Development and Evaluation of Methods

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Development and Evaluation of Methods,1 Method development. The development and evaluation of analytical methods that are useful reliable and accurate. for industrial hygiene monitoring problems require the application of some general guidelines. and evaluation criteria The guiding objective in this work requires that over a specified. concentration range the method provide a result that differs no more than 25 from the. true value 95 times out of 100 The application of consistent evaluation criteria and guidelines. is particularly important when methods are developed by different individuals and. organizations e g contractors or outside laboratories and compiled into a single manual. Adherence to guidelines should minimize overlooking potential problems in the methodology. during its development as well as provide cohesiveness and uniformity to the method that is. developed This chapter provides an outline of a generalized set of evaluation criteria prepared. by NIOSH researchers for the evaluation of sampling and analytical methodology NIOSH. In the development of a sampling and analytical method there is a logical progression of. events that cover a search of the literature to gather pertinent information and the preliminary. experimentation for selection of analysis technique and sampling medium To initiate the. development of a method the identity of the analyte must be as fully defined as possible. Physical and chemical properties of the analyte should be defined so that procedures for. proper handling and use of the analyte can be prepared These also aid in establishment of. analyte purity Potential sources of this information include chemical reference books health. hazard evaluation reports bulk sample analyses material safety data sheets chemical process. information etc, Since innovation is a key element in the sampling and analytical method development. process detailed experiments for the initial development of the sampling approach and. optimization of the analytical procedure are better left to the discretion of the researcher. During development it should be recognized that appropriate statistically designed. experiments will optimize the amount of information obtained Therefore consultation with a. statistician about appropriately designed experiments will be of value during this phase of the. a Preliminary experimentation, Several key points including calibration and selection of measurement technique and. sampling media should be studied during the initial method development experiments. The selection of sampling medium and procedure is a decision that usually is made early. in the method development process The physical state of the analyte i e gas aerosol. vapor or combination thereof plays an important factor in the selection of an appropriate. sampler Analytes which can exist in more than one physical state may require a. NIOSH Manual of Analytical Methods 5th Edition Chapter ME April 2016 Page ME 2 of ME 19. Development and Evaluation of Methods, combination of sampling media in one sampler for efficient collection NIOSH 1995. Where possible commonly available and easily used samplers should be investigated. initially As the preliminary testing of a sampling method progresses further modification. in the sampling medium or sampler design may be required and may affect the. measurement procedure Sampler design and media selection considerations should. include U S Department of Transportation regulations and restrictions for shipment back. to a laboratory for analysis, Since industrial hygiene analytical methods are geared toward measuring personal.
exposure the size weight and convenience of the sampler are important elements in. sampler design The personal sampler should allow freedom of movement and should be. unobtrusive unbreakable and not prone to leakage The pressure drop across the sampler. should not be so great as to limit sample collection times to 10 h with personal sampling. pumps For situations where only a short term sample will be required i e 15 min for. ceiling determinations this 10 h recommendations can be reduced to 1 h The use of. potentially toxic reagents should be avoided unless they can be used safely Reagents used. should not pose any exposure hazard to the worker wearing the sampler or to the. industrial hygienist taking the samples,b Recovery of the analyte from the medium. During the course of method development experiments the ability to recover the analyte. from the sampling medium should be determined A suggested experiment to accomplish. this entails the fortification of sets of 6 samplers with amounts of analyte equivalent to. sampling concentrations of 0 1 0 5 1 0 and 2 0 or higher times the exposure limit for a. minimum of 4 h at the typical sampling rate used for that type of sampler If the analyte. has a ceiling or short term exposure limit the amount of analyte fortified should be. adjusted for the shorter sampling time required for this type of exposure limit If the. sampler has a backup section then a like number of separate backup sections should be. fortified with amounts of analyte equivalent to 25 of the amount fortified on the front. sections of the samplers since this amount has been used to characterize the breakthrough. limit of useful samples Streicher et al 1994 Samples and backup sections should be. prepared for analysis and analyzed according to previously determined procedures. Results of these analyses should be expressed in terms of estimated percent recovery. according to the following formula,Percent Recovery est 100. After initial analyses of the samples the samples should be resealed and analyzed on the. following day if possible If the sample workup procedure results in a solution of the. NIOSH Manual of Analytical Methods 5th Edition Chapter ME April 2016 Page ME 3 of ME 19. Development and Evaluation of Methods, sample these solutions should be recapped after the initial analysis if possible and. reanalyzed on the following day using fresh standards. The recovery of the analyte should be calculated for the primary and backup media in the. sampler Although complete recovery of the analyte from the sampler is most desirable at. a minimum the estimated recovery of the analyte from the primary collection medium. should be greater than or equal to 75 for concentrations equivalent to sampling 0 1 0 5. 1 0 and 2 0 times the exposure limit If recovery varies with analyte loading results should. be graphed as recovery versus loading during calibration of the method so that. appropriate correction can be made to sample results as long as recovery is greater than. 75 Melcher et al 1978 If estimated recovery does not exceed 75 the method is not. suitable for monitoring at this limit, Estimated recovery from any backup media should be noted so that appropriate. corrections can be applied if breakthrough of the sampler has occurred during sampling. The recovery of the analyte from the medium in the backup section of a sampler may be. different from that of the front section since the backup section of a sorbent based. sampler usually contains only half of the sorbent of the primary section If the same. volume of desorption solvent is used for both the primary and backup sections of the. sampler the desorption equilibrium can be shifted since the backup section is being. desorbed by twice the volume i e on a mL solvent mg sorbent basis Saalwaechter et al. Reanalysis of the samples on the day after initial analysis indicates if immediate analysis. after sample preparation is required Often when processing a large number of samples it. may be necessary to prepare the samples for analysis as a batch In these instances the last. samples may not be analyzed for up to 24 h or more after preparation because of the time. required for analysis If samples prepared for analysis exhibit time dependent stability. after desorption analyses must be conducted within acceptable time constraints Analysis. and reanalysis results should agree within 5 of each other. c Stability of the analyte on the medium, An extension to the experiment described above may be performed to investigate potential.
