Optical emission spectroscopic (OES) instrument with automatic top and bottom slit curtains

An optical emission spectroscopic (OES) instrument includes a spectrometer, a processor and an adjustable mask controlled by the processor. The adjustable mask defines a portion of an analytical gap imaged by the spectrometer. The instrument automatically adjusts the size and position of an opening in the mask, so the spectrometer images an optimal portion of plasma formed in the analytical gap, thereby improving signal and noise characteristics of the instrument, without requiring tedious and time-consuming manual adjustment of the mask during manufacture or use.

TECHNICAL FIELD

The present invention relates to optical emission spectroscopic (OES) instruments and, more particularly, to such instruments that automatically adjust top and/or bottom curtains of an optical mask for a spectrometer, in response to analyses of optical signals analyzed by the spectrometer.

BACKGROUND ART

Analyzing chemical compositions of samples is important in many contexts, including identifying and segregating metal types (particularly various alloys of iron and steel) in outdoor metal recycling facilities, quality control testing in factories and forensic work. Several analytical methods are available.

Optical emission spectroscopy (OES) is a mature, robust technology for the elemental analysis of materials. In OES, a small quantity of sample material is vaporized and excited above atomic ground state. Emissions characteristic of elements in the vaporized sample are captured by a light guide, which sends the light to a spectrometer, which produces and analyzes a spectrum from the light, so as to yield information about the elemental composition.

For electrically conductive samples, prevalent techniques for generating emission spectra use either an electric arc or a spark, or both, to vaporize a small quantity of the sample to be analyzed. An electrical potential in an analytical gap between a counterelectrode and a surface of the sample breaks down gas in the gap, enabling an electrical current, in the form of a spark or an arc or both, to flow between counterelectrode and the sample surface. Typically, the spark or arc vaporizes a portion of the sample and causes the vaporized sample to move into the analytical gap, and thereafter heats the gas in the gap, thereby exciting the vaporized sample material. In the resulting plasma, the excited sample (“analyte”) produces an optical (although possibly invisible) discharge that is characteristic of the elemental composition of the excited material.

Alternatively, laser-induced breakdown spectroscopy (LIBS) or glow discharge (GD) may be used to vaporize and excite an emission sample. A survey of OES analytical techniques may be found in K. Slickers, “Automatic Atomic-Emission Spectroscopy”, Second Edition (1993), which is incorporated by reference as if fully set forth herein for all purposes.

Regardless which excitation technique is used, an image of the excited sample is projected onto an entrance slit of a spectrometer, which analyzes composition of the sample, based on wavelengths and intensities of the optical signal. Emissions from the analyte should be sampled from a volume of the analytical gap where the analyte is ionized. Optical signals from other sources, such as the heated tip of the counterelectrode or the sample surface, could confuse the analysis and should not, therefore, be allowed to enter the spectrometer.

A mask and/or a suitably short slit may be used to exclude these unwanted emissions. However, masks and short slits limit the amount of optical signal received by the spectrometer, leading to poor signal-to-noise ratios.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides a method for automatically adjusting a field of view of an optical emission spectroscopic instrument. The instrument defines an analytical gap. A spectrometer in the instrument is configured to analyze an optical signal produced within the analytical gap and to generate an output signal representative of the analysis. An adjustable mask is disposed in a light path of the instrument. The mask adjustably defines a portion of the analytical gap that is imaged by the spectrometer. The mask is adjusted under control of a processor.

A constituent of a sample may be identified, under control of the processor, based on the output signal. The mask may be adjusted, based on the identified constituent. The constituent may be identified during a surface preparation phase.

A plurality of time-separated output signals may be analyzed, under control of the processor. The mask may be adjusted between each pair of successive output signal analyses. For example, the mask may be adjusted between each pair of successive output signal analyses, such that after adjusting the mask, the spectrometer images a different, but equal sized, portion of the analytical gap, or a different sized portion of the analytical gap.

The mask may be adjusted until a predetermined criterion is met, relative to the output signal. For example, the mask may be adjusted until a predetermined noise or signal-to-noise or signal level criterion is met, relative to the output signal.

The mask may be adjusted until a predetermined signal level criterion is met, and then the mask may be further adjusted until a predetermined signal-to-noise criterion is met.

The mask may include at least one curtain. Adjusting the mask may involve opening or closing the at least one curtain until a predetermined noise or signal level or signal-to-noise criterion is met, relative to the output signal.

The mask may include at least two curtains. Adjusting the mask may involve opening or closing one of the curtains until a first predetermined noise or signal level or signal-to-noise criterion is met, relative to the output signal, and then opening or closing another of the curtains until a second predetermined noise or signal level or signal-to-noise criterion is met, relative to the output signal.

The mask defines an opening. Adjusting the mask may involve adjusting the mask so as to translate the opening until a predetermined noise or signal level or signal-to-noise criterion is met, relative to the output signal. At least one of the curtains may at least partially define the opening. The at least one curtain may be opened or closed until a first predetermined noise or signal level or signal-to-noise criterion is met, relative to the output signal.

