CA1213446A - Atomic absorption spectrophotometer providing simply derived background absorbance measurement - Google Patents
Atomic absorption spectrophotometer providing simply derived background absorbance measurementInfo
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- CA1213446A CA1213446A CA000492534A CA492534A CA1213446A CA 1213446 A CA1213446 A CA 1213446A CA 000492534 A CA000492534 A CA 000492534A CA 492534 A CA492534 A CA 492534A CA 1213446 A CA1213446 A CA 1213446A
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- background
- radiation
- absorption
- responsive
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Abstract
ABSTRACT OF THE DISCLOSURE
The invention relates to apparatus for utilizing the Zeeman (or Stark) effect for achieving background correc-tion in an atomic absorption spectrophotometer using an electromagnetic optical effect for background correction and causing the spectrophotometer to provide time varying radiation.
A radiation responsive device produces an output signal which varies in time and at certain times is proportional to the background absorption, and the radiation responsive device is responsive to a linearly increasing supply signal to produce an exponentially increasing output signal upon receiving a fixed radiation level. The specific improvement provided by this invention is a device for supplying the background signal so as to control the supply signal to the radiation responsive device, whereby the supply signal is pro-portional to the log of the background absorption, and the supply signal being used as a measure of the background absorbance.
The invention relates to apparatus for utilizing the Zeeman (or Stark) effect for achieving background correc-tion in an atomic absorption spectrophotometer using an electromagnetic optical effect for background correction and causing the spectrophotometer to provide time varying radiation.
A radiation responsive device produces an output signal which varies in time and at certain times is proportional to the background absorption, and the radiation responsive device is responsive to a linearly increasing supply signal to produce an exponentially increasing output signal upon receiving a fixed radiation level. The specific improvement provided by this invention is a device for supplying the background signal so as to control the supply signal to the radiation responsive device, whereby the supply signal is pro-portional to the log of the background absorption, and the supply signal being used as a measure of the background absorbance.
Description
AYE
TECHNICAL FIELD
This invention relates to apparatus or utilizirlg the Zeeman (or Stark) effect for achieving background correction in atomic absorption spectrophotometers (AS); moxie paretic-ularly, it relates to a unique combination of features which provide several times greater insensitivity to background absorption than heretofore attained in the prior art and enables the correction system to take the form o an accessory applicable to existing atomic absorption spectroph~tometers.
Jo ~;~139~6 BACKGROUND ART
Atomic absorption spectrophotometry is utilized to measure the concentration of a particular element in a sample.
For example: if one wishes to determine the concentration of copper in a sample, a light source producing one of the characteristic spectral lines of copper is utilized in the spectrophotometer. These sources are most often hollow cathode lamps, the cathode comprising the element to be determined, Liz., copper in this case. Electrode less discharge lamps containing a vaporizable charge of the analyze element are also used.
A monochromator customarily utilizing a diffraction grating disperses the light from the hollow cathode into a spectrum and the monochromator is adjusted so that the line of interest falls upon a detector, usually a photo multiplier tube. The amount of light falling on the photo tube is measured as a reference.
A sample of material in which one wishes to determine the amount of copper is then introduced into the path of the light from the line source to the monochromator. The sample must be 20` dissociated so that the copper atoms are free and not a part of a molecular compound in which case they would not provide their characteristic spectrum. This may be accomplished in an absorption furnace ~electrothermic sample atomizer). When the copper atoms are introduced into the light path they absorb light at the same characteristic spectral lines at which the copper atoms in the light source emit light. Thus, at the line of interest, light will be absorbed and less light will fall on the photo multiplier tube. The natural logarithm of the signal from the photo tube when there is no absorbency divided by the signal when the copper is present in the light path to absorb the light is called the absorbency, and from the absorbency the concentration of copper in the sample may be determined.
TECHNICAL FIELD
This invention relates to apparatus or utilizirlg the Zeeman (or Stark) effect for achieving background correction in atomic absorption spectrophotometers (AS); moxie paretic-ularly, it relates to a unique combination of features which provide several times greater insensitivity to background absorption than heretofore attained in the prior art and enables the correction system to take the form o an accessory applicable to existing atomic absorption spectroph~tometers.
Jo ~;~139~6 BACKGROUND ART
Atomic absorption spectrophotometry is utilized to measure the concentration of a particular element in a sample.
For example: if one wishes to determine the concentration of copper in a sample, a light source producing one of the characteristic spectral lines of copper is utilized in the spectrophotometer. These sources are most often hollow cathode lamps, the cathode comprising the element to be determined, Liz., copper in this case. Electrode less discharge lamps containing a vaporizable charge of the analyze element are also used.
A monochromator customarily utilizing a diffraction grating disperses the light from the hollow cathode into a spectrum and the monochromator is adjusted so that the line of interest falls upon a detector, usually a photo multiplier tube. The amount of light falling on the photo tube is measured as a reference.
A sample of material in which one wishes to determine the amount of copper is then introduced into the path of the light from the line source to the monochromator. The sample must be 20` dissociated so that the copper atoms are free and not a part of a molecular compound in which case they would not provide their characteristic spectrum. This may be accomplished in an absorption furnace ~electrothermic sample atomizer). When the copper atoms are introduced into the light path they absorb light at the same characteristic spectral lines at which the copper atoms in the light source emit light. Thus, at the line of interest, light will be absorbed and less light will fall on the photo multiplier tube. The natural logarithm of the signal from the photo tube when there is no absorbency divided by the signal when the copper is present in the light path to absorb the light is called the absorbency, and from the absorbency the concentration of copper in the sample may be determined.
2-.;~......
.
I
There is one basic problem in all atomic absorption ~pectrophotometry. This is the so-called background absorbency, sometimes termed "non-atomic absorption" or "molecular absorption The problem is that other atoms and molecules in the sample may also absorb fight at the characteristic spectral line of interest. This absorption will of course cause an error in the absoxbance measured Various means have been disclosed in the prier art to correct the problem and in general such systems are called "background correction".
The most common form of background correction utilized in commercial atomic absorption spectrophotometers is the continuum source system In this system light from a broad band light source, that is, one producing a continuous, rather than a line spectrum, is utilized to measure the lo absorbency of a sample. Another beam is passed through the sample from a characteristic line source the absorbency is then measured at the line of interest and it is assumed that it one subtracts the absorbency from the continuous line source one will derive the absorbency at the spectral line of interest. There are mazy problems with such systems The light from the characteristic line source and the light from the continuous source do not pass through the same path and there may be substantial di~erences in the concentrations of the sample in the two paths, leading to systematic error.
If a sequential beam system is utilized, wherein the continuous spectrum reference beam is firs passed through the sample and thereafter the line source beam, the concern- -traction may Mary over time as well as space, again intro-during systematic erroxsO
Another method of background correction has been proposed.
This utilizes the Zeeman or Stark effects In the Zoom or Stark effects, when a magnetic or an electric field is applied to the sample, the spectral lives characteristic of on atom are split into eerily spectral lines.
I
In the normal reman and Stark effects of interest here, a spectral line may be converted into two spectral lines shifted to either side of the normal spectra line by an amount proportional to the applied field, or into three spectral lines, one at the normal position and two shifted, as aforesaid.
An important feature of the Stark and Zeeman effects is that the up it spectral lines do not all have the same polarization and in particular the polarization of the central or normal central line and the shifter spectral lines will be different, thus making it possible to look at the normal line or the shiftefl lines with polarization analyzer.
Below are listed a number of prior art patents and lo publication describing various systems utilizing the Stark or reman effects for background correction in atomic absorption spectrophotometry.
Patent Number Inventor Date I. S. PATENT
-203j676,004 Plugger et at 7/11/72
.
I
There is one basic problem in all atomic absorption ~pectrophotometry. This is the so-called background absorbency, sometimes termed "non-atomic absorption" or "molecular absorption The problem is that other atoms and molecules in the sample may also absorb fight at the characteristic spectral line of interest. This absorption will of course cause an error in the absoxbance measured Various means have been disclosed in the prier art to correct the problem and in general such systems are called "background correction".
The most common form of background correction utilized in commercial atomic absorption spectrophotometers is the continuum source system In this system light from a broad band light source, that is, one producing a continuous, rather than a line spectrum, is utilized to measure the lo absorbency of a sample. Another beam is passed through the sample from a characteristic line source the absorbency is then measured at the line of interest and it is assumed that it one subtracts the absorbency from the continuous line source one will derive the absorbency at the spectral line of interest. There are mazy problems with such systems The light from the characteristic line source and the light from the continuous source do not pass through the same path and there may be substantial di~erences in the concentrations of the sample in the two paths, leading to systematic error.
