GB2066985A - Spectrophotometer with servo-positioned dispersive element - Google Patents

Abstract

A spectrophotometer 10 utilizes a multiple-wavelength electromagnetic wave source 12 whose waves are intercepted and dispersed into a spectrum by intercepting and directing means 14, 38 comprising a grating assembly 18 having a concave mirrored surface 20 which can be swung about an axis (47, Fig. 3), so that a selected centre band 30 of the spectrum passes through a specimen to be analysed, within a flow cell 24. The spectrophotometer includes a closed-loop servo positioning mechanism for swinging the grating assembly about the axis to select different bands of the spectrum for analysis. <IMAGE>

Description

SPECIFICATION Spectrophotometer The present invention relates to a novel spectrophotometer particularly useful for, but not limited to, analyzing soluble materials.

Spectrophotometry concerns itself with the measurement of the transmission or reflection of radiant energy by a body in comparison to g standard. Prior spectrophotometers generally fall into two types, namely the variable wavelength type and the fixed wavelength type.

Fixed-wavelength spectrophotometers have good sensitivity but loack the ability quickly to utilize a multiplicity of narrow spectral bands to analyze a sample. Many chemical compounds are transparent to certain bands of electromagnetic spectrum while being absorbent to other bands of the same. Existing variable-wavelength spectrophotometers possess the advantage of using many bands for detection, but they require too great a time period to accomplish this task.

A recent innovation in the art of spectrophotometry involves the use of white light being passed through the sample and dispersed into a spectrum. Each element of a linear array of detectors then resolves a narrow portion of the spectrum. The resultant data is processed and provides an accurate depiction of spectral absorbency of the sample being analyzed.

However, the linear array is quite expensive and requires passing a high intensity of white light through the sample. The brilliance of the white light tends adversely to affect the solution being analyzed, e.g. by photo-chemical reaction or the like.

Recent art also includes spectrophotometers which utilize oscillating mirrors or granting rapidly to scan the spectrum through the sample and detector. In such systems, the light transmission at a particular wavelength is derived by sampling the detector output at a precise time during the oscillation, i.e.: the wavelength measured is related to time. Due to the short sampling time at a particular wavelength the signal to noise ratio from such systems is too low for high performance liquid chromatography use. This system has poor wavelength accuracy and, therefore, exhibits poor reproducibility.

There is a need for a spectrophotometer which is able quickly to employ a plurality of wavelengths of the electromagnetic spectrum to analyze liquid or gas solutions.

It is therefore an object of the present invention to provide a spectrophotometer which enables selected bands of electromagnetic radiation to be utilized rapidly for simultaneous analyses of a sample.

It is another object of the present invention to provide a spectrophotometer which can be adapted to employ a point source of multiwavelength electomagnetic waves and transform the same into monochromatic electromagnetic radiation without the aid of lenses or slits.

It is another object of the present invention to provide a spectrophotometer with simplified geometry to minimize contamination of its optical components.

It is yet another object of the present invention to provide a spectrophotometer which may analyze by means of a plurality of bands of electromagnetic radiation and may be controlled by a microprocessor.

Still another object of the present invention is to provide a spectrophotometer which may utilize a concave grating for dispersion of the white light into a spectrum and isolate portions of the spectrum to analyze chemical components flowing in a liquid chromatographic system without interrupting the normal flow of the liquid components in that system.

Another object of the present invention to provide a spectrophotometer by which exposure of the sample to electromagnetic flux can be minimised and which will efficiently utilize an electromagnetic band for analysis.

Yet another object of the present invention is to provide a spectrophotometer which is relatively inexpensive to manufacture.

It is another object of the present invention to provide a spectrophotometer which may efficiently utilize the ultraviolet region of the electromagnetic spectrum for analysis of liquid chromatographic samples.

