Chromium trioxide (CrOg) was tested as a replacement for the potassium permanganate-sulfuric acid solutions designed to oxidize NO for conversion and detection as NO2. The CrOg oxidizer was prepared by soaking firebrick in a 17% solution of pure CrOg, draining, and drying at 105°C. The final loading was about 10% by weight. Since the optimal relative humidity (R.H.) working range of the CrOg oxidizer without sulfuric acid is higher (35-90% R.H.) than the material mixed with sulfuric acid, a humidifier with a shortened inlet tube was used so that the airstream was blown over water rather than bubbled through it. This gave a relative humidity of about 70-80%. Three to 5 grams of the hydrated oxidizer was placed in a midget impinger. An interesting and desirable feature of the CrOg oxidizer is its color change when hydrated (dull yellow) or dehydrated (dull pink) and when its oxidizing ability is spent (greenish brown). The midget impinger should be changed before more than half has depleted. At ambient levels of NO the oxidant should last at least 3 months. The oxidation efficiency is close to 100%.The development of suitable analytical procedures for determining NO paralleled the historical sequence of improvements in NO2 analysis. Chronologically, the need for trace analysis of NO started with the manufacture of city gas from coal. Methods for oxidizing NO with a mixture of KMn04 and H2SO4 were described by Guyer and Weber (1933), Rollings (1937), andShindman andYeaw (1942).Other systems for oxidizing NO to NO2 were described by Johnston (1954), Thomas et al. (1956), Remy (1956), and Ripley et al. (1964). Such systems used such compounds as ozone, heated I2O5, acidified MnC>2, periodate, persulfate, N2O5, CIO2, and oxygen. Each method, however, has certain disadvantages for continuous sampling and analysis. An excess of ozone converts nitrogen oxides all the way to nitric acid in the presence of atmospheric moisture. Therefore, ozone must be precisely metered into the gas stream with no more than a 100% excess over the amount of NO. A disadvantage of the use of heated iodine pentoxide is the fact that many other reducing pollutants also liberate iodine from this reagent. Gaseous oxygen (O2) oxidizes nitric oxide slowly and can be used only for concentrations above 100 ppm. Periodate, persulfate, and acidified permanganate are not quantitative oxidizers, and acidified manganese dioxide is not stable. Chlorine dioxide has been demonstrated to give good conversions. However, it is not readily available, and any large excess bleaches the azo dye reagent and must, therefore, be precisely metered into the air stream, similarly to ozone (Thomas et al. 1956). Remy (1956) mentioned the use of CrOg as an oxidizing

Thus far, our search for molecular carbonaceous remnants, specifically indicative of biogenic processes, has been epitomized by the isolation and identification of isoprenoid alkanes in a number of ancient sediments of various geologic types ranging from several million to over 2.7 billion years in age.'-3 We have felt that the isoprenoid alkanes could serve as "biological markers" in our quest for evidence of life among geologically well-characterized ancient shales and oils, and more specifically in carbonaceous chondrites.Earlier we reported preliminary studies on the oil shale from the Green River Formation (Eocene age, about 52 X 106 years) at Rifle, Colorado.' 2 These results, which paralleled those of Cummins and Robinson,4 established the biological history of this Cenozoic rock from the very uneven distribution of the n-alkanes and from the presence of large proportions of isoprenoid alkanes [C16-, C18-isoprenoid; C19isoprenoid (pristane); C20-isoprenoid (phytane)].We wish to report now the isolation and identification of the C27-, C28-, and C29steranes and a C3o-pentacyclic triterpane from the branched-cyclic alkane fraction of the Green River Shale. Separation of the branched-cyclic alkane fraction (extraction from shale previously described in detail2) into its individual components was achieved by an initial programed gas-liquid chromatographic run to 3000 (3% SE-30 on Gaschrom Z, 100-120 mesh, 10 ft X 1/4 in. s.s. column, program rate 40 per min) in which peaks were collected and in turn rechromatographed isothermally at temperatures ranging from 2600 to 280°(1% SE-30 on Gaschrom Z, 100-120 mesh, 10 ft X 1/4 in. s.s. column). Mass spectra of these collected samples were determined in a direct inlet system of a modified C.E.C. 21-103C mass spectrometer.' A preliminary mass spectral examination of the fractions collected as indicated in Figure 1 from a programed run revealed the molecular weights which are listed in Table 1 for the major components. From the subsequent isothermal separations, a homologous series of compounds were isolated which display mass spectral fragmentation patterns characteristic of the saturated, tetracyclic sterane carbon skeleton

paper was prepared to provide an overview of analytical activities concerning air pollution in the years of 1969 and 1970. It avoids overlapping the previous analogous review [Altshuller, A. P., Anal. Chem., 41, 1R, (1969)]. As a practical matter, most of the papers included were dated in the period June 1968 to August 1970. In most cases we had to rely on abstracts. Papers readily available to us but published after August 1970 were also included.A review such as this, is intended primarily to collate and to provide perspective on what has been published. We have found previous reviews useful not only at the time of publication but also years afterward. A review largely restricted to the information published cannot be relied upon to include the more recent advances in a rapidly progressing field.