Metastable aqueous foams and flame atomic absorption spectrometry.

Using calmagite as the compleximetric agent in spectro-photometric measurements of magnesium in serum involves a nonspecific matrix effect, which may bias results by as much as 25 to 30% if aqueous standards are used in serum assays. We have overcome this effect by simply adding 5 pL of the amphoteric detergent Empigen BB (300 g/L; Albright and Wilson Ltd., Whitehaven, U.K.) to each reaction cuvette. We used a standard diagnostic kit for assaying magnesium (cat. which is based on the calmagite method of Ingman and Rungbom (1). Like Abernethy and Fowler (2), we found that adding Empigen BB shifted the absorbance mRvimum of the blank, but not of the magnesium.-calmagite complex, thus increasing the signal-to-noise ratio. Furthermore, the positive intercepts of dilution curves for samples without Empi-gen BB became zero with Empigen BB, indicating elimination of the matrix effect. Abernethy and Fowler (2) optimized their method by using a more concentrated calinagite reagent, thereby extending linearity of standard curves up to 5 mmol/L. Our standard curve is linear only between 0 and 2 mmol/L, but we optimized our method for small sample volumes (10 L) and low magnesium concentrations (<1 mmol/L). We optimized the method for final concentration of Empi-gen BB (1.5 g/L), time of incubation (<1 h), pH (11.8), temperature (20 to 25#{176}C), measurement wavelength (529 rim), sample volume (10 pL), and order of reagent addition (to 5 pL of Empigen BB add 500 pL of color reagent and 500 pL of base reagent, then 10 uL of sample, standard, or demetallized water). With Empigen BB included, the assay performed as follows. Within physiological values for magnesium, the within-day CV ranged from 2.9 to 3.9%, and the between-day CV ranged from 3.1 to 4.3%. Analytical recovery of added magnesium averaged 98.4%. Change in ionic milieu (up to twice the normal concentration of important ions) did not change analytical outcome. Results agree well with those determined by atomic absorbance spectrophotometry (p = 0.6, by Student's paired t-test). References 1. Ingman F, Ringbom A. Spectrophotometric determination of small amounts of magnesium and calcium employing calmagite. The use of pre-heparunized syringes for blood gas analysis representsboth a rapid and convenient blood-collection method. One type of collection syringe widely used in both U.sure but contains 0.5 mL of sodium heparin solution, any excess of which must be expelled before the sampling. The danger of not expelling excess heparin or not collecting the prescribed …