The mask may include at least two curtains at least partially defining the opening. Another of the curtains may be opened or closed until a second predetermined noise or signal level or signal-to-noise criterion is met, relative to the output signal.

Another embodiment of the present invention provides a self-adjusting optical emission spectroscopic instrument for analyzing composition of a portion of a sample. The instrument includes an exciter, a spectrometer, an adjustable mask and a processor. The exciter is capable of exciting the portion of the sample within an analytical gap. The excitation produces an optical signal. The spectrometer is disposed in the instrument to receive the optical signal. The spectrometer disperses the optical signal and produces an output signal from the dispersed optical signal. Then adjustable mask is also disposed in the instrument, along a path of the optical signal. The adjustable mask adjustably defines a portion of the analytical gap imaged by the spectrometer. The processor is coupled to the spectrometer and to the mask. The processor is programmed to process the output signal and to adjust the mask.

The processor may also be programmed to identify a constituent of a sample based on the output signal, as well as to adjust the mask based on the identified constituent. The processor may also be programmed to identify the constituent of the sample during a surface preparation phase.

The processor may also be programmed to analyze a plurality of time-separated output signals and to adjust the mask between each pair of successive output signal analyses. The processor may be programmed to adjust the mask between each pair of successive output signal analyses, such that the spectrometer images a different, but equal sized, portion of the analytical gap, or such that the spectrometer images a different sized portion of the analytical gap.

The processor may be programmed to adjust the mask until a predetermined criterion is met, relative to the output signal. For example, the processor may be programmed to adjust the mask until a predetermined noise or signal-to-noise or signal level criterion is met, relative to the output signal.

The processor may be programmed to adjust the mask until a predetermined signal level criterion is met, and then to adjust the mask until a predetermined signal-to-noise criterion is met.

The mask may include at least two curtains. The processor may be programmed to open or close one of the curtains until a first predetermined noise or signal level or signal-to-noise criterion is met, relative to the output signal. The processor may be further programmed to open or close another of the curtains until a second predetermined noise or signal level or signal-to-noise criterion is met, relative to the output signal.

The mask may define an opening. The processor may be programmed to adjust the mask so as to translate the opening until a predetermined noise or signal level or signal-to-noise criterion is met, relative to the output signal. The mask may include at least one curtain at least partially defining the opening. The processor may be programmed to open or close the at least one curtain until a first predetermined noise or signal level or signal-to-noise criterion is met, relative to the output signal.

The mask may include at least two curtains at least partially defining the opening. The processor may be programmed to open or close another of the curtains until a second predetermined noise or signal level or signal-to-noise criterion is met, relative to the output signal.

Yet another embodiment of the present invention provides a computer program product for use on a computer for automatically adjusting a field of view of an optical emission spectroscopic instrument. The instrument defines an analytical gap and including a spectrometer. The spectrometer is configured to analyze an optical signal produced within the analytical gap. The spectrometer generates an output signal representative of the analysis. The instrument further including an adjustable mask in a light path of the instrument. The mask adjustably defines a portion of the analytical gap imaged by the spectrometer. A tangible computer usable medium has computer readable program code stored thereon. When the program code is executed by the computer, the computer adjusts the mask.

The tangible computer usable medium may have additional computer readable program code stored thereon. When the program code is executed by the computer, the computer identifies a constituent of a sample based on the output signal. Based on the identified constituent, the computer adjusts the mask.

Optionally or alternatively, when the program code is executed by the computer, the computer analyzes a plurality of time-separated output signals. The computer adjusts the mask between each pair of successive output signal analyses.

The computer may adjust the mask until a predetermined criterion is met, relative to the output signal. For example, the computer may adjust the mask until a predetermined noise or signal-to-noise or signal level criterion is met, relative to the output signal. The computer may adjust the mask until a predetermined signal level criterion is met, and then adjust the mask until a predetermined signal-to-noise criterion is met.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In accordance with embodiments of the present invention, methods and apparatus are disclosed for automatically adjusting a field of view of an optical emission spectroscopic (OES) instrument. The instrument includes a spectrometer and an adjustable mask controlled by a processor. As a result, the instrument may automatically adjust the size and position of an opening in the mask, so the spectrometer images an optimal portion of plasma formed in an analytical gap, thereby improving signal or noise characteristics of the instrument, without requiring tedious and time-consuming mane al adjustment of the mask during manufacture or use.

As shown inFIG. 1, in arc/spark OES analysis, plasma100is formed in an analytical gap104between a counterelectrode108and a sample surface110. Light114(typically in a range of wavelengths from about 170 nm to about 450 nm, i.e. mostly in the ultraviolet spectrum) from the plasma100is analyzed by a spectrometer118and a processor120to determine elemental composition of the sample110. An image of the plasma100is typically projected onto a slit124of the spectrometer, such that the axis126of the plasma100is aligned with the long axis of the slit124. A dispersive element128(such as a grating or a prism) produces a wavelength-dispersed optical signal (spectrum)130, which is distributed across a plurality of detectors134. Each of the detectors134is positioned to receive a different, yet narrow, range of wavelengths of the spectrum130. The detectors134produce electrical signals136that are fed to the processor120. As noted, other techniques, such as laser-induced breakdown spectroscopy (LIBS) or glow discharge (GD), may be used to vaporize and excite the sample110.

