What Is CPS?
CPS stands for counts per second. In plain terms, it is how many ions the detector catches every second. The sample is vaporized and ionized in the high-temperature plasma, turning into positive ions. The mass analyzer lets only ions of the target mass through. The ones that reach the detector each add one count. How many hits per second is the CPS on screen.
A reading of 500,000 CPS means half a million ions of that element hit the detector every second.
How CPS Relates to Concentration
On a report, what you actually want is concentration in mg/L, ug/L, ppm, or ppb. Inside the instrument linear range, CPS tracks concentration one-for-one. Run a set of standards with known concentrations, record the CPS for each, draw the calibration curve, then run the unknown sample and read the concentration off the curve.
CPS is the raw signal; concentration is what you report. If the CPS count is off, the concentration number on the report is off too.
Why CPS Can Drop Without Concentration Changing
CPS depends not only on concentration but also on instrument condition. Sensitivity: the better the instrument condition, the higher the CPS at the same concentration. Wear and contamination: a tired nebulizer, dirty sampler or skimmer cones, or weak lenses all reduce ions reaching the detector. Background noise: even a pure-water blank gives a faint signal; a good instrument keeps that background down to single digits or a few tens of CPS.
KCPS and MCPS
1 CPS is the base unit. 1 KCPS is 1,000 CPS. 1 MCPS is 1,000,000 CPS. When tuning reports show high-sensitivity elements hitting tens of MCPS, that is tens of millions of ions hitting the detector every second.
Summary
In ICP-MS, CPS is the raw signal the instrument actually sees. It is not the concentration you hand to the client, but it is the foundation underneath every result. Reading the ups and downs of CPS lets you judge instrument condition and spot bad data before it becomes a bad report.