Ionic (liquid) matrices for matrix-assisted laser desorption/ionization mass spectrometry—applications and perspectives

A large number of matrix substances have been used for various applications in matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). The majority of matrices applied in ultraviolet-MALDI MS are crystalline, low molecular weight compounds. A problem encountered with many of these matrices is the formation of hot spots, which lead to inhomogeneous samples, thus leading to increased measurement times and hampering the application of MALDI MS for quantitative purposes. Recently, ionic (liquid) matrices (ILM or IM) have been introduced as a potential alternative to the classical crystalline matrices. ILM are equimolar mixtures of conventional MALDI matrix compounds such as 2,5-dihydroxybenzoic acid (DHB), α-cyano-4-hydroxycinnamic acid (CCA) or sinapinic acid (SA) together with organic bases [e.g., pyridine (Py), tributylamine (TBA) or N,N-dimethylethylenediamine (DMED)]. The present article presents a first overview of this new class of matrices. Characteristic properties of ILM, their influence on mass spectrometric parameters such as sensitivity, resolution and adduct formation and their application in the fields of proteome analysis, the measurement of low molecular weight compounds, the use of MALDI MS for quantitative purposes and in MALDI imaging will be presented. Scopes and limitations for the application of ILM are discussed.     

Ionic matrix for matrix-enhanced surface-assisted laser desorption ionization mass spectrometry imaging (ME-SALDI-MSI)

Matrix-enhanced surface-assisted laser desorption ionization mass spectrometry imaging (ME-SALDI MSI) has been previously demonstrated as a viable approach to improving MS imaging sensitivity. We describe here the employment of ionic matrices to replace conventional MALDI matrices as the coating layer with the aims of reducing analyte redistribution during sample preparation and improving matrix vacuum stability during imaging. In this study, CHCA/ANI (α-cyano-4-hydroxycinnamic acid/aniline) was deposited atop tissue samples through sublimation to eliminate redistribution of analytes of interest on the tissue surface. The resulting film was visually homogeneous under an optical microscope. Excellent vacuum stability of the ionic matrix was quantitatively compared with the conventional matrix. The subsequently improved ionization efficiency of the analytes over traditional MALDI was demonstrated. The benefits of using the ionic matrix in MS imaging were apparent in the analysis of garlic tissue sections in the ME-SALDI MSI mode.     

On the use of DHB/aniline and DHB/<Emphasis Type="Italic">N,N</Emphasis>-dimethylaniline matrices for improved detection of carbohydrates: Automated identification of oligosaccharides and quantitative analysis of sialylated glycans by MALDI-TOF mass spectrometry

This study demonstrates the application of 2,5-dihydrohybenzoic acid/aniline (DHB/An) and 2,5-dihydroxybenzoic acid/N,N-dimethylaniline (DHB/DMA) matrices for automated identification and quantitative analysis of native oligosaccharides by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Both matrices are shown to be superior to pure DHB for native glycans in terms of signal intensities of analytes and homogeneity of sample distribution throughout the crystal layer. On-target formation of stable aniline Schiff base derivatives of glycans in DHB/An and the complete absence of such products in the mass spectra acquired in DHB/DMA matrix provide a platform for automated identification of reducing oligosaccharides in the MALDI mass spectra of complex samples. The study also shows how enhanced sensitivity is achieved with the use of these matrices and how the homogeneity of deposited sample material may be exploited for quick and accurate quantitative analysis of native glycan mixtures containing neutral and sialylated oligosaccharides in the low-nanogram to mid-picogram range.     

Liquid supports for ultraviolet atmospheric pressure matrix-assisted laser desorption/ionization

Atmospheric pressure (AP) liquid matrices for ultraviolet (UV) matrix-assisted laser desorption/ionization (MALDI) are presented. Doping a known organic chromophore, alpha-cyano-4-hydroxycinnamic acid (CHCA), into liquid media yielded a homogenous sample system with simplified sample preparation, increased sample lifetime, and added utility for APMALDI ion sources. Compared with vacuum situations, AP matrices are not as limited by vapor pressure, so liquid matrix formulations can focus on desorption and ionization versus vacuum stability and source contamination. The parameters studied include chromophore concentration, liquid support variations, and quantitation capability. Chromophore concentration adjustments provided insight into the necessary absorbance for UV-APMALDI and demonstrated the importance of laser penetration depth. Liquid support variations allowed adjustments of sample lifetime and analyte solvents. Extended sample lifetime is beneficial for instrument tuning and source optimization; however, increased liquid viscosity lowers signal intensity. The shot-to-shot reproducibility, as examined with individual ion packets, suggests that the liquid matrix can alleviate some inconsistencies seen with solid MALDI, suggesting a possibility for better quantitation. The measurements for laser penetration depth, solution viscosity, and solvent additives could add to the information on MALDI mechanisms. The liquid matrix offers advantages that complement current MALDI methods.     

