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 carbohydrate (a disaccharide). Air samples can contain complex particulate matter, such as wood and cigarette smokes, plant debris (cellulose), and products of biomass burning. In some cases, the biomass may contribute a sizable fraction of the sample carbon. About 38% of the particulate carbon in SRM 1649a (an urban dust) is derived from biomass (see Certificate of Analysis and related publication [J. Res. Natl. Inst. Stand. Technol., 107:279298, 2002] available at http://www.nist.gov/jres). For these reasons, a material more representative of the plant-derived components in air samples was sought. Various plant-derived materials were examined to determine whether any would be useful as laboratory control samples. A well-characterized, stable material that chars also would be useful for proficiency testing, and it could potentially be used to produce a standard reference material. After a preliminary screening of candidate materials (e.g., agarose, alginic acid, starch, and cellulose), alginic acid was selected for further study. Alginic acid is a polysaccharide derived from sea kelp [76], it has adequate solubility in water, and it forms a significant amount of char during the analysis. To prepare a solution, about 150 mg of alginic acid (Sigma, St. Louis, MO) was added to a test tube containing 10 mL of purified water (ultrafiltered, type I). The tube was shaken vigorously and allowed to sit overnight at room temperature. The following day, the tube was shaken again and then centrifuged to settle the suspended material. The clear supernatant was removed and syringe filtered. Aliquots of the alginic acid solution were applied to clean filter punches. The solution was analyzed over a seven-week period. Results of these analyses are plotted in Figure 4. No evidence of solution degradation was observed. A mean carbon loading of 13.98 :g /cm2 (RSD = 3.06%, n = 22) was obtained for 10-:L aliquots of the solution. This translates to a carbon concentration of 2.06 :g /:L. About half of the carbon in alginic acid remained on the filter as char before oxygen was introduced, but the mean EC result (0.04 ±0.06 :g /cm2 ) was not statistically different from that for the media blanks (0.06 ±0.03). Therefore, the pyrolysis correction was accurate. Sets of diesel soot samples spiked with the alginic acid solution also were examined. Multiple punches (1.5 cm2 ) were taken from a soot sample collected on an 8” x 10” filter. The punches were then analyzed before and after spiking with a 10-:L aliquot of the solution. Punches in a given set were taken from the same area of the filter to minimize variability, and the solution was applied in a consistent manner (dispersed evenly on one end of the punch where the laser penetrates it). Mean TC results for the unspiked and spiked samples are provided in Table 2. Results for three sample sets (A, B and C) are reported. For each set, the difference between the mean results for the unspiked and spiked samples is reported; this difference is the alginic acid carbon. Based on the mean (13.98 :g/cm2 ) for 22 spiked blanks, recoveries of alginic acid carbon from the soot samples were 96%, 101% and 110% (sets A, B, and C, respectively). The pooled recovery for all spiked samples was 97%.

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