Richard Frank petitions for review of the July 23, 1993 decision and award of the arbitrator in Grievance No. NC-AEA-93-38POU-1 (Decision and Award). The arbitrator sustained the adverse action of the Department of Transportation (DOT), Federal *1555Aviation Administration (agency), removing petitioner from his position as Air Traffic Control Specialist for tampering, adulterating, or substituting a urine specimen during a random drug test. We affirm.
BACKGROUND
Petitioner was employed by the agency as an Air Traffic Control Specialist at the Dutchess County Tower in New York. On December 18, 1992, while he was working a 7:00 A.M. to 3:00 P.M. shift, he was informed that he had been randomly selected to be drug tested that day. Around noon, petitioner was brought to a collection site, which consisted of a “paperwork area” in a conference room and the men’s restroom across the hall. Petitioner was asked to produce a urine specimen. He was unable to produce a specimen at that time, however, and was rescheduled for later in the day. A couple of hours later, petitioner returned to the collection site. At that time, he was given a specimen kit and led to the restroom. Contrary to agency regulations, the faucets in the restroom had not been taped to prevent them from being turned on, and bluing agent had not been added to the toilets.1
Petitioner was in the restroom unsupervised for about ten minutes,2 after which time he emerged with a small amount of fluid in his specimen cup. One of the two specimen collectors on duty from Olsten Healthcare, Mr. Dominick Scalereio, obtained the sample and immediately observed that a temperature indicator on the specimen cup read 88 degrees Fahrenheit, the lowest reading capable of measurement. Samples must be within the range of 90.5 to 99.8 degrees Fahrenheit to be indicative of origin within the human body. A temperature outside this range suggests tampering. Mr. Scalereio immediately notified Ms. Sandra O’Connor, the collector in charge, and reported that petitioner’s specimen was out of range. Ms. O’Connor confirmed the 88 degree reading and immediately thereafter transferred petitioner’s specimen into a second collection cup to ensure the temperature indicator on the first was not faulty. The temperature indicator on the second cup failed to register altogether, indicating that the sample had a temperature of less than 88 degrees Fahrenheit. Then, Ms. O’Connor testified, she and Mr. Scalereio poured the specimen into a bottle and sealed it, whereupon the petitioner initialed the seal.
Ms. O’Connor then discussed the problem presented by petitioner’s sample with Mr. Anthony Capaldi, who was the facility manager in charge of the drug testing program and was at the collection site at the time. After cheeking some manuals in the paperwork area, Ms. O’Connor and Mr. Capaldi went to Mr. Capaldi’s office, which was just next door to the conference room, so that they could telephone and obtain guidance from DOT’s drug program coordinator, Dr. Mary Lewis, who works out of Kennedy Airport. When they went to Mr. Capaldi’s office, Ms. O’Connor and Mr. Capaldi left Mr. Scalereio and petitioner alone in the conference room with the sealed sample. After waiting a few minutes, petitioner left the conference room and started toward Mr. Ca-paldi’s office where Mr. Capaldi and Ms. O’Connor were still on the telephone with Dr. Lewis. Mr. Scalereio followed petitioner without taking the sealed specimen container. When Ms. O’Connor saw petitioner and Mr. Scalereio in the hall outside Mr. Capal-di’s office, she told them to return to the specimen, which they did.
In due course, Mr. Capaldi requested of petitioner that he remain at the collection site to provide another specimen. Further, because it was now about 3:00 P.M., Mr. Capaldi told petitioner that the agency would pay him overtime for the time needed for the retesting. Petitioner declined to be retested at that time, relating that he was needed at home to take care of his children. Then, after being told by Mr. Capaldi that he was free to leave, petitioner left to go home.
*1556Petitioner left without signing the required chain-of-custody form. A donor is required to sign a chain-of-custody form certifying that a sample is the donor’s and that information on the form regarding the sample is correct. Failure to do so is to be noted on the form by the collector. Ms. O’Connor, who was still on the telephone with Dr. Lewis, realized that petitioner had not signed the form, but by the time she did so, petitioner had already left the facility. Per Dr. Lewis’s instructions, Ms. O’Connor had Mr. Scalereio note on the form what had occurred.
