Versatile LIBS calibration approach using metal:SPADNS/DTAB ion pairs immobilized on photographic paper

Laser-Induced Breakdown Spectroscopy (LIBS) is a powerful analytical technique that enables multielemental determinations with minimal sample preparation. However, one of the main challenges for routine quantitative analyses is the lack of suitable calibration materials due to the microsampling nature of the technique. In this study, a multielemental calibration material was developed using photographic paper as a solid substrate. This material was immersed in a buffered metal solution containing Al³⁺, Co²⁺, Cr³⁺, Cu²⁺, Mn²⁺, and Zn²⁺, a complexing agent (trisodium salt of 1,8-dihydroxy-2-(4-sulfophenylazo)-3,6-naphthalenedisulfonic acid, SPADNS) and dodecyltrimethylammonium bromide (DTAB). Metal:SPADNS/DTAB ion pairs were efficiently adsorbed onto the substrate under the optimized conditions: 1.6 × 10^-2 mol L^-1 SPADNS, 3.2 × 10^-3 mol L^-1 DTAB, phosphate buffer pH 8.0, ultrasonic bath (37 kHz, 100 % power, 30 min at 40 °C). LIBS conditions were 150 mJ laser energy, 7 accumulated laser pulses, 2.5 μs delay, and 13 μs integration time. Linear response ranges were 0.50-10.0 mg L^-1 (Al, Cr, Mn, and Zn), 1.0-10.0 mg L^-1 (Co), and 0.50-5.0 mg L^-1 (Cu), with precision ranging from 4.2 to 28 %. Using internal standard and external standard calibration strategies, the reliability of the calibration material was demonstrated by analysis of certified reference materials from different water types and blood serum, as well as Conostan® mineral oil, colored nail polishes, and printer ink, without any sample preparation. Recoveries of spiked analytes ranged from 81 % to 123 %, and relative errors of 0.40-25 % in relation to certified values, demonstrating the calibration material effectiveness for determining Al, Cr, Cu, Mn, and Zn in samples with quite different characteristics.