Abstract
Under certain conditions, diffusion was found to be a significant positive bias when sampling for gases using sorbent tubes and pumps at reduced flow rates. Based upon a theoretical analysis, this bias is predicted to be significant for inlet Peclet numbers smaller than 3. The inlet Peclet number is defined as vL1/D where: v=inlet velocity; L1=length to sorbent bed; D=diffusion coefficient. During experimental work, the analysis of sorbent tube samples collected at flow rates of 0.8 cm3/min resulted in measured concentrations as much as 75% higher than sorbent tube samples collected at flow rates of 20 cm3/min.