Investigation of enhanced ion abundances from a carrier process in high-performance liquid chromatography particle beam mass spectrometry

Enhancements in ion abundances in high-performance liquid chromatography (HPLC) particle beam (PB) mass spectrometry were observed during coelution or when certain substances (e.g., ammonium acetate) were added to the mobile phase during a reverse-phase gradient elution HPLC separation. These enhancements were observed with two commercial PB interfaces and a variety of compounds generally not amenable to separation by gas chromatography. The enhancements are attributed to both improved chromatographic efficiency and a PB carrier process.     

Enhanced detection of phosphopeptides in matrix-assisted laser desorption/ionization mass spectrometry using ammonium salts

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) has been used successfully to detect phosphorylation sites in proteins. Applications may be limited by the low response of phosphopeptides compared to nonphosphorylated peptides in MALDI MS. The addition of ammonium salts to the matrix/analyte solution substantially enhances the signal for phosphopeptides. In examples shown for equimolar mixtures, the phosphorylated peptide peaks become the largest peaks in the spectrum upon ammonium ion addition. This can allow for the identification of phosphopeptides in an unfractionated proteolytic digestion mixture. Sufficient numbers of protonated phosphopeptides can be generated such that they can be subjected to postsource decay analysis, in order to confirm the number of phosphate groups present. The approach works well with the common MALDI matrices such as α-cyano-4-hydroxycinnamic acid and 2,5-dihydroxybenzoic acid, and with ammonium salts such as diammonium citrate and ammonium acetate.     

Flame Atomic Absorption Spectrometric Determination of Cu(II), Co(II), Cd(II), Fe(III) and Mn(II) in Ammonium Salts and Industrial Fertilizers after PrEconcentration/Separation on Diaion HP-20

A sensitive and simple separation-enrichment technique for the determination of trace amounts of Cu(II), Co(II), Cd(II), Fe(III) and Mn(II) was described. Metal ions were complexed with 1-nitroso-2-naphthol at pH 9. Following solid-phase extraction on Diaion HP-20 resin, metals were determined by flame atomic absorption spectrometry. The effect of the matrix ions were investigated. The recoveries of metal ions were greater than 95%. The detection limits of the analyte ions ( k = 3, N = 21) were varying 0.18 µg/l for Cd(II) to 0.44 µg/l for Fe(III). The method was applied to a stream sediment standard reference material (GBW7309), some ammonium salts and industrial fertilizer samples for the determination of copper, cobalt, cadmium, iron and manganese. The relative standard deviations (RSD) of the determinations for analyte ions at µg/g levels varied from 1 to 10%.     

Addressing a Common Misconception: Ammonium Acetate as Neutral pH “Buffer” for Native Electrospray Mass Spectrometry

Native ESI-MS involves the transfer of intact proteins and biomolecular complexes from solution into the gas phase. One potential pitfall is the occurrence of pH-induced changes that can affect the analyte while it is still surrounded by solvent. Most native ESI-MS studies employ neutral aqueous ammonium acetate solutions. It is a widely perpetuated misconception that ammonium acetate buffers the analyte solution at neutral pH. By definition, a buffer consists of a weak acid and its conjugate weak base. The buffering range covers the weak acid pK_a ± 1 pH unit. NH₄ ⁺ and CH₃-COO^? are not a conjugate acid/base pair, which means that they do not constitute a buffer at pH 7. Dissolution of ammonium acetate salt in water results in pH 7, but this pH is highly labile. Ammonium acetate does provide buffering around pH 4.75 (the pK_a of acetic acid) and around pH 9.25 (the pK_a of ammonium). This implies that neutral ammonium acetate solutions electrosprayed in positive ion mode will likely undergo acidification down to pH 4.75 ± 1 in the ESI plume. Ammonium acetate nonetheless remains a useful additive for native ESI-MS. It is a volatile electrolyte that can mimic the solvation properties experienced by proteins under physiological conditions. Also, a drop from pH 7 to around pH 4.75 is less dramatic than the acidification that would take place in pure water. It is hoped that the habit of referring to pH 7 solutions as ammonium acetate “buffer” will disappear from the literature. Ammonium acetate “solution” should be used instead.

