BACKGROUND
On November 13, 1992, plaintiffs filed this toxic tort ease in state court at Beaumont, Texas. After receipt of answers to interrogatories, defendant General Electric (GE) removed the case to the United States District Court for the Eastern District of Texas. The Texas district court judge upheld removal jurisdiction, denied plaintiffs’ motion to remand and transferred venue to the Western District of Oklahoma as a more convenient forum. The Oklahoma district court judge granted summary judgment in favor of defendants. Plaintiffs appealed, arguing that removal was untimely and summary judgment unwarranted.
STANDARD OF REVIEW
Summary judgment is appropriate “if the pleadings, depositions, answers to interrogatories, and admissions on file, together with the affidavits, if any, show that there is no genuine issue as to any material fact and that the moving party is entitled to a judgment as a matter of law.” Fed.R.Civ.P. 56(c); see also Celotex Corp.v. Catrett, 477 U.S. 317, 322-23, 106 S.Ct. 2548, 91 L.Ed.2d 265 (1986). We review a grant of summary judgment de novo, applying the same standard as the district court. See Wolf v. Prudential Ins. Co. of Am., 50 F.3d 793, 796 (10th Cir.1995). “[We] examine the record to determine whether any genuine issue of material fact was in dispute; if not, we determine [whether] the substantive law was correctly applied,” and in so doing “we examine the factual record and reasonable inferences therefrom in the light most favorable to the party opposing” the motion. Applied Genetics Int’l, Inc. v. First Affiliated Sec. Inc., 912 *1034F.2d 1238, 1241 (10th Cir.1990). However, “where the non moving party will bear the burden of proof at trial on a dispositive issue” that party must “go beyond the pleadings” and designate specific facts so as to “make a showing sufficient to establish the existence of an element essential to that party’s case” in order to survive summary judgment. Celotex, 477 U.S. at 322, 324, 106 S.Ct. 2548. A dispute is genuine only if the evidence is such that a reasonable jury could return a verdict for the nonmoving party. See Vitkus v. Beatrice Co., 11 F.3d 1535, 1539 (10th Cir.1993).
DISCUSSION
I. REMOVABILITY TO FEDERAL COURT
— Federal Enclave Jurisdiction
The United States has power and exclusive authority “in all Cases whatsoever ... over all places purchased” by the government “for the erection of Forts, Magazines, Arsenals, Dock-Yards, and other needful Buildings,” U.S. Const, art. I, § 8, cl. 17. Such places are “federal enclaves” within which the United States has exclusive jurisdiction.1 Personal injury actions which arise from incidents occurring in federal enclaves may be removed to federal district court as a part of federal question jurisdiction. There is no dispute that Tinker Air force Base at Oklahoma City, Oklahoma is such a federal enclave.
— Federal Officer Removal
Plaintiffs argue that the removal petition was defective in that all co-defendants did not consent and join in the removal papers. In the ease at bar, defendant GE removed the case based on its status as a “person acting under” a federal officer, as well as the status of Tinker Air Force Base as a federal enclave. Federal officer removal constitutes an exception to the general removal rule under 28 U.S.C. § 1441 and § 1446 which require all defendants to join in the removal petition. The exception is set forth at 28 U.S.C. § 1442(a)(1) which provides in part that:
(a) “A civil action or criminal prosecution commenced in a State court against any of the following may be removed by them to the district court of the United States for the district and division embracing the place wherein it is pending:
(1) The United States or any agency thereof or any officer (or any person acting under that officer) of the United States or of any agency thereof, sued in an official or individual capacity for any act under color of such office ....”
Id. (emphasis added). This statutory exception allows a federal officer independently to remove a case to federal court even though that officer is only one of several named defendants. The Congressional policy permitting federal officer removal could easily be frustrated by simply joining non-federal defendants unwilling to remove if consent of co-defendant(s) were required. Thus in Bradford v. Harding, 284 F.2d 307, 310 (2d Cir.1960), the Second circuit ruled that “[t]he ‘general government’ must be able to assure each of its officers that a federal forum will be available if he wishes it, whether others sued with him wish it or no.” Likewise, in Fowler v. Southern Bell Tel. & Tel. Co., 343 F.2d 150, 152 (5th Cir.1965), the Fifth circuit held “it is settled that the filing of a petition for removal by a single federal officer removes the entire case to the federal court.” In Ely Valley Mines, Inc. v. Hartford Accident & Indem. Co., 644 F.2d 1310, 1315 (9th Cir.1981), the Ninth circuit reasoned that “[sjince the federal officer is the only one entitled to remove under § 1442, he alone can remove without other defendants joining in the petition, and the entire case is removed to the federal court.” In City of Aurora v. Erwin, this court noted the importance to the United States and the absolute nature *1035of federal officer removal jurisdiction, quoting the Supreme Court that exercise of such jurisdiction “ ‘should not be frustrated by a narrow, grudging interpretation.’ ” 706 F.2d 295, 296 (10th Cir.1983) (quoting Arizona v. Manypenny, 451 U.S. 232, 242, 101 S.Ct. 1657, 68 L.Ed.2d 58 (1981)). On the basis of the foregoing, we hold that G.E. properly removed the case to federal court without the consent of co-defendants.
