Intercomparison study

LaBrecque JJ (1990) The comparison of the results of two independent intercomparison studies (BAK-i and SLB-i) ftom the same bulk material of a lateritic soil. Fresenius J Anal Chem 338 498-500. 

S.L. Reynolds, R. Fussel, M. Caldow, R. James, S. Nawaz, C. Ebden, D. Pendhngton, T. Stijve, and H. Desirens, Intercomparison Study of Two Multi-residue Methods for the Enforcement of EU MRLs for Pesticides in Fruit, Vegetables and Grain, European Commission, BCR Information, Chemical Analysis Contract No. SMT4-CT-95-2030 Reports EUR 17870EN (1997), EUR 18639 EN (1998), EUR 19306 EN (2000) and EUR 19443 EN, European Commission, Brussels (2001). 

If we look at the intercomparison studies that have been made on the measurement of lead in seawater [21], two points become obvious first, that almost no one can measure lead accurately in seawater and second, that there is much less lead present than anyone had estimated. With so little lead present in any form, identification of the organic moiety of an organolead compound becomes a major problem. 

Take part in External Quality Assessment, formally through Proficiency Testing schemes and informally by taking part in intercomparison studies. 

International Council for the Exploration of the Sea (ICES). 1992. Report on the Results of the ICES/IOC/OSPARCOM Intercomparison Exercise on the Analysis of Chlorobiphenyl Congeners in Marine Media — Step 1 and the Intercomparison Study of the Determination of Cbs in Baltic Herring Oil. ICES Coop. Res. Rep. No. 183. 

Shatalov V., Mantseva, E., Baait, A., et al. (2004). POP Model Intercomparison Study. Stage I. Comparison of Descriptions of Main Processes Determining POP Behaviour in Various Environmental Compartments. MSC-E Technical Report 1/2004. 

A. W. T. Bristow and K. S. Webh. Intercomparison Study on Accurate Mass Measurement of Small Molecules in Mass Spectrometry. J. Am. Soc. Mass Spectrom., 14(2003) 1086-1098. 

An Intercomparison study of trace element determinations In simulated and real air particulate samples has been published by Camp, Van Lehn, Rhodes, and Pradzynskl ( ). This Involved twenty-two different laboratories reporting up to thirteen elements per sample. The simulated samples consisted of dried solution deposits of ten elements on Mllllpore cellulose membrane filters. In our data analysis a set of energy dispersive X-ray emission results restricted to eight laboratories reporting six elements (V, Cr, Mn, Fe, Zn, Cd) was... 

Source contributions assigned to the same aerosol sample have varied greatly in intercomparison studies (23) but, without the intermediate particle property classifications, it is impossible to ascribe the differences to the analytical portion or to the source assignment portion of the process. 

Infrared spectroscopy has been applied to ambient air measurements since the mid-1950s (Stephens, 1958). Indeed, PAN was first identified in laboratory systems by its infrared absorptions and dubbed compound X because its identity was not known (Stephens et al., 1956a, 1956b). It was subsequently measured in ambient air (Scott et al., 1957). Since then, IR has been applied in many areas and has provided unequivocal and artifact-free measurements of a number of compounds. Because of its specificity, it has often been used as a standard for intercomparison studies (e.g., for HNO see later). 

Figure 11.3 shows typical ambient air spectra in two regions in which HNO-, (Fig. 11.3a) and NH-, (Fig. 11.3b), respectively, have characteristic absorption bands (Biermann et al., 1988). Figure 11.4 shows, for comparison, some typical reference spectra for HN03 and NH-, taken at much higher concentrations in a 25-cm-long cell (see Problem 6). It can be seen that the absorption bands in air even in a polluted urban area are relatively weak. However, FTIR has also proven particularly useful as a standard for intercomparison studies in polluted urban atmospheres (e.g., see Hering et al., 1988). 

However, in at least one intercomparison study using diffusion denuders and transition flow reactors, different results were obtained for some important atmospheric gases such as S02, HNO-, and H+, where the TFR values were about 30, 80, and 85% higher, respectively, than those from the denuder system (Sickles et al., 1989) the researchers attributed these differences to biases in the TFR measurements. 

Several intercomparison studies for N02 have been carried out (e.g., Fehsenfeld et al., 1990). At concentra-... 

An airborne intercomparison study (Gregory et al., 1990a) was also carried out using two photolysis methods (the 353-nm laser photolysis with TP-LIF detection of NO and a Xe arc lamp photolysis with chemiluminescence detection of NO) as well as TDLS. Overall, for N02 up to 200 ppt, the techniques agreed with the average values of all three by 20% or better and with each other to within 30%. However, below 50 ppt, there was very little correlation between the various measurement techniques (Gregory et al., 1990a). 

An informal intercomparison study of N02 measurements was carried out in a remote atmosphere at Izana, Tenerife (Zenker et al., 1998). Three techniques were used TDLS, photolysis with a chemiluminescence detector, and matrix isolation-ESR. Agreement between the three methods was good, with plots of data from one technique against the others having slopes within experimental error of unity. For example, TDLS and the photolysis technique plotted against the matrix isolation measurements had slopes of 0.90 + 0.47 and 1.04 + 0.34, respectively, over a range of NOz concentrations from 100 to 600 ppt. 

