Applera Corp. v. Micromass UK Ltd. (186 F. Supp. 2d 487, 2002)

MEMORANDUM OPINION

MCKELVIE, District Judge.

This is a patent infringement case. Plaintiff Applera Corporation, formerly known as PE Corporation, is a Delaware corporation with its principal place of business in Norwalk, Connecticut. Plaintiff MDS Inc. is a Canadian corporation with its principal place of business in Toronto, Canada. Plaintiff Applied Biosys-tems/MDS Sciex, formerly known as Per-kin-Elmer Sciex Instruments, is a Canadian partnership formed under the laws of Ontario and having a place of business there. Applera and MDS are general partners of Applied Biosystems/MDS Sciex. MDS is the owner of U.S. Patent No. 4,963,736 (the ’736 patent), entitled “Mass Spectrometer and Method and Improved Ion Transmission.” Applied Bios-ystems/MDS Sciex is the exclusive licensee of the ’736 patent. The plaintiffs will be collectively referred to as AB/Sciex, although MDS, as owner of the patent, will be referred to individually when appropriate.

Defendant Micromass UK Ltd. is a British corporation with its principal place of business in Manchester, United Kingdom. Micromass UK manufactures mass spectrometers sold under the name Quattro Ultima. Defendant Micromass, Inc. is a Massachusetts corporation with its principal place of business in Beverly, Massachusetts. Micromass, Inc. distributes and sells the Quattro Ultima in the United States. When necessary, the defendants will be referred to collectively as Micro-mass.

On February 18, 2000, AB/Sciex filed its complaint in this action alleging that the defendants infringe one or more claims of the ’736 patent. On July 10, 2000, Micro-mass Inc. filed its answer, affirmative defenses, and counterclaims and Micromass UK moved to dismiss the case for lack of personal jurisdiction. Micromass UK later withdrew the motion to dismiss and filed its answer, affirmative defenses, and counterclaims. On November 15, 2000, both defendants filed their amended answer, affirmative defenses, and counterclaims. The defendants’ counterclaims seek a declaratory judgment that ’736 patent is invalid and unenforceable, and allege that AB/Sciex has filed this suit in an improper effort to maintain monopoly power in violation of section 2 of the Sherman Act, 15 U.S.C. § 2, or attempted or conspired to do so.

On December 13, 2001, the court held a hearing in accordance with Markman v. Westview Instruments, Inc., 517 U.S. 370, 116 S.Ct. 1384, 134 L.Ed.2d 577 (1996), to construe the disputed claims of the ’736 patent. The parties sought construction of almost every limitation in the patent’s two independent claims. Among the many limitations considered, the principal disputes between the parties relate to the claim terms “first” and “second,” “end to end” and “aligned,” and the structure accompanying certain means-plus-function limitations. This is the court’s construction of the disputed claims.

I. FACTUAL AND PROCEDURAL BACKGROUND

The court draws the following facts from the complaint, the ’736 patent, its prosecution and reexamination history, and the submissions of the parties.

A. Background of the Technology

Mass spectrometers analyze trace substances in a sample gas or liquid and provide information about the molecular weight or chemical structures of compounds in the trace substance. They are commonly employed in analytical chemistry for a variety of uses, including testing for the presence of drugs in bodily fluids or testing food and drink for minimum quality standards. Mass spectrometers operate by applying an electrical charge to the molecules of the substance being analyzed, resulting in charged molecules known as ions. The substance being analyzed can then be separated into its constituent parts by applying an electrical charge to the ions that separates them based on the ratio of their molecular weight to the charge.

Figure 1 from the ’736 patent can be used to illustrate the basic workings of the type of mass spectrometer, typically called a quadrupole mass spectrometer, that AB/ Sciex argues is at issue in this case.

In a quadrupole mass spectrometer, ions are generated by introducing a trace substance into a duct (14). The trace substance is then ionized in the ionization chamber (16) by applying an electric charge with an electric discharge needle (18). The desired ions are then separated from the ambient gas (introduced through duct 44) and the undesired ions by two rod sets (32 and 40). A rod set is a group of electrodes (four in a quadrupole, six in a hexapole, etc.) shaped as rods, spaced equally apart to define an elongated central space through which the ions travel. The two quadrupole rod sets are each two-dimensionally represented in Figure 1 (32 and 40).

A typical quadrupole mass spectrometer uses two types of rod sets (32 and 40), each set in a separate vacuum chamber (30 and 38). One set of rods (32), known as an ion guide, uses an alternating current (AC) to channel the ions entering the device into the central space between the rods. By alternating the positive and negative charges in adjacent rods, the ion guide forces the ions to oscillate between the rods while traveling down their length. This is known as “strong focusing.” The ambient gas, meanwhile, is pumped out of the vacuum chamber (31).

By directing ions into a vacuum chamber containing an ion guide, the ions are separated from the background gas in the chamber and channeled by the ion guide into a central stream. The central stream proceeds through a small orifice (34) and into another vacuum chamber (38), which contains another rod set (40) and vacuum pump (39). This second set of rods, known as a mass filter, applies both an AC voltage and a direct current (DC) voltage to select ions of a particular mass-charge ratio. The mass filter is arranged so that the ion stream can proceed from the ion guide rod set in the first vacuum chamber, though an orifice, and into the mass filter rod set in the second vacuum chamber. The mass filter then uses a particular voltage to separate the desired ions from the undesired, and the desired ions continue to a detector (48) that records their presence.

According to the ’736 patent, it was believed “[i]n the past” that the ion transmission through the device “increases with lowered gas pressure” in the vacuum chambers, also called cells. “For example the classical equation for a scattering cell shows that the ion signal intensity (ion current) transmitted through the cell decreases with increasing gas pressure in the cell.” ’736 Patent, Col. 1, In. 33-37.

Unfortunately the resultant need for low pressures in the region of the ion optic elements has in the case of gassy ion sources required the use of large and expensive vacuum pumps. This greatly increases the cost of the instrument and reduces its portability.

Id. at Col. 1, In. 37-41. The inventors of the ’736 patent sought to solve this problem. As more fully described below, the inventors discovered that the “classical equation” was flawed and that increasing the pressure in the ion guide, within certain parameters, could improve ion signal intensity.

