See Wikipedia for Potassium-40, abundance.

There are three (commonly encountered) isotopes of K: 39K, 40K and 41K. ICP-MS does not measure "K total", but either 39K (~93.3% of K) or 41K (~6.7% of K). It also doesn't measure 40K (0.01%) as there is too much interference from 40Ar used to generate the plasma of the ICP.

So you could potentially measure 39K and calculate that if this is 93.2581% of total K, then 40K is 0.0117% of the total.

Of course, you are assuming

• you have taken a representative sample of sediment
• you extracted 100% the K from the sediment
• the 3 isotopes are equally extracted by your process
• the ICP-MS is accurately measuring the 39K
• the 40K is actually present in the ratio 0.0117 : 93.2581 in the sediment
• your calculations are correct

Perhaps you need to find a non-ICP method to quantify the K isotopes.

Potassium (K) has three stable isotopes: K-39, K-40, and K-41. K-40 is radioactive and decays with a half-life of 1.25 billion years. It emits beta particles and gamma rays during decay, and its decay products include calcium-40 (Ca-40) and argon-40 (Ar-40).

ICP-MS (Inductively Coupled Plasma Mass Spectrometry) is a common analytical technique used to determine the total amount of K in sediment samples. However, it does not differentiate between the different isotopes of potassium.

To determine the K-40 content in sediment samples, one needs to use a radiometric dating method, such as the K-Ar (potassium-argon) or Ar-Ar (argon-argon) dating methods. These methods rely on the measurement of the ratio of K-40 to its decay product Ar-40 in the sample, and the known decay constant of K-40. The age of the sample can then be calculated from the ratio of K-40 to Ar-40, using the known decay constant.

Therefore, it is not possible to determine the K-40 content in sediment samples directly by ICP-MS. Radiometric dating methods such as K-Ar or Ar-Ar dating would need to be used to determine the K-40 content.