Artifacts, filter sampling

The traceability and uncertainty of the coordinates as measured (see section Definitions ) must be taken into account, as well of uncertainties in the used probe size, filtering, sampling, reference line or surface, etc. For 3-D measurements it is a problem that the default measurement system is a mechanical probing system. As 3-D mechanical roughness measurements are very time-consuming, most measurements are taken using optical techniques that may have artifacts that are not easily recognized. 

Sampling artifacts. The use of in situ pumps to collect water samples for " Th analysis permits simultaneous collection (and separation) of different particle fractions as well as dissolved Th. As pumping systems have been modified to permit determination of POC on the pump filters, it became possible to compare POC determined from the pump samples with conventional POC determinations made on small volume samples (0.5 - 2 L) taken from hydrocasts. The JGOFS data from multiple studies show large discrepancies between these two sample collection methods, with pump POC values 3 to 100 times lower than bottle POC values. Possible artifacts with each approach have been identified. For example Moran et al. (1999) have suggested that DOC is adsorbed onto... 

A more complex but faster and more sensitive approach is polarization modulation (PM) IRLD. For such experiments, a photoelastic modulator is used to modulate the polarization state of the incident radiation at about 100 kHz. The detected signal is the sum of the low-frequency intensity modulation with a high-frequency modulation that depends on the orientation of the sample. After appropriate signal filtering, demodulation, and calibration [41], a dichroic difference spectrum can be directly obtained in a single scan. This improves the time resolution to 400 ms, prevents artifacts due to relaxation between measurements, and improves sensitivity for weakly oriented samples. However, structural information can be lost since individual polarized spectra are not recorded. Pezolet and coworkers have used this approach to study the deformation and relaxation in various homopolymers, copolymers, and polymer blends [15,42,43]. For instance, Figure 7 shows the relaxation curves determined in situ for miscible blends of PS and PVME [42]. The (P2) values were determined... 

Because of potential glass fiber sulfate artifacts, a separate sampling for total sulfate was made using a polycarbonate filter. 

Gratton et. al. in [34] describes a least squares regression method to model heartbeat artifacts and to filter it out adaptively. Since the heartbeat rate is approximately 1 Hz, it is necessary that data is sampled at a sufficiently high rate (e.g. above the Nyquist rate) so as to represent and filter the artifacts out correctly and have minimal impact on the signal of interest itself [98]. 

A sampling system designed to reduce artifact problems was employed by Liang and co-workers (1997) in an experimental chamber study of the gas-particle partitioning of several PAHs (and //-alkanes) on three types of model aerosols and ambient urban particulate matter. The system consisted of a dual glass fiber filter system as described by Hart and Pankow (1994), followed by two parallel sampling trains for trapping... 

Fibrous mat type filters include the frequently used paper (cellulose) fiber filter, for example, the Whatman paper filter, and glass fiber filters. A common fibrous mat filter used for sample collection and air cleaning is known as the HEPA filter (/zigh efficiency particulate air filter) and is made of a combination of cellulose and mineral fibers. A widely used type of fibrous mat filter is the high-volume filter, commonly referred to as Hi-Vol, shown in Fig. 11.56. A modification of the Hi-Vol filter to increase the total air flow allows the collection of sufficient particulate matter in relatively short time periods (e.g., 2 h) to cany out chemical analysis this is important for studies of the diurnal variation of various chemical components of the aerosol as well as for minimizing sampling artifacts (Fitz el ai, 1983). 

Eatough, D. J., L. J. Kewis, M. Eatough, and E. A. Lewis, Sampling Artifacts in the Determination of Particulate Sulfate and S02(g) in the Desert Southwest Using Filter Pack Samplers, Environ. Sci. Technol., 29, 787-791 (1995). 

The degree of retention of SO on these nylon filters was also assessed 0.14 ppm S02 was sampled at 20 °C, 80% RH, and 20 L/min (total dosages for each filter, 2600 xg of SO (19). At saturation, the artifact SO values with the S S filter, 56 1 xg, was lower than that for the Sartorius and Gelman nylon 6 filters, which ranged from 65 to 70 xg. 

A prefired Pallflex 2500 QAST quartz filter (104-mm diameter) downstream of the denuder retained particle-phase PAHs together with other particulate matter at a face velocity of about 3 cm/s. The PUF retained vapor-phase PAH compounds that penetrated the denuder, together with those volatilized from the filter following collection in the particle state. The presence of a denuder is expected to enhance the extent of volatilization of PAHs from the filter relative to the unit without a denuder. The PAH concentrations volatilized during sampling from the unit without a denuder were referred to as the normal artifact, An. The corresponding concentrations from the unit with the denuder included An + Ae, where Ae indicates excess artifact caused by the presence of the denuder. The possibility that particle-phase PAH compounds could volatilize and be retained while within the denuder was not addressed. 

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