How many methods are there for quantitative mass spectrometry?

Two methods for quantification

The process of quantifying any physical quantity is nothing more than comparing it with a "ruler", and mass spectrometry analysis is no exception. There are two methods for achieving quantitative mass spectrometry analysis: the external standard method and the internal standard method. 

External standard method 

We conduct experiments using the known standard sample A and the unknown untested sample A; we will obtain the following three pieces of information: the quantity of the standard sample (known); the signal intensity of the standard sample; and the signal intensity of the sample to be tested. (Assuming that the response of the sample = constant * concentration, calculate the quantity of the sample to be tested from these three pieces of information.) 

To measure the unknown samples more accurately, we hope that the signal intensity of the standard samples is as close as possible to that of the samples to be measured (to reduce the influence of nonlinear response). Therefore, the commonly used standard method is to measure a series of known standard samples, draw a working curve, and then determine the quantity of the unknown samples through fitting methods. 

Internal standard method 

The external standard method mainly has the following two limitations: 

The samples and the samples to be tested are independently subjected to experiments, making it impossible to eliminate accidental errors between experiments. 

2.2. Unlike the matrix of the sample to be tested (i.e., all components except the analyte), the matrix may cause different effects and errors.

Then, if we directly add the known standard sample B to the sample A to be tested, we can complete the measurement of the standard sample and the unknown sample in the same experiment and the same matrix, which eliminates the errors caused by two experiments and different matrices. This is the internal standard method. (If the added standard sample and the sample to be tested are the same substance A, then since they cannot be distinguished, only one experiment cannot quantify the sample to be tested. At this time, we conduct two measurements before and after adding the sample, that is, measure the signal of the sample to be tested and the sample to be tested + sample, to calculate the quantity of the sample to be tested.)