B. The ’736 Patent

On October 16, 1990, the U.S. Patent and Trademark Office (PTO) issued the ’736 patent, entitled “Mass Spectrometer and Method and Improved Ion Transmission.” Donald J. Douglas and John B. French are its inventors and MDS Health Group Limited is the assignee. The ’736 patent describes a mass spectrometer employing both an ion guide and mass filter located in separate vacuum chambers, as shown in Figure 1. What was novel about the invention, according to its specification, was the particular parameters of pressure, rod length, and voltage used to maximize transmission of ions from the ionization chamber (16) to the detector (48), thereby improving its sensitivity. Because, in accordance with the suggested parameters, the pressure in the ion guide chamber (30) was higher than previously used, “smaller, cheaper pumps” could be used to make the device more easily transportable.

According to the ’736 patent’s abstract, the vacuum chamber (38) containing the mass filter (40) is kept at a low pressure, such as 0.02 millitorr or less. In contrast, the vacuum chamber (30) containing the AC-only rods (32) that act as an ion guide is kept at a comparatively higher pressure, defined in terms of the product of the pressure and the length (“P x L”) of the rods. The patent claims state that the product of the length of the rods and the pressure should be equal to or above 2.25 x 10 " torr cm, and the specification further explains that the P x L parameter should preferably be between 6 x 10 ~ and 15 x 10 ~ torr cm. In addition, a DC voltage employed between the inlet (26) and the AC-only rods (32) in the ion guide is kept low, “e.g. below 1 and 30 volts, preferably between 1 and 10 volts.” As a result of utilizing these parameters, the inventors found “a large enhancement in ion signal, with less focussing aberration and better sensitivity at high masses.”

The inventors reported that “the reasons for this [improvement in ion signal intensity] are not fully understood,” ’736 Patent, Col. 1, In. 49-50, but hypothesized why they thought it occurred. They described that the use of the above parameters produces “a kind of collisional focuss-ing [sic] or damping effect,” id., Col. 6, In. 66-67, that forced ions toward the center line of the ion guide. By adjusting the pressure in the first vacuum chamber (30) to a level comparatively higher than previously thought and varying it depending on length of the rods (32), and by manipulating the DC voltage between the inlet orifice (26) and the ion guide rod set (32) to a lower level than normal, the user of the invention could achieve improved ion transmission.

The ’736 patent contains 24 claims, two of which are independent and 22 are dependent. The two independent claims are 1 and 14. Claim 1, an apparatus claim, recites:

1. A mass spectrometer system comprising:

(a) first and second vacuum chambers separated by a wall, said first vacuum chamber having an inlet orifice therein,

(b) means for generating ions of a trace substance to be analyzed and for directing said ions through said inlet orifice into said first vacuum chamber,

(c) a first rod set in said first vacuum chamber extending along at least a substantial portion of the length of said first vacuum chamber, and a second rod set in said second vacuum chamber, each rod set comprising a plurality of elongated parallel rod means spaced laterally apart a short distance from each other to define an elongated space therebet-ween extending longitudinally through such rod set, said elongated spaces of said first and second rod sets being first and second spaces respectively, said first rod set being located end to end with said second rod set so that said first and second spaces are aligned,

(d) an interchamber orifice located in said wall and aligned with said first and second spaces so that ions may travel through said inlet orifice, through said first space, through said interchamber orifice, and through said second space,

(e) means for applying essentially an AC-only voltage between the rod means of said first rod set so that said first rod set may guide ions through said first space,

(f) means for applying both AC and DC voltages between the rod means of said second rod set so that said second rod set may act as a mass filter for said ions,

(g) means for flowing gas through said inlet orifice into said first space,

(h) means for pumping said gas from each of said chambers,

(i) the pressure in said second chamber being a very low pressure for operation of said second rod set as a mass filter,

(j) the product of the pressure in said first chamber times the length of said first rod set being equal to or greater than 2.25 x 10 ~ torr cm but the pressure in said first chamber being below that pressure at which an electrical breakdown will occur between the rod means of said first rod set,

(k) and means for maintaining the kinetic energies of ions moving from said inlet orifice to said first rod set at a relatively low level, whereby to provide improved transmission of ions through said interchamber orifice.

’736 Patent, Col. 14, In. 24 — Col. 15, In. 7. Claim 14, a method claim, recites:

14. A method of mass analysis utilizing a first rod set and a second rod set located in first and second vacuum chambers respectively, said first and second rod sets each comprising a plurality of rod means and defining longitudinally extending first and second spaces respectively located end-to-end with each other and separated by an inter-chamber orifice so that an ion may travel through said first space, said inter-chamber orifice and said second space, said method comprising:

(a) producing outside said first chamber ions of a trace substance to be analyzed,

(b) directing said ions through an inlet orifice in an inlet wall into said first space, first through said first space, said interchamber orifice and then through said second space, and then detecting the ions which have passed through said second space, to analyze said substance,

(c) placing an essentially AC-only RF voltage between the rod means of said first set so that said first rod set acts to guide ions therethrough, through,

(d) placing AC and DC voltages between the rod means of said second rod set so that said second rod set acts as a mass filter,

(e) admitting a gas into said first chamber with said ions,

(f) pumping said gas from said first chamber to maintain the product of the pressure in said first chamber times the length of said first rod set at or greater than 2.25 x 10 ~ torr cm but maintaining the pressure in said first chamber below that pressure at which an electrical breakdown would occur between the rods of said first set,

(g) pumping gas from said second chamber to maintain the pressure in said second chamber at a substantially lower pressure than that of said first chamber, for effective mass filter operation of said second rod set,

(h) and controlling the kinetic energy of ions entering said first rod set to maintain such kinetic energy at a relatively low value, whereby to provide improved transmission of said ions through said interchamber orifice.

’736 Patent, Col. 15, In. 51 — Col. 16, In. 29. The patent’s specification contained two preferred embodiments. The first, Figure 1, was the basic quadrupole mass spectrometer structure discussed above. The second, Figure 12, was basically a reproduction of the first, with a few slight modifications and additions, including an empty vacuum chamber (70) between the curtain gas chamber (24) and the ion guide’s vacuum chamber (30').

C. The Prosecution History

MDS filed the application for the ’736 patent on November 15, 1989. The application describes the basic structure of a quadrupole mass spectrometer and notes that much of the structure and method of operation recited within is also detailed in U.S. Patent No. 4,328,420, an earlier patent filed by one of the same inventors, John B. French. The ’736 patent application cites two articles by Dr. Richard Smith, et al., entitled “On-Line Mass Speetrometric Detection for Capillary Zone Electrophoresis,” 59 Anal. Chem. 1230 (Apr. 15, 1987) (the “1987 Smith article”), and “Capillary Zone Electrophoresis — Mass Spectrometer Using an Elec-trospray Ionization Interface,” 60 Anal. Chem. 436 (March 1, 1988) (the “1988 Smith article”).

