Jonathan W. Amy

A tandem quadrupole mass spectrometer for the study of surface-induced dissociation

Abstract A tandem quadrupole mass spectrometer has been built for the study of polyatomic ion/surface collision phenomena. Mass-selected ions collide with a surface at translational energies of 10–100 eV and the ionic products are extracted at an angle of 90° to the incoming beam direction and mass analyzed. Inelastic collisions leading to surface-induced dissociation (SID) give daughter spectra which show excellent precision and agree well with data taken using an earlier hybrid instrument. Large internal energies can be deposited upon surface collisions and this leads to extensive fragmentation of such refractory ions as the pyrene molecular ion. The distributions of internal energies acquired upon collision are estimated from the daughter spectra in appropriate cases such as that of tetraethylsilane molecular ions. From these data, the average internal energy of Et 4 Si +· ions after surface activation is found to vary linearly with collision energy in the range 10–100 eV, with some 16% of the laboratory energy appearing as average internal energy of the organic ion. Comparisons are made with internal energy distributions for gas-phase collisions under single and multiple collision conditions. Ion/surface reactive collisions can be studied using the new instrument while collision-activated dissociation can be performed in addition to or as an alternative to surface-activated dissociation.

A hybrid BEQQ mass spectrometer

Abstract A new hybrid mass spectrometer of BEQQ geometry (B, magnetic sector; E, electric sector; Q, quadrupole mass filter) is described. The high resolution (BE) stage is interfaced to the double quadrupole (QQ) stage by a deceleration lens system (dual zoom lens with intermediate d.c. quadrupole) which also shapes the beam. The four reaction regions provide capabilities for both low- and high-energy MS/MS experiments, for exact mass measurements, and for sequential reactions in different regions. Provision is also made for energy and angle-resolved measurements which provide controlled energy deposition in selected ions. Needle collision cells are used in the first and third reaction regions. The r.f.-only quadrupole is encased in a gas-tight shroud and serves as a focussing device for collisions occurring in a laboratory energy range commonly 1–100 eV but sometimes considerably more. Capabilities are in place for recording parent, daughter and neutral species loss spectra associated with reactions in the collision quadrupole as well as the first and second field-free regions. High-resolution selection of the parent ions is possible. More complex scans are accessed through links between the electric and magnetic sector controls (provided by a linked scan microprocessor) and a system of microprocessors which controls the lens and quadrupole voltages.

Performance of a hybrid mass spectrometer

Abstract The BEQQ hybrid instrument provides capabilities for tandem mass spectrometry and for high resolution measurements. Reactions can be performed at energies between 5 and 3000 eV using any one of four reaction regions. Angle-resolved experiments allow control over the energy deposited upon collision. Processes which occur in sequence in different reaction regions are easily examined. The very large data domain is interrogated selectively. This involves the use of conventional parent, daughter and neutral species loss scans as well as new experiments in which the quadrupole is used to remove interferences in conventional linked scans and MIKES scans. Transmission data and data for efficiency of collision-induced dissociation are presented.

Broad-Band Fourier Transform Quadrupole Ion Trap Mass Spectrometry

Broad-band nondestructive ion detection is achieved in a quadrupole ion trap mass spectrometer by impulsive excitation of a collection of trapped ions of different masses and recording of ion image currents induced on a small detector electrode embedded in but isolated from the adjacent end cap electrode. The image currents are directly measured using a simple differential preamplifier, filter, and amplifier combination and then Fourier analyzed to obtain broad-band frequency domain spectra characteristic of the sample ions. The use of the detector electrode provides a significant reduction in capacitive coupling with the ring electrode. This minimizes coupling of the rf drive signal, which can saturate the front-end stage of the detection circuit and prevent measurement of the relatively weaker ion image currents. Although impulsive excitation is preferred due to its broad-band characteristics and simplicity of use, results are also given for narrow-band ac and broad-band SWIFT (stored wave-form inverse Fourier transform) excitation. Data using argon, acetophenone, and n-butylbenzene show that a resolution of better than 1000 is obtained with a detection bandwidth of 400 kHz. An advantage of nondestructive ion detection is the ability to measure a single-ion population multiple times. This is demonstrated using argon as the sample gas with an average remeasurement efficiency of >90%. Tandem mass spectrometry experiments using a population of acetophenone ions are also shown.

In situ optimization of the electrode geometry of the quadrupole ion trap

Abstract The performance of the quadrupole ion trap is affected by imperfections in the quadrupolar electric fields used to trap the ions. These effects can be compensated for by stretching the trap along its axis. A systematic investigation of the effects of stretching and compressing the z 0 dimension of the ion trap under operating conditions shows improvement in resolution and signal intensity if the z 0 dimension if the quadrupole ion trap is stretched by 9%, with respect to the value for the nominally pure quadrupolar trap. A plot of the relationship between the mass shift for the nitrobenzene molecular ion and z 0 is also presented. It shows that the shift drops to zero at the geometry at which the performance is optimized.