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Particle beam sample introduction into glow discharge plasmas for speciation analysis

This paper reviews the use of the particle beam (PB) as a transport-type interface for the introduction of liquid samples into glow discharge (GD) plasmas. Emphasis is placed on the PB interface as a coupling for liquid chromatography (LC) with optical emission spectroscopy (OES) and mass spectrometry (MS) detection methods. Advantages and disadvantages of the particle beam sample introduction for LC–MS and LC–OES as well as a comparison with other interfaces (i.e. moving belt) are covered. Fundamental aspects of the particle beam such as solvent removal and analyte delivery are highlighted. Furthermore, the development of the particle beam interface is discussed regarding its potential for providing “comprehensive speciation” analysis of solution-phase samples. Specifically, the particle beam/hollow cathode–optical emission spectroscopy (PB/HC–OES) technique provides information towards metal and non-metals determinations as well molecular species identification of organic compounds, organometallics, and small biomolecules via empirical formulae determinations. Particle beam–glow discharge mass spectrometry (PB/GDMS) also provides molecular species information through fragmentation pattern analysis of plasma-produced mass spectra that are similar in structure to electron impact (EI) sources. The evolving capabilities of the PB/GD couplings deliver analytical information that is not available from any other spectrochemical source. The technique has relevance to an incredible range of analytical applications and warrants further investigation by other researchers and instrument manufacturers.     

Unusual Analyte-Matrix Adduct Ions and Mechanism of Their Formation in MALDI TOF MS of Benzene-1,3,5-Tricarboxamide and Urea Compounds

Analyte-matrix adducts are normally absent under typical matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS) conditions. Interestingly, though, in the analysis of several types of organic compounds synthesized in our laboratory, analyte-matrix adduct ion peaks were always recorded when common MALDI matrices such as 4-hydroxy-α-cyanocinnamic acid (CHCA) were used. These compounds are mainly those with a benzene-1,3,5-tricarboxamide (BTA) or urea moiety, which are important building blocks to make new functional supramolecular materials. The possible mechanism of the adduct formation was investigated. A shared feature of the compounds studied is that they can form intermolecular hydrogen bonding with matrices like CHCA. The intermolecular hydrogen bonding will make the association between analyte ions and matrix molecules stronger. As a result, the analyte ions and matrix molecules in MALDI clusters will become more difficult to be separated from each other. Furthermore, it was found that analyte ions were mainly adducted with matrix salts, which is probably due to the much lower volatility of the salts compared with that of their corresponding matrix acids. It seems that the analyte-matrix adduct formation for our compounds are caused by the incomplete evaporation of matrix molecules from the MALDI clusters because of the combined effects of enhanced intermolecular interaction between analyte-matrix and of the low volatility of matrix salts. Based on these findings, strategies to suppress the analyte-matrix adduction are briefly discussed. In return, the positive results of using these strategies support the proposed mechanism of the analyte-matrix adduct formation.

Trace metal assay of thorium oxide by atomic emission spectrometry

Summary An Atomic Emission Spectrometric (AES) method has been developed for the direct determination of twenty-two metallic elements at trace concentrations in thorium oxide. The method is based on the use of a d.c. arc — carrier distillation technique with 12% composition of a mixed carrier, viz. AgCl+SrF₂ in 5:1 proportion, and is equally applicable to both volatile and refractory analytes. A direct reading emission spectrometer with computerised data acquisition and processing facility has been used in these studies. The standardisation procedure includes study of volatilisation/excitation characteristics of the analyte elements to optimise choice and proportion of the carrier and signal integration time apart from optimisation of other experimental parameters such as arc current, viewing position of the arc plasma, sample charge etc. The detection limits for the analytes lie in the range 0.1–50 ppm while the precision of determinations is better than 15% for most of the elements as evaluated from the repetitive analyses of spiked samples.     