Ionic liquid matrix-induced metastable decay of peptides and oligonucleotides and stabilization of phospholipids in MALDI FTMS analyses

Room-temperature ionic liquid matrices (ILMs) have recently been investigated for use in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) and proven to be advantageous. Literature accounts of ILM performance for biological samples document increased sensitivity and ionization efficiency. These claims have been investigated here, and are supported for MALDI TOF applications to peptides, oligonucleotides, and phospholipids. Peptides and oligonucleotides however, do not behave in the same way when ILMs are used for MALDI FTMS. As reported here, with 3 tesla MALDI FTMS peptides and oligonucleotides fragment readily. These observations contrast with those found for MALDI time-of-flight mass spectrometry. Fragmentation is apparently slower than the time required to accelerate ions in a MALDI TOF mass spectrometer, but is readily observed by MALDI FTMS. Therefore, fragmentation of these molecules must occur on a relatively slow time scale. As trapping time is extended, increased fragmentation of peptides and oligonucleotides is seen. However, phospholipids do not fragment extensively. Furthermore, use of traditional solid matrices causes significant fragmentation for this category of compound but is suppressed by use of ILMs.     

A fast screening MALDI method for the detection of cocaine and its metabolites in hair

Matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry was used for the rapid detection of cocaine, benzoylecgonine and cocaethylene in hair. Different MALDI sample preparation procedures have been tested and the employment of a multi-layer 'graphite-sample-electrosprayed alpha-cyano-4-hydroxycinnamic acid (HCCA)' yielded the best results for standard solutions of the target analytes. The same approach was subsequently applied to hair samples that were known to contain cocaine, benzoylecgonine and cocaethylene, as determined by a classical GC-MS method. It was however necessary to extract hair samples by incubating them in methanol/trifluoroacetic acid for a short time (15 min) at 45 degrees C; 1 microl of the obtained supernatant was deposed on a metal surface treated with graphite, and HCCA was electrosprayed on it. This procedure successfully suppressed matrix peaks and was effective in detecting all the target analytes as their protonated species. The results obtained give further confirmation of the effectiveness of the MALDI for detecting drugs and their metabolites in complex biological matrices. The method can be useful as a fast screening procedure to detect the presence of cocaine and metabolites in hair samples.     

Compelling Evidence for Lucky Survivor and Gas Phase Protonation: The Unified MALDI Analyte Protonation Mechanism

This work experimentally verifies and proves the two long since postulated matrix-assisted laser desorption/ionization (MALDI) analyte protonation pathways known as the Lucky Survivor and the gas phase protonation model. Experimental differentiation between the predicted mechanisms becomes possible by the use of deuterated matrix esters as MALDI matrices, which are stable under typical sample preparation conditions and generate deuteronated reagent ions, including the deuterated and deuteronated free matrix acid, only upon laser irradiation in the MALDI process. While the generation of deuteronated analyte ions proves the gas phase protonation model, the detection of protonated analytes by application of deuterated matrix compounds without acidic hydrogens proves the survival of analytes precharged from solution in accordance with the predictions from the Lucky Survivor model. The observed ratio of the two analyte ionization processes depends on the applied experimental parameters as well as the nature of analyte and matrix. Increasing laser fluences and lower matrix proton affinities favor gas phase protonation, whereas more quantitative analyte protonation in solution and intramolecular ion stabilization leads to more Lucky Survivors. The presented results allow for a deeper understanding of the fundamental processes causing analyte ionization in MALDI and may alleviate future efforts for increasing the analyte ion yield.     

