This paper compares three analytical methods that are often used to analyze composition of atmospheric aerosol: Ion Chromatography (IC), Proton Induced X-ray Emission (PIXE), and X-ray Fluorescence (XRF). Three monitoring studies are discussed: (1) a comparison of air particulate data collected by several independent sampler/analytical technique suites run by different laboratories; (2) a study involving two identical samplers and a single suite of analytical techniques; and (3) analysis of identical aerosol samples by two different techniques (XRF vs. PIXE). While the XRF versus PIXE project shows a very good agreement for most elements, the first interlaboratory study demonstrates the "real-life" noise introduced into the final data set by various sampling complications and different collection characteristics of the samplers used. The XRF versus PIXE study also revealed an unexplained deviation in measured sulphur concentrations for very lightly loaded samples. In the five-sampler comparison, two data sets provided by IC were approximately 20% lower than the three data sets obtained by PIXE and XRF. When two identical IMPROVE-compatible samplers were used and samples were subjected to similar procedures and the same analytical techniques, the variability between the two air concentration data sets significantly decreased.