OPINION OF THE COURT
HUTCHINSON, Circuit Judge.
Appellants, Donald Bogusz (“Bogusz”) and John O’Rourke (“O’Rourke”), appeal criminal sentences imposed on them by the United States District Court for the District of New Jersey.1 The district court sentenced Bo-gusz to 120 months and O’Rourke to 168 months of imprisonment for their criminal involvement with a methamphetamine laboratory. Because the district court erroneously interpreted the United States Sentencing Guidelines (the “Guidelines”),2 it miscalculated Bogusz’s and O’Rourke’s sentences. Therefore, we will vacate both their sentences and remand for resentencing.
I. Background
Because this appeal focuses on sentencing, only a summary of the facts material to the sentencing issues is needed. On August 29, 1991, a federal grand jury returned an indictment against twelve individuals, including Bogusz and O’Rourke, charging them with participation in a scheme to manufacture and distribute methamphetamine. Bogusz located and obtained glassware and phenylacetic acid, a methamphetamine precursor, for the methamphetamine production process. O’Rourke served as a “plumber.” In that capacity, he unclogged drains that became blocked during the methamphetamine manufacturing process.
Bogusz and O’Rourke received methamphetamine as part of the consideration for their services. O’Rourke received four of the eight pounds of methamphetamine produced while he worked on the pipes and Bogusz got one pound. The methamphetamine produced was described as “sticky” and “like caramel” indicating its poor quality. In fact, Bogusz gave half of his methamphetamine to a co-conspirator and returned the other half because of its poor quality.
On' March 17, 1992, Bogusz pled guilty under a plea agreement to a conspiracy to distribute more than two pounds of phenyla-cetic acid, a listed chemical, knowing that it would be used to manufacture methamphetamine, a controlled substance, in violation of 21 U.S.C.A. § 841(d)(2) (West Supp.1994). On May 14, 1992, after a jury trial, O’Rourke was convicted of a conspiracy to manufacture methamphetamine with intent to distribute in violation of 21 U.S.C.A. § 846 (West Supp. 1994) and possession with intent to distribute in excess of one kilogram of methamphetamine in violation of 21 U.S.C.A. § 841(a)(1) (West Supp.1994).
At Bogusz’s sentencing, the district court adopted a recommendation in the probation office’s Presentence Report (the “PSR”) to apply a higher base offense level than the one stipulated in Bogusz’s plea agreement. Bogusz and the government had stipulated to a base offense level of 24, applying U.S.S.G. § 2Dl.ll(d)(3); but the PSR recommended applying U.S.S.G. § 2D1.1 with a base offense level of 34. Using a cross-reference from section 2Dl.ll(e)(l) to section 2D1.1, the district court decided the base offense level was 34. Because phenylacetic acid is not included in section 2Dl.l’s Sentencing Table, use of section 2D1.1 required conversion of the phenylacetic acid quantities to those of a substance on the table. The probation officer preparing the PSR converted the eight pounds of phenylacetic acid to two pounds of methamphetamine, the amount of methamphetamine produced from the pheny-lacetic acid.
The PSR also recommended that sentencing be based upon “methamphetamine (actu[85]*85al)” as opposed to “methamphetamine.”3 The base offense level for two pounds of methamphetamine (actual) under section 2D1.1 was 34. U.S.S.G. § 2Dl.l(c)(5) (Drug Quantity Table). This ultimately resulted in Bogusz’s 120-month sentence. Sentencing under section 2Dl.ll(d)(3), with its base level of 24, in accord with the stipulation in the plea agreement, would have resulted in a sentencing range of 51 to 63 months. See U.S.S.G. Ch. 5, Pt. A (Sentencing Table). Applying a two level reduction for acceptance of responsibility and a criminal history category of III to this offense level, the Guidelines indicated that Bogusz should be sentenced to 151 to 188 months of imprisonment. Id. Because the statutory maximum sentence under 21 U.S.C.A. § 841(d) is 120 months, the district court sentenced Bogusz to 120 months. See U.S.S.G. § 5Gl.l(a) (“Where the statutorily authorized maximum sentence is less than the minimum of the applicable guideline range, the statutorily authorized maximum sentence shall be the guideline sentence.”); see also United States v. Donley, 878 F.2d 735, 741 (3d Cir.1989) (“the underlying statute shall control in case of conflict with the Sentencing Guidelines”), cert. denied, 494 U.S. 1058, 110 S.Ct. 1528, 108 L.Ed.2d 767 (1990).
