The bioAMS resource enables rapid analysis of liquid samples, such as plasma and urine, as well as biological samples suspended in liquid. The ability to directly analyze liquid samples offers researchers several benefits. For example, it:
- Is a good fit for biomedical research, as most biochemical samples measured by Accelerator Mass Spectrometry (AMS) are already in liquid form or can be readily suspended or dissolved in appropriate solvents or buffers.
- Significantly reduces the time needed to prepare samples for AMS analysis from days to minutes (e.g., much faster than converting solid samples to graphite, which can take up to 3 days).
- Enables analysis of extremely small sample sizes (e.g., picograms), making it possible to accurately quantify low-abundance metabolites in metabolite profiles.
- Can be coupled with a high-performance liquid chromatography (HPLC) instrument to separate the components of a mixture prior to AMS analysis, or to enable sample analysis by both AMS and traditional mass spectrometry.
How it works
Following sample preparation, the carbon content of samples suspended or dissolved in a liquid is rapidly converted to carbon dioxide gas, which is directly transported to a gas-accepting ion source for real-time AMS analysis. The steps in this process include:
- A liquid sample interface automatically deposits samples from a vial onto a moving wire made of high-purity nickel.
- Before the wire receives any sample, it passes through a cleaning oven to remove surface carbon and create a nickel oxide coating. The coating enables better adherence of fluids and minimizes conversion of trace amounts of carbon within the bulk wire material to carbon dioxide during sample combustion.
- Periodic indentations in the wire attract fluid, ensuring that when liquid samples are deposited onto the wire, they attach to the indentations as droplets that are virtually equal in size.
- The moving wire passes through a drying oven in order to evaporate the liquid carrier, and then it passes through a combustion oven to convert the carbon content of the dried sample to carbon dioxide gas.
- The combustion oven is plumbed so that 100% of the gaseous combustion products are directed in a helium stream to an exit capillary, coupled to a cesium sputter, gas-accepting ion source for carbon-14 analysis by the AMS instrument.
Once inside the AMS instrument, magnetic and electrostatic fields separate the different carbon isotopes according to their nuclear charge and mass. Interfering molecular isobars of carbon-14 are destroyed. The amount of carbon-14 is measured relative to a more abundant isotope (e.g., carbon-13 or carbon-12). The AMS instrument counts individual rare isotope atoms using an ion beam. Absolute quantification comes from comparing the sample’s measured isotope ratio to that of measured standards of known ratios that are traceable through the National Institute of Standards and Technology. It takes only 60 seconds from the time a sample is deposited on the moving wire until its carbon-14/carbon-12 ratio is measured.
In addition, the HPLC instrument can be used to separate liquid samples into constituent compounds based on chemical characteristics, such as relative polarity, and deliver a fine stream of eluent to the moving nickel wire. The HPLC-AMS approach is very efficient for performing metabolism studies since the analysis is as rapid as the HPLC elution, avoiding fraction collection and analysis of discreet solid samples. Another option is to split the elution from the HPLC, and then feed one sample stream into the AMS instrument and feed the other stream into a conventional mass spectrometer.
