DRIFTS internal standards

As described in Section 13.3, using an instrument for a long period can result in variation in instrument calibration, or drift. This can be detected in solution analysis by the addition of a known amount of an internal standard to the sample solutions. This is impossible, however, for LA-ICP-MS analysis of a solid sample. One method of minimizing this is to simultaneously measure a... 

During a single run, which may take all day if a large number of samples are to be analyzed, the instrument may drift from its optimum settings. To detect this drift in solution-based techniques, and also to compensate for some matrix effects, a known amount of an element may be added to each sample before analysis. This internal standard (also called a spike) is added to all the samples and blanks, with the exception of the instrument blank (which is defined as zero concentration for all elements see below). It is important that the element (or isotope) chosen as the spike is not an element which is to be determined in the samples, and preferably which does not occur naturally in the samples. It must not be an element which will cause, or suffer from, interference with the other elements to be determined. In solution ICP-MS,... 

Drifts of migration times can partly be compensated by calculating the mobility for analyte identification and using corrected PAs or internal standards for quantitation. 

It is also possible to use an internal standard to correct for sample transport effects, instrumental drift and short-term noise, if a simultaneous multi-element detector is used. Simultaneous detection is necessary because the analyte and internal standard signals must be in-phase for effective correction. If a sequential instrument is used there will be a time lag between acquisition of the analyte signal and the internal standard signal, during which time short-term fluctuations in the signals will render the correction inaccurate, and could even lead to a degradation in precision. The element used as the internal standard should have similar chemical behaviour as the analyte of interest and the emission line should have similar excitation energy and should be the same species, i.e. ion or atom line, as the analyte emission line. 

An easy calibration strategy is possible in ICP-MS (in analogy to optical emission spectroscopy with an inductively coupled plasma source, ICP-OES) because aqueous standard solutions with well known analyte concentrations can be measured in a short time with good precision. Normally, internal standardization is applied in this calibration procedure, where an internal standard element of the same concentration is added to the standard solutions, the samples and the blank solution. The analytical procedure can then be optimized using the internal standard element. The internal standard element is commonly applied in ICP-MS and LA-ICP-MS to account for plasma instabilities, changes in sample transport, short and long term drifts of separation fields of the mass analyzer and other aspects which would lead to errors during mass spectrometric measurements. 

An advantage of ICP-MS compared to all other atomic mass spectrometric techniques including TIMS is that usually only simple sample preparation (e.g., by microwave induced digestion of solid samples) is necessary. Sample preparation steps for ICP-MS analyses are similar to those of ICP-OES. Concentrated solutions are analyzed after dilution with high purity water only. In order to correct mass drifts of the instrument, an internal standard element like In or Ir with known concentration (e.g., I Op.g 1) is added. The solution is then acidified with HN03 to stabilize the metal ions in aqueous solution. 

This simple method is used in industry for repetitive analyses. For such analyses, the chromatograph must be equipped with an autosampler, including a sample tray and an automatic injector. The single reference solution, periodically injected for control purposes, can be used to compensate for baseline drifts. It is not necessary to add an internal standard to each of the samples, as discussed below. 

CH4 and mass range, 33-500 at one scan/s. Decachlorobiphenyl (10 ng/pL) was added as an internal standard prior to derivatization and p-chlorobenzophenone was added (20 ng/pL) just prior to injection on the GC-MS to correct for any changes during sample handling and to normalize drift in MS sensitivity. These steps permitted the determination of changes in the concentrations of detected components. 

For quantitative analysis, determine the linear range required to measure the least concentrated and most concentrated analytes. If desired, select an internal standard. If the migration time or peak area of the standard changes, then you have an indication that some condition has drifted out of control. 

Although laser-ablation sample preparation and analysis are conducted with relative ease, quantification of data can prove challenging. With liquid samples, the amount of material introduced into the ICP-MS remains relatively constant, and instrument drift is usually corrected through the use of internal standards. However, in LA-ICP-MS, conditions such as the texture of the sample, ablation time, the location of the sample within the laser cell, surface topography, laser... 

To improve reproducibility of measurements, internal standardization is required to correct possible instrument drift or changes in the ablation efficiency. The isotope 65Cu was selected as an internal standard because it is present in every sample at relatively high concentrations allowing for accurate measurement. Figure 1 shows signals after normalization. 