The agency sent petitioner’s specimen to an outside testing laboratory, which determined that the sample was not urine, but water. The agency subsequently removed petitioner from his position for submitting an adulterated specimen. Following his removal, petitioner filed a grievance in accordance with the terms of the labor relations agreement between his union and the agency (Agreement). After a hearing and the submission of briefs, the arbitrator denied the grievance, thus sustaining the agency’s removal action.
OPINION
I. Standard of Review
Under Article 10, Section 2 of the Agreement, a removal by the agency must be supported “by a preponderance of the evidence.” This is the same standard of proof that is required in an appeal of an adverse personnel action before the Merit Systems Protection Board (Board). 5 U.S.C. § 7701(c) (1988).
We review the arbitrator’s decision under the same narrow standard that applies to appeals from the Board. 5 U.S.C. § 7121(f) (1988); Cornelius v. Nutt, 472 U.S. 648, 652, 105 S.Ct. 2882, 2885, 86 L.Ed.2d 515 (1985); Dixon v. Department of Transp., 8 F.3d 798, 803 (Fed.Cir.1993). Thus, we affirm the arbitrator’s decision unless it is (1) arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law; (2) obtained without procedures required by law, rule, or regulation having been followed; or (3) unsupported by substantial evidence. 5 U.S.C. § 7703(e) (1988); Dixon, 8 F.3d at 803. Substantial evidence “means such relevant evidence as a reasonable mind might accept as adequate to support a conclusion.” Consolidated Edison Co. v. National Labor Relations Bd., 305 U.S. 197, 229, 59 S.Ct. 206, 217, 83 L.Ed. 126 (1938).
II. Analysis
A.
Petitioner’s first challenge to the arbitrator’s decision focuses on the agency’s failure to adhere to DOT Order 3910.1B, which requires keeping the specimen and chain of custody forms under the control of the collector. Petitioner primarily relies upon the fact that his specimen was left unattended in the conference room for a short period of time, in direct violation of DOT procedures.3 The arbitrator concluded that this violation did not harm or prejudice petitioner because there was no evidence that any other person was in the conference room or had access to the room during the short period of time that the specimen was left unattended. Decision and Award, at 11-12. Petitioner argues that the arbitrator erroneously required that petitioner prove that someone else tampered with the specimen while it lay unattended. Petitioner further argues that the agency should have been required to prove perfect compliance with its procedures because any non-compliance is per se harmful and prejudicial error that is sufficient to invalidate a drug test. Essentially, we understand petitioner to be arguing that the failure of the agency to follow certain of its drug testing procedures should be deemed to have undermined the chain of custody over the specimen, thus rendering *1557the evidence supporting the arbitrator’s decision insubstantial. We disagree.
By no means do we wish to trivialize the importance of the chain of custody over specimens, as well as the procedures necessary to ensure the chain is not broken. We are not prepared to say, however, that a violation of procedures automatically and fatally undermines the chain of custody. Each case must be considered on its own merits. The chain of custody must be strong enough so that, on the record as a whole, the decision of the arbitrator can be found to be supported by substantial evidence. Dixon, 8 F.3d at 804. Moreover, where there is procedural error on the part of the agency, the error does not require that the agency decision be overturned unless the error is shown to have been harmful. 5 U.S.C. § 7701(c)(2)(A) (1988).4 Thus, we do not think that the adoption of the per se standard urged by petitioner is warranted.