With these objects in view, the present invention provides a spectrophotometer comprising: (a) a multiple wavelength electromagetic wave source; (b) a means for intercepting waves from said source and dispersing said waves into a spectrum of said electromagnetic waves; (c) means for detecting a selected portion of said spectrum of said electromagetic waves; and (d) means for directing a selected portion of the spectrum of said electromagnetic waves to said detecting means, characterized by: (e) closed-loop servo positioning means for moving said directing means and selected portions of the spectrum of said electromagnetic waves simultaneously to said detecting means, said closed-loop servo positioning means including sensor means for discerning the position of said directing means and translating said position of said directing means into a signal; (f) position-setting means for generating a selected signal representing a desired position of said directing means; (g) comparator means for comparing as input signals, said sensor means signal and said position setting means signal, said comparator means producing an output error signal therefrom; and h) servo motor means for moving said directing means comprising means for transforming said error signal into a movementinducing signal, means for transmitting said movement-inducing signal, and means for imparting movement to said directing means in accordance with said transmitted movementinducing signal.

The invention further provides a method of analyzing a sample, using a spectrophotometer, comprising the steps of: (a) producing a multiple wavelength source; (b) intercepting waves from said source; (c) dispersing said waves into a spectrum of said waves; (d) simultaneously directing a selected portion of the spectrum to a sample and then to detecting means by closed-loop servo positioning means, characterized by the steps of: sensing the position of said directing means; translating said position of said directing means into a signal; generating a selected signal representing a desired position of said directing means; comparing said signal derived from sensing the position of said directing means and said selected signal representing a desired position of said directing means and producing an output error signal as a result of said comparing; transforming said error signal into a movement-inducing signal; transmitting said movement-inducing signal; and imparting movement to said directing means in accordance with said transmitted movement-inducing signal.

The present invention thus provides a novel and useful spectrophotometer which employs a multiple wavelength electromagnetic wave source in the visible or invisible spectrum and means for intercepting waves from the source.

Such intercepting means disperses waves from the source into a spectrum.

A portion of the spectrum passes through a gaseous or liquid sample to detection means.

The absorptivity of the sample is determined thereby.

The device of the present invention also comprises directing means to focus the selected portion of the spectrum produced by the intercepting means. The intercepting and directing means may take the form of a prism or grating. Likewise, the intercepting means may be in the form of an interference filter and the directing means may be a mirror prism lens and the like.

The directing means may be mounted on an axis and movable-with respect to the same.

Such movement may take place substantially about the axis. In the case where the intercepting and directing means are in the form of a grating, such a grating may be curved to provide an image of the source to the detecting means without additional optical elements such as a lens or curved mirror necessary with a flat linearly-ruled grating.

Closed-loop servo positioning means permits rapid, accurate selection of portions of the spectral radiation being projected through the analysis sample. Such close-loop servo positioning may include sensor means which discerns the position of the directing means with respect to its axis. The sensor means transforms the directing means position into a signal. A position-setting means may then generate a selected signal which represents the desired position of the directing means with respect to the axis. In this manner, the directing means may be very quickly shifted from one spectral band to another.

The sensor means may use a magnetic, electrical, photometric or other media to produce its output signal which is conveyed to the comparator means. For example, the sensor may include an exciter which produces a reference electrical field. An encoder may then receive the exciters electrical fields at a specified distance therefrom. A shield may be placed between the exciter and the encoder selectively to block the electrical field leaving the exciter. The intensity of the electrical field reaching the encoder is then proportional to the axial movement of the directing means. In one embodiment of the present invention, the shield may be linked to the intercepting means as well.

One embodiment provides an encoder having a surface with electricity-conductive and electrically-non-conductive alternating portions. The shields then correspondingly have a multiplicity of spaces interposed a multiplicity of solid portions blocking the electrical field produced by the exciter. Thus, a slight turning of the shield greatly alters the electrical field reaching the encoder. Also, the shield is capable of blocking the entire electrical field emanating from the exciter. The sensor means is relatively unaffected by movement of the shield and the directing means along the axis of the directing means, i.e. away from and towards the encoder.