If a sequential beam system is utilized, wherein the continuous spectrum reference beam is firs passed through the sample and thereafter the line source beam, the concern- -traction may Mary over time as well as space, again intro-during systematic erroxsO
Another method of background correction has been proposed.
This utilizes the Zeeman or Stark effects In the Zoom or Stark effects, when a magnetic or an electric field is applied to the sample, the spectral lives characteristic of on atom are split into eerily spectral lines.
I
In the normal reman and Stark effects of interest here, a spectral line may be converted into two spectral lines shifted to either side of the normal spectra line by an amount proportional to the applied field, or into three spectral lines, one at the normal position and two shifted, as aforesaid.
An important feature of the Stark and Zeeman effects is that the up it spectral lines do not all have the same polarization and in particular the polarization of the central or normal central line and the shifter spectral lines will be different, thus making it possible to look at the normal line or the shiftefl lines with polarization analyzer.
Below are listed a number of prior art patents and lo publication describing various systems utilizing the Stark or reman effects for background correction in atomic absorption spectrophotometry.
Patent Number Inventor Date I. S. PATENT
-203j676,004 Plugger et at 7/11/72
3,811,778 Headache 1/74 3,914,9~4 Kadeishi 10~21/75 3,937,577 Borsch 2/10/76
4,035,083 Woodruff et at 7/12/77 254~171,912 It et at 10/23/79 _. PATENTS
918,878 Isaac 2/20/63 grow, 8i9 Isaac 2/20/63 1,27i,170 Zeiss Stiftung 4~19/72 301~385,791 Parker and Pearl 2/26/75 1,420,044 I ERDA . 1/7~76 - ARTICLES
science, "Hyperfine reman Effect Atomic Absorption Spectrometer for Mercury, Headache, T. and McLaughlin, ROD.; Sol. 174, Oust 22, 1971, pp. 404-407.
Lo Analytical Chemistry, "New Czarina Method for Atomic absorption Spectrophotometry~, Koizumi, H. and Yessed, K.;
VQ1. 47, I 9, Us 1975, PP. 1679-1682.
Atlanta, Jan Application of the Zoom Effect to analytical Atomic Spectroscopy-II", Stephens, R. and Ryan, D. En; Vol. 22, pp. 659-662~ Pergamon Press, 1975; printed in Great Britain.
Atlanta, "An Application of the Zeeman Effect to Analytical Atomic Spectroscopy-I n Stephens, R. and Ryan, D.
E.; Vol. 22, pp. 655-658; Pergamon Press, lg75; Printed in Great Britain.
The prior art may be categorized as providing systems having a number of possible characteristics. The magnetic or electric field may be applied at tune wine source or at the absorption chamber. The field may be DC, that is on or off; it may be AC/ for example sinusoidal varying; it may be unpiler, or bipolar, that is never going negative, or alternately going negative and positive; the polarization analyzer may be located before or after the absorption chamber; it may be static or rotating; and the optical axis Do the system through which the light passes may be parallel to or transverse to the applied field. Clearly, there is a vast number of combinations of possible elements to provide systems utilizing the Zeeman or the Stark effect for back-ground correction.
However, we have found that all of the prior art systems do no utilize or suggest what we have discovered to be the ideal combinations of elements for such systems For example, in the early British Patent numbers on 918t 878, and 918~79, a double beam system is proposed subject Jo all of the aforesaid problems of double beam systems. British Paterlt number 1, 38S,791 describes a multiplicity of possible systems, but does not indicate any advantage or disadvantage, depending on whether the field it applied at the absorption chamber or at the line source (except for lamp non linearities which have lately been overcome as described below); or where the polarization ~2~L3~
analyzer is placed in the system. U. S. Patent 4,035,083 discloses an AC full wave magnetic system end a rotating polarizer system. No practical differences butter the systems are discussed. U, S. Patent ODE and cores-pounding British Patent 1,271,170 discloses systems in which a magnetic field is applied at the line source and a rotating polarization analyzer is employed. U. 5. Patent 3,914,054 and corresp~ndiny British Patent 1,420,044, I. S. Patents 3,937,577, 3,B11,778 and the articles by Headache and McLaughlin, Stephens and yo-yo, and ~oizumi and Yessed _ .
all disclose foxed fields. Many of these systems have rotating analyzers. All of these systems apply the field at the line source.
I. S. Patent 4,171,912 is concerned with double peak detection; utilizes polarizers both before and after the absorption cell; and applies the field at the sample. The article by Stephens and describes a DC discharge lamp which will maintain a s able plasma in a Munich field and thus overcome the previously expressed objections to applying the field to the light source.
. S. Patents 3,413,382, 3,544,789 and 3,689,158 disclose conventional non-Zeeman or Stark background eon- .
reaction.
If one applies the field to the line source rather than US the absorption chamber one has all of the disadvantages previously described in continuum source systems. That is, what one does t utilizing the yield a the line source, is to pass alternately through the absorption chamfer the line of interest, thus providing a measure of the absorption plus I the background, and then the shifted reman lines or line, to obtain the background absorption. Perturbing the light source causes the same types of errors as the sequential continuum source systems previously described. It is not believed that this disadvantage of applying the f yield to the line source has been recognized in the prior Argo , - .
I
If one uses a DC field, which is turned on and off to provide synchronous detective, or a field which it bipolar, energy must be stored alternately as the field collapses and restored to the field as the field is established, leading to the utilization of large capacitors our inductors, which add Jo the bulkiness and cost of the unit. Furthermore if h magnetic field is used, the alternate magnetization of the poles and fore of the magnet requires increased energy due to hysteresis.
The polarization analyzers normally used in Zeeman systems have disadvantages and rotating polarizers have severe disadvantages. In the Zeeman system it is normally desirable to operate at least part of the time in the ultraviolet portion of the spectrum. The bire~ringent I polarization analyzers for this portion of the spectrum have restricted fields of view; that is, they only operate when light reaches them prom very small angles off the optical axis; they tend to have some non-uniformities in their crystalline structure which, when they are rotated, changes Jo the amount ox hi passing through them xegar~less of the polarization providing a false signal. frocks gratings used in the monochr~mators are not uniformly sensitive to light of different polarizations, having a preferred polarization or blazed direction, and therefore rotating any polarizer and then looking at it with the do fraction grating in the monochromator leads to a false signal .
furthermore, prior art rotating polarizers exhibit non-uniform light transmission across their apertures. We have therefore found that the polarization analyzer in a Zeeman atomic absorption system -should be static in order to obtain real improvement in background correction accuracy relative to continuum source systems ~L%~39~46 There appears to be no discussion in the prior art as to where the aperture stop or the field Stop of a reman atomic absorption spectrophotometer should be located. We have found that where are certain ideal positions for these elements of the system which lead to maximum utilization of the light available, rejection of black body radiation from the furnace of the absorption cell, maximum utilization of the field of view of the polarizer, and independence of the size of the light source.
We have further discovered that if the high voltage power supply to the photo multiplier detector is controlled ho an automatic gain control circuit responsive to the background signal in order, in the first instance, to increase the dynamic range of the instrumerlt, that the high voltage signal applied to the photo multiplier is in fact proportional to the background absorption signal derived with no Tom-pupation whatsoever We have also found that a Zeeman atomic absorption system may be conveniently provided as an adapter for existing atomic absorption spectrophotometers, such as the Perking Elmer 5000~
OBJECTS OF THE INVENTION
It is therefore an object of the invention to provide an atomic absorption spectrophotometer exhibiting improved background correction.
Another object of the invention is to provide an atomic absorption spectrophotometer of the above character employing the Zeeman or Stark effect for background correction in which the background signal and the absorption plus background signals are measured over the same optical path and at the same spectral line.
A further object of the invention is to provide a spectrophotometer of the above character employing very simple field generating means.
Still another object of the invention is to provide a spectrophotometer of the above character utilizing no moving parts.
still further object of the invention is to provide a spectrophotometer of the above character employing synchronous detection.
Yet another object of the invention is to provide a spectrophotometer of the above character providing a simply derived background absorbency measurement Still yet another object ox the invention is to provide a spectrophotometer of the above character wherein the background signal from the photo detector is kept constant.
