Technology

Accelerator Mass Spectrometer (AMS)

Drug developers currently employ a number of bioanalytical platforms to evaluate pharmacokinetic (PK), biodistribution and metabolism of new drugs candidates. These include isotope-tagged liquid scintillation counting (LSC) methods, HPLC, and more recently, LC/MS/MS. Accelerator Mass Spectrometry (AMS) is a highly-sensitive instrument designed to quantify the amount of isotope present in a sample. It is used in drug development to detect trace amounts of ¹⁴C-labeled compounds in biological samples.

AMS is a type of isotope ratio mass spectrometry (IRMS) developed 25 years ago to directly count individual ions of very rare isotopes. Acceleration of the ions to million electron volt energies was the key to collisional destruction of any molecular isobars while supplying ions of sufficient energy for identifying the resulting atomic ions by their characteristic interactions with nuclear particle detectors. AMS is so sensitive that it is only used for isotopes having concentrations of parts per billion to parts per quintillion in the isolated elemental sample. Recently, AMS instruments have been developed to quantify long-lived radioisotopes that are particularly suitable for pharmaceutical research, particularly ¹⁴C, at natural concentrations independently of their decay properties ^1,2,3.

AMS - the Ultrasensitive Bioanalytical Platform

Accelerator Mass Spectrometry (AMS) is the platform of choice when extreme sensitivity is required for early stage drug development. AMS quantifies the amount of radiocarbon-labeled compound in a biological sample with attomole (10^-18 M) sensitivity. Traditional LSC methods only count the isotopes that decay during the detection period. In contrast, AMS counts every radioisotope present in the sample, whether it decays or not. Since radiocarbon is relatively stable (5,730 half-life), there is a 10⁶-fold increase in sensitivity compared to LSC.

AMS traces very low doses of compounds (micrograms) using extremely low radiation (<100 nanoCurie) in animal models and human subject. Absorption, metabolism, distribution, binding, and elimination are all quantifiable with high precision after appropriate sample definition.

System Description

The National Electrostatics Corporation (NEC) Model 1.5SDH-1 Pelletron Accelerator is a dual acceleration (tandem) electrostatic accelerator. This accelerator along with the other items described in this proposal form an Accelerator Mass Spectrometry (AMS) System especially constructed to measure the amount of¹⁴C in small modern graphite samples to a design aim precision of 0.5% or better. This system employs the fast, 10 Hz, sequential energy pulsing technique first used in the early 1980's by the group at Eidgenössische Technische Hochschule, ETH, Zurich.

In this system a beam of C- ions is produced by bombarding the cool cesiated surface of a graphite sample with about 5 keV Cs+ ions. The C- beam produced by the sputtering of the sample by the Cs⁺ beam is accelerated, focused, and mass analyzed into mass 14, and 13 amu beams.

These beams are then accelerated to high energy in sequence by successively changing their energy as they pass through the mass analyzer so that they are on the correct trajectory for transmission into the 1.5SDH-1 Pelletron accelerator. The energy changing sequencer is adjusted about 10 times a second so that about 1 part in 10³ of the mass 13 beam, and 99.9% of the mass 14 beam passes into the accelerator keeping average accelerated and beam loading currents very low and X-rays produced directly or indirectly by high energy ions also very low. The beam of negative ions is 500 keV in energy when it reaches a region of relatively high argon gas pressure, the stripper canal, located in the high voltage terminal of the 1.5SDH-1 Pelletron. The fast moving negative ions lose electrons and become predominantly C⁺ ions when passing through the stripper canal. Also critical to the AMS process, negative molecular ions such as CH^- and CH^-₂ are broken into C⁺ⁿ and H⁺ ions by the argon gas. This eliminates interferences that might be caused by molecular ions when counting ¹⁴C⁺ ions later in the system.

The charge 1 positive ions are accelerated from the high voltage terminal to ground gaining an additional 0.5 MeV in energy for a total of about 1 MeV. The ions are magnetically deflected and focused at 90° by the analyzing magnet so that the pulses of ¹³C⁺ can be separated from the ¹⁴C⁺ and measured in a Faraday cage. The ¹⁴C⁺ ions and a small number of ¹²C⁺ or ¹³C⁺ ions from the molecular breakup in the terminal that have changed charge state at exactly the right places in the accelerating tube so that their energy is enough greater than the ¹⁴C⁺ ions to be transmitted around the 90° magnet are then allowed to pass into a 90° electrostatic spherical analyzer (ESA) which deflects the faster ¹²C⁺ and ¹⁴C⁺ ions away from the ¹⁴C⁺ ion beam path. The ESA also provides a final focusing so that the ¹⁴C⁺ ions are transmitted to a solid state detector where they are counted. By recording the ¹³C current and ¹⁴C counts as known and unknown samples are sputtered, the amount of ¹⁴C present in a sample is determined to high accuracy.

With appropriate corrections for how the ¹⁴C came to be in the sample, the years of radioactive decay and a chronological age can then be found. With an optional item, ¹²C ion current will also be sequentially injected into the accelerator and measured.

Core AMS Facility

Accium BioSciences is the first company to operate a commercial bioanalytical AMS facility in the U.S. At the core of Accium’s facility is a National Electrostatics Model 1.5SDH-1 Pelletron Accelerator, a dual acceleration (tandem) electrostatic accelerator. This accelerator along with other associated items form an AMS system especially constructed to measure the amount of ¹⁴C in small biological samples.

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References

(1) Vogel, J.S., 2005. Accelerator mass spectrometry for quantitative in vivo tracing. BioTechniques 38:S25-S29 (June 2005)

(2) Nelson, D.E., R.G. Korteling, and W.R. Stott. 1977. Carbon-14: direct detection at natural concentrations. Science 198:507-508.

(3) Bennett, C.L., R.P. Beukens, M.R. Clover, H.E. Gove, R.B. Liebert, A.E. Litherland, K.H. Purser, and W.E. Sondheim. 1977. Radiocarbon dating using electrostatic accelerators: negative-ions provide key. Science 198:508-510.