Analytical Currents Synopses of significant analytical articles from other publications
VOCs in SEPARATIONS urban air
Volatile organic compounds, of which nonmethane hydrocarbons are a significant fraction, are important precursor compounds that, in conjunction with nitric oxide and nitrogen dioxide, photocatalyze the formation of tropospheric ozone. Peter J. Milne and colleagues at the University of Miami have developed an automated system for the near-continuous, hourly field measurement of C2-C10 hydrocarbons and other VOCs at the sub-part-per-billion level in ambient air. Air is collected for up to 55 min of each hour into a holding canister so that a timeintegrated sample can be taken without the use of solid sorbent traps or excessive cryogen. Subsequent analysis, using both cryotrapping and cryofocusing steps with either GC-FID or GC/MS, allows identification and quantitation of nonmethane hydrocarbons on a single capillary GC column. The system was used during a six-week field study in Atlanta, GA. (Environ. Sci. Technol. 1994, 28, 238-45)
LC/MS smokes out nicotine metabolites Metabolites such as cotinine and cotinine-V-glucuronide are measured in urine to assess nicotine uptake in smokers. G. D. Byrd and co-workers at R. J. Reynolds Tobacco Co. and colleagues at the University of Kentucky compared direct and indirect thermospray LC/MS methods for determining-the glucuronide. In the direct method a radiolabeled internal standard was used, and ions of protonated and adducted forms of cotinine produced by the glucuronide peak from LC were monitored. In the indirect method a sample was analyzed for cotinine before and after enzymatic cleavage of the glucuronic acid from the metabolite. The direct method needed less sample preparation, and samples were run only once. The researchers found that the two methods agreed well in a test of four smokers' urine samples, although values for the glucuronide were 6-14% higher when the direct method was used. The limit of detection in the direct method was 0.70 pmol/L. (Biol. Mass Spectrom. 1994, 23,103-07)
Indirect vitamin B 1 2 assay Vitamin B12 deficiency is sometimes determined in tissues and body fluids by measuring the concentration of methylmalonic acid (MMA), a metabolite in one of the vitamin B12-mediated enzymatic pathways. Most GC and HPLC methods for assaying MMA in plasma and urine either lack sensitivity or are too cumbersome for routine clinical use. P. J. Babidge of the Central Veterinary Laboratories and W. J. Babidge of the University of Adelaide in Australia have developed an HPLC method using fluorescence derivatization of MMA with monodansylcadaverine and dicyclohexylcarbodiimide. They demonstrated its use in ovine urine, plasma, rumen fluid, and liver and found a linear measurable range from submicromolar to millimolar concentrations of MMA. (Anal. Biochem. 1994,226, 424-26)
ELECTROANALYTICAL
Flow injection coulometry of corn syrup
Starch in the form of oe-amylose must be completely broken down to its D-glucopyranoic units during the processing of corn syrup for commercial use; sodium metabisulfite is then added as a preservative. Richard H. Taylor, Jon Rothermund, Gary Christian, and Jaromir Ruzicka of the Center for Process Analytical Chemistry at the University of Washington have developed an automated coulometric flow injection titration (CFIT) method to determine starch and sodium metabisulfite, both individually and together. Sodium metabisulfite is determined by CFIT with generated iodine using spectrophotometric endpoint detection at 530 nm.
Starch is determined from the absorbance after reaction of the sample with a specific amount of iodine. In the presence of sodium metabisulfite, starch is determined from the rate of accumulation of the starch/iodine interaction product after the metabisulfite endpoint. The method demonstrates the ability to perform an analysis in which an analyte reacts with an electrochemically generated reagent to form a readily detectable species. (Talanta 1994, 41, 31-38)
Polarography of selenium in fish Selenium is an essential nutrient at low concentrations in biological systems but is toxic at higher concentrations. Because there is a relatively small difference between essential and toxic levels, it is important to have accurate methods for determining selenium in biological tissue. Ming Khong Wong and colleagues at the National University of Singapore have developed a microwave digestion technique that increases the speed and safety of the differential pulse polarographic method for determining selenium in fish tissue. The sample is decomposed with an HN0 3 /H 2 S0 4 /H 2 0 2 mixture in a closed PFTE digestion vessel with microwave heating. After selenate is reduced to selenite with hot HC1, the samples are analyzed using DPP. The detection limit is 0.06 pg/dm 3 , with a linear calibration curve up to 8 pg/dm 3 . Results obtained for reference samples using DPP are in good agreement with those obtained using hydride generation AAS. (Talanta 1994,41, 53-58)
SPECTROSCOPY ICP-AES of rare gases The emission spectra of plasma gases other than argon in the inductively coupled plasma have not been well characterized. In the first step toward investigating rare gases as alternative discharge gases for the ICP, Ramon Barnes and
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Tracey Jacksier of the University of Mas sachusetts and American Air Liquide have reported the spectra of pure xenon, kryp ton, and neon ICP discharges at atmo spheric pressure. Discharges are initiated in pure flow ing argon in a sealed quartz container. After the rare gas is added to the argon stream, the flow of argon is stopped and the rare gas plasma forms. Background spectra of the pure gases are then mea sured over the spectral range of 200900 nm. Typical flow rates for the sealed ICP of < 20 mL/min facilitate recording the emission spectra with < 2 L of each gas. Spectral features observed include resonance emission lines as well as emis sion from gas impurities such as hydro gen, oxygen, nitrogen, and argon. (Appl. Spectrosc. 1994, 48, 65-71)
