Generation of a square wave inductively coupled plasma scanning mass spectrometry signal using electrothermal vaporization sample introduction

A device was designed to convert the typical skewed gaussian shaped signal from an inductively coupled plasma mass spectrometer (ICP-MS) using electrothermal vaporization (ETV) into a square wave signal. The basis for such a transformation is centered on the assumption that the aerosol particles produced from ETV are aerodynamically small, and thus, have diffusive properties similar to gases, which makes them easy to mix and resistant to loss by settling. The ETV square wave signals were generated through the use of a stainless steel cylinder equipped with a motor driven piston assembly, which was used to trap the transient ETV signal and deliver a uniform density aerosol to the ICP-MS. The length of the signal was controlled by the rate of travel of the piston during expulsion of the sample aerosol. A constant flow rate was achieved and maintained using a pressure sensor feedback to the motor driving the piston. The system was evaluated by quantitative full mass scans on a variety of signal shapes including square wave signals, normal ETV signals, and broadened ETV signals generated with the use of a single bead string reactor (SBSR). The square wave signals yielded precisions for quantitative full mass scans of better than 10% RSD for most metals compared to approximately 15% for SBSR broadened signals and 40% for normal signals. The improved precision was attributed to an increased duty cycle, but the increase did not become significant until more than 100 masses were monitored due to the relatively small amount of "temporal overhead time'' (fly-back time, amplifier settling time, etc.) relative to data collection periods. Because the ETV generated aerosol consists of aerodynamically small particles, there was only a minimal decrease (<2-11%) in the integrated signal area as a result of using the square wave generator.