Figure 4: Comparison of stored (a, c) and mailed (b, d) samples with the reference Tenax tube samples for both application methods: liquid (a, b) and gaseous (c, d). Difference plots with stored samples (22°C blue dots; 4°C red triangles) vs. direct measured reference samples (a, c) and mailed samples vs. direct measured reference vs. (turquoise rectangles) (b, d). The dashed lines represent the mean of the differences and the solid lines represent the mean of the difference ± 1.96 SD.