As heretofore described, a prim or grating may serve as the intercepting and directing means. The directing means may include a grating supported by a pair of axially-spaced springs. Such springs may take the form of band springs fixed in juxtaposed relations'hip.

The close-loop servo positioning means may then embrace servo motor means for moving the directing means with respect to the axis.

The servo motor means may include a frame wrapped with a conductive material to form a conductive coil thereabout. A shaft may link the frame to the guide. Further, the shield of the servo positioning means may also be linked to the guide of the directing means such that the servo motor effectively positions the shield in the sensor.

Reference signal means may be incorporated into the encoder by providing thereupon a conductive portion unaffected by the shield.

In this manner compensation is provided for variations in the electric field originating from the exciter. A reference capacitor may be substituted to perform this function also.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a preferred embodiment of the spectrophotometer of the invention; Figure 2 is a block diagram depiciting the closed-loop servo positioning means of the spectrophotometer of Fig. 1; Figure 3 is a side elevational view, partially in section, of a portion of the spectrophotometer of Figs. 1 and 2; Figure 4 is a section taken along the line 4-4 of Fig. 3; Figure 5 is a view taken along the line 5-5 of Fig. 3; Figure 6 is a schematic diagram illustrating in more detail parts of the control circuitry of the embodiment of the invention of Figs. 1 to 5; and Figure 7 is a schematic diagram similar to Fig. 6 but illustrating another of an embodiment of the spectrophotometer of the invention.

The preferred embodiment of the spectrophotometer of the invention is indicated as a whole in the drawings by the reference character 10 and includes, as one of its components, a multiple-wavelength electromagnetic wave source 1 2 (Fig. 1). The source 1 2 may consist of any relatively-broad band lamp. For example, a deuterium lamp may be used to emphasize the ultraviolet region of the electromagnetic spectrum. Likewise, incandescent, xenon, phosphor-mercury vapour or other suitable broad band electromagnetic wave sources may be employed.

The spectrophotometer 10 also includes intercepting means 1 4 which receives waves emanating from the source 1 2 and disperses the same into a spectrum 16. The intercepting means may take the form of an interference filter, a prism, or a grating. In the form shown in Fig. 1, the intercepting means 1 4 comprises a concave grating assembly 1 8 having a concave mirrored surface 20 with closelyspaced grating lines thereupon. For example, a model 12H10 Holographic spectrographic grating, manufactured by J.Y. Optical, of Metuchen, New Jersey will serve this purpose.

Ideally, the source 1 2 is a point source of electromagnetic waves. In reality, the source 1 2 has a finite dimension and may be projected through a circular opening 22. A flow cell 24 has an opening, usually a circular opening. which may be about one millimeter in diameter, for admission of the electromagnetic waves from the intercepting means 14.

The spectrum 16, in the case of a grating, may be the first order type produced thereby.

The flow cell 24 will then accept a narrow band of spectrum 1 6 which will pass through material to be analyzed therewithin. Detection means 26 will record the absorbency of the material being analyzed within the flow cell 24.

In accordance with Beer's Law, the absorptive capacity of the material within the flow cell 24 is directly proportional to the concentration of the solute in a solution. In addition, Bouguer's law states that each layer of equal thickness of the absorbing medium absorbs an equal fraction of the radiant energy traversing it. The combined laws may be expressed as follows: Log,O P0/P = Log,O 1 /T = A = abc where P0 and P are the radiant power (flux) transmitted by a standard and by the unknown, respectively; T is the transmittance which is equal to P/PO; A is the absorbency; and a is the absorptivity, b the thickness, and C the concentration of solute.The absorptivity a depends on the particular wavelength of electromagnetic radiation being employed for the analysis. In other words, the fluid solutions being analyzed absorb different amounts of photons from different portions of the electric magnetic spectrum. Thus, it is important to test for absorbency at different selected portions of the spectrum 16, and to use selected portions of the spectrum 16, represented by centre wave lengths 28, 30 and 32, in as narrow a band width as possible. In this manner, the detection means 26 will record well-defined absorbency characteristics, which is extremely useful in the quantitative and qualitative analysis of a plurality of unknowns within the flow cell 24.

