Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
1998-11-06
2001-05-22
Kim, Robert H. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S326000
Reexamination Certificate
active
06236457
ABSTRACT:
TECHNICAL FIELD
This invention relates to spectroscopic analysis of a sample for the purpose of determining the presence of elements of interest in that sample. The invention is particularly concerned with analysis of a sample to determine the presence of two or more elements of interest.
It will be convenient to hereinafter describe the invention with particular reference to atomic absorption (AA) spectroscopy, but it is to be understood that the invention has wider application.
BACKGROUND
In elemental analysis, it is common for an analyst to be interested in determining concentrations of several elements for each sample. Since the focus for the analyst is the sample, the optimum arrangement would be for the system to determine all results pertaining to each sample as the sample is presented. Notwithstanding this, most current multi-element AA instrumentation operates by analysing all samples in sequence for the first element, followed by analysis of all samples in sequence for the second element, and so on. This method of operation while well entrenched is of considerable inconvenience to an analyst.
For this reason, a simultaneous multi-element AA system, capable of simultaneously analysing for all elements of interest as a sample is presented, has long been a goal of instrument manufacturers. Unfortunately such a system has proven extremely difficult to achieve at reasonable cost.
Prior to the present invention it has not been practical to change quickly some of the instrument operating conditions from those appropriate to one element of interest to those appropriate to another element of interest. In particular, a significant period of time is required to:
change from one lamp to another,
change the fuel and oxidant gas flows (in flame atomisation spectrophotometers), and
change from one wavelength to another.
The long delays involved in making changes of the foregoing kind have been considered unacceptable because of the consequent reduction in overall throughput and the excessive use of each sample.
SUMMARY OF THE INVENTION
According to the invention there is provided spectroscopic apparatus for sequentially detecting the presence of a plurality of elements in a sample including, a plurality of lamps, each of which is for generating a beam of light for detecting the presence of a respective at least one predetermined element of a plurality of elements, a beam selector and operating means for the beam selector, and an analysis zone at which a sample to be analysed is presentable, wherein each of the plurality of lamps occupies a fixed position relative to the analysis zone, and wherein the beam selector is operable. by the operating means to direct a beam of light from any one of the plurality of lamps to the analysis zone, and further including a monochromator for separating the analysis beam of light into different wave lengths, and controllable drive means for the monochromator for driving the monochromator to a predetermined setting which corresponds to a peak setting for each wavelength of interest.
A method according to the invention involves the use of a plurality of lamps, each of which is appropriate for detecting the presence of a respective at least one of a plurality of elements of interest and has a fixed position relative to a zone at which a sample to be analysed is presented for analysis. The method includes the steps of energising each lamp, either sequentially or simultaneously, so as to generate a beam of light from each lamp, and operating a beam selector so that it receives a selected one of the beams and directs that beam to the analysis zone. The arrangement is such that each beam of light can be directed to the analysis zone in turn so that a sample at that zone can be sequentially analysed for the presence of two or more elements of interest before another sample is presented to the analysis zone. The method additionally includes predetermining peak settings of the monochromator for each wavelength of interest and storing those peak settings for quickly driving the monochromator to a peak setting for a measurement.
An apparatus and method as described above substantially reduces the period of time required to change from one lamp to another and, for embodiments of the invention which provide for the sequential analysis of a number of samples, provides a relatively convenient and inexpensive solution for undertaking sequential spectroscopic multi-element analysis of those samples. In conventional AA spectroscopy the monochromator is driven to a wavelength of interest and is then peaked at that wavelength. Peaking involves scanning the monochromator over a small wavelength range and noting the setting which yields the highest light throughput. The monochromator is then driven to that setting for the measurement. This is a time consuming operation and cannot be started until the lamp of interest is stably positioned in the optical beam. The time taken to peak as well as being long is also additive to the time taken to change the lamp and typically is repeated for each measurement. Repetitive peaking time of the foregoing kind is avoided by executing a peaking routine for each element at the start of the batch analysis and storing in a memory the exact drive motor position corresponding to each peak. For subsequent monochromator movements the motor can be driven to a predetermined position without further peaking being required. By way of example, the monochromator drive motor may be a stepping motor and the motor position corresponding to a peak setting of the monochromator may be stored in software as the motor step count. In the case of very long batch runs there is a possibility of monochromator drift and this may require periodic readjustment of the peak settings. Such occasional re-adjustment does not add substantially to the overall batch time, whereas re-peaking on every movement does add very substantially to the batch run time.
In a preferred arrangement the beam selector includes a mirror which is movable relative to each of the lamps and which can adopt any one of a number of positions, each of which enables the mirror to receive a beam of light from a respective one of the lamps and direct that beam to the analysis zone of the apparatus.
Adoption of the foregoing technique enables lamp selection time to be reduced to less than 1 second, whereas conventional systems typically require a change over time of more than 10 seconds.
According to one arrangement the lamps are arranged radially in a circle or an arc of a circle, and the beam selector is located at the center of the circle and is movable about the circle axis so as to adopt any one of a plurality of operative positions. At each of the operative positions the beam selector is operative to receive a beam of light from a respective one of the lamps and direct that beam to the analysis zone. Other arrangements are clearly possible. By way of example, the lamps could be arranged in an arc or a straight line so that each has a fixed positlon relative to the others and relative to the analysis zone, and the beam selector is movable relative to the array of lamps so as to adopt any one of a plurality of operative positions as previously mentioned. It will, of course, be realised that for lamp arrangements wherein the lamps do not have an equal length optical path to the analysis zone additional focussing means to the focussing means that is normally present will be required for ensuring a lamp source is correctly imaged at the analysis zone.
In a form of the apparatus which uses a flame for atomising a sample, the apparatus can include means for feeding a fuel gas and means for feeding an oxidant gas to the analysis zone, wherein the fuel and oxidant gas flows are controlled by high speed regulating valves, preferably oscillating metering valves. In this form of the apparatus, the on to off time ratio of each valve can be adjusted as required to suit a respective one of two or more elements of interest. Such an arrangement enables appropriate adjustment of a valve to be achieved in a sin
Allen Brian Lawrence
Hammer Michael Ron
Berkowitz Edward H.
Fishman Bella
Kim Robert H.
Stafira Michael P.
Varian Australia Pty. Ltd.
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