Chemistry 1LC Practical Practice Test

The key idea here is to determine an unknown concentration in a sample where the signal depends on the sample’s matrix by using standard additions. You take a fixed amount of the unknown sample and add known amounts of a standard solution containing the same analyte. For each addition, you measure the instrument response. Then you plot the response versus how much standard you added. If the response is linear, the resulting line will cross the x-axis at a value that corresponds to the negative of the unknown concentration in the sample. The magnitude of that intercept gives the unknown concentration in the original sample. This approach works well because each measurement is made in the same matrix, so any matrix effects are carried through and cancel out when you extrapolate to zero added standard. So the described method—adding known volumes of a standard to the unknown, measuring the response, plotting response against added standard, and extrapolating to zero added to find the initial concentration—is the standard addition approach. The other ideas don’t isolate the unknown concentration in the same way: calibrating with the unknown alone doesn’t correct for matrix effects, diluting to match a standard changes the signal without revealing the original amount, and adding the unknown to a standard (or extrapolating to infinite addition) doesn’t provide the same intercept-based determination.