It should be noted that the detection means 26 may perform the function of referencing its transmitted beam 36 to a standard beam.

This dual beam technique is well known in the art and may be accomplished therewith.

The spectrophotometer 10 may also include directing means 38 to project the spectrum 1 6 and centre wave lengths 28, 30 and 32 towards the flow cell 24 and the detection means 26. Where the intercepting means 14 is a prism or interference filter, the directing means 38 may take the form of a lens, mirror, prism, or the like. As shown in Fig. 1, the frating assembly 1 8 serves the functions of both the intercepting means 14 and the directing means 38 without additional optical devices.

Turning now to Fig. 3, the intercepting and directing means 14 and 38, in the form of the grating assembly 1 8, of the preferred embodiment is shown therein. The grating assembly 1 8 includes a grating 40 which has grating lines 42 which are depicted in a greatlyenlarged portion 44. Surface 46 of the grating 40 is slightly concave and has a mirrored finish. The grating 40 moves about an axis 47 as will be more fully explained as the specification continues. The grating lines 42 may be present to the general order of four thousand to twelve thousand lines per centimetre. This, of course, will vary, depending upon the portion of the electromagnetic spectrum 1 6 being used for analysis.

The spectrophotometer 10 in one embodiment may include close-loop servo positioning means 48 to move the directing means 38, i.e.: the grating 40 of the grating assembly 1 8. Fig. 2 shows, in a block diagram form, the components of such closed-loop servo positioning means 48. Sensor means 50 discerns the orientation of the grating 40 about the axis 47 and translates the physical position of the grating 40 into an output signal 56 representing the same. Comparator means or error amplifier 58 compares the output signal 56 of the sensor means 50 and an output signal 60 of position setting means.

62. An error signal 64 activates a servo motor 66 which moves the grating 40 until the error signal 64 equals zero. Thus, the position setting means 62 determines the orientation of the grating 40 and the spectral band of the spectrum 1 6 which enters the flow cell 24 for spectrophotometric measurement.

As shown in Fig. 1, centre wavelength band 30 of the spectrum 1 6 passes through the flow cell 24. It has been found that the grating 40 of the grating assembly 18, turning about 25 of arc, will make available the first order spectrum 1 6 ranging from about 1 95 nanometres to 700 nanometres. As heretofore stated, different intercepting means 1 4 may be used to produce a spectrum 1 6 which includes different portions of the electromagnetic spectrum necessary for analysis of the sample within the flow cell 24. The band width of the centre wavelength band 30 actually passing through the flow cell 24 would be about 7 nanometres.Further, the position setting means 62 may include a microprocessor unit which serves rapidly to change the orientation of the grating 40 and the centre wavelength band 30 of the electromagnetic radiation passing through the flow cell 24. For example, the spectrophotometer 10 of the present invention may select and determine the absorbency at five distinct centre wavelengths of spectrum 1 6 within a second. Such a feature is critical where the components being analyzed in the flow cell 24 are moving at a fixed rate.

The sensor means 50 shown in Figs. 3 and 5 includes, in one of its embodiments, an exciter or sensor plate 68 producing a standard signal. As shown in the drawings, such a signal may be an electric field. An encoder or conductive element 70 receives the exciter's signals and transducer the same into the electrical signal 56 which serves as an input to the error amplifier 58 (Fig. 2).