At O’Rourke’s sentencing, the district court again adopted the PSR’s recommendation to apply U.S.S.G. § 2D1.1 and again decided that the methamphetamine was methamphetamine (actual). Based on the eight pounds of methamphetamine produced when he worked on the pipes, O’Rourke received a base offense level of 38, see U.S.S.G. § 2Dl.l(c)(3), but the district court granted O’Rourke a four point offense level reduction for his mitigating role. See U.S.S.G. § 3B1.2(a). Using a criminal history category of II and an offense level of 34, the Guidelines put O’Rourke in a sentencing range of 168 to 210 months imprisonment. See U.S.S.G. Ch. 5, Pt. A (Sentencing Table). O’Rourke was sentenced to concurrent sentences of 168 months imprisonment on each count. Both Bogusz and O’Rourke filed timely notices of appeal.
II. Jurisdiction and Standard of Review
The district court had subject matter jurisdiction over these criminal eases pursuant to 18 U.S.C.A. § 3231 (West 1985). We have appellate jurisdiction over this consolidated appeal under 28 U.S.C.A. § 1291 (West 1993) (review of final decisions) and 18 U.S.C.A. § 3742 (West 1985) (review of sentences).
Under the Guidelines, we review a district court’s findings of fact for the limited purpose of determining whether they are clearly erroneous. United States v. Miele, 989 F.2d 659, 663 (3d Cir.1993); United States v. Belletiere, 971 F.2d 961, 964 (3d Cir.1992); see also 18 U.S.C.A. § 3742(e) (West Supp.1994) (reviewing courts “shall accept the findings of fact of the district court unless they are clearly erroneous”). Findings of fact are clearly erroneous when “the reviewing court on the entire evidence is left with the definite and firm conviction that a mistake has been committed.” United States v. U.S. Gypsum Co., 333 U.S. 364, 395, 68 S.Ct. 525, 542, 92 L.Ed. 746 (1948). Findings involving mixed questions of law and fact are subjected to a more demanding scrutiny “approaching de novo review as the issue moves from one of strictly fact to one of strictly law.” Belletiere, 971 F.2d at 964 (quoting United States v. Murillo, 933 F.2d 195, 198 (3d Cir.1991)). When the essential facts are not in dispute, our review of the district court’s interpretation of the Guidelines, like our review of a statute’s interpretation, is plenary. See United States v. Rosen, 896 F.2d 789, 790-91 (3d Cir.1990). We must, however, defer to the Sentencing Commission’s interpretation of the Guidelines unless “it violates the Constitution or a Federal Statute, or is inconsistent with, or a plainly erroneous interpretation of, that [provision].” Stinson v. United States, — U.S. -, -, 113 S.Ct. 1913, 1915, 123 L.Ed.2d 598 (1993).
III. Discussion
Bogusz raises four challenges to the district court’s sentences. O’Rourke joins with [86]*86him in two. First, both contend that the district court erred in finding the unanalyzed methamphetamine, upon which their sentencing was based, to be methamphetamine (actual). Second, both argue that the district court erred in tacitly finding that the methamphetamine was Dextro-methamphetamine (“D-methamphetamine”) as opposed to'Levo-methamphetamine (“L-methamphetamine”). Third, Bogusz argues that U.S.S.G. § 2D1.1 does not apply to violations of 21 U.S.C.A. § 841(d)(2) (West Supp.1994), and that U.S.S.G. § 2D1.11 is the only Guidelines’ provision applicable to this offense. Fourth, Bogusz contends that the government is violating its plea agreement with him by arguing for affirmance of the district court’s sentence. We will address each challenge in turn.
A. Guidelines’ Treatment of Methamphetamine
To apply U.S.S.G. § 2D1.1, a sentencing court must first determine whether the substance in question is methamphetamine or methamphetamine (actual). This determination involves two related issues. We must first consider whether the methamphetamine that Bogusz and O’Rourke helped produce was “pure” methamphetamine, a necessary condition for its classification as methamphetamine (actual), and then the more complex question of whether the government must also prove that the substance is D- or L-methamphetamine.
1. Methamphetamine (Actual)
The difference between methamphetamine and methamphetamine (actual) is highly significant for sentencing purposes: methamphetamine (actual) is subject to an offense level ten times greater than methamphetamine. See U.S.S.G. § 2D1.1, comment, (n. 10) (Drug Equivalency Table) (one gram of methamphetamine (actual) is treated as the equivalent of ten grams of marijuana while one gram of methamphetamine is equivalent to one gram of marijuana); see also United States v. Lande, 40 F.3d 329, 330 n. 1 (10th Cir.1994); United States v. Carroll, 6 F.3d 735, 744 (11th Cir.1993) (discussing the effect on sentencing) (citing United States v. Brown, 921 F.2d 785, 789 & n. 2 (8th Cir.1990)), cert. denied sub. nom., Jessee v. United States, — U.S. -, 114 S.Ct. 1234, 127 L.Ed.2d 577 (1994).
