Addition of internal standards to particulate sample matrices for routine trace analyses of semivolatile organic compounds: A source of systematical and random errors

This article is a criticism of the strategy of adding (isotope labelled) internal standards of semi volatile hydrophobic organic compounds directly on the surface of particulate samples matrix such as sediment, soil and fly ash, etc. in a small aliquot (mL) of solvent, before trace level analysis. The use of the internal standard is intended to compensate for incomplete extractions, clean-up losses, dilution errors and instrument variations. However, direct addition of internal standards to sample matrices creates two possibilities for inaccurate results by processes only affecting the internal standard: First, evaporation losses of standard from the sample matrix during evaporation of the carrier solvent. Second, the native analyte and internal standard sorb to the sample matrix with differing force. Both processes can introduce systematic and random error to the result. A systematic error of 74% due to evaporation losses of tetra chlorinated dibenzo-p-dioxins is observed, while the corresponding error for octa chlorinated dioxin is 0%. The associated random error is 45% for tetra down to 1–4% relative standard deviations for hepta and octa chlorinated dioxins. For laboratory staff the evaporation losses of standard (and native) compounds causes, besides dust, an additional risk of inhalation exposure. The internal standard should instead be added to the extraction solvent after the extraction. Smaller systematical errors (10–20%) and associated random errors due to irreversible sorption are discussed. Received: 4 September 1997 / Revised: 26 January 1998 / Accepted: 31 January 1998     

New Spectroscopic Information on 98Pd

Neutron deficient ⁹⁸Pd nuclei were produced in the ⁵⁰Cr(⁵⁸Ni,2p2α)⁹⁸Pd reaction at a beam energy of 261 MeV. A revision and extension of the level structure of ⁹⁸Pd is proposed. About 40 new transitions have been assigned to, and placed in the level scheme of this nucleus. A sequence of states are candidates for the negative parity 4—quasiparticle states built on the πg _9/2 ^?3 πp _1/2 ^?1 configuration predicted in the framework of the nuclear shell model.     

Extraction of a quality assurance sediment sample for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F)

An experimental design for two variables at three and four levels was used to investigate the Soxhlet extraction of polychlorinated dibenzo-p-dioxins/ dibenzofurans (PCDD/F) from a quality assurance sediment. The first variable was the solvent: toluene/methanol, toluene (both Soxhlet) and toluene in Soxhlet-Dean-Stark equipment. The second variable were four different bulks: silica, sulfuric acid treated silica, sodium carbonate and no bulk. Extractions with toluene/methanol and sulfuric acid provided only a small contribution to the overall extraction efficiency. Toluene/methanol preferably improved the extraction of PCDD, and the sulfuric acid improved the PCDF extraction. This was likely to reflect improved extraction efficiency of substructures in the sediment as pulp effluent remains (fiber) and fly-ash particulate. A previously found PCDD formation in extractions using toluene/methanol in presence of sodium carbonate was reproduced. A designed supercritical fluid extraction (SFE) experiment was also accomplished, however even under the best used conditions some PCDD/F were retained on the particulate compared to Soxhlet extraction. Variations in extraction efficiency in Soxhlet and SFE indicated that different subfractions of PCDD/F are connected to the matrix with different mechanisms, thus indicating that different PCDD/F fractions had different abilities to equilibrate also in the real environment. Received: 5 December 1997 / Revised: 9 February 1998 / Accepted: 15 February 1998     

Extraction of a quality assurance sediment sample for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F)

An experimental design for two variables at three and four levels was used to investigate the Soxhlet extraction of polychlorinated dibenzo-p-dioxins/ dibenzofurans (PCDD/F) from a quality assurance sediment. The first variable was the solvent: toluene/methanol, toluene (both Soxhlet) and toluene in Soxhlet-Dean-Stark equipment. The second variable were four different bulks: silica, sulfuric acid treated silica, sodium carbonate and no bulk. Extractions with toluene/methanol and sulfuric acid provided only a small contribution to the overall extraction efficiency. Toluene/methanol preferably improved the extraction of PCDD, and the sulfuric acid improved the PCDF extraction. This was likely to reflect improved extraction efficiency of substructures in the sediment as pulp effluent remains (fiber) and fly-ash particulate. A previously found PCDD formation in extractions using toluene/methanol in presence of sodium carbonate was reproduced. A designed supercritical fluid extraction (SFE) experiment was also accomplished, however even under the best used conditions some PCDD/F were retained on the particulate compared to Soxhlet extraction. Variations in extraction efficiency in Soxhlet and SFE indicated that different subfractions of PCDD/F are connected to the matrix with different mechanisms, thus indicating that different PCDD/F fractions had different abilities to equilibrate also in the real environment.     

