Optimizing the multielement analysis capabilities of an ICP quadrupole mass spectrometer using electrothermal vaporization sample introduction

A mathematical solution and an experimental procedure are described for calculating the maximum number of transient signals, such as those obtained using ETV sample introduction into an ICPMS, that can be monitored by a scanning spectrometer, such as a quadrupole mass analyzer. The total number of masses that can be monitored per ETV firing is shown to be dependent on the values chosen for data collection (i.e., scan time and dwell time) and the necessary limits of detection required by the method. The theory shows the effect that statistical noise, peak shapes, and inconsistent peak appearance times have on the overall variance calculated for the sample concentration based on total counts or "peak area". Also included in the variance calculation is the contribution made by the autosampler as an example of one type of error that is not associated with the data collection parameters. The theory is validated by two experiments where 21 and 68 transient signals are monitored per ETV firing; and as predicted, a 10 ppb sample is accurately quantified with precision better than 9% in both cases.     

Preliminary investigation of electrothermal vaporization sample introduction for inductively coupled plasma time-of-flight mass spectrometry

The coupling of an electrothermal vaporization (ETV) apparatus to an inductively coupled plasma time-of-flight mass spectrometer (ICP-TOFMS) is described. The ability of the ICP-TOFMS to produce complete elemental mass spectra at high repetition rates is experimentally demonstrated. A signal-averaging data acquisition board is employed to rapidly record complete elemental spectra throughout the vaporization stage of the ETV temperature cycle; a solution containing 34 elements is analyzed. The reduction of both molecular and atomic isobaric interferences through the temperature program of the furnace is demonstrated. Isobaric overlaps among the isotopes of cadmium, tin, and indium are resolved by exploiting differences in the vaporization characteristics of the elements. Figures of merit for the system are defined with several different data acquisition schemes capable of operating at the high repetition rate of the TOF instrument. With the use of both ion counting and a boxcar averager, the dynamic range is shown to be linear over a range of at least 6 orders of magnitude. A pair of boxcar averagers are used to measure the isotope ratio for silver with a precision of 1.9% RSD, despite a cycle-to-cycle precision of 19% RSD. Detection limits of 10-80 fg are calculated for seven elements, based upon a 10-microL injection.     

Quantitative thin-layer chromatography/mass spectrometry analysis of caffeine using a surface sampling probe electrospray ionization tandem mass spectrometry system

Quantitative determination of caffeine on reversed-phase C8 thin-layer chromatography plates using a surface sampling electrospray ionization system with tandem mass spectrometry detection is reported. The thin-layer chromatography/electrospray tandem mass spectrometry method employed a deuterium-labeled caffeine internal standard and selected reaction monitoring detection. Up to nine parallel caffeine bands on a single plate were sampled in a single surface scanning experiment requiring 35 min at a surface scan rate of 44 mum/s. A reversed-phase HPLC/UV caffeine assay was developed in parallel to assess the mass spectrometry method performance. Limits of detection for the HPLC/UV and thin-layer chromatography/electrospray tandem mass spectrometry methods determined from the calibration curve statistics were 0.20 ng injected (0.50 muL) and 1.0 ng spotted on the plate, respectively. Spike recoveries with standards and real samples ranged between 97 and 106% for both methods. The caffeine content of three diet soft drinks (Diet Coke, Diet Cherry Coke, Diet Pepsi) and three diet sport drinks (Diet Turbo Tea, Speed Stack Grape, Speed Stack Fruit Punch) was measured. The HPLC/UV and mass spectrometry determinations were in general agreement, and these values were consistent with the quoted values for two of the three diet colas. In the case of Diet Cherry Coke and the diet sports drinks, the determined caffeine amounts using both methods were consistently higher (by approximately 8% or more) than the literature values.     