Formulation of matrix solutions for use in matrix-assisted laser desorption/ionization of biomolecules

We report a simple method for converting solid matrices into useful matrix solutions for matrix-assisted laser desorption/ionization (MALDI). This method is based on the dissolution of the solid matrix in a liquid support of low volatility such as glycerol. An appropriate solubilizing reagent was added to promote the dissolution of the matrix materials into the liquid support. Selection of the solubilizing reagent is empirically related to an acid-base relationship, i.e., an acidic solid matrix requires a basic organic compound to form a stable matrix solution in the liquid support and vice versa. A tenfold increase in the solubility can be obtained for many solid matrices when appropriate solubilizing reagents are added into the glycerol support. This solubility enhancement is tentatively attributed to the ion-pair formation in a polar nonvolatile liquid support. In addition, the hydrophobicity of the solid matrix seems to play an important role in the efficiency of the resulting matrix solution. By using glycerol as liquid support, a hydrophilic matrix, such as 2,5-dihydroxybenzoic acid (DHB), showed a substantial “peripheral effect,” in which good analyte ion signals could only be recorded at the peripheral region of the sample droplet. More hydrophobic matrices, such as α-cyano-4-hydroxycinnamic acid (α-CCA), exhibit better and more homogeneous responses at different regions of the droplets. The performance of these matrix solutions was evaluated in terms of the durability, reproducibility, sensitivity, high mass capability, and generality. A typical sample droplet can afford more than an hour of repeated sampling with excellent shot-to-shot reproducibility. A low picomole sensitivity was demonstrated using a luteinizing hormone releasing hormone (LHRH) in a Fourier transform ion cyclotron resonance mass spectrometer with a homemade external MALDI ion source. By using a commercial MALDI time-of-flight mass spectrometer, proteins with masses as high as 66,000 Da were successfully analyzed by using these matrix solutions.     

高效液相色谱-串联质谱法测定食品中曲酸

建立了食品中新型防腐剂曲酸的高效液相色谱-串联质谱的定量测定方法。动物禽肉、鱼虾甲壳类、酱菜类、水果蔬菜、面制品等固体样品经乙腈提取;酱及酱油、醋、酒、饮料、糖浆等液体样品经水稀释,乙酸锌和亚铁氰化钾沉淀蛋白;以C18柱为分离柱,流动相为乙腈和5 mmol/L乙酸铵甲酸溶液,采用电喷雾串联四极杆质谱进行检测。选择1个母离子和2个子离子进行选择反应监测,以13C6-曲酸作为内标,选择信号最强的子离子进行定量测定。固体类基质中的定量限(按信噪比(S/N)大于10计)为0.1 mg/kg;液体类基质中的定量限为2.5 mg/kg。在0.1～2.0 mg/L范围内呈现良好的线性关系,相关系数r＞0.99。各种基质在3个添加水平的平均回收率在72.6%～114%之间,相对标准偏差均小于11.4%。本方法简单实用,准确可靠,适用范围包括了食品中可能使用曲酸这种食品添加剂的大部分基质,可以满足进出口食品中曲酸的定性和定量要求。     

Ionic liquid matrices with phosphoric acid as matrix additive for the facilitated analysis of phosphopeptides by matrix-assisted laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a powerful tool for the analysis and characterization of protein phosphorylation on the peptide level. In this study, the applicability of ionic liquid matrices (ILM) formed by combination of the crystalline MALDI matrix 2,5-dihydroxybenzoic acid (DHB) with pyridine or n-butylamine was tested for the analysis of phosphopeptides. Low ionization efficiency in both positive and negative ion mode was observed in acid-free sample preparations. Upon addition of 0.1% trifluoroacetic acid (TFA), ion formation was increased, but analogously to the situation described earlier for pure DHB, best results were obtained upon use of 1% phosphoric acid as matrix additive. The samples prepared in this way were significantly more homogeneous than preparations with pure DHB, thus avoiding the need for time-consuming search for hot spots. Other characteristics like metastable fragmentation of phosphopeptides did not differ from that observed in classical preparations. The limits of detection for synthetic phosphopeptides and singly or multiply phosphorylated peptides from tryptic digests of alpha- and beta-casein were comparable with those obtained when using pure DHB; in some cases even higher signal intensities could be observed in the ILM. The use of ILM in combination with 1% phosphoric acid as matrix additive significantly facilitates analysis of phosphopeptides by MALDI-MS.     

