Question

Correcting for matrix effects with an internal standard. The appearance of pharmaceuticals in municipal wastewater (sewage) is an increasing problem that is likely to have adverse effects on our drinking water supply. Sewage is a complex matrix. The drug carbamazepine can be measured at low levels in sewage by liquid chromatography with mass spectral detection. When carbamazepine was spiked into sewage at a concentration of $\mathbf{5}\mathbf{,}$
chromatographic analysis gave an apparent spike recovery of $\mathbf{154}{\mathbf{\%}}^{\mathbf{16}}$Deuterium (D) is the hydrogen isotope $\mathbf{154}{\mathbf{\%}}^{\mathbf{16}}$. Deuterated carbamazepine can be used as an internal standard for carbamazepine. The deuterated compound has the same retention time as undertreated material in chromatography, but is distinguished by its higher mass in the mass spectrum. When deuterated carbamazepine was used as an internal standard for the analysis, the apparent recovery was $\mathbf{98}\mathbf{\%}$Explain how the internal standard is used in this analysis and rationalize why it works so well to correct for matrix effects.$\mathbf{5}\mathbf{,}$

Short answer

Its resemblance to the original analyte, carbamazepine, the deuterated carbamazepine spike was a viable candidate as an internal standard to account for matrix effects.

Step-by-step solution

define the spike recovery

A spike, also called a fortification, is a known quantity of analyte added to a sample to test whether the response to the spike is the same as that expected from a calibration curve.

Spiked samples are analysed in the same manner as unknowns.

For example, if drinking water is found to contain o$10.0\mu \mathrm{g}/\mathrm{L}$f nitrate, a spike of$5.0\mu \mathrm{g}/\mathrm{L}$could be added. Ideally, the concentration in the spiked portion found by analysis will be$15.0\mu \mathrm{g}/\mathrm{L}$If a number other than $15.0\mu \mathrm{g}/\mathrm{L}$is found, then the matrix could be interfering with the analysis.

localid="1663340785205" $\%\mathrm{recovery}=\frac{{\mathrm{C}}_{\mathrm{spiked}}-{\mathrm{C}}_{\mathrm{unspiked}\mathrm{sample}}}{{\mathrm{C}}_{\mathrm{added}}}\times 100$

Step 2:

Because of its near resemblance to the original analyte, carbamazepine, the deuterated carbamazepine spike was a viable candidate as an internal standard to account for matrix effects.

Because they are chemically nearly identical, the matrix effects that cause the increased spike recovery would also boost the internal standard recovery.

To cancel out the matrix effects, this rise in recovery for the internal standard would be reduced from the recovery for the analyte.