In most situations, we use “external standards” whereby the unknown is in one bottle, the standard is in another bottle, we run both through a test, and compare them.
Internal standards are used when you’re not sure that the AMOUNT of sample and standard solutions can be controlled precisely enough. For example, we are trying to measure something precisely, but the fluid is delivered by an irregular or pulsating pump, or where the amount delivered is too small to measure precisely. The ICP machine, fed by a peristaltic pump is our best example.
Ideally, the standard and unknown are mixed together, and then the responses of each are compared on a relative basis, thus eliminating the effect of sample size.
Our ICP actually uses a hybrid system. The Yttrium is introduced with a separate tube, but since the same pulsating peristaltic pump drives both the standard and the sample tube, the effect of the pump is erased.