The district court defined methamphetamine (actual) as “pure methamphetamine.” 4 The court then explained:
Well, pure is how you define “pure.” I’m defining it, “pure,” as uncut product, not whether the product was good product or bad product. Now that may be erroneous, in which case I’ll be reversed on appeal. But the fact of the matter is that a caramel-like mess to me is not the critical point; the point is, that’s what came out of the manufacturing process, and it had not yet been cut.
Bogusz Appendix at 85. Bogusz and O’Rourke argue that methamphetamine (actual) refers to the percentage purity of the end product. That is, they argue that methamphetamine (actual) refers to the net amount of methamphetamine hydrochloride present in the substance upon which sentencing is based.
The Guidelines’ commentary defines methamphetamine (actual) as “the weight of the controlled substance, itself, contained in the mixture or substance.” U.S.S.G. § 2Dl.l(c), comment.(n.*).5 The Guidelines also provide the following illustrative example: “a mixture weighing 10 grams containing PCP at 50% purity contains 5 grams PCP (actual).” Id. (under the Guidelines, PCP and methamphetamine are treated identically).
Unfortunately, the commentary to the Guidelines is susceptible to either interpretation of “pure,” and each has case law support. Compare United States v. Macklin, 927 F.2d 1272, 1282 (2d Cir.) (holding that “pure” [87]*87merely means uncut or unadulterated), cert. denied, — U.S. -, 112 S.Ct. 146, 116 L.Ed.2d 112 (1991); United States v. Patrick, 983 F.2d 206 (11th Cir.1993) (same in dicta) with Carroll, 6 F.3d at 746 (“the only way to calculate the quantity of ‘pure methamphetamine’ in determining a defendant’s base offense level under § 2Dl.l(c) is to multiply the purity of the mixture times the weight”), cert. denied sub. nom., Jessee v. United States, — U.S. -, 114 S.Ct. 1234, 127 L.Ed.2d 577 (1994); United States v. Rusher, 966 F.2d 868, 880 (4th Cir.) (same), cert. denied, — U.S. -, 113 S.Ct. 351, 121 L.Ed.2d 266 (1992); United States v. Alfeche, 942 F.2d 697, 699 (9th Cir.1991) (same); United States v. Brown, 921 F.2d 785, 789-90 (8th Cir.1990) (same); see also United States v. Spencer, 4 F.3d 115, 122 (2d Cir.1993) (noting that pure methamphetamine does not include the weight of impurities).
At oral argument, the government argued that adoption of the appellants’ interpretation would reward them for being “poor cooks.” Transcript of Oral Arguments at 46. This contention reflects a fundamental misunderstanding of methamphetamine production and the Guidelines’ treatment of it. Sentencing for methamphetamine drug offenses is intended to punish all cooks equally. Sentencing under methamphetamine (actual) punishes particularly good cooks and then-employers more severely. Methamphetamine, as produced through normal chemical processes, contains a number of impurities. See Spencer, 4 F.3d at 121 (noting that methamphetamine results from a “chemical reaction which yields a mixture of methamphetamine and various impurities”); United States v. Stoner, 927 F.2d 45, 47 (1st Cir.) (noting that methamphetamine “virtually never is completely pure”), cert. denied, — U.S. -, 112 S.Ct. 129, 116 L.Ed.2d 96 (1991). The initial product can then be processed further to remove these impurities. The purified product, being more concentrated, can then be cut into larger quantities for resale. The interpretation Bogusz and O’Rourke urge on us does not reward bad cooks; instead, it merely punishes more severely the sophisticated cooks who could otherwise manipulate the Guidelines by producing smaller quantities of more concentrated methamphetamine.
The government’s reliance on Chapman v. United States, 500 U.S. 453, 461, 111 S.Ct. 1919, 1925, 114 L.Ed.2d 524 (1991), and its discussion of Congress’ “market-oriented” approach is also misplaced. As the Supreme Court noted in Chapman, Congress and the Guidelines identified methamphetamine and PCP as drugs warranting differential treatment with regard to purity and thus provided for their unique sentencing scheme. See id. at 459-60, 111 S.Ct. at 1924. An interpretation of purity that relies upon the treatment of other controlled substances conflicts with the Guidelines’ unique treatment of methamphetamine.
