The Importance of Repeatability: Quantitative and Qualitative Analysis - Gaschromatography

The gas chromatograph is one of the most versatile, selective and sensitive of all analytical instrumentation, especially for hydrocarbon applications. This is because the analysis consists of 3 parts: injection, separation and detection. The selectivity aspect results primarily from the separation when using detectors, like FID and TCD, and gives the user confidence that the measured component is really what is expected. The retention time from injection to component peak is used during methods development to make certain the peak identity is reliable and free from other component interference.

 

Figure 1: Falcon Ultrafast Gas Chromatography System Diagram

Thus, retention time repeatability becomes critically important for reliable quantitative analysis. Run to run retention time variance must be stable enough to have confidence that each component is being properly identified. To be clear, all gas chromatography exhibits retention time variance at some level. The question is how much variance can be tolerated to be confident that components are appropriately identified and measured. This specification is defined at the point of the methods development and the peaks named in the calibration file for run to run concentration results.

Figure 2: Obvious and Unacceptable Retention Time Variance

Figure 2 demonstrates an extreme example of what happens when the retention time variance is unacceptable: misidentified peaks and erroneous results. Modern gas chromatography systems have become very good at delivering constant flow and constant temperature for injectors, detectors and column ovens in the case of isothermal methods or reproducible run to run programmed temperature column oven profiles… all critical for retention time stability.
 

But what about ULTRAFAST GAS CHROMATOGRAPHY? There are those who question whether the ultrafast GC can deliver the retention time stability of high thermal mass, slow, research grade big box GCs. The answer to the question is of course yes, the ultrafast GC can equal or beat any traditional GC retention time stability. Careful attention to thermal management, extra-column dead volume, sample size and other operational details can actually deliver extremely low retention time variance.
 

The best way to demonstrate the Falcon Ultrafast GC’s retention time stability is to look at a real-world application over time. In this rather complex application, retention time slices were used to quantify sample composition. The original method development used GC/MS data to define which time slice areas were proportional to the desired result. It should be obvious that at the 99 second cut point any significant run to run variance would cause an erroneous result.

Figure 3a: Process Falcon Ultrafast GC 1 from the End of Q1 from Startup

Figure 3b: Process Falcon Ultrafast GC 1 from End of Q1 & Q2 Overlaid

Figure 3c: Process Falcon Ultrafast GC 1 from the End of Q1, Q2 and Q3 Overlaid

Figure 3d: Process Falcon Ultrafast GC 1 from the End of Q1, Q2, Q3 and Q4 Overlaid

 

In the example, the area under the curve prior to 99 seconds accounts for low boiling components. Two other cut points define the balance of the desired result but fall in well separated areas in the chromatogram. For the 99 second cut point, big area differences would result with very small change in retention time due to the slope of the curve. No variance can be tolerated. Use of Infometrix LineUp™ uses the "gold standard" chromatogram from startup to make sure that any variance is removed from each run.

The answer to the question, "can the ultrafast GC deliver repeatable retention times" is a resounding YES! Using retention time alignment from LineUp™ insures proper results. The pre-alignment raw retention time variance further demonstrates that the ultrafast GC is extremely repeatable. Over an entire year and more than 175,000 injections, the shortest retention time subtracted from the longest retention time was only 0.6 seconds or just under 0.5%... peak to peak… for a whole year. For comparison, a typical application specification is 0.5% relative standard deviation over 24 hours.

One might ask how this translates into benefits for simpler applications. Whether in the lab or the process, misidentified components occur even in simple applications. Having excellent retention time stability makes the data QA process much easier and, in the process, virtually eliminates the dreaded "regating the peaks." And superior retention time stability leads to the ability to have instrument to instrument reproducibility as demonstrated in the ASTM standard method D7798. In an industrial plant the combination of excellent retention time stability and alignment can result in statistically identical instruments as instrument to analyzer pairs. For more about this, click on this link http://www.teledyne-ai.com/Products/Gas-Analyzers/Documents/pgs_boiling-range-distributions-jgp3.pdf to see another real world application example. 
 

Please don’t hesitate to contact Inventech to learn more about the ultrafast, ultra-user-friendly, highly durable, practical, reliable, versatile, compact and economical Teledyne Falcon Series Ultrafast Gas Chromatography System.