With regard to the chain of custody in this case, the arbitrator found that the violation of DOT chain-of-eustody procedures did not harm or prejudice petitioner because there was no evidence that any other person had access to the specimen during the short time it was left unattended. Decision and Award, at 12. In other words, there was no evidence that someone other than petitioner had tampered with the specimen while it lay unattended. Substantial evidence supports this finding. First, there is substantial evidence that the specimen container was sealed before it was left unattended. In this regard, Ms. O’Connor testified that it is the policy of Olsten Healthcare that after visual and temperature observations of the specimen in the collection cup are made, the specimen is immediately poured into a specimen container which is then sealed. She also testified that this is what was done with petitioner’s specimen.5 Later in her testimony, Ms. O’Connor made a statement that suggested the specimen was not sealed before it was left unattended. This statement was made while Ms. O’Connor was testifying about receiving instructions from DOT’s drug program coordinator, Dr. Lewis, and about realizing that petitioner had left the facility without signing the custody-and-control form. Ms. O’Connor testified that she:
followed her [Dr. Lewis’s] instructions from there which was to take the urine sample with Mr. Capaldi—with Mr. Scaler-cio into the conference room, into the paperwork area, note on the custody form what had occurred, seal the bottle. I mean the bottle’s already sealed. Put the bottle into the plastic bag that you close and then you put that in with the drug custody form, the sections that should go to the lab. Put those in the box, seal the box.
We cannot say that her statement emphasized above was anything more than a misstatement, whieh she immediately and without any prompting corrected. As such, we do not think that this misstatement undermines her earlier testimony, where she explained in detail when and how the specimen was seated in the container.
The only other evidence in the record that contradicts Ms. O’Connor’s testimony that the specimen was in the container and seated before it was left unattended is the petitioner’s testimony before the arbitrator. Petitioner testified as follows:
A THE WITNESS [Petitioner]: .... At this point, the both of us [Mr. Scalereio and petitioner] went back there [the conference room where the specimen was unattended] and, at that point, I initiated the tag for the sample bottle.
Q BY MR. MORIN [Representative of petitioner’s union]: Where was the tag?
A Well, when I initialed the tag, it was on the form that I believe they peeled it off. Now, I don’t remember where the tag came from, to be honest with you.
Q But the tag was not on the bottle?
A No, not that I recall.
Thus, what we have is one person’s word (Ms. O’Connor’s) against another’s (petitioner’s). We note that on this particular point—whether the specimen was seated in a *1558container before it was left unattended — the arbitrator made no finding. Nevertheless, the arbitrator did find that there was no evidence that anyone else had access to the sample while it lay unattended. Decision and Award, at 12. This finding is supported by the substantial evidence that the sample was sealed before it lay unattended (Ms. O’Connor’s testimony), in combination with the absence of any evidence that there was any tampering with the seal before it arrived at the testing laboratory.
Moreover, the arbitrator’s finding that petitioner was not harmed or prejudiced by the chain-of-custody violation is further supported by evidence regarding the briefness of the time the sample lay unattended, as well as the shortness of the distance that Mr. Scalercio and petitioner strayed from the sample. Petitioner himself testified that he and Mr. Scalercio were only away from the sample for “about a minute.” Also, it is undisputed that Mr. Capaldi’s office, the place where Mr. Scalercio and petitioner strayed, is just next door to the conference room.6
Petitioner points to other errors in the collection process: (1) the fact that, contrary to DOT procedures, the restroom had not been secured against the possibility of adulteration before petitioner entered it the second time; (2) the fact that the collectors failed to note on the custody and control form certain “unusual behavior” of petitioner (that petitioner appeared nervous, in that when he gave his specimen cup to Mr. Scal-ercio, his hand was shaking), as they were required to do; and (3) the fact that the custody and control form was not signed by petitioner before he left the facility. Petitioner claims that the removing officer was unaware of these anomalies, and that if he had been aware of them, he would not have removed petitioner. We are not persuaded. Petitioner was removed because it was found that he had submitted a specimen that was not mine. Substantial evidence supports the finding that the specimen petitioner submitted was not urine. None of the asserted errors have any bearing on this finding; thus, petitioner has failed to show that he was prejudiced or harmed by the asserted errors.7
Accordingly, petitioner’s complaints regarding chain of custody and the collection procedures do not warrant reversal of the removal action.
B.
