Digital Subtractive Method (DSM) CD
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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

OLIS 20 & 17: fewer than one
OLIS 1000: up to 62.5

Kinetic fitting methodology

Not available 

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’; others say ‘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

Integration time

Response time, usually given as 0.5 (or 1) msec to 15 (or 60) seconds

0.001 to 64,000 seconds per datum

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.