Excited elements in the plasma100emit light at characteristic wavelengths and relative intensities.FIG. 2contains a graph representing a hypothetical emission spectrum projected onto the detectors134. Emissions characteristic of the sample110appear as relatively tall lines, exemplified by lines200, that indicate by their heights the amount of light detected at respective wavelengths. By identifying the wavelengths and relative heights of some or all of the lines200, etc., in the spectrum130, the processor120may ascertain the composition of the sample110, including identifying relative concentrations of various elemental constituents, thus identifying an alloy or other classification to which the sample110belongs.

Other emissions, such as from material eroded from the counterelectrode108and from environmental gases present in the analytical gap104, are typically present in relatively small quantities and, in a well-adjusted analytical instrument, contribute to a relatively low strength background signal (“noise”)204. Noise is also caused by recombination phenomena at the sample surface110. The ratio of the height of the lines200, etc., to the amplitude of the noise204is commonly referred to as a signal-to-noise ratio (“S/N”).

In a poorly adjusted instrument, the noise level300may be high, as shown inFIG. 3. The noise level300may be high enough to overwhelm some or all of the lines200, etc., thus precluding analysis of the sample, or at least reducing accuracy of the analysis.

Returning toFIG. 1, the temperature of the plasma100varies along its length. The portion of the plasma100close to the tip of the counterelectrode108is typically cooler (at about 1,000° K) than the portion of the plasma100close to the sample surface110(which may be about 10,000° K). Optimum analysis of the spectrum130generally requires a relatively high signal-to-noise ratio, which involves allowing as much light as possible from a desired portion138of the plasma100to reach the spectrometer118, while preventing optical signals from the ends of the plasma100, the counterelectrode108and the surface110from reaching the spectrometer118. This is often accomplished by disposing a mask along an optical path of the light114, such that an opening in the mask admits light from the desired portion138of the plasma100, and the mask blocks light from the unwanted portions of the plasma100. The mask may, for example, be disposed on or near the slit124to define effective top and bottom extents of the slit124.

The location and size of the desired portion138of the plasma100varies depending on several factors, including the size of the analytical gap104; the amount of electrical power introduced into the analytical gap; and the base material (such as iron, aluminum, zinc or titanium) of the sample110. Thus, the optimum size of the mask opening, as well as the optimum locations of the top and bottom portions of the mask, also vary with these factors.

In conventional OES instruments, the positions of the top and bottom portions of the mask, and therefore the size of the mask opening, are fixed, typically during manufacture of the instrument or thereafter when the instrument is serviced. However, such a fixed size and positions combination represents, at best, a compromise among several competing objectives. For example, the mask may be configured to facilitate analyzing both iron-based and aluminum-based samples, although such a mask configuration may not be optimum for either type of sample, inasmuch as a different mask configuration would yield a larger signal-to-noise ratio when analyzing an iron-based sample, and a yet different mask configuration would yield a larger signal-to-noise ratio when analyzing an aluminum-based sample. Furthermore, determining the mask configuration for a given OES instrument typically involves a labor-intensive process due, at least in part, to slight mechanical and other variations among OES instruments. Thus, fixed-size and fixed-position masks pose problems.

These and other problems associated with the prior art may be solved by automatically adjusting the mask under control of the processor120in response to the signals136received from the spectrometer118. These automatic adjustments may be made as part of a manufacturing or repair process and/or during normal use. For example, the mask may be adjusted until a predetermined criterion, such as a maximum signal-to-noise ratio, is met. An embodiment of an adjustable mask300is shown schematically inFIG. 4. The mask400includes two independently adjustable curtains402and404. The positions of the curtains402and404may be adjusted, as indicated by arrows408and410, to raise or lower edges414and418, respectively, of the curtains402and404, thereby adjusting the size and/or position of the opening420in the mask400. A dashed outline422indicates the outer extent of the light114, i.e., an image of the plasma100, that would otherwise be processed by the spectrometer118.

Collectively, the curtains402and404limit a field of view of the spectrometer118, i.e., an amount and portion of the analytical gap104that is imaged by the spectrometer118. “Imaged by the spectrometer” means impinging on the dispersive element128that produces the spectrum130that impinges on the detectors134. Thus, the curtains402and404limit the amount and portion of the plasma100that is (or would be) imaged by the spectrometer118. One curtain402limits the amount of the counterelectrode108side of the plasma100that can be imaged by the spectrometer118, and the other curtain404limits the amount of the sample110side of the plasma100that can be imaged by the spectrometer118. For example, lowering curtain402cuts off progressively more of the plasma100, beginning with the counterelectrode108end of the plasma100.

The curtains402and404may, but need not, be capable of opening slightly wider than necessary to image the entire analytical gap104. That is, the curtains402and404may be capable of opening wide enough to image a portion of the counterelectrode108or a portion of the sample110, respectively. This capability may be useful when, for example, the counterelectrode108becomes shorter through erosion or an operator inadvertently mis-orients the instrument so as to leave a gap between the instrument and the sample surface110. (Ideally, a snout of the instrument is typically brought into contact with the sample surface110, leaving no gap between the instrument and the sample surface110.)

