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4). In the broad sense deconvolution is the process of extracting one signal from a complex mixture of signals [2] (Figure 5).
There are several sources of deconvolution programs which are either built into the GC-MS data analysis software by the instrument manufacturer or are available as an add-on and are capable of processing most GC-MS data files produced using many different instruments. Some suppliers give details of how their software performs the deconvolution but all of them use different algorithms and therefore it is possible to get slightly different results using each one.
Figure 3: Spectral skewing/tilting is mostly seen in scanning instruments, in particular when a high number of data points are not obtained across the peak. It is caused by the concentration of the ions changing while the mass filter scans across the mass range. For example, if scanning from low to high mass, at the front of the peak when allowing low mass ions to the detector there is a low concentration of ions being produced, as the scan progresses to higher m/z there is an increase in concentration of ions being produced. The opposite is seen at the back of the peak, where there is a high concentration of ions being produced when scanning at the low masses, which decreases when scanning for the higher masses. Deconvolution algorithms must de-skew the data first, to ensure these changes don’t affect the determination of co-eluting peaks [3].
The main steps in the deconvolution process, the exact steps and order are software dependent, are to define and handle the noise, de-skew the data, find the peak apexes for each ion (ions belonging to the same peak should have the same apex), track the rate of rise and fall for each ion profile (check the peak shape) and finally produce a deconvoluted mass spectrum for each peak (Figure 6). Most software can also automate the library search against commercial or user-created databases and use the retention index (or time) from the database as a qualifier, to check that the correct isomer has been identified.
Figure 4: Chromatogram showing the EICs for the unique masses of co-eluting peaks, along with the non- deconvoluted (raw) mass spectrum, deconvoluted mass spectrum and library hit for diazinon [3].
elute with the target analytes. If their mass spectra contain one or more of the target analytes’ unique masses this could result in mis-identification or most commonly false negatives due to the ion ratios for the target being incorrect, the target analyte is not identified and therefore deemed to be not present in the sample.
So, whether target analysis is being performed in more complex samples or unknowns are being identified, ensuring that a pure mass spectrum or ions are extracted is important for identification.
Deconvolution is an automated process,
capable of detecting differences in the mass spectra across a peak which can indicate co-elutions. Mass spectra across a peak should be identical (after de-skewing spectra from scanning instruments (Figure 3)), with extracted ions for the analyte having both the same apex (retention time) and peak shape. Changes in the peak apex or peak shape could indicate another peak is chromatographically co-eluting (Figures 2 and 6). An algorithm in the deconvolution program, then derives a mass spectrum for each peak which is high quality and library searchable, ready for identification (Figure
In terms of optimising the deconvolution software, again this is very software dependent, but could result in some peaks not being deconvoluted (mixed mass spectra still occur) or too many ions being removed and entered as additional peaks when they all belong to the same peak (too many peaks found and too few ions for identification). It might be possible (or necessary) to optimise integration parameters or change the sensitivity to determine how small the ions or peaks are to be deconvoluted, this could also be applied as a filter after deconvolution has taken place. Parameters that consider the degree of chromatographic co-elution (or resolution) could be present. The sensitivity of the peak shape – should the ions be a similar shape or exactly the same shape for them to be included/excluded in the mass spectrum?
No matter which deconvolution algorithms are used, they all benefit from obtaining high quality GC-MS data. There should always be some chromatographic resolution, as total co-elution of peaks with the same apex and shape cannot be deconvoluted. The best chromatography for the application should be achieved, as sharp, Gaussian peaks are easier to deconvolute. The mass range acquired should be applicable to the application, deconvolution cannot
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