Method development for the analysis of N-nitrosodimethylamine and other N-nitrosamines in drinking water at low nanogram/liter concentrations using solid-phase extraction and gas chromatography with chemical ionization tandem mass spectrometry

N-nitrosodimethylamine (NDMA) is a probable human carcinogen of concern that has been identified as a drinking water contaminant. U.S. Environmental Protection Agency Method 521 has been developed for the analysis of NDMA and 6 additional N-nitrosamines in drinking water at low ng/L concentrations. The method uses solid-phase extraction with coconut charcoal as the sorbent and dichloromethane as the eluent to concentrate 0.50 L water samples to 1 mL. The extracts are analyzed by gas chromatography-chemical ionization tandem mass spectrometry using large-volume injection. Method performance was evaluated in 2 laboratories. Typical analyte recoveries of 87-104% were demonstrated for fortified reagent water samples, and recoveries of 77-106% were demonstrated for fortified drinking water samples. All relative standard deviations on replicate analyses were < 11%.     

Controlled self-assembly of re-engineered insulin by Fe(II)

Self-assembly of proteins mediated by metal ions is crucial in biological systems and a better understanding and novel strategies for its control are important. An abiotic metal ion ligand in a protein offers the prospect of control of the oligomeric state, if a selectivity over binding to the native side chains can be achieved. Insulin binds Zn(II) to form a hexamer, which is important for its storage in vivo and in drug formulations. We have re-engineered an insulin variant to control its self-assembly by covalent attachment of 2,2'-bipyridine. The use of Fe(II) provided chemoselective binding over the native site, forming a homotrimer in a reversible manner, which was easily followed by the characteristic color of the Fe(II) complex. This provided the first well-defined insulin trimer and the first insulin variant for which self-assembly can be followed visually.     

Scattering properties of salt-free wormlike micellar solutions

Static and dynamic light scattering and conductivity experiments were carried out on salt-free aqueous micellar solutions of cetyltrimethylammonium n-hexane sulfonate (CTAC_6SO_3) and cetyltrimethylammonium n-heptane sulfonate (CTAC_7SO_3) as a function of surfactant concentration. This study confirms the analogy between the behavior in the semi-dilute regime of elongated micellar systems and “classical” polyelectrolyte solutions. Time-resolved scattering experiments performed after a variation of concentration from about twice the overlap volume fraction to less than half of it revealed the existence of a structural relaxation with a characteristic time of several hours. Received 21 December 1999     

Precise mass measurement of a double-stranded 500 base-pair (309 kDa) polymerase chain reaction product by negative ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS) has been used to determine the mass of a double-stranded 500 base-pair (bp) polymerase chain reaction (PCR) product with an average theoretical mass of the blunt-ended (i.e. unadenylated) species of 308 859.35 Da. The PCR product was generated from the linearized bacteriophage Lambda genome which is a double-stranded template. Utilization of ethanol precipitation in tandem with a rapid microdialysis step to purify and desalt the PCR product was crucial to obtain a precise mass measurement. The PCR product (0.8 pmol/μL) was electrosprayed from a solution containing 75% acetonitrile, 25 mM piperidine, and 25 mM imidazole and was infused at a rate of 200 nL/min. The average molecular mass and the corresponding precision were determined using the charge-states ranging from 172 to 235 net negative charges. The experimental mass and corresponding precision (reported as the 95% confidence interval of the mean) was 309 406 +/- 27 Da (87 ppm). The mass accuracy was compromised due to the fact that the PCR generates multiple products when using Taq polymerase due to the non-template directed 3'-adenylation. This results in a mixture of three PCR products with nearly identical mass (i.e. blunt-ended, mono-adenylated and di-adenylated) with unknown relative abundances that were not resolved in the spectrum. Thus, the experimental mass will be a weighted average of the three species which, under our experimental conditions, reflects a nearly equal concentration of the mono- and di-adenylated species. This report demonstrates that precise mass measurements of PCR products up to 309 kDa (500 bp) can be routinely obtained by ESI-FTICR requiring low femtomole amounts. Copyright 1999 John Wiley & Sons, Ltd.