It should be noted that the analytical gap104increases in size as the counterelectrode108erodes. The size of the analytical gap104also depends on the location of the sample surface110, relative to the tip of the counterelectrode108. As used herein, the term “analytical gap” means all or any portion of a region that might reasonably be expected to contain plasma when an instrument is in reasonable use, i.e., all or any portion of a region from approximately the tip of the counterelectrode (allowing for reasonable variations in counterelectrode location due to variability of installation of the counterelectrode, and allowing for reasonably expected wear of the counterelectrode) to approximately the sample surface (allowing for reasonable variation in positioning of the instrument, relative to the sample). “Analytical gap” does not necessarily mean the entire region that might contain plasma when an instrument is in reasonable use.

The curtains402and404may, but need not, be capable of completely closing the opening420. Furthermore, the maximum excursions of the curtains402and404may overlap. That is, the maximum downward excursion of the top curtain402may place the lower edge414of the top curtain402lower than the maximum upward excursion of the bottom curtain404would place its upper edge418. However, the curtains402and404may be operated such that, in use, the curtains never actually overlap each other. The curtains need not be coplanar.

The curtains402and404also may, but need not, be moved together. For example, both curtains402and404may be moved the same distance and in the same direction, thereby maintaining a constant size opening402between the edges414and418of the curtains. Such coordinated movement of the two curtains402and404essentially translates the opening420, thereby essentially scanning the opening402across a portion of the analytical gap104.

Collectively, the curtains402and404determine the amount and portion of the analytical gap104that may be imaged by the spectrometer118. Thus, the mask400adjustably defines a portion of the analytical gap104that may be imaged by the spectrometer118.

In some embodiments, the curtains402and404are operated by respective stepper motors424and428through respective rack-and-pinion gear couplings. Screw drives or other suitable couplings may be used. The motors424and428may be driven by respective motor drive circuits430and434under control of the processor120. In other embodiments, the curtains402and404are operated by any suitable mechanical, hydraulic, pneumatic, piezoelectric, electromagnetic or other actuators, microactuators or combinations thereof. In one embodiment, the analytical gap104is about 3-5 mm, and the curtains402and404and the actuators are configured to move the curtains402and404in increments of about 0.1 mm. Other (larger or smaller) analytical gaps and curtain increments may be used.

Optionally, position sensors, encoders or resolvers (not shown) may be disposed to detect the positions of the curtains402and404or the actuators and to provide signals representing these positions to the processor120. Thus, the curtains424and428may be controlled by the processor120in an open-loop fashion or as part of a servomechanism. The curtains402and404may be flat, as shown inFIG. 4, or the curtains402and404may be curved or another suitable shape, based on optical and mechanical considerations, such as fitting the curtains and actuators in an instrument without interfering with light paths. Optionally (not shown), only one of the curtains402or404may be operated by a first actuator, and the other curtain may be fixed, with respect to a moveable carriage to which the first actuator is attached. A second actuator may be coupled to the carriage to control its position.

The processor120may execute program code to process the signals136from the detectors134and, in response, cause one or both of the curtains402and404to move in order to meet a predetermined criterion or to facilitate analysis of the sample110. For example, the size and/or position of the opening420in the mask400may be adjusted to maximize the heights of the lines200, etc. (FIG. 2). This may be accomplished by enlarging the opening402to admit more light114, or by translating the opening402such that a more fruitful portion of the plasma100is imaged by the spectrometer118. In general, the mask400may be adjusted to maximize the detected strengths of the emissions that characterize the sample110. Optionally or alternatively, the size and/or position of the mask400may be adjusted to minimize the amplitude of the noise204or300(FIGS. 2 and 3), to maximize the signal-to-noise ratio, or to meet another predetermined criterion or several predetermined criteria or according to another algorithm or heuristic (collectively referred to herein as a “criterion”).

In general, unwanted emissions caused by the counterelectrode108and by the surface110result in a broad “hump” of noise300(FIG. 3), with the noise level being highest near the center of the wavelength range depicted, and the noise level being lower at longer and at shorter wavelengths, as shown. The general hump shape of the noise300profile may be used by the processor120to distinguish noisy spectral data from spectral data that is less noisy, as exemplified by the relatively flat noise profile204inFIG. 2. The cooler region of the plasma100may exhibit a more well-defined “hump” as a result of thermal recombination than the hotter region of the plasma100.

In some embodiments, the processor120analyzes the signals136from the detectors134and, based on results of the analysis, causes one or more of the curtains402and404to move a specified amount in a specified direction. For example, if analysis of the signals136indicates that the signal strength, i.e., the heights of the lines200, etc., (FIG. 2) is insufficient to meet the criterion, the processor120may cause one or both of the curtains402and404to open by a desired step amount (such as about 0.1 mm), i.e., to increase the size of the opening420, thereby admitting more of the light114to be processed by the spectrometer118. After the curtains402and/or404achieve their new positions, the processor120may analyze fresh signals136and again determine if the criterion is met. If the criterion is not met, the processor120repeats by further opening the curtains402and/or404(unless the curtains402and404can not be opened further) and analyzes fresh signals136. Thus, the mask400, the spectrometer118and the processor120operate as a feedback system, as indicated by arrow438.