A shield 74 mounted on a shaft 76 selectively blocks the signal from tho exciter 68 to produce an electrical signal of varying strength. Referring to Fig. 5, t'le encoder 70 includes a surface 78 with electrically-conductive portions or elements 80, 82, 84 and 86 which alternate with grounded electrically-conductive portions 88, 90, 92 and 94. The shield 74 is constructed to have a multiplicity of solid portions 96, 98, 100 and 102 each separated from one another by a space which does not block the electric field from the exciter 68. Thus, the solid portions 96, 98, 100 and 102 are capable of completely blocking the electric field emanating from the exciter 68. Likewise, a relatively-slight movement of the shield 74 will permit the encoder 70 to receive an electric field from the exciter 68.As may be apparent, the greater the exposure of the conductive portions 80, 82, 84 and 86 to the electric field originating in the exciter 68, the greater will be the strength of the signal 56. A conductive portion 104, which lies on the perimeter of the encoder 70, will be more fully explained as the specification continues.

Returning to Fig. 3, it will be apparent that the sensor means 50 is mounted on a plate 11 6 by the use of mounting screws 11 8 and 1 20 and bushings 1 22 and 1 24. A mounting screw 1 26 secures the shield 74 to a shaft 76. The grating 40 is held within a frame 1 28. The lower portion of the frame 1 28 is fixed to a block 1 30 by machine screws 1 32 and 1 34. The upper portion of the frame 1 28 is fixed to a block 1 36 by machine screws 1 38 and 140.The block 1 36 may be integrally connected to a shaft 142, but in the illustrated case is shown as united with the shaft 142 by means of a screw 144 allows the disconnection of the shaft 142 from the block 136.

The servo motor 66 may comprise a rigid frame 1 46 constructed, for example, of aluminium. Coils 148 are wrapped around the frame 146. A pole piece 1 50 is fitted within the frame 146 by screws 1 52 and 1 54.

Permanent magnets 1 56 and 1 58 interact with a movement inducing signal, i.e. the magnetic field produced and transmitted by the coil frame 146 according to the error signal 64. Plates 1 60 and 1 62 retain a cylindrical member 1 64 by means of threaded bolts 1 66 and 1 68 in conjunction with nirts 170 and 172. The plate 162 is fixed to a plate 1 74 by bolts 1 78 and 1 80. Coil mounting 1 82 integrally connects with a member 184 affixed to the frame 146. Thus, the shaft 142 and the frame 146 impart affixed to the frame 146. Thus, the shaft 142 and the frame 146 impart movement to the grating 40 and the shield 74. The grating 40 is supported by a pair of band springs 1 86 and 188 spaced along the axis 47.

Fig. 4 shows the method of fixing the band spring 1 86 which, in the embodiment shown, is the same for the band spring 1 88. The machine screws 1 38 and 140 fix the band spring 1 86 to the block 1 36 and the upper portion of the frame 1 28. Thus, the bock 1 36 and the upper portion of the frame 1 28 and movable in unison with the motion of the grating 40 and the frame 146. The band spring 1 86 is also fastened with screws 1 92 and 1 94 between a block 1 96 and the plate 174.Thus, the block 1 96 and the band spring 1 86 sandwiched between the block 1 96 and the plate 1 74 are immobile. The grating 40 of the grating assembly 1 8 turns with respect to the axis 47, although such turning deviates slightly from axial rotation. It has been found that such deviation does not affect the accuracy of the projection of the band 30 to the flow cell 24, since a relatively small turning arc is required to present the entire spectrum 1 6 to the flow cell 24. It should be noted that the lower portion of the grating 40 supported by the band spring 1 88 also has a fixed portion fixed by a block 198, the plate 116, and screws 200 and 202.The portion of band spring 1 88 held between the block 1 30 and the frame 1 28 is movable. The resultant support of the grating 40 of the grating assembly 1 8 has very little frictional resistance to turning. Thus, the grating 40 is quickly positioned at selected places within a very short time span.

The sensor means 50 (Fig. 2) may comprise the more specific embodiment shown by Fig.

