CZ304648B6 - Method of preparing samples for atomic spectrometric techniques - Google Patents

Abstract

The present invention relates to a method of sample preparation for atomic spectrometric techniques, wherein a liquid analyte or analyte solution, emulsion or suspension is applied onto a solid substrate and is subsequently sampled by desorption pulses, which may be pulses of laser radiation or discharge. At the same time, the substrate is made of a material or covered by a layer of material that absorbs the used desorption pulses. Alternatively, a liquid starting sample along with at least one auxiliary substance that absorbs the said desorption pulses can be applied to the substrate.

Description

Technical field

The invention relates to a method of preparing samples from liquid precursor samples for atomic spectrometric techniques.

BACKGROUND OF THE INVENTION

Atomic spectrometry is the part of optical spectrometry that uses the signal produced by electron transitions in atoms of the determined elements and inorganic mass spectrometry. In analytical practice, atomic spectra in the optical (UV / VIS) and X-ray regions are used for the determination of individual elements; emission, absorption or fluorescence is measured. To use optical atomic spectrometry, it is necessary to convert the determined elements of the analyzed sample to the state of free atoms (atomization), because the formation of optical spectra is related to the transition of valence electrons. In the case of inorganic mass spectrometry, the atoms formed must be additionally ionized. Atomization, resp. Ionization usually takes place at very high temperatures in so-called atomization environments. Methods of optical atomic spectrometry are divided according to electron transitions between energy levels of atoms into emission, absorption and fluorescence. Optical emission spectrometry includes several methodologies suitable for elementary qualitative and quantitative analysis. These methodologies differ according to the excitation source used in which the sample atoms are vaporized, atomized and excited. These sources are characterized by a very high temperature (up to 10 ⁴ K). Frequently used excitation sources include inductively coupled plasma (ICP) with a plasma temperature of up to 11000 K, a glow discharge (GD) and recently also laser radiation (laser induced plasma spectrometry, LIBS) has been used for atomization of solid samples. Atomic spectrometric techniques further include flame atomic absorption / emission spectrometry (FAAS / FAES) and atomic fluorescence spectrometry (AFS) for which samples are prepared in the same way as for ICP spectrometry, ie by misting or solid ablation.

Inductively coupled plasma spectrometry - inductively coupled plasma mass spectrometry (ICP MS) or inductively coupled plasma emission spectrometry (ICP OES) are one of the groups of highly sensitive atomic spectrometric techniques. ICP MS is a type of mass spectrometry suitable for the determination of a wide range of metals and some non-metals at concentrations below 1 ppt. It is based on the connection of the inductively coupled plasma technique as a method of ion preparation (ionization) with mass spectrometry as a method of ion separation and detection. Inductively coupled plasma optical emission spectrometry (ICP OES) is a type of inductively coupled plasma emission spectrometry to produce excited atoms emitting electromagnetic radiation whose wavelength is typical of each element and whose intensity determines the concentration of that element in the sample. The inductively coupled plasma discharge is initiated by a spark discharge from the transformer and maintained in a plasma head consisting of three concentric tubes made of quartz glass. The end of this head is inserted into an induction coil, which is guided by an alternating current of radio frequency. Between the two outer tubes, a stream of argon is conducted into which an electrical spark is briefly applied to generate free electrons. These electrons interact with the magnetic field induced by the coil and are accelerated alternately in both directions. Accelerated electrons collide with argon atoms, which are ionized after collision and released electrons are again accelerated by a magnetic field. This process continues avalanche until the rate of electron release is equal to the rate of recombination of free electrons and argon atoms. This creates a plasma of the order of 10 ⁵ K, which is maintained in the head due to the flow of external plasma gas (Ar) between the outer and middle tubes. The central plasma gas (Ar) flowing between the central and inner tubes serves to isolate the plasma from the central tube. A carrier gas (Ar or a mixture of Ar and He) flows through this, passing through the center of the plasma and forming an analytical channel in which the sample aerosol is desolvated, most molecules atomized, atoms excited and ionized (mostly one charged cation). To this stream of carrier gas, before

The sample is introduced in the form of an aerosol. If the liquid sample is misted, it is a wet aerosol. When solid material is sampled using, for example, laser ablation, it is a dry aerosol, where a laser ablation auxiliary gas (Ar) can be added to the carrier gas stream (He).