Determinations of geniposide using LC/MS/MS methods via forming ammonium and acetate adducts

In this paper, we report method development work to determine geniposide using LC/MS/MS via the formation of positive and negative ion adducts. Geniposide, which has been recognized to have choleretic effects, is the major iridoid glycoside component of Gardenia herbs. To enhance the sensitivity of LC/MS detection of geniposide, a small amount of volatile additives such as ammonium acetate and acetic acid are added into mobile phase solvents to form positive and negative adducts, which can then ionize via electrospray processes. The formation of positive adducts is due to the complexation between geniposide and ammonium ions ([M + NH₄]⁺). The formation of anionic adducts [M + CH₃COO]⁻ is believed to occur via hydrogen bonds bridging acetate ions and glucose groups on the geniposide molecule. Mobile phase solvents containing acetonitrile and aqueous solution (0.2 mM ammonium acetate or 0.1% acetic acid) at the ratio 15: 85 are employed to elute geniposide using C8 reverse phase liquid chromatography columns with electrospray tandem mass spectrometry determinations. Using geniposide standards, the methods are validated at the concentration ranges of 5 to 1000 ng/mL and 20 to 5000 ng/mL using ammonium and acetate adducts respectively. The correlation coefficients of the standard curves are 0.9999 using both ammonium and acetate adducts. The detection limits of using ammonium and acetate adducts are 1 and 5 ng/mL respectively. The measurement accuracy and precision of using ammonium adducts are within 12% and 3% respectively, whereas the accuracy and precision are within 6 and 11% respectively using acetate adducts. When the validated calibration curves of the ammonium adduct of geniposide are used to determine spiked control samples in rat blood dialysates, the determination errors of accuracy and precision are within 12% and 10% respectively.     

The Enrichment/Separation of Fe, Co, Pb, Cd, and Cr on Ambersorb 563 Prior to Their Flame Atomic Absorption Spectrometric Determinations

A sorbent extraction method for the separation/preconcentration of Fe, Co, Pb, Cd, and Cr was developed. The analyte metal ions were retained on a column of Ambersorb 563 from a buffered sample solution. The flow rates of the sample and eluent solution were controlled by a peristaltic pump. The analyte ions were quantitatively retained at pH 9 by using an ammonia/ammonium chloride buffer solution, and were then eluted with 5 mL of 0.25 M HNO₃ at 5 mL/min flow rate. The detection limits were in the range of 0.33 and 72 g/L for Cd and Pb, respectively. The relative standard deviations were less than 10%. Recoveries of spike addition to drinking water and seawater were quantitative. The method presented was applied for the determination of Fe, Co, Pb, Cd, and Cr ions in drinking and seawater samples with satisfactory results (recoveries >95%).     

Spectrophotometric determination of osmium with ethylisobutrazine hydrochloride

The sheath flow cuvette is used for refractive index determinations of neat solutions within picoliter probe volumes. In this detector, a sample stream is injected as a narrow stream into the center of a flowing sheath stream under laminar flow conditions. The sample stream retains its identity as a thin cylinder through the center of a 250-m square flow chamber. The propagation properties of a focused helium-neon laser are perturbed by interaction with the sample stream. Detection limits of RI=3×10^–6 were obtained within a 20 micrometer radius sample stream, corresponding to about a 400 picoliter probe volume. For analyte with refractive index significantly different than the solvent, detection limits are possible which correspond to a few picograms of analyte within the probe volume.     

(E)-Propyl α-Cyano-4-Hydroxyl Cinnamylate: A High Sensitive and Salt Tolerant Matrix for Intact Protein Profiling by MALDI Mass Spectrometry

