Chemical Noise Research Articles

Ionization in matrix-assisted laser desorption/ionization: singly charged molecular ions are the lucky survivors.

A new model for the ionization processes in UV matrix-assisted laser desorption/ionization (MALDI) which accounts for the major phenomena observed is presented and discussed. The model retains elements of earlier approaches, such as photoionization and photochemical reactions, but it redefines these in the light of new working questions, most importantly why only singly charged ions are detected. Based on experimental evidence, the formation of singly and multiply charged clusters by a deficiency/excess of ions and also by photoionization and subsequent photochemical processes is pointed out to be the major ionization processes, which typically occur in parallel. The generation of electrons and their partial loss into the surrounding vacuum and solid, on the one hand, results in a positively charged ion-neutral plume facilitating a high overall ionization yield. On the other hand, these electrons, and also the large excess of protonated matrix ions in the negative ion mode, induce effective ion reneutralization in the plume. These neutralization processes are most effective for the highly charged cluster ions initially formed. Their fragmentation behaviour is evidenced in fast metastable fragmentation characteristics and agrees well with an electron capture dissociation mechanism and the enthalpy transfer upon neutralization forms the rationale for the prominent fragmentation and intense chemical noise accompanying successful MALDI. Within the course of the paper, cross-correlations with other desorption/ionization techniques and with earlier discussions on their mechanisms are drawn.

Analysis of sphingomyelin, glucosylceramide, ceramide, sphingosine, and sphingosine 1-phosphate by tandem mass spectrometry

Free sphingoid bases such as sphingosine, sphinganine, and the respective phosphorylated bases, as well as the complex sphingolipids ceramides, glucosylceramide, and sphingomyelin, all dissociate to form structurally distinctive product ions. For sphingomyelin these ions are characteristic of their phosphorylcholine headgroup and are observed at m/z 184. The other sphingolipids dissociate to form carbocations characteristic of their sphingoid base. For common mammalian sphingoid bases such as d18:1 or d18:0 these product ions are detected at m/z 264 or 266, respectively. However, changes in the sphingoid base chain length, degree of unsaturation, or other modifications may correspondingly result in a shift in m/z of [figure: see text] this product ion. Additionally, the kinetics that govern the formation of these product ions is affected by the presence of a delta 4 double bond. Thus, internal standards for each type of sphingoid base are required for quantitative data. Structurally distinctive product ions, when used with either precursor ion or constant neutral loss scans allow highly specific and sensitive methods for sphingolipid analysis. They serve to greatly reduce background chemical noise, and enhance detection of sphingolipids at very low concentrations. This occurs by allowing only those ions that dissociate to yield a specific product ion or neutral loss to be passed to the detector. Additionally, these scans reveal the exact combinations of headgroup, sphingoid base, and fatty acid in a complex mixture by mass. The free sphingoid bases and Cer readily decompose in the ion source, whereas GlcCer and SM do not. Finally, each individual sphingolipid species fragmented optimally at a different collision energy, precluding the use of either precursor ion or neutral loss scans for quantitation. Multiple reaction monitoring (MRM) experiments directly address the issues regarding accurate quantitation of sphingolipids that precursor ion and neutral loss scans cannot. In these experiments both ionization and dissociation parameters are optimized for each individual species. By detecting only specific precursor and product ion pairs instead of scanning wide m/z ranges maximum sensitivity is attained. Furthermore, relative ion abundance data are not biased with regard to instrumental parameters. At this point simple loop injections can be used with the MRM scanning methods developed to observe changes in sphingolipid type and quantity in crude extracts on a class-by-class basis. This, however, is labor intensive requiring multiple injections and multiple runs for each class in order to obtain a complete picture of all sphingolipids present. As an alternative [figure: see text] to loop injections, HPLC-MS/MS methods are being developed. In these methods sphingolipids are eluted by class, thus, each individually optimized MRM method can be used at specific times in an LC run. This provides a highly sensitive and accurate quantitation as well as a complete picture of all sphingolipids in a single run (Fig. 6). Additionally, this methodology is amenable to automation and can be used for high-throughput screening of multiple samples.

Investigating the use of an octupole ion guide for ion storage and high-pass mass filtering to improve the quantitative performance of electrospray ion trap mass spectrometry.

