From the peak shape, it is possible to obtain estimates of the beam width, slit width, and working resolution of the instrument. The peak is scanned and the positions of the 5% and 95% peak-heights are noted. As an example, a voltage scan performed across m/z 44 on a 6-cm radius instrument is shown in Figure 5a. The 5% points occur at 2696.7 (V₁) and 2723.3 V (V₂), and the 95% points at 2703.3 (V₃) and 2716.7 V (V₄). From these are calculated

Note the alternative expression for resolution,

R = radius/(slit + beam width)

A different method of measuring the resolution, useful if the absolute scale of the x-axis is unknown, is shown in Figure 5b. Here the width of the peak at 5% height and the separation between adjacent mass numbers are measured directly from the spectrum. In this example, showing peaks at m/z 45 and 44 where s = 99 and d = 43 units, the resolution is calculated as

Abundance Sensitivity

In the preceding discussion, it was assumed that the ion beam distribution is well defined in space, and the overlap between adjacent beams for m/z well below the resolution was assumed to be negligible. This is not the case — there are many reasons why the ions of a particular mass might contribute to the adjacent mass number. The measure of this is termed the abundance sensitivity, and it is quite simply defined as the fractional intensity of a peak appearing at the neighboring position. Typically, the abundance sensitivity of the instrument should be a few parts per million (ppm).