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USP 36
  Second Supplement to USP 36–NF 31 General Information / 〈 1251 〉  Weighing on an Analytical Balance6217 microorganisms that pass the 0.45- µ m filter are of interest ment, Validation, and Routine Control of a Sterilization because they present the greatest potential challenge to the Process for Medical Devices.  Arlington, VA: AAMI;sterilizing filtration process. They should be evaluated2006.against the upstream bioburden. Bioburden of greatest con-7.AAMI/ISO. 11737. Sterilization of Medical Devices—Mi- cern includes Pseudomonas  , Brevundimonas  , Ralstonia  , and crobiological Methods—Part 1: Estimation of PopulationMycoplasma  . of Microorganisms on Products.  Arlington, VA: AAMI;2006.  2S ( USP36 ) Identification Recovered microorganisms can be identified to an appro-priate level using the methods described in Microbial Identifi-cation, Characterization, and Strain Typing 〈 1113 〉 . Only thosemicroorganisms that present a potential risk to the productor the patient require identification to the species level or beyond. For a detailed discussion of the methods used to 〈 1251 〉  WEIGHING ON AN grow and identify microorganisms, see 〈 1113 〉 . It is not nec-essary for the purposes of evaluating presterilization bi-  ANALYTICAL BALANCE oburden to identify all isolates to the species level, althoughthis can be helpful in some investigations. Analysts mustconduct all evaluative work with pure cultures, and theymust apply normal microbiology laboratory procedures for the selection and maintenance of pure cultures. Change to read: Bioburden Control  INTRODUCTION  A typical bioburden-control program includes review andanalysis of potential sources of contamination as well as Weighing is a frequent step in analytical procedures, andsound process design and preventive and monitoring meas-the balance is an essential piece of laboratory equipment.ures. The microbiological contamination-control programThe general information described here applies directly toshould be developed to identify and control bioburden andelectronic balances used in analytical procedures. Althoughto assess product risk based on a formal assessment of riskmany portions of the chapter are applicable to all balances,modalities. The bioburden risk assessment should result insome are applicable only to analytical balances. This chapter the establishment of critical control points and should in-should not be considered all-inclusive, and other sources of clude consideration of the following elements:information (e.g., the US National Institute for Science andãMicrobiological attributes of materials before steriliza-Technology and balance manufacturers) may be useful andtion and the manufacturing process used for the materi-applicable when analysts perform a weighing operation or als (if applicable)implement a weighing procedure. 1,2  The information givenãInherent antimicrobial properties of the materialsin this chapter is applicable not only to balances used for ãTime limits for process executionmaterials that must be accurately weighed (see Balances  ãWater activity of the material 〈 41 〉 ) but also to balances used in all analytical procedures.ãEnvironmental conditions within the facilityãEquipment design and cleaningãSanitization, decontamination, and other active micro- QUALIFICATION bial control processes (such as prefiltration, tempera-ture, pH, osmolarity, etc.)Users should consult Analytical Instrument Qualification Controlling the bioburden of materials and products to be 〈 1058 〉 , standard operating procedures, and recommenda-sterilized will ensure conformance to the levels required bytions from manufacturers when they devise qualificationthe sterilization process validation. Additionally, controllingplans.the bioburden levels of the items to be sterilized assures thatresiduals (e.g., allergens, endotoxins, and exotoxins) fromthat population will also be controlled. This is important be- Installation cause direct detection of these materials is challenging.The balance’s performance depends on the conditions of the facility where it is installed. Analysts should consult infor- REFERENCES mation provided by the manufacturer before they install abalance.1.21 CFR  807, Subpart E. Premarket Notification Support surface: The balance should be installed on aProcedures.solid, level, nonmagnetic surface that minimizes the trans-2.FDA. Updated 510(k) sterility review guidance K90-1;mission of vibration (e.g., a floor-mounted, granite weigh final guidance for industry and FDA. 2002. http://bench). If a metallic support surface is used, the be grounded in order to prevent the buildup of uidance/GuidanceDocuments/ucm072783.htm.static electricity.3.Murray CJ. Sampling and data analysis for environ- Location: If possible, the balance should be located in amental microbiology. In: Hurst CJ, Knudsen GR, McIn-room that is temperature and humidity controlled. The loca-erney J, eds. Manual of Environmental Microbiology. tion should have a clean, consistent electrical power supply.2nd ed. Washington, DC: ASM Press; 2002:166–177.4.Pflug IJ. Microbiology and Engineering of Sterilization  1 National Physical Laboratory. Good practice guide No. 70, weighing in the Processes.  7th ed. Minneapolis, MN: Environmental pharmaceutical industry. 