A second‐generation ionic liquid matrix‐assisted laser desorption/ionization matrix for effective mass spectrometric analysis of biodegradable polymers

A second generation ionic liquid matrix (ILM), N,N‐diisopropylethylammonium α‐cyano‐4‐hydroxycinnamate (DEA‐CHCA), was developed for the characterization of polar biodegradable polymers. It is compared with five solid matrices typically used for the characterization of these polymers and one other new ILM. It is shown that use of the ILM, DEA‐CHCA, allows maximum signal with minimum laser intensity which minimizes polymer degradation. In these conditions, the DEA‐CHCA ILM is able to assist in the ionization of analytes in an efficient but soft manner. These qualities produce spectra that allow an accurate and sensitive determination of the number average molecular weights, weight average m.w., and polydispersity index of labile polar polymers. With such polymers, many solid matrices produce spectra showing extensive polymer degradation leading to the underestimation of molecular weights. The distribution of intact analyte peaks obtained with the ILM DEA‐CHCA allows for identification of the fine structure of complex copolymers. ILMs were much less susceptible to effects of extraction delay times on molecular weight determination than were solid matrices. The liquid nature of the matrix is an important reason for the outstanding results obtained for labile analyte polymers. No comparable results could be obtained with any known solid matrices or other ILMs. In many cases, the manufacturers' listed molecular weights and polydispersity measurements for biodegradable polymers are determined by size‐exclusion chromatography and the data obtained by that method may differ considerably from the high‐precision matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF‐MS) results presented here. Copyright © 2009 John Wiley & Sons, Ltd.     

Uncovering matrix effects on lipid analyses in MALDI imaging mass spectrometry experiments

The specific matrix used in matrix‐assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) can have an effect on the molecules ionized from a tissue sample. The sensitivity for distinct classes of biomolecules can vary when employing different MALDI matrices. Here, we compare the intensities of various lipid subclasses measured by Fourier transform ion cyclotron resonance (FT‐ICR) IMS of murine liver tissue when using 9‐aminoacridine (9AA), 5‐chloro‐2‐mercaptobenzothiazole (CMBT), 1,5‐diaminonaphthalene (DAN), 2,5‐Dihydroxyacetophenone (DHA), and 2,5‐dihydroxybenzoic acid (DHB). Principal component analysis and receiver operating characteristic curve analysis revealed significant matrix effects on the relative signal intensities observed for different lipid subclasses and adducts. Comparison of spectral profiles and quantitative assessment of the number and intensity of species from each lipid subclass showed that each matrix produces unique lipid signals. In positive ion mode, matrix application methods played a role in the MALDI analysis for different cationic species. Comparisons of different methods for the application of DHA showed a significant increase in the intensity of sodiated and potassiated analytes when using an aerosol sprayer. In negative ion mode, lipid profiles generated using DAN were significantly different than all other matrices tested. This difference was found to be driven by modification of phosphatidylcholines during ionization that enables them to be detected in negative ion mode. These modified phosphatidylcholines are isomeric with common phosphatidylethanolamines confounding MALDI IMS analysis when using DAN. These results show an experimental basis of MALDI analyses when analyzing lipids from tissue and allow for more informed selection of MALDI matrices when performing lipid IMS experiments.     

Liquid matrix deposition on conductive hydrophobic surfaces for tuning and quantitation in UV-MALDI mass spectrometry

With its highly fluctuating ion production matrix-assisted laser desorption/ionization (MALDI) poses many practical challenges for its application in mass spectrometry. Instrument tuning and quantitative ion abundance measurements using ion signal alone depend on a stable ion beam. Liquid MALDI matrices have been shown to be a promising alternative to the commonly used solid matrices. Their application in areas where a stable ion current is essential has been discussed but only limited data have been provided to demonstrate their practical use and advantages in the formation of stable MALDI ion beams. In this article we present experimental data showing high MALDI ion beam stability over more than two orders of magnitude at high analytical sensitivity (low femtomole amount prepared) for quantitative peptide abundance measurements and instrument tuning in a MALDI Q-TOF mass spectrometer. Samples were deposited on an inexpensive conductive hydrophobic surface and shrunk to droplets <10 nL in size. By using a sample droplet <10 nL it was possible to acquire data from a single irradiated spot for roughly 10,000 shots with little variation in ion signal intensity at a laser repetition rate of 5-20 Hz.     