The processor120may, but need not, directly or indirectly control operation of a power supply440that provides electrical power to the counterelectrode108to cause sparks and/or arcs between the counterelectrode108and the sample surface110. If the processor120directly or indirectly (such as through another processor) controls the power supply440, the processor may coordinate generating sparks/arcs with the above-described analysis of the signals136from the detectors134. For example, the processor120may cause one or more sparks/arcs to be generated each time fresh signals136are required, i.e., before or after each adjustment of the positions of the curtains402and404, until the criterion is met. If the polarity of the counterelectrode108and the sample surface110are reverses, such as to clean the counterelectrode108, the hotter and cooler ends of the plasma100exchange positions. Thus, the curtain positions may need to be adjusted correspondingly.

An arc/spark-based OES analysis sequence typically begins with about 200 to 500 sparks to prepare the surface110of the sample. These sparks bum off or evaporate surface contaminants, as well as melt and re-melt a portion of the surface110to blend the sample, thereby yielding a more representative elemental composition of the sample. After this surface preparation phase has been completed, additional sparks/arcs are generated to analyze the sample, as described above.

Conventionally, no analysis of the sample is performed during the surface preparation phase. However, in some embodiments, light114resulting from some or all of the surface preparation sparks may be analyzed to adjust the curtains402and404. In one embodiment, some or all of the surface preparation sparks are used to determine the base material, such as iron, aluminum, zinc or titanium, of the sample110. This determination need not necessarily identify other constituents or the alloy of the sample110. However, identifying the base material of the sample110enables the processor120to at least preliminarily adjust the mask400.

For example, because the portion of the plasma100close to the sample surface110is hotter than the portion of the plasma100close to the counterelectrode108, the mask400may be adjusted to image the portion of the analytical gap104that is of a preferred temperature, based on the base material of the sample110. Iron-based materials require higher temperatures than aluminum-based materials to emit comparable intensities of light114. Thus, if the base material is determined to be iron, the curtains402and404may be adjusted so the opening402images a hotter portion of the plasma100. However, if the base material is determined to be aluminum, the curtains402and404may be adjusted so the opening402images a cooler portion of the plasma100.

Similarly, hard line emissions emanate from the hotter portion of the plasma100. Conversely, soft line emissions emanate from the cooler portion of the plasma100. A given sample may include both hard line elements and soft line elements. After the surface preparation phase, while the sample110is being analyzed, the opening420may be translated to image different temperature portions of the plasma100, essentially sweeping through the available plasma temperatures, so as to generate the maximum amount of light114from each element in the sample110, although not necessarily generating the maximum amount of light114from all of the elements at the same time. This sweeping ability is one factor that obviates the need for a compromise mask size and position combination used in the prior art.

FIG. 6contains a flowchart depicting operations that may be performed, according to one embodiment. In general, the flowchart ofFIG. 6depicts a process for adjusting a mask until a predetermined criterion is met. At600, an adjustable mask is disposed in a light path, such that the mask adjustably defines a portion of an analytical gap imaged by a spectrometer. This operation may be performed when an OES instrument is manufactured or retrofitted with the adjustable mask. At602, one or more sparks and/or arcs are generated, thereby creating plasma. At604, light from the analytical gap is analyzed by the spectrometer. At608, if a predetermined criterion is met, control passes to610, where analysis of the sample continues. However, if the predetermined criterion is not met, control passes to614, where the mask is adjusted under control of a processor. Thereafter, control returns to600.

As indicated at618, the criterion may require: receipt of at least a minimum signal level; at least a minimum average signal level over a specific range of wavelengths; at least a minimum signal level for one or more specific wavelengths; at most a maximum noise level; at most a maximum average noise level; at most a maximum noise level at a specific wavelength or within a specific range of wavelengths; at least a minimum signal-to-noise (S/N) ratio; or at least a minimum S/N ratio at a specific wavelength or within a specific range of wavelengths. The c riterion may involve adjusting the mask to: maximize signal level; minimize noise; maximize S/N; or another criterion. For example, the bottom curtain may be opened progressively wider until the noise level (presumably from surface recombination) begins to rise or the noise reaches a predetermined value or the S/N ratio reaches a predetermined value. Optionally, after reaching such a point, the curtain may be closed by a small predetermined amount or until the noise level drops.

A criterion may involve varying a mask parameter, such as the location of the top edge of the bottom curtain, throughout a range of possible values (such as the entire range of possible curtain edge positions) and analyzing the output signals from the spectrometer after each or some of the possible curtain-edge locations, then setting the mask parameter according to which curtain position value provided the best signal level, the best noise level, the best S/N, etc. If a range of curtain edge positions provided equally good results, the mask parameter may be set to the middle of the range. As noted, the criterion may include a combination of criteria.