At present, nebulizers are used to introduce the liquid sample into the inner tube of the head. Another method of introducing the sample into the plasma, laser ablation, is used for the analysis of solid samples (steel, ceramics, compressed tablets of powdered materials, etc.). The essence of laser ablation is the evaporation of the sample layer by means of a focused laser beam, wherein the vaporized sample is carried by the carrier gas to the plasma head.

Another method using focused laser radiation for solids sampling is LIBS (Laser Induced Plasma Spectrometry). The principle of the method is the interaction of the high-energy pulse of the laser with the sample and subsequent detection of the emission of the resulting plasma formation. Since the plasma contains not only ambient particles, but also excited and ionized particles of the sample, its elemental composition can be inferred from the emission spectra. The advantage of this approach is the speed and simplicity of measurement, where we can obtain spectral information for qualitative and quantitative evaluation of the content of elements even from a single laser pulse using a suitable detection system.

In the flame atomic spectrometry, FAES, FAAS, and AFS (flame atomic fluorescence spectrometry) techniques, a sample aerosol is introduced into the flame, a column of a gas fuel / oxidant mixture formed at the end of the burner (eg acetylene). -air). As with ICP, a moist or dry aerosol can be introduced into the flame. The emission, absorption or fluorescence of atoms in the flame is measured.

In the GD (glow discharge) technique, the analyte is atomized and atomized, followed by excitation and ionization of atoms by an electric discharge. It is an important method for analysis of chemical composition of metallic and non-metallic materials such as nitrides, carbides, oxides, etc. Emission of excited atoms by optical spectrometer or ion signal by mass spectrometer is detected.

Only online nebulizers are used to combine ICP MS / OES and related atomic spectrometry flame techniques with column separation techniques. The use of nebulizers for on-line connection with microcolumn separation techniques is often associated with difficulties such as induced flow in the separation column, loss of separation efficiency, memory effects or dilution with additional liquid.

The disadvantages of prior art sample preparation methods from liquid starting samples are overcome by the present invention.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for preparing samples from liquid precursor samples for atomic spectrometric techniques, which comprises depositing a liquid precursor sample on a solid support and sampling with desorption pulses, wherein the support is made of material or coated with a material absorbing said desorption or a liquid starting sample is applied to the support together with at least one excipient absorbing said desorption pulses.

The desorption pulses are selected from the group consisting of laser pulses and discharges.

The liquid starting sample may be the liquid analyte itself, an analyte solution, an analyte emulsion, or an analyte suspension. The liquid starting sample may be applied in one of two forms:

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a. the liquid starting sample itself, optionally with admixtures resulting from prior treatment and analysis of the sample (eg, separation buffer components).

b. a liquid starting sample with at least one excipient to aid in its evaporation by the desorption pulses used. Substances which absorb the used desorption pulses may be used as excipients. Suitable excipients can be aromatic compounds, colorants and suspensions of carbon or metal in an organic solvent. Examples of aromatic compounds are aromatic acids such as 2,5-dihydroxybenzoic acid, α-cyano-4-hydroxy cinnamic, nicotinic, picolinic and the like. Examples of dyes are fluoresceins, coumarins. Examples of a suspension of metal or carbon in an organic solvent are ultrafine cobalt or carbon powder in glycerine. The liquid starting sample may be mixed with the excipient prior to application to the carrier, or the excipient and the liquid starting sample may be applied separately.

The support may be a probe, in particular a plate, disc or belt shape, but also another suitable shape. The carrier may be of a variety of materials, preferably a material selected from the group consisting of metal, ceramic, glass, plastic. Advantageously, a carrier made of a material that absorbs used desorption pulses may be used, or a carrier coated with a layer of a material used to absorb used desorption pulses, such as plastic.

The application of desorption pulses to the sample carrier results in ablation or desorption of the sample. When absorbing excipients are used, desorption is promoted, i.e. the sample evaporates at a lower laser power density. In the case of desorption of the analyte mixed with the absorbing excipient, only the applied sample is vaporized by laser radiation, the carrier then remains intact and can be reused. If the absorbing carrier is ablated, the analyte deposited thereon is vaporized with the carrier.

