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FLEXO Magazine : April 2009
TECHNOLOGIES & TECHNIQUES of component present and is measured by the data system in a process called integration. To calculate this area, the data system must also accurately determine the baseline under the peak. The peak areas for the standard are stored in the PC and used to calculate the amount of each solvent identifi ed in your samples. IMPORTANCE OF OPERATOR REVIEW In order to check that a GC is performing properly, the opera- tor must inject a known sample. The easiest way of doing this is to inject the same standard that was used to initially set up the data system. This is called a check standard. Operator review of check standard results is important. Both the actual chromatogram and the component identities and areas in printed report should be reviewed. Remember that the data system can only report what it sees and is not able to identify or integrate the peaks correctly if there is a problem or the GC analysis conditions change. On the other hand, the operator can see if there is a problem with the check standard data if he or she knows what to look for. If, after careful review, the check standard results show no problems, you will be able to trust your unknown sample results. Always correct any GC problems before proceeding to analyze samples. REVIEW YOUR RESULTS Review the check standard and sample runs by looking for the following: • Correct component identity/retention times match. • Stable detector output signal/baseline drift. • Off -scale detector response. • Reported peak areas and amounts. COMPONENT IDENTITY Your check standard chromatogram should match the refer- ence chromatogram. If there are 12 components in the standard, all 12 peaks should be present in your current check standard in- jection. Missing peaks in check standard runs indicate a problem that should be corrected before running samples. Review your check standard chromatogram for extra peaks. Sample identity and the reported amounts can be adversely af- fected if unknown extra peaks are present. Unknown or “ghost” peaks can interfere with your sample results. Run a system blank (no injection) or an empty sample vial (sample blank) and check the chromatogram for the appearance of extra peaks. NEGATIVE BASELINE DRIFT A common chromatograph data problem is caused by negative baseline drift. Note in Figure 3a how the detector signal is drift- ing down for the fi rst one-third of the chromatogram and then becomes perfectly fl at in the last two-thirds. This fl at section of the chromatogram indicates that the signal has indeed gone off - scale, or below the detector zero. The bottom part of the area for all of the peaks in the last two-thirds of the chromatogram has not been collected and your report will show lower-than-actual sample amounts for these peaks. POSITIVE BASELINE DRIFT If the detector signal or baseline shows excessive positive drift as in Figure 3b, this may indicate a column or system problem. This should be corrected before running sample for analysis. Positively drifting detector signals will make it very diffi cult for the data system to draw the correct baseline under any peaks riding of the upslope of the detector signal. Integrated areas for these peaks will usually be too large, resulting in incorrect high sample amounts. FIGURE 2. Chromatogram of a solvent standard used as a reference. 32 FLEXO APRIL 2009 www. f le xography. org