stability problems early in the experimentation An additional set of fortified samples at. each of the 4 concentrations should be prepared and analyzed after 7 days storage at room. temperature Recovery should be similar to the above results within experimental error. Discrepancies larger than those expected by experimental error indicate sample stability. problems that will need correcting by additional developmental effort e g refrigerated. storage Comparison of results can be performed with statistical tests such as an analysis. NIOSH Manual of Analytical Methods 5th Edition Chapter ME April 2016 Page ME 4 of ME 19. Development and Evaluation of Methods, of variance ANOVA Posner and Okenfuss 1981 test of the Day difference or a paired. t test Box et al 1978 of the means of the Day 1 and Day 7 storage results. 2 Method evaluation, After the initial development experiments for the method have been completed and a method. has been proposed the sampling and analysis approach should be evaluated to ensure that the. data collected provides reliable precise and accurate results Specifically the goal of this. evaluation is to determine whether on the average over a concentration range of 0 1 to 2. times the exposure limit the method can provide a result that is within 25 of the true. concentration 95 of the time For simplification the true concentration is assumed to be. represented by an independent method An experimental approach for collecting the data. necessary for this determination is described below. As part of the evaluation of a method the sampling of a generated atmosphere is needed to. more adequately assess the performance of a method NIOSH 1984 Nelson 1971 Nelson. 1992 This allows the determination of 1 the capacity of the sampler 2 the efficiency of. analyte collection by the sampler 3 the repeatability of the method 4 the bias in the method. 5 interferences in the collection of the sample Concentration ranges to be used in the. evaluation of the method should be based on several factors These ranges at a minimum. should cover 0 1 to 2 0 times the exposure limit In some instances higher multiples of the. exposure limit can be added if needed e g 10 times the exposure limit In situations where. multiple exposure limits i e from different authorities exist for an analyte the lowest. exposure limit should be used to set the lower limit of the evaluation range 0 1 times lowest. exposure limit and the highest limit used to calculate the upper limit of evaluation range 2. times the highest exposure limit Intermediate evaluation concentrations should be within. these exposure limits The toxicity of an analyte e g suspected carcinogenicity may indicate. that a concentration lower than that calculated by the exposure limit should be included in the. measurement and evaluation ranges Previous monitoring information from other methods. may indicate that typical concentrations of the analyte may be below or above a concentration. range based on the exposure limit In this case this lower or upper level may be included in. the method evaluation,a Feasibility of analyte generation. In order to provide a realistic test of the method under study air concentrations covering. the range from 0 1 to 2 times the exposure limit of the analyte should be generated The. generated atmospheres should be homogeneous in concentration and representative of the. environment encountered when sampling for the analyte in the workplace. NIOSH Manual of Analytical Methods 5th Edition Chapter ME April 2016 Page ME 5 of ME 19. Development and Evaluation of Methods, When attempting to generate a concentration of an analyte the impact of environmental. conditions such as temperature pressure humidity and interferences on sampler. performance and or generation should be considered The effect of elevated temperature. on the collection medium of a sampler may decrease the capacity of the sampler or may. decompose the analyte during generation and sampling Reduced pressure may also. reduce the capacity of a sampler High relative humidity in many instances has been. observed to reduce sampler capacity Melcher et al 1978 In other instances it has. increased sampler capacity Cassinelli 1991 A typical interference s should be generated. along with the analyte to approximate a typical workplace sampling environment. Generation of particulate material can be extremely complex Willeke 1980 Hinds 1982. especially if particles of a required size range must be generated for the evaluation of a. specified sampler inlet design The aerodynamic performance of the generator is a factor. in the generation of this type of atmosphere and should be evaluated carefully. Appropriate independent methods should be available to verify particle size if this is a. critical element in the generation, The concentration of the generated atmosphere should be verified either by well.
characterized gravimetric volumetric means or by analysis of replicate samples if. possible by an independent method at each concentration used Further details on this. verification are included in the literature NIOSH 1995 Ashley 2015 A statistician should. be consulted for advice on the design and sample sizes to accomplish this validation. Ideally the independent method should not be biased and should provide an accurate. NIOSH Manual of Analytical Methods 5th Edition Chapter ME April 2016 Page ME 5 of ME 19 Development and Evaluation of Methods of variance ANOVA Posner and Okenfuss 1981 test of the Day difference or a paired t test Box et al 1978 of the means of the Day 1 and Day 7 storage results 2 Method evaluation After the initial development experiments for the method have been completed and

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