After consideration of the text and commentary of the Guidelines, existing case law and the peculiar sentencing scheme for methamphetamine, we hold that methamphetamine (actual) refers to the net amount of methamphetamine hydrochloride produced and not the gross amount of uncut methamphetamine. Thus, methamphetamine (actual) refers to the net amount of methamphetamine hydrochloride after all impurities, waste, byproducts, or cutting agents are removed.
The government argues that Bogusz and O’Rourke did not show that the methamphetamine “contained a cutting agent, waste product, or any substance other than the controlled substance itself.” Brief of Appellee at 46. In essence, the government is arguing that defendants bear the burden of showing the portion of the substance that is not methamphetamine (actual). This argument fails. Although the purity of a methamphetamine product does not bear on a defendant’s guilt or innocence and, thus, does not invoke the “beyond a reasonable doubt” standard of In re Winship, 397 U.S. 358, 360, 90 S.Ct. 1068, 1070, 25 L.Ed.2d 368 (1970), it does have a profound effect on the sentence imposed, and the government bears the burden of proving it, albeit by only a preponderance of the evidence. See United States v. Miele, 989 F.2d 659, 663 (3d Cir.1993).
In some situations, a chemical analysis of the substance that indicates its purity may be required for the government to meet this [88]*88burden. In others, circumstantial evidence of purity may be sufficient. We hold only that the government must produce evidence of the quantity of methamphetamine hydrochloride the mixture in question contains if a defendant is to be sentenced under U.S.S.G. § 2D1.1 for methamphetamine (actual).
In this ease, some of the evidence produced at trial supports the district court’s tacit finding of 100% purity. Trial testimony showed that the defendants were in possession of a functional recipe, proper equipment, and requisite chemicals. There was, however, other evidence on the color and consistency of the product which indicated poor quality and could have supported a finding of impurity. Manufactured methamphetamine is not 100% pure regardless of the sophistication of the equipment. Therefore, the government cannot rely solely on the nature of the production process and assume that the total product is pure methamphetamine which calls for sentencing under methamphetamine (actual). Instead, we think there should be a finding, based on evidence, on how much methamphetamine hydrochloride is included in the mixture that constitutes the end product. Because the district court failed to make such a finding, we will remand for further fact finding on the purity of the product.6
2. Organic Composition
On the second aspect of methamphetamine sentencing, both Bogusz and O’Rourke challenge the district court’s tacit assumption that the methamphetamine was D-methamphetamine as opposed to L-meth.amphetamine. The two are grossly different in physiological effect and, as we shall see, this difference is reflected in the Guidelines drug equivalency tables by a factor of 250 to 1. An initial failure of the parties to appreciate the chemistry involved and thus to inform the district court of the scientific basis for this contention requires us to consider whether Bogusz and O’Rourke have waived any issue regarding the distinction between D- and L-methamphetamine. Discussion of the principles of organic chemistry that underlie this issue is necessary before the problem created by the distinction between D- and L-methamphetamine can be understood. See United States v. Ammar, 714 F.2d 238, 261-64 (3d Cir.) (pre-Guidelines case discussing chemical difference between D- and L-heroin), cert. denied sub nom. Stillman v. United States, 464 U.S. 936, 104 S.Ct. 344, 78 L.Ed.2d 311 (1983).
The methamphetamine molecule, like most organic molecules, exists in different “isomeric” forms. Isomers “are compounds that have the same molecular formula but different structural formulas.” Harold Hart, Organic Chemistry: A Short Course 15 (6th ed. 1983) (“Organic Chemistry Text”). Just as people are either right- or left-hand dominant, a molecule can sometimes exist in right- and left-handed forms. See Organic Chemistry Text at 125-26; Roger A. Heg-strom & Dilip K. Kondepudi, The Handedness of the Universe, Scientific American, Jan. 1990, at 108 (“Hegstrom & Kondepudi Article”); United States v. Patrick, 983 F.2d 206, 209 (11th Cir.1993). A molecule “that exhibits the property of handedness” is called a chiral molecule.7 The two forms of the chiral molecules are called enantiomers.8
Each enantiomer is labelled either Dextro or Levo, or D or L. Hegstrom & Kondepudi Article at 109. The difference is determined by the optical rotation of light. D is right-[89]*89handed and L is left-handed. One is the mirror image of the other; that is, they are mirror symmetrical. Id. Although enan-tiomers only differ with respect to chirality, the human body “is highly sensitive to enan-tiomeric differences.” Id. For example, the thalidomide birth defects of the 1960’s resulted because one enantiomer of thalidomide stopped morning sickness while the other caused birth defects. Id. at 109-10.9
Methamphetamine exists in these two isomeric forms.10 L-methamphetamine is a compound that produces little or no physiological effect when ingested. Carroll, 6 F.3d at 743. D-methamphetamine, on the other hand, produces the physiological effect desired by its users. Id.
