Atmospheric Pressure Chemical Ionization Ion Trap Mass Spectrometer

Why an Ion Trap?

A major thrust of our instrumentation development efforts is the development of an ion trap. While our present "5/8" triple quadrupole mass spectrometer is a powerful analytical tool, it can only measure one ion mass or daughter ion mass at a time. Thus, the measurement of many complex organic compounds with the triple requires scanning a daughter, or at least part of a daughter ion spectrum, for each compound to be measured. This means that hundreds of individual measurements of mass need to be made to track several compounds, so less than 1% of the total measurement time can be spent on any one mass. For studying the ammonium and sulfate content of particles, this posses few problems. However, to identify and measure the organic molecules present in 2-10 nm particles is orders of magnitude more time consuming. The use of an ion trap will allow the collection of all ions of interest simultaneously for 70-85% of the total measurement time. The ion trap can then analyze both parent and daughter ion spectra from numerous compounds of interest in the remaining 15-30% measurement time. Thus, for large, complex compounds that are expected to make up most of the organic mass of particles, most requiring MS/MS analysis, an ion trap with a similar throughput to a triple mass spectrometer can increase usable measurement time by over 2 orders of magnitude. It should also remove much of the present background measurement and electronic noise.

ion trap photo

Technical Details

The ion trap mass spectrometer that we are developing at NCAR is being constructed in-house, and is unique by the measures that will be taken to improve ion throughput above that provided by commercial instruments. The trap itself is a commercially available item (R.M. Jordan Co). The electronics for generating the waveforms for the endcaps and ring electrode are based on a plug-in arbitrary waveform generator (National Instruments Corp). The chamber was built in-house at NCAR's Design and Fabrication Services facility, and features a main chamber with high pumping speed (1000 l/s turbo pump), which under normal operation is at a pressure of a mtorr of He, and a second chamber for the discrete dynode electron multiplier (ETP), which is pumped by a 70 l/s pump and is at a pressure of 10-5 torr. A/D and D/A converters located next to the chamber convert all signals into digital words for transfer to the computer, thus minimizing noise. The control software is written in Labview.

In the first phase of development (figure 1), we have used simple ion transfer optics to focus ions into the trap. The first spectrum, shown in figure 2, was obtained in June, 2002. The next phase of development will focus on developing means for improving instrument sensitivity. These will include conventional techniques such as high efficiency ion transfer optics (octopoles) and large sampling orifices as well as more experimental state-of-the-art techniques such as broadband nondestructive ion detection and the use of asymmetric trapping fields for ion ejection.

Figure 1: NCAR ion trap phase I development

Figure 2. First spectrum from NCAR ion trap. Ions were produced by irradiating laboratory air with an Am241 foil.




ACOM | Atmospheric Chemistry Observations & Modeling