As indicated at620, the mask adjustment may include: enlarging the opening defined by the mask; reducing the size of the opening; raising or lowering one or the other of the curtains; translating the opening; or a combination thereof. The flowchart depicts a loop. Additional stopping criteria (not shown), such as reaching an adjustability limit of the mask or performing a predetermined maximum number of iterations, may be employed, as is well known in the art of computer programming.

FIG. 6depicts a process for adjusting a mask until a predetermined criterion is met. Thereafter, full analysis of the sample may commence at610. In another embodiment, as depicted in a flowchart inFIG. 7, the mask may be adjusted until one criterion is met, then the mask may be further adjusted until a second criterion is met, before full analysis commences. In particular, each curtain may be separately adjusted. For example, the top curtain may be progressively raised to a position where the admitted signal level is high, but optical signals from the counterelectrode's influence are blocked. Then, the bottom curtain may be progressively lowered to a position where the admitted signal level is high, but optical signals from the surface's influence remain blocked.

At700, an adjustable mask is disposed in a light path, such that the mask adjustably defines a portion of an analytical gap imaged by a spectrometer. At704, one or more sparks and/or arcs are generated, thereby creating plasma. At708, light from the analytical gap is analyzed by the spectrometer. At710, if a first predetermined criterion is met, control passes to714. However, if the first predetermined criterion is not met, control passes to718, where the mask is adjusted under control of a processor. Thereafter, control returns to704.

As noted at720, the first predetermined criterion may be similar to any of the criteria discussed with respect toFIG. 6. As noted at724, the mask may be adjusted by raising or lowering the curtain.

At714, one or more sparks and/or arcs are generated, thereby creating plasma. At728, light from the analytical gap is analyzed by the spectrometer. At730, if a second predetermined criterion is met, control passes to734, where full analysis of the sample may commence. However, if the second predetermined criterion is not met, control passes to738, where the mask is adjusted under control of a processor. Thereafter, control returns to714.

As noted at740, the second predetermined criterion may be similar to any of the criteria discussed with respect toFIG. 6. As noted at744, the mask may be adjusted by raising or lowering the other curtain. The flowchart depicts two loops. Additional stopping criteria (not shown), such as reaching an adjustability limit of a curtain or performing a predetermined maximum number of iterations, may be employed, as is well known in the art of computer programming.

It is possible that each of the two adjustments, i.e., operations (704,708,710and718) and operations (714,728,730and738), influences the other adjustment. Thus, if the second criterion is met at730, a check may be performed at732to determine if the first criterion is still met. If not, control may return to704. Optionally, as indicated at748, the two adjustments may be repeated a predetermined number of times or until a third criterion (such as a change smaller than a predetermined amount is made during a given iteration) is met.

In yet other embodiments, the mask may be adjusted in two phases, although both curtains may be adjusted during each phase. One such embodiment is described with reference to a flowchart inFIG. 8. At800, the mask is adjusted to meet a signal level criterion, such as maximizing signal level, and then at804, the mask is adjusted to meet a S/N criterion, such as maximizing S/N. Each operation,800and804, may be performed according to the description provided above, with respect toFIG. 6. After both criteria are met, full analysis of the sample may commence at808.

FIG. 9contains a flowchart depicting operations that may be performed, according to another embodiment. In general, the flowchart ofFIG. 9depicts adjusting a mask during a surface preparation phase. At900, an adjustable mask is disposed in a light path, such that the mask adjustably defines a portion of an analytical gap imaged by a spectrometer. At904, one or more surface preparation sparks and/or arcs are generated. At908, light from the analytical gap is analyzed by the spectrometer and a processor, and the processor thereby identifies a constituent (such as a base material) of a sample. At910, the mask is adjusted by the processor, based on the identified constituent. As indicated at914, the mask adjustment may include: enlarging the opening defined by the mask; reducing the size of the opening; raising or lowering one or the other of the curtains; translating the opening; or a combination thereof For example, for iron-based samples, the mask opening may be translated toward the hotter portion of the plasma, whereas for aluminum-based samples, the mask may be translated toward the cooler portion of the plasma.

FIG. 10contains a flowchart depicting operations that may be performed, according to yet another embodiment. In general, the flowchart ofFIG. 10depicts a process for scanning the analytical gap to image different temperature portions of a plasma, so as to generate the maximum amount of light from a number of different elements in the sample, where each element's emissions may peak at a different temperature. At1000, an adjustable mask is disposed in a light path, such that the mask adjustably defines a portion of an analytical gap imaged by a spectrometer. At1004, one or more sparks and/or arcs are generated, thereby creating the plasma. At1008, light from the analytical gap is analyzed by the spectrometer and a processor. At1010, if the analysis set is complete, control passes to1014, otherwise control passes to1018, where the mask is adjusted.

As indicated at1020, the mask adjustment may include translating the opening defined by the mask; enlarging the opening; reducing the size of the opening; raising or lowering one or the other of the curtains; or a combination thereof. An analysis set may include a number of spark/arc-spectral analysis operation pairs, i.e., operation1004followed by operation1008. For example, data from a number of spark/arc-spectral analysis operation pairs may be averaged together or otherwise statistically processed to improve signal-to-noise. As indicated at1018, the mask may be adjusted, such as by translating the opening to image a different temperature portion of the plasma, after each spark/arc-spectral analysis operation pair.

