U.S. patent number 3,885,155 [Application Number 05/411,801] was granted by the patent office on 1975-05-20 for mass spectrometric determination of carbon 14.
This patent grant is currently assigned to Stanford Research Institute. Invention is credited to Michael Anbar.
United States Patent |
3,885,155 |
Anbar |
May 20, 1975 |
Mass spectrometric determination of carbon 14
Abstract
A determination of the amount of carbon 14 in a substance is
made simpler and more accurate by introducing the carbon 14 in the
form of CO.sub.2 into a duoplasmatron negative ion source for a
mass spectrometer together with .sup.15 N.sub.2 producing CN.sup.-,
as the predominate ion. The spectrometer can then accurately detect
the amount of carbon 14.
Inventors: |
Anbar; Michael (Palo Alto,
CA) |
Assignee: |
Stanford Research Institute
(Menlo Park, CA)
|
Family
ID: |
23630394 |
Appl.
No.: |
05/411,801 |
Filed: |
November 1, 1973 |
Current U.S.
Class: |
250/283;
250/288 |
Current CPC
Class: |
H01J
49/288 (20130101); H01J 49/126 (20130101) |
Current International
Class: |
H01J
49/12 (20060101); H01J 49/28 (20060101); H01J
49/26 (20060101); H01J 49/10 (20060101); H01j
039/28 () |
Field of
Search: |
;250/281,282,283,288 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Characteristics of a Low Energy Duoplasmatron Negative Ion
Source," Aberth et al., Rev. of Sci. Inst., June 1967, pp.
745-748..
|
Primary Examiner: Borchelt; Archie R.
Assistant Examiner: Anderson; B. C.
Attorney, Agent or Firm: Lindenberg, Freilich, Wasserman,
Rosen & Fernandez
Claims
What is claimed is:
1. A method of measuring carbon 14 content in a sample
comprising
converting carbon in said sample to CO.sub.2,
mixing said CO.sub.2 with .sup.15 N.sub.2,
converting said mixture of CO.sup.2 and .sup.15 N.sub.2 into
CN.sup.- ions having different masses including mass 29,
separating said mass 29 CN.sup.- ions from the ions having other
masses, and
counting the number of mass 29 ions which have been separated from
which count the concentration of carbon 14 in said sample can be
determined.
2. A method as recited in claim 1 wherein said step of converting
said mixture of CO.sub.2 and .sup.15 N.sub.2 into CN.sup.- ions
comprises introducing said mixture into a duoplasmatron negative
ion source.
3. A method as recited in claim 1 wherein said CN.sup.- ions having
different masses including mass 29 include ions having masses 27
and 28,
said method further includes the steps of separating said ions
having masses of 27 and 28 from each other, and
counting the number of ions having a mass of 27 and a mass of
28.
4. A method of determining the concentration of .sup.14 C in a
specimen comprising
converting the carbon in said specimen to CO.sub.2,
introducing said CO.sub.2 together with .sup.15 N.sub.2 into a
plasma negative ion source, to form CN.sup.- ions having different
masses including masses 27, 28 and 29,
separating said CN.sup.- ions having said different masses into
separate ion beams respectively containing CN.sup.- ions 27, 28 and
29, and
counting the number of ions in said separate ion beams with masses
27, 28 and 29, from which counts the concentration of .sup.14 C in
said sample can be determined.
Description
BACKGROUND OF THE INVENTION
Radiobiological effects of radio isotopes make them undesirable as
tracers in biological systems. The use of stable isotopes as
tracers is therefore more desirable but, their use is limited by
the background of their natural abundance, making detection
extremely difficult if not impossible. Although, in many cases this
limitation can be overcome by the use of multi-labeled molecular
tracers, the latter cannot be used for a large number of small
molecules, e.g., glycine, acetic or lactic acids, or for
degradation studies where the products are small molecules, for
example, CO.sub.2, formaldehyde, acetadehyde, or methylamine.
.sup.14 C is the ideal and only practical tracer for such cases,
only that the microcurie doses of such a long lived radioisotope
make its use in routine clinical tests highly undersirable.
The same system as described above can also be used for radiocarbon
dating if desired.
OBJECTS AND SUMMARY OF THE INVENTION
An object of this invention is to provide an analytical procedure
that gives a more precise determination of .sup.14 C content than
is available heretofore.
