HPLC-inductively coupled plasma

Encinar, J. R., Schaumloffel, D., Ogra, Y., and Lobinski, R., Determination of selenomethionine and selenocysteine in human serum using speciated isotope dilution-capillary HPLC-inductively coupled plasma collision cell mass spectrometry, Analytical Chemistry 76(22), 6635-6642, 2004. 

An independent method should be used to verify the results of routine analysis. If the results of both methods are in good agreement, it can be concluded that the results of the routine analysis are unlikely to be affected by a contribution of a systematic nature (e.g. insufficient extraction). This conclusion is stronger when the two methods differ widely, such as derivatiza-tion/gas chromatography (GC)/atomic absorption spectrometry (AAS) and HPLC/inductively coupled plasma mass spectrometry (ICP-MS). 

A number of very useful and practical element selective detectors are covered, as these have already been interfaced with both HPLC and/or FIA for trace metal analysis and spe-ciation. Some approaches to metal speciation discussed here include HPLC-inductively coupled plasma emission, HPLC-direct current plasma emission, and HPLC-microwave induced plasma emission spectroscopy. Most of the remaining detection devices and approaches covered utilize light as part of the overall detection process. Usually, a distinct derivative of the starting analyte is generated, and that new derivative is then detected in a variety of ways. These include HPLC-photoionization detection, HPLC-photoelectro-chemical detection, HPLC-photoconductivity detection, and HPLC-photolysis-electrochemical detection. Mechanisms, instrumentation, details of interfacing with HPLC, detector operations, as well as specific applications for each HPLC-detector case are presented and discussed. Finally, some suggestions are provided for possible future developments and advances in detection methods and instrumentation for both HPLC and FIA. 

The most common detectors in HPLC are ultraviolet, fluorescence, electrochemical detector and diffractometer. However, despite all improvements of these techniques it seems necessary to have a more selectivity and sensitivity detector for the purposes of the medical analysis. It should be therefore improvements to couple analytical techniques like infrared IR, MS, nuclear magnetic resonance (NMR), inductively coupled plasma-MS (ICP-MS) or biospecific detectors to the LC-system and many efforts have been made in this field. 

In modern times, most analyses are performed on an analytical instrument for, e.g., gas chromatography (GC), high-performance liquid chromatography (HPLC), ultra-violet/visible (UV) or infrared (IR) spectrophotometry, atomic absorption spectrometry, inductively coupled plasma mass spectrometry (ICP-MS), mass spectrometry. Each of these instruments has a limitation on the amount of an analyte that they can detect. This limitation can be expressed as the IDL, which may be defined as the smallest amount of an analyte that can be reliably detected or differentiated from the background on an instrument. 

An inductively coupled plasma formed by passing argon through a quartz torch is widely used for the mass spectroscopic analysis of metal compounds separated by online HPLC.6 Samples are nebulized on introduction into the interface. Plasma impact evaporates solvent, and atomizes and ionizes the analyte. Applications include separation of organoarsenic compounds on ion-pairing F4PLC and vanadium species on cation exchange. 

The interaction between Ag+ and selenium in the bloodstream has been studied in vitro by means of the HPLC-inductively coupled argon plasma-mass spectrometry (ICPMS) method. The metal ions and selenide form the unit complex (AgSe) and then this unit binds to selenoprotein to form the ternary complex [(AgSe)ra] selenoprotein in the bloodstream.1042... 

Atomic techniques such as atomic absorption spectrometry (AA), inductively coupled plasma-optical emission spectrometry (ICP-OES), and inductively coupled plasma-mass spectrometry (ICP-MS), have been widely used in the pharmaceutical industry for metal analysis.190-192 A content uniformity analysis of a calcium salt API tablet formulation by ICP-AES exhibited significantly improved efficiency and fast analysis time (1 min per sample) compared to an HPLC method.193... 

Five different detectors are common in HPLC (1) UV, (2) refractive index (RI), (3) conductivity, (4) inductively coupled plasma atomic emission, and (5) mass spectrometry. A UV detector passes a specific wavelength of UV light... 