Optionally, operations1004and1008may be repeated a number of times before operation1010. In other words, operations1004and1008may be repeated a number of times while the mask remains unchanged, then the mask may be adjusted before operations1004and1008are again repeated a number of times.

The flowchart depicts a loop. Additional stopping criteria (not shown), such as reaching an adjustability limit of the mask or performing a predetermined maximum number of iterations, may be employed, as is well known in the art of computer programming.

The above-described adjustable mask and methods for automatically adjusting a field of view of an OES instrument may be used in bench-top and hand-holdable OES instruments.FIG. 11is a cut-away view perspective view of a self-adjusting, hand-holdable OES instrument1100for analyzing composition of a portion of a sample, according to one embodiment.

In operation, an electrically-conductive flat portion1102of the instrument1100is pressed against an electrically-conductive sample surface (not shown). An electrically insulated block1104defines a bore1106, in which the counterelectrode100is disposed. A spark from a counterelectrode100to the sample excites a portion of the sample, thereby producing an optical signal. The optical signal enters a port1108and may be reflected by one or more mirrors (one of which is visible at1110) into a spectrometer1114inside the instrument1100. A processor (not visible) is coupled to a set of detectors (not visible) in the spectrometer1114. The processor is programmed to process signals from the detectors.

The processor analyzes at least a portion of a spectrum produced by the spectrometer1114to identify and quantify elemental composition of the sample. The processor may displays results of the analysis on a touchscreen1118. A user initiates analysis by the instrument1100via a trigger switch1120. Additional pushbuttons1124enable the user to further interact with the processor. A (typically rechargeable) battery1128powers the instrument1100.

Aspects of the spectrometer1114, as well as integration of the adjustable mask into the instrument1100, are described below. Additional information about such an instrument is available in U.S. patent application Ser. No. 12/036,039, titled “Hand-Held, Self-Contained Optical Emission Spectroscopy (OES) Analyzer,” filed Feb. 22, 2008, which is incorporated by reference as if fully set forth herein for all purposes.

FIG. 12is a perspective view of the spectrometer1114with its cover removed. The block1104and bore1106ofFIG. 11are shown in phantom. An optical signal1200from a plasma within an analytical gap enters a port1204and is reflected by a first mirror (not visible, but indicated at1206) into a bore1208(shown in phantom) and exits a second port1210, thus following an optical path1212. A second mirror1214(shown removed for clarity) reflects the optical signal into a third port1218, and then the optical signal then travels through a second bore1220(shown in phantom) to an optical subassembly1224.

In some cases (an example of which is shown inFIG. 12), the spectrometer1114is cross-dispersed, although this aspect of the spectrometer is not germane to the present invention. The optical subassembly1224is also shown removed from the spectrometer1114and with its cover removed at1228. The optical signals1230pass through a prism1238. A dispersed optical signal1240impinges on a grating1244, and a cross-dispersed optical signal1248is projected on a plurality of detectors1250. In non-cross-dispersed cases, the prism1238may be omitted. The detectors1250are electrically coupled (not shown) to the processor (not shown).

In some cases, the first mirror1206is flat, and in other cases the mirror1206is powered. Similarly, the second mirror1214may be flat or powered. The mirrors1206and1214may be parabolic, toroidal or shaped according to another simple or compound curve. The two mirrors1206and1214need not have identical shapes. The choice of number of mirrors, mirror placement, mirror shape, mirror size and other parameters may be informed by overall optical, size, weight and other objectives of the spectrometer1114.

In the optical subsystem1228, the optical signal1230enters an adjustable mask1254having upper and lower curtains1256and1258, respectively. The adjustable mask1254should be located at a focal point along the optical path1208and1230or close to the emission origin. Depending on the parameters of the mirrors1206and1214, one or more focal points may lie along the optical path1208and1230. For example, instead of, or in addition to, a focal point where the adjustable mask1254is shown inFIG. 12, a focal point may lie at another point along the optical path1208and1230. Thus, the adjustable mask need not necessarily be located as shown inFIG. 12.

Furthermore, depending on the parameters of the mirrors1206and1214, an image produced at one of the focal points may be larger than an image produced at the other focal point(s). It may be advantageous to locate the adjustable mask1254at the focal point that produces the larger image, because the amount of the image passed or blocked by an adjustable mask so located can be controlled with more precision, without requiring more precise curtain positioning. For example, if a 4 mm tall image is produced at one focal point, and a 2 mm image is produced at another focal point, 0.1 mm increments in curtain positioning provide more precise control over the amount of the 4 mm tall image admitted by the mask than over the amount of the 2 mm tall image admitted by the mask.

FIG. 5is a block diagram of major components and subsystems of the test instrument1100ofFIG. 11. Instructions for a processor1300, as well as spectral feature prototypes, may be stored in a memory1302. Analytical results from samples may also be stored in the memory1302and may be displayed on the touchscreen1118and/or provided to an external device via a wired or wireless data port1304. In addition, the memory1302may store tables of compositions of known materials (such as alloys) for comparison to compositions of test samples, and results of this comparison may be displayed on the screen1118and/or provided via the port1304.