An octupole radio frequency (rf) ion guide was evaluated for storage and filtering of ions generated by electrospray prior to introduction into an ion trap mass spectrometer. The control of the rf potential on the ion guide enabled its operation as a high-pass mass filter, removing low-mass chemical noise that would normally fill the ion trap and result in reduced sensitivity, mass resolution, and dynamic range. Also, the ion guide can serve to store the high-pass filtered ions during the ion trap mass analysis, enabling nearly 100% duty cycle and reduction of the cycle time by a factor of two. The linearity, precision, and detection limits of the liquid chromatography (LC) electrospray - ion guide-ion trap MS/MS system were evaluated for the determination of ceftiofur in milk. A linear calibration (linear correlation coefficient of 0.986) from 2-200 ppb was obtained with a relative standard deviation for replicate analysis of less than 8%. A quantitation detection limit of 100 pg of ceftiofur on-column (2 ppb) was achieved from a milk extract. This detection limit is nearly a factor of 10 lower compared with the determination on the same electrospray ion trap system not using a rf ion guide for high-pass mass filtering and ion storage.

Mass spectrometric and tandem mass spectrometric behavior of nitrocatechol glucuronides: a comparison of atmospheric pressure chemical ionization and electrospray ionization

The mass spectrometric (MS) and tandem mass spectrometric (MS/MS) behavior of six nitrocatechol-type glucuronides using atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) was systematically studied, and the effect of operation parameters on the fragmentations are presented. The positive ion APCI- and ESI-MS spectra showed an intense protonated molecule and the respective negative ion spectra a deprotonated molecule with minimal fragmentation. The main fragment ions in the MS/MS spectra of the protonated and deprotonated molecules were [M + H − Glu] + and [M − H − Glu] −, respectively, formed by the loss of the glucuronide moiety. The measured limits of detection indicated that ESI is a significantly more efficient ionization method than APCI in the negative and positive ion modes for the compounds studied. MS/MS was found to be less sensitive, but more reliable and simple than MS due to the absence of chemical noise.

Negative-ion electrospray tandem mass spectrometry of peptides derivatized with 4-aminonaphthalenesulphonic acid

The value of derivatizing peptides at the C-terminus with 4-aminonaphthalenesulphonic acid (ansa) to aid in the de novo sequencing of peptides by mass spectrometry was assessed. Negative-ion electrospray of peptides is enhanced by derivatization witb ansa. The derivatizing group also has the effect of increasing the mass of the peptides by 205 Da, often shifting their deprotonated molecules to regions of the mass spectrum with reduced chemical noise. Collision-induced dissociation of naphthalenesulphonated peptide [M-H] - ions results in abundant fragment ions formed by charge-remote fragmentations. The resulting fragmentation patterns are less complex than those of the underivatized analogues and are easier to interpret. Peptides were successfully derivatized at the low picomole level and sequenced on the hundred femtomole level using nano-electrospray tandem mass spectrometry.

Fully-automated systematic toxicological analysis of drugs, poisons, and metabolites in whole blood, urine, and plasma by gas chromatography–full scan mass spectrometry

The availability of automated, rapid and reliable methods for the systematic toxicological analysis (STA) of drugs and poisons in biosamples is of great importance in clinical and forensic toxicology laboratories. Gas chromatography–continuous scan mass spectrometry (GC–MS) possesses a high potential in STA because of its selectivity and identification power. However, in order to develop a fully automated STA method based on GC–MS two main obstacles have to be overcome: (a) sample preparation is rather sophisticated owing to the need to isolate analytes from the aqueous matrix and to allow a correct GC repartition of polar analytes; (b) the large amount of information collected within a single analysis makes it difficult to isolate relevant analytical information (mass spectra of analytes) from the chemical noise. Using a bench-top GC–MS system equipped with a laboratory robot for sample preparation (the Hewlett-Packard 7686 PrepStation) and an original method for mass spectral purification, a fully automated STA procedure was developed involving isolation of drugs from the sample (whole blood with minimal pretreatment, plasma, urine) by means of solid-phase extraction, derivatization (trimethylsilylation) of the acidic–neutral and of the basic extracts, GC–MS analysis, processing of data, and reporting of results. Each step of the procedure, and the method for data analysis in particular, can be easily integrated with other existing STA methods based on GC–MS.

