Digital Subtractive Method (DSM) CD
OLIS INDIRECT METHOD
OLIS DIRECT METHOD
Means of acquiring CD information
Analog: modulate polarization of one beam of light, isolate and amplify signal with lock-in amplifier, measure also DC level. Calculate, including ‘k,’ the calibration constant:
CD = k(IAC/IDC)
Digital: the absorption for each rotation of light is measured; the measurement for right circularly polarized light is directly subtracted from the measurement for left circular polarized light:
CD = abs(L) – abs(R)
Calibration against a standard
Required
Not Required
Lock-In Amplifier
Required
Not Used
Light Source
Xe, usually 150 or 250 watt water or air cooled
Xe, 150 watt water cooled
Spectral Range
Most quote 165-900 or 1100 nm
OLIS 20: 170-700 nm,
OLIS 17: 185-1700 nm,
OLIS 1000: 167-1700 nm,
depending on gratings for RSM
Mechanical Range
Most quote 165-900 or 1100 nm
OLIS 20: 170-700 nm,
OLIS 17: 185-1700 nm,
OLIS 1000: 0-1700 nm
Interrogation Method
Single PMT
Dual beam, constantly modulating left/right, right/left, with 2 beams 180° out of phase with each other (“phase coherent”)
Mode of Detection
Single PMT
Two PMTs, either UV/Vis optimized or red sensitive; option of photon counting for fluorescence and FDCD
Dispersive elements
Prisms; or prism and grating
OLIS 20: double grating
OLIS 17: prism and grating
OLIS 1000: two 50 mm2 gratings, select for span and resolution
Number of scans per second
Fewer than one
Global fits using Matheson’s Simplex and Matrix Exponentiation
Slew rate
Most quote fixed rates of up to 5000 nm/minute
OLIS 17 & 20: 40 nm/second
OLIS 1000: Moving the grating end to end takes 10 seconds. For a 400 line grating, this results in 240 nm/sec; for a 2400 line grating, 40 nm/sec
Wavelength accuracy
Most quote 180-300 nm ±0.2 nm
300-400 nm ± 0.5 nm
OLIS 20: ±0.25 nm below 300; ±0.5, 300-500 nm
OLIS 17: < 0.05 nm to 800 nm
OLIS 1000: 170-450 nm, < 0.125nm
Slits
Some state ‘variable’ while others state ‘manual’
OLIS 20 & 1000: Manual setting, 0.12 – 6 mm
OLIS 17: Automatic and continuously variable to provide constant bandpass.
Spectral bandpass
Some state ‘variable’ while others state ‘manual’
OLIS 20 & 1000: Fixed setting, based on chosen slit width and gratings, 0.1 to 20+ nm.
OLIS 17: Automatic and continuously variable
Modulator
18.5 or 50 kHz
50 kHz
Autoscale
Up to 3300 m°
Arbitrary
RMS Noise
Measured without sample, 1nm bandpass, 4 sec integration time: commonly quoted values, 0.04 m° at 185 nm, 0.003 m° at 500nm
Measured without sample, 3 nm bandpass, ~3 sec integration time: 0.07 m° at 180 nm 0.02 m° at 185-190 0.02 m° at 200 nm 0.01 m° at 220 and up
Baseline stability
Typically quoted per hour
<0.1 m° per day
Absorbance mode
Single beam
Dual beam
Upgradable to LD
Unknown
Yes
Upgradable to fluorescence
Unknown
Yes, including options of milliosecond emission scanning, fixed wavelength photon counting and others
Nitrogen consumption
Quoted from 55 to 22 l/m at startup, down to 3 l/m maintenance level in visible
Startup, 24 l/minl <190 nm, 15 l/m 190-250 nm, 7 l/m <250 nm, 4-6 l/m. Separate flow valves to lamp housing, monochromator, and sample compartment.
Footnotes
1) A dual beam system provides the correct answer, obviating the need for calibration against a standard.
2) The Olis DSM 1000 CD (based on the DeSa RSM monochromator) uses two matched 50 mm² gratings. These gratings can be any available gratings. That is, one might choose a pair blazed at 200 nm with 2400 lines/nm, or one might choose a pair blazed at 800 nm with 300 lines/nm. The choice of gratings determines current spectral range, spectral resolution, and bandpass. All Olis CDs use two matched photomultiplier tubes; these PMTs can be UV/Vis or red-sensitive, as the situation requires. For NIR work, InGaAs detectors are used.
3) There is no CD which can provide subsecond spectra other than those built around the DeSa monochromator.
4) Since all Olis systems can acquire spectral scans as a function of time, they are ideally suited for use in kinetic analyses. The 3D global fitting software provided with these systems is the fastest and most accurate and robust available in the world. This software uses the simplex method and matrix exponentiation to solve the rate equations describing the reaction being studied; these new (but in the literature) methods have no connection to outmoded methods such as Levenberg-Marquardt and numerical integration techniques. The Global approach can be applied also to situations wherein the spectral changes are induced by non-kinetic means, such as temperature and titration.
5) DSM 20 CD: Mechanical characteristics of its grating mount include 4 steps/nm; therefore, the 1200 line grating results in a spectral range of 170-700 nm and a resettability of 0.25 nm/step below 300 nm and 0.5 nm/step from 300-500 nm.
DSM 17 CD: Optimized for high resolution work and including a double prism grating design, the DSM 17 has an available range of 185-1700 nm resulting in a resettability of 0.05 nm/step to 800 nm and ~0.1 nm/step into the NIR.
DSM 1000 CD: Mechanical characteristics of our grating mount include 8 steps/nm; therefore, the 2400 line grating results in a spectral range of 170-450 nm and a resettability of 0.125 nm/step.
* Every CD spectrophotometer by every CD manufacturer is a single beam instrument, other than the three models by Olis, Inc. Thus, Them is every non-Olis CD.