For example, capillaries, piezoelectric pipettors, sprays and the like can be used to apply a liquid starting sample to the support. Many of these systems are currently available for sample preparation for matrix-assisted desorption and ionization mass spectrometry (MALDI MS).

After application of the liquid starting sample, the solvent may evaporate if the liquid starting sample was an analyte solution, emulsion or suspension.

Atomic spectrometry techniques for which samples can be prepared by the method of the present invention may be selected in particular from the group of methods including inductively coupled plasma spectrometry (ICP, OES, ICP MS), laser-induced plasma spectrometry (LIBS), glow discharge (GD), flame atomic absorption / emission spectrometry (FAAS / FAES), atomic fluorescence spectrometry (AFS). A preferred atomic spectrometric technique for practicing the invention is inductively coupled plasma spectrometry (ICP OES, ICPM MS) and desorption pulses are laser pulses.

In the case of combining the above techniques with column separation techniques, the liquid starting samples are individual fractions taken from the column (eluates). The method of the invention can be used to analyze liquid starting samples similar to the on-line nebulizer of ICP MS / OES, FAAS, FAES, and AFS techniques, but at the same time has the advantages of off-line analysis by laser ablation, ie archiving capabilities. The whole sample does not have to be consumed during the analysis, for example the possibility of re-analysis or the use of another analyzer. Current combinations of column separation techniques and atomic spectrometric techniques do not allow for the storage of a liquid starting sample.

In a preferred embodiment of the invention, the atomic spectrometric technique is ICP OES or ICP MS and the desorption pulses are laser pulses. The carrier with the loaded liquid starting samples is placed in an ablation / desorption cell in which it is sampled by laser pulses and the resulting aerosol is entrained by the carrier gas into the plasma head of the ICP MS / OES spectrometer.

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Sample evaporation is performed by scanning the pulsed laser across the area of the deposited liquid starting sample, or by focusing the laser beam at the center of the deposited liquid starting sample, the number of pulses and raster shape selected according to sample type and analysis requirement (sensitivity, speed, archiving the rest of the sample).

The highest sensitivity (the most complete use of the sample) is generally achieved by applying a small volume of the liquid starting sample to the support and evaporating the entire sample with a single laser pulse. Attention should be paid to factors affecting application (eg solvent selection, organic solvent content, temperature, pressure, excipient concentration, etc.); ideally, a thin film or layer of tiny analyte crystals on a support. A low (submicrolitre) volume of the liquid starting sample is sufficient for analysis.

The sample preparation method of the present invention is particularly suitable for analyzing small volumes of liquid starting samples and for off-line coupling of atomic spectrometry techniques to column and microcolumn separation techniques (liquid chromatography, capillary electrophoresis, etc.), in which case the eluate fractions are collected . Achieving optimal analysis results (sensitivity, separation efficiency) assumes that small amounts of eluate are deposited on the support so as to avoid separation of the separated zones and that the resulting fractions can be quantitatively evaporated by one or more laser pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

Giant. 1 shows the intensity of the ICP MS signal (53 amu) in a raster across the sample of 1 pmol ² CrO _"4 in the procedure of the Example 1.

Giant. 2 shows the integrated ICP MS signal intensities (53 a.m.u) for a series of fractions in the procedure of Example 2.

Giant. 3 shows an ICP MS signal (53 a.m.u) of a sample of chromate alone in the procedure of Example 3.

Giant. 4 shows an ICP MS signal (53 a.m.u) of a chromate sample with an adjuvant in the procedure of Example 3.

Giant. 5 shows an electropherogram of chromium speciation with absorption detection at 214 nm using the procedure of Example 4.