A yet further object of the invention is to provide a spectrophotometer of the above-character employing sub Stan-tidally no energy storage devices and substantially unit polar fields.
I
Another object of the invention is two provide specs trophotometer of the above character insensitive to polarization effects in the monochromatorD
A further object of the invention is to provide a spectrophotometer of the above character which rejects black body radiation from the absorption furnace thereof.
Y t still another object of the invention is to provide a spectrophotorneter of the above character utilizing a polarization analyzer having a restricted field of view.
I Yet still a further object of the invention is to provide a spectrophotometer of the above character employing a novel form of polarization analyzer which depolarizes the light exiting therefrom.
Still another object of the invention is to provide a spectrophotometer of the above character that wakes maximum use of the light throughout.
Yet another object of the invention is to provide a spectrophotometer of the above character which operates without regard to the size of the light source employed.
Still a further object of the invention is to provide a spectrophotometer of the above character which stray light effects are minimized.
Yet still another object of the invention is to provide a spectrophotometer of the above character employing the 25 Zeeman effect for background correction.
Another object of the invention is to provide a novel polarization analyzer the output of which is depolarize.
A further object of the invention is to eliminate deleterious polarization effects in spectrometers employing 30 polarization analyzers. .
Other important objects ox the invention are to provide a spectrophotometer ox the above character providing increased sensitivity and accuracy, ease of operation, low manufac-luring and operating cost, and which my utilize existing microcomputer architecture and programs.
I
Other objects of the invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises features of con-striation particular elements, arrangements of parts and a system which will ye exemplified in the elements, construe-lions, and system hereinafter set forth. The scope of the invention will be indicated in the claims.
,, --11--I
THE DRAWINGS
For a fuller understanding of the Norway and objects of the invention, reference should be had to the hollowing detailed description, taken in connection with the accom-paying drawings, it which:
FIGURE 1 is a front view of an adapter according to the invention for providing Zeeman background correction applied to a Perkin-Elmer Model 5000 atomic adsorption spectrophotometer;
FIGURE 2 is a diagrammatic top view of the optimal system of the adapter of the invention and of the Model 5000 spectrophotometer of FIGURE l;
FOGGIER is a schematic diagram of the system of the invention employing Zeeman background correction;
FIGURE 4 is a timing diagram of the system of FIGURE 3 and comprises FIGURES PA through YE, each showing a separate signal emp~oyea in the system;
FIGURE 5 is a series of plots of absorban~e over time produced by the system of FIGURE 3, illustrating the system's sensitivity in measuring the presence of I micrograms per milliliter ox lead in a .5% sodium chloride solution;
FIGURE I series of plots similar to FIGURE
illustrating the system's sensitivity in measuring the presence of Owe micrograms per milliliter of lead in a 1%
sodium chloride 501ution~o FIGURE 7 it a series of plots produced by the prior art Perkin-Elmer Model 4000 spectrophotDmeter in measuring the presence owe micrograms per milliliter of lead in a .5%
and in a 1% sodium chloride solution;
FIGURE is a top view of the polarization analyzer of FIGURE 3 with the direction of light passage reversed from thaw shown it I GORE 3; and I
3~6 FIGURE 9 is an end view of the exit face of the Polaris zatic~n analyzer of FIGURES 3 and 9.
The same reference characters refer to the save elements - throughout the several views of the drawings.
~%~34~q~
DISCLOSURE OF THE INVENTION
The system of the invention for background correction employs a field at the absorption furnace so that ~11 Abe sorbance measurements, with and without the sample signal, are made at the same spectral line The field is sub Stan-tidally unpiler. For convenience a magnetic field is utilized and the electromagnet is connected directly to the alternating current power lines through a diode to provide the unpiler field. When the field is ON the background absorbency is measured. When the field is OF the absorbency measured is the sum of the absorbency due to the sample and the background. Thus the absorbency of the sample may be measured by simple subtraction.
It order that the absorbency at substantially zero field may be measured over a significant length of time, the field is made to go slightly negative, 50 that over the measurement period the integral of the field is substantially zero. This us accomplished by connecting a small capacitor across the coil ox the electromagnet.
I The output of the photo multiplier when he field is ON
it integrated and supplied to an automatic gain control circuit which controls the high voltage power supply to the photo-multiplier. the result is that the high voltage potential supply to the photo multiplier is proportional tooth log of the background signal and directly proportional to the background "absorbency and this signal may be derived through a voltage divider and utilized directly by the operator.
A bixefringent polarizer is employed Jo that the instrument may be posted in the ultraviolet. Materials that may be used include quartz, magnesium fluoride and sapphire. Artificial crystal quartz it the preferred material. The polarizer is used in a unique orientation . .
5L3~4~
which causes the undeviat~d ray along the optical axis to be depolarized as it exits from the polarizer, thus fang the monochromator from polarization effects.
The polarizer is of the type which deviates thy extra-ordinary rays from the optical axis and therefore the polar-sizer has a rather restricted field of view. The polarizer has exit and entrance faces which are perpendicular to the optical axis.
The polarizer is located between the absorption furnace and ho monochromator so that light from the furnace will not be reflected off the exit surface of the polarizer, as would be the case if it were located before the furnace.
Such stray light would pays through the absorption furnace twice and thus adversely affect the accuracy of the absorption measurement.
A field stop is employed between the absorption furnace and the polarization analyzer which restricts light reaching the analyzer to the annualizer effective field of view, The field stop and polarization analyzer are located between the furnace and the monochromator such that the black body radiation from the wall of the absorption furnace are excluded from the field seen by the monochromator. furthermore;
the line source is looted in the optical system such that the field stop restricts toe field of view of the polarization analyzer to the active light source; that is the glowing hollow cathode in a hollow cathode lamp for example,.
The entrance face of the polarization an lousier acts as the aperture stop of the system and the optical system is arranged such that this entrance faze fin the directive ox the slit) is imaged ox and co-extensive with the difxaction grating of the monochromator or maximum light utilization efficiency.
Those skilled in the art will understand that many of the future the invention could be accomplished in a ~2~L3~
Stark background correction instrument as jell as in the Zeeman background correction instrument disclosed it a sufficiently go electric yield were produced at the furnace.
We therefore use the expression electromagnetic optical effect" in order to cover both the Stark and the Zeeman effects; that is both electric field effects and magnetic field effects We also use the expression "Electra-magnetic field" TV mean both the electric field used in the Stark effect and the magnetic field used in the Zeeman effect.
Reflection optics, i. e. mirrors, are employed through-out the system, Father than lenses, in order to minimize the effects of dispersion and stray light.
~39~
BEST MODE POX CARRYING OUT THE INVENTION
Now referring to FIGURE 1, instrument 20 is a Perking Elmer Motel ODE atomic adsorption spectrophotometer.
Zeeman background correction adapter 22 according to the invention is located to the right of the spectrophotometer 20. The magnet and absorption furnace are generally in-dilated a 24. A three-position switch generally indicated a 26~ urns the magnet ON or OFF or allows it to be con-trolled remotely from the spectr~photometer 20.
lo Now referring to FIGURE 2, the spectrophotom~ter 20 it provided with a carousel 28 in which a plurality of hollow : cathode line sources 30 may be mounted. The carousel is rotatable about the axis 32 to briny the desired line source into alignment with the optical axis generally indicated at .15 34. Normally a two-positioned mirror 36 is located in it dotted position and light from the line source 30 proceeds to the.monochromator and photo multiplier section of the instrument generally indicated at 3B. The two-positioned mirror 36 is a new element added to the system so that it I may be utilized with the adapter 22.
When the mirror 36 is at the svlld line position the optical axis is diverted as show at 34' to accept light from the adapter 22 into the mon~chromator photomultipliex section 380 In order to supply light from a line source to the - adapter 22, one or more of the hollow cathode lamps 30' are reversed on the carousel 28 so that their light proceeds out of the spectrophotometer 20 and into the adapter along optical axis 34'. The optical axis is stepped downwardly by mirror optics ge~exally indicated at 40, so that the light is focused on the absorption furnace 42 (which may be a heated graphite analyzer) located between the poles of the magnet 24. The light exiting in the absorption furnace 42 I
is then diverted by mirrors 44 and 46 and supplied to the entrance face 48 of the polarization analyzer OWE The polarization analyzer 50 is oriented such that the or polarization of the normal absorption line is blocked and
918,878 Isaac 2/20/63 grow, 8i9 Isaac 2/20/63 1,27i,170 Zeiss Stiftung 4~19/72 301~385,791 Parker and Pearl 2/26/75 1,420,044 I ERDA . 1/7~76 - ARTICLES
science, "Hyperfine reman Effect Atomic Absorption Spectrometer for Mercury, Headache, T. and McLaughlin, ROD.; Sol. 174, Oust 22, 1971, pp. 404-407.
