Measuring > 8A using the PerkinElmer Lambda 1050 UV/Vis/NIR Spectrometer
Introduction
The PerkinElmer Lambda 1050 is a state of the art research UV/Vis/NIR spectrometer utilizing 3D (three detector) technology. Because the Lambda 1050 incorporates holographic gratings in a true Littrow double monochromator design, the stray radiation level is specified at < 0.00007 %T in the UV/Vis range. Because the amount of stray radiation limits the dynamic range of a spectrometer, the ultra-low stray light level of the Lambda 1050 allows the instrument dynamic range to be specified to 8A. High absorbance measurements to 8A require reference beam attenuation, a process where neutral density screens are inserted in the reference beam path to help balance the sample and reference beam energies when high absorbance samples are being measured. Reference beam attenuation will improve the signal-to-noise levels when high absorbance samples are measured. The capability to measure to 8A is incorporated in the Lambda 1050 by the inclusion of automated sample/reference beam attenuator screens. The neutral density screens included allow attenuation levels of 1%T (2A) and 0.1 %T (3A) to be programmed in the method. When required, the screens will be automatically inserted in the sample or reference beam paths, or both. The 1%T attenuator works well with sample up to 5-6A, and the 0.1%T attenuator works well when sample absorbance reaches 6-8A range. Though the Lambda 1050 is specified to 8A, valid data can be acquired in the 8-10A range as long as certain procedures are followed. The instrument’s parameters need to be optimized for wide bandpasses, slow scan speeds (high integration times), enable dark current (0%T) correction, and the use of supplemental reference beam attenuation. Discussed in this application note are the proper procedures for acquiring absorbance data >8A. A bandpass filter with out of band blocking specified at grater that 8A will be used as an example. A PerkinElmer Lambda 1050 (S/N 1050L1109146) was used for measurements.
Experimental
MicroCoatings™ 337 nm bandpass filter was used an as example. This filter is pictured in Figure 1.
This filter is specified with an out of band blocking greater than 8A. The filter is a sandwich type, with one side mirrored, the other side a glass surface. For measurements, the glass side surface was inserted to face the beam. A standard solid sample holder (PE # B0080822) was used to hold the filter properly in the beam.
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| Figure 1. MicroCoatings 337 nm Bandpass Filter |
A reference beam attenuator kit (PE Part Number L1160560), was used to provide supplemental reference beam attenuation (Figure 2). This kit consists of 5 circular magnetic ringed screens having different attenuation levels (32, 14, 6, 4, and 1%T) which can be adhered to the reference beam magnetic window. The attenuators in this kit provide a uniformly flat response as a function of wavelength. If needed, multiple reference beam attenuators can be stacked to produce a specific level of attenuation in the reference beam.
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| Figure 2. Supplemental Reference Attenuation Screen Kit (PE # L1160560) |
To construct a method to properly scan above 8A, the use of wide slits (bandpasses), slow scan speeds-high integration times, enabling dark current correction (0%T correction), and use of reference beam attenuation are all required. Because the Lambda 1050 incorporates motorized screen attenuators in sample and reference beams, a unique software feature can be used where the attenuation level is selected on the reference side, and the sample side attenuator set to “Automatic”. When the method is executed, this enables a three step measurement of the internal attenuators 1) Sample side, 2) Reference side, and 3) Attenuators in both sample and reference. This type of correction solves one of the problems of only using reference beam attenuation alone. By just adding screens to the reference beam path, the signal-to-noise at lower absorbance levels will be significantly degraded to obtain the benefit of better S/N at higher absorbance. By correcting with attenuators in both sample and reference, and then correcting for the contributions of the attenuators, the signal-to-noise will be enhanced not only at high absorbance but at low absorbance as well.
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Figure 3. Scan parameter selections influencing high absorbance scanning include data interval (1), scan speed (2), response time (3), slit (4), and attenuator settings (5),
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Under the Corrections menu in UVWinlab V6, is a check box that allows the 0%T dark current correction to be done. Correction at 0%T allows better accuracy when the sample absorbance exceeds 8A. In this example the MicroCoatings filter was scanned from 750 to 350 nm, using a 5 nm data interval, a 5 second response time, a 5 nm slit, and a reference attenuation setting of 0.1% and the front attenuation setting of Automatic. With the goal of verifying out of band blocking greater that 8A, it is shown in Figure 4 that that was accomplished without the use of supplemental reference beam attenuation. Because no part of the scan in the 670 to 400 nm range dipped below 8A, the conclusion that this filter meets is stated requirements can be proven.
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Figure 4. Scan of MicroCoatings 337 nm bandpass filter for verification of out of band blocking performance. The filter does exceed 8A in the range of 670 to 400 nm.
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To help reduce the noise above 8A, additional reference beam attenuation was applied by adding the 1%T screen from the attenuator screen kit, affixing this to the reference beam magnetic window. Additional reference attenuation often will reduce the noise seen above 8A. The spectrum shown in Figure 5 is the same MicroCoatings filter scan with an additional 1%T screen added to the reference beam. Note that the spectra of the screens from the attenuator kit were scanned earlier to allow the absorbance from the selected screen to be easily added to the sample spectrum (reference attenuation will reduce the absorbance by the amount of the screen – to derive the actual absorbance the screen absorbance is added to the sample absorbance spectrum).
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Figure 5. The same filter shown in Figure 4 is rescanned with an additional 1.8A (1%T) attenuator screen inserted in the reference beam. The additional reference attenuation has the benefit of improving the signal to noise at high absorbance. In this example, not only can it be shown that the filter exceeds the stated performance of >8A, but it actually exceeds 9A!
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Figure 6. The mean absorbance of this filter calculated from 660 to 400 nm, was determined to be 9.4613 A.
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Conclusion
The Lambda 1050 is a state-of-the-art double monochromator research UV/Vis/NIR spectrometer with ultra-low stray radiation levels In this system the use of holographic gratings used in a double monochromator Littrow design allows the dynamic range to be specified to 8A. In instances where the absorbance levels exceed 8A, valid data can still be obtained when the instrument parameters are properly configured. In the examples presented here, verification of > 8A out of band blocking can be achieved using the standard automated attenuators. To achieve better signal-to-noise when the sample absorbance start to approach 10A, additional attenuators can be used in the reference beam path, allowing sample absorbance measurements to exceed 9A
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