Giant. 6 shows an LD ICP MS electropherogram of chromium speciation with ⁵² Cr detection using the procedure of Example 4.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

Laser desorption of chromate from plastic substrate

A 50 x 26 x 0.5 mm PETG plate (polyethylene terephthalate glycol, Vivak, Bayer) was coated with droplets of a liquid starting sample - 10 μΜ of an aqueous solution of K2C1O4 in a subatmospheric chamber (Rejtar, T .; Hu, P .; Juhasz) , P .; Campbell, JM; Vestal, ML; Preisler, J .; Karger, BL, J. Proteome Res. 1, 171-179, 2002). At a pressure difference of 80 kPa, a capillary internal diameter of 51 µm, a capillary length of 296 mm and a collection time of one drop of 2 s, the droplet volume was -100 mL, corresponding to a substance amount of 1 pmol K ₂ CrO ₄ per drop. The diameter of the individual samples after solvent evaporation was about 0.2 mm, the center-to-center spacing was 1.5 mm. For laser desorption, a commercial ablation system, New Wave, model UP213, emitting laser radiation with a wavelength of 213 nm and a pulse frequency of 10 Hz was used. An S-shaped grid over a 400 x 400 pm area was selected, a screen speed of 100 pm / s, a desorption beam was blurred to 300 pm (beam diameter on the plate), a power density of 4.8 MW / cm ² (desorption beam energy 35%) in focused beam mode). The desorbed material was entrained with carrier gas (He) at a flow rate of 1.0 L / min from the ablation cell to the plasma head of an ICP MS spectrometer (Agilent, model 7500CE), laser ablation auxiliary gas (Ar), medium plasma gas (Ar) and the external plasma gas (Ar) was 0.6; 0.9 and 15.0 L / min, RF power 1390 W and plasma sampling depth of 8.3 mm. The ion signal (ICP MS signal) was recorded by a mass spectrometer at a mass of m = 52 amu with a signal integration time of 0.1 s. The collision cell with helium was used to reduce the ion signal of multi-atomic compounds, especially ArC ⁺ (m = 52 amu). mL / min). The recording of the ⁵² Cr ⁺ ion signal from one fraction is seen in Figure 1; ⁵² Cr ⁺ ion signal appears shortly after turning on the laser at ~ 10 s and disappears after turning off the laser and washing the ablation cell at ~ 28 s.

Example 2

Laser desorption of chromate from plastic substrate - verification of reproducibility of the technique

The reproducibility of the technique was investigated for a series of 100 nL volumes of the liquid starting sample - 1 μΜ of an aqueous solution of K ₂ CrO ₄ (0.1 pmol K ₂ CrO ₄ in fraction) deposited on a PETG-plate carrier according to Example 1. In this case linear linear motion in small steps passed over a series of samples and the resulting ion signal integrals for each of the 21 applied fractions are shown in Figure 2 for the two major Cr isotypes (m = 52 and 53 amu). The relative standard deviation (RSD) was 13%.

Example 1 and Example 2 demonstrate the use of a UV laser to ablate a strongly absorbing support on which a sample is deposited. Together with the carrier, the sample is then evaporated into the carrier gas stream (He), which is subsequently analyzed by ICP MS. The trajectory of the laser beam across the sample zone may be, for example, in the form of a raster (scan) covering the entire surface of the sample, lines intersecting the sample zone or a point lying in the sample zone. The fluctuation of the ion signal during the raster is not a problem; the signal value integrated over the whole raster area is important for the analyte content evaluation. With a suitable arrangement, a high reproducibility analysis with an RSD of the integrated signal of ~ 13% or less was achieved.

Example 3

Laser desorption of chromate in the presence of absorbing excipients

Two samples were prepared in a 44 x 26 x 0.5 mm glass plate in a subatmospheric deposition chamber; first by applying 100 nL of a 10 μΜ K ₂ CrO ₄ solution, the second by applying 100 nL of a 10 mg / mL solution of α-cyano-4-hydroxycinnamic acid excipient and then 100 nL of the 10 μΜ K ₂ CrO ₄ solution. Both prepared samples were analyzed under the above conditions in S-shaped raster mode, the laser was switched on at t - 0 s for 30 s. Figures 3 and 4 show the ⁵³ Cr ion signal in the absence and presence of excipient. Comparing Figs. 3 and 4, it is clear that the presence of an absorbing excipient, like the absorbing carrier, greatly aided the desorption of the analyte.

