Matrix Suppression Effects Research Articles

Is there a ‘matrix suppression effect’ under fast‐atom bombardment liquid secondary ion mass spectrometry of ionic surfactants in glycerol?

AbstractSome features of a ‘matrix suppression effect’ caused by ionic surface‐active compounds under fast‐atom bombardment (FAB) liquid secondary ion mass spectrometry (LSIMS) are being revised. It is shown that abundant transfer of the glycerol matrix molecules to the gas phase does occur under FAB‐LSIMS of ionic surfactants, contrary to popular belief. This process can be obscure because of the dependence of the charge state of the glycerol‐containing cluster ions on the type of ionic surfactant. It is revealed that, while glycerol matrix signals are really completely suppressed in the positive ion mass spectra of cationic surfactants (decamethoxinum, aethonium), abundant deprotonated glycerol and glycerol‐anion clusters are recorded in the negative ion mode. In the case of an anionic surfactant (sodium dodecyl sulfate), on the contrary, glycerol is completely suppressed in the negative ion mode, but is present in the protonated and cationized forms in the positive ion mass spectra. It is suggested that such patterns of positive and negative ion FAB‐LSIMS spectra of ionic surfactants solutions reflect the structure and composition of the electric double layer formed at the vacuum‐liquid interface by organic cations or anions and their counterions. Processes leading to the formation of the glycerol‐containing ions preferentially of positive or negative charge are discussed. The most obvious of them is efficient binding of glycerol to inorganic counterions of the salts Cl− or Na+, which is confirmed by data from quantum chemical calculations. The high content of the counterions and relatively small content of glycerol in the sputtered zone may be responsible for the charge‐selective suppression of neat glycerol clusters of opposite charge to the counterions. In the case of a mixture of cationic and anionic surfactants the substitution of inorganic counterions by organic ones was observed. The dependence of the exchange rate in the surface layer is not a linear function of the bulk solution concentration, and an effect of abrupt recharging of the surface can be registered. No both positively or negatively charged pure glycerol and glycerol‐inorganic counterion clusters are recorded for the mixture. Correlations between the mass spectrometric observations and some phenomena of surface and colloid chemistry and physics are discussed. Copyright © 2007 John Wiley & Sons, Ltd.

Capillary HPLC–ICP MS mapping of selenocompounds in spots obtained from the 2-D gel electrophoresis of the water-soluble protein fraction of selenized yeast

A method based on ICP collision-cell MS detection in capillary HPLC was developed to gain an insight into the purity and identity of selenium-containing proteins separated by 1-D and 2-D electrophoresis. The bands and spots obtained after the separation of water-soluble proteins in selenized yeast were digested with trypsin prior to chromatography. Selenium could be detected down to the subpicogram level. The method, assisted by information obtained by MALDI TOF MS on the 5000 Da cut-off fraction, permitted the purity of bands and spots to be estimated and the efficiency of tryptic digestion and the quantity of selenium present in individual peptides to be evaluated. Owing to the high sensitivity and the lack of matrix suppression effects, the method provided chromatograms with signal-to-noise ratios of 10-1000 in conditions where the common ES Q-TOF MS detection failed.

Direct determination of rare earth impurities in high purity erbium oxide dissolved in nitric acid by inductively coupled plasma mass spectrometry

A novel method was developed for the direct determination of trace quantities of rare earth elements (REEs) in high purity erbium oxide dissolved in nitric acid by inductively coupled plasma mass spectrometry (ICP-MS) in this work. The mass spectra overlap interferences arose from Er matrix on the neighbouring and monoisotopic analytes of 165Ho(100) and 169Tm(100) were eliminated by adjusting instrumental peak resolution value from 0.7 to 0.3 amu. The matrix suppression effect of Er on the ion peak signals of REEs impurities was effectively compensated with spiking In as internal standard element. The limit of quantitation (LOQ) of REEs impurities was from 0.0090 to 0.025 μg g −1, the recoveries of spiked sample for REEs were found to be in the range of 90.3–107% through using the proposed method and relative standard deviation (R.S.D.) varied between 2.5% and 6.7%. The novel methodology had been found to be suitable for the direct determination of trace REEs impurities in 99.999–99.9999% high purity Er 2O 3 and the results obtained from this method keep in good agreement with that acquired from high resolution ICP-MS.

