Speeding up Lonestar/FAIMS Core Data Acquisition

It is possible to speed up the data acquisition on the Lonestar or PAD units by altering some key parameters in the configuration file via the configuration editor.

WARNING – Setting any values incorrectly in the configuration file could permanently damage your system. In particular some versions of the configuration editor have a bug which means that the 'RF Pulse Frequency, Mhz' can reset itself to a default value of 28. The system MUST NOT be run with the RF set like this, you must check that it is set to either 25 or 26   before saving the configuration. The problem tends to arise if you go to open a configuration, and then click cancel. So do watch out and always check the RF Pulse Frequency field before saving a configuration.

As the scan speed is increased there will be more and more distortions seen in the FAIMS spectra. Note that most of these distortions are due to the limits of the ion detector and not the FAIMS chip, therefore you may be able to tolerate these distortions if, for example, you are using the FAIMS system to pre-filter the ions before passing them into a Mass Spec.

All the settings that you can change are under the heading ‘CV Sweep’ – but don’t forge to check that the RF Pulse Frequency is 25 or 26 before saving the configuration!.

Description of functions and recommended settings:

CV start and CV end – These define the range that the compensation voltage sweeps over. These settings must be symmetric, e.g. +6 and – 6, +3 and -3 etc.  Changing these settings won’t actually alter the speed at which data is collected, but reducing the CV range will reduce the signal distortion effects caused by speeding the system up

• Maximum               +/- 8.0
• Default                    +/- 6.0
• Minimum               +/- 0.1

CV number of samples -  Defines the number of compensation voltage data points to take. The smaller the number the faster the system will scan. Reducing the number of samples will increase the distortion slightly and will result in less resolution on the FAIMS spectra.

• Maximum              2056
• Default                   512
• Minimum               128

Sampling Period – Defines the rate of change of compensation voltage. The smaller the number the faster the system will scan. Reducing the sampling period will result in signal distortion and ‘ringing’ appearing on the FAIMS spectra. Running below 1ms will result in significant signal degradation.

• Maximum              10
• Default                   2
• Minimum               0.5

Oversweep – Switching the ion detection mode switches between positive and negative ions results in an induced current on the FAIMS detector. The oversweep function allows the detector time to settle to a steady state before starting to collect a FAIMS spectra. Reducing the oversweep time will increase the scan speed but will result in a curve or bend being observed in the baseline. If the oversweep is set to 0 then large amounts of ringing will be observed at the start and ends of the FAIMS spectra.

• Maximum              240
• Default                   120
• Minimum               0

Here are some recommended settings for achieving different scan speeds and signal quality.

NOTE – ALWAYS CHECK THE RF PULSE FREQUENCY SETTING WHEN ALTERING THE CONFIGURATION FILE -  IT MUST BE SET TO EITHER 25 or 26,  ANY OTHER VALUES WILL PERMANTELY DAMAGE THE SYSTEM.

The variables that you can change are as follows:

CV - Keep symmetric, reducing to +/-3V is a good start

Steps - Can be reduced to 100 (256 and 128 are recommended)

Sample Time (ms) - Can be reduced to 0.5

Oversweep (ms) - Can be reduced to 0 (120 or 60 recommended)

The fastest combination that I found whilst still being able to see some sort of fingerprint was:

CV = +/-3V

Steps = 128

Sample Time = 0.5

Oversweep = 60

However you could improve this even further (by dropping the Oversweep to 0 and the number of steps to 100) if you aren't interested in what the FAIMS spectra is doing, all the distortion that you will see on the fingerprint is due to the detector, not the FAIMS chip itself. The only exception to this is that you might start to see the peak position shifting slightly, as its doubtful that the CV bias amps can keep up with the requested rate of change (i.e. the actual CV bias will lag the set point slightly).

Caution: There is a bug in the configuration editor which means that the 'RF Pulse Frequency, Mhz' can reset itself to a labview default value of 28. The system MUST NOT be run with the RF set like this, you must check that it is set to either 26 or 25 (it doesn't matter which) before saving the configuration. The problem tends to arise if you go to open a configuration, but change your mind and click cancel. So do watch out and always check that field.

Please note that when you load a new configuration you may get a couple of warning messages about the positive mode and negative mode models being invalid. No need to worry about this, just click OK.