Low-abundance samples and salt interference are always of great challenges for the practical protein profiling by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Herein, a series of carboxyl-esterified derivatives of α-cyano-4-hydroxycinnamic acid (CHCA) were synthesized and evaluated as matrices for MALDI-MS analysis of protein. Among them, (E)-propyl α-cyano-4-hydroxyl cinnamylate (CHCA-C3) was found to exhibit excellent assay performance for intact proteins by improving the detection sensitivity 10 folds compared with the traditional matrices [i.e., super2,5-dihydroxybenzoic acid (superDHB), sinapic acid (SA), and CHCA]. In addition, CHCA-C3 was shown to have high tolerance to salts, the ion signal of myoglobin was readily detected even in the presence of urea (8 M), NH₄HCO₃ (2 M), and KH₂PO₄ (500 mM), meanwhile sample washability was robust. These achievements were mainly attributed to improved ablation ability and increased hydrophobicity or affinity of CHCA-C3 to proteins in comparison with hydrophilic matrixes, leading to more efficient ionization of analyte. Furthermore, direct analysis of proteins from crude egg white demonstrated that CHCA-C3 was a highly efficient matrix for the analysis of low-abundance proteins in complex biological samples. These outstanding performances indicate the tremendous potential use of CHCA-C3 in protein profiling by MALDI-MS.

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Determination of Abacavir in maternal plasma, amniotic fluid, fetal and placental tissues by a polarity switching liquid chromatography/tandem mass spectrometry method

A rapid and efficient high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) method for the determination of concentrations of the carbocyclic nucleoside antiviral Abacavir in maternal rat plasma, amniotic fluid, placental and fetal tissue samples has been developed and validated. All tissue samples were homogenized in water prior to analysis and all samples were prepared by acetonitrile precipitation followed by dilution with HPLC-grade water. Separation of the analyte and internal standard from the matrices was achieved on a C(8) analytical column (2.1 x 150 mm, 5 microm particle size). The mobile phase consisted of 10 mM ammonium acetate/acetonitrile using a gradient method at a flow rate of 0.25 mL/min for all matrices. The method yields retention times of approximately 3.4 and 5.1 min for the internal standard (Azidouridine) and Abacavir, respectively. For all matrices the limit of detection was approximately 1 ng/ml. Recoveries from the different matrices ranged from 53-87% for Abacavir and from 69-84% for Azidouridine. Within- and between-run precision (%RSD) and accuracy (%Error) were under 15% for all matrices.     

Application of cobalt ammine complexes for the simultaneous determination of traces of As, Fe, Ti, V and Zr in high-purity cobalt metal by lanthanum hydroxide coprecipitation and inductively coupled plasma-atomic emission spectrometry

Co is not coprecipitated with lanthanum hydroxide in ammonium hydroxide solution due to the formation of soluble salts of cobalt ammine complexes. This complexation reaction has been applied to the simultaneous determination of five trace impurities (As, Fe, Ti, V and Zr) in high-purity Co metal by inductively coupled plasma-atomic emission spectrometry. Warming the solution and double coprecipitation are effective for separating the trace elements simultaneously from a large amount of cobalt ammine complexes. Matrix-matched solutions are used for calibration. The detection limits of the five elements are in the range of 0.06 and 0.6 g/g and the recoveries are quantitative.     

On-Line Solid Phase Extraction and Simultaneous Determination of Hafnium and Zirconium by ICP–Atomic Emission Spectroscopy

A new simple flow injection analysis (FIA) system is described for on-line preconcentration by solid phase extraction and simultaneous determination of Hf and Zr in different samples using inductively coupled plasma atomic emission spectroscopy with a charge coupling detector (CCD). Quinalizarin (QA) was loaded on an octadecyl silica-polyethylene mini-column for the retention of Hf and Zr ions in complexed form. A 0.3 M ammonium acetate was used as buffer for providing suitable conditions for complexation and increasing reproducibility. Retained ions on the solid phase were then eluted by a solution containing 3.0 M HCl and 0.5 M HNO₃. In this work, for reducing bandwidths of eluted ions, elution of minicolumn was carried out from opposite direction. The same solution was used as both carrier and eluent, in order to increase the reproducibility. The eluted ions were introduced into the conventional nebulizer of ICP–AES instrument. Effects of different parameters, including instrumental parameters of ICP and FIA were optimized. An enrichment factor of 330 for each analyte ion was obtained at a concentration level of 80 ppb. The detection limits of the proposed method for Hf and Zr were 0.16 ng mL⁻¹ and 0.04 ng mL⁻¹ respectively. The ability of the method for the recovery of Hf and Zr ions was tested in the presence of several diverse metal ions in a synthetic mixture and some real matrices. It was also applied to the determination of Zr and Hf ions in a standard soil and in a standard alloy as real samples.     

