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A compositional heterogeneity method to simplify LC data analysis

By Susan Goodchild and Jeff Hurlbut

A custom routine that automates calibration and curve fitting reduces the time to analyze polymer compositional distribution liquid chromatography data by 50%. Researchers at Solutia, Inc. (Indian Orchard, MA) use the compositional heterogeneity method to determine the weight percent composition of two copolymers, monomer A and monomer B. The routine has largely automated what was previously a time-consuming manual process that involved the selection of baselines from calibration chromatograms, calculating statistical parameters, fitting a second-order polynomial curve to the data, locating peaks on the sample chromatogram, and reformatting the data for presentation purposes.

The company's Physical and Analytical Sciences Center (PASC) performs basic and applied research to support synthetic fibers, plastic interlayer for laminated glass, phosphorus-based food ingredients and performance products, high-performance specialty chemicals, and chemical intermediates businesses. The center uses a wide range of polymer characterization techniques to meet the needs of product development and laboratory-scale manufacturing operations. One of its standby methods is the compositional distribution LC method, which gives percent composition and a measure of the polydispersity of a copolymer, based on the weight percentage of one of the monomer units. This method differs from the traditional LC, in which the components of the sample elute (move along the column) at varying speeds based on their affinity to the stationary phase in the column. Instead, the sample is deposited on the column, and elutes based on its solubility with the mobile phase. Initially, the LC analysis is performed on a series of standards with varying proportions of two different monomers. The resulting chromatograms are used to prepare a calibration curve that is used to assess the composition of the sample.



Figure 1 Calibration data fitted to a second-order polynomial. The quadratic is used to map retention time to weight fraction.

Figure 2 Sample chromatogram shows calculated results for heterogeneity and asymmetry indices.

 

This method has provided more than satisfactory performance in terms of accurately characterizing a number of copolymers. However, the required statistical analysis was time consuming and so difficult that it required the attention of a professional with an advanced degree. In order to generate the calibration curves, the scientist had to choose the baseline points at the beginning and end of the peak for each calibration chromatogram. Then, with the statistical analysis software used, data had to be entered on a number of different screens to enter the calibration data. The process took about 10 min for each calibration chromatogram.

From this point, a statistical program took over and generated the necessary output parameters. However, the output from the program was available only in a rigid format that did not match the researchers' needs. In particular, researchers usually need to create a single-page report that shows the sample chromatogram along with the calibration data and curves, statistical output, and other parameters. This meant that several different plots had to be cut out and taped together and finally photocopied to produce the final report.

Performing the statistical analysis on a typical run of eight samples, which are run in duplicate, took a total of 16 hr. Since this analysis was run on a regular basis, it occupied a considerable amount of scarce researchers' time. In an effort to streamline the process, researchers investigated commercial data analysis software to determine whether one would have the necessary statistical routines, graphical output capabilities, and a programming language that would allow them to automate the data analysis process.

They discovered Origin™ version 5.0, a Windows-based technical graphics and data analysis software package (Microcal Software, Inc., Northampton, MA), which provides several crucial advantages that make it well suited for this application. The software offers a wide range of statistical capabilities, including a fitting function category designed specifically for chromatography applications (Figure 1). It features a wide range of graphing capabilities, allowing the user to adjust virtually any parameter of the graph simply by clicking on it with a mouse. Finally, it utilizes an extremely powerful programming language, LabTalk™, that provides access to virtually every function in the program

How Origin Was Used

Working with Origin application programmers, the researchers developed a LabTalk program that largely automates the statistical analysis process. A custom Origin tool provides the user interface and manages the process. The user starts the process by clicking the NEW CALIBRATION DATA button to open a worksheet into which he or she can enter chromatogram readings. The user then clicks the MEAN AND STANDARD DEVIATION button to calculate the parameters for each standard and then simply clicks the GRAPH button to plot the mean and standard deviation versus retention time.

The user specifies a request number and date, which Origin uses to data-stamp the plot. The software fits the data to a second-order polynomial and later uses the coefficients to calculate the percent composition based on the retention time of the sample. Next, the user clicks the OPEN CHROMATOGRAM button and loads the sample chromatogram in the form of an ASCII file. The routine parses the report file, removes index numbers that are not needed, and saves the rate values that are specified by the user. The rate values are used to calculate the independent x-axis of the output curve as retention time.

After the software plots the curve, the user takes advantage of specialized automatic and manual tools to zoom in and set a baseline, which automatically snaps to the curve. He or she then clicks PROCESS to generate the final graph, which contains the calculated values for the analysis: the percentage composition of the sample, a heterogeneity index (HI), and an asymmetry index (AI) (Figure 2). The percent composition is a weight average of the total polymer. The HI and AI refer to the shape of the composition distribution. The HI is a measure of the breadth of the distribution of the polymer, and the AI refers to the skewness of the distribution. An AI of 1 means that the polymer is symmetrical with respect to monomer A distribution; an AI of greater than 1 means that the polymer is skewed to high A content, and an AI of less than 1 means that the polymer is skewed to a low A content.

The routine significantly reduces the time required to produce the needed output. It automatically places the calibration worksheet and calibration graph on the same page, eliminating the need to tape multiple sheets of paper together and photocopy. In addition, researchers can interact dynamically with the output and customize it in any way they wish using the program's point-and-click interface. For example, they can easily select and customize color, size, fonts, markers, ticks, text labels, line styles, and background colors. Additional data sets can easily be added to the page when desired.

Conclusion

Origin's facility for custom programming has dramatically improved the speed and ease of LC data analysis. Reducing the time required to generate output for eight samples in duplicate from 16 to 8 hr saves a considerable amount of researchers' time that can be devoted to other tasks. The routine has simplified the data analysis task such that it can be delegated to a technician in the future for even greater time savings.


Ms. Goodchild is Senior Research Chemist and Mr. Hurlbut is Senior Research Physicist, Solutia, Inc., 730 Worcester St., Sprinfield, MA 01151, U.S.A.; tel.: 413-730-2402; fax: 413-730-2196.

AMERICAN LABORATORY : 44 : SEPTEMBER 1998

 

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