2004. Sterilization Laboratory; 1990. ing_panel/pharmaweigh.pdf. Accessed 21March 2012. [ N OTE — Nomenclature 5.Block, S. Disinfection, Sterilization & Preservation.  5th in this chapter tends to follow this document, except where it conflicts with ed. Philadelphia, PA: Lippincott, Williams and Wilkins; USP terms.] 2001.  2  American Society for Testing and Materials. E898 Standard test method of testing top-loading, direct-reading laboratory scales and balances. 2005. 6.ANSI/AAMI/ISO. 11137. Sterilization of Health Care Accessed 21March 2012. Products—Radiation—Part 1: Requirements for Develop- ffcl from December 1 2013oyrt c 2013 Te nted ttes rmcoel onenton All rts resered Accessed from 208 85 168 103 by mettler2 on Mon Jun 03 12:43:04 EDT 2013  6218  〈 1251 〉  Weighing on an Analytical Balance  / General Information Second Supplement to USP 36–NF 31 The location should be free of drafts and should not be neardiation, magnetic fields, or changes in temperature or hu-ovens, furnaces, air conditioner ducts, or cooling fans frommidity, the assessment should be conducted with those sys-equipment or computers. The balance should be positionedtems operating in order to duplicate a worst-case scenario.away from outside windows so that direct sunlight does notstrike the balance. The balance should not be installed near  Operational Qualification sources of electromagnetic radiation such as radio-frequencygenerators, electric motors, or hand-held communication An operational qualification should be performed either devices (including cordless telephones, cellular telephones,by the user or by a qualified third-party vendor after theand walkie-talkies). The balance should not be located near equipment has been installed.magnetic fields induced by laboratory instrumentation or  As a minimum, the power should be turned on and theother equipment.balance should be allowed to equilibrate according to theThe performance of the balance should be assessed fol-manufacturer’s instructions (1–24 h, depending on the typelowing installation and before use in order to demonstrateof balance) before use. Depending on the balance, analystsadequate performance. In some situations, it may not beshould include the following procedures in the operationalpossible to position the balance in an optimum environ-qualification:ment. Examples of potential facility issues include the1.Mechanical mobility of all moveable parts following:2.Control of stable indication1.Air currents sometimes are present in the laboratory.3.Manually triggered or automatic adjustment by2.Temperatures in the laboratory vary excessively (checkmeans of built-in weightsthe manufacturer’s literature about temperature4.Operation of ancillary equipmentsensitivity).5.Tare function3.Humidity is either very low or very high. Either condi-6.Initial calibrationtion may increase the rate at which the sampleSeveral types of electronic analytical balances use built-inweight varies because of pickup or loss of water. Lowweights for manually triggered or automatic adjustment.humidity increases the buildup of static electricity.This adjustment usually is applied to reduce the drift of the4.Adjacent operations are causing vibration.balance over time and to compensate for drifts caused by5.Corrosive materials are used nearby or are routinelyvariations in the ambient temperature.weighed.Calibration normally is performed as part of the opera-6.The balance is located within a fume hood because ittional qualification, but it also can be performed periodicallyis used to weigh corrosive or hazardous materials.thereafter. Calibration should be performed at the location7.The balance is adjacent to equipment that produces awhere the balance is used in normal operation.magnetic field (e.g., a magnetic stirrer).8.Direct sunlight strikes the balance.In situations when the balance is located near equipmentor systems that induce vibration, drafts, electromagnetic ra- Table 1. Suggested Performance Tests and Acceptance CriteriaPropertyDefinitionExamplesAcceptance Criteria Change in weighing value divided bythe change in load, usually measuredNMT 0.05% deviation where 〈 41 〉  isbetween zero and the capacity of theThe test load at or sufficiently close toapplicable. For other uses, respectiveSensitivitybalance.the capacity of the balance.tolerance requirement divided by 2. Ability of a balance to follow the linear relationship between a load and theindicated weighing value. Nonlinearityusually is expressed as the largestNMT 0.05% deviation where 〈 41 〉  ismagnitude of any linearity deviationFrom 3 to 6 points over the range ofapplicable. For other uses, respectiveLinearitywithin the test interval.the balance.tolerance requirement divided by 2.Deviation in the measurement valuecaused by eccentric loading—in other words, the asymmetrical placement ofPerformed in the center of gravity andthe center of gravity of the load rela-the four quadrants (for rectangular tive to the load receiver. Eccentricityplatter shapes) or at analogous loca-usually is expressed as the largesttions for other platter shapes. Testmagnitude of any of the deviationsload usually should be 30% of the ca-between an off-center reading andpacity of the balance or higher (referNMT 0.05% deviation where 〈 41 〉  isthe center reading for a given testto the