Charge manipulation for improved mass determination of high-mass species and mixture components by electrospray mass spectrometry

The manipulation of the charge states of high-mass ions can facilitate mass determination in electrospray (ES) mass spectrometry. Specifically, the reduction of charge (which leads to ions of higher mass-to-charge ratios) can significantly reduce peak overlap. Signals associated with various charge states of high-mass ions are more easily resolved at low charge states and chemical noise tends to be significantly lower at high mass-to-charge ratios than in the normal mass-to-charge window typically associated with electrospray. Algorithms that transform ES mass spectra to zero-charge spectra are most likely to yield unambiguous results when charge states are clearly resolved and when signal-to-noise ratios are relatively high. Charge manipulation can enhance the value of the transformation algorithms in cases in which compromises their utility. Such situations include ES mass spectra of high-mass species that yield charge states that are not baseline resolved, mixtures with many components and mixtures in which the signals from major components overwhelm signals from minor components. Examples of improved mass determination are illustrated for proteins using ion-ion chemistry as the means for charge state manipulation and the quadrupole ion trap as the mass analyzer.

Investigation of the quantitative capabilities of an electrospray ionization ion trap/linear time-of-flight mass spectrometer

The quantitative analysis of pharmaceutical or environmental compounds represents an important application area of electrospray ionization (ESI) mass spectrometry (MS). Typically, these analyses are carried out using quadrupole mass analyzers. Recently, several other mass spectrometric configurations have been developed for ESI, including time-of-flight (TOF), which offers high sensitivity and fast data acquisition capabilities. This work investigates the capabilities and limitations of using ESI ion trap/TOFMS for quantitation. The analysis of sulfonamides, an important class of compounds used in the food industry, was selected for this study. It is demonstrated that a calibration curve exhibits a linear response for almost 3 orders of magnitude. The lower limit of the linear range is restricted by the chemical background noise while the upper limit is governed by the ESI source. The short term and long term reproducibility of the instrument are both demonstrated to be better than 8% relative standard deviation. The detection limits and linear dynamic range of this instrument are comparable with those obtained using a single quadrupole mass spectrometer for a single compound and superior for multicomponent analyses. For the analysis of five components, the detection limits using ESI ion trap/TOFMS are about three times lower. It is argued that improving the detection limits, through the use of tandem mass spectrometry, should further extend the linear dynamic range of the instrument for quantitation. © 1998 John Wiley & Sons, Ltd.

Investigation of the quantitative capabilities of an electrospray ionization ion trap/linear time‐of‐flight mass spectrometer

The quantitative analysis of pharmaceutical or environmental compounds represents an important application area of electrospray ionization (ESI) mass spectrometry (MS). Typically, these analyses are carried out using quadrupole mass analyzers. Recently, several other mass spectrometric configurations have been developed for ESI, including time-of-flight (TOF), which offers high sensitivity and fast data acquisition capabilities. This work investigates the capabilities and limitations of using ESI ion trap/TOFMS for quantitation. The analysis of sulfonamides, an important class of compounds used in the food industry, was selected for this study. It is demonstrated that a calibration curve exhibits a linear response for almost 3 orders of magnitude. The lower limit of the linear range is restricted by the chemical background noise while the upper limit is governed by the ESI source. The short term and long term reproducibility of the instrument are both demonstrated to be better than 8% relative standard deviation. The detection limits and linear dynamic range of this instrument are comparable with those obtained using a single quadrupole mass spectrometer for a single compound and superior for multicomponent analyses. For the analysis of five components, the detection limits using ESI ion trap/TOFMS are about three times lower. It is argued that improving the detection limits, through the use of tandem mass spectrometry, should further extend the linear dynamic range of the instrument for quantitation. © 1998 John Wiley & Sons, Ltd.

Phosphate-specific fluorescence labeling with BO-IMI: reaction details

Previously we reported that BO-IMI, a reagent which contains a BODIPY fluorophore linked to an imidazole group, can be used to covalently label a phosphomonoester in a single step under aqueous conditions [P. Wang, R.W. Giese, Anal. Chem. 65 (1993) 3518]. The reaction was conducted in the presence of a water-soluble carbodiimide 1-ethyl-3-(3′-N,N′-dimethylaminopropyl)carbodiimide [EDC] to activate the phosphomonoester, and the coupling took place onto both the N1 and N3 imidazole nitrogens of BO-IMI. Whether the two BO-IMI-phosphomonoester regioisomers migrated separately or together during capillary electrophoresis depended on the pH, due to a difference in their p K a values. Since then, we have studied the reaction in more detail leading to the information reported here. First, we have learned that the regioisomer ratio changes during the course of the reaction, and found that the mechanism involves both spontaneous and BO-IMI-catalyzed hydrolysis of the less stable isomer. Second, there is a background reaction in which BO-IMI becomes attached to EDC. Third, the BO-IMI-phosphomonoester product (a mixture of two isomers), that is observed by capillary electrophoresis at an alkaline pH, is found to no longer contain the two fluorine atoms present in the starting BO-IMI reagent. This is because they are replaced by hydroxy groups at high pH. Finally, an event was discovered which complicates the detection of less than about 60 fmol of a phosphomonoester with BO-IMI: hydrolysis of a tiny fraction of the BO-IMI takes place during the coupling reaction, which leads to chemical noise in the capillary electropherogram.