High‐throughput biological sample analysis using on‐line turbulent flow extraction combined with monolithic column liquid chromatography/tandem mass spectrometry

A high-throughput liquid chromatography/tandem mass spectrometry (LC/MS/MS) method, which combines on-line sample extraction through turbulent flow chromatography with a monolithic column separation, has been developed for direct injection analysis of drugs and metabolites in human plasma samples. By coupling a monolithic column into the system as the analytical column, the method enables running 'dual-column' extraction and chromatography at higher flow rates, thus significantly reducing the time required for the transfer and mixing of extracted fraction onto the separation column as well as the time for gradient separation. A strategy of assessing and reducing the matrix suppression effect on the on-line extraction LC/MS/MS has also been discussed. Experiments for evaluating the resolution, peak shape, sensitivity, speed, and matrix effect were conducted with dextromethorphan and its metabolite dextrorphan as model compounds in human plasma matrix. It was demonstrated that the total run time for this assay with a baseline separation of two analytes is less than 1.5 min.

Increase of the LC–MS/MS sensitivity and detection limits using on-line sample preparation with large volume plasma injection

Large volume injection (LVI) has systematically been studied to improve LC-MS/MS sensitivity (signal-to-noise ratio, or S/N) and detection limits. The method of LVI was combined with on-line solid phase extraction (on-line SPE) and LC-MS/MS detection for analysis of compounds directly in plasma. It was demonstrated that LVI of plasma with on-line SPE-LC-MS/MS allows for improvement of sensitivity and detection limits without compromising chromatographic peak shape and resolution and inducing significant matrix and signal suppression effects. Furthermore, sensitivity and detection limits improve linearly with the injection volume up to 100 microL. Quantification of the model compounds in plasma demonstrated comparable calibration curve statistics, precision and accuracy for 5, 50 and 100 microL plasma injections.

MALDI-TOF MS Analysis of Urinary Nucleosides

As RNA turnover seems to be impaired in cancer patients, modified nucleosides have been evaluated as potential tumor markers. Modified nucleosides are mainly formed post-transcriptionally in tRNA, set free during RNA metabolism, and excreted in urine. Especially methylated nucleosides play an important role, as their levels are higher in urine from cancer patients. For structural elucidation of known and unknown nucleosides from urine samples of cancer patients, MALDI-TOF MS and MALDI-PSD were used for the first time. This technique generally ensures high sensitivity, mass resolution, and accuracy. In our analytical approach we prepurified nucleosides from urine by affinity chromatography and subsequently separated them by semipreparative high performance liquid chromatography. The different fractions were collected separately and analyzed by MALDI-TOF MS and PSD-MALDI using a mixture of six low molecular weight calibrants for internal or external calibration. The molecular totals formulas based on a mass accuracy of 10 ppm and below were calculated and a systematic data base search was performed. The inherent problem of the MALDI-technique, the reduced sensitivity for low molecular weight substances caused by matrix suppression effects, was reduced by our technique. We identified several nucleosides in urine, which were previously identified via retention times and UV spectra of standards after HPLC analysis. Eight further nucleosides were observed. This work demonstrates for the first time the potential of MALDI-TOF and PSD-MALDI in combination with semipreparative HPLC for assignment of nucleosides in urine. The particularly high mass accuracy of this mass spectrometric method provides opportunities for identifying unknown compounds.

