## 3 Results
### Hydrogen Cyanide Recovery  
The recovery of HCN was determined by extracting and assay HCN-free blood samples fortified to contain 1.3 and 2.6 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) of KCN and by comparing the absorbencies with those obtained by pouring directly equivalent loads of the KCN stock solution into the inner well of the microdiffusion device (the outer well was kept empty). The results for recovery rate (mean \(\pm\) S.D.) of six replicates at this concentration were 84.4 \(\pm\) 6.6%, when 0.1 N KOH solution was used for capturing the HCN removed by the 20% phosphoric solution, and 78% \(\pm\) 8.3% in the case of 2% K\({}_{2}\)CO\({}_{3}\) absorbent solution. This recovery was found to be satisfactory and quite similar to those obtained following a longer time period (2-3 h) with conventional Conway units [43]. The HCN from blood samples was recovered in a proportion ranging from 75% to 85%, determined by internal standards with known concentrations of KCN (1.3 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) and 2.6 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) KCN). When 1 mL of 1.3 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) KCN was added to each blood sample as an internal standard, HCN in the blood was determined more accurately. A better recovery of HCN was observed when the aqueous solutions of 1.3 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) and 2.6 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) KCN were placed in the Conway external wall instead of cyanide-free blood with the addition of KCN. Comparing the absorbance values for HCN in blood samples with those in a calibration curve performed with aqueous KCN solutions in the range of concentrations from 0 to 2.6 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\), a blood HCN concentration of 0.75 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) and a total HCN of 1.37 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) were calculated for a specific blood sample taken from a fire victim. However, a blood HCN concentration of 1.043 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) and another one of 1.904 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) for total hydrogen cyanide were determined if the calculation was made with the two internal standards (1.3 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) and 2.6 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) KCN). From Figure 4, for the absorbance value of 1.432 at 493 nm, which corresponds to a concentration of 2.6 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) KCN or 1.08 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) HCN, a high molar absorptivity of 3.58 \(\times\) 10\({}^{6}\) L mol\({}^{-1}\) cm\({}^{-1}\) was calculated. Therefore, the sensitivity of this assay is comparable with that of the fluorescence methods. A derivative of the absorbance spectra showed a small variation in the absorbance values during the kinetic measurements. In general, the reference sample had the absorbance values in a narrow range close to zero. On the contrary, in Figure 4a, a negative value was observed, probably due to the experimental errors. However, the slope of the reference spectrum was very low (Slope 0.060), while its value increased with the concentration of KCN or HCN (Slope 0.5029-0.5348 for 2.6 \(\mathrm{\SIUnitSymbolMicro m}\)\(\mathrm{L}^{-1}\) KCN in two different days). Since the absorbance values and slopes of kinetic spectra slightly differ from one day to another, a standard with KCN is needed for each series of determinations per day. Head-space sampling of HCN demonstrated the high sensitivity of this method (A\({}_{493}\) > 2.3, at 30 min from the reagent mixing).