According to the ’736 patent, the two Smith articles demonstrate the “classic theory” that the ion signal is improved by keeping the pressure in the ion guide’s vacuum chamber relatively low. The 1987 Smith article shows operation of an AC-only rod set in a vacuum chamber at 8 x 10 ^ torr and the 1988 Smith article shows an AC-only rod set in a vacuum chamber at 1 x 10 "6 torr.

On May 8, 1990, the patent examiner issued a final office action allowing all 24 claims of the ’736 patent, stating that “pri- or art does not teach to operate an AC only quadrupole, used to guide ions to a mass analyzing quadrupole in a high vacuum chamber, at a pressure such that the product of the length of the AC only quadrupole times the pressure in its chamber is greater than or equal to 2.25 x 10 ~ torr cm.” The ’736 patent issued on October 16,1990.

D. The Reexamination History

On January 10, 1997, attorneys representing MDS sent a letter to Micromass UK, asserting that an employee of Micro-mass UK had recently published an article describing a hexapole rod set in a vacuum pressure similar to that disclosed in MDS’s ’736 patent. The letter commented that “this device, if sold, will infringe the claims of the” ’736 patent and its Canadian counterpart. Micromass UK responded by letter dated April 16, 1997, stating that it did not believe its product infringes and citing several references published prior to the application for ’736 patent, including: (1) French, European Patent Application, Publication No. 0 023 826, February 11, 1981 (the “French application”); (2) Boit-nott et al., Optimization of Instrument Parameters for Collision Activated Decomposition (CAD) Experiments for a Finnigan Triple Stage Quadrupole GC/MS/MS/DS, 1981 Pittsburgh Conference On Analytical Chemistry and Applied Spectroscopy, Abstract No. 782 (the “Finnigan abstract”); (3) Boitnott et al., Optimization of Instrument Parameters for Collision Activated Decomposition (CAD) Experiments for a Triple Stage Quadrupole (TSQ ™ GC/MS/MS/DS, Finnigan Topic 8160 (the Finnigan paper)); (4) Caldecourt et al., An Atmospheric-Pressure Ionization Mass Spectrometer/Mass Spectometer, International Journal of Mass Spectrometery and Ion Physics, Vol. 49, p. 233-251 (1983) (the “Caldecourt article”).

On September 30, 1997, MDS filed a request for reexamination with the PTO, citing the four references mentioned in Micromass UK’s letter and four additional references, including another European patent application and three articles.

1. Ion Trap References

In its request for reexamination, MDS described the four new references not disclosed by Micromass as “ion trap” references and distinguished them from the ’736 patent on the basis that the claimed invention did not trap ions in the system for analysis. This distinction is relevant to the court’s claim construction because Micromass now argues that MDS disclaimed that the ’736 patent “traps” ions for a significant period of time, as the ion trap references of prior art would suggest. For example, one of the ion trap references was Schaaf et al., Trapped Ion Density Distribution in the Presence of He-Buffer Gas, Applied Physics, Vol. 25, pp. 249-251 (1981) (the Schaaf article). MDS explained that “Schaafs ion trap operates on a fundamentally different principle than the claimed mass spectrometer. With an ion trap, ions of a selected range of mass to charge ratios are trapped or stored for a period of time (which can be quite lengthy) due to electric fields generated with electrodes.” Request for Reexamination at 6. In contrast, MDS argued that in the claimed invention, “[t]he first rod set receives essentially only an AC voltage so that ions are guided through the first vacuum chamber without being trapped there.” Id. at 7.

2. Tandem References

MDS also distinguished the four references provided by Micromass, describing the structure they reveal as a “tandem mass spectrometer.” Micromass refers to them instead as “triple stage mass spectrometers,” and states that the Quattro Ultima is a “triple stage” or “tandem” mass spectrometer.

According to MDS, in a tandem mass spectrometer ions proceed through a quadruple AC-DC mass filter (10, in the figure below), then a collision cell containing an AC-only rod set (14), and then another quadrupole AC-DC mass filter (12). The collision cell accepts ions not filtered out by the first mass filter, then collides those ions into a gas at high energy, causing them to fragment. The fragments are called “daughter ions” and then proceed into the second mass filter for further filtering for the desired fragments. The basic structure of a tandem mass spectrometer is shown here.

Micromass argues that MDS’s distinction between a tandem mass spectrometer and the claimed invention is significant to the construction of the claim terms “first” and “second.” For example, MDS explained that the “first rod set” (ion guide) in the claimed invention is comprised of AC-only rods, while the “second rod set” (mass filter) in the claimed invention are AC-DC rods. It also explained that the “first vacuum chamber” in the claimed invention has a product of the pressure and length of the rods of 2.25 x 10 ~ torr cm, while the “second vacuum chamber” has a lower pressure. Given these characteristics of the claimed invention, MDS distinguished the tandem references as follows:

The French application also differs from the system of the invention in other ways. For instance, whereas the first rod set in the invention receives essentially an AC-only voltage, the first section in the French application receives both AC and DC voltages. Whereas the first vacuum chamber of the invention has a product of its pressure with the length of the first rod set equal or greater than 2.25 x 10 "2 torr cm, whereby the pressure is at least 1.5 millitorr for a 15 cm rod set, the first section in the French application states that the pressure must be maintained low, typically at 10 "5 torr. Further, whereas the second rod set in the invention receives both AC and DC voltages to act as a mass filter, the second section in the French application receives an AC only voltage and is for inducing dissociation of ions. The second chamber of the invention is at very low pressure while the French application states that the pressure in the second section may be varied from 0.1 millitorr to 10 millitorr.

Id. at 13-14; see also id. at 16 (Finnegan abstract), 19 (Finnigan paper), and 21 (Caldecourt article).