The text of U.S.S.G. § 2D1.1 differentiates only between methamphetamine and methamphetamine (actual). We have previously discussed that distinction as it involves drug purity, not organic structure. The question now posed is whether the isomeric structure of methamphetamine, as well as the net quantity of methamphetamine hydrochloride, is relevant to Guidelines sentencing. The Guidelines do not differentiate between the D- and L-isomers of methamphetamine in the text of section 2D1.1, but only in the commentary to it. There, in the Drug Equivalency Tables, L-methamphetamine is treated far less severely than either methamphetamine or methamphetamine (actual): methamphetamine (actual) by a factor of 250, methamphetamine by a factor of 25. See U.S.S.G. § 2D1.1 comment, (n. 10) (Drug Equivalency Table) (one gram of L-methamphetamine is equivalent to 40 grams of marijuana, one gram of methamphetamine is equivalent to one kilogram of marijuana, and one gram of methamphetamine (actual) is equivalent to 10 kilograms of marijuana).11 With this chemical background in mind, we consider first whether the issue raised by this distinction in their physiological effect was fairly raised before the district court. Bogusz and O’Rourke never used precise chemical terms in arguing this question. They lumped this issue together with their arguments on purity when they objected to sentencing based upon an unanalyzed substance. Nevertheless, we conclude that Bo-gusz’s and O’Rourke’s objections to sentencing based on the unanalyzed substance produced at the methamphetamine laboratory fáirly raised and preserved the issue for appeal.
Even if Bogusz and O’Rourke were raising the issue for the first time on appeal, we could nevertheless review the trial court’s findings for plain error. See Fed.R.Crim.P. 52(b) (“Plain errors or defects affecting substantial rights may be noticed although they were not brought to the attention of the court.”). We believe that under these circumstances the district court’s interpretation of the Guidelines would be plain error. Because of the objections at sentencing, this case differs factually from United States v. Deninno, 29 F.3d 572, 580 (10th Cir.1994), in which the United States Court of Appeals for the Eleventh Circuit refused to consider a similar claim because of the appellant’s complete failure to object at sentencing. Moreover, considering the gross disparity in sentencing, we disagree with the Deninno [90]*90court’s holding that the determination of methamphetamine type is entirely a factual question that cannot rise to the level of plain error. Id.
The Fifth Circuit recently defined a plain error as one “so obvious that [a] failure to notice it would seriously affect the fairness, integrity, or public reputation of the judicial proceeding and result in a miscarriage of justice.” United States v. Hoster, 988 F.2d 1374, 1380 (5th Cir.1993) (quoting United States v. Surasky, 974 F.2d 19, 21 (5th Cir.1992), cert. denied, — U.S. -, 113 S.Ct. 1948, 123 L.Ed.2d 653 (1993)); see also United States v. Olano, — U.S. -, -, 113 S.Ct. 1770, 1776, 123 L.Ed.2d 508 (1993). We recognize that the term “plain error” normally implies an error that is apparent as well as unjust. See United States v. Atkinson, 297 U.S. 157, 160, 56 S.Ct. 391, 392, 80 L.Ed. 555 (1936).12 Here, however, we think that the egregiousness of the injustice that would result if the distinction between the two isomers is not recognized outweighs the failure of Bogusz and O’Rourke to articulate clearly the principles of organic chemistry that underlie their objections to the district court’s application of the Guidelines.13
Thus, considering the magnitude of the difference' in sentencing that could result from the application of the wrong organic isomer, we think the sentencing court’s failure to make this determination would result in a grave miscarriage of justice.14 We will thus consider whether the distinction between the left- and right-handed isomers of methamphetamine is material to the Guidelines sentences that can be legally imposed on Bogusz and O’Rourke.