5-Ethyl-2-mercaptothiazole as matrix for matrix-assisted laser desorption/ionization of a broad spectrum of analytes in positive and negative ion mode

A preliminary investigation of the use of 5-ethyl-2-mercaptothiazole as matrix in matrix-assisted laser desorption/ionization (MALDI) of a broad spectrum of analytes is reported. The analytes studied are substance P, insulin, beta-cyclodextrin, triacylglycerols of coconut oil and polypropylene glycol 2000 (PPG 2000). In the positive ion mass spectra of the matrix/analyte combinations, the formation of [M + H]+ and [M + cation]+ species were observed and compared with those obtained by using well-established matrices such as alpha-cyano-4-hydroxycinnamic acid, genticic acid, sinapinic acid and dithranol. In addition, the usefulness of this new matrix for MALDI in negative ion mode is also described using substance P and beta-cyclodextrin as examples.     

Hyphenation of capillary high-performance liquid chromatography with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for nano-scale screening of single-bead combinatorial libraries

This paper focuses on the technical aspects of chemical screening from 384-well plate nano-scale single-bead combinatorial libraries. The analytical technique utilized is a combination of capillary liquid chromatography with ultraviolet detection and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. The HPLC/MALDI-MS hyphenation is achieved by means of a micro-fraction collector with a peak detection system that automatically collects the peaks onto the MALDI targets for subsequent characterization. Several experimental parameters such as type of 384-well plate, well-plate sealing foils, and a column-switching procedure were investigated using a small test library of nine components. Additionally, the influence of different MALDI matrices, different MALDI targets and sample-spotting techniques on the MALDI detection sensitivity as well as the ruggedness and sample throughput capacity of this technique were studied. Optimum results for the analytes investigated were obtained with 2,5-dihydroxybenzoic acid using on-line mixing of HPLC effluent and matrix solution. To demonstrate the potential of this capillary HPLC/MALDI-TOFMS method, its application to several single-bead libraries was investigated. The instrumental method allowed for the rapid identification and purity assessment of combinatorial libraries with detection limits down to the higher femtomole level using both UV detection and MALDI mass spectrometry.     

norHarmane containing ionic liquid matrices for low molecular weight MALDI‐MS carbohydrate analysis: The perfect couple with α‐cyano‐4‐hydroxycinnamic acid

Cinnamic acid derivatives, particularly α‐cyano‐4‐hydroxycinnamic acid (E‐α‐cyano‐4‐hydroxycinnamic acid or (E)‐2‐cyano‐3‐(4‐hydroxyphenyl)prop‐2‐enoate; CHCA), have been extensively used especially for protein and peptide analysis. Together with the introduction of ionic liquid MALDI matrix (ILM) started the study of applications of IL prepared with CHCA and a counter organic base (ie, aliphatic amines) in which CHCA moiety is the chromophore responsible of UV‐laser absorption. Despite the extensive studies of norharmane (9H‐pyrido[3,4‐b]indole; nHo) applications as matrix and its peculiar basic properties in the ground and electronic excited state, nHo containing ILM was never tested in MALDI‐MS experiments. This pyrido‐indole compound was introduced as MALDI matrix 22 years ago for different applications including low molecular weight (LMW) carbohydrates (neutral, acidic, and basic carbohydrates). These facts encouraged us to use it as a base, for the first time, for ILM preparation. As a rational design of new IL MALDI matrices, E‐α‐cyanocinnamic acid.nHo and E‐cinnamic acid.nHo were prepared and their properties as matrices studied. Their performance was compared with that of (a) the corresponding IL prepared with butylamine as basic component, (b) the corresponding crystalline E‐α‐cyanocinnamic and E‐cinnamic acid, and (c) the classical crystalline matrices (2,5‐dihydroxybenzoic acid, DHB; nHo) used in the analysis of neutral/sulfated carbohydrates. The IL DHB.nHo was tested, too. Herein, we demonstrate the outstanding performance for the IL CHCA.nHo for LMW carbohydrate in positive and negative ion mode (linear and reflectron modes). Sulfated oligosaccharides were detected in negative ion mode, and although the dissociation of sulfate groups was not completely suppressed the relative intensity (RI) of [M ? Na]^? peak was quite high. Additionally, to better understand the quite different performance of each IL tested as matrix, the physical and morphological properties in solid state were studied (optical image; MS image).     