Addition of internal standards to particulate sample matrices for routine trace analyses of semivolatile organic compounds: A source of systematical and random errors

This article is a criticism of the strategy of adding (isotope labelled) internal standards of semi volatile hydrophobic organic compounds directly on the surface of particulate samples matrix such as sediment, soil and fly ash, etc. in a small aliquot (mL) of solvent, before trace level analysis. The use of the internal standard is intended to compensate for incomplete extractions, clean-up losses, dilution errors and instrument variations. However, direct addition of internal standards to sample matrices creates two possibilities for inaccurate results by processes only affecting the internal standard: First, evaporation losses of standard from the sample matrix during evaporation of the carrier solvent. Second, the native analyte and internal standard sorb to the sample matrix with differing force. Both processes can introduce systematic and random error to the result. A systematic error of 74% due to evaporation losses of tetra chlorinated dibenzo-p-dioxins is observed, while the corresponding error for octa chlorinated dioxin is 0%. The associated random error is 45% for tetra down to 1–4% relative standard deviations for hepta and octa chlorinated dioxins. For laboratory staff the evaporation losses of standard (and native) compounds causes, besides dust, an additional risk of inhalation exposure. The internal standard should instead be added to the extraction solvent after the extraction. Smaller systematical errors (10–20%) and associated random errors due to irreversible sorption are discussed.     

Absorption and fluorescence spectroscopic characterization of cryptochrome 3 from Arabidopsis thaliana

The blue light photoreceptor cryptochrome 3 (cry3) from Arabidopsis thaliana was characterized at room temperature in vitro in aqueous solution by optical absorption and emission spectroscopic studies. The protein non-covalently binds the chromophores flavin adenine dinucleotide (FAD) and N5,N10-methenyl-5,6,7,8-tetrahydrofolate (MTHF). In the dark-adapted state of cry3, the bound FAD is present in the oxidized form (FAD(ox), ca. 38.5%), in the semiquinone form (FADH., ca. 5%), and in the fully reduced neutral form (FAD(red)H2) or fully reduced anionic form (FAD(red)H-, ca. 55%). Some amount of FAD (ca. 1.5%) in the oxidized state remains unbound probably caused by chromophore release and/or denaturation. F?rster-type energy transfer from MTHF to FAD(ox) is observed. Photo-excitation reversibly modifies the protein conformation causing a slight rise of the MTHF absorption strength and an increase of the MTHF fluorescence efficiency (efficient protein conformation photo-cycle). Additionally there occurs reversible reduction of bound FAD(ox) to FAD(red)H2 (or FAD(red)H-, FAD(ox) photo-cycle of moderate efficiency), reversible reduction of FADH. to FAD(red)H2 (or FAD(red)H-, FADH. photo-cycle of high efficiency), and modification of re-oxidable FAD(red)H2 (or FAD(red)H-) to permanent FAD(red)H2 (or FAD(red)H-) with low quantum efficiency. Photo-excitation of MTHF causes the reversible formation of a MTHF species (MTHF', MTHF photo-cycle, moderate quantum efficiency) with slow recovery to the initial dark state, and also the formation of an irreversible photoproduct (MTHF').     

An out-of-plane force field for p-benzoquinone and some structurally related molecules

A simplified out-of-plane valence force field in the harmonic approximation has been constructed for ethene, butadiene, glyoxal, acrolein, and p-benzoquinone using the overlay technique. p-Benzoquinone is considered to consist of two conjugated OCCHCHCO systems and force constants from the first four mentioned molecules are shown to be successfully transferable to p-benzoquinone. The force field has 21 independent parameters and gives a root-mean-square frequency error of 7.61 cm^?1.     

Inclusion of electric octupole contributions explains the fast radiative decays of two metastable states in Ar+

A laser probing investigation has yielded the lifetimes of the 3s(2)3p(4)(1D)3d (2)G(7/2,9/2) metastable doublet states of Ar+. The results, obtained with the CRYRING ion storage ring of Stockholm, are 3.0+/-0.4 and 2.1+/-0.1 s, respectively. Comparisons with theoretical values calculated with two independent theoretical approaches, i.e., the pseudorelativistic Hartree-Fock method and the multiconfiguration Breit-Pauli approach, have allowed us to establish the unexpected and extraordinary strong contribution of an electric octupole (E3) transition to the ground state, in addition to the M1 decay channels to the 3d ;{2,4}F states and the E2 contributions to the 4s 2P, 2D states.     

Raman scattering from p-benzoquinone

The Raman spectrum of powdered p-benzoquinone and p-benzoquinone-d₄ has been investigated at liquid nitrogen temperature using Kr⁺-laser excitation. Several new lines were observed. To facilitate the assignment, depolarization ratios have been measured in the melt and in acetone solution. Overlapping peaks were resolved with the aid of a computer program.     

Importance of an M2 depopulating channel for a Kr II metastable state

An experimental investigation of the radiative lifetime of the metastable 4s24p4(3P)4d4D7/2 level in Kr II shows an unusual situation regarding the importance of an M2 depopulation channel. While the first order M1 and E2 channels are expected to contribute in a dominant way to the decay, the experimental result, obtained using a laser probing technique on a stored ion beam, tau = 0.57+/-0.03 s, is far too short to be due to these channels according to our relativistic multiconfiguration Dirac-Fock calculation. Only if second order contributions to the decay branches (including essentially the M2 contribution) are taken into account in the calculations could the unexpected short lifetime be explained.