Determination of talinolol in human plasma using automated on-line solid phase extraction combined with atmospheric pressure chemical ionization tandem mass spectrometry

A specific LC–MS/MS assay was developed for the automated determination of talinolol in human plasma, using on-line solid phase extraction system (prospekt 2) combined with atmospheric pressure chemical ionization (APCI) tandem mass spectrometry. The method involved simple precipitation of plasma proteins with perchloric acid (contained propranolol) as the internal standard (IS) and injection of the supernatant onto a C 8 End Capped (10 mm × 2 mm) cartridge without any evaporation step. Using the back-flush mode, the analytes were transferred onto an analytical column (XTerra C 18, 50 mm × 4.6 mm) for chromatographic separation and mass spectrometry detection. One of the particularities of the assay is that the SPE cartridge is used as a column switching device and not as an SPE cartridge. Therefore, the same SPE cartridge could be used more than 28 times, significantly reducing the analysis cost. APCI ionization was selected to overcome any potential matrix suppression effects because the analyte and IS co-eluted. The mean precision and accuracy in the concentration range 2.5–200 ng/mL was found to be 103% and 7.4%, respectively. The data was assessed from QC samples during the validation phase of the assay. The lower limit of quantification was 2.5 ng/mL, using a 250 μL plasma aliquot. The LC–MS/MS method provided the requisite selectivity, sensitivity, robustness accuracy and precision to assess pharmacokinetics of the compound in several hundred human plasma samples.

Metabolome analysis: the potential of in vivo labeling with stable isotopes for metabolite profiling

Metabolome analysis technologies are still in early development because, unlike genome, transcriptome and proteome analyses, metabolome analysis has to deal with a highly diverse range of biomolecules. Combinations of different analytical platforms are therefore required for comprehensive metabolomic studies. Each of these platforms covers only part of the metabolome. To establish multiparallel technologies, thorough standardization of each measured metabolite is required. Standardization is best achieved by addition of a specific stable isotope-labeled compound, a mass isotopomer, for each metabolite. This suggestion, at first glance, seems unrealistic because of cost and time constraints. A possible solution to this problem is discussed in this article. Saturation in vivo labeling with stable isotopes enables the biosynthesis of differentially mass-labeled metabolite mixtures, which can be exploited for highly standardized metabolite profiling by mass isotopomer ratios.

Accurate mass measurements for the confirmation of Sudan azo-dyes in hot chilli products by capillary liquid chromatography–electrospray tandem quadrupole orthogonal-acceleration time of flight mass spectrometry

The potential of capillary liquid chromatography (microLC)–quadrupole/time-of-flight mass spectrometry (Q-TOF MS) for the confirmation of Sudan I, II, III and IV azo-dyes as contaminants in hot-chilli food products was demonstrated. Using the microLC–electrospray ionization (ESI)–Q-TOF MS technique, accurate mass measurements of Sudan dyes were performed both on standard solutions and on matrices. Precision of exact mass measurements was calculated taking into account the ion statistics according to the number of ion sampled in the measurement. Accurate mass measurements by MS/MS experiments were performed to elucidate azo-dye fragmentation patterns. Selectivity of the microLC–Q-TOF MS method was assessed by evaluating matrix suppression effects by pre-column injection of blank hot chilli tomato sauce matrices. The results were compared with those obtained on a LC–triple quadrupole–MS system. Confirmation of Sudan I present in hot chilli tomato sauce samples was obtained by accurate mass measurements. In real samples trueness of exact mass measurements was estimated to be 1.6 and 4.4 ppm when calculated for hot chilli tomato sauce and hot chilli tomato with cheese sauce samples, respectively; precision was calculated around 9.5 ppm.

Small-Molecule MALDI Using the Matrix Suppression Effect To Reduce or Eliminate Matrix Background Interferences

The matrix suppression effect (MSE) can lead to high-quality MALDI mass spectra: strong analyte signals and weak or negligible matrix background peaks. Experiment and theory suggest that MSE should be widespread and, therefore, generally applicable to measurement of low molecular weight (LMW) substances. These are otherwise impractical with MALDI due to interference from matrix. Appropriate conditions for MSE were investigated and tested on a variety of LMW substances. Straightforward and semiautomated interpretation was possible for 87.7% of these. Another 3.5% gave poor MSE due to sodium cationization rather than protonation of the analyte, but interpretation was possible. MALDI imaging shows that MSE varies significantly across a typical sample. Selective data accumulation could further increase the utility of the method. Samples containing more than one analyte were also studied. Analyte-analyte suppression was not found to be excessive, and moderately abundant minority species can be adequately detected.