MDS did not, however, distinguish the tandem mass spectrometers based solely on the placement of the various rod sets and vacuum chambers. It also described operational differences. For example, MDS distinguished the French application by stating:

The French application differs from the system of the invention in that it relates to a three-stage mass spectometer having two end sections for acting as mass filters and a center quadrupole section for producing collision induced dissociation of parent ions into fragment or daughter ions. A collision cell, such as the one described in the French application, dissociates a parent ion into fragment ions by creating conditions whereby a high energy parent ion collides with a high pressure gas. The use of a high pressure gas is therefore well known with mass spectrometers that have collision cells. A mass spectrometer according to the invention, on the other hand, is intended to improve the transmission of ions through a cell. The mass spectrometer according to the invention uses an increased pressure to improve ion transmission and maintains ‘the kinetic energies of ions moving from said inlet orifice to said first rod set at a relatively low level’ (claim 1). The French application would therefore teach away from the invention since it collides ions at high kinetic energies into a high pressure region to dissociate the ions into daughter ions, which is in contrast to the invention which uses low kinetic energy ions and an increased pressure to produce an improved transmission of ions entering the device.

Request for Reexamination at 13. MDS identified similar distinctions for the Finni-gan abstract, see id. at 15-16, the Finnigan paper, see id. at 18, and the Caldecourt article, see id. at 21-22.

3. PTO Proceedings

The PTO examiner granted MDS’s reexamination request, stating that there “is a substantial likelihood that a reasonable examiner would consider these teachings important in deciding whether or not the claims are patentable.” The PTO examiner discussed with particularity the relevance of the French application, the Finni-gan abstract, and the Finnigan paper to the patentability of the claims of the ’736 patent.

In an Office Action on February 3, 1998, the PTO examiner rejected all the claims of the ’736 patent as obvious under paragraph 2 of 35 U.S.C. § 103(a). The examiner considered the structure disclosed in the French application and the voltage parameters disclosed in the Finnigan abstract and paper and concluded, “[i]t would have been obvious to a person having ordinary skill in the art to control the energies of the ions entering the French apparatus in accordance with the teachings of [the Finnigan abstract and paper] by providing DC voltage between the rods of the first quadrupole and the inlet wall.”

On March 11, 1998, the PTO examiner met with Donald Douglas, inventor of the technology in the ’736 patent, to discuss the patentability of its claims. In his interview summary, the examiner indicated that he and Douglas reached agreement on all claims. He described that agreement as follows.

Applicant could remove French et al. as a reference by establishing that the product of the pressure and length of • the AC only quadrupole described in the reference when that quadrupole was used as a collision cell between two mass analyzing quadrupoles to fragment ions was not intended to be used when that quadrupole was used only as an ion guide and not to fragment the ions.

Thus, the examiner acknowledged the distinction between an ion guide and collision cell.

On April 30, 1998, MDS filed an Amendment to the ’736 patent to add new depen-dant claims 25-30, which will be discussed separately. The Amendment was accompanied by a declaration from Dr. French, the second inventor indicated on the ’736 patent and the inventor of the ’420 patent and the French European patent application. In that declaration, French recited the distinction drawn during the interview with Douglas. “The French application does not suggest that the recited pressure range and rod length may be used in a quadrupole section which acts as an ion guide and which is not intended to fragment the ions.”

On June 2, 1998, the PTO issued a final Office Action in which it stated that claims 1-24 of the ’736 patent were patentable, but rejected the new claims 25-30 as indefinite under paragraph 2 of 35 U.S.C. § 112. With respect to claims 1-24, the examiner stated that the “declaration of Dr. French filed on June 3, 1998 establishes that the apparatus disclosed in the French application does not operate with a product of pressure and rod length greater than or equal to 2.25 x 10 “ torn cm in a chamber containing a rod set operated with only AC voltages applied.”

On August 12, 1998, MDS filed a Response After Final Office Action in which it submitted that claims 25-30 are patentable. The Response also listed further reasons, not cited by the examiner, why MDS believed claims 1-24 were not suggested by the French application. MDS listed the following four reasons why the French application did not suggest the claimed invention: “(1) the French application teaches away from the invention by suggesting that pressure be reduced in the first chamber, (2) the French application does not suggest the product of pressure and rod length in the first chamber, (3) the French application does not suggest collisional focusing, (4) the French application does not suggest improving the transmission of ions entering the first chamber.”

Following further correspondence between the PTO and MDS, the PTO issued a Reexamination Certificate dated May 25, 1999 which confirmed the patentability of claims 1-24 and allowed claims 25-30.

E. The New Claims

As noted, the PTO examiner initially rejected all of MDS’s new claims as indefinite under 35 U.S.C. § 112. Of the six new claims, claims 25 and 26 are depen-dant on claim 1 and the remainder are dependant on claim 14. Although neither party seeks construction of the terms in the new claims, Micromass argues that statements made during the prosecution of claims 25 and 26 are relevant to the construction of the terms “end to end” and “aligned” in claim 1. Claims 25 and 26 state:

25. The mass spectrometer system as set forth in claim 1, wherein a first longitudinal axis of the first rod set intersects a second longitudinal axis of the second rod set.

26. The mass spectrometer system as set forth in claim 1, wherein the first rod set is parallel to the second rod set.

The examiner stated that claim 25 was rejected because its new limitation, the intersection of the longitudinal axes of the rod sets, “contradicts the limitation set forth in parent claim 1 that the rods in each rod set ‘define an elongated space ... extending longitudinally though such rod set’ and the two rod sets are located end to end with each other ‘so that said first and second spaces are aligned.’ ” To make his point, he then posed the question, “[h]ow can the two longitudinal axes of the rod sets intersect if they are aligned?”

In the Response After Final Office Action filed by MDS on August 12, 1998, MDS sought to answer the examiner’s question. It argued that the term “aligned” did not require alignment on one parallel axis:

The use of the term “aligned” in claim 1, however, does not necessarily mean that the first and second rod sets are parallel to each other. Webster’s Ninth New Collegiate Dictionary, for instance, defines “align” as “to bring into alignment” and also as “to be in or come into precise adjustment or correct relative position ” (emphasis added). The term “alignment,” moreover, is defined as “the act of aligning or state of being aligned, esp: the proper positioning or state of adjustment of parts ... in relation to each other. ”

Response After Final Office Action, at 2 (emphasis added). After noting that the term “aligned” did not require the rod sets to be parallel, MDS argued that therefore the longitudinal axes of the rod sets “could be at an angle relative to each other while at the same time the first and second spaces are aligned to each other.” Id. at 3. Alternatively, MDS argued that even if the longitudinal axes were parallel, the two axes would nonetheless “intersect” because “ ‘intersect’ can mean that the axes cross over each other or are parallel to each other.” Id.

