5. System Optimisation

Theo Wilson -

Due to the versatility and dynamics of the LONESTAR system there may be some instances when the spectral output can become crowded and difficult to read.  This is a result of many variables but various adjustments can be made to optimize the spectral output. 

 

No Peak – If a system produces no peak at all a number of issues could be the cause and can show itself in a number of ways which are listed below: 

  • When running the system at 0 % DF, if there is simply a noise line at or around 0 au in height with no sign of a peak this indicates there are no ions passing through the chip to the detector.  To rectify, first, check the main pump or regulator is on and flow is present in the system.  Second, check whether the mode of operation requires some fixtures to be removed or replaced.  In an extreme situation the particulate filter could be blocked and may need replacing.
  • Sometimes when running at 0 % DF there is a peak but when running at elevated DF values the peaks are no longer observable.  This is due to ion attenuation and is a perfectly normal phenomenon related to the size of the sample analyte passing through the device and its concentration.  Large molecular ions will give a response over the full range of DF values whereas small ions will only have a short range.  To overcome these problems try focussing the DF value from 25 to 45 % in order to observe the smaller ions.  
  •  When running the system at any DF value, if the response is a flat line at or around 10 au then an OWLSTONE technician should be contacted.  This may indicate that the FAIMS chip is damaged – although this is extremely rare.

  

System saturation – this occurs when there too many ions striking the detector plate, and as a result the peak(s) may have the top ‘chopped off’ or clipped, this is caused by the ion count overwhelming the detecting electronics.  Another example of saturation can be observed from peak height and width; if either of these are excessively large then saturation has occurred. 

  • Clipping – When running the system at 0 % DF with no sample flow, if the only peak is clipped then the flow rate is too high.  The needle valve on the LONESTAR exhaust controls the flow rate and this should be adjusted until the correct flow is displayed in the software.  The pressure within the LONESTAR will also change as the flow is decreased, check the pressure reading and adjust to the required set point.

 

Clipping – When running the system at 0 % DF with the sample flow on, if the only peak present is clipped this may indicate a very high concentration of sample vapour entering the system.  Check the sample and reduce to the lowest setting (lowest flow) in steps until the signal no longer clips. 

  • Excessive peak size – When operating the system at DF values of 30 % and above, if any peak within the positive section of the compensation voltage sweep is wider than 2V at the peak base and over 7 au in height this could be obscuring extra information.  Either reduce the flow rate as covered in the Clipping instructions above or reduce the sample concentration.  This can be done by preparing the sample at a lower concentration or splitting off some of the analyte before the LONESTAR sample inlet.   Turn down the sample wheel, in steps until this peak has reduced sufficiently.

 

Peak Mirroring – In extreme circumstances the peak in one polarity can be large enough to impact on the opposite polarity spectrum.  This is due to high ion concentrations of one polarity interfering with the detection electronics.  Mirroring is only detrimental if there is information in both detection polarities with one polarity overwhelming the other. 

  • Mirroring – High ion counts can be limited by changing the sample flow through the sample wheel, as mentioned in the clipping section, and by changing the main pump voltage or flow.

 

Minimal Peak Movement – All peaks within the spectral window should move left or right as the DF values are increased.  Very few ions will remain on a straight line in the spectrum, such an observation may indicate a problem (charged particulates may be an exception to this rule).  

  • If a peak that does not move left or right is genuine then its height and width will change with increasing DF value.  This can be monitored using the offline software where the integrated ion current should show a general downward trend.  If, however, this result is constant it may indicate the RF generation stage has broken and needs repair.
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