Chemically assisted laser ablation ICP mass spectrometry

A new laser ablation technique combined with a chemical evaporation reaction has been developed for elemental ratio analysis of solid samples using an inductively coupled plasma mass spectrometer (ICPMS). Using a chemically assisted laser ablation (CIA) technique developed in this study, analytical repeatability of the elemental ratio measurement was successively improved. To evaluate the reliability of the CLA-ICPMS technique, Pb/U isotopic ratios were determined for zircon samples that have previously been analyzed by other techniques. Conventional laser ablation for Pb/U shows a serious elemental fractionation during ablation mainly due to the large difference in elemental volatility between Pb and U. In the case of Pb/U ratio measurement, a Freon R-134a gas (1,1,1,2-tetrafluoroethane) was introduced into the laser cell as a fluorination reactant. The Freon gas introduced into the laser cell reacts with the ablated sample U, and refractory U compounds are converted to a volatile U fluoride compound (UF6) under the high-temperature condition at the ablation site. This avoids the redeposition of U around the ablation pits. Although not all the U is reacted with Freon, formation of volatile UF compounds improves the transmission efficiency of U. Typical precision of the 206Pb/238U ratio measurement is 3-5% (2sigma) for NIST SRM 610 and Nancy 91500 zircon standard, and the U-Pb age data obtained here show good agreement within analytical uncertainties with the previously reported values. Since the observed Pb/U ratio for solid samples is relatively insensitive to laser power and ablation time, optimization of ablation conditions or acquisition parameters no longer needs to be performed on a sample-to-sample basis.     

A graphite furnace electrothermal vaporization system for inductively coupled plasma atomic emission spectrometry

An improved graphite furnace electrothermal vaporization device equipped with an autosampler for inductively coupled plasma atomic emission spectrometry is presented. The transport losses of eight selected analytes in the individual segments of the device were determined by means of the radiotracer technique by applying amounts traced comparable to those to be determined in real samples. The results obtained from the radiotracer study were the basis for further improvement of the interface design, leading to considerable increase of the total transport efficiency, which finally was found to be between 26 (for Cr) and 57% (for Ga). The whole system consists of a graphite furnace vaporizer, a power supply, a gas flow box, and an autosampler with incorporated microbalance. The temperature program, gas flows, and autosampler functions are controlled by a data station which also provides the data acquisition and processing of the transient signals. The performance parameters of the developed system were evaluated using aqueous standard solutions. Absolute limits of detection for most analytes were between 0.1 and 1 ng, and for As, K, Ni and Pb, they were between 2 and 3.2 ng.     

Application of quantitative chemometric analysis techniques to direct sampling mass spectrometry

This paper explores the use of direct sampling mass spectrometry coupled with multivariate chemometric analysis techniques for the analysis of sample mixtures containing analytes with similar mass spectra. Water samples containing varying mixtures of toluene, ethyl benzene, and cumene were analyzed by purge-and-trap/direct sampling mass spectrometry. Multivariate calibration models were built using partial least-squares regression (PLS), trilinear partial least-squares regression (tri-PLS), and parallel factor analysis (PARAFAC), with the latter two methods taking advantage of the differences in the temporal profiles of the analytes. The prediction errors for each model were compared to those obtained with simple univariate regression. Multivariate quantitative methods were found to be superior to univariate regression when a unique ion for quantitation could not be found. For prediction samples that contained unmodeled, interfering compounds, PARAFAC outperformed the other analysis methods. The uniqueness of the PARAFAC model allows for estimation of the mass spectra of the interfering compounds, which can be subsequently identified via visual inspection or a library search.     

Diode laser thermal vaporization inductively coupled plasma mass spectrometry

An approach of sample introduction for inductively coupled mass spectrometry (ICPMS), diode laser thermal vaporization (DLTV) is described. The method allows quantitative determination of metals in submicroliter volumes of liquid samples. Laser power is sufficient to induce pyrolysis of a suitable substrate with the deposited sample leading to aerosol generation. Unlike existing sample introduction systems based on laser ablation, it uses a NIR diode laser rather than an expensive high-energy pulsed laser. For certain elements, this sample introduction technique may serve as an alternative to solution analysis with conventional nebulizers. Using a prearranged calibration set, DLTV ICPMS provides rapid and reproducible sample analysis (RSD ~ 10%). Sample preparation is fast and simple, and the prepared samples can easily be archived and transported. The limits of detection for Co, Ni, Zn, Mo, Cd, Sn, and Pb deposited on the preprinted paper were found to be in the range of 0.4-30 pg. The method was characterized, optimized, and applied to the determination of Co in a drug preparation, Pb in whole blood, and Sn in food samples without any sample pretreatment.     

On-line separation and determination of bromate in drinking waters using flow injection ICP mass spectrometry

A flow injection (FI) system with a microcolumn of anion exchanger has been used to effect rapid on-line separation of bromate and bromide prior to quantitation by ICP mass spectrometry. Basic performance studies are described including the effect of key FI parameters, i.e., sample injection volume, carrier stream flow rate, and eluent concentration on system response. The new approach permitted ultratrace determinations of bromate in drinking waters, the main benefits being low limit of detection (0.13 microg/L based on a 500-microL sample injection), rapid analysis time (10 min/sample), and good precision (2.8% at the 5 microg/L level). Accuracy was checked via an EC-sponsored interlaboratory trial.     

