Tutorial on estimating the limit of detection using LC-MS analysis,part I: Theoretical review

A large body of literature exists on the limit of detection (LOD), but there is still a lot of confusion about this important validation parameter. This confusion mainly stems from its statistically complex background. The goal of this two-part tutorial is to discuss and clarify the topic of LOD for practitioners. The two main conclusions of this tutorial are: (1) the choice of how to estimate LOD should be based on the purpose of the analytical method that is being validated (e.g. considerable effort should not be made to estimate LOD for a method that is not used for detecting traces in the vicinity of LOD), and (2) LOD estimates are strongly dependent on different assumptions and the approach used, and therefore caution must be exercised when using the estimate or when comparing different estimates.     

Practical Organocatalytic Synthesis of Functionalized Non‐C2‐Symmetrical Atropisomeric Biaryls

An organic acid catalyzed direct arylation of aromatic C(sp²)? H bonds in phenols and naphthols for the preparation of 1,1′‐linked functionalized biaryls was developed. The products are non‐C₂‐symmetrical, atropoisomeric, and represent previously untapped chemical space. Overall this transformation is operationally simple, does not require an external oxidant, is readily scaled up (up to 98 mmol), and the structurally diverse 2,2′‐dihydroxy biaryl (i.e., BINOL‐type), as well as 2‐amino‐2′‐hydroxy products (i.e., NOBIN‐type) are formed with complete regioselectivity. Density‐functional calculations suggest that the quinone and imino‐quinone monoacetal coupling partners are exclusively arylated at their α‐position by an asynchronous [3,3]‐sigmatropic rearrangement of a mixed acetal species which is formed in situ under the reaction conditions.     

Separation of selected metal-binding proteins with capillary zone electrophoresis

Several metal-binding proteins, including albumin, carbonic anhydrase, conalbumin, cytochrome c, ferritin, hemoglobin, myoglobin, plasma amine oxidase, superoxide dismutase and transferrin were separated with capillary zone electrophoresis (CZE) in uncoated and coated capillaries. Phosphate and tetraborate buffers achieved complementary separation selectivities. Optimised pre-wash protocols for uncoated capillaries using 0.1 M HCl as a rinsing solution for the borate buffer and a combination of 0.1 M NaOH and 0.1 M HCl for the phosphate system improved the stability of migration times considerably with coefficients of variation between 0.10 and 0.77% (n=7) instead of up to 2.92% with inappropriate rinsing conditions. Capillaries coated with poly(vinyl alcohol) and equipped with a 150 μm i.d. bubble cell increased the signal-to-noise ratio by a factor three, additionally improving the resolution. For commercial protein standards, which gave several peaks in CZE with UV detection, MS data proved the presence of proteinaceous contaminants. Molecular weights (M_r) of proteins experimentally determined from MS data showed deviations from theoretical M_r as small as 0.002-0.021%. Applicability of the developed separation for clinical samples is shown for human serum.     

On the general transformation from molecular geometric parameters to cartesian coordinates

This article presents a general method and accurate algorithm for calculating the Cartesian coordinates (x_a, y_a, z_a) from an arbitrary triple of distances r(a,i), angles, θ(a,j,k), or dihedral angles ?(a,l,m,n), specifying the position of the nucleus a relative to nuclei i, ?, n with known Cartesian coordinates. There is a brief discussion of the requirements on the 3N-6 geometric parameters in order for them to determine the shape of an N-atomic molecule.     

Capillary zone electrophoretic determination of hydroxymetabolites of atrazine in potable water using solid-phase extraction with Amberchrom resins

A CZE-separation system for hydroxyatrazine, desisopropylhydroxyatrazine, desethylhydroxyatrazine and ammeline is presented that allows their determination in environmental waters in the subgL^–1 domain after solid-phase extraction/enrichment with styrene-divinylbenzene and methacrylate macroporous (Amberchrom) resins. The CZE separation and determination system uses the stacking effect and an increased light path for UV-detection (bubble cell). The etching of the bubble cell was done in our laboratory. A ruggedness check of the CZE-system revealed the resolution of theN-dealkylated metabolites to be sensitive to pH, temperature and current variations as well as to changes of capillary dimensions. Fine-tuning of the separation is performed by variation of the current intensity in the constant-current mode. Recoveries from fortified tap-water samples were 95% for hydroxyatrazine, 70% for desethylhydroxyatrazine and approximately 20% for desisopropylhydroxyatrazine at the 0.2 g L^–1 concentration level. Due to its high polarity, ammeline is only slightly adsorbed on Amberchrom resins and cannot be enriched using these materials.     