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Example 4

Speciation of Cr compounds by off-line coupling of capillary electrophoresis to ICP MS

This example demonstrates the use of the proposed sample preparation for coupling the microcolumn separation technique to ICP MS / OES. First, a simple capillary electrophoresis method for Cr speciation was developed with the Spectra 100 absorption detector (Spectra-Physics, USA). A solution of 1 mM Cr (NO ₃ ) 3 in 10 mM ammonium formate (pH = 3) was mixed 1: 1 with a 20 mM EDTA solution in 10 mM ammonium formate and heated to 70 ° C for 5 minutes. By mixing the solution of the thus prepared Cr complex with EDTA with a solution of 500 μΜ K ₂ CrO ₄ followed by dilution with a solution of 10 mM formic acid, a mixture for analysis containing 100 μΜ Cr ⁱⁿ and 100 μΜ Cr ^VI was prepared. A quartz separation capillary of 260 mm long, effective length 200 mm and an inner diameter of 75 µm was used for separation. The mixture was dosed for analysis at the difference in height of the dosing and detection vials, A / z = 20 mm for 5 s, corresponding to a dosed amount of 200 fmol (10 µg) for each Cr species. A 10 mM formic acid solution was used as the separation electrolyte; the separation voltage was 11.4 kV. In the electropherogram of a sample with absorption detection at 214 nm in Fig. 5, in addition to peaks of chromium species, peaks of other absorbing substances (EDTA, impurities), which are not analytically important, can be observed.

Under the same conditions, separation and application of capillary electrophoresis eluate to a PETG plate in an application chamber for off-line detection by laser desorption / ablation and ICP MS was performed. In this case, the effective and total length of the separation capillary was the same (200 mm). The separation capillary was coupled to the deposition capillary via a liquid coupling (Preisler, J .; Foret, F .; Karger, BL, Anal. Chem. 70, 5278, 1998), which was also filled with separation buffer (10 mM formic acid). The deposition conditions were the same as in the previous cases, i.e. the length of the deposition capillary was 296 mm, the internal diameter was 51 µm, and the deposition chamber pressure was 20 kPa. Fractions of the eluate (liquid starting sample) were collected at intervals of 2 s, the distances between the corresponding samples on the support were 1.5 mm. After loading, the samples were analyzed under the conditions of Example 1 by ICP MS, but in this example, point desorption from the center of each sample was used, with a desorption time of 2 s and a wash time of 3 s between samples. ⁵² Cr in individual fractions depending on migration time; both chromium species were successfully separated and detected. Compared to the electropherogram with absorption detection, the peak count was reduced in the electropherogram because only chromium species are detected.

Industrial applicability

The preparation of samples for atomic spectrometric techniques by the method of the present invention is applicable in a wide range of applications in the analysis of liquid samples by atomic spectrometric techniques as well as in combining these spectrometric techniques with column and microcolumn separation methods.

Claims (9)

PATENT CLAIMS A method for preparing samples from liquid precursor samples for atomic spectrometric techniques, wherein the liquid precursor sample is applied to a solid support and sampled by desorption pulses, characterized in that the support is made of or coated with a material absorbing said desorption pulses, or the carrier applies a liquid starting sample together with at least one excipient absorbing said desorption pulses. The method of claim 1, wherein the desorption pulses are selected from the group consisting of laser pulses and discharges. Method according to claim 1 or 2, characterized in that the liquid starting sample is a liquid analyte itself, an analyte solution, an analyte emulsion or an analyte suspension, optionally containing impurities resulting from the pretreatment and analysis of the sample. The method of claim 3, wherein the liquid starting sample is a column separation or microcolumn eluate. The process according to any one of the preceding claims, wherein the excipient is selected from the group consisting of aromatic compounds, colorants and suspensions of carbon or metal in an organic solvent. Method according to any one of the preceding claims, characterized in that the support is made of a material selected from the group consisting of metal, ceramic, glass and plastic. Method according to any one of the preceding claims, characterized in that the liquid starting sample is applied to the support by means selected from the group consisting of a capillary, a piezoelectric pipettor and a spray. Method according to any one of the preceding claims, characterized in that the atomic spectrometric technique is selected from the group comprising inductively coupled plasma spectrometry, laser-induced plasma spectrometry, glow discharge, flame atomic absorption / emission spectrometry and atomic fluorescence spectrometry. Method according to any one of the preceding claims, characterized in that the atomic spectrometric technique is inductively coupled plasma spectrometry and the desorption pulses are laser pulses.