Elimination of aliasing in LA-ICP-MS by alignment of laser and mass spectrometer†

Insufficient sampling of periodic signals results in an unwanted phenomenon known as aliasing. When measuring by LA-ICP-MS it is widely observed that aliasing between laser pulse rates and sampling by sequential ICP-MS instruments creates erroneous variations in the measured element concentrations in the sample. Smoothing the sample flow to the ICP-MS can largely eliminate this variation but reduces spatial resolution of the time-resolved signal and is thus detrimental for imaging with the increasingly popular fast response sample cells. We have developed a fire control circuit that fires the laser in alignment with the measurement cycle of the mass spectrometer to lessen or eliminate aliasing. We describe the device and show that with a conventional quadrupole ICP-MS the technique is able to maintain measurement precision when the extent of mixing between individual laser pulses is reduced by an order of magnitude.