Pigment identification in artwork using graphite furnace atomic absorption spectrometry.

The use of a sampling technique is described for the identification of metals from inorganic pigments in paint. The sampling technique involves gently contacting a cotton swab with the painted surface to physically remove a minute quantity ( approximately 1-2mug) of pigment. The amount of material removed from the painted surface is invisible to the unaided eye and does not cause any visible effect to the painted surface. The cotton swab was then placed in a 1.5ml polystyrene beaker containing HNO(3) to extract pigment metals prior to analysis using graphite furnace atomic absorption spectrometry (GFAAS). GFAAS is well suited for identifying pigment metals since it requires small samples and many pigments consist of main group elements (e.g. Al) as well as transition metals (e.g. Zn, Fe and Cd). Using Cd (cadmium red) as the test element, the reproducibility of sampling a paint surface with the cotton swab was approximately 13% in either a water or oil medium. To test the feasibility of cotton sampling for pigment identification, samples were obtained from paintings (watercolour and oil) of a local collection. Raman spectra provided complementary information to the GFAAS, which together are essential for positive identification of some pigments. For example, GFAAS indicated the presence of Cu, but the Raman spectra positively identified the modern copper pigment phthalocyanine green (Cu(C(32)Cl(16)N(8)). Both Raman spectroscopy and GFAAS were useful for identifying ZnO as a white pigment.

Escherichia coli beta-galactosidase is heterogeneous with respect to a requirement for magnesium.

Commercially obtained E. coli beta-galactosidase was stored at 25 degrees C in buffer containing 1 mM MgCl2 and in buffer containing no added MgCl2. Samples were removed at set times and the activity of individual enzyme molecules assayed. When stored in the presence of 1 mM magnesium, the number of active molecules did not change over a 2.5-h period. When stored in the absence of added MgCl2, over half the enzyme molecules became inactive within the first hour. However, those molecules which retained activity remained active for the duration of the experiment. This indicates that there may exist two populations of E. coli beta-galactosidase, one which requires storage in the presence of the higher concentration of Mg2+ in order to remain active. There was no observed correlation between this requirement for magnesium and reaction rate. Additionally, the presence of the 1 mM MgCl2 was found to decrease the average activity of the beta-galactosidase molecules under the conditions employed.

Rapid dissolution of metal in aqueous media using a modified commercial spark source unit.

In this preliminary study the feasibility of a modified spark source was investigated as a method of rapidly dissolving solid conductive samples in aqueous media prior to analysis by atomic spectroscopy. The spark source, originally designed for spark emission spectroscopy in air, was modified by the installation of a spark ablation vessel. This spark ablation vessel was designed for spark ablating samples in aqueous solution, such as deionised water. Samples such as mild steel and brass were ablated in 5-10 ml of deionised water for 2-30 s producing a colloidal suspension. The suspension was readily dissolved by adding 100 mul of concentrated HCl or HNO(3). In this paper the spark ablation vessel is described as well as some of the properties of spark ablation in aqueous solutions. Spark ablation rates on mild steel were measured with respect to spark ablation parameters such as applied current (power), polarity and spark time. Using mild steel as a test sample, spark ablation rates varied from 40 mug s(-1), with 2.5 A of applied current, to 70 mug s(-1) with 10 A of applied current to the electrodes. The feasibility of using this technique for analysing trace levels (mug g(-1)) of elements in solid samples was also demonstrated for elements such as Ni in brass with inductively coupled plasma mass spectrometry (ICP-MS). Quantification of selected elements (Cr, Ni, Mn and Cu) in a certified alloy (SRM 663) and a non-certified stainless steel showed good agreement between the measured values using spark ablation and the accepted values.

Activation parameters for the reconstitution of apotyrosinase by copper.

The reaction of apotyrosinase with divalent copper to give enzymatically active tyrosinase has been studied at pH 8.2 and temperatures from 278 to 303 K. At a 10-fold excess of Cu(II) over enzyme, the pseudo-first order rate constants range from 1.32 x 10(-3) s-1 to 2.93 x 10(-2) s-1 and yield activation parameters of delta H not equal to = 85 +/- 3 kJ.mol-1 and delta S not equal to = 5 +/- 20 J.mol-1.K-1. The near zero value for the entropy of activation is discussed.

The pH dependence of the reconstitution reaction of apotyrosinase: the question of Cu(I) versus Cu(II).

The rates of reconstitution of apotyrosinase by CuSO4 at various pH values have been determined. The rate at pH 6.0 is 130 times faster than that at pH 9.0. These results show that the idea of reconstitution by Cu(I) being a faster process than by Cu(II) by a factor of 200 should be discarded.