A small accelerator mass spectrometer with a gas chromatographic inlet interface
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A small accelerator mass spectrometer with a gas chromatographic inlet/interface. Barbara Hughey, Bob Klinkowstein, Ruth Shefer Newton Scientific, Inc. Paul Skipper, John Mehl, Pete Wishnok, Steve Tannenbaum MIT Division of Bioengineering and Environmental Health. 10 μg carbon.

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A small accelerator mass spectrometer with a gas chromatographic inlet interface

A small accelerator mass spectrometerwith a gas chromatographic inlet/interface.

Barbara Hughey, Bob Klinkowstein, Ruth Shefer

Newton Scientific, Inc.

Paul Skipper, John Mehl, Pete Wishnok, Steve Tannenbaum

MIT Division of Bioengineering and Environmental Health


Comparing ams to scintillation counting

10 μg carbon

6 X 105 atoms 14C

1 attomole 14C

(t= 5730 years)

ß-

AMS

scintillation counter

1000 counts

in

14 years

1000 counts

in

2 minutes

Comparing AMS to scintillation counting

AMS counts the number

of atoms in tbe sample,

while scintillation

counters measure the

infrequent radioactive

decay events in the sample.


Applications of accelerator mass spectrometry

Applications of accelerator mass spectrometry

AMS can be used for any experiment that is currently done by scintillation counting, but is faster and requires much less radioactivity.

Drug development

Toxicology (low-level, I.e., ambient dose-response)

Human metabolism and distribution

Trace analysis by post-labeling

Geochemistry - radiocarbon dating

  • Experiments can be done on humans

  • Minimum precautions needed during synthesis of reagents

  • No regulations involved in disposal


Why is accelerator mass spectrometry not widely used

Why is accelerator mass spectrometry not widely used?

1. Current instruments are generally large and expensive, and often require

dedicated facilites and an operational staff.

2. Sample preparation is time-consuming and skill-intensive.


Schematic of a gc ams system

gas

chromatograph

oxidizer

gas-fed

ion-source

detector

  • GC:

  • Oxidizer:

  • AMS system:

sends pure compounds into the oxidizing interface.

converts these compounds into a chemical form amenable to gas-fed ion-source.

measures the amount of carbon-14 in the sample.

Schematic of a GC-AMS system

complex

organic

molecules

C-

and other

negative ions

CO2

14C2+

AMS

accelerator system

with

low- and high-

energy analyzers

AMS accelerator and detection system

sample separation and injection interface


Q why can such low levels of 14 c be quantitated

Q. Why can such low levels of 14C be quantitated?

A. Because the natural abundance is also low.

With a low backround, anything much above

background can be measured; there is very little interference.

This is especially useful in experiments with samples

that have been enriched with carbon-14 since very

little enrichment is needed in order to have detectable signal.


Q but why does it take a complex expensive system to quantitate the 14 c

Q. But why does it take a complex, expensive system toquantitate the 14C?

A. Because - even though there is very little 14C relative to 12C,

there is a very high abundance of other substances with very

similar atomic or molecular weights:

12CH213CH N

A combination of negative-ion formation followed by

high-energy collisions with gas or thin foils eliminates

interference from these substances.


Schematic of the nsi mit gc ams

Schematic of the NSI/MIT GC-AMS

Cesium Sputter

Negative Ion Source

People standing around

Oxidizer

GC

Detector

High Energy

Analyzing

Magnet

Low Energy

Analyzing

Magnet

Low Energy Accelerating Tube

Electrostatic analyzer

Carbon Stripping Foil Carousel

High Energy Accelerating Tube


Oxidation and ionization

Oxidation and ionization

The CO2 (and ambient nitrogen)

is converted to negative ions by

bombardment with high-energy Cs ions.

16O-

Cs sputter

source

oxidizer

12C-

sample

CO2

13C-

CuO

750oC

14C-

13CH-

12CH2-

N-

As each compound elutes from the GC,

it’s converted to CO2 by an on-line oxidizer.

N- is unstable, and decomposes.


Oxidation and ionization1

Oxidation and ionization

The CO2 (and ambient nitrogen)

is converted to negative ions by

bombardment with high-energy Cs ions.

16O-

Cs sputter

source

oxidizer

12C-

sample

CO2

13C-

CuO

750oC

14C-

13CH-

12CH2-

As each compound elutes from the GC,

it’s converted to CO2 by an on-line oxidizer.

N- is unstable, and decomposes.


Isolation of the 14 da isobars

Isolation of the 14 Da isobars

16O-

The negative ions of higher and lower weight are easily

removed with a low-energy magnetic sector, sending only

the 14-dalton substances into the accelerator.