6, in which a source of sinusoidal voltage, such as an oscillator 204, feeds into the exciter 68. This sinusoidal signal capacitively couples with the conductive surface 78 and the conductive portion 104 of the encoder 70. It should be noted that the conductive portion 104 lies outside the shadow of the shield 74. The amplitude of the electrical signals from the portion 104 is constant while the amplitude of the electrical signals of portions 80 of plate 78 is proportional to the position of the shield 74. Terminals 108 and 110 (Fig. 5) conduct the electrical signals from the plate 78 and the portion 104, respectively. Terminals 11 2 and 11 4 connect to ground.

The signals from the conductive portion 104 and the conductive surfaces of the plate 78 alternatively feed into A.C. amplifier 206 by activation of switches 208 and 210. Output signal 212 from the A.C. amplifier 206 is rectified and amplified by a detector 214. The output signal of the detector 214 is applied to switches 216 and 218 which operate in synchronism with the switches 208 and 210; the switches 208 and 21 6 turn on and off together, as do the switches 210 and 21 8. A frequency divider 220 provides complementary signals 22? and 224 which perform synchronous ol4eration of the switches 208, 210, 216 and 218. By this it is ensured that the magnitude of the signal at the sensor plate 68 controls the magnitude of the position signal 56.D.C. amplifiers 226, 228 and 230 amplify the signals coming from the detector 214, the switch 216, and the switch 21 8 respectively.

Fig. 6 also depicts how a reference signal 232 may be fed into the position setting means 62 to compensate for drift therein.

Such correction is well known in the art. An automatic gain control 234 functions to stabilize the reference signal 232 by adjusting the gain of the A,C. amplifier 206. As such, the need for the reference signal 232 is greatly diminished.

Turning to Fig. 7, yet another embodiment of sensor means 50 is shown which uses the same mechanical assembly of the Fig. 6 embodiment. However, these mechanical components are utilized electrically in a different manner.

For instance, conductive portion 80 of the encoder or conductive element 70 connects to a source of A.C. voltage via the terminal 108 (Fig. 5). This voltage originates from the action of electronic switch means 236 which rapidly alternately connects with a reference voltage 238 and ground 240. The resulting signal 242 travels to conductive portion 80 of the element 70. A second signal 250 is produced by switch means 252, similar to the switch means 236, except that the signal 250 is of opposite polarity to the signal 242. Thus, signal 56, the generation of which will be hereinafter described as the input to the switch means 252, is opposite in polarity to the signal 242. Switch control means 256 rapidly switches the switch means 236 and 252 between the ground 240 and their respective D.C. voltage sources to generate the signals 250 and 242.A signal 254 is produced by summation of the transmitted portions of the signals 242 and 250 on the plate 68. The transmitted portion of the signal 242 to the plate 68 is proportional to the angular position of the rotatable shield 74. The second signal 254 becomes the signal 56 after being amplified and rectified by an A.C. amplifier 244, a detector 246 and a D.C. amplifier 248. Thus, the magnitude of the signal 56 passing to the switch means 252 serves as a negative feedback or null signal and is proportional to the position of the shield 74, and, therefore, of the grating 40.

In operation, the user determines, by the position setting means 62, the proper position of the grating 40 of the grating assembly 1 8.

The error amplifier 58 will produce an error signal which will move the servo motor 66 by the closed-loop servo positioning means 48.

The frame 146 will turn the shaft 142 and orientate the grating 40 of the grating assembly 1 8 at the same time. The sensor 50 will produce its output signal 56 which it supplies to the error amplifier 58. The output signal 56 of the sensor 50 will match the output signal 60 of the position setting means 62 such that the error signal 64 from the error amplifier 58 will hold the servo motor 66 at the desired position. At this point, a selected centre wavelength band 30 of the spectrum 1 6 will pass through the flow cell 24 and can be detected and compared to a reference signal in the detection means 26. Position setting means may programme a series of selected centre wavelength bands 30 of the spectrum 1 6 within a short time period for the purpose of analysis

Claims (26)

