Low MS sensitivity on QTof mass spectrometers - WKB91752

SYMPTOMS

• MS sensitivity is around three orders of magnitude lower than expected for leucine enkephaline (1E3 instead of 1E6) and other analytes.
• Occurs in both positive and negative resolution modes (customer is not using any other modes).
• Detector setup fails.

ENVIRONMENT

• SYNAPT G2-Si
• SYNAPT G2-S
• SYNAPT G2
• Xevo G2-XS Qtof
• Xevo G2-S Qtof
• Vion IMS QTof

CAUSE

Customer uses tune file created for troubleshooting a low-sensitivity problem on high m/z ions. In this tune file, the quad profile is set to a manual profile in both positive and negative resolution mode.

FIX or WORKAROUND

Set quad profile to auto profile or load a tune file with an auto quad profile.

ADDITIONAL INFORMATION

The default quadrupole automatic profile suits general mass spectroscopy. Experts can adjust the quadrupole radio frequency (RF) voltage configuration, for fine tuning.

The transmission window of the quadrupole in non-resolving mode (used for MS acquisition) is a function of the quadrupole set-mass. A fixed quadrupole set-mass results in a limited mass range for MS acquisitions. To overcome this limitation, the quadrupole set-mass scans during acquisitions. Enabled by a default setting of Auto Profile, this scan function is automatic.

Where appropriate for your application, it is also possible to manually define a scanning profile or set a fixed quadrupole set-mass. You can observe the effect of changing the parameter settings in real time on the peak display.

In the Quad Profile dialog, select Manual Fixed mass, Manual profile, or Auto profile, as follows:

Fixed mass 

The graph below shows a typical quadrupole transmission window for a fixed mass. For a mass-to-set-mass ratio of 0.8, transmission cuts off sharply and, at the high-mass end, as the mass approaches seven times the quadrupole set-mass, the cutoff is more gradual. Thus for a typical fixed set-mass of 500 Da, no ions are transmitted below 400 Da (0.8 × set-mass), the transmission remains high, at 2500 Da (5 × set-mass) and diminishes above 3500 Da (7 x set-mass).

Typical fixed mass transmission profile:

Auto profile 

In Auto Profile mode the RF voltage on the quadrupole is scanned, giving an optimal transmission profile over the mass range of interest. The RF scanning range is a function of the acquisition start and end mass values.

Manual profile 

When you select the Manual Profile mode, you can ramp the quadrupole set-mass, enabling the acquisition of mass ranges wider than the quadrupole transmission window. Specify appropriate values for mass, dwell time, and ramp time to suit the acquisition mass range.

Manual quad RF profile:

Typical manual quad RF profile parameter values:

Requirement: Dwell and Ramp times must add to 100%.

Status information from the front of the instrument: