Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods393mixed sample through a glass fiber filter and drying the filter to constantweight at 103–105 o C.The microbiological testing of water also uses particulate gravimetry.One example is the analysis for coliform bacteria in which an appropriatevolume of sample is passed through a sterilized 0.45-mm membrane filter.The filter is placed on a sterilized absorbent pad saturated with a culturingmedium and incubated for 22–24 hours at 35 ± 0.5 o C. Coliform bacteriaare identified by the presence of individual bacterial colonies that form duringthe incubation period (Figure 8.14). As with qualitative applications ofprecipitation gravimetry, the signal in this case is a visual observation ratherthan a measurement of mass.Total airborne particulates are determined using a high-volume airsampler equipped with either cellulose fiber or glass fiber filters. Samplesfrom urban environments require approximately 1 h of sampling time, butsamples from rural environments require substantially longer times.Grain size distributions for sediments and soils are used to determinethe amount of sand, silt, and clay in a sample. For example, a grain size of2 mm serves as the boundary between gravel and sand. The grain size forthe sand–silt and the silt–clay boundaries are 1/16 mm and 1/256 mm,respectively.Several standard quantitative analytical methods for agricultural productsare based on measuring the sample’s mass following a selective solventextraction. For example, the crude fat content in chocolate can be determinedby extracting with ether for 16 hours in a Soxhlet extractor. Afterthe extraction is complete, the ether is allowed to evaporate and the residueis weighed after drying at 100 o C. This analysis has also been accomplishedindirectly by weighing a sample before and after extracting with supercriticalCO 2 .Quartz crystal microbalances equipped with thin film polymer filmsor chemical coatings have found numerous quantitative applications inenvironmental analysis. Methods have been reported for the analysis of avariety of gaseous pollutants, including ammonia, hydrogen sulfide, ozone,sulfur dioxide, and mercury. Biochemical particulate gravimetric sensorsalso have been developed. For example, a piezoelectric immunosensor hasbeen developed that shows a high selectivity for human serum albumin, andis capable of detecting microgram quantities. 11Figure 8.14 Colonies of fecal coliformbacteria from a water supply.Source: Susan Boyer (www.ars.usda.gov).Qu a n t i t a t i ve Ca l c u l at i o n sThe result of a quantitative analysis by particulate gravimetry is just the ratio,using appropriate units, of the amount of analyte relative to the amountof sample.11 Muratsugu, M.; Ohta, F.; Miya, Y.; Hosokawa, T.; Kurosawa, S.; Kamo, N.; Ikeda, H. Anal.Chem. 1993, 65, 2933–2937.