The absenceof a meaningful time-trend (reason 3) might also be linked to sensitivity if TPs exceed the detection limit only in a few inconsecutive samples. Although we can assume that the integrated experimental and screening approach produces a substantial number of false negatives, the compounds “overlooked” in this way should all beat low concentrations in the samples and it seems that quantitatively dominant TPs were not discarded. Furthermore, one last type of false negatives that is inherent to such methods cannot be evaluated, i.e. compounds that are not ionized in the electrospray source or that are fragmenting excessively in the source. This
limitation can be potentially overcome if complementary analytical techniques such as GC/MS are used to screen for additional TPs.The potential of obtaining false negative results can be illustrated with the incubation series at a lower initial concentration(200 g L−1; experiments with Roter Main downstream sediment and Gründlach sediment). No TP could be identified by the in silico procedure in both experiments. The lower concentration of parent compounds implies also lower concentrations of TPs and consequently less abundance of monoisotopic molecular and 13C isotopic peaks. This increases the likelihood that especially the latter ones are discarded in the noise removal step. Further reducing the noise threshold would retain more peaks, butin turn it would negatively affect the non-monoisotopic massassignment. In the case of the two experiments with the lowerinitial concentration, ion chromatograms of the 16 identified TPswere extracted (not shown). Results showed that 8 out of 16 TPswere detected in both experiments and one (Met 210) was solelydetected in Gründlach sediment. The absence of others (i.e. Bez 223,Bez 238, carbamazepine-10,11-epoxide, 4-hydroxydiclofenac, -hydroxymetoprolol, 1-naphthol and Pro 176) might be explained by their comparatively low levels. These findings are qualitatively consistent between the three experiments, but the performance in experiments with the lower initial concentration is unsatisfactory.This can be improved when an LC/HRMS instrument of newer generations is used, as these instruments have a substantially increased sensitivity as compared to the one used in the present study (pur-chased in 2005).
Absenceof ที่มีความหมายเวลาแนวโน้ม (เหตุผล 3) อาจจะเชื่อมโยงไปยังไวถ้า TPs เกินการตรวจเพียงบางตัวอย่าง inconsecutive ถึงแม้ว่าเราสามารถสมมติว่า วิธีทดลอง และการตรวจกรองที่รวมผลิตจำนวนฟิล์มไม่พบ สารประกอบ "มองข้าม" ในลักษณะนี้ควรทั้งหมดตีความเข้มข้นต่ำในตัวอย่าง และดูเหมือนว่าที่ quantitatively TPs ตัวถูกไม่ละทิ้ง นอกจากนี้ หนึ่งต้องเท็จสิ่งที่โดยธรรมชาติกับวิธีดังกล่าวไม่สามารถประเมิน เช่นสารประกอบที่ถูก ionized ไม่ต้นวิธีพ่นละอองไฟฟ้าหรือที่ มีหลัก fragmenting มากเกินไปในแหล่ง นี้ limitation can be potentially overcome if complementary analytical techniques such as GC/MS are used to screen for additional TPs.The potential of obtaining false negative results can be illustrated with the incubation series at a lower initial concentration(200 g L−1; experiments with Roter Main downstream sediment and Gründlach sediment). No TP could be identified by the in silico procedure in both experiments. The lower concentration of parent compounds implies also lower concentrations of TPs and consequently less abundance of monoisotopic molecular and 13C isotopic peaks. This increases the likelihood that especially the latter ones are discarded in the noise removal step. Further reducing the noise threshold would retain more peaks, butin turn it would negatively affect the non-monoisotopic massassignment. In the case of the two experiments with the lowerinitial concentration, ion chromatograms of the 16 identified TPswere extracted (not shown). Results showed that 8 out of 16 TPswere detected in both experiments and one (Met 210) was solelydetected in Gründlach sediment. The absence of others (i.e. Bez 223,Bez 238, carbamazepine-10,11-epoxide, 4-hydroxydiclofenac, -hydroxymetoprolol, 1-naphthol and Pro 176) might be explained by their comparatively low levels. These findings are qualitatively consistent between the three experiments, but the performance in experiments with the lower initial concentration is unsatisfactory.This can be improved when an LC/HRMS instrument of newer generations is used, as these instruments have a substantially increased sensitivity as compared to the one used in the present study (pur-chased in 2005).
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The absenceof a meaningful time-trend (reason 3) might also be linked to sensitivity if TPs exceed the detection limit only in a few inconsecutive samples. Although we can assume that the integrated experimental and screening approach produces a substantial number of false negatives, the compounds “overlooked” in this way should all beat low concentrations in the samples and it seems that quantitatively dominant TPs were not discarded. Furthermore, one last type of false negatives that is inherent to such methods cannot be evaluated, i.e. compounds that are not ionized in the electrospray source or that are fragmenting excessively in the source. This
limitation can be potentially overcome if complementary analytical techniques such as GC/MS are used to screen for additional TPs.The potential of obtaining false negative results can be illustrated with the incubation series at a lower initial concentration(200 g L−1; experiments with Roter Main downstream sediment and Gründlach sediment). No TP could be identified by the in silico procedure in both experiments. The lower concentration of parent compounds implies also lower concentrations of TPs and consequently less abundance of monoisotopic molecular and 13C isotopic peaks. This increases the likelihood that especially the latter ones are discarded in the noise removal step. Further reducing the noise threshold would retain more peaks, butin turn it would negatively affect the non-monoisotopic massassignment. In the case of the two experiments with the lowerinitial concentration, ion chromatograms of the 16 identified TPswere extracted (not shown). Results showed that 8 out of 16 TPswere detected in both experiments and one (Met 210) was solelydetected in Gründlach sediment. The absence of others (i.e. Bez 223,Bez 238, carbamazepine-10,11-epoxide, 4-hydroxydiclofenac, -hydroxymetoprolol, 1-naphthol and Pro 176) might be explained by their comparatively low levels. These findings are qualitatively consistent between the three experiments, but the performance in experiments with the lower initial concentration is unsatisfactory.This can be improved when an LC/HRMS instrument of newer generations is used, as these instruments have a substantially increased sensitivity as compared to the one used in the present study (pur-chased in 2005).
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