Article pubs.acs.org/ac
Diode Laser Thermal Vaporization Inductively Coupled Plasma Mass Spectrometry Pavla Foltynová,
†
Viktor Kanický,
†
and Jan Preisler*,
†
†
Central European Institute of Technology, CEITEC MU and Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic ABSTRACT: An approach of sample introduction for inductively coupled mass spectrometry (ICPMS), diode laser thermal vaporization (DLTV) is described. The method allows quantitative determination of metals in submicroliter volumes of liquid samples. Laser power is sufficient to induce pyrolysis of a suitable substrate with the deposited sample leading to aerosol generation. Unlike existing sample introduction systems based on laser ablation, it uses a NIR diode laser rather than an expensive high-energy pulsed laser. For certain elements, this sample introduction technique may serve as an alternative to solution analysis with conventional nebulizers. Using a prearranged calibration set, DLTV ICPMS provides rapid and reproducible sample analysis (RSD ∼ 10%). Sample preparation is fast and simple, and the prepared samples can easily be archived and transported. The limits of detection for Co, Ni, Zn, Mo, Cd, Sn, and Pb deposited on the preprinted paper were found to be in the range of 0.4−30 pg. The method was characterized, optimized, and applied to the determination of Co in a drug preparation, Pb in whole blood, and Sn in food samples without any sample pretreatment.
Inductively coupled plasma mass spectrometry (ICPMS) is a sensitive tool for the elemental analysis of liquid, solid, and gaseous samples. The sample introduction is the critical step in ICPMS; routinely used sample introduction techniques1 include many types of nebulizers, laser ablation (LA), and direct introduction for liquid, solid, and gaseous samples, respectively. The combination of pneumatic nebulizer and spray chambers is primarily used in ICPMS because of its simplicity and low cost; a detailed overview devoted to basic types of pneumatic nebulizers and spray chambers was published by Sharp.2 However, these setups are characterized by low analyte transport efficiency (typically 1−6%) and high sample consumption (typically 0.5−2 mL min−1) or a high matrix load of the ICP discharge which is acceptable for ICPAES but not for ICPMS. Micronebulizers working at extremely low liquid flow rates and with high efficiencies were designed primarily for the coupling of microcolumn separation techniques to ICPMS.3−12 Transport efficiencies of some micronebulizers reach up to 100% at flow rates ∼μL min−1. Nevertheless, these nebulizers are very prone to clogging or stalling, glass nebulizers might break, and their cost is high compared to conventional nebulizers. Common nebulizers also suffer from a memory effect; this effect may be reduced by choosing a suitable type of nebulizer or by adding a diluent.10,13 The micronebulizers available for the introduction of liquid microsamples into the ICP were reviewed by Todoli and Mermet;14 the review also includes studies of aerosol generation and transport and a selection guide for a micronebulizer and aerosol transport device for a given application. © 2012 American Chemical Society
Sample introduction for analysis of liquid samples without the use of nebulizers are electrothermal vaporization (ETV)15,16 and substrate-assisted laser desorption (SALD).17 In ETV ICPMS, a small amount of sample is placed in a conducting reservoir, which is subsequently heated resistively in order to vaporize the sample. The formed sample aerosol is transported into the ICP by a carrier gas. In addition to analysis of liquid samples, ETV may also be used for other types of samples: conductive or nonconductive powders, pellets, or slurries.18 However, variable transport efficiency was observed for elements of different volatilities or present in different matrices, which leads to very strong matrix effects in ETV ICPMS measurements. The calibration is often difficult due to the occurrence of matrix effects and also the sample inhomogeneity which, especially with small sample quantities, can cause larger uncertainties. ETV ICPMS allows off-line coupling with a microcolumn separation method. Sample introduction by laser ablation is most frequently employed for local microanalysis and for direct analysis of bulk solid samples such as steels, glasses, ceramics, and alloys. Movement of the ablation cell during the laser ablation also allows elemental distribution mapping in biological tissue sections and thin metal layer profiling. The quantification in LA ICPMS is difficult because of the variable sample matrix and its different ablation rate and because of elemental fractionation.19 Recently presented sample introduction for liquid samples SALD ICPMS20 allows fast analysis of submicroliter-volume Received: November 1, 2011 Accepted: January 18, 2012 Published: January 18, 2012 2268
dx.doi.org/10.1021/ac202884m | Anal. Chem. 2012, 84, 2268−2274
Analytical Chemistry
Article
as certified reference materials in 2% nitric acid solutions (Analytika, Czech Republic) were used as analytes for ICPMS. Suspension of near-infrared dyes dimethyl{4-[1,5,5-tris(4dimethylaminophenyl)-2,4-pentadienylidene]-2,5-cyclohexadien-1-ylidene}ammonium perchlorate (IR800); 2-[2-[2chloro-3-[2-(1,3-dihydro-1,1,3-trimethyl-2H-benzo[e]-indol-2ylidene)ethylidene]-1-cyclohexen-1-yl]-ethenyl]-1,1,3-trimethyl-1H-benzo[e]indolium perchlorate (IR813); 1,1′-diethyl-4,4′dicarbocyanine iodide (IR814) from Sigma-Aldrich, Germany, commercial printer black ink (HP CB316E, Hewlett-Packard, Ireland) in 50% ethanol (Lachema, Czech Republic) were used as absorber. Potassium chlorate (Lachema) was used as additive. Cyanocobalamin preparation (Vitamin B12 Leciva 300 MCG INJ SOL 5 × 1 ML/300RG) was obtained from Zentiva, Czech Republic. Human blood (BCR-634, SigmaAldrich, Czech Republic) has been certified by The Community Bureau of Reference. All chemicals were analytical-reagent grade except the printer black ink and cyanocobalamin. Sample Preparation and Experimental Arrangement. Defined submicroliter volumes of samples are deposited on a carrier with an absorber. Several experimental arrangements were tested (Figure 1).
samples. A well-defined volume of sample is deposited on an absorbing substrate, desorbed with pulses of 213-nm laser at a relatively low laser power density (
