Verified Hydrocarbon Analysis: Synergy Between DHA and VUV

My colleague Dan Wispinski recently wrote a series of blog posts sharing his experience with detailed hydrocarbon analysis, or DHA. In part one, he notes that DHA’s simplicity and reliability have cemented it as a gold standard in petrochemical analysis. However, Dan also points out that DHA, which hasn’t changed much in several decades, has its fair share of drawbacks. Our approach to DHA using the Verified Hydrocarbon Analyzer™ and the Verified Hydrocarbon Analysis (VHA) application addresses these issues while maintaining DHA’s analytical utility. What is VHA and how can it save you valuable resources? Let’s take a look!

The First Change: Introduce VUV to the Equation

The flame ionization detector (FID) is the de facto detector used with DHA. It’s inexpensive, relatively sensitive, and easy to operate. On the other hand, one major drawback FID brings to the benchtop is the lack of information provided by the detector. Because it provides no information beyond compound retention time, DHA relies on compounds’ individual retention index (RI) values as the sole means of compound identification. Even with DHA’s extensive run times, there’s still room for error.

Let’s start with one change: hook up the GC to a Verified Hydrocarbon Analyzer™ rather than an FID. By switching to Vacuum Ultraviolet (VUV) Spectroscopy, we already gain some key advantages:

1. Individual compound identification
2. Unknown compound classification

Looking at Figure 1 below, we can see both advantages at work. Each compound in the VUV spectral library has its own unique spectral shape; for example, it’s possible to distinguish between m-xylene and p-xylene, despite having similar structures and coeluting under most GC conditions. In addition, compounds within the same class have the same general spectral shape, so even if a peak can’t be specifically identified using the VUV spectral library, its spectral shape can be used for compound classification.

Figure 1. Comparison of various compounds in each of the PIONA classes. Individual compounds can be identified by their unique VUV spectra, distinguishing them from others with similar structures. Even if a compound can’t be identified specifically, its spectral shape can be used for PIONA classification.

With this one change, we can access an entire third dimension of analytical power, radically improving the accuracy of the analysis. Plus, the ability to classify unknown compounds based on spectral shape means that there are few, if any, truly unknown compounds in a VHA analysis.

Next: Shorten the Run Time

A single DHA run can take hours. It needs that long to achieve the chromatographic separation necessary for RI-based peak identification. However, using the Verified Hydrocarbon Analyzer instead of a GC-FID setup allows us another advantage: we can analyze the samples in a much shorter amount of time.

DHA uses a 100-meter long column to ensure adequate chromatographic separation. Using a method translator such as the one provided by Restek®, we can obtain GC conditions that achieve a nearly identical run on a shorter 60-meter column. Figure 2 below shows a comparison of a gasoline sample analyzed on both columns. Shortening the column shortens the runtime from nearly 3 hours to under 50 minutes – 3x to 4x faster – and yet the separation is almost identical!

Figure 2. Comparison of a gasoline sample run on a 100-meter column (orange) versus a 60-meter column (blue) (a). Despite a significant reduction in column length, we observe extremely similar chromatographic separation, even when we zoom in and examine the finer points of the chromatogram (b-c).

Those of you with keen eyes might notice that, despite the similarity between the runs, the 60-meter column does yield some coelutions. For traditional DHA, this likely wouldn’t be acceptable, as chromatographic separation is necessary for accurate compound identification. However, these coelutions emphasize the true power behind VUV spectroscopy: spectral deconvolution and three-dimensional data. Even if two or more compounds coelute, the differences in their unique VUV spectra can be used to distinguish them. My colleague, Alex Hodgson, wrote a detailed blog post explaining deconvolution. Figure 3 shows an example coelution and deconvolution.

Figure 3. Example deconvolution of acetone and isopentane. Despite the chromatogram showing a single peak (a), these two closely coeluting compounds can be distinguished using spectral deconvolution (b-c). Deconvolution is made possible by differences in the compounds’ unique VUV absorbance spectra.

VHA gives us comparable separation in a much shorter amount of time, and any coelutions can be addressed thanks to spectral deconvolution. In a single day, we can now get DHA-level speciation for 3x-4x the number of samples!

One Last Step: Automate Data Analysis with VUV Analyze™ Software

We’ve shortened the runtimes needed to speciate the compounds in our sample; how about we streamline the analysis process as well? Historically, DHA data has needed extensive manual review to match compounds using a large RI table. Even though DHA is easier today with the use of dedicated software, the data still needs to be manually checked to confirm compound matches. However, since the matches carried out using the VHA application are spectrally verified, we can streamline the analysis to a simple push-button approach.

Suppose your lab runs reformate samples on a regular basis. With a little work on the front end, you can develop a reformate analysis “template” based on a representative reformate sample. Using VUV Analyze Software, you can build a template using the representative sample. After confirming the generated speciation list covers the compounds you’re interested in and making some minor tweaks to ensure total analyte coverage, the template can be saved and re-used for future reformate samples. Now, instead of spending hours poring over DHA data, you can load your reformate template and let VHA automate the analysis of each sample in under a minute.

With the Verified Hydrocarbon Analyzer, DHA-level speciation can be achieved in a significantly shorter time with greater accuracy and less resources. Does this sound like something you might be interested in? Have any questions about VHA? Let us know in the comments or contact us directly. Stay tuned for more VHA updates in the future!