Single Particle Analyzer of Mass: A Charge Detection Mass Spectrometer with a Multi-Detector Electrostatic Ion Trap

Abstract

A new charge detection mass spectrometer that combines array detection and electrostatic ion trapping to repeatedly measure the masses of single ions is described. This instrument has four detector tubes inside an electrostatic ion trap with conical electrodes (cone trap) to provide multiple measurements of an ion on each pass through the trap resulting in a signal gain over a conventional trap with a single detection tube. Simulations of a cone trap and a dual ion mirror trap design indicate that more passes through the trap per unit time are possible with the latter. However, the cone trap has the advantages that ions entering up to 2 mm off the central axis of the trap are still trapped, the trapping time is less sensitive to the background pressure, and only a narrow range of energies are trapped so it can be used for energy selection. The capability of this instrument to obtain information about the molecular weight distributions of heterogeneous high molecular weight samples is demonstrated with 8 MDa polyethylene glycol (PEG) and 50 and 100 nm amine modified polystyrene nanoparticle samples. The measured mass distribution of the PEG sample is centered at 8 MDa. The size distribution obtained from mass measurements of the 100 nm nanoparticle sample is similar to the size distribution obtained from transmission electron microscopy (TEM) images, but most of the smaller nanoparticles observed in TEM images of the 50 nm nanoparticles do not reach a sufficiently high charge to trigger the trap on a single pass and be detected by the mass spectrometer. With the maximum trapping time set to 100 ms, the charge uncertainty is as low as ±2 charges and the mass uncertainty is approximately 2% for PEG and polystyrene ions.

Fig. 1

Schematic diagrams of a) the single particle analyzer of mass (SPAM) instrument and b) the combined ion trap and detector array.

Fig. 2

Comparison of trapping performance of cone trap and electrostatic ion trap (EIT), showing a) position of the ion inside the two traps as a function of time, b) the length of time an ion is trapped relative to the maximum trapping time of 50 ms at different trapping voltages, and c) entrance radii, and d) the length of time an ion is trapped for at different entrance angles at pressures of 10⁻⁵, 10⁻⁶, and 10⁻⁷ Torr.

Fig. 3

Transient recorded for a single polystyrene nanoparticle ion that was trapped for 63 ms. The upper transient shows the data from channel A, and the lower transient the data from channel B. The inset is an expansion of both channels between 10.6 ms and 12.75 ms.

Fig. 4

Data showing percent uncertainty in measured charge versus the length of ion trapping time for 376 PEG ions with a nominal 8 MDa mass. The inset shows the trapping time of all ions with an absolute charge uncertainty less than 10 charges, as low as 2 charges for ions trapped for approximately 100 ms.

Fig. 5

Histogram of 376 PEG ion masses making up a mass spectrum of a sample of nominal molecular weight 8 MDa.

Fig. 6

Data from a nominal 50 nm amine-modified polystyrene nanoparticle sample: a) mass spectrum measured with SPAM, b) size distribution determined from mass spectrum, c) size distribution determined from TEM images, and d) TEM image of the nanoparticle sample.

Fig. 7

Measured charge of ions from the 50 nm nanoparticle sample versus their diameter determined from the mass data obtained with SPAM. The line shows the Rayleigh limit charge for a water droplet of a given size. Relatively few ions are detected below ~400 charges, indicating the threshold for detecting an ion from a single induced signal.

Fig. 8

Data from a nominal 100 nm amine-modified polystyrene nanoparticle sample: a) mass spectrum measured by SPAM, b) size distribution determined from the mass spectrum, c) size distribution determined from TEM images, d) relative trapping rate at each particle size, e) TEM image of the nanoparticle sample.

Fig. 9

Size distribution of ions from a nominal 100 nm amine-modified polystyrene nanoparticle sample measured at different trapping voltages, a) 300 V, b) 310 V, c) 320 V, d) 330 V, e) 340 V, f) 350 V, g) 360 V, h) 370 V, i) 380 V, and j) 390 V.