Lo Analytical Chemistry, "New Czarina Method for Atomic absorption Spectrophotometry~, Koizumi, H. and Yessed, K.;
VQ1. 47, I 9, Us 1975, PP. 1679-1682.
Atlanta, Jan Application of the Zoom Effect to analytical Atomic Spectroscopy-II", Stephens, R. and Ryan, D. En; Vol. 22, pp. 659-662~ Pergamon Press, 1975; printed in Great Britain.
Atlanta, "An Application of the Zeeman Effect to Analytical Atomic Spectroscopy-I n Stephens, R. and Ryan, D.
E.; Vol. 22, pp. 655-658; Pergamon Press, lg75; Printed in Great Britain.
The prior art may be categorized as providing systems having a number of possible characteristics. The magnetic or electric field may be applied at tune wine source or at the absorption chamber. The field may be DC, that is on or off; it may be AC/ for example sinusoidal varying; it may be unpiler, or bipolar, that is never going negative, or alternately going negative and positive; the polarization analyzer may be located before or after the absorption chamber; it may be static or rotating; and the optical axis Do the system through which the light passes may be parallel to or transverse to the applied field. Clearly, there is a vast number of combinations of possible elements to provide systems utilizing the Zeeman or the Stark effect for back-ground correction.
However, we have found that all of the prior art systems do no utilize or suggest what we have discovered to be the ideal combinations of elements for such systems For example, in the early British Patent numbers on 918t 878, and 918~79, a double beam system is proposed subject Jo all of the aforesaid problems of double beam systems. British Paterlt number 1, 38S,791 describes a multiplicity of possible systems, but does not indicate any advantage or disadvantage, depending on whether the field it applied at the absorption chamber or at the line source (except for lamp non linearities which have lately been overcome as described below); or where the polarization ~2~L3~
analyzer is placed in the system. U. S. Patent 4,035,083 discloses an AC full wave magnetic system end a rotating polarizer system. No practical differences butter the systems are discussed. U, S. Patent ODE and cores-pounding British Patent 1,271,170 discloses systems in which a magnetic field is applied at the line source and a rotating polarization analyzer is employed. U. 5. Patent 3,914,054 and corresp~ndiny British Patent 1,420,044, I. S. Patents 3,937,577, 3,B11,778 and the articles by Headache and McLaughlin, Stephens and yo-yo, and ~oizumi and Yessed _ .
all disclose foxed fields. Many of these systems have rotating analyzers. All of these systems apply the field at the line source.
I. S. Patent 4,171,912 is concerned with double peak detection; utilizes polarizers both before and after the absorption cell; and applies the field at the sample. The article by Stephens and describes a DC discharge lamp which will maintain a s able plasma in a Munich field and thus overcome the previously expressed objections to applying the field to the light source.
. S. Patents 3,413,382, 3,544,789 and 3,689,158 disclose conventional non-Zeeman or Stark background eon- .
reaction.
If one applies the field to the line source rather than US the absorption chamber one has all of the disadvantages previously described in continuum source systems. That is, what one does t utilizing the yield a the line source, is to pass alternately through the absorption chamfer the line of interest, thus providing a measure of the absorption plus I the background, and then the shifted reman lines or line, to obtain the background absorption. Perturbing the light source causes the same types of errors as the sequential continuum source systems previously described. It is not believed that this disadvantage of applying the f yield to the line source has been recognized in the prior Argo , - .
I
If one uses a DC field, which is turned on and off to provide synchronous detective, or a field which it bipolar, energy must be stored alternately as the field collapses and restored to the field as the field is established, leading to the utilization of large capacitors our inductors, which add Jo the bulkiness and cost of the unit. Furthermore if h magnetic field is used, the alternate magnetization of the poles and fore of the magnet requires increased energy due to hysteresis.
The polarization analyzers normally used in Zeeman systems have disadvantages and rotating polarizers have severe disadvantages. In the Zeeman system it is normally desirable to operate at least part of the time in the ultraviolet portion of the spectrum. The bire~ringent I polarization analyzers for this portion of the spectrum have restricted fields of view; that is, they only operate when light reaches them prom very small angles off the optical axis; they tend to have some non-uniformities in their crystalline structure which, when they are rotated, changes Jo the amount ox hi passing through them xegar~less of the polarization providing a false signal. frocks gratings used in the monochr~mators are not uniformly sensitive to light of different polarizations, having a preferred polarization or blazed direction, and therefore rotating any polarizer and then looking at it with the do fraction grating in the monochromator leads to a false signal .
furthermore, prior art rotating polarizers exhibit non-uniform light transmission across their apertures. We have therefore found that the polarization analyzer in a Zeeman atomic absorption system -should be static in order to obtain real improvement in background correction accuracy relative to continuum source systems ~L%~39~46 There appears to be no discussion in the prior art as to where the aperture stop or the field Stop of a reman atomic absorption spectrophotometer should be located. We have found that where are certain ideal positions for these elements of the system which lead to maximum utilization of the light available, rejection of black body radiation from the furnace of the absorption cell, maximum utilization of the field of view of the polarizer, and independence of the size of the light source.
We have further discovered that if the high voltage power supply to the photo multiplier detector is controlled ho an automatic gain control circuit responsive to the background signal in order, in the first instance, to increase the dynamic range of the instrumerlt, that the high voltage signal applied to the photo multiplier is in fact proportional to the background absorption signal derived with no Tom-pupation whatsoever We have also found that a Zeeman atomic absorption system may be conveniently provided as an adapter for existing atomic absorption spectrophotometers, such as the Perking Elmer 5000~
OBJECTS OF THE INVENTION
It is therefore an object of the invention to provide an atomic absorption spectrophotometer exhibiting improved background correction.
Another object of the invention is to provide an atomic absorption spectrophotometer of the above character employing the Zeeman or Stark effect for background correction in which the background signal and the absorption plus background signals are measured over the same optical path and at the same spectral line.
A further object of the invention is to provide a spectrophotometer of the above character employing very simple field generating means.
Still another object of the invention is to provide a spectrophotometer of the above character utilizing no moving parts.
still further object of the invention is to provide a spectrophotometer of the above character employing synchronous detection.
Yet another object of the invention is to provide a spectrophotometer of the above character providing a simply derived background absorbency measurement Still yet another object ox the invention is to provide a spectrophotometer of the above character wherein the background signal from the photo detector is kept constant.
A yet further object of the invention is to provide a spectrophotometer of the above-character employing sub Stan-tidally no energy storage devices and substantially unit polar fields.
I
Another object of the invention is two provide specs trophotometer of the above character insensitive to polarization effects in the monochromatorD
A further object of the invention is to provide a spectrophotometer of the above character which rejects black body radiation from the absorption furnace thereof.
Y t still another object of the invention is to provide a spectrophotorneter of the above character utilizing a polarization analyzer having a restricted field of view.
I Yet still a further object of the invention is to provide a spectrophotometer of the above character employing a novel form of polarization analyzer which depolarizes the light exiting therefrom.
Still another object of the invention is to provide a spectrophotometer of the above character that wakes maximum use of the light throughout.
Yet another object of the invention is to provide a spectrophotometer of the above character which operates without regard to the size of the light source employed.
Still a further object of the invention is to provide a spectrophotometer of the above character which stray light effects are minimized.
Yet still another object of the invention is to provide a spectrophotometer of the above character employing the 25 Zeeman effect for background correction.
Another object of the invention is to provide a novel polarization analyzer the output of which is depolarize.
A further object of the invention is to eliminate deleterious polarization effects in spectrometers employing 30 polarization analyzers. .
Other important objects ox the invention are to provide a spectrophotometer ox the above character providing increased sensitivity and accuracy, ease of operation, low manufac-luring and operating cost, and which my utilize existing microcomputer architecture and programs.
I
Other objects of the invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises features of con-striation particular elements, arrangements of parts and a system which will ye exemplified in the elements, construe-lions, and system hereinafter set forth. The scope of the invention will be indicated in the claims.