Application of gas chromatography/electron capture negative chemical ionization high-resolution mass spectrometry for analysis of DNA and protein adducts.

The analytical potential of gas chromatography/electron capture negative chemical ionization high-resolution mass spectrometry (HRMS) for characterization and quantitation of DNA and hemoglobin adducts was demonstrated using three model compounds: N2, 3-ethenoguanine (EG), 7-(2-hydroxyethyl)guanine (7-HEG), and N-(2-hydroxyethyl)valine (HEV). At a resolving power of 10 000, the signal-to-noise (S/N) ratios obtained from quantitative selected ion monitoring (SIM) experiments using biological samples were comparable to or better than existing unit mass resolution experiments due to the reduction of chemical noise from the use of narrower mass windows. The specificity gained by HRMS was essential for quantitation of ultratrace amounts near the limit of detection since coeluting interferences of the analyte or internal standard can lead to inaccurate measurement of response factors. The limit of detection (LOD) was 100 amol (S/N = 5) using a pure standard of TTB2-EG. The LOD for complete assays using spiked samples was 500 amol (S/N = 5) for EG and 600 amol (S/N = 5) injected for 7-HEG. The standard deviation (SD) for the HRMS quantitative measurements was typically less than 10%. The SD for the complete biological assays as determined by spiking replicate samples was less than 15%. This method has adequate sensitivity and specificity to accurately measure DNA and protein adducts as low as endogenous concentrations in rodent and human tissues.

Sample handling for proteome analysis.

The main factor limiting the sensitivity range for the identification of proteins isolated by two-dimensional (2-D) electrophoresis is sample handling: protein detection limits on the gel, losses during extraction and digestion, as well as interference of gel contaminants and detergents with the mass spectrometry (MS) detection increasing background noise. At the one hundred picomole level, losses are fairly negligible but when the amounts drop below 1 picomole (and subfemtomole peptide detection limits have been reported recently by MS), the losses become a critical point. In order to extend proteome analysis to include very low copy number proteins, methods must be developed to minimize losses and handling steps, maximize digestion and extraction yields, as well as to lower chemical noise. We present several methods that we have developed in our laboratory to: (i) increase the amount of material available in a sodium dodecyl sulfate (SDS)-free form which does not require staining, (ii) increase protein extraction and digestion yields and lower the contamination by autoproteolytic products, and (iii) allow direct modification of the peptide mixture to generate sequence tags.

Negative ion electrospray high-performance liquid chromatography–mass spectrometry method development for determination of a highly polar phosphonic acid/sulfonic acid compound in plasma: Optimization of ammonium acetate concentration and in-source collision-induced dissociation

A method, based on negative ion electrospray ionization (ESI) single-stage mass spectrometry coupled with HPLC, was developed for the determination of a squalene synthase inhibitor, BMS-187745, in human plasma. BMS-187745, a highly polar compound with both phosphonic acid and sulfonic acid groups, presented difficulties in developing plasma extraction and HPLC procedures. Precipitation of the plasma protein with methanol was finally chosen as the basis for sample preparation since extraction with water-immiscible solvents or with solid-phase extraction columns failed. It was essential to add ammonium acetate to the HPLC mobile phase, not only to enhance the retention of BMS-187745 but also to ensure a well-shaped chromatographic peak. While the use of ammonium acetate had the desired chromatographic effects, it had the undesirable consequence of suppressing the negative ion ESI signal. With the plasma extracts, the [M–H 2O–H] − ion ( m/ z 367) showed significantly lower chemical noise than the [M–H] − ion ( m/ z 385), and was thus chosen as the analytical ion for the selected ion monitoring. The signal of the m/ z 367 ion was significantly enhanced by the optimization of the in-source collision-induced dissociation (CID) of m/ z 385 to m/ z 367.

