Which Source Configuration is Right for your Application?

If you have an Agilent 5973, 5975, or 5977 mass spec, there is a simple change to the configuration you can make to optimize its performance. By default, these systems come with a 3 mm drawout plate (or extraction lens) in their source assemblies. According to the manufacturer, this 3 mm lens is ideally suited for trace level (low pg to low ng) analysis. There are two other draw out plate options offered by Agilent, the 6 mm ID and the 9 mm ID. Whether you are using the standard Inert Source or the Extractor source, the draw out plate (or extractor lens) aperture (internal diameter) determines the initial diameter of your ion beam, as shown below (Figure 1).

http://www.chem.agilent.com/Library/technicaloverviews/Public/5991-2106EN.pdf

Figure 2 - Extractor lenses on the top and draw out plates on the bottom

The areas of the apertures for the 3, 6, and 9 mm ID lenses  are 7.07, 28.27, and 63.62 mm² respectively (Figure 2). This extra surface area on the lenses with the smaller apertures can get fairly dirty. Figure 3 shows how the majority of the material buildup on the 3 mm extraction lens occurs inside the 6 mm aperture area. This is important, because low volatility compounds can adsorb onto metal surfaces in the ion source, and after a brief delay, diffuse back into the ion beam. This is most likely the cause of the "source tailing" seen with high molecular PAHs.

Figure 3 – 6 mm and 3 mm extraction lenses

Figures 4, 5, and 6 are comparisons of PAH peak shapes when collected with 3, 6, and 9 mm aperture extraction lenses when holding the source temperature, column, and acquisition parameters constant. Figure 4 shows an overlay of m/z 252 acquired on the 30 m x 0.25 mm x 0.10 µm Rxi-PAH column. Benzo[b]fluoranthene is well resolved from the k and j benzofluoranthene isomers, and the peak shapes are similar for all three extractor lenses.

Figure 4 - extracted ion chromatogram overlay of m/z = 252 - the benzofluoranthene isomers

Figure 5 - extracted ion chromatogram overlay of m/z = 279, 276, and 278 (dibenzacridines, indeno[123-cd]pyrene, and dibenz[ah]anthracene respectively)

Figure 5 shows a significant difference in peak tailing between the 3 mm extractor lens and the other two. While the 9 mm extractor lens is showing a higher response, the peak shapes for the 6 and 9 mm extractor lens runs are still rather symetrical. It isn't until we look at compounds with a m/z = 50 higher than the benzofluoranthenes that we start to see tailing with the 6 and 9 mm extractor lenses.

Figure 6 is an overlay of three dibenzopyrenes. The 6 and 9 mm extractor lenses show moderate tailing while the 3 mm extractor lens has tailing so bad it is difficult to determine where the final peak ends.

Figure 6 - extracted ion chromatogram overlay of m/z = 302 - dibenzopyrene isomers

Recommendations from the manufacturer are to use the 3 mm lens for trace analysis applications, the 6 mm lens with purge and trap (e.g. EPA 8260), dynamic headspace, and standard semivolatiles applications (e.g. EPA 8270). The inert 9 mm drawout plate is a custom order, but the 9 mm extractor lens is a standard part (and ships as the standard lens in the PAH analyzer). The first thing I do when I'm operating a new MSD is replace the 3 mm lens with the 6 mm option. That being said, I've been experimenting with the 9 mm extractor lens and EPA 8270 using split injection and haven't encountered any sensitivity or tuning issues (more on that to follow in a different blog).