cient of 0.999725. Because no cu-vettes were needed, assay time was very short.

The excellent signal-to-noise of the dipping probe accessory fitted with a LAMBDA 35 allows very low concentration of analyte to be meas-ured with precision. Low concen-trations of benzene in water (25, 50, 100, and 150 ppb) were prepared in flasks and measured with the dip-ping 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

Table 2. Typical peak-to-peak and

RMS noise of the LAMBDA 35 with dipping probe. Measured with a 4 nm slit at 0 Abs.

Wavelength

250 500 750

P/P 0.00069 0.00056 0.00043

RMS 0.00013 0.00012 0.00007

Figure 4. Monofiber transfer optic stage

mounted in the sample compartment of the

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 ±0.0001 A.

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.

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w w w. p e r k i n e l m e r. c o m Conclusion

The dipping probe accessory is a new optional accessory available for the PerkinElmer line of LAMBDA UV/Vis and UV/Vis/NIR spectro-photometers. 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 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 0.040 A. 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.

of very efficient silica monofibers, allows sample measurements that approach the precision of cuvette measurements. The ability to meas-ure in any container, and in full room light, allows the purity of raw materials to be assayed rapidly.

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.

Figure 9. Standard curve for the Evan’s Blue dye standards. 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 8. Concentration results for Evan’s Blue dye. Excellent photometric

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