The processor1300controls a power supply1306to generate sparks/arcs, as needed. The processor1300receives output signals from detectors1308within the spectrometer1114. The processor controls motor drivers1310, which drive actuators1312and1314, which operate the upper and lower curtains, respectively.

Embodiments of the present invention provide advantages over the prior art. For example, use of an adjustable mask facilitates conserving electrical power. Power conservation is important in hand-holdable, battery-powered analytical instruments. Light output from the plasma is typically related to power input into the analytical gap. However, as noted, in conventional OES instruments, fixed masks are typically configured according to a compromise between several competing objects (such as being able to analyze iron-based materials and aluminum-based materials), and the masks are not optimized for either objective. Consequently, to generate sufficient light for a spectrometer to analyze, a conventional instrument must use more power than would be necessary if the mask were optimized. The present invention enables the mask to be optimized. Thus, for each base material, the mask may be adjusted, and a minimum amount of electrical power may be used, thereby extending battery life.

Designing and manufacturing hand-holdable OES instruments present greater challenges than those faced in relation to bench-top OES instruments. For example, counterelectrode-to-sample gaps are typically smaller (about 2 mm) in hand-holdable instruments than in bench-top instruments (about 3-5 mm). Thus, mask opening size and position are more critical in hand-holdable instruments. For example, a placement error of, say, 0.3 mm is about twice as significant in a hand-holdable instrument than the same error is in a bench-top instrument. The ability of a hand-holdable instrument equipped with an adjustable mask to image an automatically selected portion of an analytical gap enables the instrument to dynamically select an optimum mask opening size and position, thereby streamlining manufacturing and service of the instrument, because critical adjustments to the mask need not be made manually.

In addition, hand-holdable instruments are typically used in the field where samples are typically not prepared as well as for bench-top analysis and where they are sometimes not prepared at all. Field samples typically have more surface irregularities than polished bench-top samples. The flexibility to image a dynamically-selected portion of an analytical gap enables a hand-holdable instrument to compensate for variabilities in field samples.

Furthermore, counterelectrode-to-sample distances are likely to vary more when hand-holdable instruments are used than for bench-top instruments. The flexibility to image a dynamically-selected portion of an analytical gap enables hand-holdable instruments to accommodate these variations, as well as other variations that are likely to occur with both hand-holdable and bench-top instruments, such as: shortening of counterelectrodes over time due to wear; instrument-to-sample misalignment by operators; instrument aging; thermal expansion and contraction of input optics; and lack of rigid mechanical coupling between input optics and spectrometer.

A self-adjusting OES instrument has been described as including a processor controlled by instructions stored in a memory. The memory may be random access memory (RAM), read-only memory (ROM), flash memory or any other memory, or combination thereof, suitable for storing control software or other instructions and data. Some of the functions performed by the instrument have been described with reference to flowcharts and/or block diagrams. Those skilled in the art should readily appreciate that functions, operations, decisions, etc. of all or a portion of each block, or a combination of blocks, of the flowcharts or block diagrams may be implemented as computer program instructions, software, hardware, firmware or combinations thereof. Those skilled in the art should also readily appreciate that instructions or programs defining the functions of the present invention may be delivered to a processor in many forms, including, but not limited to, information permanently stored on non-writable storage media (e.g. read-only memory devices within a computer, such as ROM, or devices readable by a computer I/O attachment, such as CD-ROM or DVD disks), information alterably stored on writable storage media (e.g. floppy disks, removable flash memory and hard drives) or information conveyed to a computer through communication media, including wired or wireless computer networks. In addition, while the invention may be embodied in software, the functions necessary to implement the invention may alternatively be embodied in part or in whole using firmware and/or hardware components, such as combinatorial logic, Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs) or other hardware or some combination of hardware, software and/or firmware components.

These embodiments are discussed in the context of analytical techniques and test instruments that employ OES; however, the teachings of this application are applicable to other types of analytical test instruments and techniques that employ spectral analysis, including test instruments that employ optical absorption spectroscopy. Furthermore, although the disclosed embodiments are discussed in the context of arc/spark excitation, other forms of excitation, including laser-induced breakdown (LIB) and glow discharge (GD) may be used. In addition, the adjustable mask for a spectrometer and the methods described above may be used in other contexts, such as terrestrial or extraterrestrial astronomy, including in combination with or within telescopes and satellites.

While the invention is described through the above-described exemplary embodiments, it will be understood by those of ordinary skill in the art that modifications to, and variations of, the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. For example, although some aspects of a self-adjusting OES instrument and methods for automatically adjusting a field of view of an OES instrument have been described with reference to flowcharts, those skilled in the art should readily appreciate that functions, operations, decisions, etc. of all or a portion of each block, or a combination of blocks, of the flowcharts may be combined, separated into separate operations or performed in other orders. Furthermore, disclosed aspects, or portions of these aspects, may be combined in ways not listed above. Accordingly, the invention should not be viewed as limited by the specific embodiments described herein.