Determination of triforine using high-performance liquid chromatography with tandem mass spectrometry.

A method for determination of triforine using high-performance liquid chromatography with electrospray tandem mass spectrometry was developed. A simple ethyl acetate extraction with solvent exchange into water/methanol was used for sample preparation. The method was validated at 0.01 and 0.05 mg kg(-1) levels in apple, tomato and tinned blackcurrants. Recoveries were in the range 56.6-99.8% and no matrix suppression or enhancement effects were observed.

Ultra-fast quantitative bioanalysis of a pharmaceutical compound using liquid chromatography-tandem mass spectrometry

This paper describes the ultra-fast determination of a pharmaceutical compound using TurboIonSpray™ LC-MS-MS on an API 4000 mass spectrometer. Sample preparation consisted of plasma protein precipitation, centrifugation and dilution of the supernatant. The use of small analytical column dimensions ( 2.1 mm×10 mm) and high eluent flow rates (up to 2.2 ml/min) in isocratic mode led to a retention time of 9 s. A sample-to-sample cycle time of only 10 s was achieved by coupling two autosamplers. Partial separation of the drug and its main metabolite could be obtained. The d5-labeled drug used as internal standard compensated for matrix suppression effects. The assay was linear in the concentration range 1–1000 ng/ml, using standards prepared in human plasma. Inter-assay accuracy and precision were 98.5 and 6.2%, respectively. Mean intra-assay accuracy and precision calculated from quality control (QC) samples in human, rat and dog plasma at 3, 30 and 800 ng/ml were 100.8 and 3.8%, respectively. The ultra-fast LC-MS-MS method was successfully cross-validated against a commonly used column-switching LC-MS-MS assay with 2.3 min run time by analyzing real study samples.

Evaluation of Pb-Spec ® for flow-injection solid-phase extraction preconcentration for the determination of trace lead in water and wine by flame atomic absorption spectrometry

Two manifold designs were evaluated. Water samples and wine digests in 10% nitric acid were pumped through a column containing a commercially available resin (Pb-Spec ®), an immobilized crown ether with a cavity size selective for Pb 2+. The column was rinsed with 2% HNO 3 and the eluent, 0.1 mol l −1 ammonium oxalate was injected via a six-port rotary valve. The eluted lead was delivered to the flame atomic absorption spectrometer at 4.0 ml min −1. The following flow-injection (FI) parameters were optimized: sample acidity and volume, loading and elution flow rates, and eluent composition and volume. The detection limit for the water samples, estimated from the noise on the signal obtained for 250 ml of 10 μg l −1 loaded at 19.1 ml min −1 was 1 μg l −1. For 50 ml of wine digest loaded at 4 ml min −1, the value was 3 μg l −1. The roles of loading flow rate and sample volume were investigated in detail. The variation in retention efficiency with loading flow rate showed that the amount of lead retained (during a fixed loading time) increased with flow rate until the upper performance limit of the peristaltic pump was reached. The variation of detection limit with sample volume followed the expected hyperbolic relationship and showed that only small improvements in LOD were obtained for volumes greater than 50 ml. The method was evaluated through spike recovery for both water and wine. The lead contents of tap (0.24 μg l −1), pond (0.40 μg l −1), and river waters (not detected) were determined. The concentrations of lead in three Port wine samples ranged from not detected to 190 μg l −1. No significant matrix suppression effects were observed.

A quantitative model of ultraviolet matrix-assisted laser desorption/ionization including analyte ion generation.

A quantitative model of ionization in ultraviolet matrix-assisted laser desorption/ionization (Knochenmuss, R. J. Mass Spectrom. 2002, 37, 867) is extended to include secondary ion-molecule reactions. Matrix-to-analyte charge-transfer reaction kinetics are described by a hard-sphere Arrhenius expression. The activation energy is derived from the reaction exoergicity using a nonlinear free energy relationship. The approach is applied to the specific case of proton-transfer reactions. With no adjustable parameters, the model correctly predicts the existence and characteristics of the matrix and analyte suppression effects, the shapes of the two-pulse time-delayed yield curves, and the dependence of analyte yields on laser fluence, molecular weight, relative concentrations, and reaction exoergicity.