These studies were carried out in a polluted urban atmosphere where the concentrations are relatively high it might be expected that the agreement (or lack thereof) would certainly not improve at the much smaller concentrations found in rural and remote regions. Similar disagreements have been observed in other intercomparison studies (e.g., Anlauf et al., 1985 Gregory et al., 1990b Huebert et al., 1990), even in synthetic atmospheres (e.g., Fox et al., 1988). 

Mass spectrometiy. Several instruments based on chemical ionization mass spectrometry (CIMS) have been developed and applied to ambient air (Huey et al., 1998 Mauldin et al., 1998). Figure 11.32 shows two such instruments that have undergone an informal intercomparison study between themselves and a filter pack (nylon) method (Fehsenfeld et al., 1998). 

A brief informal intercomparison study of the mass spectrometry methods with a nylon filter pack method near Boulder, Colorado, gave average levels of 0.38-1.6 ppb when the wind carried air from the direction of the greater metropolitan Denver urban area and 0.14-0.56 ppb when the wind was downslope and westerly, where there are fewer emissions sources previous filter pack... 

Intercomparison studies Because of the importance of accurate measurement of individual organics in air, there have been several intercomparison studies carried out in which a number of different laboratories have analyzed common samples, either synthetic or ambient air. In one such study (Apel et al., 1994), a... 

FIGURE 11.39 Results of intercomparison study of a mixture of organics found in ambient air. The concentrations (in ppbC) are shown as unfilled bare, along with the coefficient of variation ( ) for these measurements (adapted from Bernardo-Bricker et at., f995). 

A number of intercomparison studies of the various methods of measurement of HCHO have been carried out. As might be expected given the specificity of spectroscopic methods, the results of FTIR, DOAS, and TDLS are generally in good agreement, within 15% of their mean value in one study in a polluted atmosphere (Lawson et al., 1990). During the same study, the diacetyldihydrolutidine derivative method and the DNPH method were lower by 15-25% than the spectroscopic mean, whereas the enzymatic method was higher by 25%. 

For example, Cantrell and co-workers (1993) estimate the efficiency of conversion of simple alkyl peroxy radicals to vary from 0.93 for CH3CH202 to 0.47 for (CH3)2C02, and it may be even less for larger alkyl peroxy radicals. This may be the reason that in some intercomparison studies, the matrix isolation-ESR technique (vide infra), which measures the sum of ROz, gives some higher concentrations for some individual measurements than the chemical amplifier method (e.g., Zenker et al., 1998). 

Fewer intercomparison studies have been carried out for peroxy radicals than for OH. Two chemical amplification methods were compared during a measurement campaign in Brittany, France (Cantrell et al., 1996). Although the measurements tended to track one another, there is more scatter than might be expected, given the similar nature of the instruments. For example, a plot of the data from one instrument against those from the second had a slope of 0.71 but a correlation coefficient of only r = 0.36. In another study (Zenker et al., 1998), comparison of three chemical amplifier techniques to matrix isolation-ESR gave... 

In summaty, HOz and ROz radical concentrations are substantially greater than those of OH, typically by several orders of magnitude. There are several different approaches to measuring these peroxy radicals, and the results from these are in overall agreement as to the magnitude of the concentrations and their diurnal variation. However, there have not been a significant number of intercomparison studies of these methods, so evaluation of the absolute accuracies will require further work. 

Intercomparison studies. A number of intercomparison studies have been carried out to determine the accuracy and precision of measurements of various elements found in particles. For example, Nejedly et al. 

Mackay, G. I., H. I. Schiff, A. Wiebe, and K. Anlauf, Measurements of N02, H2CO, and HNO, by Tunable Diode Laser Absorption Spectroscopy during the 1985 Claremont Intercomparison Study, Atmos. Enriron., 22, 1555-1564 (1988). 

Similar results have been reported in an intercomparison study of models used to predict tropospheric ozone on a global scale (Olson et al., 1997). Agreement for 03 and NO. was reasonably good for relatively clean atmospheres, with a larger spread for predicted H202. However, introduction of VOC chemistry increased the range of model predictions substantially,... 

A DS-based NH3 instrument, similar to that just described, was among several others tested in a European intercomparison study in Rome in 1989 and was found to perform well (95). 

Several potential peroxy radical measurement techniques exist in the realm of atmospheric chemistry studies, although most have been used only in the laboratory. The techniques are summarized in Table I. Possibly, some laboratory methods could be applied to atmospheric measurements. The database for ambient peroxy radical concentrations in the troposphere and stratosphere is meager. Much of the available stratospheric data yield concentrations of H02 higher than those calculated with computer models. The reasons for this systematic difference are not known. In the troposphere, more measurements are called for in conjunction with other related species such as ozone, NO, NOjNo2 andjcv It wiH also t>e appropriate to develop multiple methods, and, when they have reached maturity, to perform intercomparison studies. 

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