With respect to claim 26 and its requirement that the rod sets be parallel, MDS argued that it had established, in its discussion of claim 25, that the requirement that the first and second spaces be aligned did not require that the rod sets that define the spaces be parallel to one another. “Instead, the first space could be at an angle relative to the second space and the two spaces could be aligned by having the ends precisely located relative to each other so that their ends abut.” Id. at 4. Therefore, MDS submitted that claim 26’s limitation of parallel rod sets was not already part of claim 1 and could be patented.

On May 25, 2000, the PTO issued the Reexamination Certificate for the ’736 patent allowing the new claims.

F. The Accused Product: The Quattro Ultima

AB/Sciex argues that Micromass’s Quat-tro Ultima satisfies all of the claim limitations of at least claims 1 and 14 of the ’736 patent. As noted, the Quattro Ultima is a tandem mass spectrometer that contains, among other structures, a quadrupole mass filter, then a collision cell (in this case, a hexapole collision cell), and another quadrupole mass filter. Those structures are shown in vacuum chamber 4 in the schematic below, which was furnished by Micromass as a simplified example of the Quattro Ultima’s structure.

Ions enter the Quattro Ultima from a source and then enter an initial vacuum chamber (vacuum chamber 1). That vacuum chamber has an orifice in it, permitting ions and gas to flow into vacuum chamber 2, which contains what Micromass refers to a “hexapole ion bridge.” AB/Sciex contends that the hexapole ion bridge is a set of AC-only rods and therefore is an ion guide satisfying the relevant claim limitations of the ’736 patent. It also claims that there is a DC voltage applied between the orifice of vacuum chamber 2 and the hexapole ion bridge. Next, ions pass through an orifice into vacuum chamber 3, which contains another hexapole ion bridge. Ions then pass through another orifice into vacuum chamber 4, which contains the aforementioned tandem mass spectrometer, including an AC-DC quadrupole mass filter, an AC-only hexapole collision cell, and another AC-DC quadrupole mass filter. After proceeding through the tandem mass spectrometer section of the Quattro Ultimata, the ions reach a detector.

In July 2001, Micromass replaced the hexapole ion bridges in vacuum chambers 2 and 3 with “ion tunnels.” An ion tunnel is a series of ring-shaped electrodes arranged so that ions travel through their empty center. AB/Sciex continues to contend that the post-July 2001 Quattro Ulti-ma infringes the claims of the ’736 patent.

II. DISCUSSION

A. Basic Principles of Claim Construction

The construction of the claims in a patent is a matter left to the province of the court. Markman v. Westview Instruments, Inc., 617 U.S. 370, 391, 116 S.Ct. 1384, 134 L.Ed.2d 577 (1996). In construing a patent’s claims, the court must begin with intrinsic evidence, such as the patent itself, the patent specification, and the prosecution history. “It is well-settled that, in interpreting an asserted claim, the court should look first to the intrinsic evidence of record, i.e., the patent itself, including the claims, the specification and, if in evidence, the prosecution history. Such intrinsic evidence is the most significant source of the legally operative meaning of disputed claim language.” Vitronics Corp. v. Conceptronic, Inc., 90 F.3d 1576, 1582 (Fed.Cir.1996). Among these types of intrinsic evidence, the court “look[s] first to the claim language itself to define the scope of the patented invention.” Bell Atlantic Network Servs., Inc. v. Covad Communications Group, Inc., 262 F.3d 1258, 1267 (Fed. Cir.2001). The court must “give[ ] claim terms their ordinary and accustomed meaning as understood by one of ordinary skill in the art.” Nockerson-Halberstadt, Inc. v. Avia Group Int’l, Inc., 222 F.3d 951, 955 (Fed.Cir.2000). This requirement extends to technical terms, which must be furnished “the meaning that [they] would be given by persons experienced in the field of the invention, unless it is apparent from the patent and the prosecution history that the inventor used the term with a different meaning.” Hoechst Celanese Corp. v. BP Chems. Ltd., 78 F.3d 1575, 1578 (Fed.Cir.1996).

After looking to the patent claims themselves, the court considers the remaining intrinsic evidence presented, including the patent’s specification and its prosecution history. Interactive Gift Express, Inc. v. Compuserve Inc., 256 F.3d 1323, 1331 (Fed.Cir.2001). “If the claim language is clear on its face, then [the court’s] consideration of the rest of the intrinsic evidence is restricted to determining if a deviation from the clear language of the claims is specified.” Id. There are typically two such potential deviations. First, a patentee may choose to be his own lexicographer and use a claim term in the specification in a manner other than its plain and ordinary meaning. See Vitronics Corp., 90 F.3d at 1582. Second, the patentee may forfeit a particular construction if he or she “relinquished [a] potential claim construction in an amendment to the claim or in an argument to overcome or distinguish a reference.” Elkay Mfg. Co. v. Ebco Mfg. Co., 192 F.3d 973, 979 (Fed.Cir.1999).

After consideration of the plain and ordinary meaning of the claim limitations, the court considers the patent specification and prosecution history. Interactive Gift Express, Inc., 256 F.3d at 1332. The patent specification is helpful in construing claims because it is the patentee’s written description of the invention. There are two general guidelines for the use of the patent specification: “(a) one may not read a limitation into a claim from the written description, but (b) one may look to the written description to define a term already in a claim limitation, for a claim must be read in view of the specification of which it is a part.” Renishaw PLC v. Marposs Societa’ per Azioni, 158 F.3d 1243, 1248 (Fed.Cir.1998). The court may also consider the patent’s prosecution history. “The prosecution history limits the interpretation of claim terms so as to exclude any interpretation that was disclaimed during prosecution.” Southwall Techs. Inc. v. Cardinal IG Co., 54 F.3d 1570, 1576 (Fed.Cir.1995). If, after consideration of the prosecution history and patent specification, “the meaning of the claim limitations is apparent from the totality of the intrinsic evidence, then the claim has been construed.” Interactive Gift Express, Inc., 256 F.3d at 1332.

“Only when the claim language remains genuinely ambiguous after consideration of the intrinsic evidence,” may the court consider extrinsic evidence presented by the parties. Bell & Howell Document Mgmt. Prods. Co. v. Altek Sys., 132 F.3d 701, 706 (Fed.Cir.1997). All evidence other than the claims themselves, the patent specification, and prosecution history is extrinsic evidence. There are few limits on the court’s use of extrinsic evidence, but it is well-established that “extrinsic evidence may never be used ‘for the purpose of varying or contradicting the terms in the claims.’ ” Interactive Gift Express, Inc., 256 F.3d at 1332 (citing Markman, 52 F.3d at 981).

