4.2.2 Detection limit
Compared with other MRTOF-MS in nuclear physics, API-MRTOF-MS suffers from extra background, such as the chemical noise from sample injection from the atmosphere, and the electromagnetic interference from the ambient environment leaving noise to the spectrum55. Thus, the detection limit is another crucial parameter. Taking CsI as the sample, we diluted the sample into concentrations of 1 ppm and 100, 10 and 1 ppb. Using the ESI to ionize the samples, we conducted a series of experimental tests. A signal-to-noise ratio (S /N ) ≥ 3:1 was accepted as the detection limit.
Fig. 8 shows the spectrum of 133Cs+recorded directly with the oscilloscope at a concentration of 10 ppb. The intensity of the 133Cs+ peak is 9.84 × 10–2 mV, the calculated standard deviation of the noise is 1.23 × 10–2 mV, and theS /N is 8. Thus, we obtain a detection limit of 10 ppb, which is a meaningful value in a situation where a flight distance of more than 1 km is implemented. In this detection level, API-MRTOF-MS can be used in many scientific fields.
Fig. 8. Spectrum of 133Cs+ at a concentration of 10 ppb.