2.6. Isotope calculation
Isotope enrichment of acetate was calculated from the ions intensities of M0, M0þ1 and M0þ2, which were monitored at m/z 60, 61and62,respectively.The ion intensity ratio of labeled-to-unlabeled species measured by the GC/MS was presented as tracer-to- trace ratio (TTR). In the study presented herein, the Rosenblatt approach [21] was used for calculating the TTR of acetate, which is a widely used approach in the tracer studies of SCFA. According to Rosenblatt et al. [21], several corrections need to be included in the calculation of TTR. The following three corrections were considered in our TTR calculation. First, the contribution of labeled molecule from the background sample was corrected for the sample data. Second, the abundance in the spectrum of labeled molecule is different from those in the unlabeled molecule, and this proportionality “skew” was corrected. The “skew” abundance distribution correction can be approximated by the value [1/(1þnA)], where A is the natural abundance of the element used as the label and n is the number of these atoms labeled. The “skew” correction factors are 0.9890 and 0.9783 for the [2-13C] acetate and [U-13C] acetate, respectively (A¼0.01108). Third, the isotope over lapping of neighboring peaks was corrected. Each compound studied (Table1) not only contributed to its major peak, but also contributed to the mass isotopomer distribution of the neighboring peaks(M0, M0þ1 and M0þ2) significantly. Taking all the corrections stated above, Eqs. (2) and(3) were used for calculating the TTR of both [2-13C] acetate and [U-13C] acetate.
2.6. ไอโซโทปคำนวณ Isotope enrichment of acetate was calculated from the ions intensities of M0, M0þ1 and M0þ2, which were monitored at m/z 60, 61and62,respectively.The ion intensity ratio of labeled-to-unlabeled species measured by the GC/MS was presented as tracer-to- trace ratio (TTR). In the study presented herein, the Rosenblatt approach [21] was used for calculating the TTR of acetate, which is a widely used approach in the tracer studies of SCFA. According to Rosenblatt et al. [21], several corrections need to be included in the calculation of TTR. The following three corrections were considered in our TTR calculation. First, the contribution of labeled molecule from the background sample was corrected for the sample data. Second, the abundance in the spectrum of labeled molecule is different from those in the unlabeled molecule, and this proportionality “skew” was corrected. The “skew” abundance distribution correction can be approximated by the value [1/(1þnA)], where A is the natural abundance of the element used as the label and n is the number of these atoms labeled. The “skew” correction factors are 0.9890 and 0.9783 for the [2-13C] acetate and [U-13C] acetate, respectively (A¼0.01108). Third, the isotope over lapping of neighboring peaks was corrected. Each compound studied (Table1) not only contributed to its major peak, but also contributed to the mass isotopomer distribution of the neighboring peaks(M0, M0þ1 and M0þ2) significantly. Taking all the corrections stated above, Eqs. (2) and(3) were used for calculating the TTR of both [2-13C] acetate and [U-13C] acetate.
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