Determination of Pt, Pd, Ru and Ir in geological samples by ID-ICP-MS using sodium peroxide fusion and Te co-precipitation

A method is proposed for the determination of Pt, Pd, Ru and Ir in geological samples by isotope dilution-inductively coupled plasma mass spectrometry (ICP-MS). Mono-isotopic elements Rh and Au are determined by external calibration using Lu as the internal standard. Sodium peroxide fusion in corundum crucibles (instead of glassy carbon or zirconium crucibles) is used to decompose the samples. After dissolution of the fused cake in water, the solution is spiked with enriched stable isotopes of Pt, Pd, Ru and Ir, and then acidified with HCl. This solution is then evaporated to dryness promoting the formation of anhydrous silicate from colloidal material. PGEs and Au are then pre-concentrated by Te co-precipitation. The reagents HCl and SnCl2 are purified by Te co-precipitation. PGE blank levels are thus only derived from the Na2O2 which has PGE concentrations of <0.02 ng/g. Lutetium is added as the internal standard to correct for instrument drift and matrix suppression effects for the determination of the mono-isotopic elements. Method detection limits range from 0.02 to 0.3 ng/g. The results obtained for the CCRMP (CANMET, Ottawa, Canada) certified reference materials WGB-1 (gabbro), TDB-1 (diabase), UMT-1 (ultramafic ore tailings), WPR-1 (altered peridotite), WMG-1 (mineralized gabbro), and WMS-1 (massive sulphide) are in good agreement with certified values.

Application of column-switching liquid chromatography-tandem mass spectrometry for the determination of pharmaceutical compounds in tissue samples

Information on plasma-tissue distribution which is important for drug development may be obtained by “in silico” prediction tools. To support the validation of computer models, drug concentrations in rat plasma and tissues (brain, liver, kidney, testes, spleen, gut, lung, heart, muscle, skin and fat) had to be determined. In our work, we established analytical assays for a variety of substances including nicardipine, nitrendipine, felodipine and benzodiazepines. Sample preparation had to be simple and method development as well as analytical run time short to allow a high sample throughput and to minimize resources. Column-switching HPLC after homogenization and protein precipitation served as an efficient, easy and rapid sample preparation method, followed by selective MS–MS detection. Optimization of the trapping procedure was performed in order to reduce the influence of endogenous interferences and to obtain good recovery. Chromatographic separation was necessary to increase the selectivity. The use of small analytical column dimensions (2.1×10 mm) was investigated to achieve higher sample throughput without compromising the assay quality. Mass spectrometric parameters, such as ionization modes (positive vs. negative) and ion source types (TurboIonSpray vs. APCI) were screened to find suitable conditions for sensitive analysis of the compounds. Matrix suppression effects were taken into consideration. Calibration samples were prepared in plasma only, whereas quality control samples were prepared in both plasma and tissues to save animals and time. Accuracy and precision were in the range of 84.4–119.1% and 1–16.5%, respectively. Limits of quantification were in the range of 0.5–2.5 ng/ml for plasma and 2–10 ng/ml for tissues. Run times as short as 2.2 min could be achieved.

Determination of rare earth impurities in high purity gadolinium oxide by inductively coupled plasma mass spectrometry after 2-ethylhexylhydrogen-ethylhexy phosphonate extraction chromatographic separation

A method was developed for the determination of rare earth impurities in high purity Gd2O3 by inductively coupled plasma mass spectrometry (ICP-MS). The matrix suppression effect of Gd2O3 on signals of rare earth impurities was compensated for by Re internal standardization. The spectra overlap interferences from GdH, GdO, GdOHn (n=1–3) on Tb, Tm, Yb and Lu were eliminated by 2-thylhexylhydrogen-2-ethylhexy phosphonate (EHEHP) extraction chromatographic separation. The detection limits for REEs were 0.005–0.017 ng ml−1 in solution and 0.002–0.05 μg g−1 in solid. Recoveries of spiked sample for REEs were from 88 to 121% with the precision of 1.0–7.5% RSD. Determination of trace REEs in two Gd2O3 samples were performed. The method can be applied to analysis of 99.99–99.9999% high purity Gd2O3.

