# The Evaluation of the Detection of Cr(VI) in Leather
## 3 Results
The use of a guard column packed with polystyrene-divinylbenzene (PS-DVB) and variation of the ionic strength of the eluent can further improve identification of false positives due to colourants [12]. This methodology is currently optional in BS EN ISO 17075-2:2017, and it is recommended that this approach becomes a fixed feature of the method [22]. Dyes are a significant interference in colorimetry and the use of a C18 separation medium (SPE) is recommended by ISO 17075-1:2017 to mitigate against this [11]. However, previous studies have shown SPE media can be retentive of Cr(VI) and the SPE may not be 100% efficient at removing the coloured interference [23]. From the above analyses, the chromatographic methods efficiently mitigate against interferences occurring in the colorimetric method and are an advancement in the detection of Cr(VI) in leather goods. However, none of the established methods detail the concentration of Cr(VI) in solution at the current 3 mg kg\({}^{-1}\) limit in dry leather. In all established methods, several dilution factors must be considered, and the concentrations in solution are significantly smaller than the value quoted in leather [11,12]. Table 1 lists the concentrations of Cr(VI) at the current and proposed compliance limits and shows the chromatographic method has a fundamental advantage versus the colorimetric method with a 6-fold concentration increase. Figure 6: Normalised absorbance vs. wavelength of the selected peaks from Figure 5: solution containing Cr(VI) (peak 1) (blue), extract from leather containing a yellow dye (peak 2) (green), extract from leather containing a red dye (peak 3) (red). The values in solution relating to the colorimetric method are significantly below 1 \(\upmu\)M for both compliance limits. Conventional spectrophotometry becomes unreliable at analyte concentrations below 1 \(\upmu\)M, and specialist forms of spectrophotometry are recommended to achieve a quantification limit on the 0.1 \(\upmu\)M scale [24]. The values in solution relating to the chromatographic methods are approximately 1 \(\upmu\)M for the current compliance limit; however, the proposed compliance limit equates to 0.4 \(\upmu\)M, a concentration at which conventional spectrophotometry would be unreliable to quantify [24]. Although larger quantities of Cr(VI) in solution lead to greater signal intensity, the absorptivity coefficient of each method must also be considered when detecting significantly small quantities of analyte. Figure 7 compares the calibration gradient (peak absorbance or chromatogram peak area versus Cr(VI) concentration) for the established methods, which illustrates that the absorptivity coefficient of the Cr(III)-carbazone complex (Part 2 Annex C) is 14 times larger compared to the Cr\({}_{2}\)O\({}_{4}\)\({}^{2-}\) detection (Part 2, Annex B) and the colorimetric method (Part 1). The absorptivity of the CrO\({}_{4}\)\({}^{2-}\) ion at 372 nm was calculated to be 9.25 \(\times\) 10\({}^{-2}\) mL \(\upmu\)\({}^{-1}\) cm\({}^{-1}\), whereas the Cr(III)-carbazone complex at 540 nm had an absorptivity of 8.25 \(\times\) 10\({}^{-1}\) mL \(\upmu\)\({}^{-1}\) cm\({}^{-1}\). Despite the advantages of the larger detection quantities for the chromatographic method, the lower absorptivity coefficient of the CrO\({}_{4}\)\({}^{2-}\) ion results in the calibration gradient of the direct detection method being comparable to the colorimetric method. This comparison suggests that there is no advantage in the detection limit between the colorimetric (Part 1) and Direct Cr\({}_{2}\)O\({}_{4}\)\({}^{2-}\) detection (Part 2, Annex B)