— Right to Remove Must be Clearly Determinable
The right to remove a ease to federal court is determined from allegations set forth in the initial pleading, “or other paper from which it may first be ascertained that the ease is one which is or has become removable ...” 28 U.S.C. § 1446(b) (emphasis added).
Appellants argue that removal was untimely because the initial pleading provided defendants with all necessary information to ascertain removability.2 Appellees argue that plaintiffs’ allegations of injuries sustained “while working at” Tinker Air Force Base and “while working for” the United States were ambiguous.3 The district judge in Texas ruled that the word “at” in the phrase “while' working at” could serve as either a “geographical modifier” or a “dura-tional modifier” (Order at 12, 13), and that only after receipt of “other paper”—in this case answers to interrogatories—were defendants provided sufficient notice that the conduct sued upon took place wholly within the enclave, and under federal direction.4
We agree that the initial pleading in this ease was ambiguous in that it did not provide unequivocal notice of the right to remove, and that the first clear notice of removability was given in answer to an interrogatory.5
In DeBry v. Transamerica Corp., 601 F.2d 480, 489 (10th Cir.1979), we held that “EJf the statute is going to run, the notice ought to be unequivocal. It should not be one which may have a double design.” We further ruled that “ascertained” as used in section 1446(b) means a statement that “should not be ambiguous” or one which “requires an extensive investigation to determine the truth.” Id. at 490. DeBry is con*1036sistent with our prior ruling in Ardison v. Villa, 248 F.2d 226 (10th Cir.1957), in which we interpreted the predecessor provision of § 1446(b), holding that the key to determining the date from which the clock begins to run is when the defendant is able to “intelligently ascertain removabilityId. at 227. We disagree with cases from other jurisdictions which impose a duty to investigate and determine removability where the initial pleading indicates that the right to remove may exist.6 Rather, this court requires clear and unequivocal notice from the pleading itself, or a subsequent “other paper” such as an answer to interrogatory.
— Proceedings in State Court Before Removal
Plaintiffs contend that defendant Chemical Specialist was barred from consenting to removal because it had previously filed a motion for summary judgment in state court, which motion was pending when removal was granted, citing Scholz v. RDV Sports, Inc., 821 F.Supp. 1469, 1471 (M.D.Fla.1993). That case is inapposite because actions in state court by defendant Chemical Specialist in this case were taken before it was unequivocally apparent that the case was removable.7 This court rules that a defendant who actively invokes the jurisdiction of the state court and interposes a defense in that forum is not barred from the right to removal in the absence of adequate notice of the right to remove.
— Proceedings in Federal Court after Removal
After removal of the case to federal court and the motion to remand was denied, plaintiffs voluntarily amended their complaint, asserting a cause of action in federal court against defendants Dow Chemical, Ash-land Chemical and McGean-Rohco, Inc. This court holds that plaintiffs cannot voluntarily invoke, and then disavow, federal jurisdiction. In Bernstein v. Lind-Waldock & Co., the Seventh Circuit stated:
But once [plaintiff] decided to take advantage of his involuntary presence in federal court to add a federal claim to his complaint he was bound to remain there. Otherwise he would be in a position where if he won his case on there merits in federal court he could claim to have raised the federal question in his amended complaint voluntarily, and if he lost he could claim to have raised it involuntarily and to be entitled to start over in state court. He “cannot be permitted to invoke the jurisdiction of the federal court, and then disclaim it when he loses.”
738 F.2d 179, 185-86 (7th Cir.1984) (citations omitted); see also Barbara v. New York Stock Exch., Inc., 99 F.3d 49 (2d Cir.1996); Tolton v. American Biodyne, Inc., 48 F.3d 937 (6th Cir.1995). In Johnson v. Odeco Oil & Gas Co., 864 F.2d 40 (5th Cir.1989), an amendment in federal court after removal was found to constitute a waiver of later objection to removal. In the circumstances of this case, the amendment adding parties also amounted to a waiver of alleged defective removal.
