Measuring and testing – Gas analysis – Gas chromatography
Reexamination Certificate
2001-02-09
2002-08-06
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
Gas chromatography
C095S087000
Reexamination Certificate
active
06427522
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to gas chromatographs (GCs) and more particularly to a temperature programmed GC that has a shorter time required for heating and cooling of the GC column.
DESCRIPTION OF THE PRIOR ART
Temperature programmed GCs are typically used in process applications for two reasons. The first is to provide faster analysis times for long, complex analyses. These applications are traditionally developed from laboratory techniques, where temperature programming is used routinely to speed the elution time. In this instance, the laboratory analysis is merely “dropped” into the analyzer.
The second category of applications are those where the temperature programming is used to influence the desired elution. In this category are distillation analyses of mixtures with a broad boiling point range. The application of heat to these columns causes a shift in equilibrium between the gas and the liquid phase toward the gas phase which in turn causes the components to elute from the columns in a more complete and timely manner.
None of the foregoing applications are known for their speed. In some cases, the slowness of the application precludes the use of an analyzer in the control loop, relegating the analyzer to be used in an advisory capacity to the control algorithm. Therefore, the applications using these analyzers are not typically run near their limits, nor are they necessarily optimized for maximum revenue by the refiner or chemical producer.
Efforts have been made in the past to speed up the analysis times of GCs. Initial efforts were concentrated in studying the fundamentals of the partitioning process in the GC column. For example, B. O. Ayers and D. D. DeFord in “High Speed Process Gas Chromatograph,” Analytical Chemistry, 32, p 698, (1960) describe the design constraints necessary to optimize operational parameters of chromatographs to the point where a group of six hydrocarbons can be analyzed in 25 seconds.
The heart of a GC system is the column. Column performance sets limits to the separations attainable and helps determine the speed of analysis. Three basic types of columns are used in gas chromatography.
The first type of column are conventionally packed columns which have been used since the introduction of gas chromatography by A. T. James and A. J. P. Martin in 1952. By 1960, R. J. Loyd et al. reported in “Optimization of Resolution—Time Ratio with Packed Chromatographic Columns,” Analytical Chemistry, Volume 32, Number 6, p 698 a ninefold improvement in the time required to obtain a given chromatographic separation using columns containing a low proportion of partitioning agent and a carrier gas of low viscosity and high diffusivity.
Other parameters which affect the resolution and speed of packed columns such as liquid loading, solid support characteristics, column diameter and length have been investigated and reported in numerous articles and presentations.
The second type of column is the micropacked column which is a packed column with an internal diameter of 0.5 to 1 mm and the same packing density as a conventional packed column. Micropacked columns have been used in gas chromatography since 1963. Because of the advantages micropacked columns possess, numerous process applications have been done using this column type. The advantages include reproducibility, a small carrier gas flow rate and high efficiency. Supports may be coated with any stationary phase in the desired quantity. The column packing may be prepared in large batches to ensure reproducible properties. The pressure drop is not excessive while the number of theoretical plates per unit length is high. The major problem associated with this type of column is difficulty in packing longer lengths (>10 feet). These columns are normally packed in {fraction (1/16)} inch stainless tubing and it is visually impossible to determine if there are empty spaces within the column.
The third type of column is the capillary (0.1 to 1.0 mm I.D.) or wall coated open tubular (WCOT) columns introduced by Golay in 1957. Numerous articles have been written and extensive research has been conducted to define the benefits of capillary columns in process gas chromatography for resolution and speed of analysis which these columns provide. Parameters such as column diameter/length, stationary film thickness, column material, and carrier flows/pressures have been studied and optimized for reduced analysis time in process applications. The wide-bore capillary has been of particular interest for process applications because it can be used as a direct replacement for a packed column without changing operating parameters or sample preparation. The associated benefit is a dramatic decrease in analysis time without changing sample size.
Another unique approach to providing faster analysis times without sacrificing sensitivity and requiring small sample volumes is the multicapillary column. This column was introduced by Alltech in the late 1990's by combining over 900 liquid phase coated, 40 &mgr;m capillaries in a single glass tube. Compared to conventional capillary columns, multicapillary columns maintain high efficiency across a broader flow rate range, operate at lower temperatures and provide faster analyses.
Although numerous advances have been made relative to speed of analysis by manipulating the column types and parameters, there is a theoretical limit to what can be done to decrease time for the sample to reach equilibrium between the mobile and stationary phases. Or more precisely, reduce analysis time without sacrificing separation.
Two of the factors, that affect this equilibrium time, are temperature and carrier gas pressure/flow. By increasing either or both of these parameters there will be a decrease in analysis time.
Temperature programming which is a controlled change in the temperature surrounding the column has been used to speed up the analysis time of wide boiling range samples since the early 1960's [see for example A. J. Martin, “Linear Programmed Temperature Gas Chromatography to 500° C.,” Edinburgh Symposium, London, Butterworths, 208-10 (1960)]. The most common application is the use of temperature programming for simulated distillation of fuel products. By increasing the temperature, the time spent by a sample in the liquid phase is decreased which shifts the equilibrium to the gas phase which reduces the time of analysis. One point to be considered with temperature programming is cycle times, which is the length of time from sample inject for one analysis to sample inject for the next analysis and includes cool down time. Although the analysis time may have. been significantly reduced by temperature programming, the consideration of the time it takes to cool down to the initial temperature diminishes the benefit gained. Regardless, this approach has been used to reduce the analysis time for many complex process samples and would have greater benefit if the heating and cooling cycles can be reduced.
Pressure/flow programming of the carrier gas has more recently become available on process gas chromatographs and can also be used to reduce analysis times. By increasing the pressure/flow in a controlled manner the time for the sample to reach equilibrium is reduced and the sample is swept through the column to the detector by the faster flow of the carrier gas. Pressure/flow can be used independently of temperature or both can be used simultaneously to speed the analysis. There is a special benefit to using pressure programming when the liquid phase in the column has reached its maximum operating temperature.
In the mid to late 1980's the microchip gas chromatograph, also known as the “GC on a chip”, was developed and introduced by Microsensor Technology, Fremont, Calif. The major benefit associated with this development was speed of analysis which was gained through miniaturization of each of the chromatographic components, including the column which was etched on a silicon wafer. Factors such as no backflush, no liquid inject, limited column/de
Bennett Ronnie D.
Thomas Thomas J.
ABB Automation Inc.
Politzer Jay L
Rickin Michael M.
Williams Hezron
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