B. “comprising”

AB/Sciex and Micromass dispute the construction of terms in the ’736 patent’s two independent claims, 1 and 14, for purposes of this Markman proceeding. Claims 1 and 14 both use the term “comprising” in their prefatory statements, before the claims go on to enumerate further claim limitations. AB/Sciex suggests that the court should define “comprising” as “including, but not limited to.” Micromass does not disagree with AB/Sciex’s proposed construction and admits that the term “comprising” permits the inclusion of additional elements beyond those recited, but argues that the court’s construction of the term should not be used by AB/Sciex to evade the primary requirement of the term “comprising” — that the invention must include all enumerated claim limitations.

It is well-established that “ ‘comprising’ is a term of art used in claim language which means that the named elements are essential, but other elements may be added and still form a construct within the scope of the claim.” Genentech, Inc. v. Chiron Corp., 112 F.3d 495, 501 (Fed.Cir.1997); see also Phillips Petroleum Co. v. Huntsman Polymers Corp., 157 F.3d 866, 874 (Fed.Cir.1998) (“The use of ‘comprising’ and ‘which comprises’ in the composition and process claims generally would mean that the claims require the presence of [the listed element], but that additional elements or process steps may be present.”); Regents of Univ. of California v. Eli Lilly & Co., 119 F.3d 1559, 1572 (Fed.Cir.1997) (“The word ‘comprising,’ as UC argues and as is well-established, permits inclusion of other moieties.”); Moleculon Research Corp. v. CBS, Inc., 793 F.2d 1261, 1271 (Fed.Cir.1986) (“In every case, the court has held that the open term ‘comprising’ does not exclude additional unrecited elements, or steps ... ”). Thus, “comprising” can neither narrow nor broaden the meaning of the claim limitations subsequently recited. It simply requires the presence of the enumerated claim limitations enumerated without prohibiting other unrecited elements, structures, or steps from being present in the invention. Because the court finds that AB/Sciex’s construction of “including, but not limited to” is consistent with this well-understood construction of the term, the court hereby adopts that construction.

C. “first” and “second”

Claims 1 and 14 of the ’736 patent use “first” or “second” to modify various claim elements, such as “vacuum chamber,” “rod set,” and “space.” The construction of “first” and “second” is important because the tandem mass spectrometer asserted as prior art is alleged to have elements similar to those in the ’736 patent, but in a different order of ion travel. The construction is also important because Micro-mass’s Quattro Ultima has an empty vacuum chamber before the hexapole ion bridge chamber alleged by AB/Sciex to be the “first vacuum chamber” in the ’736 patent. Thus, the construction of “first” and “second” could dictate which element must come first in the claimed invention and the order in which subsequent elements must follow.

Micromass proposes that the plain meaning of “first” is “preceding all others in time, order, or importance,” and the plain meaning of “second” is “next to the first in place or time.” Webster’s Ninth New Collegiate Dictionary 466, 1060 (1991). In the context of the ’736 patent therefore, “first” and “second” sets where each element is located in the path an ion travels in the device. For example, the “first vacuum chamber” must be “the very first vacuum chamber encountered by the ions” and “second vacuum chamber” must be “the very next vacuum chamber encountered by the ions.” In this way, Micromass contends “first” and “second” dictate the absolute position of each particular element in the device.

AB/Sciex proposes that “first” and “second” only identify separate, but distinct, elements. That is, the ’736 patent discloses two vacuum chambers and the terms “first” and “second” should be understood as separately identifying “a vacuum chamber” and “another distinct vacuum chamber,” respectively, without specifying a particular order.

AB/Sciex’s position that “first” and “second” do not establish positions in the claimed invention and are mere identifiers is premised on the following three arguments. First, AB/Sciex argues that it is well-established practice among the paten-tees to use “first” and “second” as identifiers of similar, but distinct, elements. It notes that numerous cases and treatises demonstrate that patent drafters use the terms “first” and “second” to identify separate elements. See, e.g., Envirco Corp. v. Clestra Cleanroom, Inc., 209 F.3d 1360, 1365-66 (Fed.Cir.2000) (distinguishing “the ‘second’ from the ‘first baffle means’ ”); Canon Computer Sys., Inc. v. Nu-Kote Int’l, Inc., 134 F.3d 1085, 1089-90 (Fed. Cir.1998); NeoMagic Corp. v. Trident Mi-crosystems, Inc., 98 F.Supp.2d 538, 544 (D.Del.2000); Robert C. Faber, ed., Landis on Mechanics of Patent Claim Drafting, § 19, at III — 16 (4th ed.1999); 2 Irving Kayton et al., Patent Practice § 10.22(f) (6th ed.1998). None of the cases or treatises cited, however, state that “first” and “second” are only identifiers and that they do not also explain the position of elements. Indeed, most of the cases and treatises have no discussion of the meaning of “first” or “second,” or the terms thereby modified, at all. Thus, it is not clear that patent drafters using “first” and “second” do not also intend to impart positional significance to those terms.

Second, AB/Sciex notes the claims state, for example, either “first and second vacuum chamber” or “a first rod set” and “a second rod set.” The claims do not state “the first rod set” or “the second rod set.” AB/Sciex argues that the because the claims do not use the definite article “the,” the claims cannot be interpreted to mean “the very first rod set” or “the very next rod set.” This argument is unpersuasive, however, because AB/Sciex does not explain how the use of either no article, the indefinite article “a,” or the definite article “the” explains how “first” and “second” should be properly construed.

Looking only at these arguments, Micro-mass’s reliance on the plain meaning of “first” and “second” might be persuasive. Were “first” and “second” merely identifiers, as AB/Sciex suggests, the drafters of the ’736 patent could just as easily have said, for example, “a vacuum chamber” and “another vacuum chamber,” or “vacuum chamber A” and “vacuum chamber B.” Either would have identified separate vacuum chambers without also suggesting a positional hierarchy.

But relying on the plain meaning of “first” as “preceding all others in time, order, or importance” and the plain meaning of “second” as “next to the first in place or time” does not necessarily provide a correct construction of the use of those terms in the patent. Nowhere in the claims themselves is it stated that “first” must mean “preceding all other in the path of ion travel,” as opposed to, for example, “preceding all others in importance.” While the use of “first” and “second” in the patent is consistent with “first” and “second” in the path of a traveling ion, this construction is not required by the claims themselves. Therefore, Micromass’s plain meaning argument does not necessarily support its contention that “first” and “second” establish the absolute position of the elements.