Synthesis and relative stability of 3,5-diacyl-4,5-dihydro-1H-pyrazoles prepared by dipolar cycloaddition of enones and alpha-diazoketones

An unusual reaction process that produced unexpected heterocyclic systems by a fragmentation-recombination mechanism is described. Thus treatment of the triketone, 3-acetyl-2,6-heptanedione, 1, with methanesulfonyl azide gave, in addition to the expected alpha-diazo ketone 3a, the dihydropyrazole 3c and its oxidation product, the pyrazole 3d. We propose that the initially formed alpha-diazo ketone 3a fragments into the simple alpha-diazomethyl ketone and methyl vinyl ketone which then undergo an intermolecular [2,3]-dipolar cycloaddition. Analogous treatment of the trifluoromethyl trione 2 again afforded a pyrazole 4c. Further experiments were carried out to lend evidence to our mechanistic hypothesis. Thus alpha-diazoacetophenone 5 and MVK underwent a [2,3]-dipolar cycloaddition under mild conditions to give the two regioisomeric dihydropyrazoles 6a and 6b. Interestingly these were formed in a 2:1 ratio, which suggested that 6a was more stable than 6b. The structures of 6a and 6b were optimized by using the B3LYP density functional method and the 6-31G* basis set and isomer 6a was predicted to be 1.5 kcal/mol more stable than isomer 6b. This energy difference could be rationalized by the greater capacity of the acetyl group than the benzoyl group to conjugate with the hydrazone. This difference in conjugation is reflected by key bond length differences. Thus we have discovered a novel fragmentation-cycloaddition process. We have also presented evidence for the mechanism of the formation of the dihydropyrazoles and carried out calculations to support these findings.     

Radical Anions of Cyclic Azoalkanes: An ESR,ENDOR, and TRIPLE-Resonance Study

Radical anions of ten monocyclic and bicyclic azoalkanes containing the azo group in (Z)-conformation, have been fully characterized by their hyperfine data with the use of ESR, ENDOR, and general-TRIPLE-resonance spectroscopy. These azoalkanes are represented by 3,3,5,5-tetramethyl-1-pyrazline ( 1 ), 2,3-diazabicyclo[2.2.1]hept-2-ene ( 4 ), and 2,3-diazabicyclo[2.2.2]oct-2-ene ( 9 ), as well as by their derivatives 2, 3, 5–8 , and 10. For all radical anions $1^{- \atop \dot{}}-10^{- \atop \dot{}}$, the ¹⁴N-coupling constant, a_N, is in the range of +0.83 to +0.97 mT; this finding indicates that the spin population is essentially restricted to the π system of the azo group. The ¹⁴N-hyperfine anisotropy largely affects the width of ESR lines, particularly at low temperatures. Substantial coupling constants of ⁷Li-, ²³K-, and ¹³³Cs-nuclei point to a close association of the radical anions with their alkakimetal counterions. With the exception of ³⁹K, these nuclei give rise to readily observable ENDOR signals which appear along with those stemming from protons. The prominent hyperfine features of $1^{- \atop \dot{}}-10^{- \atop \dot{}}$ are discussed.     

Electricity generation by Geobacter sulfurreducens attached to gold electrodes

The versatility of gold for electrode manufacture suggests that it could be an ideal material for some microbial fuel cell applications. However, previous studies have suggested that microorganisms that readily transfer electrons to graphite do not transfer electrons to gold. Investigations with Geobacter sulfurreducens demonstrated that it could grow on gold anodes producing current nearly as effectively as with graphite anodes. Current production was associated with the development of G. sulfurreducens biofilms up to 40 microm thick. No current was produced if pilA, the gene for the structural protein of the conductive pili of G. sulfurreducens, was deleted. The finding that gold is a suitable anode material for microbial fuel cells offers expanded possibilities for the construction of microbial fuel cells and the electrochemical analysis of microbe-electrode interactions.