Compositions – Light transmission modifying compositions – Ultraviolet
Reexamination Certificate
2000-06-22
2002-07-23
Tucker, Philip (Department: 1712)
Compositions
Light transmission modifying compositions
Ultraviolet
C252S584000, C359S350000, C359S361000, C356S051000, C356S234000
Reexamination Certificate
active
06423249
ABSTRACT:
FIELD OF INVENTION
The present invention relates to techniques and mechanisms for calibration of optical analyzing devices or systems and more particularly to rare-earth doped optical mediums to calibrate UV absorbance detectors, methods for making such optical mediums and methods for calibrating devices using such optical calibration mediums.
BACKGROUND OF THE INVENTION
Ultraviolet (UV) absorbance detectors or detection systems, such as monochromator based liquid/gas chromatographic detectors or spectrographs, typically are used in manufacturing facilities, hospitals and laboratories to analyze a sample to determine its chemical composition or make-up. The sample being analyzed can be an unknown material sample, e.g., a forensic analysis sample or a sample of a known material that is analyzed to verify its chemical composition, e.g. a sample of the raw material being used in a manufacturing process (e.g., pharmaceuticals). As such, these detectors or detection systems are calibrated by the manufacturer for delivery to the user and periodically thereafter to assure the detector/detection system is repeatedly and accurately sensing the spectral emissions representative of the material sample being analyzed. There are a number of techniques that can be used for field calibration of UV absorbance detectors. For purposes of the subject application, field calibration shall be understood to mean calibration of an instrument, detector or detection system at the end users location and not in a dedicated laboratory, manufacturing or testing facility, which generally is referred to as shop or lab testing.
One calibration technique involves the use of a light source, such as mercury pen-ray lamp, having a known spectral emission to calibrate the detector or detection system (i.e., calibration light source). Such calibration light sources provide for accuracy in wavelength calibration because of the generous range of their spectral features (e.g., emission peaks). For example, the range of spectral features for a mercury pen-ray lamp covers the region from 254 nm to 580 nm. Simply, a calibration light source has a number of well defined and known spectral peaks or valleys that can be easily and repeatedly identified by a detector.
Notwithstanding its advantages, this technique is inconvenient and time consuming particularly when used for field calibration. To calibrate a UV detector or detection system in the field, it is shutdown and then disassembled so the light source normally used for analysis (i.e., analysis light source) can be removed and the calibration light source installed in its place. In other words, the detector or detection system is re-configured with the calibration light source specifically for the purposes of its calibration.
After re-configurement is completed, the detector or detection system is turned on and the calibration light source is run for a sufficient period of time to stabilize the lamp's spectral emissions. For example, it is typically recommended that a mercury pen-ray lamp be on for about 30 minutes to 1 hour before starting any calibration actions.
Thereafter, the detector or detection system is operated to determine the spectral emissions of the light source in relation to the detector's/system's performance or operation. For example, each position of a rotating diffraction grating of a monochromator detector or detection system is related to the wavelength of light reaching the UV sensor. In this way, the end user can correlate each position of the diffraction grating to a specific wavelength and the related bandpass of radiation that would irradiate a sample for analysis.
When the above actions are completed, the technician shuts the detector or detection system off, removes the calibration light source and re-installs the analysis light source. The technician then turns the detector back on and allows it to equilibrate to a stable operating condition.
The analysis light source, e.g., a deuterium lamp, typically has a characteristic spectral feature (e.g., see FIG.
5
). After the unit has stabilized for purposes of spectral emissions, a quick test is typically run to see if the spectral characteristic of the analysis light source is seen where it is supposed to be. For example, the spectral emissions about and at a given rotational position of the rotating diffraction grating, corresponding to this wavelength characteristic, are evaluated to see if the position does corresponds to the wavelength of the analysis light source's characteristic.
It is not uncommon to see a technician take about 2-3 hours, and more if there are adjustments or problems, to perform the above described calibration testing process. Because of the testing process and the need for a calibration light kit, it is also not uncommon to see this type of calibration test done by the manufacturer's field representatives. As such, this technique does not allow “on-demand” tests by the end user to be performed easily or without undue complexity.
In a second technique, a holmium doped glass filter is selectively disposed between the analysis light source and a UV sensor of the system. In one configuration, a holmium doped glass optical filter is disposed between the light source and the entrance slit for the detector. In another configuration, the holmium doped glass optical filter is disposed between the detector's/system's sample cell and UV sensor. The holmium glass filter in conjunction with the analysis light source generates an emission spectrum with distinct spectral features that can be used for wavelength calibration of a spectrophotometer and some HPLC detectors. In contrast to the first calibration technique described above, the holmium glass filter based calibration technique can be incorporated into the design and function of the instrument so the user can make an “on-demand” type of test.
However, there are inherent shortcomings when using the holmium glass filter for UV instruments. Specifically, the holmium doped glass lacks far UV spectral features. Although holmium in a solution does exhibit spectral features in the range from about 240 nm to about 880 nm, as a practical matter holmium doped glass is only useful down to about 330 nm (e.g., see FIG.
6
). Spectral features below 345 nm are difficult, if not impossible, to resolve because of the transmission cutoff of the base glass doped with the holmium material.
Conventional methods of doping optical glass requires the melting of the base glass, adding the required dopants and letting the glass cool and solidify. The solidified glass is then further processed (e.g., machined/ polished) to obtain the finished part geometry. To overcome the poor UV transmission characteristic inherent in the base glass material described above, one could use a base glass such as quartz or fused silica. However, the extreme high temperatures required to melt quartz or fused silica, e.g. greater than about 1800° C., restricts the selection of suitable dopants. In particular, these high temperatures essentially preclude doping base glass with a rare-earth material because the end product will not exhibit the desired spectral characteristic(s).
The absence of a useful spectral feature in the far UV range means that algorithms must be used to extrapolate the wavelength scale of the instrument over the spectral region between 190 nm and 345 nm. This is the spectral wavelength region in which the vast majority of UV absorbance detectors are operated in.
In a third technique, the detection system is initially calibrated using a calibration light source lamp at the manufacturer's site or by a field service representative in the manner described above. The end user then periodically checks calibration by using spectral features inherent in the light source used for analysis. For example, in the case of deuterium lamp, one uses the 486 nm and 656 nm spectral lines (see FIG.
5
). Although this method is convenient and accurate for the spectral region close to and between these lines, its accuracy and repea
Michaelis Brian
Serio John
Tucker Philip
Waters Investments Limited
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