Importantly, Micromass’s proposed construction of “first” and “second” as setting the absolute position of elements is inconsistent with one of the preferred embodiments in the specification. Were the court to adopt Micromass’s position that “first” and “second” must mean “the very first” and “the very second,” Figure 12 would be excluded from coverage under the claims. While Figure 1 of the ’736 patent shows the more basic embodiment of the invention with only two vacuum chambers (30 and 38), Figure 12 presents a slight variation in which an empty vacuum chamber (70) is added, after the ionization chamber (16') but before the ion guide chamber (30'). If the term “first vacuum chamber” is construed to mean the very first vacuum chamber in the path of ions, the preferred embodiment in Figure 12 would be excluded from coverage by the claims because claim 1 requires “a first rod set in said first vacuum chamber.” AB/Sciex correctly notes that a claim construction that excludes a preferred embodiment “is rarely, if ever, correct and would require highly persuasive evidentiary support.” Vitronics Corp., 90 F.3d at 1583.

Micromass argues that regardless of whether Figure 12 is covered by the claims, its construction is compelled by the prosecution history. See Elekta Instrument S.A. v. O.U.R. Scientific Int\ Inc., 214 F.3d 1302, 1308 (Fed.Cir.2000) (preferred embodiment may be excluded from patent’s claims when patentee disclaimed the construction that would cover the embodiment). Micromass contends that when MDS distinguished the tandem mass spectrometer references during reexamination, it adopted a construction of “first” and “second” inconsistent with the position it now takes. As noted previously, ions traveling in a tandem mass spectrometer first encounter an AC-DC rod set in a low pressure vacuum chamber, then an AC-only rod set in a high pressure collision cell, and finally another AC-DC rod set in a low pressure vacuum chamber. In distinguishing this structure, MDS stated:

The French application also differs from the system of the invention in other ways. For instance, whereas the first rod set in the invention receives essentially an AC-only voltage, the first section in the French application receives both AC and DC voltages. Whereas the first vacuum chamber of the invention has a product of its pressure with the length of the first rod set equal or greater than 2.25 x 10 “2 torr cm, whereby the pressure is at least 1.5 millitorr for a 15 cm rod set, the first section in the French application states that the pressure must be maintained low, typically at 10 ~5 torr. Further, whereas the second rod set in the invention receives both AC and DC voltages to act as a mass filter, the second section in the French application receives an AC only voltage and is for inducing dissociation of ions. The second chamber of the invention is at very low pressure while the French application states that the pressure in the second section may be varied from 0.1 millitorr to 10 millitorr.

Request for Reexamination at 13-14 (emphasis added); see also id. at 16 (Finnegan abstract), 19 (Finnigan paper), and 21 (Caldeeourt article). Because MDS distinguished the tandem references based on which elements were “first” and “second,” Micromass argues that the correct meaning for those terms must be “preceding all other elements in the path of ion travel” and “next to the first element in the path of ion travel.”

It is apparent from the manner in which MDS uses “first” and “second” in the above passage that it intended to refer to “first” and “second” in the path of ion travel relative to each other. If MDS were only using “first” and “second” as identifiers of separate elements, and not the order of those elements, MDS’s distinction would fail because the mere presence of the elements, in any order, would satisfy the claim limitations. Indeed, MDS made clear that it was using “first” and “second” as positions in the order of ion travel in distinguishing the Finnigan abstract, another tandem mass spectrometer reference.

The Finnigan abstract does not disclose or suggest that ions having a relatively low kinetic energy travel through an inlet orifice into a first vacuum chamber having a first rod set for receiving essentially only an AC voltage. The Fin-nigan abstract further does not disclose or suggest that ions then travel through an interchamber orifice to a second chamber having a second rod set receiving both AC and DC voltages.

Id. at 16 (emphasis added). From these statements, it is clear that MDS is relying on positional differences to distinguish the tandem references, and therefore disclaimed a more broad construction that the terms are mere identifiers of separate elements. In such cases, the Federal Circuit “has endorsed narrowing the interpretation of the claim to be consistent with a narrow claim scope urged by the applicant during the prosecution of the patent.” Pall Corp. v. PTI Techs., Inc., 259 F.3d 1383, 1392-93 (Fed.Cir.2001).

Although MDS disclaimed its proposed construction of “first” and “second” as mere identifiers, this “disclaimer” does not compel the court’s adoption of Micromass’s “absolute position” construction. AB/ Sciex, in explaining its comments from the reexamination, set forth an alternative construction — that “first” and “second” only define where in the invention the element is located relative to the other listed element. That is, regardless of how many vacuum chambers and rod sets there might be in the structure and where they are, the invention only requires that “first” come before “second.” Or, put simply, the ion guide elements must precede the mass filter elements. MDS did not disclaim this construction of “first” and “second” on reexamination and, in fact, its comments were consistent with this construction.

This “relative positioning” construction of “first” and “second” is persuasive because it would cover Figure 12. The existence of an empty vacuum chamber prior to the vacuum chamber containing the ion guide is immaterial to whether the “first vacuum chamber” claimed in the invention precedes the “second vacuum chamber” claimed in the invention. Thus, the court will construe “first” to mean “an element” and “second” to mean “an element coming after, in the path of ion travel, the first such element.” This construction sets a relative relationship between the “first vacuum chamber” and “second vacuum chamber” consistent with the plain meaning of those terms and the reexamination history. The court does not believe that this construction reads the word “first” out of the claims. Rather, the court’s constructions of “first” and “second” together establish the relative positions of those elements listed in the claims without regard to the existence or placement of similar elements not mentioned in the claims.

Therefore, the court finds that the construction of the terms “first” and “second” that best comports with the plain meaning of those terms, the patent’s specification, and the reexamination history, is that they define the position, in the path of ion travel, of the elements in the invention relative to the similar elements also mentioned in the claims. Thus, “first” is construed to mean “an element.” “Second” is construed to mean “an element coming after, in the path of ion travel, the first such element.”

