Primarily on the basis of testimony by an undercover police officer, a jury found appellant guilty of unlawful distribution of cocaine (D.C.Code § 33-541(a)(l) (1988)). On appeal he contends that the trial judge erred in refusing to allow him to question the officer about the fact that, on this occasion and others, he earned overtime pay for testifying in court. Appellant further contends that the evidence that he distributed a usable amount of narcotics was insufficient as a matter of law. We reject both contentions and affirm.
I.
Bruce Faison, a member of the Narcotics Task Force of the Metropolitan Police, was operating undercover with his partner Officer Bush when they approached appellant in the Potomac Gardens area of Southeast Washington, D.C., at around 5:00 p.m. on August 31, 1990. Faison’s attention was drawn to appellant because the latter was standing near Building 714 in Potomac Gardens exclaiming “shake, shake, got that shake,” which Faison understood to be a street name for a powder form of cocaine. Faison asked appellant, “Who got that shake?”, and appellant replied, “Come on, Shorty, I will take you.” The two men walked toward the building where a third man, Willis Baum, joined them. Appellant told Baum what Faison wanted, and Baum asked, “How many?”, to which Faison replied, “Two dimes,” meaning two ten-dollar packets of white powder. As appellant stood beside the men, Baum gave Faison *903two plastic bags containing white powder from a bundle secured around his finger by a rubber band. Faison in turn gave Baum twenty dollars in pre-recorded police department money.
After the sale, Faison and Officer Bush, who had remained in a nearby doorway during the transaction, returned to their unmarked police vehicle, where Faison radioed descriptions of appellant and Baum to a waiting arrest team. He described appellant as a black male, tall, wearing a black baseball-style cap, a green short-sleeve shirt, blue jeans, and black tennis shoes.1 When appellant and Baum were stopped approximately four minutes later, Faison drove by and positively identified both men as the sellers. Identifying appellant in court as well, Faison was “very sure” of his recollection of the events, remembering the incident particularly because appellant had been advertising “shake” aloud.2 A search of Baum on the scene yielded the twenty dollars in pre-recorded funds used to buy the cocaine.
A forensic chemist from the Drug Enforcement Administration testified that the two plastic bags contained a total of 180 milligrams of powder, of which seventeen percent, or 30 milligrams, was cocaine. Detective Joseph Brenner testified that this was a usable amount of cocaine because it could be ingested into the body in the way powder cocaine normally is used, i.e., by snorting or injection.
The day before appellant’s trial began, codefendant Baum pleaded guilty to the charge of distributing cocaine on August 31, 1990. He took the stand on behalf of appellant and explained (in limited testimony) that he had had no arrangement with appellant to sell cocaine on the day in question, he had never met appellant before they were arrested, and appellant had come no closer to him than 40 to 50 feet before the arrest.3
II.
Appellant contends that the trial judge deprived him of his Sixth Amendment right to establish bias on the part of Officer Faison by precluding cross-examination about whether the officer received overtime pay for his testimony in court. When appellant’s counsel asked Faison, “[A]re you on overtime now?”, the government objected on grounds of relevancy, and a lengthy discussion ensued out of the presence of the jury. Ultimately the trial judge sustained the objection on the ground that the proposed questioning lacked probative value and could only distract and confuse the jury. We uphold the trial court’s ruling.
Appellant’s claim of bias was anything but straightforward. As he concedes on appeal, the financial incentive he sought to establish bore only indirectly on Officer Faison’s veracity at trial, for if — as defense counsel proffered — the officer was receiving time and a half pay for his appearance in court, he would earn that pay for all overtime work whether it involved testifying in court or not. Moreover, he would receive it whether or not his courtroom testimony incriminated appellant. What counsel sought to establish was that there was a self-interested, indeed a corrupt, link between the fact of appellant’s arrest and Faison’s testimony. He maintained that Faison was motivated from the beginning to arrest appellant and other persons who were either “marginally” involved in drug sales or not involved at all, in order to secure for himself time in court as a witness and overtime pay in consequence. Counsel proffered (and expressed confidence he could elicit from the government’s own police expert in the case) that police *904officers, particularly those from the Narcotics Task Force such as Faison, could earn “thousands of dollars a year” in overtime from their courtroom testimony. And this interest, counsel alleged, was linked directly to the potential innocence of defendants such as appellant, because among all persons arrested on drug charges, those most likely to put the government to its proof at trial were defendants believing in their innocence, whereas factually guilty defendants were more likely to enter guilty pleas to lesser drug charges before trial in order to avoid mandatory minimum sentencing.
