Verifying constant response for an internal standard. When we develop a method using an internal standard, it is important to verify that the response factor is constant over the calibration range. Data are shown below for a chromatographic analysis of naphthalene (C10H8), using deuterated naphthalene (C10D8 in which D is the isotope 2H) as an internal standard. The two compounds emerge from the column at almost identical times and are measured by a mass spectrometer, which distinguishes them by molecular mass. From the definition of response factor in Equation 5-11, we can write

Prepare a graph of peak area ratio (C10H8/C10D8) versus concentration ratio ([C10H8]/[C10D8]) and find the slope, which is the response factor. Evaluate F for each of the three samples and find the standard deviation of F to see how "constant" it is.

Area of analyte signal Area of standard signal concentration concentration of analyte of standard C1oHs peak area C1oDs peak area 2 992 6 141 2 819 Sample (ppm) ppm) 10.0 10.0 10.0 1.0 303 3 519 3 023 3 0.0