ICP-OES (or AES) can be complicated to interpret.
I have run it myself on my own (well, my company's) machine. There is not always just a single answer from a single run of a single sample. I was doing it with much more careful oversight of the actual data than I'm sure any analysis company does, and this was my comment in one case:
http://reefkeeping.com/issues/2005-04/rhf/index.php
from it:
"For initial testing I chose to use as the "standard" a sample of artificial seawater that was mixed to an approximate salinity of S=35. I mixed a 44-gallon batch using Instant Ocean artificial salt mix and reverse osmosis/deionized (RO/DI) water to a conductivity of 52.7 mS/cm, and allowed it to settle for three weeks. I then proceeded to measure its calcium concentration by ICP-AES (inductively coupled plasma-atomic emission spectroscopy, an $80,000 analytical instrument. I was somewhat disappointed with my inability to use this sophisticated technique to get a precise answer. Despite taking five different samples and analyzing them at eight different emission wavelengths using two different calibration methods (five standard additions of known calcium concentrations to each sample, as well as comparison to a fixed 1000 ppm commercial calcium standard), I was unable to get consistent values. Some of the samples were acidified or filtered through submicron filter membranes to determine if solid materials were impacting the result (they were not). Overall, I took more than 200 measurements, each involving three replicate observations of the emission intensity. Nevertheless, the result was not very satisfying, with a substantial variation occurring between the different values. The average of every measurement taken was 336 ppm. With the uncertainty involved, however, I'd conclude that the true value was probably 340 ± 40 ppm. I also measured the same sample once with a Salifert brand test kit and got 330 ppm calcium."