Sample preparation for analysis by hyphenated methods requires some additional planning when compared to nonhyphenated methods. All steps, extraction, concentration, and final solvent selection must take into consideration and be compatible with all the components of the hyphenated instrumentation. For gas chromatographic methods, all the components in the mixture must be in the gaseous state. For liquid chromatography (LC) or high-performance liquid chromatography (HPLC), the samples of the analytes of interest can be solids or liquids, neutral or charged molecules, or ions, but they must be in solution. If the follow-on analysis is by MS, then each of the analytes may require a different method of introduction into the MS. Metals and metal ions may be introduced by HPLC if they are in solution but commonly are introduced via AAS or inductively coupled plasma (ICP). Other analytes may be directly introduced from HPLC to MS [2],... 

Determination of iodo amino acids by HPLC with inductively coupled plasma (ICP)-MS detection had LOD 35-130 pg of I, which is about one order of magnitude lower than with UVD usually applied for these compounds175. Amino acids and peptides containing sulfur, such as cysteine, cystine, methionine and glutathione, can be determined after HPLC separation by pulsed electrochemical detection, using gold electrodes176. 

Because ICP-MS with different instrumentations and sample introduction systems (besides solution nebulization, also laser ablation or hyphenated methods, such as HPLC, CE, SPME) is today the most frequently used analytical technique for precise and accurate isotope ratio measurements, the following section will mainly focus on this form of mass spectrometry with an inductively coupled plasma source. 

AAS = atomic absorption spectroscopy HPLC = high performance liquid chromatography ICP/AES = inductively coupled plasma atomic emission spectroscopy... 

Jensen et al. [125] investigated an HPLC/ICP-MS (inductively coupled plasma mass spectrometry) with sulfur-specific detection, as a method for obtaining metabolite profiles for omeprazole administered as a 1 1... 

HPLC units have been interfaced with a wide range of detection techniques (e.g. spectrophotometry, fluorimetry, refractive index measurement, voltammetry and conductance) but most of them only provide elution rate information. As with other forms of chromatography, for component identification, the retention parameters have to be compared with the behaviour of known chemical species. For organo-metallic species element-specific detectors (such as spectrometers which measure atomic absorption, atomic emission and atomic fluorescence) have proved quite useful. The state-of-the-art HPLC detection system is an inductively coupled plasma/MS unit. HPLC applications (in speciation studies) include determination of metal alkyls and aryls in oils, separation of soluble species of higher molecular weight, and separation of As111, Asv, mono-, di- and trimethyl arsonic acids. There are also procedures for separating mixtures of oxyanions of N, S or P. 

Figure 6.1 Bar-graph of MeHg in CRM 580. The results correspond to six replicate determinations as performed by different laboratories using various methods. MEANS indicates the mean of laboratory means with 95% confidence interval. Abbreviations-. CVAAS, cold vapour atomic absorption spectrometry CVAFS, cold vapour atomic fluorescence spectrometry ECD, electron capture detection GC, gas chromatography HPLC, high-performance liquid chromatography ICPMS, inductively coupled plasma mass spectrometry MIP, microwave induced plasma atomic emission spectrometry QFAAS, quartz furnace atomic absorption spectrometry SFE, supercritical fluid extraction.

A direct injection nebuliser (DIN) was used to interface LC with ICP-MS (Shum et al., 1992a). The DIN transferred all of the sample into the inductively coupled plasma. Microscale LC separations in small packed columns were studied because the column flow rates of about 30 ml min 1 were compatible with the DIN. The low dead volume (less than 1 ml) of the interface prevented excessive band broadening. Eluents containing up to 85% methanol were accommodated. The analyte signal varied by about 20% as the eluent changed from 20% to 80% methanol in water. Detection limits for arsenic and tin species using the HPLC-DIN-ICP-MS system were 0.2-0.6 and 8-10pg, respectively. 

GC, gas chromatography HPLC, high-performance liquid chromatography MS, mass spectroscopy AA, atomic absorption GFAA, graphite furnace atomic absorption ICP, inductively coupled plasma UV-VIS, ultraviolet-visible molecular absorption spectroscopy IC, ion chromatography. 

Figure 4 Schematic diagram of high-pressure liquid chromatography inductively coupled plasma mass spectrometry (HPLC-ICP-MS) system with dual-inlet system for isotope dilution analyses. (From Ref. 32.)...

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