,, --11--I
THE DRAWINGS
For a fuller understanding of the Norway and objects of the invention, reference should be had to the hollowing detailed description, taken in connection with the accom-paying drawings, it which:
FIGURE 1 is a front view of an adapter according to the invention for providing Zeeman background correction applied to a Perkin-Elmer Model 5000 atomic adsorption spectrophotometer;
FIGURE 2 is a diagrammatic top view of the optimal system of the adapter of the invention and of the Model 5000 spectrophotometer of FIGURE l;
FOGGIER is a schematic diagram of the system of the invention employing Zeeman background correction;
FIGURE 4 is a timing diagram of the system of FIGURE 3 and comprises FIGURES PA through YE, each showing a separate signal emp~oyea in the system;
FIGURE 5 is a series of plots of absorban~e over time produced by the system of FIGURE 3, illustrating the system's sensitivity in measuring the presence of I micrograms per milliliter ox lead in a .5% sodium chloride solution;
FIGURE I series of plots similar to FIGURE
illustrating the system's sensitivity in measuring the presence of Owe micrograms per milliliter of lead in a 1%
sodium chloride 501ution~o FIGURE 7 it a series of plots produced by the prior art Perkin-Elmer Model 4000 spectrophotDmeter in measuring the presence owe micrograms per milliliter of lead in a .5%
and in a 1% sodium chloride solution;
FIGURE is a top view of the polarization analyzer of FIGURE 3 with the direction of light passage reversed from thaw shown it I GORE 3; and I
3~6 FIGURE 9 is an end view of the exit face of the Polaris zatic~n analyzer of FIGURES 3 and 9.
The same reference characters refer to the save elements - throughout the several views of the drawings.
~%~34~q~
DISCLOSURE OF THE INVENTION
The system of the invention for background correction employs a field at the absorption furnace so that ~11 Abe sorbance measurements, with and without the sample signal, are made at the same spectral line The field is sub Stan-tidally unpiler. For convenience a magnetic field is utilized and the electromagnet is connected directly to the alternating current power lines through a diode to provide the unpiler field. When the field is ON the background absorbency is measured. When the field is OF the absorbency measured is the sum of the absorbency due to the sample and the background. Thus the absorbency of the sample may be measured by simple subtraction.
It order that the absorbency at substantially zero field may be measured over a significant length of time, the field is made to go slightly negative, 50 that over the measurement period the integral of the field is substantially zero. This us accomplished by connecting a small capacitor across the coil ox the electromagnet.
I The output of the photo multiplier when he field is ON
it integrated and supplied to an automatic gain control circuit which controls the high voltage power supply to the photo-multiplier. the result is that the high voltage potential supply to the photo multiplier is proportional tooth log of the background signal and directly proportional to the background "absorbency and this signal may be derived through a voltage divider and utilized directly by the operator.
A bixefringent polarizer is employed Jo that the instrument may be posted in the ultraviolet. Materials that may be used include quartz, magnesium fluoride and sapphire. Artificial crystal quartz it the preferred material. The polarizer is used in a unique orientation . .
5L3~4~
which causes the undeviat~d ray along the optical axis to be depolarized as it exits from the polarizer, thus fang the monochromator from polarization effects.
The polarizer is of the type which deviates thy extra-ordinary rays from the optical axis and therefore the polar-sizer has a rather restricted field of view. The polarizer has exit and entrance faces which are perpendicular to the optical axis.
The polarizer is located between the absorption furnace and ho monochromator so that light from the furnace will not be reflected off the exit surface of the polarizer, as would be the case if it were located before the furnace.
Such stray light would pays through the absorption furnace twice and thus adversely affect the accuracy of the absorption measurement.
A field stop is employed between the absorption furnace and the polarization analyzer which restricts light reaching the analyzer to the annualizer effective field of view, The field stop and polarization analyzer are located between the furnace and the monochromator such that the black body radiation from the wall of the absorption furnace are excluded from the field seen by the monochromator. furthermore;
the line source is looted in the optical system such that the field stop restricts toe field of view of the polarization analyzer to the active light source; that is the glowing hollow cathode in a hollow cathode lamp for example,.
The entrance face of the polarization an lousier acts as the aperture stop of the system and the optical system is arranged such that this entrance faze fin the directive ox the slit) is imaged ox and co-extensive with the difxaction grating of the monochromator or maximum light utilization efficiency.
Those skilled in the art will understand that many of the future the invention could be accomplished in a ~2~L3~
Stark background correction instrument as jell as in the Zeeman background correction instrument disclosed it a sufficiently go electric yield were produced at the furnace.
We therefore use the expression electromagnetic optical effect" in order to cover both the Stark and the Zeeman effects; that is both electric field effects and magnetic field effects We also use the expression "Electra-magnetic field" TV mean both the electric field used in the Stark effect and the magnetic field used in the Zeeman effect.
Reflection optics, i. e. mirrors, are employed through-out the system, Father than lenses, in order to minimize the effects of dispersion and stray light.
~39~
BEST MODE POX CARRYING OUT THE INVENTION
Now referring to FIGURE 1, instrument 20 is a Perking Elmer Motel ODE atomic adsorption spectrophotometer.
Zeeman background correction adapter 22 according to the invention is located to the right of the spectrophotometer 20. The magnet and absorption furnace are generally in-dilated a 24. A three-position switch generally indicated a 26~ urns the magnet ON or OFF or allows it to be con-trolled remotely from the spectr~photometer 20.
lo Now referring to FIGURE 2, the spectrophotom~ter 20 it provided with a carousel 28 in which a plurality of hollow : cathode line sources 30 may be mounted. The carousel is rotatable about the axis 32 to briny the desired line source into alignment with the optical axis generally indicated at .15 34. Normally a two-positioned mirror 36 is located in it dotted position and light from the line source 30 proceeds to the.monochromator and photo multiplier section of the instrument generally indicated at 3B. The two-positioned mirror 36 is a new element added to the system so that it I may be utilized with the adapter 22.
When the mirror 36 is at the svlld line position the optical axis is diverted as show at 34' to accept light from the adapter 22 into the mon~chromator photomultipliex section 380 In order to supply light from a line source to the - adapter 22, one or more of the hollow cathode lamps 30' are reversed on the carousel 28 so that their light proceeds out of the spectrophotometer 20 and into the adapter along optical axis 34'. The optical axis is stepped downwardly by mirror optics ge~exally indicated at 40, so that the light is focused on the absorption furnace 42 (which may be a heated graphite analyzer) located between the poles of the magnet 24. The light exiting in the absorption furnace 42 I
is then diverted by mirrors 44 and 46 and supplied to the entrance face 48 of the polarization analyzer OWE The polarization analyzer 50 is oriented such that the or polarization of the normal absorption line is blocked and
5 the ox polarization of the shifted absorption lines are undeviated and proceed along the optical axis 34 ' through another set of stepped optics generally indicated at 52 to bring the optical axis 34' into alignment with flip mirror 36 and thence to the monochromator and photo multiplier section generally indicated at 38. However, the line source 30' provides substantially Jo light at the displaced lines with the result that the absorption of the normal wine in the polarization state is due to background alone. The chopper generally indicated at 54 of the spectrophotometer 20 is not used whey the Reman adapter 22 is in use.
Now referring to FIGURE 3. In schematic terms list from toe line source 30' is focused by mirror optics 40 to within the absorption furnace 42 located between the poles ox an electromagnet 56. The optic axis 34 ' is perpendicular 20 to the field. - Light is then refocused by mirror optics 44 off an apest~ge stop 47. Iota from the aperture stop 47 passe through a polarization analyzer So is then refocused by mirror optics generally indicated at 58 on the suit 59 of a monochromator 60 which thence focuses light around the 25 spectral line of interest upon a photo multiplier So.
The coils 64 of the electromagnet 56 are connected across Argo ordinary At: power line generally indicated at 66 . - .
The coils 6g are energized through a series connected diode 68. This causes the current to the coils, and thus the magnetic field between the poles, to be essentially unpiler.
I order that the field not be merely instantaneously zero but substantially zero for a lunger length of time a small two micro farad capacitor 70 it connected across the coils 64.
Multi-position switch 26 is shown in its off position.
When connected to pole 72~ relay 74 is energized losing the circuit to the electromagnet 56. Surge protec~ion-is provided by urge protector 76. Simultaneously, lamp 78, which may be convenient lye located on the front panel (FIGURE
I energizes.
