Note: Descriptions are shown in the official language in which they were submitted.
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.
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
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
--5--
~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
, - .
--6--
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
--7--
~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~
-8-
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.
--10--
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.
--13--
~%~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
. .
-14-
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
-15-
~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.
-16-
~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
-17-
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
-20-
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.
-21-
~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.
-22-
~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.
-2
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.
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