All non-Olis CDs
CD = (IAC/IDC)
adjusted by calibration constant |
All Olis DSM CDs
CD = abs(L) - abs(R) |
Complex electronics find, isolate, and amplify CD information from a single, non-referenced beam of light.
Thus,the correct answer is attainable only if the electronic settings are correct and calibration is correct and the lamp is stable and the signal is <2 degrees. |
Powerful software collects raw data from two sources of abs(L) and abs(R) information. The correct answer is the only possible answer! |
Dates to 1960
"CD is proportional to (IAC/IDC)" |
Premiered 1994
"CD = abs(L) - abs(R)" |
| Indirect: Variable Dependent |
Direct: Absolutely Correct |
The spectrophotometer must be calibrated correctly, or the wrong answer is obtained. The operator must know the approximate CD answer in advance to set the electronics correctly, or the wrong answer is obtained. There is no real-time reference, so external influences cannot be isolated as such, making the wrong answer possible. The CD is calculated from decoupled and modified AC and DC signals, so all data available to the user is already influenced by electronic settings. Half of the light from the lamp is not used, so the device's sensitivity is less than its potential.
|
Calibration is not required. No sensitivity settings are used; response is linear from at least 0.5 to 5,000 m0. Raw data are acquired and used in the direct calculation of abs(L) - abs(R). Each beam is a reference for the other, so all non-CD information is identified as such and is kept from influencing the answer. All of the light from the lamp is used. No potential sensitivity is wasted.
|
CD = (IAC/IDC)
adjusted by calibration constant |
CD = abs(L) - abs(R) |
|
Old 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 required1 |
| 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 gratings2 for RSM |
| Mechanical Range |
Most quote 165-900 or 1100 nm |
Olis 20: 170-700 nm,
Olis 17: 185-2600 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 & resolution |
| Number of scans per second |
Less than 13 |
Olis 20 & 17: less than 1
Olis 1000: up to 62.5 |
Kinetic fitting methodology4 |
Not available |
Global fits using Matheson's Simplex and Matrix Exponentiation |
Slew rate |
Most quote fixed rates of up to 5000 nm/minute |
Olis 20 & 17: 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. |
Scan Rate |
Fixed |
Entirely variable, to 15,000 nm/second, with the speed being determined by the difficulty of the measurement. |
Wavelenth5 accuracy |
Most quote 180-300 nm,
±0.1 or 0.2 nm
300-400 nm,
±0.3 to 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, <0.125 nm |
Slits |
Some state 'variable'; 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 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 some limit ranging from 1000 to 3300 m° |
Arbitrary |
RMS noise |
Measured without sample, 1 nm bandpass, 4 sec integration time: commonly quoted values,
0.04 m° at 185 nm, 0.003 m° at 500 nm |
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 |
Absorption range |
Quote as 0-4 or 0-5 OD |
0-3 OD, without additional filters |
Absorbance mode |
Single beam |
Dual beam |
Upgradable to fluorescence |
Unknown |
Yes, including options of milliosecond emission scanning, fixed wavelength photon counting and others |
Upgradable to LD |
Unknown |
Yes |
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.
