Errors, Malfunctions, and Design Issues

A number of malfunctions and errors were encountered in early experiments with the Velox 360, ranging in severity from minor issues requiring only resetting a component to a consistent error that required an overhaul by Prosolia engineers. Of the issues encountered,

Figure 4.6: Mass spectrum of 1 ppm atrazine in

acetonitrile, analyzed using the Velox 360, illustrating the major background ions observed. Ions at m/z 604.3 and 609.4 are observed consistently when using the Velox 360. A variety of ions at lower mass are also regularly observed, albeit at lower intensity.

the least severe was an intermittent “stacker position unknown” error. This error refers to the linear actuator used to load cartridges from the vertical magazine into the ion source. If for some reason the linear actuator fails to retract fully or return completely to its initial position, as may happen if a cartridge is improperly loaded, this error may occur. It is readily dealt with by eliminating the source of the problem, such as a jammed cartridge, and resetting the position of the stacker using the diagnostic tools available via the instrument soft panel. This issue can generally be attributed to user error.

A more persistent and difficult to resolve problem was observed immediately upon installing the Velox 360 on the LTQ-FT XL mass spectrometer. Signal was intermittent and very low intensity at expected spray voltages (3-4 kV), and remained unstable at significantly elevated voltages (up to 8 kV) although signal intensity improved. Upon investigation it was observed that the spray voltage measured at the output from the mounting flange was

significantly different from the voltage set in the instrument control software. At a voltage setting of 4 kV, the measured voltage output from the mounting flange was only 1.5 kV. Testing the Velox 360 with the mounting flange on another instrument yielded performance as expected at 5 kV, indicating that the problem was likely at the interface between the flange and our mass spectrometer. Investigating further, it was discovered that the high voltage contact at the mass spectrometer inlet had been displaced slightly, yielding poor electrical contact with the pin in the mounting flange. The high voltage contact in Thermo Scientific mass spectrometers is mounted in a flexible insulator. Through use, the contact may be displaced back into the instrument housing. This can be corrected fairly easily by removal of the front housing of the mass spectrometer and pushing the high voltage cable forwards, returning the contact to its original position.

The ultimate cause of this malfunction is a slight difference between the design of the Thermo Scientific electrospray ionization source and the mounting flange of the Velox 360. The difference between the two is shown in Figure 4.7: the entire high voltage contact assembly extends approximately 2 mm further from the surface of the flange in the Thermo Scientific ESI source than in the Velox 360 mounting flange. The extra 2 mm in the ESI source is sufficient to maintain good contact even when the high voltage contact on the mass spectrometer is slightly displaced. This difference was reported to Prosolia, Inc. and is expected to be incorporated in future revisions of the Velox 360 mounting flange design.

The most persistent issue which arose was a “cartridge position unknown” error which began to occur shortly after receipt of the Velox 360. This error indicates that the carousel has not completed its rotation to the next position properly. If this error occurs while operating in manual mode, simply pressing “next” will frequently cause the carousel to rotate to the proper position and allow the experiment to continue. However, in automatic mode the error cannot be handled without stopping the experiment, causing the loss of any cartridges which may already have been wetted. The failure of the carousel to complete its rotation Figure 4.7: Differences between the high voltage contacts on the Velox and Thermo ESI sources. The high voltage contact extends 2 mm further from the face of the flange on the ESI source.

properly is likely due to friction between the cartridges and the metal clip used to apply the spray voltage, exacerbated by loose pulleys in the carousel drive system (shown in Figure 4.8). Tightening the pulley mounting clamps in the drive system reduced but did not eliminate the problem, and the friction between the clip and cartridge could not be easily adjusted. The ion source was shipped to Prosolia, Inc. and overhauled by engineers there. Several minor issues were corrected, and the drive pulleys and belts were adjusted. After this, the problem was greatly reduced, and found to occur very infrequently. There appears to be some minor slippage of the carousel over time, but routine calibration is all that is required to prevent this from developing into a significant problem. Prosolia engineers have indicated that they plan to modify the pulley assembly design to incorporate a locking, D-shaped fitting between the pulleys and the drive shafts, which should minimize slippage.

These errors and malfunctions, while certainly problematic, were not significant enough to impede testing of the ion source.

All hardware problems were effectively alleviated with the assistance of Prosolia engineers, and have not recurred with

significant frequency. Our assessment is that these reflect the relatively early stage of commercialization of this technology, and the reliability of the Prosolia paper spray ion sources is likely to improve as further effort is made to refine and enhance the design.

Figure 4.8: The Velox 360 carousel drive system. a) carousel; b) carousel drive pulley mounted on drive shaft; c) drive motor with pulley.