Self-aliquoting micro-grooves in combination with laser ablation-ICP-mass spectrometry for the analysis of challenging liquids: quantification of lead in whole blood

We present a technique for the fast screening of the lead concentration in whole blood samples using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The whole blood sample is deposited on a polymeric surface and wiped across a set of micro-grooves previously engraved into the surface. The engraving of the micro-grooves was accomplished with the same laser system used for LA-ICP-MS analysis. In each groove, a part of the liquid blood is trapped, and thus, the sample is divided into sub-aliquots. These aliquots dry quasi instantly and are then investigated by means of LA-ICP-MS. For quantification, external calibration against aqueous standard solutions was relied on, with iron as an internal standard to account for varying volumes of the sample aliquots. The 208Pb/57Fe nuclide ratio used for quantification was obtained via a data treatment protocol so far only used in the context of isotope ratio determination involving transient signals. The method presented here was shown to provide reliable results for Recipe ClinChek® Whole Blood Control levels I–III (nos. 8840–8842), with a repeatability of typically 3 % relative standard deviation (n = 6, for Pb at 442 μg L−1). Spiked and non-spiked real whole blood was analysed as well, and the results were compared with those obtained via dilution and sectorfield ICP-MS. A good agreement between both methods was observed. The detection limit (3 s) for lead in whole blood was established to be 10 μg L−1 for the laser ablation method presented here. Graphical Abstract Micro-grooves are filled with whole blood, dried, and analyzed by laser ablation ICP-mass spectrometry. Notice that the laser moves in perpendicular direction with regard to the micro-grooves Electronic supplementary material The online version of this article (doi:10.1007/s00216-016-9717-3) contains supplementary material, which is available to authorized users.

Quantification of the lead concentration in spiked whole blood samples using conventional nebulizer-based ICP-MS.

Sample preparation
Whole blood samples (either native, or spiked with increasing amounts of lead) were diluted 100 times using 1% v/v nitric acid. All dilutions were done gravimetrically. The samples were homogenized thoroughly, and each sample was divided in five sub-samples which were spiked with adequate amounts of lead to allow for standard addition quantification. At this stage, indium was added as internal standard to all samples with a final concentration of 2 ng mL -1 . Samples were analysed immediately after dilution. Recipe ClinChek® Whole Blood Control level II was reconstituted freshly, and treated as unknown sample using the same dilution and standard addition conditions.

Instrumental
Measurements were performed on an Element XR sectorfield ICP-MS (Thermo) using nickel cones and a quartz concentric nebulizer connected to a quartz cyclonic spray chamber. The instrument was operated in low resolution mode, and tuned for best sensitivity (Li, In, U) and low oxide rates (UO/U). Further instrumental details can be found in table S1. Samples were analysed in the order of increasing lead concentration. In-between each individual sample, the system was rinsed for 1 minute with 5% v/v nitric acid. After each full block of samples (i.e., all standard addition solutions corresponding to one blood sample), the system was rinsed for 5 minutes with a solution containing 5% v/v nitric acid and 0.5% v/v isopropanol. Aqueous control standards containing lead and indium as well as blank solutions were analysed periodically to monitor the performance of the instrument. No significant trend of the internal standard or the control standards over the entire measurement session was observed. Therefore, no normalization to the internal standard was carried out and signals of 208 Pb were directly used for quantification. The method detection limit was calculated from aqueous standard solutions and was found to be 0.6 ng g -1 in the undiluted whole blood.

Results
The correlation coefficient of the derived standard addition curves was > 0. 9 in all cases, and therefore the data was quantified using the approach described by Meija et al. (doi: 10.1021/ac5014749) which uses coordinate swapping to allow for a straight-forward assessment of the uncertainty of the calculated result obtained from standard addition.
For the Recipe ClinChek® Whole Blood Control level II, a good agreement with the certified value was found (certified value: 228 ng g -1 with acceptable values between 182 ng g -1 -274 ng g -1 ). All calculated data can be found in table S2.
The iron concentration in the whole blood sample was compared with the iron concentration in the Recipe CleanCheck® reference material. To this end, both samples were diluted 100 times, spiked with indium, and analysed in medium resolution mode using 57 Fe and 115 In in medium mass resolution using the detector in analog mode. Other instrumental conditions were similar to the ones summarized in table S1. The iron found in the whole blood amounts to 470 ± 60 mg L -1 .