In United States v. Carroll, 6 F.3d 735 (11th Cir.1993), the Court of Appeals for the Eleventh Circuit sought to separate methamphetamine’s purity from the effect of its isomers. In that case, the defendant’s sentence was based on methamphetamine that contained 50% D- and 50% L-methamphetamine. Id. at 743. The Court of Appeals held, “the distinction between methamphetamine and [methamphetamine (actual)] refers to the relative purity of any methamphetamine [91]*91compound; it does not refer to a particular form of methamphetamine.” Id. at 744. Thus, the 50% D- and 50% L-methamphetamine compound could be 100% pure for purposes of calculating methamphetamine (actual). Judge Bright dissented from the majority’s “drug quality issue.” Id. at 747. Because the Guidelines “caused great confusion due to the convoluted chemical rhetoric” required by their application in this area, Judge Bright would have affirmed the lower court’s conclusion that purity should be based on the quantity of D-methamphetamine. Id. at 749.15
An isolated literal reading of U.S.S.G. § 2Dl.l(e) does offer some support to the Carroll majority’s separation of the purity problem from the difference in the effect of the two isomers. We think, however, that such a separation for purposes of Guidelines’ sentences would obliterate the distinction between the effect of the two isomers that the Guidelines’ commentary recognizes in any case in which the Drug Equivalency Tables are not used. Considering the difference between the physiological effect of the two isomers, along with the Sentencing Commission’s recognition of that difference in its use of conversion factors with a 1 to 250 ratio, the disparity in sentencing that would result seems to us contrary to one of Congress’s primary goals in passing the Sentencing Reform Act — the substitution of uniformity for disparity in sentencing.16 We think the Guidelines should not be construed in a way that results in so greatly irrational a disparity. To illustrate, under Carroll, a defendant convicted of one gram of pure L-methamphetamine would have a base offense level of 16 with a Guidelines range of 21 to 27 months imprisonment. See U.S.S.G. Ch. 5, Pt. A (Sentencing Table) (assuming a criminal history category of I). Another defendant sentenced for one gram of pure L-methamphetamine and an additional 200 grams of marijuana (thus, requiring conversion under the Drug Equivalency Table) would have a base offense level of 6 and a sentencing range of zero to six months. See U.S.S.G. Ch. 5, Pt. A (Sentencing Table) (again, assuming a criminal history category of I).
Accordingly, we hold that the references to methamphetamine and methamphetamine (actual) in the Drug Quantity Tables of U.S.S.G. § 2Dl.l(c) refer solely to quantities of D-methamphetamine. In order to calculate a base offense level under section 2Dl.l(c) for L-methamphetamine, the substance in question must first be converted into marijuana equivalents. See U.S.S.G. § 2D1.1, comment, (n. 10) (noting that the Drug Quantity Tables do not include all substances and that the Drug Equivalency Tables should be used for those that are not included).
Because no determination of the isomeric composition of methamphetamine was made at sentencing, this issue must also be considered on remand. We again remind the government that it has the burden of production and persuasion on this issue and that the proper standard for the burden of persuasion is a preponderance of the evidence. The type of proof required to satisfy this standard will also vary from case to case.17 In [92]*92some cases, the evidence will include a chemical analysis or expert testimony. In others, circumstantial evidence of which isomer is present may be sufficient to meet the preponderance of the evidence standard. See United States v. Koonce, 884 F.2d 349, 352-53 (8th Cir.1989) (affirming D-methamphetamine determination based on circumstantial evidence of defendant’s prior methamphetamine shipment).
B. Section 2Dl.ll’s Cross Reference to 2D1.1
Bogusz alone raises the next Guidelines issue. He argues that the district court erred in applying U.S.S.G. § 2D11 under section 2Dl.ll’s cross-reference to it. He contends that section 2D1.1 does not apply to violations of 21 U.S.C.A. § 841(d). Bogusz pled guilty to distribution of a precursor chemical knowing that it would be used to manufacture a controlled substance but was sentenced for conspiracy to unlawfully manufacture the quantity of the controlled substance that was produced from the precursor chemicals he delivered.
Bogusz relies primarily on United States v. Voss, 956 F.2d 1007 (10th Cir.1992), a case decided under the Guidelines in effect before the amendment adopting section 2D1.11. In Voss, over a strong dissent by Judge Ebel, the court held U.S.S.G. § 2D1.1 inapplicable to violations of 21 U.S.C.A. § 841(d). Id. at 1012. The majority reasoned that its application “would insure that almost all violators of section 841(d) would be sentenced to the ten year maximum imprisonment, thus turning a statutory maximum into a mandatory sentence.” Id. at 1010 (citations omitted). The majority refused to interpret the Guidelines in a manner that would achieve this result, fearing that such an interpretation “would effectively nullify the various sections of the Guidelines geared to a particular defendant’s offense specific conduct.” Id.