Some fundamental and technical aspects of the quantitative analysis of pharmaceutical drugs by matrix-assisted laser desorption/ionization mass spectrometry

The purpose of the present paper was to study some of the underlying physical and technical aspects of high-throughput quantitative matrix-assisted laser desorption/ionization (MALDI) of small drug molecules. A prototype MALDI-triple quadrupole instrument equipped with a high repetition rate laser was employed. Initially, the detection limits and dynamic ranges for the quantitation of four drugs (quinidine, danofloxacin, ramipril and nadolol) were determined. Internal standards were carefully chosen for each of these analytes in terms of structure similarity and fragmentation pathways. Three organic matrices were tested for these assays, resulting in different crystallization behaviors and measurement reproducibilities. alpha-Cyano-4-hydroxycinnamic acid yielded the best results and was subsequently employed for the quantitative determination of all four analytes. Further experiments considered the role of laser energy and pulse rate on the ablated areas as well as ion signals. Light microscope and scanning electron microscope images allowed the examination of the ablated area of the MALDI spots. The images showed convincing evidence that the ablated area was virtually void of crystals after analysis, with no preferential removal of material in the center of the laser's path. Average values for the amount of material ablated were determined to be 3.9+/-0.5% of the total spot size, and as low as 19.5 attomoles of analyte were detectable for our most sensitive analyte, ramipril. It was calculated that, under these assay conditions, it was possible to accurately quantify less than 1 femtomole of all analytes with the use of appropriately pure internal standards. These studies showed very promising results for the quantitative nature of MALDI for small molecules with molecular weights less than 500 Da.     

Sublimation as a method of matrix application for mass spectrometric imaging

Common organic matrix-assisted laser desorption/ionization (MALDI) matrices, 2,5-dihydroxybenzoic acid, 3,5-dimethoxy-4-hydroxycinnamic acid, and alpha-cyano-4-hydroxycinnamic acid, were found to undergo sublimation without decomposition under conditions of reduced pressure and elevated temperature. This solid to vapor-phase transition was exploited to apply MALDI matrix onto tissue samples over a broad surface in a solvent-free application for mass spectrometric imaging. Sublimation of matrix produced an even layer of small crystals across the sample plate. The deposition was readily controlled with time, temperature, and pressure settings and was highly reproducible from one sample to the next. Mass spectrometric images acquired from phospholipid standards robotically spotted onto a MALDI plate yielded a more intense, even signal with fewer sodium adducts when matrix was applied by sublimation relative to samples where matrix was deposited by an electrospray technique. MALDI matrix could be readily applied to tissue sections on glass slides and stainless steel MALDI plate inserts as long as good thermal contact was made with the condenser of the sublimation device. Sections of mouse brain were coated with matrix applied by sublimation and were imaged using a Q-q-TOF mass spectrometer to yield mass spectral images of very high quality. Image quality is likely enhanced by several features of this technique including the microcrystalline morphology of the deposited matrix, increased purity of deposited matrix, and evenness of deposition. This inexpensive method was reproducible and eliminated the potential for spreading of analytes arising from solvent deposition during matrix application.     

Sample preparation with solid phase microextraction and exhaustive extraction approaches: Comparison for challenging cases

In chemical analysis, sample preparation is frequently considered the bottleneck of the entire analytical method. The success of the final method strongly depends on understanding the entire process of analysis of a particular type of analyte in a sample, namely: the physicochemical properties of the analytes (solubility, volatility, polarity etc.), the environmental conditions, and the matrix components of the sample. Various sample preparation strategies have been developed based on exhaustive or non-exhaustive extraction of analytes from matrices. Undoubtedly, amongst all sample preparation approaches, liquid extraction, including liquid–liquid (LLE) and solid phase extraction (SPE), are the most well-known, widely used, and commonly accepted methods by many international organizations and accredited laboratories. Both methods are well documented and there are many well defined procedures, which make them, at first sight, the methods of choice. However, many challenging tasks, such as complex matrix applications, on-site and in vivo applications, and determination of matrix-bound and free concentrations of analytes, are not easily attainable with these classical approaches for sample preparation.     

Ionic-liquid matrices for quantitative analysis by MALDI-TOF mass spectrometry

Ionic liquid matrices (ILMs) were tested as MALDI matrices for quantification of oligodeoxynucleotides (ODNs), peptides, and small proteins. Good calibrations with high linearity and reproducibility were achieved over a broad concentration range for all the tested ILMs in spite of their different physical states. However, the standard deviation is higher for ILMs that are solid with visible crystals. The experimental results indicate various ILMs have different sensitivity owing to changes in their cation components. More importantly, we found that the slopes of the calibration curves correlate with the inverse of the peptide molecular weights, presenting an opportunity to predict a priori, the relative sensitivities (slopes of calibration plots) for various analytes that have similar hydrophobicites.