Establishing Priorities for Occupational Health Research among Women Working in the Maquiladora Industry.

In order to estimate the prevalence of potentially hazardous exposures and the frequencies of, types of, and reasons for job transitions among women working in the Mexican maquiladora industry, an age- and occupation-stratified sample of 479 women, aged 16-40 years, living in 12 neighborhoods in Tijuana were interviewed. Considerable transition between industrial sectors was observed. About 20% had left their last job in a maquiladora because of hazardous or stressful working conditions and work-related health problems. Chemical exposures, ergonomic risks, noise, and stress were reported. The sociodemographic characteristics and occupational risk profile of the maquiladora workforce, considered in the broader context of Mexico's development, indicate that occupational health research in the maquiladora should focus on musculoskeletal injuries, adverse reproductive outcomes, neurologic damage, and mental illness. Appropriate methods for investigating occupational risks in a highly mobile workforce will need to be developed in order to avoid sampling bias and identify appropriate comparison groups.

Losing your temper: the effect of mineral inclusions on pottery analyses

AbstractPottery from Buka Island, Papua New Guinea, was subjected to chemical characterisation studies using both the electron microprobe and PIXE‐PIGME. I report here on the problems and limits of using chemical analyses on prehistoric pottery to assess questions concerning technology and production patterns, and address in particular the problem of compensating for the chemical noise that arises when mineral inclusions are added to a clay in the manufacturing process of pottery. This chemical noise, if not taken into account, can lead to erroneous models of production and exchange. The results demonstrate that the electron microprobe is a useful tool in overcoming the problem of chemical noise and when used in conjunction with PIXE‐PIGME can also be useful in identifying whether inclusions found in a sherd were manually added or found naturally in the original clay. This allows archaeologists another dimension to identify technological changes between assemblages.

Quantification of a potent 5-HT 2a antagonist and an active metabolite in rat plasma and brain microdialysate by liquid chromatography-tandem mass spectrometry

A method based on liquid chromatography-tandem mass spectrometry and microbore column separation was developed for the quantification of a potent 5-HT2a receptor antagonist (R)-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)etnyl]-4-piperidine-methanol (I) and the desmethyl metabolite (II) in rat brain extracellular fluid (ECF) following microdialysis sampling. The analytical method was also applied to determining plasma concentrations of these compounds. The lower limit of quantification (LLQ) for each compound in microdialysis perfusate is 500 pg/mL, which translates to <7 fmol (injected). The recovery of I and II for the microdialysis probe in brain ECF was 18.5 and 22.7%, respectively. The LLQ for each compound in plasma is 1 ng/mL. The inherent selectivity offered by tandem mass spectrometry eliminated chemical noise, thereby improving the detectability of these compounds. These methods were used to confirm that I and II penetrated the blood-brain barrier following administration of I to rats and enabled comparison of plasma and brain ECF concentrations.

Manual microcolumn chromatography for sample cleanup before mass spectrometry.

BioTechniquesVol. 22, No. 2 BenchmarksOpen AccessManual Microcolumn Chromatography for Sample Cleanup Before Mass SpectrometryPaul L. Courchesne & Scott D. PattersonPaul L. CourchesneAmgen, Thousand Oaks, CA, USASearch for more papers by this author & Scott D. Patterson*Address correspondence to Scott D. Patterson, Amgen, Amgen Center, Mail Stop 14-2-E, 1840 DeHavilland Drive, Thousand Oaks, CA 91320-1789, USA. Internet: E-mail Address: spatters@amgen.comAmgen, Thousand Oaks, CA, USASearch for more papers by this authorPublished Online:7 Aug 2018https://doi.org/10.2144/97222bm10AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinkedInRedditEmail FiguresReferencesRelatedDetailsCited ByHigh-Throughput Glycomic Methods7 July 2022 | Chemical Reviews, Vol. 19Negative detection of biomolecules separated in polyacrylamide electrophoresis gels9 November 2006 | Nature Protocols, Vol. 1, No. 3Suppression of α-Cyano-4-hydroxycinnamic Acid Matrix Clusters and Reduction of Chemical Noise in MALDI-TOF Mass Spectrometry13 April 2004 | Analytical Chemistry, Vol. 76, No. 10Direct Atmospheric Pressure Coupling of Polyacrylamide Gel Electrophoresis to Mass Spectrometry for Rapid Protein Sequence Analysis29 July 2003 | Journal of Proteome Research, Vol. 2, No. 6Stop and Go Extraction Tips for Matrix-Assisted Laser Desorption/Ionization, Nanoelectrospray, and LC/MS Sample Pretreatment in Proteomics24 December 2002 | Analytical Chemistry, Vol. 75, No. 3Chapter 29 Sample cleanup and concentration techniques for capillary electrophoresisA simplified device for protein identification by microcapillary gradient liquid chromatography-tandem mass spectrometryElectrophoresis, Vol. 21, No. 9Better approaches to finding the needle in a haystack: Optimizing proteome analysis through automationElectrophoresis, Vol. 21, No. 6Protein identification methods in proteomicsElectrophoresis, Vol. 21, No. 6Protein Micro-Characterization by Mass Spectrometry: Sample Handling and Data FlowTechniques for sample preparation including methods for concentrating peptide samplesIdentification of Proteins by Mass SpectrometryExamination of micro-tip reversed-phase liquid chromatographic extraction of peptide pools for mass spectrometric analysisJournal of Chromatography A, Vol. 826, No. 2Introduction Vol. 22, No. 2 Follow us on social media for the latest updates Metrics Downloaded 78 times History Published online 7 August 2018 Published in print February 1997 Information© 2018 Author(s)PDF download

