Gas separation: processes – Chromatography – With heating or cooling
Reexamination Certificate
2001-12-10
2003-06-17
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Chromatography
With heating or cooling
C095S082000, C096S102000
Reexamination Certificate
active
06579345
ABSTRACT:
TECHNICAL FIELD
The present invention relates to apparatus for chromatography in which the capillary column is heated directly and to a method of controlling the temperature of the capillary column.
BACKGROUND ART
It is known in the art that direct heating of the column confers various advantages, among which is a considerable reduction in the quantity of electrical power required to heat the column by means of an element made of electrically conductive material which encircles the same capillary column.
An example of a portable gas chromatography apparatus with direct heating of the column is described in U.S. Pat. No. 5,611,846 by Overton et al. In order to directly heat the column, this document suggests inserting the column into a sheath together with a conductive filament, or inserting the column directly into a tube made of conductive material. The hypothesis of using columns covered with conductive material—such as, for example, columns in fused silica covered with a thin layer of aluminum—had already been rejected because of frequent breakage of the column or the covering conductor due to the different coefficients of thermal expansion of the materials.
As a temperature sensor, the Overton patent suggests to use a filament made of conductive material inserted into the sheath or the conductive tube in close contact with the column but insulated electrically from the other conductive element (tube or filament) used to heat the column. A control device able to determine the temperature of the conductor as a function of its resistance is employed to regulate the heating of the column. The control device cyclically compares the set temperature with that calculated on the basis of the temperature sensor signal and corrects the power supplied to the heating conductor by varying the voltage applied to the same. Moreover, the possibility of using the same heating conductor as temperature sensor Is also suggested, without however specifying what kind of control device could be suitable for this particular embodiment.
While not specifying which type of control was used In the system of the cited US patent, it was asserted that the system is able to control temperature ramps of slope up to 10° C./sec. This parameter confirms that the direct heating of the column can be efficient from the point of view of the rapidity of response of the system, but is opportune to point out that obtaining similar heating speeds with an Inadequate control device could compromise the stability and the precision of the system in Its entirety.
In fact, it has been subsequently found that this type of system has shortcomings-related to both the precision of temperature measurement with respect to conventional chromatography apparatus with oven heating, and to the repeatability of the set temperature profiles for Identical analyses (see “Novel Column Heater for Fast Capillary Gas Chromatography”; Overton et al—-Journal of Chromatographic Science—Vol.34 —December 1996, for example). U.S. Pat. No. 5,114,439 by Yost et al describes a chromatography apparatus in which the column is covered with a film of conductive material. This document confirms substantially the effectiveness of the column direct heating technology, but it is opportune to emphasize that the use of columns covered with conductive films involves the disadvantages already identified by Overton referred to previously. As temperature control device, the Yost patent suggests the use of a PID-type industrial controller to control by feedback the electrical supply to the conductor associated with the column.
A system for heating the column by means of a tube conductor which contains the same column is illustrated in U.S. Pat. No. 5,808,178 and in the corresponding international patent application n. WO 97/14957 in the name of Thermedics. The temperature control system alternates cycles of supplying constant high voltage to heat the tube conductor, with measurement cycles of more limited constant voltage to measure the resistance of the tube conductor and, consequently, the temperature of the tube/column system. Moreover, the use of standard PI or PID type standard industrial controllers is suggested as an improvement of a similar system.
However, it should be noted that even PID type standard controllers are inadequate to guarantee the necessary temperature control precision when particularly high heating speeds are applied.
A slightly improved column temperature control system is described in U.S. Pat. No. 5,005,399 by Holtzclaw et al. Also in this case, the heating system provides for the use of a column covered with a conductive film, therefore subject to all the disadvantages already previously cited. However, to control the heating temperature, a control device is suggested in which a pseudo-derivative correction factor is introduced into the feedback control of the temperature or, rather, of the voltage applied to the conductive material which covers the column. One of the main disadvantages of this system lies in the fact that, to maintain the correct operation of the column within the specified limits of error (±1° C.), particularly precise calibration of the gain of every component of the control system is required. The calibration operations necessarily demand a certain skill and experience, and they are necessary every time the column is replaced.
The object of the present invention is to propose a chromatography apparatus with direct heating of the column that gives particularly precise control of the temperature of the column.
Another object of the present invention is to propose a chromatography apparatus with direct heating of the column that can guarantee high response speed, maintaining in any case the necessary precision with respect to the set temperature profiles, be they linear or not linear (e.g. exponential and polynomial)
DISCLOSURE OF THE INVENTION
These objects are achieved by the present invention, that relates to a chromatography apparatus, of the type comprising at least one capillary column and means to control the temperature of the column, characterized by comprising at least one element made of electrically conductive material to heat directly the capillary column, and by means for controlling the temperature of the capillary column comprising a control device operating according to a mathematical model having one component of predictive type, or of Feed Forward type, which describes the thermodynamic behavior of the assembly comprising the electrically conducting element and the column at least as a function of the thermal resistance and the thermal capacity of the cited assembly to regulate the supply of electrical energy to the conductive element.
The behavior of the conductive element is therefore simulated by one model component that takes account of the thermodynamic characteristics of the conductor/column assembly to determine what power must be applied to the conductive element to obtain the pre-established temperature at a given time. That in particular allows a fast response to unexpected variations of the temperature profiles set up for the analysis.
Beyond the predictive or Feed Forward type component, the model comprises also one component preferably of corrective or Feed Back type, to correct any errors of temperature that may be Introduced by the predictive component of the model.
Contrary to what happens in the known art, In which the control systems operate in feedback alone trying continuously to reach a series of equilibrium conditions, the system of the present invention operates according to a mathematical model that describes the thermodynamic behavior of the system under the form of a transfer function determined by two very well-defined algorithms.
The advantage of this approach is given by the greater contribution of power being determined a priori by the Feed Forward type model component, while a smaller contribution of power Is determined by the Feed Back type model component on the basis of the temperature error. In this way, the power determined by the Feed Back type model is c
Facchetti Riccardo
Magni Paolo
Munari Fausto
Raffaeta Roberto
Zillioli Giancinto
Spitzer Robert H.
Thermo Finnigan Italia S.p.A.
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