II WARNINGS CONCERNING HAZARDOUS MATERIALS NEED NOT BE PROVIDED TO “SOPHISTICATED” AND KNOWLEDGEABLE PURCHASERS
— State Law Applies in Failure to Warn Cases
This products liability action is premised on the manufacturer’s failure to warn purchaser’s employees of the danger of low level exposure to certain chemicals. We apply Oklahoma law in such cases. See Chiles v. Ceridian Corp., 95 F.3d 1505, 1510 (10th Cir.1996). We review the district court’s interpretation of Oklahoma law de novo, see Salve Regina College v. Russell, 499 U.S. 225, 231, 111 S.Ct. 1217, 113 L.Ed.2d 190 (1991), and “as a matter of *1037independent federal procedure we utilize the normal federal standards of appellate review to examine the district court’s decision process.” Mid-America Pipeline Co. v. Lario Enters., 942 F.2d 1519, 1524 (10th Cir.1991).
— Sophisticated Purchaser Defense
Plaintiffs contend that they were injured while cleaning jet engine parts due to low-level, chronic exposure to defendants’ chemicals. Plaintiffs argue that defendants breached their duty to warn potential users of the dangerous propensities of these chemicals even though the chemicals supplied were not improperly manufactured or contaminated.
Under Oklahoma law, the general rule applies that chemicals will be considered defective only if the following three elements are satisfied: (1) the product was unreasonably dangerous; (2) there was a failure to warn of its dangerous characteristics; and (3) the failure to warn was the cause of the plaintiffs injury. See Cunningham v. Charles Pfizer & Co., 532 P.2d 1377 (Okla.1974). But Oklahoma law authorizes an important exception to the second element of the general rule, namely that there is no duty to warn members of a profession against dangers generally known to members of that profession. Hence, in Mayberry v. Akron Rubber Mach. Corp., the court said that “where the danger or potentiality of danger is known or should be known to the user, the duty (to warn) does not attach.” 483 F.Supp. 407, 413 (N.D.Okla.1979)(citing Berry v. Eckhardt Porsche Audi, Inc., 578 P.2d 1195 (Okla.1978); Nicholson v. Tacker, 512 P.2d 156 (Okla.1973)); see also Davis v. Fox River Tractor Co., 518 F.2d 481 (10th Cir.1975); Marshall v. Ford Motor Co., 446 F.2d 712 (10th Cir.1971). We read Oklahoma case law to impose no duty to warn a purchaser as knowledgeable as the United States Air Force of the potential dangers of low-level chemical exposure. Plaintiffs rely on the argument that the Air Force did not actually know of the risks involved in low-level chemical exposure. However, Oklahoma law clearly imposes a “should have known” standard as well, applicable to “knowledgeable purchasers,” Mayberry, 483 F.Supp. at 413. This is tantamount to the familiar “sophisticated purchaser defense” exception which is based upon the principles set forth in the Restatement (Second) of Torts.8 This exception absolves suppliers of the duty to warn purchasers who are already aware or should be aware of the potential dangers. See O’Neal v. Celanese Corp., 10 F.3d 249, 251-52 (4th Cir.1993); see also Davis v. Avondale Indus., 975 F.2d 169, 171 (5th Cir.1992) (a manufacturer is not required to provide an adequate warning about his product when the user or handler of the product already knows or reasonably should be expected to know of the characteristics of the product that may cause damage and the danger of such characteristics.) In Apperson v. E.I. du Pont de Nemours & Co., 41 F.3d 1103, 1108 (7th Cir.1994), the Seventh Circuit stated: “a duty to warn arises only when there is unequal knowledge with respect to the risk of the harm.”
Because of the wealth of research available, the ability of the Air Force to conduct studies, and its extremely knowledgeable staff, we find that the Air Force easily qualifies as a “knowledgeable purchaser” that should have known the risks involved with low-level chemical exposure. Employees of the Air Force are also deemed to possess the necessary level of sophistication, so that defendants had no duty to warn the Air Force or its employees of the potential hazards. See Mayberry, 483 F.Supp. at 413.
Based upon the foregoing, this court concludes from uneontroverted evidence viewed *1038in a light most favorable to the plaintiffs that the district court’s decision granting defendants’ Motion for Summary Judgment should be affirmed.
AFFIRMED.