Changes in Peak Shape (Part 1)

Part 1: Injection and Data Acquisition

Achieving good peak shape is essential for accurate quantitation and reliable identification in LCMS. Distorted peak shapes not only indicate chromatographic issues but can also signal problems affecting ionisation and MS detection (e.g., ion suppression, detector saturation).
This topic is divided into two parts:

1. 1. Injection & data acquisition effects on peak shape.
2. 2. Causes of peak shape changes (fronting, tailing, broadening) and MS-specific factors.

A. Effect of the Sample Solvent

In LCMS, the sample solvent must be compatible with both the LC separation and MS ionisation process.

• Solutions

   
   Re-dissolve samples in weaker solvents (preferably matching initial gradient).
   Use a "sandwich" injection (water-sample-water) to minimise diluent effects.
   If solubility is poor, adjust sample prep to prevent precipitation in weaker solvents. 
• 

  Fig. 1 Visualisation of the sandwich injection

B: Impact of Injection Volume

Excessive injection volumes can overload the column and ion source, distorting peaks and reducing MS signal stability.

C: Data Acquisition Rate & Detector Response

A frequently neglected point is data acquisition. However, this has a significant impact on the quality of chromatograms and should therefore always be adapted to the respective chromatography. A representation of different data acquisition times is shown in Fig. 2, varying from 1 to 10 ms. The 1 and 2 ms acquisition settings adequately describe the peak shape. The 5 ms acquisition rate begins to poorly describe the peak and is quite angular which could impact on quantitation. The 10 ms results insufficiently describes the peak and misses significant portions of the peak. Fig. 3 also illustrates the variability which could be expected if insufficient data points are collected.

Peaks with a low data acquisition rate are not sufficiently described due to too few data points. This can lead to strong peak area fluctuations, as no data point can lie at the peak maximum with slight fluctuations. But too high a data acquisition rate can also have a negative effect, such as the noise increasing and fewer compounds which can be detected in a given time frame. 

• 

  Fig. 2 Example of improved peak description with more data points as dwell time increases 

   

• 

  Fig. 3 Illustration of the variability of peak area when using a low dwell time (10 ms)

 

Another setting option is the response of the detector. If the response is set too high, the peaks are artificially broadened. If it is set too low, the peak becomes narrower but the noise increases.

Unfortunately, there are no standard values for the data acquisition rate and the response of the detector. They should always be adapted to the application's chromatography. As a guideline, it should be as fast as possible with an acceptable signal-to-noise ratio.

 

 Low acquisition rate: Too few points across a peak = inaccurate area intergration and poor peak shape representation.

 High acquisition rate: Excessive noise, which is particularly problematic for low-abundance ions.

 Detector response settings:
High Response Time = Artificially broadened peaks (loss of resolution)
Low Response Time = Sharper peaks but increased noise

Best Practice for LCMS

Your Shimadzu LCMS Team

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