Since Voss, the Guidelines have been amended and section 2D1.11 now clearly applies to violations of section 21 U.S.C.A. § 841(d). See U.S.S.G. § 2D1.11 (Unlawfully Distributing, Importing, Exporting or Possessing a Listed Chemical; Attempt or Conspiracy). We sympathize with the concern the Voss majority expressed,18 but we cannot reconcile it with the text of the amended Guidelines. See United States v. O’Leary, 35 F.3d 153, 154 (5th Cir.1994). Section 2Dl.ll(c)(l) states: “If the offense involved unlawfully manufacturing a controlled substance, or attempting to manufacture a controlled substance unlawfully, apply § 2D1.1 (Unlawful Manufacturing, Importing, Exporting, Trafficking) if the resulting offense level is greater than that determined above.” U.S.S.G. § 2D1.11(c)(1). The Guidelines explain that section 2Dl.ll(c)(l) applies when “the defendant, or a person for whose conduct the defendant is accountable under § 1B1.3 (Relevant Conduct), completed the actions sufficient to constitute the offense of unlawfully manufacturing a controlled substance or attempting to manufacture a controlled substance unlawfully.” Id. § 2D1.11, comment.(n. 2). The relevant conduct referred to in section 1B1.3 includes “all acts ... committed, aided, abetted, counselled, commanded, induced, procured, or willfully caused by the defendant; and in the case of a jointly undertaken criminal activity ... all reasonably foreseeable acts and omissions of others in furtherance of the jointly undertak[93]*93en criminal activity.” Id. § lB1.3(a)(l)(A & B).
Unless there is a showing of contrary intent, we must “follow the clear unambiguous language of the Guidelines.” United States v. Wong, 3 F.3d 667, 670 (3d Cir.1993). Therefore, we apply the Guidelines as they were written, not as we think they should have been written. Id. The district court had before it sufficient evidence to determine that Bogusz’s conduct satisfied the requirements of U.S.S.G. § 2Dl.ll(c)(l). Accordingly, we hold that the district court did not err in its reliance on section 2D1.1 by cross-reference from section 2D1.11.
However, the use of section 2D1.11 requires us to consider a constitutional issue. If the Voss majority’s interpretation of the pre-1991 Guidelines was correct, Bogusz’s sentence would conflict with the Ex Post Facto Clause. U.S. Const, art. I, § 9, cl. 3. The prohibition against the passage of ex post facto laws includes, inter alia, “[ejvery law that changes the punishment and inflicts a greater punishment than it was when committed.” Calder v. Bull, 3 Dall. 386, 390, 1 L.Ed. 648 (1798). In Miller v. Florida, 482 U.S. 423, 107 S.Ct. 2446, 96 L.Ed.2d 351 (1987), the Supreme Court, in striking down the use of a state sentencing guideline, held that an application of a sentencing provision to conduct occurring before its passage or promulgation violates the Ex Post Facto Clause whenever “the law [is] retrospective, that is, it ... applies] to events occurring before its enactment, and ... it ... disadvantage^] the offender affected by it.” Id. at 430, 107 S.Ct. at 2451 (internal quotations and citations omitted).
Bogusz was sentenced under the 1991 Guidelines for conduct that occurred from early April to early May 1990. The 1991 Guidelines became, effective November 1, 1991. Therefore, if the 1990 Guidelines would have resulted in a lower sentence, the Ex Post Facto Clause would require its application. See United States v. Spiropoulos, 976 F.2d 155, 160 n. 3 (3d Cir.1992) (“district courts are required to apply the time-of-offense [G]uidelines rather than the time-of-sentence [G]uidelines when ... the time-of-offense [GJuidelines are more favorable to the defendant”). Thus, if the Foss interpretation of the earlier Guidelines were correct, Bogusz could have received a less severe sentence under the pre-1991 revision of the Guidelines. See Voss, 956 F.2d at 1013 (district court should base sentencing on the purposes of 18 U.S.C.A. § 3553(b) (1988) without regard to the Guidelines’ sentencing table).
The Ex Post Facto Clause requires us to interpret the pre-1991 Guidelines and meet the question posed by Foss, one this Court has not previously decided under the pre-1991 Guidelines. After thorough consideration, we find ourselves in agreement with those courts that have rejected the view of the Foss majority.19 Following Judge Ebel’s reasoning in dissent, we hold that, under the pre-1991 Guidelines, violators of 21 U.S.C.A. § 841(d) could properly be sentenced under U.S.S.G. § 2D1.1. Accordingly, we conclude that application of the 1991 Guidelines did not disadvantage Bogusz and thus no constitutional infirmity exists under the Ex Post Facto Clause.
