The Benefits of Ion–Molecule Chemistry for the Determination of Titanium in Whole Blood and Serum Using Quadrupole-Based Collision–Reaction Cell ICP-MS Technology

Nov 01, 2013
Volume 28, Issue 11

This article highlights the benefits of using ion-molecule chemistry in a quadrupole-based collision–reaction cell to determine titanium levels in the presence of problematic polyatomic spectral interferences encountered in the analysis of human blood and serum samples from patients who have undergone artificial joint replacement surgery. The study specifically focuses on the ability of ammonia gas to react with titanium in the cell to form an interference-free titanium molecular ion, which can then be used for quantitation. Detection limits and spike recovery data on serum reference materials will be presented that suggest that this methodology offers a viable alternative to double focusing, magnetic sector inductively coupled plasma–mass spectrometry technology for this analysis.

Table I: Common polyatomic spectral interferences on the stable isotopes of titanium
Titanium metal and its alloys are commonly used in metallic knee and hip replacement joints. Wear and tear over time often can cause metal particulates from the joint to contaminate the surrounding tissue. In addition, various electrochemical and corrosion processes taking place in the body can lead to serious leakage of titanium into the bloodstream. As a result, it is important to be able to monitor titanium levels in whole blood and serum in patients that have undergone, or are due to undergo, knee or hip replacement surgery. Unfortunately, the determination of titanium in these kinds of biological matrices by quadrupole-based inductively coupled plasma–mass spectrometry (ICP-MS) involves many challenges because of significant spectroscopic interferences from isobaric overlaps from calcium isotopes and other molecular ions; these interferences include ions such as PO+, SO+, CO2 +, ArC+, NO2 +, and ClC+ produced by a combination of matrix and solvent components in the sample. As a result, real-world detection capability by this technique is severely compromised (1). Table I provides a summary of these potential problematic polyatomic spectral interferences on the five titanium isotopes.

Moreover, interference-reduction techniques using collision–reaction cell technology have also proved inadequate to quantify titanium in these matrices because of the difficulty to reduce these problematic spectral interferences (2). Consequently, achievable titanium detection limits in these samples using quadrupole ICP-MS have been on the order of ≥200 µg/L (1,3). For this reason, researchers have turned their attention to magnetic sector ICP-MS to resolve the polyatomic interferences away from the analyte mass (4,5). This technique offered the potential of much lower detection limits, but in practice, proved to be somewhat problematic, because the only practical isotope available for quantitation was 47Ti (7.4% abundant) as a result of the interferences from 48Ca+ and 32S16O+ on the major isotope of 48Ti (73.7% abundant). In addition, to achieve satisfactory separation of 47Ti from the 31P16O+ interference, medium resolving power (R s = 4000) was required, which also compromised the achievable detection capability. However, the instrument detection limit (IDL) and method detection limit (MDL) values obtained in one of these studies (5) were 0.01µg/L and 0.07 µg/L, respectively, which are still significantly better than those achieved using quadrupole ICP-MS.

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