Figure 1 Click to enlarge.

Figure 2 Click to enlarge.

Figure 3 Click to enlarge.

Figure 4 Click to enlarge.

Near IR with the DSM CD

The Olis® DSM CD 17 and 1000 support NIR detection to 1700 nm (optionally to 2500 nm) using large area (12.5 mm²) InGaAs detectors with unique and innovative preamplifier circuitry. An exclusive photoelastic modulator (PEM) made for Olis, Inc. provides the left and right circularly polarized to 2500 nm.

Unlike all other CD spectrophotometers, the Olis DSM 17 and 1000 CD spectrohotometers produce useful low noise CD results deep into the NIR. And, the Olis NIR-CDs still operate in the modern dual beam mode, with no lock-in amplifier or calibration required.

These 10 scans of 0.12 Molar nickel tartrate (Figure 1) were collected in a grand total of 34 minutes, 42 seconds using an integration time of 0.2 seconds per datum across the 1300-600 nm span.

The variable noise levels along this wide span are reminders of the sharp lines of the 150 watt xenon arc lamp we are using. The emission in the lamp output varies by about 30 fold over this range, with the sharpest peaks being in the regions with the least noise (circled). Double beam spectrophotometers, like all Olis CD instruments, deal with this high variance correctly, such that only the noise level is affected.

Single beam spectrophotometers, such as all non-Olis CDs, which must form the ratio of the CD signal and the DC signal, are considerably troubled by the sharp and dramatic intensity variations in the NIR. Or, to use the words of Mr. Castigliono, an internationally respected CD expert who publishes applications notes for Jasco Europe, "Xe lamps, while showing a continuous spectra in most of the UV-Vis range, over 750 nm have strong emission lines, which may create problems in obtaining proper CD signals (which is coming from AC/DC ratio).¹ The Olis DSM CD spectrophotometers—collecting CD not from an AC/DC ratio, but from a direct dual beam acquisition of ABS(L) and ABS(R)—deal with strong emission lines correctly and easily.

In your hands these ten scans might be averaged together with Olis software (Figure 2). The Olis software also supports applying global analysis (SVD, etc.) to remove the noise (Figure 3). And, if these ten scans were changing because the sample was undergoing a chemical or thermal reaction, you could use the Olis global fitting software to calculate rate constants, enthalpy values, transition wavelengths, and other equilibrium and kinetic values associated with changing spectra (Figure 4).

Changing the DSM CD Detectors for NIR

The detector panel for NIR detection is nearly identical to that for normal CD measurements.	First, the screws holding the panel in place are removed.
Then, the PMT cables are released by pressing the small lever on the side of each.	Remove the detector panel as shown. Replace with the new panel by reversing these steps.