LC/ESI-MS/MS method for quantification of 28 synthetic cannabinoids in neat oral fluid and its application to preliminary studies on their detection windows

Serum and urine samples are commonly used for the analysis of synthetic cannabinoids in biofluids; however, their utilization as analytical matrices for drug abstinence control features some substantial drawbacks. While for blood collection invasive sampling is inevitable, the urinary analysis of synthetic cannabinoids is limited by the lack of available reference standards of the respective major metabolites. Moreover, the long detectability of synthetic cannabinoids in both matrices hampers the identification of a recent synthetic cannabinoid use. This article describes the development, validation and application of an LC/ESI-MS/MS method for the quantification of 28 synthetic cannabinoids in neat oral fluid (OF) samples. OF samples were prepared by protein precipitation using ice-cold acetonitrile. Chromatographic separation was achieved by gradient elution on a Luna Phenyl Hexyl column (50?×?2 mm, 5 μm), while detection was carried out on a QTrap 4000 instrument in positive ionization mode. The limits of detection ranged from 0.02 to 0.40 ng/mL, whereas the lower limits of quantification ranged from 0.2 to 4.0 ng/mL. The method was applied to authentic samples collected during two preliminary studies in order to obtain insights into the general detectability and detection windows of synthetic cannabinoids in this matrix. The results indicate that synthetic cannabinoids are transferred from the blood stream into OF and vice versa only at a very low rate. Therefore, positive OF samples are due to contamination of the oral cavity during smoking. As these drug-contaminations could be detected up to approximately 2 days, neat oral fluid appears to be well suited for detection of a recent synthetic cannabinoid use.

Multivariate optimization of an axially-viewed inductively coupled plasma multichannel-based emission spectrometer for the analysis of environmental samples

An experimental design procedure was applied to optimize the operating conditions of an axially-viewed inductively coupled plasma emission spectrometer instruments equipped with echelle optics with cross dispersion and charge transfer device. The multivariate effect of carrier gas flow rate and r.f. power on several analytical figures was investigated and discussed. Both ultrasonic and pneumatic nebulization were used. For the final choice of the optimum, different criteria were taken into account, mainly plasma robustness, instrumental precision, analyte and background net emission, detection limits and signal-to-background ratios. It was found that the use of moderate power (1100W) and mean carrier gas flow rate (0.75 L/min) allows to obtain sufficient plasma robustness, satisfactory precision, and excellent signal-to-background ratios and limits of detection, favorable for ultratrace element determinations in environmental matrices.     

Ethynyl carbonium ions. 2. Reactions of 4-ethynylpyrylium perchlorates with alcohols and nitrogen-containing nucleophiles

Derivatives of -alkoxy- and -(phenylamino)styrylpyrylium salts, the hydrolysis of which leads to phenacylpyrylium salts and 4-benzoylmethylenepyrans, are formed in the reaction of 4-phenylethynylpyrylium salts with alcohols and aromatic amines. Ethynyl-substituted pyridines and pyridinium salts were synthesized by the action of ammonium acetate and methylamine, benzylamine, and butylamine. A -methylaminostyryl-1-methyl-substituted pyridinium salt is formed with excess methylamine.See [1] for Communication 1.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 299–306, March, 1990.     

Influence of Alkylammonium Acetate Buffers on Protein–Ligand Noncovalent Interactions Using Native Mass Spectrometry

We investigate the influence of three volatile alkylammonium acetate buffers on binding affinities for protein–ligand interactions determined by native electrospray ionization-mass spectrometry (ESI-MS). Four different types of proteins were chosen for this study. A charge-reduction effect was observed for all the cases studied, in comparison to the ions formed in ammonium acetate solution. When increasing the collision energy, the complexes of trypsin and the ligand were found to be more stable when sprayed from alkylammonium acetate buffers than from ammonium acetate. The determined dissociation constant (K_d) also exhibited a drop (up to 40%) when ammonium acetate was replaced by alkylammonium acetate buffers for the case of lysozyme and the ligand. The prospective uses of these ammonium acetate analogs in native ESI-MS are discussed in this paper as well.

