The new PerkinElmer Lambda 650/850/950/1050 UV/Vis and UV/Vis/NIR spectrometers incorporate the latest technological advancement in scanning, termed “Step-Scan Integration”. This technology is enabled by a unique CSSC (Chopper Segment Signal Correction) high-speed chopper design having 4 segments. The ultimate goal of advancing scan technology is the elimination of photometric errors. When scanning a monochromator in a conventional time shared optical system, the wavelength changes during the measurement cycle and can lead to distortions in the spectral data. At each wavelength, a conventional double beam spectrometer records a Sample signal, a Reference signal, and a Dark signal. This is most commonly accomplished by use of a three segment chopper.
The problem with a conventional instrument design, is that the chopper spins continuously, collecting readings from the three chopper segments, while the monochromator is actually moving. The result of this is that the Sample (S), Reference (R) and Dark (D) readings are actually taken at three different wavelengths. This produces a derivative error as scan speed is increased, causing shifts in absorption peaks towards the blue, and a suppression of the true absorption. This effect is commonly known as “tracking error”. Below is a holmium oxide doped glass sample scanned with a conventional wavelength drive spectrophotometer illustrating the “tracking error” effect, where peaks positions will shift to the blue with increasing scan speed, and peak values decrease.
Step-Scan Technology
Step Scan Integration is a digital scan drive system that synchronizes the chopper and grating movement. This yields two benefits not available on conventional drive systems…
1) Elimination of “tracking error”, regardless of scan speed selected.
2) The exact integration time at each digital wavelength step can be controlled by the researcher.
The Lambda 650/850/950/1050 spectrometers use an advanced chopper design, called CSSC which is an ultra-high speed chopper (operating at a 20 millisecond measurement phase) utilizing four segments, a Sample, Reference, and two Dark portions. In the PerkinElmer CSSC chopper design, the chopper spins first to collect the Sample, Reference, and Dark signals, and then the gratings move on the next non-measurement spin of the chopper. This design is a significant improvement over the earlier “Dark Cycle Stepping” technology, in that the readings for all photometric components are recorded while the gratings are truly stationary.
The earlier “Dark Cycle Stepping” design would actually move the grating while integrating the Dark portion of the signal. Unfortunately, this design had the effect of compromising the accuracy at higher absorbance levels (low %T).
The unique PerkinElmer patented CSSC chopper design having two dark chopper segments provides the following additional benefits…
1) Real-time dark current corrections are taken 2X more frequently than a conventional chopper, yielding better accuracy for optically dense samples, with less signal fluctuation.
2) The ultra high-speed chopper allows the instrument to be operated with the cover open. Most conventional systems (slower chopper, less frequent dark current correction) utilize a cut-off switch on the sample compartment door to prevent detector damage. The Lambda 650/850/950 spectrometers are immune to damage from room light.
3) Digitally synchronizing the scan drive to the chopper allows the Sample/Dark and the Reference/Dark signals to be collected while the monochromators are stationary, completely eliminating tracking error.
Shown below are an overlaid holmium oxide scans acquired with a Lambda 950 with slow to fast scan speeds. The peak positions and absorbance values remain constant when moving from slow to high scan speeds. Digital Step Scan Technology eliminates “tracking error”.
