In ICP-MS laboratories, phrases such as “this element is only a few thousand CPS” or “did tuning reach how many MCPS?” are used every day. CPS is the raw signal the instrument works with, and understanding how it relates to the concentration shown on a report makes it easier to read a certificate and to recognize when an instrument, rather than a sample, has changed.
- CPS means counts per second: the number of ions reaching the detector each second.
- Concentration is obtained from CPS through a calibration curve built from standards.
- If concentration should be unchanged but CPS drops, the instrument and its sampling path are the usual suspects.
Understanding CPS
CPS is counts per second, the number of ions the detector receives each second after the sample has been ionized in the plasma, separated by mass and directed to the detector. A higher rate of ions of a chosen mass gives a higher CPS. The signal is therefore a direct measure of how much of that mass is arriving at the detector, but it is not yet a concentration: it depends on the sensitivity of the particular instrument at the time of measurement.
How CPS becomes concentration
The conversion is made through a calibration curve. Standard solutions of known concentration are measured to obtain their CPS values, and a linear relationship between concentration and CPS is fitted within the working range. The unknown sample’s CPS is then measured and back-calculated to a concentration. Within the linear range, more of the element gives a higher CPS; outside it, the relationship can no longer be assumed to be linear and the sample may need dilution.
| Step | Purpose | Output |
|---|---|---|
| Measure standards | Relate known concentration to signal | CPS at each level |
| Fit calibration | Establish the linear relationship | Calibration curve |
| Measure unknown | Obtain the sample CPS | Signal for each element |
| Back-calculate | Convert signal to amount | Concentration, e.g. ppb |
Why CPS can drop while concentration is unchanged
If a sample of the same concentration gives a lower CPS than before, three causes are common. First, instrument sensitivity may have declined and the daily tuning no longer gives the expected response. Second, the nebulizer or the interface cones may be clogged or dirty, so less material reaches the detector. Third, background noise may have risen because of reagent contamination, which changes the signal that can be attributed to the sample. Each points to maintenance rather than to a change in the product.
One KCPS is 1,000 counts per second and one MCPS is 1,000,000 counts per second. A tuning figure of tens of MCPS means tens of millions of ion impacts each second.
Why this matters for DMSO qualification
Electronic-grade DMSO is specified by identified metals at low ppb levels using ICP-MS. Reading the report is easier with the signal chain in mind: a reported concentration depends on calibration, on the element list and detection limits, and on the instrument being properly tuned and maintained. Asking how sub-limit values are expressed, and which elements were measured, gives more confidence than treating a single number as the complete result.
Confirm metals reporting on your DMSO
Send the element list and limits, and we will confirm the ICP-MS basis and provide the batch COA.
Frequently asked questions
What does CPS stand for?
Counts per second, the number of ions reaching the ICP-MS detector each second for the mass being measured.
How is CPS converted to ppb?
Through a calibration curve: standards of known concentration give the signal-to-concentration relationship, and the unknown is back-calculated from its CPS.
Why would the same concentration give a lower signal?
Common causes are reduced sensitivity after tuning, a dirty nebulizer or interface cones, or elevated background from contaminated reagents.
What is the difference between KCPS and MCPS?
KCPS is thousands of counts per second and MCPS is millions; 1 MCPS equals 1,000 KCPS.
Does a higher CPS always mean more contamination?
Only after calibration and within the linear range, and only when the instrument is properly tuned. The concentration, not the raw CPS, is compared between runs.