Applied Biosystems ABI 4000 LC-MS-MS
Overview -
The Applied Biosystems (ABI) LC-MS-MS is a modern, research-grade, instrument designed for the separation and identification of the componenets of a mixture of organic compounds. Since it is capable of separating a large number of components, as well as observing a high mass range (2800 amu) the ABI LC/MS/MS is particularlly well suited to analyzing large molecules, such as protiens.
Theory -
As with all hypenated techniques, the LC-MS is actually two instruments in one: a Liquid Chromatograph ("LC"), which separates the components of the sample, and a Mass Spectrometer ("MS") which is used to identify the separated components.
The ABI API 4000 uses the Tempo nanoMDLC binary pump system for separating the constituent components of the sample. An Eksigent Autosampler loads the samples onto the LC, and allows for unattended operation.
Standard Operating Procedure
Methods -
As the separated componnts of the sample elute off of the LC, they enter the MS portion of the instument. In most LC applications, the components exist in the sample in an uncharged state. The components of the sample must be ionized before entering onto the MS. Depending of the sample size and the structure of the analytes, either Electrospay Ionization (ESI) or Atmospheric Pressure IOnization (API) can be used to effect the ioniztion. After ionization, sample is passed into the entrance of the MS.
The ABI 4000 contains 4 distinct quadrapoles, along with a series of lenses, which can be engaged and configured to provide several different types of anlysis. The physical layout of the quads and lenses is shown in this figure.
Detection Limits -
The Detection Limit for the PS200 II is in the low parts-per-trillion range. With other analytical techniques, this might be problematic, as samples with higher mercury concentrations would need to be diluted to bring the analyte into the concentration range corresponding to a linear absorbance response. With the PS200 II, though, the amount of sample analyzed can be decreased, and/or the flow rate of the stripping gas can increased, decreasing the amount of ground state mercury ending up in the absorbance cell. This procedure allows mercury concentrations into the parts-per-million to be determined.
Sample Matrix -
The sample must be in a liquid matrix. Samples with a high salt (buffer) concentration are hard on the instrument. Any samples with an appreciable salt concentration must be passed throug a desalting trap before injection onto the LC column. Depending on the speciics of the sample and the instrument operating parameters, the use of the desalter may affect the attainable detection limits.
Sample Size -
Typically, 2-3 uL of sample is required for each replicate analysis, but the autosampler requires a minimum of about 200 uL in the vial for the sampling needle to reach the sample.
Approximate C0st -
The cost matrix for the LC-MS is still being established.