Another object of this invention is to provide an analytical
procedure that enables a greater sensitivity in the determination
of .sup.14 C content heretofore available.
Yet, another object of this invention is to provide a radiocarbon
assay technique which provides less extensive pre-treatment and
chemical processing than is required by present techniques.
Still another object of the invention is to provide a radiocarbon
assay technique for .sup.14 C which requires smaller amounts of a
sample and takes significantly less time than heretofore
possible.
These and other objects of the invention are achieved by measuring
the .sup.14 C content of a sample in the form of CN.sup.- rather
than in the form of CO.sub.2 ions. This is accomplished by
introducing a mixture of CO.sub.2, derived from a sample desired to
be analyzed, and into a plasma negative ion source such as
duoplasmatron negative ion source. This produces CN.sup.- as the
predominate carbon containing ion. In the highly intense and hot
plasma of the ion source, CO.sub.2 and N.sub.2 are atomized and the
CN radicals formed are readily converted to CN.sup.-, owing to the
high electron affinity of CN. The other ionization products formed
are of O.sup.-, NO.sup.- and NO.sub.2 .sup.-. Because N.sub.2
.sup.- is energetically unstable, it is not formed. Included among
the ionic peaks which are formed is .sup.14 C .sup.15 N (mass 29).
Using .sup.15 N.sub.2 predominant peaks which are formed have mass
27 and 28, with mass 29 uniquely representing the abundance of
.sup.14 C. The background of mass 29 in the negative ion spectrum
is nill, unlike the positive ion spectrum counterpart where
fragments of hydrocarbons and their oxidation products contribute
to the mass 29 peak, making detection inaccurate. Therefore, the
duoplasmatron negative ion spectrometer readily provides
information whereby the amount of .sup.14 C present in the original
sample can be determined.
The novel features of the invention are set forth with
particularity in the appended claims. The invention will best be
understood from the following description when read in conjunction
with the accompanying drawing which illustrates the use of a
negative ion mass spectrometer, in accordance with this invention,
for determining the amount of carbon 14 in a sample.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preparation of a sample for analysis, in accordance with this
invention, utilizes standard and well known techniques. The carbon
sample which is desired to be analyzed is converted to CO.sub.2 by
any well known technique such as by burning in the presence of
oxygen or mixing with copper oxide and then burning. The CO.sub.2
is then mixed with .sup.15 N.sub.2 and thereafter is introduced
into a duoplasmatron negative ion source. In the highly intense and
hot plasma of this ion source, CO.sub.2 and N.sub.2 are atomized
and the CN radicals formed are readily converted into CN.sup.-,
owing to the high electron affinity of CN.
The other ionization products which are formed are O.sup.-,
NO.sup.-, O.sub.2 .sup.-, and NO.sub.2 .sup.-. Because N.sub.2
.sup.- is energetically unstable, it is not formed.
Now, the ionic peaks which are formed as a result of mixing the
CO.sub.2 and N.sub.2 are .sup.12 C .sup.14 N (mass 26), .sup.13 C
.sup.14 N and .sup.12 C .sup.15 N (mass 27), .sup.14 C .sup.14 N
and .sup.13 C .sup.15 N (mass 28) and .sup.14 C .sup.15 N (mass
29).
The N.sub.2 which is mixed with the CO.sub.2 is highly enriched in
.sup.15 N (greater than 98 percent .sup.15 N, is commercially
available at about 50 cents per milliliter). The predominant peaks
which will be formed have masses 27, 28 and 29 which latter mass
uniquely represents the abundance of .sup.14 C. Under these
conditions, the desired .sup.14 C/.sup.12 C isotope ratio is
measured by comparing mass 29 and 27. The contribution of .sup.13 C
.sup.14 N to mass 27 is less than 2 .times. 10.sup..sup.-4, and its
range of variation is only less than 1 .times. 10.sup..sup.-5 ;
thus, it does not affect the precision desired, which is on the
order of 0.1 percent, in the assessment of .sup.14 C present. The
small amount of .sup.14 N accompanying .sup.15 N lowers the overall
detection sensitivity of .sup.14 C by a proportional amount (about
2%) without affecting the precision of the determination of the
.sup.14 C/.sup.12 C isotope ratio. It should be noted that there is
not background of mass 29 in the negative ion spectrum, unlike the
positive ion spectrum counterpart where fragments of hydrocarbon
C.sub.2 H.sub.5 .sup.+ and their oxiditation products (CHO.sup.+)
contribute to the mass 29 peak.