Direct analysis of single rat peritoneal mast cells with laser vaporization/ionization mass spectrometry

A linear time-of-flight mass spectrometer was used as a detector for flow cytometry. These two techniques were coupled by a laser vaporization/ionization interface. The estimated mass detection limit of the combined system was 20 amol of serotonin standard with one laser pulse. An aqueous buffer at physiological pH was used to ensure compatibility with cells. Rat peritoneal mast cells (RPMCs) were dispensed into the mass spectrometer in a single file confined within a 20-micron-i.d. capillary. By using the mass spectrometer as a detector, no precolumn staining or derivatization is required. Determination of serotonin and histamine in individual cells was demonstrated. With this method, hundreds of cells can be analyzed within a few minutes. The average amounts of histamine and serotonin per RPMC were found to be 0.75 +/- 0.33 and 0.11 +/- 0.06 fmol, respectively. No correlation was found between the amounts of the two amines in each cell.     

Thin-layer chromatography and electrospray mass spectrometry coupled using a surface sampling probe

A combined surface sampling probe/electrospray emitter was used for the direct readout of thin-layer chromatography plates by electrospray mass spectrometry. The technique was demonstrated with reversed-phase C18 plates using a three-dye mixture composed of methylene blue, crystal violet, and rhodamine 6G for positive ion mode detection and a separate dye mixture containing fluorescein, naphthol blue black, and fast green FCF for negative ion mode detection. Acquisition of mass spectra of components of individual bands on the plate was shown by manual stepping to and sampling from specific locations within the bands. Computer-controlled scanning of development lanes on the plate was illustrated by using multiple ion monitoring in both positive and negative ion modes. Commercial TLC plates were used and no post-separation processing other than drying of the plates was required prior to mass spectrometric analysis. Readout resolution, the limits of scan speed, detection levels, TLC phase, and eluting solvents were investigated and discussed.     

Metal powder substrate-assisted laser desorption/ionization mass spectrometry for polyethylene analysis

Polyethylene is one of the most important industrial polymers and is also one of the most challenging polymers to be characterized by mass spectrometry. We have developed a substrate-assisted laser desorption/ionization (LDI) mass spectrometric method for polyethylene analysis. In this method, cobalt, copper, nickel, or iron metal powders are used as a sample substrate and silver nitrate is used as the cationization reagent. Using a conventional UV LDI time-of-flight mass spectrometer, intact oligomer ions having masses up to 5000 u can be detected. Cobalt is found to produce spectra with the highest signal-to-noise ratio and the lowest level of fragmentation. Cobalt powder size is shown to have some effect on the spectra produced. The best results are obtained with the use of cobalt powders with diameters ranging from 30 to 100 microm. Fragmentation cannot be totally eliminated, but the fragment ion peaks can be readily discerned from the intact polyethylene ions in the substrate-assisted LDI spectrum. Thus, the average molecular masses of low-mass polyethylene samples can be determined by using this method. A rapid heating model is used to account for the effectiveness of using the coarse metal powders to assist the analysis of intact polyethylene molecules by LDI.     

Helium plasma source time-of-flight mass spectrometry: off-cone sampling for elemental analysis

In this paper, an atmospheric pressure, helium microwave-induced plasma (MIP) ion source coupled with an orthogonal acceleration time-of-flight mass spectrometer (TOFMS) is explored for elemental analysis. Studies of the relationship between ion signals and sampling distance of the MS reveal that background signals can be suppressed dramatically without sacrificing the signal intensities of analytes when the microwave plasma plume is off the tip of the sampler orifice. This "off-cone" ion sampling mode provides a technique to obtain nearly "clean" background spectra and, thus, eliminates the spectral interference from entrainment air and the working-gas species, making it possible to sensitively determine isotopes that suffered from spectral interference in ICPMS and MIPMS (such as 40Ca, 52Cr, 55Mn, and 56Fe). On the other hand, since the high-temperature plasma is kept away from the sampler aperture, off-cone sampling places little demand on the cooling device and the lifetime of the sampler plate can be extended. The instrumental system can provide a fairly good mass resolution of 1100 (fwhm). The detection limits (3sigma) in the tens of picograms per milliliter level for the elements studied can be achieved with a digital oscilloscope. These detection limits can be easily improved with an advanced detection system, which is currently available in commercial markets.     