To be able to produce quantitative data, an internal standard and external standards are required. Internal standardization corrects for possible instrument drift or changes in the efficiency of the ablation and thus improves the... 

An internal standard is needed to compensate for differences in physical properties (such as viscosity) between the calibration standard and the test samples and drift caused by thermal changes in the laboratory that will affect the instrument optics. An appropriate internal standard element should not be naturally present in the test samples in appreciable concentrations and should not present spectral interferences with any analyte. In addition, the internal standard should be a strong emitter so that its relative concentration can be kept low, and be as chemically similar to the analyte as possible. 

The response factor for each analyte in the calibration standard, often referred to as the relative response factor, is calculated relative to the response of the internal standard as shown in Equation 9, Appendix 22. Unlike the response (calibration) factor used in the external standard calculations, relative response factor is unitless. Once the relative response factors have been calculated, the average relative response factor, the RSD, the percent difference or drift are calculated according to Equations 2-5, Appendix 22. 

Isotope Ratios and Internal Standardization The ability to extract all ions from an incoming ion beam simultaneously allows the TOF-MS to provide better precision than sequentially scanned ICP-MS instruments. Provided that the dominant source of noise is multiplicative in nature, all elements and isotopes should experience the same perturbations. Therefore, ratioing techniques such as isotopic dilution should allow compensation for drift and source noise, and isotope-ratio measurements should improve in precision. 

The transfer line consists of an electrically heated stainless steel tube, through which an uncoated, yet deactivated fused silica transfer capillary is passed until the end of the plasma injector. All parts of the stainless steel transfer tube are heated, including the part inside the torch box. The ICP-MS instrumentation is prone to signal suppressions and/or instrumental drift. These problems can be compensated by the use of internal standards. In the case of GC-ICP-MS the internal standard can be added to the carrier gas of the GC apparatus. A suitable internal standard is Xenon (Xe) [41]. The 126Xe signal is monitored simultaneously with the other isotopes of interest. In this way instrumental drift and signal suppression can be corrected. 

Use of Internal Standards The use of internal standards envisages different possibilities. The procedure described here is based on two internal standards. Once thawed, fish sample were dissolved in TMAH, ethylated with NaBEt4, extracted into iso-octane and subjected to GC-ICP-MS for the identification and quantification of Me-Hg and inorganic Hg2+. For the correction of procedural errors two internal standards were used. The sample pretreatment was corrected by the recovery factor of the spiked dibutyl-dipentyl-Sn (DBT-pe), while the GC-ICP-MS measurements were controlled by the signal stability of Xe added to the GC carrier gas [47], In another application propyl-Hg was used as an internal standard to correct for matrix-induced ion signal variation and instrumental drift [65]. 

With some instruments, the instability of the analyser is such that there is a drift in mass on relatively short times, say less than 1 day. When this drift is large, the use of an internal standard or a dual source to inject ions alternately from the sample and from the standard is needed. The first solution increases the number of ions in the analyser, and thus the importance of the space charge effect. The second increases the acquisition time. Frequent recalibration of the spectrometer may also be required. If the instrument is stable, a lock mass can be used either from a compound added to the sample or from a known ion from the solvent or the background. 

The first coupling of a capillary gas chromatography to a sector field ICP-MS for the speciation of organometallic compounds present in a synthetic sample was described by de Smaele et cd Transient ion signals of °Sn, ° Hg+ and ° Pb+ were measured using Xe+ as an internal standard (to correct possible drifts of the magnetic field and plasma instabilities). 

Quantitative analysis is normally performed by preparing calibration curves using external standards. To compensate for instrument drifts, instabilities, and matrix effects, an internal standard is usually added to the standards and to the sample. Multiple internal standards are sometimes used to optimize matching of the characteristics of the standard to those of various analytes. 

Through the use of more than one diode and tuning them to several analyte lines, multielement determinations and the use of an internal standard become simple. In AAS work the latter also enables instrumental drift to be overcome and/ or the short-term precision to be improved as well. For the detection, all that is required is pulsing of the primary source and the use of lock-in amplification or a Fourier analyzer. 

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