manufacturer’s manual for anyapplicable. For other uses, respectiveEccentricityload.possible upper limit).tolerance requirement divided by 2. Ability of a weighing instrument to dis-play identical measurement values for repeated weighings of the same ob-jects under the same conditions, e.g.,the same measurement procedure,same operator, same measuring sys-tem, same operating conditions, andsame location over a short period of time. Repeatability usually is expressed10 replicate weighings (using a testRequirement from 〈 41 〉  where applica-as the standard deviation of multipleweight that is a few percent of theble. For other uses, user specified re-Repeatabilityweighings.nominal capacity of the balance).quirements will apply. ffcl from December 1 2013oyrt c 2013 Te nted ttes rmcoel onenton All rts resered Accessed from 208 85 168 103 by mettler2 on Mon Jun 03 12:43:04 EDT 2013  Second Supplement to USP 36–NF 31 General Information / 〈 1251 〉  Weighing on an Analytical Balance6219  Depending on the acceptance criterion, it may be suffi-cient to consider only the nominal weight value of the test Performance Qualification weights. If the nominal value of the test weight is consid-ered, analysts should ensure that the maximum permissible Table 1  provides a list of the most important balanceerror does not exceed one-third of the acceptance that should be assessed during performance qual- Alternatively, if the certified value of the test weight is con-ification. Depending on the risk of the application and thesidered, its calibration uncertainty should not exceed one-required weighing process tolerance, some of these teststhird of the acceptance criterion. If more than one weight ismay be omitted. Tests also can be omitted if there is evi-used to perform the test, the calibration uncertainties of thedence that the property in question has only minimal effectweights must be summed and the sum should not exceedon the weighing performance. Any procedures used shouldone-third of the acceptance criterion. For tests such as ec-be consistent with in-house standard operating procedures,centricity or repeatability, the use of certified weights is op-applicable for the specific balance, and adequately justified.tional, but analysts must ensure that the mass of the weightPerformance qualification should be performed periodicallydoes not change during the described in standard operating procedures, and the fre-The tests described above also can be included in formalquency of each of the individual tests can vary dependingperiodic calibration in order to fulfill applicable cGMPon the criticality of the property.requirements.The weights that are used to perform the tests should bestored and handled in a manner that minimizes contamina-tion. Before executing the tests, the analyst should place the Balance Checks weights in the vicinity of the balance for an appropriatetime to reach sufficient thermal equilibrium. If possible, all A balance check using an external weight helps ensuretests should be carried out with a single test weight in order that the balance meets weighing tolerance minimize handling errors, but multiple test weights areThe balance check is performed at appropriate intervalspermitted.based on applicable standard operating procedures. The fre-The tests should be recorded in such a manner that thequency of the balance check depends on the risk of thedata can be used to easily track balance performance and toapplication and the required weighing tolerance. Checksassist in laboratory investigations as needed. Meaningful ac-with external weights can be replaced partially using auto-ceptance criteria can be set depending on the requiredmatic or manually triggered adjustment by means of built-inweighing tolerance, i.e., the maximum allowed deviationweights. When analysts perform the balance check with anpermitted by specifications, regulations, etc., of a quantityexternal weight, the same acceptance criteria may apply asto be weighed from its target value. Procedures should bedescribed in the sensitivity test place to address test results that are outside acceptableranges and to provide assurance that balance cleanliness Minimum Weight and environment have not affected the result. Also, a proce-dure should be in place for removing a balance from opera-The minimum net sample weight, m min , of a balance cantion when observed results fall outside acceptable expressed by the equation:Sensitivity, linearity, and eccentricity all account for sys-tematic deviations; i.e., they limit the accuracy of the bal- m min  = k    ×   s  /required weighing toleranceance (based on the definition of accuracy in Validation of Compendial Procedures 〈 1225 〉  and ICH Q2). In the Interna-where k   is the coverage factor (usually 2 or larger) and s   istional Vocabulary of Metrology (VIM) and documents of thethe standard deviation (in a mass unit, e.g., in mg) of notInternational Organization for Standardization, this concept fewer than 10 replicate measurements of a test weight. Theis referred to as trueness. Because deviations are largely in-minimum weight describes the lower limit of the balancedependent from each other, it is not likely that all deviationsbelow which the required weighing tolerance is not adheredoccur simultaneously and have the same algebraic The