Petitioner next argues that we must overturn his removal because the agency violated his union representation rights. Federal employees enjoy a statutory right to union representation, upon request, at investigatory interviews with agency representatives whenever the employee reasonably believes disciplinary action may result. 5 U.S.C. *1559§ 7114(a)(2)(B) (1988); see also National Labor Relations Bd. v. J. Weingarten, Inc., 420 U.S. 251, 263, 95 S.Ct. 959, 966-67, 43 L.Ed.2d 171 (1975). Employees can bargain and modify their statutory rights to some extent through collective bargaining. See 5 U.S.C. § 7106(b)(2) (1988); Cornelius, 472 U.S. at 652, 659, 105 S.Ct. at 2885, 2889.
Petitioner argues that his union representation rights were violated in two respects. First, he contends that when the specimen collectors found problems with his specimen at the collection site on December 18, 1992, the agency was required at that time to advise him of his right to union representation.8 Petitioner has raised this argument for the first time, however, in his petition to this court. We do not address petitioner’s argument as we do not consider issues that were not raised in the proceedings below. Oshiver v. Office of Personnel Mgt., 896 F.2d 540, 542 (Fed.Cir.1990) (“Our precedent clearly establishes the impropriety of seeking a reversal of the board’s decision on the basis of assertions never presented to the presiding official or to the board.” (quoting Rockwell v. Department of Transp., 789 F.2d 908, 913 (Fed.Cir.1986))); Allred v. Department of Health and Human Servs., 786 F.2d 1128, 1130 (Fed.Cir.1986) (holding that the issue of notice of petitioner’s proposed suspension “has not been preserved and is not before this court” because petitioner “failed to show that that argument was presented to the presiding official”).9
Second, petitioner argues, as he did before the arbitrator, that his union representation rights were violated on January 15, 1993, when an agency Medical Review Officer (MRO) telephoned petitioner to question him about the problem with the specimen. It is undisputed that the MRO telephoned petitioner before he had been advised of his union representation rights.10 According to petitioner, it was only when the MRO began to question him about the events of December 18, 1992, that he realized the substance of the charges that could possibly be brought against him. At that point, thinking that he needed to speak with a union representative or his manager, petitioner asked the MRO if he could telephone her at a later time. The MRO agreed and the telephone conversation ended. Petitioner then telephoned his union representative. The arbitrator found that, even accepting petitioner’s version of the facts, petitioner was not harmed or prejudiced by not having union representation during his brief conversation with the MRO. Decision and Award, at 12 n. 5. This finding is supported by substantial evidence. Petitioner was removed for submitting an adulterated specimen. Nothing resulting from the telephone conversation was relied upon by the agency in taking the removal action.
*1560Petitioner also argues that the Agreement requires that any disciplinary action must be set aside if there has been a violation of the employee’s union representation rights. In other words, petitioner argues that the agency contracted away the statutory harmful error rule of 5 U.S.C. § 7701(c)(2)(A). Petitioner states that this mandate is in Article 6, Section 2 of the Agreement, which provides that “[n]o disciplinary action may result from any such meeting unless the provisions of this Article have been met.” We disagree with petitioner." We interpret Section 2 as providing that evidence (for example, an admission) obtained at a meeting where an employee’s union representation rights are violated cannot be used against the employee. In this case, the only relevant evidence relied upon by the agency (other than evidence that was favorable to petitioner) was the adulterated specimen.
Accordingly, petitioner’s arguments regarding union representation also do not warrant reversal of the arbitrator’s Decision and Award.
CONCLUSION
Because we conclude that the arbitrator’s Decision and Award is supported by substantial evidence, and because petitioner has either waived his union representation claim or failed to establish harm because of a lack of union representation, the Decision and Award is affirmed.
COSTS
Each side shall bear its own costs.
AFFIRMED.
. The faucets had been taped and the toilets had bluing agent in them during petitioner’s earlier attempt to provide a specimen.
. The government tells us that petitioner was unaccompanied in the restroom so as to respect his privacy. If the restroom in the collection site contains individual stalls, a collector must accompany the donor into the restroom and wait outside the stall. However, where, as in this case, the restroom has no stalls, the collector waits outside the restroom.
