The component of the coefficient of variation of the method that arises from analytical variability, determined from the analysis of spiked quartz fiber filters, was 0.7% to 3.2% for phosphoric acid and 0.5% to 2.6% for sulfuric acid [3]. An interlaboratory study with 26 participants found negligible biases and interlaboratory relative standard deviations of 12 to 15% for sulfuric acid and phosphoric acid concentrations between 0.05 and 1 mg/m3