12C-

13C-

magnet 1

14C-

13CH-

12CH2-

14C-

13CH-

12CH2-

16O-

12C-

13C-


Conversion to atomic ions

Conversion to atomic ions

accelerator/stripper

14C-

13CH-

12CH2-

14Cn+

13Cn+

12 Cn+

The isobars are then accelerated to(a maximum of) 1 MEV and collided

with a thin foil.

  • Polyatomic structures are destroyed.

  • Some electrons are stripped, leaving positive ions.


Detection of 14 c

Detection of 14C

electrostatic sector

magnetic sector

14Cn+

13Cn+

12 Cn+

14Cn+

These ions - now with different m/z values - are brought down to ground

potential and sent through an electrostatic analyzer and a high-energy magnetic

sector to send (finally) only carbon-14 into the detector.


Our original test mixture

Our original test mixture.

These were chosen because they were handy, because they were aromatic and

thus potentially difficult to oxidize, and because they contained a variety of heteroatoms.

2

acetanilide

C8H9NO

3

diethylphthalate

C12H14O4

4

4-chlorodiphenyl ether

C12H9ClO

1

methyl phenyl sulfide

C7H8S

5

benzophenone

C13H10O

6

9-fluorenone

C13H8O

7

phenanthrene

C14H10


Online oxidation of organics to co 2

7

5

6

4

3

1

2

m/z = 44

4.00

5.00

6.00

7.00

8.00

Retention time

Online oxidation of organics to CO2

In this experiment, the previous mixture was separated by capillary GC and then

sent through the CuO oxidizer into a small mass spectrometer that was set to

detect only m/z 44. The number of carbons in the mixture was the same for each

component. The oxidizer cleanly and efficiently converted each component to CO2.


Conversion of co 2 to c

350

CO2

CO2

CO2

CH4

CO2

CO2

CH4

CH4

CH4

CH4

300

12C- current (μA)

250

each injection = 100 pmoles

200

0

1

2

3

4

5

6

Time (min)

Conversion of CO2 to C-

In this experiment, alternate injections of CO2 and methane were flow-injected

through the oxidizer and into the Cs sputter ion source. The methane was

quantitatively converted into CO2, giving essentially identical C- signals for

each substance.


New test mixture old gc column

New test mixture, old GC column.

A new version of the test mixture was prepared without methylphenyl sulfide.

(It decomposed, and it smelled bad.) This experiment is identical with the earlier one

using the small mass spectrometer as a CO2 detector. The oxidizer is working well,

but the chromatography has deteriorated - peaks 3 and 4 are no longer resolved.

3

4

7

5

6

2

m/z = 44

3.00

4.00

5.00

6.00

Retention time


Online conversion of co 2 from organic molecules into c

12C- current (nA)

110

3+4

105

100

5

7

95

6

90

2

85

80

75

70

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

Time (min)

Online conversion of CO2 from organic molecules into C-

The previous test mixture was separated by capillary GC, sent through the CuO

oxidizer and through the Cs sputter ion-source. The negative ion current from

carbon-12 was detected by a Faraday cup after the low-energy magnet. The

chromatographic peak shapes are acceptable.


Detection of a 14 co 2 pulse

Carbon-14 window

Mass-13 Faraday cup

Detection of a 14CO2 pulse.

Enriched CO2 was flow injected into the Cs sputter ion source, carbon-13 was detected

by a Faraday cup after the low-energy magnet and carbon-14 was detected as positive

ions at the end of the entire AMS system with essentially no memory effect.

1400

0.6

1200

0.5

1000

Counts in 14C window

Mass-13 negative ion current (μA)

800

0.4

600

0.3

400

200

0.2

0

0

10

20

30

40

50

Time (sec)


Summary

Summary

  • We’ve shown so far that:

  • The oxidizing interface works;

  • The chromagraphy is pretty good;

  • The ion-source works;

  • The accelerator/stripper works;

  • The device transmits 14C.

  • I.e., that all the parts are in place.

  • What we’ll do next:

  • Optimize 14C ion transmission;

  • Characterize the complete GC-AMS system;

  • Finish development of LC-AMS interface;

  • Run some real samples.


Thanks

Thanks

Tom Doucette, Dennis Clarke, and Andrew Dart are NSI engineers who’ve

helped with design and construction.

Naomi Fried and Kaisheng Jiao were postdoctoral fellows who helped with the early

exploratory experiments.

Financial support has been primarily through small business grants from

The National Institutes of Health and the National Science Foundation


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