Characterization and optimization of a field-portable mass spectrometer for in situ analysis
The high demand for field chemical analysis has created a need for smaller and lighter analytical instruments, which are capable of fast analytical determination. Field-portable analytical instrumentation has advantages in chemical analysis over instruments in offsite laboratories, especially in terms of more rapid analysis, easier chain of custody and faster adaptation of the analytical protocol in response to emerging analytical data. Real-time detection, chemical identification and quantification of target compounds is important for public safety purposes, environmental monitoring, and industrial process optimization. ^ In response to the growing need for in situ analysis, the Mini-10.5, a handheld mass spectrometer based on a rectilinear ion trap mass analyzer, was developed at Purdue University. The instrument features unit resolution, an mass limit of m/z 550, detection limits ranging from high parts per trillion to low parts per million and the capability for MS and MSn experiments. In the research presented here, the miniature mass spectrometer was coupled with various sampling and ionization methods, which demonstrated its versatility and provide d a platform with broad applications for field studies. Optimization of instrument settings and a systematical study of mass spectrometer performance were attained prior to determination of analytical figures of merit.^ The primary focus of the research was on the development and operation of the sample delivery/ionization interfaces for the field-portable mass spectrometer. The objective of the study was to facilitate trace analysis and chemical speciation over a wide range of analytes present in vapors and solution phase samples. Tandem mass spectrometry (MS/MS) and higher-stage mass spectrometry experiments were carried out to achieve both sensitive detection of molecules of interests and high specificity of chemical assignments.^ Initial studies were performed using a handheld mass spectrometer equipped with a membrane inlet MS (MIMS) as a means of introducing analytes, as well as with a sorbent trap for sample preconcentration and introduction. The mass spectrometer was applied to fuel-related problems, specifically to monitor vapor-phase and aqueous solutions in order to detect traces of hydrocarbons. The aim of the work was to achieve sampling, detection identification and quantitation in times of less than 1 minute. Detection limits, sensitivity, specificity and quantitative performance for these analytes in the types of sample matrices indicated, were also characterized.^ In the following studies a miniature mass spectrometer was equipped with an atmospheric pressure ionization (API) interface, which required significant changes within the instrumental design as well as its operation. This modification allowed for the mass analysis of ions generated externally, using API methods, and expanded the number of molecules that could be detected. The system was characterized using several target compound classes, including alcohols, amines, amino acids, nitrated esters, nitramines, purines, pyrimidines, nucleo sides and peptides. Results are discussed using the following analytical figures of merit: limits of detection (LOD), limits of quantitation (LOQ), linear dynamic range, accuracy, and reproducibility.^
R. Graham Cooks, Purdue University.