This theory of bias, as the trial judge recognized, rested on a series of assumptions unsupported by any evidentiary proffer. The ultimate assumption, of course, was that an officer like Faison would jeopardize his career (and risk civil if not criminal liability) by falsely arresting innocent persons for the future gain of overtime pay. The underlying factual assumption was that most “guilty” drug defendants would plea bargain (hence offer little promise of remuneration for Faison) while defendants innocent in fact would commonly insist on their right to trial.4 Appellant proffered no evidence at all about the frequency of guilty pleas in drug cases, nor about the reasons (assuming these could be established empirically) why individual defendants go to trial — i.e., because they truly believe themselves innocent; or, innocence aside, because they are confident they can beat the government’s case, perhaps by a motion to suppress; or simply because they have not been offered a plea to a reduced charge because of recidivism or other reasons subsumed under prosecu-torial discretion. In particular, as the trial judge noted and defense counsel agreed, the government commonly “wired” plea offers in the case of jointly arrested defendants such as appellant and Baum, so much so that — in this case — until the very day before trial when Baum pled guilty to the charged offense, Officer Faison’s attendance at trial was not contingent on appellant’s decision to stand trial. Yet the theory of bias was that the officer had to cast his net broadly, arresting the guilty and innocent alike, to insure that at least one among codefendants would proceed to trial.
Aside from the lack of an evidentiary proffer, the flaw the trial judge perceived in appellant’s theory was that between the putative motive to arrest innocent or “marginal” defendants5 to earn extra courtroom pay and its ultimate realization lay numerous steps in the criminal justice process over which the officer had little or no control, and which in the aggregate made the imputed motive imaginary. Assuming, for example, that the officer could predict how prosecutors would exercise their charging and plea discretion in individual cases, there remained the grand jury as an obstacle before a putatively corrupt officer could have any confidence that innocently arrested defendants would be forced to stand trial.6 Without detailed explanation of how these factors operate in the process, a jury could not intelligently begin to evaluate the theory of bias. And even then, as the trial judge concluded, the result of the inquiry would be “so speculative and so interdependent upon other information that otherwise would have no place in the trial” that jury confusion would outweigh any possible utility on the issue of bias.
We find no reason to disturb this ruling. There is, of course, “no constitutional right to present irrelevant evidence.” Roundtree v. United States, 581 A.2d 315, 321 *905(D.C.1990) (quoting Gibson v. United States, 536 A.2d 78, 82 (D.C.1987)). “Although possible bias of a principal government witness is always a proper subject for cross-examination,” a proposed line of bias questioning must satisfy standards of relevancy: “[t]he party posing the question must proffer to the court some facts which support a genuine belief that the witness is biased in the manner asserted, that there is a specific personal bias on the part of the witness, and that the proposed questions are probative of bias.” Porter v. United States, 561 A.2d 994, 996 (D.C.1989) (citations omitted). Having received no foundational proffer other than that narcotics officers generally earn substantial overtime for court attendance, the trial judge did not err in finding Officer Faison’s pay status to be “[injadequately probative of the fact it [was injtended to establish,” i.e., his bias, and thus “[insufficiently relevant to be admissible.” Reavis v. United States, 395 A.2d 75, 78-79 (D.C.1978); Punch v. United States, 377 A.2d 1353, 1358 (D.C.1977), cert. denied, 435 U.S. 955, 98 S.Ct. 1586, 55 L.Ed.2d 806 (1978).
Appellant points out that in a seemingly analogous setting we held (or at least implied) that a proffer of facts suggesting the existence of a police policy linking the number of arrests to an officer’s job advancement might require allowing some inquiry into that possible motive for bias. Van Ness v. United States, 568 A.2d 1079, 1082 (D.C.1990); id. at 1083-89 (Schwelb, J., concurring in part and dissenting in part). But in Van Ness it was the existence of a “quota” of arrests without more that provided the asserted motive to arrest indiscriminately. And the bias theory was supported by a specific factual proffer. Id. at 1087. Appellant’s theory, in vivid contrast, depended on a web of assumptions about who among the broad class of arrestees for drug offenses would predictably go to trial and who would plead guilty — and it was supported only by a proffer that overtime for narcotics officers is commonplace. Both the speculativeness of these assumptions and the danger that the jury would ruminate on them despite any answers the officer might give to the questioning justified its exclusion. As was stated in Van Ness:
Once the question whether officers are promoted, in part, on the basis of the number of arrests they make has been posed, it may be difficult even for the most conscientious juror not to muse on the subject, no matter what the officer’s answer may be. Under these circumstances, trial judges should surely be reasonably exacting in requiring counsel to establish a good faith basis for pursuing such a line of inquiry.
Id. at 1087 (Schwelb, J., concurring in part and dissenting in part). While we need not hold that a sufficient predicate could never be laid for the proposed bias questioning here, appellant’s proffer fell far short of satisfying the threshold standard of relevance. Porter v. United States, supra.7
III.
