NIOSH Manual of Analytical Methods

“ 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. ”

“ Laboratory-based methods include atomic spectrophotometric methods following hot plate acid digestion: NIOSH methods 7082 (flame atomic absorption spectrophotometry) [2] , 7105 (graphite furnace atomic absorption spectrophotometry) [3] , and 7300 (inductively coupled plasma atomic emission spectrophotometry) [4] . A field-portable analytical method for lead air filter samples using ultrasound/ASV has been developed, NIOSH Method 7701 [5] . A field-portable screening method by spot test kit has been developed, NIOSH Method 7700 [6] . ”

“ However, any compound with a similar retention time as the analytes of interest may interfere. OTHER METHODS: OSHA method 99 [1] and OSHA method 101 [2] can also be used for sampling glycol ethers at or above the REL and PEL. NIOSH Method S69, from the 2nd edition of the NIOSH Manual of Analytical Methods, was used previously for the collection and analysis of dipropylene glycol monomethyl ether and had acceptable recovery at a concentrations at or above the REL/PEL [4] . ”

“ Sample stability was not determined. The method does not meet the 10% bias criterion for a valid NIOSH method. Breakthrough of the front section of the coated XAD-2 tube was not observed after sampling 53 L of dry test atmosphere containing 0.83 mg/m 3 for 240 min at 0.22 L/min. Desorption efficiencies for samples spiked with methyl octanoate were 1.01 to 1.05. The collection efficiency and reaction of diazomethane with the octanoic acid-coated resin may be strongly dependent on sample flow rates; therefore, all samples must be collected at a flow rate of 0.2 L/min only. ”

“ In some instances, generation of an analyte may be difficult and even hazardous. As an alternative to direct generation in these cases, samplers may be fortified with an amount of analyte expected to be sampled over a specified period of time at a specific flow rate. When this is necessary, fortification of the sampler by vaporization of a known amount of analyte onto the sampling medium is a more appropriate method, since this approach more closely approximates a generated atmosphere. ”

“ QUALITY ASSURANCE IN MEASUREMENT
Certain quality control checks should be performed with each sample set [2] [4] to further support the reported results on actual field samples. The exact number and nature of these checks depend on the specific method and circumstances under consideration and should be thought of as an integral part of the method itself (i.e., a measurement should not be considered completed without the quality control checks also being completed) . Each analyst must take an independent responsibility for assuring that the analytical quality control system works. ”

“ An accredited analytical laboratory should be used to conduct analysis of collected samples, and it is essential to consult with the analytical laboratory before sampling to ensure that the measurement methods available can meet the defined sampling needs. ”

“ The addition of bulk samples can often make the difference between a successful or unsuccessful sampling effort. This is especially true where there is mixed solvent exposure or unknown dust exposure and for determining silica content of dusts. The primary purpose of bulk samples is to provide the analytical laboratory with a large enough sample for qualitative and sometimes quantitative analysis. ”

“ If cylinder standards are not available for a compound, the analyst may prepare reference gases based on gravimetric, barometric, or dynamic methods. In any case, the related mass, pressure, and flow measurements should be made with NIST-traceable equipment whenever possible. In general, methods which provide a stream of standard gas (dynamic methods) provide more reliable results than static methods, since the concentrations of static gases are more prone to change when reactions between the standards and the sampling system or absorption cell surface can occur. ”

“ Because calculation of the absorbance area involves many points in the sample spectrum, Equation C3 leads to much more accurate results than the single-point calculation represented by Equation C2.
However, when many absorbing compounds are present in a sample, the absorption patterns of the various compounds often overlap. In this case, there is usually not an isolated analytical region for each compound in which only that compound absorbs infrared radiation; no single absorbance point and no simple absorbance area is suitable for determining any of the component concentrations. ”

“ Discard the gloves. 4. Clean the template in preparation for the next wipe sample . 5. Include two blank pads (moistened and placed in bags) with each sample set. Use the procedure of NIOSH Method 7105, including final sample dilution to 10 mL. NOTE: Additional portions of nitric acid may be needed for complete digestion of the sample, including the pad. Include appropriate media and reagent blanks.
MEASUREMENT: Screening of all samples by flame AAS or ICP, followed by graphite furnace AAS for those samples giving "Not Detected" is an efficient scheme. ”

“ 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 combination of sampling media in one sampler for efficient collection [1] . 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. ”

“ The 5% breakthrough volume for sampling a 16-ppm atmosphere of EtO at 0.15 L/min was 39 L. No significant storage effects were observed for samples in the 0.1 to 16 ppm range at high humidity and stored at ambient temperature for a minimum of two weeks. The precision, Sr, of chromatographic response of working standards in the range 18 to 71 μg EtO per sample was 0.028 [1] . Recovery of EtO spikes in NIOSH laboratories averaged 75% at 11, 22, 33, and 44 μg EtO per sample. ”