Depth profiling keratin fibers Although it is known that the surfaces of wool fiber and human hair play an impor tant role in dyeing and processing, little is known about the surface composition of these fibers. Kenneth P. Ghiggino and co lleagues at the University of Melbourne, Dulux Australia, and Bio-Rad Laboratories have used FT-IR photoacoustic spectros copy to distinguish spectroscopically be tween the surface layer (cuticle) and the interior (cortex) of intact keratin fibers. The thermal diffusion lengths of wool and hair fibers, calculated as a function of the wavenumber and optical scanning ve locity of the interferometer, indicate a depth resolution of ~ 1 μπι. Shifts in the amide I and II bands to lower wavenumbers, observed with increasing velocity of the interferometer, are consistent with differences in protein structure between the bulk and surface regions of the fiber. (Appl. Spectrosc. 1994,48,44-49)
XPS and NMR of C 6 0 The properties and composition of polyfluorinated fullerenes are of interest because of their novel structure and potential use as lubricants. D. M. Cox and a group of researchers form Exxon Research and Engineering Company, The University of Tennessee, Oak Ridge National Labora tory, and the University of Pennsylvania have used X-ray photoelectron spectros copy to characterize several different polyfluorinated fullerenes synthesized in two different laboratories. Quantitative measurement of the F l s , 0 l s , and C l s XPS signals was used to es tablish the elemental stoichiometry of 398 A
each sample. The F/C ratios are in gen eral agreement with previous results de termined by MS. In addition, one sample was examined using solid-state 19F-13C cross polarization/magic angle spinning 13 C NMR spectroscopy to provide addi tional information about the degree of fluorination and the level of C-C bond breaking. (J.Am. Chem. Soc. 1994,116, 1115-20)
Cutting the fat from BIOANALYTICALhydrophobic protein quantitation Pulmonary surfac tant, essential for breathing, is a complex mixture of lipids and lipopro teins. Quantitation of surfactant-associated protein C (SP-C) by the Lowry protein assay and other usual methods can be a slippery business—SP-C is small, very hydrophobic, and stains poorly on polyacrylamide gels. Antibodies have not been produced against SP-C because its sequence is highly conserved among species. Riad Qanbar and Fred Possmayer of the University of Western Ontario in Lon don, Ontario, have treated partially puri fied SP-C from crude extracts with mild alkali to remove the lipid chains from the two cysteine residues and reacted the pro tein with [14C]iodoacetamide. The radio labeled protein can be quantitated at nano gram levels with little interference from organic solvents and free lipids. (Anal. Biochem. 1994,216,262-70)
Removing iron from the body Excess amounts of iron are toxic to almost all living things. Iron chelating agents are commonly used as drugs to treat iron overload caused by Cooley's anemia or in emergencies in which children have acci dentally ingested dietary iron supple ments meant for adults. Kenneth N. Ray mond and co-workers at the University of California, Berkeley, have prepared and characterized four new derivatives of the most commonly used iron chelating agent, desferrioxamine Β (DFO). Potentiometric, spectrophotometric, and JH NMR studies show that all four DFO derivatives form six-coordinate 1:1 ferric complexes in aqueous solution. Sta bility constants of the four ferric com plexes can be determined by spectropho tometric competition titrations. (J. Am. Chem. Soc. 1994,116, 840-46)
Analytical Chemistry, Vol. 66, No. 7, April 1, 1994
Drug bioavailability by GC/MS/MS
MASS SPECTROMETRY The percentage of an orally adminis tered drug that reaches the circula tory system intact is an important factor in the drug's effec tiveness. Oral bioavailability is usually measured by comparing the results from intravenous and oral dosing on a single subject at different times, but physiologi cal conditions can fluctuate during the test and skew results. R.L.M. Dobson, G. R. Kelm, and D. M. Neal of Procter and Gamble have devised a GC/MS/MS method in which they ad ministered a 13C- and 180-colabeled drug intravenously and at the same time dosed the subject orally with the unlabeled drug. Parent-to-product ion transitions were selected so that the unlabeled drug could be present in the sample at a concentra tion up to 200-fold higher than that of the labeled drug before naturally occurring isotopes interfered with the results. Lownanogram amounts of the labeled and un labeled drugs could be detected in the plasma of dogs given low-milligram per kilogram doses with < 8% RSD. (Biol. Mass Spectrom. 1994,23, 75-81)
MALDI-TOF sees sugar shock The presence of serum transferrins lack ing one or more of their usual oligosac charide chains is one biomarker of carbo hydrate-deficient glycoprotein syndrome (CDGS), a multisystemic metabolic disor der that can cause severe mental and de velopmental deficiencies. Detection of the abnormal transferrins is usually carried out by isoelectric focusing, but the results aren't specific to CDGS, and 2D gel elec trophoresis is both complicated and gives inaccurate molecular weights for the pro teins. Electrospray ionization with MS fails to detect the minor nonsialylated spe cies because its molecular ion peak over laps with the normal transferrin peak. Yoshinao Wada and co-workers at the Osaka Medical Center and Koji Inui of the Osaka University Medical School used MALDI-TOFMS to distinguish di- and nonsialylated serum transferrins from the normal tetrasialylated species. Molecular ion peaks for the abnormal species present in patient sera appeared as identi fiable shoulders on the main normal trans ferrin peak. (Biol. Mass Spectrom. 1994, 23,108-09)
