9.1 Target Concentrations for General Purpose Analysis

• The HP 6890/5972 performance standard requires that the instrument produce a signal-to-noise ratio (S/N) of greater than or equal to 10:1 for 10 pg of hexachlorobenzene injected onto the column. If a 60:1 split injection is used, the actual amount deposited onto the column is approximately 10 $^{\textrm{-2}}$ of the amount injected. If 1 $\mu$L is injected, the solution concentration would have to be greater than or equal to 1 ng/$\mu$L to detect hexachlorobenzene with the recommended S/N. This is the taken as the lower limit target concentration for 60:1 split methods, such as method GEN1.M.
• Though some detectors do not have an upper limit target concentration, the mass spectrometer does. If too much sample is injected,
  • the vacuum system may be compromised, causing oil contamination, filament failure or detector failure.
  • the detector may become saturated, which can shorten detector life or cause failure.
  • the amplifier or analog to digital converter may become saturated, which will make ion relative abundance measurements meaningless.
  • chromatographic separation efficiency decreases due to column overloading.
• The problems associated with injecting too much sample can be avoided for the 60:1 split methods, such as GEN1.M, by keeping the amount of sample injected less than 500 ng.