Time-Domain Quantification of Amplitude, Chemical Shift, Apparent Relaxation TimeT*2, and Phase by Wavelet-Transform Analysis. Application to Biomedical Magnetic Resonance Spectroscopy

The wavelet-transform method is used to quantify the magnetic resonance spectroscopy (MRS) parameters: chemical shift, apparent relaxation timeT*2, resonance amplitude, and phase. Wavelet transformation is a time-frequency representation which separates each component from the FID, then successively quantifies it and subtracts it from the raw signal. Two iterative procedures have been developed. They have been combined with a nonlinear regression analysis method and tested on both simulated and real sets of biomedical MRS data selected with respect to the main problems usually encountered in quantifying biomedical MRS, specifically “chemical noise,” resulting from overlapping resonances, and baseline distortion. The results indicate that the wavelet-transform method can provide efficient and accurate quantification of MRS data.

Trace determination of iron in water at the μg/l level by on-line coupling of capillary isotachophoresis and capillary zone electrophoresis with UV detection of the EDTA-Fe(III) complex

The determination of iron in water at the trace level by on-line coupled capillary isotachophoresis and capillary zone electrophoresis (CITP-CZE) with a commercial column coupling device is described. Iron is determined as the negatively charged complex with EDTA which is highly UV-absorbing and thus enables photometric detection at 254 nm. The analyses are performed using 10 mmol/l HCl+20 mmol/l l-histidine+0.1% hydroxypropylmethyl cellulose (pH 6.0), 5 mmol/l MES, and 25 mmol/l MES+10 mmol/l bis-tris-propane (pH 6.6) which served as leading, terminating and background electrolyte, respectively. Samples are acidified with HNO 3, diluted and EDTA added (to a final concentration of 10 −4 mol/l) prior to CITP-CZE analysis. The detection limit of Fe(III) is 10 μg/l, and is given by the chemical noise due to the impurities of the chemicals used enriched by the ITP preconcentration step. The precision of the CE measurement, expressed by the relative standard deviation, is about 3% (at the 400 μg/l level); the recovery is between 80 and 115% depending on the iron concentration level (40–400 μg/l).

The Reduction of Chemical Noise in an Atmospheric Pressure Ionization/Ionspray Interface for Mass Spectrometry

The reduction of chemical noise and the consequent gain in sensitivity on an ionspray interface to an ion-trap mass spectrometer for capillary electrophoresis/mass spectrometry (CE/MS) applications is described. The principal sources of chemical noise were found in ionized airborne contaminants and in solvent cluster adducts. The reduction of these unwanted signals was achieved by the application of a cover to the heated-shield atmospheric pressure ionization interface and the removal of all plastic components near the heated zones. By the use of a clean bath gas and curtain gas, the ionization region was constantly purged with nitrogen gas to prevent the deposition of contaminants on the inside surfaces and to protect the orifice from obstruction. The cover of the heated-shield interface together with the presence of the bath and curtain gas also increased the desolvation of solvent clusters, most likely by the application of more concentrated heat during the ionization process.