. The DOT’S directive regarding a drug-free departmental workplace provides that "[t]he collector and the individual providing the specimen shall always have the specimen within sight prior to being sealed and labeled.” DOT Order 3910.-1B, Chapter III—8—a(4) (Oct. 23, 1991). In addition, DOT's Drug Testing Guide provides that “[b]oth the individual being tested and the collector should keep the specimen in view at all times until it has been packaged and sealed for shipment." DOT Drug Testing Guide, Chapter II-B-1-m (Nov. 30, 1991). As noted above, Ms. O'Connor testified that she observed Mr. Scaler-cio pour the specimen into a bottle and seal it in the presence of petitioner, who initialed it. Thus, there does not appear to be an issue of compliance with DOT Order 3910.IB.
. Section 7701(c)(2)(A) provides that "the agency's decision may not be sustained ... if the employee ... shows harmful error in the application of the agency's procedures in arriving at such decision.”
. Ms. O’Connor’s testimony was consistent with her January 11, 1993 report.
. Petitioner makes much of the fact that Mr. Scalercio testified that a “lot of people” were in the collection area at the time the specimen was left unattended. The record reflects, however, that the "lot of people" to which petitioner refers were Mr. Capaldi, Ms. O'Connor, Mr. Scalercio, petitioner, and possibly one other would-be donor. There is no suggestion that these people tampered with the specimen, or even had any opportunity or motive to do so during the very brief time that the specimen lay unattended.
. The problems with the chain of custody in the present case do not rise to the level of those in Dixon, where we held that the arbitrator's finding that Dixon submitted a sample of water to the on-site collector was not supported by substantial evidence. 8 F.3d at 806. In Dixon, the collector had taped the water faucets in the restroom sink and had poured blue dye into the toilet. Id. In addition, the on-site collector did not find any problem with the sample Dixon submitted to him. Dixon's temperature strip in-cheated that his sample was in the acceptable temperature range, and the on-site collector testified that the color of the sample "looked okay" to him. Id. At the same time, there was no indication that Dixon had with him the kind of apparatus that would have been required in order to make a specimen that was not urine have a temperature that was in the acceptable temperature range. Id. at 806. In the present case, however, the problem with petitioner's sample was discovered immediately upon petitioner giving his sample to Mr. Scalercio, one of the on-site collectors. Finally, in Dixon, unlike in the present case, there were two additional problems: (1) there was evidence, in the form of a wrinkled label, suggesting that the specimen container’s seal had been tampered with before the container was opened by testing laboratory personnel; and (2) the testing laboratory receiving clerk, who first identified the problem with the color of the sample, was not identified on the custody-and-control form, and thus could not provide testimony. Id. at 806-08.
. Article 6, Section 1 of the Agreement provides in pertinent part: "If during the course of a meeting it becomes apparent for the first time that discipline or potential discipline could arise, the Employer shall stop the meeting and inform the employee of his/her right to representation if he/she so desires....”
. Petitioner asserts that the alleged violation was pointed out by the union representative, Mr. Byrnes, during his testimony before the arbitrator. That does not appear to be the case, however. Mr. Byrnes did not identify the alleged violation in his testimony. Further, the post-hearing statement of the union in support of petitioner does not identify the alleged violation either. This explains why the arbitrator did not mention the alleged violation in his opinion.
We also do not think that petitioner raised the issue through a statement in his February 24, 1993 written response to the agency’s Notice of Proposed Removal. In that statement, petitioner said that, if he had known or been advised that he was being suspected of tampering, he would have made arrangements so that he could provide another sample. In the statement, however, petitioner did not present his argument that the agency’s failure to so advise was a violation of the Agreement. In any event, we are not prepared to hold that an argument made in the response to a notice of proposed removal is, without more, deemed raised in a subsequent arbitration proceeding relating to the removal, merely because the response is part of the record before the arbitrator.
.Article 6, Section 1 of the Agreement provides in pertinent part: "When it is known in advance that the subject of a meeting is to discuss or investigate a disciplinary, or potential disciplinary situation, the employee shall be so notified in advance. The employee shall also be notified of his/her right to be accompanied by a Union representative if he/she so desires, and shall be given a reasonable opportunity to obtain such representation before the beginning of the meeting, if requested."