When switch 26 is connected to pole 80 the elertro-magnet may be turned ON by a remote switch 82 located within the spectrophotometer 20. Circuit protection is provided by circuit breaker 84 and fuse 86.
The primary coil of a signal transformer 88 is Con-netted in series with the relay 74. One side of the second Mary of transformer 88 is connected Jo ground and the other side ~xovides synchronization signal on line 90 to a lo synchronizer 92.
When the magnet 56 is OFF the photo multiplier 62 - provides a normal atomic absorption signal on its output line 94 referenced to ground across resistor 96. This signal is supplied to a preamplifier 98. During the period when the magnet is OFF switch 100 is closed and the signal prom the photomuptiplier 62 which has been reduced my the sam~le-~bsor~ance and the background absorbency, passes through coupling capacitor 102 and it supplied to a linear integrator indicated schematically us capacitor 104 and I amplifier 106. The signal from the integrator is supplied to a logarithmically scaled analog to digital converter 108.
the digital signal is supplied to a microcomputer 110 and -I
.~he-mierocomputer then supplies the information to a display generally indicated at 112. All of this is essentially the tame as the situation when the same measurement is being made in the normal atomic absorption spectropho~meter 20 of FUGUE 1.
~Z~3~
When the magnet So is ON however, absorption by the element being measured occurs at one polarization at the central line and at the opposite polarization to either side of the central line. The polarization analyzer dyes oriented so as to reject the central line polarization called and to accept the deviated line polarization called . In this way light from the line source 30' in the polarization goes through the sample, is not absorbed by the sample whey the magnet is ON, but passed through the plower-Zion analyzer to the monochromator I and pho~omultiplier62. Light of the or polarization is deviated by the Polaris ration analyzer and does not reach the monochromator 60.
Since the spectral line from the line source I is narrower than the distance between the deviated lines when the '15 magnet is ON, the absorbency of the sample measured is essentially due to the non-atomic species of the sample and the signal on line 94 from the photo multiplier 62 is the background signal.
Synchronizer 92 is arranged to close switch 114 during this period. The signal it linearly integrated by capacitor 116 and amplifier 106 and supplied to log analog to digital converter 108 and then to the microcomputer 110. The sub-traction of the field ON and field OFF system is made and the result displayed on display 112.
The integrated background signal is also supplied on line 118 to an automatic gain control circuit 120 which has a response time of abut 100 milliseconds for a 60 Hertz magnetic field frequency. The output of the automatic gain control circuit controls a high voltage power supply 122 supplying high voltage on line 124 to the photo multiplier 62. Thus the photo multiplier 62 is caused to produce the same output signal or any background absorbency, greatly increasing its range of sensitivity. We have discovered that the high voltage supplies on line 124 is in fact the I
log of the background signal that is the background absvrbance and therefore such a signal may be provided by a stage divider 124 on line 128 9 digitalized by a linear analog to digital connector snot shown and supplied to the micro-processor 110.
As is normal in synchronous detection the integra~ingcapacitor 102 is referenced to ground before each measurement during the successive energizations of the magnet 56. This is accomplished by switch 130 which is energized when no I light is being received by the photo multiplier tune 62.
This is accomplished by turning lamp 130 off by meats of a supply signal on line 132 from synchronizer 92.
The electrical operation of the Zeeman background correction instrument illustrated in FIGURE 2 can be under-lo stood with reference to FIGURE 4. FIGURE PA shows a plot of the magnetic field 13q which is essentially unpiler except that it goes slightly negative between times 136 and 138 where the integral of the field between times 136 and 138 is substantially zero.
FUGUE 4B shows a plot of the signal utilized Jo turn the lamp 30' on and off supplied OX line 1320 As can be see, the lamp is Tory on curing maximum field and during minimum field periods.
FIGURE 4C is a plot of the operation of switch 130~ the dark time clamp, which it on during a period while the line source 30' is of, as illustrated in FIGURE 4B.
FIGURE ED is an illustration of the operation of the bac~grou~a or magnet ON switch 114 which is turned ON during the magnet ON period.
FIGURE YE is an illustration of the operation of the magnet OFF or sample plus background switch 100 which is closed during the magnet OFF period.
Those skilled in the art will understand that switches loo 11~ and 130 are ideal electronic switches controlled by the synchronizer 92 as indicated by the dotted lives.
~2~3~
FIGURE 5 it a series of plots provided by the readout device 112 under control of the microprocessor lo FIGURE
I The microprocessor 110 is the same microprocessor provided in the Perkin-Elmer model 5000 and may operate with S the same program. It is desirable however that rollover detection be provided when the output fed Jo the micro-computer 110 is from the Zeeman adapter 22, in order to prevent double valued readings.
FIGURE 6 is a series of plots taken at the spectral line 28~.3 nanometers utilizing a 10 micro liter sample having a background absorbency of 1. The samples were supplied to the furnace 42 of the Zeeman adapter 22 of the invention. Curve 136 was produced when the sample contained .05 micrograms per milliliter of lead. Plot 138 is the result of a screen having an absorbency of one. Plot 140 is the result of incorporating .5% sodium chloride in the 10 micro liter sample. Plots 142 an 144 are further runs with .5% sodium chloride. The final plot 146 is the result when .05 micrograms per milliliter of lead are incorporated in a I sodium chloride 10 micro liter sample. It should be noted that the lead is as easily recognized as when there was no sodium chloride present, as illustrated in plot 136.
FIGURE 6 is a series of runs also at 283.3 nanometers, of a 10 micro liter sample with a background absorbency of 1~7. At this level nearly 99% of the light passing through the sample is absorbed by the background Plot 148 is the result of a screen having an absorbency of lo 7. Plot 150 is of a run i;. which the sample included 1% sodium chloride as are plots 152, 154 and 156. Plot 158 was generated when .05 micrograms per milliliter of lead were incorporated in the 1% sodium chloride solution. Again it should be noted how easy it is to measure the absorbency Do the lead from plot 158, basically as easy as if there were no background, as in plot 136 of FIGURE 5.
~3~4~
FUGUE 7 is a series of plots also at 293.3 nanometers a 10 micro liter sample However these plots were derived from supplying the sample to the Perkin-Elmer model 40~0 atomic absorption spectrophotometer which is similar to the model 5000, utilizing conventional continuum source background correction. When .05 micrograms per milliliter of lead are incorporated it a I sodium chloride solution sample plots 160 and 162 were derived in separate 6 second runs. Note how much cleaner plot 146 of FIGURE 5 is when the Zeeman adapter is utilized.
Plots 164 and 166 ox FIGURE 7 are two runs in the Perkin-Elmer 4000 where the sample included .05 micrograms per milliliter of lead in a 1% sodium chloride solution.
Note that in plot 164 the lead absorbency is just barely greater than the background, whereas in plot 158 made on the Zeeman adapter the background is still greatly suppressed.
Thus the Zeeman adapter of our invention is able to measure the absorbency of elements when the background absorbanc~ is much greater than can ye done in prior art I instruments We have achieved this in an instrument with a magnetic field strength of 8 kilogauss. sigher fields would increase separation of thy and lines and thus increase insensitivity to background absorbency The polarization analyzer 50 of the invention is illustrated in detail in FIGURES 8 and 9. It may be menu-lectured of any birefringent material usable at toe spectral lines ox interest. Because it it desirable to use the --Zeeman adapter at the ultraviolet range from 190 nanometers to 850 nanometers, the material of the polarization analyzer I 50 is preferably artificial crystal quart or other transparent birefringent material suitable for use in this range, e.g.
crystalline magnesium fluoride, or sapphire.
_ I
A quartz analyzer is illustrated in FIGURES 8 and 9 and comprises a wedge 168 having an isosceles triangle cross section, and a pair of right triangular cross section wedges 170, 172 in optical contact therewith. The principal optical axis of element 168 is shown at 174. The principal optical axis of elements 170 and 172 is at right angles thereto and is illustrated at 176. Optical axis 176 is aligned with the magnetic field of thy electromagnet 56~
The length 178 of the polarization analyzer ED utilizing quartz having a principal optical index of refraction of 1.64927 and an extraordinary index of 1.6627 is 40 plus or minus 0.2 millimeters. As shown in FIGURE 9 the analyzer 50 is square and the side dimensions 180-180 are 22 plus or minus 0.2 millimeters. Since the wedge 168 cannot come to a knife point, the flat on the apex thereof has a dimension 182 ox about ~.50 millimeters.