equation above takes into account that the perfor-Therefore the arithmetic addition of all individual deviationsmance of laboratory balances at the lower end of the meas-to assess the balance accuracy would constitute a rather urement range is limited by the finite repeatability.conservative approach. A quadratic addition of the individ-For materials that must be accurately weighed, 〈 41 〉  stipu-ual deviations is a more realistic approach. By allocatinglates that repeatability is satisfactory if two times the stan-50% of the weighing tolerance budget to the acceptancedard deviation of the weighed value, divided by the nomi-criteria of the individual properties, e.g., sensitivity, linearity,nal value of the weight used, does not exceed 0.10%. For and eccentricity, analysts ensure adherence to the requiredthis criterion the equation above simplifies to:weighing tolerance. Therefore, the acceptance criteria for the individual properties that account for the systematic de- m min  = 2000 ×   s  viations are set to weighing tolerance divided by 2. Theseproperties—or a subset of them—also can be taken to fulfillIf not subject to the requirements of 〈 41 〉 , the minimumthe accuracy requirement described in 〈 41 〉 . In this case theweight value may vary depending on the required weighingacceptance criteria thus allow a maximum deviation of tolerance and the specific use of the balance.0.05% for sensitivity, linearity, and eccentricity. RepeatabilityTo facilitate handling, the test weight that is used for thepreferably is tested with a test weight of a few percent of repeatability test does not need to be at the minimumthe balance capacity. At the lower end of its measurementweight value but can be larger because the standard devia-range, the performance of laboratory balances is limited bytion of repeatability is only a weak function of the testthe finite repeatability, and limitations induced by system-weight value.atic deviations normally can be neglected. Therefore, theIn order to satisfy the required weighing tolerance, whenwhole weighing tolerance budget can be allocated to thesamples are weighed the amount of sample mass (i.e., theacceptance criterion of the repeatability weight) must be equal to or larger than the minimumFor the sensitivity and linearity tests as described above,weight. The minimum weight applies to the sample weight,the analyst should use certified weights with an appropriatenot to the tare or gross weight.weight class (e.g., according to International OrganizationFactors that can influence repeatability while the balanceof Legal Metrology R111 or American Society for Testingis in use include:and Materials E617, available from and www.1.The performance of the balance and thus the, respectively). [ N OTE — If a differential method ismum weight can vary over time because of changingused for the linearity test, certified weights may not beenvironmental conditions.required.] ffcl from December 1 2013oyrt c 2013 Te nted ttes rmcoel onenton All rts resered Accessed from 208 85 168 103 by mettler2 on Mon Jun 03 12:43:04 EDT 2013  6220  〈 1251 〉  Weighing on an Analytical Balance  / General Information Second Supplement to USP 36–NF 31 2.Different operators may weigh differently on the bal-with a small opening, and the enclosure should be replacedance—i.e., the minimum weight determined by dif-rapidly following material transfer. Special precautions ferent operators may be different.should be taken to be certain that the receiver and the en-3.The standard deviation of a finite number of replicateclosure are constructed from a material that is compatibleweighings is only an estimation of the true standardwith the liquid sample. The receiver and enclosure mustdeviation, which is unknown.have a seal that is sufficient to prevent leaks from a liquid4.The determination of the minimum weight with athat is of low viscosity or has low surface tension or a lowtest weight may not be completely representative forboiling point.the weighing application.5.The tare vessel also may influence minimum weight Types of Weighing because of the interaction of the environment withthe surface of the tare vessel. Weighing for quantitative analysis: The initial step for For these reasons, when possible, weighings should bemany quantitative analyses is to accurately weigh a specifiedmade at larger values than the minimum weight.amount of a sample. Section 6.50.20 in the General Notices  stipulates that solutions for quantitative measures must be OPERATION OF THE ANALYTICAL BALANCE prepared using accurately weighed analytes: i.e., analystsmust use a balance that meets the criteria in 〈 41 〉 . ErrorsSelect the appropriate balance for the quantity and per-introduced during the weighing of a sample can affect the formance needed. General chapter 〈 41 〉  provides require-accuracy of all subsequent analytical measurements.ments for balances used for materials that must be accu- Addition weighing:  Addition weighings typically arerately weighed.The balance user should check the balanceused for solid samples or liquid samples for which volatilityenvironment (vibration, air currents, and cleanliness) andis not an issue. The receiver is placed on the balance. After status of calibration before use.the balance display stabilizes, the analyst should tare thebalance; add