1. A spectrophotometer comprising: (a) a multiple wavelength electromagnetic wave source; (b) means for intercepting waves from said source and dispersing said waves into a spectrum of said electromagnetic waves; (c) means for detecting a selected portion of said spectrum of said electromagnetic waves; and (d) means for directing a selected portion of the spectrum of said electromagnetic waves to said detecting means, characterised by: (e) closed-loop servo positioning means for moving said directing means and selected portions of the spectrum of said electromagnetic waves simultaneously to said detecting means, said closed-loop servo positioning means including sensor means for discerning the position of said directing means and translating said position of said directing means into a signal; (f) position-setting means for generating a selected signal representing a desired position of said directing means; (g) comparator means for comparing, as input signals, said sensor means signal and said position setting means signal, said comparator means producing an output error signal therefrom; and (h) servo motor means for moving said directing means comprising means for transforming said error signal into a movementinducing signal, means for transmitting said movement-inducing signal, and means for imparting movement to said directing means in accordance with said transmitted movementinducing signal.

2. A spectrophotometer as claimed in Claim 1 in which said directing means is movable with respect to an axis and said closed-loop servo positioning means moves said directing means with respect to the axis.

3. A spectrophotometer as claimed in Claim 1 or 2 in which said wave-intercepting means and said directing means comprises a grating.

4. A spectrophotometer as claimed in Claim 1, 2 or 3 in which said grating includes a concave-mirrored surfaced for intercepting and dispersing waves from said wave source and for directing said dispersed waves to said detecting means.

5. A spectrophotometer as claimed in any preceding claim which additionally includes a flow cell disposed between said directing means and said detecting means such that said detecting means receives said selected portion of said spectrum of said electromagnetic waves.

6. A spectrophotometer as claimed in any preceding claim in which said wave-interce,pt- ing means comprises a prism.

7. A spectrophotometer as claimed in any of Claims 1 to 5 in which said wave-internept- ing means comprises at least one filter.

8. A spectrophotometer as claimed in any preceding claim in which said directing means includes a grating supported by a pair of axially-spaced springs and in which said servo motor means moves said directing means with respect to said axis, said means for imparting movement to said directing means in accordance with said transmitted movement-inducing signal comprising a frame having a conductive coil and a shaft linked to said frame and said grating.

9. A spectrophotometer as claimed in claim 8 in which said sensor means discerns the orientation of said grating with respect to said axis and translates the position of said grating into a signal.

1 0. A spectrophotometer as claimed in any preceding claim in which said sensor means comprises: (a) an exciter producing a signal; (b) an encoder adapted for receiving said exciter signal and transducing the same into another signal; and (c) a shield adapted for selectively blocking said exciter signal received by said encoder in accordance with the movement of said directing means with respect to said axis.

11. A spectrophotometer as claimed in Claim 10 in which said shield is linked to said grating for movement therewith.

1 2. A spectrophotometer as claimed in Claim 10 or 11 in which said encoder includes a surface having alternating electricallyconductive and electrically-non-conductive portions and said shield includes a multiplicity of solid portions, said solid portions being generally geometrically coincident with said electrically-conductive portions of said encoder surface.

1 3. A spectrophotometer as claimed in Claim 10, 11 or 1 2 in which said encoder also comprises a conductive portion receiving said excitor signal, said encoder being unaffected by said blocking of said shield.

1 4. A spectrophotometer as claimed in any of Claims 1 to 9 in which said sensor means comprises: (a) an exciter producing a sinosoidal signal; (b) an encoder adapted for receiving said sinosodal signal and transducing the same into another signal; (c) a shield adapted for selectively block ing said exciter sinosoidal signal received by said encoder in accordance with the movement of said directing means with respect to said axis; (d) reference signal means for providing a reference signal received by said encoder from said exciter unblocked by said shield; and (e) switch means for synthronously transmitting said reference signal and said another signal selectively blocked by said shield.

1 5. A spectrophotometer as claimed in any preceding claim which said sensor means additionally comprises automatic gain control means for preventing drift in said position setting means.