The matrix suppression effect in matrix-assisted laser desorption/ionization: application to negative ions and further characteristics

As recently shown for positive ions (Rapid Commun. Mass Spectrom. 10, 871 (1996)), small to moderate-sized analyte ions (<20 000 Da) can fully suppress all matrix signals in matrix-assisted laser desorption/ionization (MALDI) mass spectra at appropriate matrix:analyte mixing ratios. The technique is useful for removing matrix interferences and led to a model for primary ion formation mechanisms. We here extend the method to negative ions, and investigate the effects of analyte type and size on the matrix suppression concentration range. For a given matrix/analyte pair, suppression was observed in either positive- or negative-ion mode, but not both. Some matrices do not show the effect at all, which is attributed to unfavourable thermodynamics. The analyte concentration where suppression appears is correlated with the analyte molecular weight and supports the proximity requirement of the model. The dependence of matrix signals on analyte concentration is very similar in both modes, and may have implications for the role of plume reactions at low analyte concentrations. Delayed extraction was found to extend matrix suppression to lower analyte concentrations. Lack of homogenous matrix/analyte co-crystallization at low analyte concentrations is shown to prevent the appearance of matrix suppression or to modify the shape of the concentration curves in some cases. Evidence is presented for the activity of electronic excitations in IR as well as in UV MALDI. © 1998 John Wiley & Sons, Ltd.

Influence of sample matrix components on the selection of calibration strategies in electrothermal vaporization inductively coupled plasma mass spectrometry

Quantification of both digested and slurry samples were studied using ultrasonic slurry electrothermal vaporization inductively coupled plasma mass spectrometry (USS-ETV-ICP-MS). The results of external calibration using aqueous standards, method of additions, and In as an internal standard were compared. The elements studied include: Mn, Ni and Cu and the materials analyzed include: NIST SRM 1548 total diet and SRM 1549 milk powder. Palladium was used as a physical carrier and oxygen ashing was used to remove the organic part of the slurry matrix. Different degrees of matrix suppression effects were observed when different skimmer cones were employed. Aging of the skimmer cone and consequent loss of its original circular symmetry and decrease in orifice size resulted in differences in sampling of the ion beam and changes in the degree of matrix effects were observed as the skimmer cone was rotated. The presence of matrix suppression effects is evidenced by strong suppressions in the Ar 2, C and analyte signals. When matrix suppression effects were present, the method of external calibration provided low recoveries (average accuracy 73 ± 12%), therefore it was necessary to use the method of additions to compensate for these problems, providing an average accuracy of 108 ± 13%. When matrix effects were absent, the external calibration method resulted in an average accuracy of 101 ± 16%.

The Matrix Suppression Effect and Ionization Mechanisms in Matrix-assisted Laser Desorption/Ionization

At appropriate matrix:analyte mixing ratios, small to moderate sized analyte ions (1000–20 000 u) can fully suppress positively charged matrix ions in matrix-assisted laser desorption/ionization (MALDI) mass spectra. This is true for all matrix species, including radical cations and adducts with protons or alkali-metal ions. Full matrix suppression is also observed, regardless of the preferred analyte ion form, be it protonated or an alkali adduct. These facts lead us to propose a mechanism for prompt, primary (not secondary gas-phase) MALDI ionization in which excited matrix molecules are the key species. At least two such excited molecules are believed to be necessary for free ion generation. This model is found to be consistent with the available data, as well as making several predictions which are confirmed by new observations. The model also predicts that the matrix suppression effect will not be observable with heavy analytes because their large excluded volume precludes desorption at the necessary mixing ratios.