the desired amount of material to the receiver;allow the balance display to stabilize; record the weight; Receivers and quantitatively transfer the material to an appropriatevessel or, if it cannot be guaranteed that the entire amountTo ensure suitable performance in measuring the weighthas been transferred, weigh the receiver again and note theof a specimen, analysts should consider selection of a proper weight difference.receiver for the material. Dispense weighing: Dispense weighing typically is General characteristics:  All receivers must be clean,used for weighing emulsions or viscous liquids such as oint-dry, and inert. The total weight of the receiver plus thements. In these situations it is not practical to weigh thespecimen must not exceed the maximum capacity of thematerial into a typical receiver. Accordingly, the analystbalance. With a properly maintained and adjusted labora-should tare the balance; place the sample on the balance intory balance, weighing uncertainty for small samples, i.e.,a suitable container (e.g., a bottle, tube, transfer pipet, or net weights with a mass not exceeding typically a few per-syringe) that has been wiped clean on the outside; recordcent of the capacity of the balance, essentially is determinedthe weight after the balance display stabilizes; transfer theby the repeatability. However, repeatability depends on thedesired amount of sample to an appropriate receiving ves-size and surface area of the weighed object. For this reasonsel, such as a volumetric flask; and place the pipet or sy-large or heavy receivers introduce a deviation from the con-ringe back onto the balance. The difference in the twoditions under which the repeatability was determined with-weighings is equal to the weight of the transferred speci-out considering the receivers. Therefore, either receivers of amen.low mass and small surface should be used (especially in Gravimetric dosing: Gravimetric dosing typically iscases when specimens of low weight are being measured)used for sample and standard preparations or capsule filling.or the repeatability test should be performed with the re-For such weighing the analyst places the volumetric flask,ceiver placed on the weighing pan as a preload. Receiversvial, or capsule shell on the balance; tares the balance after should be constructed from nonmagnetic materials in order the balance display stabilizes; adds the solid or liquid com-to prevent magnetic interference with electronic balanceponents into the receiver by means of dosing units; andcomponents. Receivers should be used at ambient tempera-records the respective weights.ture in order to prevent the formation of air currents withinthe weighing chamber. Solid samples: Receivers for weighing solid materials in- Problem Samples clude weighing paper, weighing dishes, weighing funnels,or enclosed vessels, including bottles, vials, and flasks. Hy- Electrically charged samples and receivers: Dry, finelygroscopic papers are not recommended for weighing be-divided powders may be charged with static electricity thatcause they may have a detrimental effect on the observedcan make the powder either attracted to or repelled by theresults.receiver or the balance, causing inaccurate weight measure- Weighing dishes typically are constructed from a polymer ments and specimen loss during transfer. A drift in the bal-or from aluminum. Antistatic weighing dishes are availableance readings should alert the operator to the possibility for measuring materials that retain static electricity. Weigh-that the material has a static charge. Commercially availableing funnels typically are constructed from glass or from abalances with a built-in antistatic device can be used topolymer. The design of this type of receiver combines attrib-remedy the problem. Such devices may use piezoelectricutes of a weighing dish and a transfer funnel, which cancomponents or a very small amount of a radioactive ele-simplify the analytical transfer of a weighed powder to ament (typically polonium) to generate a stream of ions thatnarrow-necked vessel such as a volumetric flask. For soliddissipate the static charge when passed over the powder samples that are volatile or deliquescent, analysts mustbeing weighed. Antistatic weigh boats, antistatic guns, andweigh the material into an enclosed vessel. Where practical,antistatic screens also are commercially available. The staticanalysts should use an enclosed vessel with a small openingcharge depends also on the relative humidity of the labora-in order to reduce sample weight loss from volatilization or tory, which in turn depends on atmospheric conditions.weight gain from the adsorption and absorption of atmos-Under certain conditions, static charge is caused by the typepheric water.of clothing worn by the operator and this charge can cause Liquid samples: Receivers for liquid samples typicallylarge errors in the weighing. Borosilicate glassware andare inert, enclosed vessels. For liquid samples that are vola-plastic receivers have a well-known propensity for pickingtile or deliquescent, analysts should use an enclosed vessel ffcl from December 1 2013oyrt c 2013 Te nted ttes rmcoel onenton All rts resered Accessed from 208 85 168 103 by mettler2 on Mon Jun 03 12:43:04 EDT 2013
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