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.

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.

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.