Appellant also contends that the government failed to prove that the substance purchased by Officer Faison, although admittedly containing 30 milligrams of cocaine (one-sixth of the total substance), “could produce a narcotic [or pharmacological] effect upon the user” and so contained a usable amount of cocaine. The argument *906that the government must adduce testimony specifically about narcotic effect in a case such as this has been laid to rest by our recent decisions. In Judge v. United States, 599 A.2d 417 (D.C.1991), we explained:
Appellant argues, nonetheless, that proof of usability requires proof that the amount was sufficient “to have a pharmacological effect on the user,” citing Singley v. United States, 533 A.2d 245, 247 (D.C.1987). However, as we noted in Davis v. United States, [590 A.2d 1036 (D.C.1991) ], distinguishing Singley, that case involved a chemical analysis which stated only that the material analyzed contained a “small amount” of heroin. The expert had testified that in his opinion a powder containing only “trace amounts” would not be usable, and that he did not know whether in a report of chemical analysis, “small amounts” differed from “trace amounts.” In Singley, we cited Edelin v. United States, 227 A.2d 395, 399 (D.C.1967), which also involved traces and where we noted that there was “no additional proof” of its usability as a narcotic. Likewise, in Singley, we concluded that since “a trace amount is insufficient to convict whenever it cannot produce a narcotic effect in any form,” the bare record presented could not support a jury finding of usability. There is no warrant to read Sing-ley as imposing a requirement of proof of narcotic effect regardless of the quantity of the controlled substance and other proof of its usability. Davis v. United States, supra.
Id. at 420 (footnote omitted). In other decisions as well, we have narrowly construed Edelin’s requirement of proof of utility as a narcotic. Thus in Wishop v. United States, 531 A.2d 1005 (D.C.1987), we stated: “Edelin holds only that if the quantity of a drug is too small to be capable of quantitative analysis, there must be ‘additional proof of its usability as a narcotic’ in order to sustain a conviction.” Id. at 1008 (emphasis added) (quoting Edelin, 227 A.2d at 399). Most recently we said that Edelin “goes no further than to recognize that the common sense application of the narcotics laws does not reach a seized amount ‘so inconsiderable as to make it of no utility to a user and unmarketable....’” Gray v. United States, 600 A.2d 367, 369 (D.C.1991) (quoting Wishop, 531 A.2d at 1008). And in Wishop we expressly stated that “the fact that a drug is measurable — i.e., capable of quantitative analysis — will usually suffice to prove that it is usable.” 531 A.2d at 1008. See also Johnson v. United States, 611 A.2d 41, 43 (D.C.1992); D.C.Law 8-138 (1990) (amending D.C.Code § 33-516’s placement of cocaine in list of controlled substances).
The essence of our decisions, therefore, is that the government’s proof of usable amount will not fail unless “there is only a trace of a substance, a chemical constituent not quantitatively determined because of minuteness, and there is no additional proof of its usability as a narcot-ic_” Edelin, 227 A.2d at 399 (emphasis added). Here the testimony plainly established that the substance recovered contained more than a trace amount of cocaine. As the DEA chemist explained, the thin-layer chromatography test demonstrated “a fingerprint^] ... a pattern of cocaine”; “[c]ocaine [was] heavily suggested.” Even as to the residue of cocaine (20 milligrams) remaining after the test, the chemist noted that “you can see it ... you can weigh it also.” In his opinion, “[n]o other substance [than cocaine] would have yielded [the] results” obtained from the testing. Detective Brenner, also called as an expert, testified that 30 milligrams of cocaine was a usable amount because it could be ingested into the body in the way powder cocaine is normally used, i.e., by snorting or injection. Finally, the evidence showed that the cocaine purchased by Fai-son was packaged in two small plastic bag containers similar to those in which cocaine is usually sold on the street. All told, these facts were easily sufficient to satisfy the usable amount requirement.
The judgment of the Superior Court is
Affirmed.
. At trial Faison stated that the description included the fact of a “medium complexion,” but acknowledged that the tape of the radio broadcast, which was played to the jury, did not mention a medium complexion.
. Faison was impeached with the fact (among others) that in a contemporaneous report he had written that he had paid forty dollars for one ziplock bag rather than twenty dollars for two.
.Baum’s assertion of his Fifth Amendment privilege with respect to any further questioning was upheld by the trial court.
. Since Faison could never be certain a particular defendant would proceed to trial or whether he would testify in a given case while on overtime or regular duty status, the number of arrested "marginal” or innocent defendants would have to be sizeable under this theory.
. Appellant did not explain how far beyond innocent bystanders the notion of "marginal” defendants extended, whether it included, for example, aiders and abettors.
.Indeed, depending on when most guilty pleas by drug defendants are entered (about which appellant proffered no evidence), an honest police officer not willing to jeopardize his career might still expect to earn overtime pay for testimony before the grand jury and for investigative conferences with the prosecutor.
. It is suggested that, at a minimum, the trial judge was required to permit “a more limited cross-examination" regarding Faison’s financial interest to satisfy Sixth Amendment requirements. But appellant nowhere suggests what that more limited impeachment might be, and at trial the only “financial stake in the fact that cases go to trial" he proffered was the fact that Faison was "being paid overtime.” Nor was the trial judge required to admit an otherwise irrelevant line of questioning because the prosecutor argued in rebuttal closing — in response to appellant’s argument that "what we have here is Officer Faison versus [the testimony of] Baum," and that Baum had no motive to fabricate — that "Officer Faison was doing his job that day when he went into Potomac Gardens and was doing his job when he came in here and testified as to what he saw and what he did.” That Faison may have been receiving overtime pay was entirely consistent with his "doing his job” (either at the time of arrest or in court) absent a proffer of facts tending minimally to link the pay to an ulterior motive and unlawful conduct on his part.