Measuring and testing – Gas analysis – Gas chromatography
Reexamination Certificate
2002-05-16
2003-10-21
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
Gas chromatography
C210S659000, C703S002000
Reexamination Certificate
active
06634211
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to gas chromatography (GC). More particularly, the present invention relates to method translation, method development automation, method optimization, solute identification, improving reproducibility of elution pattern (elution pattern locking), method porting, and the like.
BACKGROUND OF THE INVENTION
Gas Chromatography
A typical gas chromatography (GC) system is comprised of a gas chromatograph and a computer controller as described in U.S. Pat. No. 5,405,432. The prime task of a chromatographic analysis of a sample mixture of chemical compounds (also known as analytes or solutes) by a GC system is to separate the solutes from each other, to identify them, and to quantify their amounts.
The separation of individual solutes in a sample mixture takes place in a chromatographic column also described in U.S. Pat. No. 5,405,432. Due to the different interaction of different solutes with the stationary phase in the column, it takes a different amount of time for the different solutes to travel through the column. As a result, the solutes, simultaneously injected in the column as a single mixture, elute from the column at different times, thus causing the separation of the solutes from each other. A detector converts the sequence of the solutes eluting from the column into a chromatogram—a sequence of chromatographic peaks—that can be electronically stored in a computer memory and/or displayed (e.g., electronically, on paper, etc.). Several examples of chromatograms are shown in U.S. Pat. No. 5,405,432.
The identity of each analyte, separated by the column, is typically associated with retention time (also known as elution time), t
R
, of the corresponding peak in a chromatogram. A typical computer aided identification of the separated solutes makes use of a calibration table—an electronic list of records for all solutes of interest. Each record contains a solute identification and a retention time of the corresponding peak for that solute. A calibration table may also include information regarding tolerance windows for the retention times, as well as other information. A solute of interest can be correctly identified if its actual retention time in a particular analysis falls within the tolerance window for its retention time. To prevent a misidentification of a given peak with its immediately preceding or a following neighbor, the tolerance windows must have relatively narrow widths. This, in turn, may require a relatively high reproducibility of retention times for all peaks.
As described in U.S. Pat. No. 5,405,432, a solute's retention time depends on many parameters such as the column length, L, internal diameter, d
c
, stationary phase film thickness, d
f
, stationary phase type, carrier gas type, column pressure, column flow rate, and column temperature, T. Some of these parameters, such as the gas pressure or flow rate and the column temperature, can either remain constant during a given analysis or can change according to predetermined programs. The column parameters together with the calibration table and with the parameters of other components (injectors, detectors, etc.) of a GC system comprise a particular method of analysis of a particular mixture. Any change in the relevant method parameters can lead to the change in the retention times of some or all peaks. If the retention time changes are not accompanied by the necessary changes in the calibration table, misidentification of some or all solutes can occur.
Generally, it is difficult to predict the retention time changes caused by arbitrary changes in the method parameters. However, there are practically important exceptions. A concept of void time, t
M
, is useful for the description of these exceptions. The void time is a retention time of a so-called unretained solute—that is, one that does not interact with the stationary phase, and, as a result, travels with the same velocity as the velocity of a carrier gas. Methane is frequently used as an unretained solute in practical measurements of t
M
.
Method Translation
In some cases, the changes to retention times caused by a change in a method parameter can be predicted. The first case is referred to as method translation and may be applied to methods employing a constant pressure, as well as some other restrictions described below. To illustrate method translation, let T
1
(t) and T
2
(t), where t is time since injection of the mixture in a column, be the temperature programs in methods 1 and 2, respectively. Let also t
M1,ref
and t
M2,ref
be, respectively, reference void times in methods 1 and 2. These quantities should be isothermally measured at the same reference temperature. If the following conditions are met:
a. the capillary columns have the same type of a liquid stationary phase;
b. the ratio of a column internal diameter and the stationary phase film thickness is the same, i.e. d
c1
/d
f1
=d
c2
/d
f2
;
c. column inlet and outlet pressure remain constant during the analysis (this is known as a constant pressure mode); and
d. temperature programs T
1
(t) and T
2
(t) relate as T
2
(t)=T
1
(G×t) where G=t
M2,ref
/t
M1,ref
(for a piece-wise linear temperature program, this means that all temperature plateaus in method 2 are G-fold shorter than their counterparts in method 1, and all temperature ramps in method 2 have G-fold higher heating rates than their counterparts in method 1),
then retention time, t
R2
, of any peak in method 2 can be found as t
R2
=t
R1
/G where t
R1
is retention time of the peak corresponding to the same solute in method 1. Two methods satisfying this set of conditions are known as mutually translatable methods, and each of the two methods is known as a translation of the other, where quantity G=t
M2,ref
/t
M1,ref
is known as a speed gain in method 2 relative to method 1.
While the above-mentioned restrictions disallow some differences in parameters for the methods to be mutually translatable, the restrictions do not prevent many other important differences. Thus, mutually translatable methods may use columns with different diameters and lengths, may use different types of carrier gas (helium, hydrogen, nitrogen, etc.), may have different flow rates as well as different inlet pressures and outlet pressures. The latter includes the cases when, in one of the mutually translatable methods, outlet is at the vacuum while in another the outlet is at an ambient or at any other constant pressure. A theoretical analysis of the method translation is described in Blumberg, L. M. and Klee, M. S., “Method Translation and Retention Time Locking in Partition GC”, Analytical Chemistry, vol. 70, number 18, Sept. 15 1998, pp. 3828-3839. An algorithm for the calculation of the temperature program in a translated method from that of an original method, and from the ratios of the column dimensions and other relevant parameters of the two methods, is also described in U.S. Pat. No. 5,405,432.
Method translation has several useful properties. It can be viewed, for example, as a G-fold compression (stretching, if G<1) of the time axis of a temperature program in a translated method compared to that in the original method while keeping the temperature axis unchanged. More specifically, method translation does not change initial and final temperatures preceding and following each temperature ramp. It only reduces the duration of each temperature plateau and increases the heating rate of each temperature ramp by the same factor equal to the speed gain, G.
Method translation has similar time scaling effect on peak retention times. Specifically, it reduces (increases, if G<1) all retention times in a translated method by the same speed gain G. This suggests that the calibration table for a translated method can be regenerated from the calibration table of the original method by simply dividing each retention time entry in the original calibration table by the same factor G. The fact that the translated method runs G times faster (if G>1) than the origin
Luchs James K.
Williams Hezron
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