Referring now to the drawing, it is a schematic illustration of one
type of mass spectrum showing how it may be used in accordance with
this invention. The carbon sample, which is to be analyzed, is
oxidized to CO.sub.2 by conventional microanalytical techniques.
This is here represented as a "source of CO.sub.2 ", 10. It is
introduced into the negative ion duoplasmatron source 12, of the
mass spectrometer 14, along with an appropriate amount of .sup.15
N.sub.2, from a source 16. The CN.sup.- ions are extracted and
formed into a keV beam. The relatively high energy of the beam (10
to 20 keV) is needed to obtain a high transmission with the
required high resolution. By differential pumping and the use of
high vacuum techniques, the background pressure and the mass
deflection chamber can be reduced to 10.sup..sup.-9 torr. The ion
beam is electrostatically deflected by deflector 18, into the mass
separator chamber 20.
Fast moving neutral atoms and molecules formed primarily in the
high pressure region of the ionization source by charge exchange of
the beam ions with background gas, continue undeflected and are
thus prevented from entering into the mass separation chamber.
The mass separation is accomplished by means of a Wien velocity
filter 22. This commercially available device achieves mass
resolution by applying uniform magnetic and electrostatic fields
that are normal to each other and to the ion beam. Ions with a
predetermined velocity have equal but opposite electric and
magentic forces acting on them and consequently continue
undeflected. Those ions with a lesser or greater velocity will be
deflected in the plane of the electrostatic field. Since, for a
given energy, the beam velocity is proportional to m/2, the Wein
filter provides mass selection.
The Wien filter is adjusted to deflect mass 29 (.sup.14 C .sup.15
N.sup.-) off axis to prevent neutrals from accompanying the beam
and to increase the mass resolution. The off axis beam from mass 29
is represented by the line 30. Mass 28 and 27 (.sup.13 C .sup.15
N.sup.- and .sup.12 C .sup.15 N.sup.-) are deflected and detected
simultaneously in two Faraday cup collectors, respectively 31 and
32. The mass 27 and 28 beams are indicated by the lines,
respectively 34,36. The neutral beam component is represented by
the line 38. The magnetic deflector 22 separates ion species
according to momentum and thus mass 27 or 28 ions that have
scattered into the magnetic field are deflected by a greater angle
than mass 29 ions.
An interceptor 40, blocks the neutral ion beam component from
entering into the chamber of the mass spectrometer where the
desired beam 30 enters through an opening in the interceptor 40.
The ion beam 30 which passes through the opening in the interceptor
40 is then deflected again by a magnetic deflector 42 into an
electron multiplier arrangement 44 which amplifies the ions, in the
beam, converts them to electrical pulses and applies the pulses to
a counter 46 to be counted.
The Faraday cup collectors, respectfully 31 and 32, of mass 27 and
28 ion beams, are connected through the respective thermostatically
compensated resistors 48 and 50, to ground. The voltage developed
at the top of these resistors, in response to the collection of
ions, are applied to the respective voltage to frequency converters
52,54. The output of the respective frequency converters is applied
to frequency counters, respectively 56,58. Thus, the system
described produces a total of the .sup.14 C .sup.15 N ion counts,
integrated by a scaler, 46, as well as the integrated secondary
counts of masses 27 and 28. From this information, the isotope
ratios .sup.14 C/.sup.12 C and .sup.13 C/.sup.12 C can be
calculated, after correction for any background from mass 28. From
these ratios, using the appropriate half life value for .sup.14 C
and any necessary corrections for .sup.13 C fractionation, the data
of this sample can be determined.
There has accordingly been described and shown here and above a
novel and useful system for measuring the .sup.14 C content of
carbon in the form of CN.sup.- using a negative ion duoplasmatron
mass spectrometer. This uniquely performs measurements using a
biatomic ionic species that can be produced with a high yield and
in which the second single atom is an enriched heavy isotope with
no heavier isotopes in existance. Further, operation is conducted
in a negative ion region where there is no other negatively charged
species with a mass equivalent to that of .sup.14 C .sup.15 N.
* * * * *