As previously mentioned the analyzer 50 not only deviates the light from the furnace 42 which is polarized perpendicularly to the magnetic field, and passes the light which is parallel to the magnetic field, it also depolarizes the latter This depolarization occurs because light passing from wedges 170 and 172 into wedge 16~ after passing the bound Aries threaten, is rotated in its plane of polarization by the birefringent wedge 163 in proportion to the distance it travels within it. This occurs because birefringent material acts as a circular rotator on polarized light traveling along its principal axis and the analyzer 50 is made long enough for several 360 rotations Since various rays travel various distances, the exiting light include ~11 polarizations an is in effect depolarized, eliminating all polarization effects in he monochromator 60 and photo-multiplier 62.
I
If other forms of polarization analyzers are used, for example, dichroic sheet at visible light frequencies, the same result of monochr~mator insensitivity may be produced by using a depolarizer after the analyzer.
Another advantage of the analyzer SOD is that the angle in FIGURE B is one half of thaw in a conventional Russian analyzer Since the deviation of the rejected rays is inversely prDp~rtional to , great deviation is achieved White lengthening the analyzer along the optic axis with its accompanying restriction of the field of view end increased use of expensive material.
This analyzer constructed out of three wedges has four times the light throughput of a Russian analyzer constructed of two wedge of the same volume.
lo Those skilled in the art will understand that this depolarizing analyzer may be used in many other optical systems and in various spectrometers employing analyzers where it is desired to eliminate polarization effects at the monochromator.
It will thus be seen that the objects set forth above among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the above described element t constructions, and systems without departing from the scope of the invention, I it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted a illustrative and not in a limiting sense.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention, which, as a matte ox language, might be said to fall there between.
--US--
Now referring to FIGURE 3. In schematic terms list from toe line source 30' is focused by mirror optics 40 to within the absorption furnace 42 located between the poles ox an electromagnet 56. The optic axis 34 ' is perpendicular 20 to the field. - Light is then refocused by mirror optics 44 off an apest~ge stop 47. Iota from the aperture stop 47 passe through a polarization analyzer So is then refocused by mirror optics generally indicated at 58 on the suit 59 of a monochromator 60 which thence focuses light around the 25 spectral line of interest upon a photo multiplier So.
The coils 64 of the electromagnet 56 are connected across Argo ordinary At: power line generally indicated at 66 . - .
The coils 6g are energized through a series connected diode 68. This causes the current to the coils, and thus the magnetic field between the poles, to be essentially unpiler.
I order that the field not be merely instantaneously zero but substantially zero for a lunger length of time a small two micro farad capacitor 70 it connected across the coils 64.
Multi-position switch 26 is shown in its off position.
When connected to pole 72~ relay 74 is energized losing the circuit to the electromagnet 56. Surge protec~ion-is provided by urge protector 76. Simultaneously, lamp 78, which may be convenient lye located on the front panel (FIGURE
I energizes.
When switch 26 is connected to pole 80 the elertro-magnet may be turned ON by a remote switch 82 located within the spectrophotometer 20. Circuit protection is provided by circuit breaker 84 and fuse 86.
The primary coil of a signal transformer 88 is Con-netted in series with the relay 74. One side of the second Mary of transformer 88 is connected Jo ground and the other side ~xovides synchronization signal on line 90 to a lo synchronizer 92.
When the magnet 56 is OFF the photo multiplier 62 - provides a normal atomic absorption signal on its output line 94 referenced to ground across resistor 96. This signal is supplied to a preamplifier 98. During the period when the magnet is OFF switch 100 is closed and the signal prom the photomuptiplier 62 which has been reduced my the sam~le-~bsor~ance and the background absorbency, passes through coupling capacitor 102 and it supplied to a linear integrator indicated schematically us capacitor 104 and I amplifier 106. The signal from the integrator is supplied to a logarithmically scaled analog to digital converter 108.
the digital signal is supplied to a microcomputer 110 and -I
.~he-mierocomputer then supplies the information to a display generally indicated at 112. All of this is essentially the tame as the situation when the same measurement is being made in the normal atomic absorption spectropho~meter 20 of FUGUE 1.
~Z~3~
When the magnet So is ON however, absorption by the element being measured occurs at one polarization at the central line and at the opposite polarization to either side of the central line. The polarization analyzer dyes oriented so as to reject the central line polarization called and to accept the deviated line polarization called . In this way light from the line source 30' in the polarization goes through the sample, is not absorbed by the sample whey the magnet is ON, but passed through the plower-Zion analyzer to the monochromator I and pho~omultiplier62. Light of the or polarization is deviated by the Polaris ration analyzer and does not reach the monochromator 60.
Since the spectral line from the line source I is narrower than the distance between the deviated lines when the '15 magnet is ON, the absorbency of the sample measured is essentially due to the non-atomic species of the sample and the signal on line 94 from the photo multiplier 62 is the background signal.
Synchronizer 92 is arranged to close switch 114 during this period. The signal it linearly integrated by capacitor 116 and amplifier 106 and supplied to log analog to digital converter 108 and then to the microcomputer 110. The sub-traction of the field ON and field OFF system is made and the result displayed on display 112.
The integrated background signal is also supplied on line 118 to an automatic gain control circuit 120 which has a response time of abut 100 milliseconds for a 60 Hertz magnetic field frequency. The output of the automatic gain control circuit controls a high voltage power supply 122 supplying high voltage on line 124 to the photo multiplier 62. Thus the photo multiplier 62 is caused to produce the same output signal or any background absorbency, greatly increasing its range of sensitivity. We have discovered that the high voltage supplies on line 124 is in fact the I
log of the background signal that is the background absvrbance and therefore such a signal may be provided by a stage divider 124 on line 128 9 digitalized by a linear analog to digital connector snot shown and supplied to the micro-processor 110.
As is normal in synchronous detection the integra~ingcapacitor 102 is referenced to ground before each measurement during the successive energizations of the magnet 56. This is accomplished by switch 130 which is energized when no I light is being received by the photo multiplier tune 62.
This is accomplished by turning lamp 130 off by meats of a supply signal on line 132 from synchronizer 92.
The electrical operation of the Zeeman background correction instrument illustrated in FIGURE 2 can be under-lo stood with reference to FIGURE 4. FIGURE PA shows a plot of the magnetic field 13q which is essentially unpiler except that it goes slightly negative between times 136 and 138 where the integral of the field between times 136 and 138 is substantially zero.
FUGUE 4B shows a plot of the signal utilized Jo turn the lamp 30' on and off supplied OX line 1320 As can be see, the lamp is Tory on curing maximum field and during minimum field periods.
FIGURE 4C is a plot of the operation of switch 130~ the dark time clamp, which it on during a period while the line source 30' is of, as illustrated in FIGURE 4B.
FIGURE ED is an illustration of the operation of the bac~grou~a or magnet ON switch 114 which is turned ON during the magnet ON period.
FIGURE YE is an illustration of the operation of the magnet OFF or sample plus background switch 100 which is closed during the magnet OFF period.
Those skilled in the art will understand that switches loo 11~ and 130 are ideal electronic switches controlled by the synchronizer 92 as indicated by the dotted lives.
~2~3~
FIGURE 5 it a series of plots provided by the readout device 112 under control of the microprocessor lo FIGURE
I The microprocessor 110 is the same microprocessor provided in the Perkin-Elmer model 5000 and may operate with S the same program. It is desirable however that rollover detection be provided when the output fed Jo the micro-computer 110 is from the Zeeman adapter 22, in order to prevent double valued readings.
FIGURE 6 is a series of plots taken at the spectral line 28~.3 nanometers utilizing a 10 micro liter sample having a background absorbency of 1. The samples were supplied to the furnace 42 of the Zeeman adapter 22 of the invention. Curve 136 was produced when the sample contained .05 micrograms per milliliter of lead. Plot 138 is the result of a screen having an absorbency of one. Plot 140 is the result of incorporating .5% sodium chloride in the 10 micro liter sample. Plots 142 an 144 are further runs with .5% sodium chloride. The final plot 146 is the result when .05 micrograms per milliliter of lead are incorporated in a I sodium chloride 10 micro liter sample. It should be noted that the lead is as easily recognized as when there was no sodium chloride present, as illustrated in plot 136.