1. “first vacuum chamber” and “second vacuum chamber”

Consistent with the court’s conclusion, it will further define the specific applications of “first” and “second” in the various elements. The court construes “first vacuum chamber” as “a vacuum chamber.” The court construes “second vacuum chamber” as “a vacuum chamber coming after, in the path of ion travel, the first vacuum chamber.” The parties agree that the term “vacuum chamber” means a chamber held at a pressure lower than atmospheric pressure.

2. “first rod set" and “second rod set”

The court construes “first rod set” as “a rod set.” Similarly, the court construes “second rod set” as “a rod set coming after, in the path of ion travel, the first rod set.”

Other than the adjective “first,” Micro-mass raises two additional limitations that it argues are in the term “rod set.” First, Micromass argues that “rod set” must be comprised of just that — rods—and that other shapes of electrodes, such as the rings of the latest Quattro Ultima design, cannot infringe the claims. It maintains that MDS, in distinguishing the ion trap references that use AC-only voltage during reexamination, disclaimed that the term “rod set” permits anything other than “rods.” AB/Sciex agrees, but believes such a construction by the court to be unnecessary because “a rod is a rod.” The court agrees and believes the proper construction of rod to be self-evident.

Micromass also contends that the “rod set” must be arranged as a quadrupole. It notes that the patent specification repeatedly refers to the arrangement of rods as a quadrupole in the preferred embodiment. See ’736 Patent, Col. 4, In. 21-23. Nowhere, however, do the claims of the ’736 patent use the word quadrupole. Instead, claim 1 only requires “a plurality of elongated parallel rod means spaced laterally apart a short distance from each other.” Claim 14 has a similar requirement. It is well-established that limitations not existing in the claims cannot be imported from specification. See Dayco Prods., Inc. v. Total Containment, Inc., 258 F.3d 1317, 1326 (Fed.Cir.2001) (“although we construe claims in light of the teaching of the specification, we do not treat characteristics of a preferred embodiment as claim limitations”). Thus, the court finds that the term “rod set” in the claims of the ’736 patent require only a plurality, meaning two or more, of rods in each rod set and do not require a quadrupole.

3.“first space” and “second space”

Claim 1(c) of the ’736 patent discusses a space within each vacuum chamber and rod set such that “each rod set comprising a plurality of elongated parallel rod means spaced laterally apart a short distance from each other to define an elongated space therebetween extending longitudinally through such rod set.” The preamble of claim 14 is similar. The court construes “first space” in both claims as “a space.” Similarly, the court construes “second space” in both claims as “a space coming after, in the path of ion travel, the first space.”

D. “inlet orifice”

Claim 1(a) of the ’736 patent requires “first and second vacuum chambers separated by a wall, said first vacuum chamber having an inlet orifice therein.” Claim 14(b) requires the “directing said ions through an inlet orifice in an inlet wall into said first space.” Micromass contends that because the claims require that the inlet orifice must be in the first vacuum chamber, the inlet orifice must be the beginning of that part of the mass spectrometer held below atmospheric pressure. Put differently, Micromass is relying on its interpretation of “first vacuum chamber” as “the vacuum chamber proceeding all other vacuum chambers,” and arguing that because the inlet orifice must be the inlet to the first vacuum chamber and because the first vacuum chamber must be the first chamber held below atmospheric pressure, the inlet orifice must be the inlet to the first chamber held below atmospheric pressure.

AB/Seiex contends that the term “inlet orifice” refers to “an orifice that provides an inlet into the claimed first vacuum chamber for the passage of ions and neutral gas molecules.”

Micromass’s proposed construction is unconvincing because it is premised upon its construction of “first vacuum chamber,” which has been rejected by the court. Essentially, Mieromass seeks a definition of “inlet orifice” such that if an empty vacuum chamber were to precede the ion guide (as in the Quattro Ultima), the “first vacuum chamber” could not possess both an inlet orifice and a rod set, as required by the claims of the patent. But the court has construed “first vacuum chamber” to mean “a vacuum chamber.” Therefore, the ion guide vacuum chamber can be the “first vacuum chamber” and any preceding vacuum chamber does not alter this result. Thus, the “inlet orifice” to the “first vacuum chamber” need not be the entrance to the first chamber held at less than atmospheric pressure. Instead, the court will adopt AB/Sciex’s proposed construction, which is consistent with the court’s earlier construction of “first” and “second.”

E. “separated by a wall” and “inter-chamber orifice”

Claim 1(a) describes “first and second vacuum chambers separated by a wall.” Claim 1(d) then requires “an interchamber orifice located in said wall and aligned with said first and second spaces so that ions may travel through.” Similarly, the preamble in claim 14 describes “first and second spaces ... separated by an inter-chamber orifice so that an ion may travel through said first space, said interchamber orifice and said second space ...” Thus, while claim 14 does not identify the wall discussed in claim 1, it does identify the interchamber orifice separating the first and second spaces.

Micromass’s proposes a construction of “separated by a wall” and “interchamber orifice” that would require the wall and interchamber orifice to join or link the two vacuum chambers and spaces. In support of this construction, Micromass points to Figures 1 and 12 and notes that in both there is only a solitary wall and intercham-ber orifice dividing the two vacuum chambers and spaces. Micromass also notes that the description of Figure 1, which states, in part, “[t]he vacuum chamber 30 is connected by an interchamber orifice 34 in a separator plate 36 to a second vacuum chamber 38 pumped by a vacuum pump 39.” Noting the specification’s use of the term “connected” to describe the inter-chamber orifice, Micromass maintains that wall and interchamber orifice must “join or link together” the two structures.

AB/Sciex contends that “separated by a wall” should be construed to mean only that “there is at least a wall between the first and second vacuum chambers” and that “interchamber orifice” should be construed to mean “an orifice in a wall that is between the first and second vacuum chambers.” It criticizes Micromass’s proposed construction because by requiring that the wall and interchamber orifice to join or link the two vacuum chambers or spaces, Micromass would be creating a requirement that no other structure, such as the multiple walls between vacuum chambers in the Quattro Ultima, be between the “first vacuum chamber” and the “second vacuum chamber.”

AB/Sciex’s proposed construction is well-founded. Claim 1 requires only that the first and second vacuum chambers be separated by a wall with an interchamber orifice. Because the claim uses the term “comprising,” other structures may be present between the two vacuum chambers at well. See Genentech, Inc. v. Chiron Corp., 112 F.3d 495, 501 (Fed.Cir.1997) (“ ‘Comprising’ is a term of art used in claim language which means that the named elements are essential, but other elements may be added and still form a cons