Determination of phosphorous and sulfur in environmental samples by electrothermal vaporization inductively coupled plasma atomic emission spectrometry

From the viewpoint of selective introduction of the analyte from its solvent and matrices, electrothermal vaporization (ETV) is useful for the sample introduction into the inductively coupled plasma (ICP). By using a tungsten boat furnace (TBF) vaporizer system, the loss of analyte phosphorus, which normally occurs during the drying and ashing stages, is suppressed. The phosphate ion is reacted with the tungsten supplied from the surface of the TBF to form stable tungsten phosphate species. Regarding the determination of sulfur, additional chemical modifiers such as copper(II), lead(II), etc., are necessary to retain the analyte on the TBF. The furnace-fusion (FF) method or wet-digestion technique on the TBF is applied to unify the chemical forms of the analytes. Various oxidative and reductive inorganic compounds as well as organic compounds of phosphorus and sulfur show the same sensitivities after the FF digestion with hydrogen peroxide. The detection limits are 1.5 ng and 0.12 ng for phosphorous and sulfur, respectively. The repeatabilities in terms of the relative standard deviations of 10 replicate measurements of phosphorus and sulfur are 4.2% and 2.0%, respectively. Finally, the established method is applied to the determination of several environmental waters.     

Valved sampling cell for membrane introduction mass spectrometry

Membrane extractors comprising a membrane house inside of a valve have been developed to separate compounds of interest from a sample matrix and introduce these compounds into a mass spectrometer. Experimental control over parameters that affect permeability or that may damage the membrane, such as the membrane temperature, is provided with the valve. The valve was tested for response and response times with the valve separated from the mass spectrometer by various interface tube lengths. Data for steady state response measurements showed no significant change with the valve at different distances from the ion source. Polar compounds show a strong response time dependency on the interface tube length. This adsorption phenomenon is minimized by simply heating the interface tube. Other factors affecting the performance of the device are discussed.     

Nanovolume analysis with secondary ion mass spectrometry using massive projectiles

A variant of secondary ion mass spectrometry is presented where the surface is bombarded with individual gold nanoparticles each resolved in time and space with a corresponding event-by-event detection of the secondary ions (SIs). The projectile used, Au400(4+), with impact energy of 136 keV, generates high SI yields. Typically, there is co-emission of multiple SIs from a single impact, i.e., emission of SIs from molecules co-located within a nanovolume with dimensions in the 10-nm range. The ability to detect co-located molecules was tested on samples consisting of alternating nanometric layers of oppositely charged polyions, poly(diallyldimethylammonium chloride), poly(styrenesulfonate) (PSS), and clay nanoplatelets. To achieve signal statistics, the chemical analysis was carried out with a sequence of stochastic impacts making this method suitable for characterization of similar nanoparticles or spots dispersed on a surface. Attomole detection sensitivity was achieved for PSS. The homogeneity of assembled layers could be assessed with approximately 10-nm resolution.     

Selective sampling of phosphopeptides for detection by MALDI mass spectrometry

The analysis of phosphopeptides by mass spectrometry (MS) is one of the most challenging tasks in proteomics. This is due to the lower isoelectric point (pI) of phosphopeptides, which leads to inefficient sample ionization in MS, particularly when competing with other peptides. The problem is compounded by the typical low abundance of phosphopeptides in biological samples. We describe here a simple nonsorptive method to isolate phosphopeptides based on their pI. A voltage is applied to selectively migrate the phosphopeptides into a capillary, which are negatively charged at acidic pH. The selectively sampled fraction is directly deposited onto MALDI sample target in nanoliter volumes (7-35 nL) for highly sensitive MS detection. No significant sample loss is evident in this procedure; hence, the MS was able to detect the isolated phosphopeptides at trace quantity. In this case, attomole-level detection limit is achieved for synthetic phosphopeptides (nM concentration and nL volume), from a mixture containing other peptides at up to 1 million times higher in concentration. Selective sampling was also applied to the tryptic digest of beta- and alpha-caseins to reveal the multiple phosphorylated peptides at the low-femtomole level using MALDI MS. Knowledge of pI based on the rejection/injection of peptides was found to be useful in peak assignment. To confirm the sequence of the selectively sampled peptides, fraction collection was performed for offline ESI MS/MS analysis.