Performing Blank Subtraction in OpenLab CDS

Answer

This Information Applies To: Agilent OpenLab CDS 

Issue

When analyzing a sample, the chromatogram may show baseline drift. The obtained signal may be caused by analytes as well as by dilution solvents, mobile phases, additives etc. To get a cleaner chromatogram with contribution of the analytes only, perform the blank subtraction.

Background

Blank signals are obtained from:

• A blank sample within the sequence
• A blank sample outside of the sequence (for example, a single run)

The new signal is calculated by subtracting the blank signal:
New signal = sample signal - blank signal

For extracted chromatograms using a specific wavelength:
New signal = sample signal at wavelength - blank signal at wavelength

Prerequisites

• To be able to carry out blank subtraction, the privilege of Processing method > Edit signal parameters is required. Privileges are configured in the Control Panel.
• If the project enforces method approval, the method status must be Generic. For details, see Method approval. 
• Users have acquired the signal for a blank run, that is, a solution with the same solvent, additives, etc. as the samples, but without the compounds of interest.
• Users have acquired the blank run with the same detector (same signal name or same signal description) as the samples.  
  • The signal name is the short name of a detector, for example, DAD1A.
  • The signal description typically includes more details, for example, DAD1A, Sig=235,10 Ref=550,60.
  • By matching the one or the other, users define how flexible the processing method can be used with different acquisition methods.
• Blank subtraction is not supported for MS signals.

Steps to Follow

1. In the Processing Method window, click the General > Signals node and the Blank Subtraction tab (Figure 1).
2. Under Blank subtraction applied on, from the drop-down list, select for which injections users want to subtract a blank.
   If users prepare calibration standards differently, they may not show the same matrix effects as the samples. In this case, choose Calibration standards only or Samples only (exclude cal. standards) (Figure 1).
   Note: Calibration standards only option is available in OpenLab CDS version 2.7 or above.
   

   
   
   Figure 1. Choose which injections to perform blank subtraction 
   1. Processing Method window, 2. General, 3. Signals node, 4. Blank Subtraction tab, 5. Blank subtraction applied on 
   
   

3. Choose how users want to use blanks:
   1. To use blanks as defined in the sequence:
      • In the Blank Subtraction tab, select Use blanks defined in the sequence. The signals of blank runs in the sequence will be subtracted.
      • The subtracted blank is either:
        • The blank (sample type Blank or Double blank) last acquired before the current injection (Figure 2), or
        • The blank defined in the Blank column of the Injection List (Figure 2).
          

          
          
          Figure 2. Blank column in Injection List
          1. Last blank in the current injection, 2. Other blank option in the current injection under Blank column
          
          

   2. To use a specific blank outside of the sequence:
      • In the Blank Subtraction tab, select Use specific blank, and click Browse to open a list of all currently loaded injections. Select the injection to use as a blank.
      • The injection must be loaded so that users can select it in the method. It does not need to be loaded during reprocessing (Figure 3).
        

        
        
        Figure 3. External blank selection
        
        

4. If necessary, select Perform blank subtraction also if data rates are different.
   If a blank and a sample have different data rates, the data rate of the blank is adjusted. Data points are removed or created by spline interpolation. A corresponding warning is shown in the injection tree.
   Independent of the status of this check box, if a blank and a sample have different run times, blank subtraction is only performed on the common time range. A corresponding warning is shown in the injection tree. The new signal will contain corrected and not corrected data points.
5. Select a suitable signal matching option. By default, Perform blank subtraction if signal names match is selected.
6. Check the blanks in the Blank column of the Injection List window. By default, the blank last acquired before the current injection (Last Blank) is selected. If necessary, select a different blank from the current sequence.
7. Reprocess the data with the changed processing method (Figure 4).
   

   
   
   Figure 4. Example of Blank Subtraction using a Last Blank in a Current Injection
   
   

   Once blank subtraction is performed, chromatograms are displayed with subtracted signal instead of original signal. Description of a subtracted signal has an asterisk * as prefix, [b] as suffix to the signal name to differentiate it from original signal (Figure 5).
   

   
   
   Figure 5. Chromatogram with background noise removed 
   1. Before Blank Subtraction, 2. After Blank Subtraction
   
   

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