C. Bogusz’s Plea Bargain
Finally, Bogusz argues that the government violated the terms of his plea bargain by arguing for affirmance of the district court’s sentence in this appeal.20
[94]*94In the plea bargain agreement between Bogusz and the government, the parties stipulated that the base offense level would be 24 under U.S.S.G. § 2Dl.ll(d)(3). The probation office, in Bogusz’s PSR, considered and rejected application of section 2Dl.ll(d)(3) and instead recommended application of section 2Dl.ll(e)(l) which, by cross-reference, requires the application of section 2D1.1. The district court’s decision to accept the PSR recommendation raised Bogusz’s base offense level to 34. Bogusz does not appeal the district court’s refusal to honor the stipulation, however. See United States v. Torres, 926 F.2d 321 (3d Cir.1991) (allowing the sentencing judge to consider evidence outside the stipulation but requiring an opportunity for plea withdrawal); United States v. Wagner, 994 F.2d 1467, 1475 (10th Cir.1993) (“It is well settled the terms of a plea agreement are not binding on the sentencing court.”).
In Santobello v. New York, 404 U.S. 257, 92 S.Ct. 495, 30 L.Ed.2d 427 (1971), the Supreme Court held that plea bargains are governed by the law of contracts and, therefore, the parties’ must strictly adhere to then-promises. United States v. Badaracco, 954 F.2d 928, 939 (3d Cir.1992); United States v. Hayes, 946 F.2d 230, 233 (3d Cir.1991). Courts use a three-step analysis to review plea bargains: first, they determine the agreement’s terms and the conduct alleged to violate it; second, they determine if the conduct violated the plea agreement; and third, if the plea agreement is violated, they determine the remedy. Hayes, 946 F.2d at 233 (quoting United States v. Moscahlaidis, 868 F.2d 1357, 1360 (3d Cir.1989)). Here, the terms of the plea bargain are clear. Therefore, we focus on the second step. Determining whether the government’s conduct violated the plea agreement is a question of law over which we have plenary review. Id.
The stipulations attached to the plea bargain between Bogusz and the government state that “the applicable federal sentencing guideline is section 2D1.11(3) carrying a base offense level of 24.” The government argues that the district court was correct in sentencing Bogusz based on methamphetamine (actual) as opposed to methamphetamine. Brief of Appellee at 41-49. Bogusz claims that the government’s presentation of this argument violates the plea agreement.
Bogusz’s plea agreement did not explicitly address the post-conviction conduct of either party. See, e.g., United States v. Gonzalez, 981 F.2d 1037 (9th Cir.1992) (plea agreement with a no-appeal clause). Nevertheless, be-, cause “the government cannot resort to a rigidly literal approach in the construction of language,” we are not limited to the express language of the agreement. Badaracco, 954 F.2d at 939 (quoting United States v. Crusco, 536 F.2d 21 (3d Cir.1976)). Rather, “[i]n determining whether the terms of a plea agreement have been violated, the court must determine whether the government’s conduct is inconsistent with what was reasonably understood by the defendant when entering the plea of guilty.” Id. (quoting United States v. Nelson, 837 F.2d 1519, 1521-22 (11th Cir.), cert. denied sub nom. Waldhart v. United States, 488 U.S. 829, 109 S.Ct. 82, 102 L.Ed.2d 58 (1988)).
A reasonable person in Bogusz’s position may have understood the stipulation to include an agreement not to argue against the stipulation on appeal. Cf. Moore v. Foti, 546 F.2d 67, 68 (5th Cir.1977) (defendants “successful challenge to his plea bargained sentence is a tacit repudiation of the bargain”). Bogusz, however, did not appeal the district court’s refusal to follow the stipulation.
Moreover, the government does not directly argue against the stipulation; instead, it contends only that the district court’s definition of methamphetamine (actual) is correct. This issue is separate and distinct from the stipulation. Similarly, Bogusz has not breached his plea agreement by arguing directly for his own interpretation of methamphetamine after he had stipulated to an unrelated Guidelines’ provision. Our adversarial system of justice relies heavily on the presentation of opposing views by both parties, on appeal as well as at trial. Thus, we are reluctant to deny any party the right to advance its interpretation of law. Under the circumstances before us, we hold that the government did not violate the letter or the [95]*95spirit of the plea agreement by its arguments on this appeal.
IV. Conclusion
For the reasons discussed above, the sentences imposed on Bogusz and O’Rourke will be vacated and their cases remanded for resentencing in a manner consistent with this opinion. In all other respects, the orders of the district court are affirmed.