By Susan Goodchild and Jeff Hurlbut
A compositional heterogeneity method to simplify LC data
analysis
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