FIGURE 6 is a series of runs also at 283.3 nanometers, of a 10 micro liter sample with a background absorbency of 1~7. At this level nearly 99% of the light passing through the sample is absorbed by the background Plot 148 is the result of a screen having an absorbency of lo 7. Plot 150 is of a run i;. which the sample included 1% sodium chloride as are plots 152, 154 and 156. Plot 158 was generated when .05 micrograms per milliliter of lead were incorporated in the 1% sodium chloride solution. Again it should be noted how easy it is to measure the absorbency Do the lead from plot 158, basically as easy as if there were no background, as in plot 136 of FIGURE 5.
~3~4~
FUGUE 7 is a series of plots also at 293.3 nanometers a 10 micro liter sample However these plots were derived from supplying the sample to the Perkin-Elmer model 40~0 atomic absorption spectrophotometer which is similar to the model 5000, utilizing conventional continuum source background correction. When .05 micrograms per milliliter of lead are incorporated it a I sodium chloride solution sample plots 160 and 162 were derived in separate 6 second runs. Note how much cleaner plot 146 of FIGURE 5 is when the Zeeman adapter is utilized.
Plots 164 and 166 ox FIGURE 7 are two runs in the Perkin-Elmer 4000 where the sample included .05 micrograms per milliliter of lead in a 1% sodium chloride solution.
Note that in plot 164 the lead absorbency is just barely greater than the background, whereas in plot 158 made on the Zeeman adapter the background is still greatly suppressed.
Thus the Zeeman adapter of our invention is able to measure the absorbency of elements when the background absorbanc~ is much greater than can ye done in prior art I instruments We have achieved this in an instrument with a magnetic field strength of 8 kilogauss. sigher fields would increase separation of thy and lines and thus increase insensitivity to background absorbency The polarization analyzer 50 of the invention is illustrated in detail in FIGURES 8 and 9. It may be menu-lectured of any birefringent material usable at toe spectral lines ox interest. Because it it desirable to use the --Zeeman adapter at the ultraviolet range from 190 nanometers to 850 nanometers, the material of the polarization analyzer I 50 is preferably artificial crystal quart or other transparent birefringent material suitable for use in this range, e.g.
crystalline magnesium fluoride, or sapphire.
_ I
A quartz analyzer is illustrated in FIGURES 8 and 9 and comprises a wedge 168 having an isosceles triangle cross section, and a pair of right triangular cross section wedges 170, 172 in optical contact therewith. The principal optical axis of element 168 is shown at 174. The principal optical axis of elements 170 and 172 is at right angles thereto and is illustrated at 176. Optical axis 176 is aligned with the magnetic field of thy electromagnet 56~
The length 178 of the polarization analyzer ED utilizing quartz having a principal optical index of refraction of 1.64927 and an extraordinary index of 1.6627 is 40 plus or minus 0.2 millimeters. As shown in FIGURE 9 the analyzer 50 is square and the side dimensions 180-180 are 22 plus or minus 0.2 millimeters. Since the wedge 168 cannot come to a knife point, the flat on the apex thereof has a dimension 182 ox about ~.50 millimeters.
As previously mentioned the analyzer 50 not only deviates the light from the furnace 42 which is polarized perpendicularly to the magnetic field, and passes the light which is parallel to the magnetic field, it also depolarizes the latter This depolarization occurs because light passing from wedges 170 and 172 into wedge 16~ after passing the bound Aries threaten, is rotated in its plane of polarization by the birefringent wedge 163 in proportion to the distance it travels within it. This occurs because birefringent material acts as a circular rotator on polarized light traveling along its principal axis and the analyzer 50 is made long enough for several 360 rotations Since various rays travel various distances, the exiting light include ~11 polarizations an is in effect depolarized, eliminating all polarization effects in he monochromator 60 and photo-multiplier 62.
I
If other forms of polarization analyzers are used, for example, dichroic sheet at visible light frequencies, the same result of monochr~mator insensitivity may be produced by using a depolarizer after the analyzer.
Another advantage of the analyzer SOD is that the angle in FIGURE B is one half of thaw in a conventional Russian analyzer Since the deviation of the rejected rays is inversely prDp~rtional to , great deviation is achieved White lengthening the analyzer along the optic axis with its accompanying restriction of the field of view end increased use of expensive material.
This analyzer constructed out of three wedges has four times the light throughput of a Russian analyzer constructed of two wedge of the same volume.
lo Those skilled in the art will understand that this depolarizing analyzer may be used in many other optical systems and in various spectrometers employing analyzers where it is desired to eliminate polarization effects at the monochromator.
It will thus be seen that the objects set forth above among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the above described element t constructions, and systems without departing from the scope of the invention, I it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted a illustrative and not in a limiting sense.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention, which, as a matte ox language, might be said to fall there between.
--US--
Claims (4)
1. In an atomic absorption spectrophotometer employ-ing time varying means for causing an electromagnetic optical effect for background correction and causing the spectrophoto-meter to provide time varying radiation, comprising: a radiation responsive device producing an output signal which varies in time and at certain times is proportional to the background absorption and wherein said radiation responsive device is responsive to a linearly increasing supply signal to produce an exponentially increasing output signal upon receiving a fixed radiation level and means for controlling said supply signal; the improvement comprising: means for supplying the background signal to said means for controlling said supply signal to said radiation responsive device whereby said supply signal is proportional to the log of the back-ground absorption, and means utilizing said supply signal as a measure of the background absorbance.
2. An atomic absorption spectrophotometer as defined in claim 1 further defined in comprising means responsive to said time varying means for providing a time varying signal corresponding thereto, a synchronizer responsive to said time varying signal to provide three switching signals in syn-chronism therewith, and a first switch responsive to a first switching signal from said synchronizer to reference the output signal from said radiation responsive device to ground during periods when said output signal is substantially zero, a second switch for supplying said output signal to said supply signal controlling means, said second switch responsive to a second switching signal from said synchronizer to do such during periods when said electromagnetic optical effect is at a maxima and the radiation received from said radiation responsive device is the result of background absorption;
analyzer means responsive to said background signal means; and a third switch for supplying said output signal to said analyzer means and responsive to a third switching signal produced by said synchronizer during the period when said electromagnetic optical effect is at its opposite maxima and the radiation received from said radiation responsive device is the result of sample and background absorption.
analyzer means responsive to said background signal means; and a third switch for supplying said output signal to said analyzer means and responsive to a third switching signal produced by said synchronizer during the period when said electromagnetic optical effect is at its opposite maxima and the radiation received from said radiation responsive device is the result of sample and background absorption.
3. The atomic absorption spectrophotometer defined in claim 2 wherein said analyzer means comprises integrator means to which said sample plus background signal and said background signals are provided alternately, an analog to digital converter to which the output of said integrator is provided, and a microcomputer to which the output signal of said analog to digital converter is supplied.
4. The atomic absorption spectrophotometer defined in claim 2 or 3, further comprising a source of atomic line radiation and an energization source therefore; said improvement further defined in that said synchronizer provides a fourth signal to said energization source to turn the source on only during periods when said electromagnetic optical effect is at its opposite maxima.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000492534A CA1213446A (en) | 1981-02-23 | 1985-10-08 | Atomic absorption spectrophotometer providing simply derived background absorbance measurement |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/237,199 US4457623A (en) | 1981-02-23 | 1981-02-23 | Atomic absorption spectrophotometer providing background correction using the Zeeman effect |
| CA000392016A CA1195857A (en) | 1981-02-23 | 1981-12-10 | Atomic absorption spectrophotometer providing background correction using the zeeman effect |
| CA000492534A CA1213446A (en) | 1981-02-23 | 1985-10-08 | Atomic absorption spectrophotometer providing simply derived background absorbance measurement |
| US237,199 | 1988-08-29 |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000392016A Division CA1195857A (en) | 1981-02-23 | 1981-12-10 | Atomic absorption spectrophotometer providing background correction using the zeeman effect |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1213446A true CA1213446A (en) | 1986-11-04 |
Family
ID=25669506
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000492534A Expired CA1213446A (en) | 1981-02-23 | 1985-10-08 | Atomic absorption spectrophotometer providing simply derived background absorbance measurement |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1213446A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112729543A (en) * | 2020-12-18 | 2021-04-30 | 上海安杰环保科技股份有限公司 | Background correction method, equipment and medium for narrow-band absorption in optical detection |
-
1985
- 1985-10-08 CA CA000492534A patent/CA1213446A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112729543A (en) * | 2020-12-18 | 2021-04-30 | 上海安杰环保科技股份有限公司 | Background correction method, equipment and medium for narrow-band absorption in optical detection |
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