1 6. A spectrophotometer as claimed in any of Claims 1 to 9 in which said sensor means comprises: (aj a conductive element producing a signal; (b) a shield adapted for selectively blocking said element signal in accordance with the movement of said directing means with respect to said axis; (c) a sensor plate adapted for receiving said element signal selectively blocked by said shield; and (d) null circuit means for producing a signal of opposite polarity to said element signal.

1 7. A method of analyzing a sample, using a spectrophotometer, comprising the steps of: (a) producuing a multiple wavelength source; (b) intercepting waves from said source; (c) dispersing said waves into a spectrum of said waves; (d) simultaneously directing a selected portion of the spectrum to a sample and then to detecting means by closed-loop servo positioning means, characterized by the steps of: sensing the position of said directing means; translating said position of said directing means to a signal; generating a selected signal representing a desired position of said directing means; comparing said signal derived from sensing the position of said directing means and said selected signal representing a desired position of said directing means and producing an output error signal as a result of said comparing; transforming said error signal into a movement-inducing signal; transmitting said movement-inducing signal; and imparting movement to said directing means in accordance with said transmitted movement-inducing signal.

18. A method of analyzing a sample as claimed in Claim 1 7 in which said step of dispersing said waves includes dispersing said waves into a spectrum with a grating.

1 9. A spectrophotometer comprising: (a) 3 multiple-wavelength electromagnetic wave source; (b) means for intercepting waves from said source and dispersing said waves into a spectrum of said electromagnetic waves; (c) means for detecting a selected portion of said spectrum of said electromagnetic waves; and (d) means for directing a selected portion of the spectrum of said electromagnetic waves to said detecting means characterized by: (e) closed-loop servo positioning means for moving said directing means and selected portions of the spectrum of said electromagnetic waves simultaneously to said detecting means; (f) said closed-loop servo positioning means including sensor means for discerning the position of said directing means and transforming said position of said directing means into a signal comprising: (g) an exciter producing a signal; (h) an encoder adapted for receiving said exciter signal and transducing the same into another signal; and (i) a shield adapted for selectively blocking at least a portion of said exciter signal received by said encoder in accordance with the movement of said directing means.

20. A spectrophotometer as claimed in Claim 1 9 in which said encoder includes a surface having alternating electrically-conductive and electrically-non-conductive portions ana said shield includes a multiplicity of solid portions, said solid portions being geometrically substantially coincident with said electrically-conductive portions of said encoder surface.

21. A spectrophotometer as claimed in Claim 1 9 or 20 in which said encoder also comprises a conductive portion receiving said exciter signal, said encoder being unaffected by said blocking of said shield.

22. A spectrophotometer as claimed in Claim 19, 20 or 21 in which said shield is linked to said directing means for movement therewith.

23. A spectrophotometer comprising: (a) a multiple-wavelength electromagnetic wave source; (b) means for intercepting waves from said source and dispersing said waves into a spectrum of said electromagnetic waves; and (c) means for detecting a selected portion of said spectrum of said electromagnetic waves; characterized by (d) directing means for directing a selected portion of the spectrum of said electromagnetic waves to said detecting means, said directing means being movable with respect to an axis and supported by a pair of axially spaced springs; and (e) closed-loop servo positioning means for moving said directing means and selected portions of the spectrum of said electromagnetic waves simultaneously to said detecting means.

24. A spectrophotometer as claimed in Claim 23 in which said encoder includes a surface having alternating electrically-conductive and electrically-non-conductive portions and said shield includes a multiplicity of solid portions, said solid portions being geometrically substantially coincident with said electrically-conductive portions of said encoder surface.

25. A spectrophotometer as claimed in Claim 23 or 24 in which said encoder also comprises a conductive portion receiving said exciter signal, said encoder being unaffected by said blocking of said shield.

26. A spectrophotometer substantially as hereinbefore described with reference to and as illustrated in Figs. 1 to 6 or Fig. 7 of the accompanying drawings.