“ The difference between the "found" and "true" concentrations may not represent a bias in the sampling and analytical method, but rather a random variation from the experimentally determined "true" concentration. Therefore, the method has no uncorrected bias. Storage stability studies on samples collected in a test atmosphere at a concentration of 5.80 mg/m 3 indicated that collected samples are stable for at least 7 days at 8 °C. Collection efficiency for the midget bubbler was determined to be 0.96 for an average of 24 samples and a correction must be applied. ”

“ Mount the filter (sample, standard, or blank) in the XRD instrument and perform the following: a. Determine net intensity, I or , of the reference specimen before filter is scanned. Select a convenient scale factor, N, which is approximately equivalent to the net count for the reference specimen peak; use this for all analyses. b. Step-scan the most intense, interference-free diffraction peak of each compound to be determined, integrating the counts. NOTE: Useful analytical lines for the analytes are given in Table 1. Use strongest line of the analyte which does not have a matrix interference. ”

“ 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. ”

“ Flow rates typical for the media selected should be used. These may range from 0.01 - 4 L/min, depending on sampler type. At extremely low flow rates (ca. 5 mL/min) , the effect of diffusion of the analyte into the sampler must be considered. Flow rates should be kept at a high enough rate to prevent diffusion from having a positive bias in the sampler. Sampling should be performed at three different flow rates covering the range appropriate for the particular sampler type, unless the sampler is designed to operate at only one flow rate. ”

“ This method is based on the sampling procedure of Method S347 [2] , the automated analytical procedure of EPA Method 350.1 [3] , and Standard Methods 417G [4] . NIOSH Method 6701 [5] was less sensitive, employing a passive li quid sorbent badge for collection followed by ion chromatography. NIOSH Method P&CAM 205 [6] used impinger collection and Nessler's reagent for manual colorimetric analysis. OSHA has both impinger collection with ion specific electrode analys is (ID-164) [7] and sulfuric acid-impregnated beaded carbon collection followed by ion chromatography analysis (ID-188) [8] . ”

“ For gases, the measured absorbance of a single component is often completely independent of the concentrations of other gases comprising the sample, and single-component reference spectra are usually employed. (For condensed phases, there are often strong interactions between components, and reference spectral libraries of mixtures are usually required.) Reference libraries may be used for quantitative measurements of analyte concentrations, for the mathematical removal of spectral features of interferants in a mixture, or simply for the identification of compounds in a mixture. ”

“ 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. ”

“ Also the precision and bias of the independent method should be homogeneous over the concentrations investigated. (See Reference 1 for the definitions of these attributes.) In instances where the concentration of the generator can be based only on calculations using flow rates in the generator and the amount of analyte injected, the generation system should be well characterized [1] so that analyte losses are minimized. ”

“ Care must be exercised in the selection of internal standards for each analyte because of differences in derivatization efficiencies due to structural differences. a. Deuterated analogs of each target analyte may be acceptable as internal standards if they are isotopically pure enough and their ions do not interfere with the quantification ions (usually base peaks) of the target analyte, especially at the limit of detection for the target analyte. ”

“ The exact sampling time must be known in order to accurately estimate the sampled volume. Recording only the start and stop time assumes that the pump functions properly over the entire sampling period. Occasional spot checks for proper operation should be made throughout the sampling period.
Since many modern analytical techniques are extremely sensitive, special care must be taken to avoid contamination of field samples. [8] Samples must not be stored or shipped with bulk materials which might spill or otherwise present the possibility for contamination. ”

“ If a signal is obtained at the approximate retention time of the latter two compounds, or if both compounds are known to be present in the sample, perform a second analysis using Eluent B (lower ionic strength) . If either of the two compounds is known not to be present, Eluent A will effectively determine the remaining compounds. With Eluent B the other species have very long retention times and will accumulate on the column, tying up active resin sites. Therefore, flush the column with Eluent A after each 10 to 15 samples and reequilibrate with Eluent B before further analysis. ”

“ Every compound-specific quantitative analytical technique, including FTIR spectrometry, can fail to provide accurate results when interfering compounds appear in a sample. However, the results of a mathematical FTIR spectral analysis designed for a particular set of compounds provides clues to its failure in the presence of interferants. An experienced analyst can often adjust the analysis to accommodate the interferants and provide accurate results. ”

“ The working range of this method is up to 100 mg/m3 for each element in a 500-L sample (the minimum range depends on the LOD for each sample; see Table 1) . The analysis is not compound specific. Certain elemental compounds are known to be acceptable or unacceptable by this method (see Table 3) . For unverified compounds, a test run should be conducted using a known amount of the compound in question to determine acceptability. ”