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Ammonium chloride salting out extraction/cleanup for trace-level quantitative analysis in food and biological matrices by flow injection tandem mass spectrometry

A sample extraction and purification procedure that uses ammonium-salt-induced acetonitrile/water phase separation was developed and demonstrated to be compatible with the recently reported method for pesticide residue analysis based on fast extraction and dilution flow injection mass spectrometry (FED-FI-MS). The ammonium salts evaluated were chloride, acetate, formate, carbonate, and sulfate. A mixture of NaCl and MgSO₄, salts used in the well-known QuEChERS method, was also tested for comparison. With thermal decomposition/evaporation temperature of <350 °C, ammonium salts resulted in negligible ion source residual under typical electrospray conditions, leading to consistent method performance and less instrument cleaning. Although all ammonium salts tested induced acetonitrile/water phase separation, NH₄Cl yielded the best performance, thus it was the preferred salting out agent. The NH₄Cl salting out method was successfully coupled with FI/MS/MS and tested for fourteen pesticide active ingredients: chlorantraniliprole, cyantraniliprole, chlorimuron ethyl, oxamyl, methomyl, sulfometuron methyl, chlorsulfuron, triflusulfuron methyl, azimsulfuron, flupyrsulfuron methyl, aminocyclopyrachlor, aminocyclopyrachlor methyl, diuron and hexazinone. A validation study was conducted with nine complex matrices: sorghum, rice, grapefruit, canola, milk, eggs, beef, urine and blood plasma. The method is applicable to all analytes, except aminocyclopyrachlor. The method was deemed appropriate for quantitative analysis in 114 out of 126 analyte/matrix cases tested (applicability rate = 0.90). The NH₄Cl salting out extraction/cleanup allowed expansion of FI/MS/MS for analysis in food of plant and animal origin, and body fluids with increased ruggedness and sensitivity, while maintaining high-throughput (run time = 30 s/sample). Limits of quantitation (LOQs) of 0.01 mg kg⁻¹ (ppm), the ‘well-accepted standard’ in pesticide residue analysis, were achieved in >80% of cases tested; while limits of detection (LODs) were typically in the range of 0.001–0.01 mg kg⁻¹ (ppm). A comparison to a well-established HPLC/MS/MS method was also conducted, yielding comparable results, thus confirming the suitability of NH₄Cl salting out FI/MS/MS for pesticide residue analysis.     

On-line SPE-Nano-LC-Nanospray-MS for rapid and sensitive determination of perfluorooctanoic acid and perfluorooctane sulfonate in river water

An instrumental set up including on-line solid-phase extraction, nano-liquid chromatography, and nanospray mass spectrometry is constructed to improve the sensitivity for quantitation of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) in surface water. Sample volumes of 1000 microL are loaded onto a microbore 1.0-mm i.d. x 5 mm, 5 microm Kromasil C(18) enrichment column by a carrier solution consisting of 10mM ammonium acetate in acetonitrile-water (10:90, v/v) at a flow rate of 250 microL/min, providing on-line analyte enrichment and sample clean-up. Backflushed elution onto a 0.1-mm i.d. x 150 mm, 3.5 microm Kromasil C(18) analytical column is conducted using an acetonitrile-10mM ammonium acetate solvent gradient from 30% to 70% acetonitrile. Water samples are added with internal standard (perfluoroheptanoic acid) and filtrated prior to injection. The mass limits of detection of PFOA and PFOS are 0.5 and 1 pg, respectively, corresponding to concentration limits of detection of 500 pg/L and 1 ng/L, respectively. The total time spent on sample preparation, chromatography, and detection is approximately 12 min per sample. The method was employed for the determination of PFOS and PFOA in urban river water.