External calibrations

With the FT-ICR mass spectrometer it is possible to obtain high mass accuracy (about 1-5 ppm calibrated externally, 0.5-1-0 ppm if calibrated internally here an external calibration has been carried out using a tune mix containing compounds with m/z from... 

Standardize the reagents prepare the needed calibration (external, internal or standard addition) standards... 

There are different concepts for the calibration. External calibration uses the measurement of separate samples containing known amounts of analyte and we compare the signal from both, the calibration sample and our unknown sample. This method requires that the differences in the matrix of sample and calibration sample does not significantly influence the response. The use of an internal... 

True profile analysis requires scanning over the whole mass range for the acquisition of all data on excreted compounds. Quantitation has been more challenging on a quadrupole instrument because total ion current peaks are seldom a single component and extracted-ion chromatograms (EICs) when recovered from scanned data are of poor quality due to the lower sensitivity of scanning GC-MS. Thus, we developed profile analysis based on SIM of selected analytes but tried to ensure the components of every steroid class of interest were included. For ion traps the fundamental form of data collection (in non-MS/MS mode must be full -scans). Thus, the quantitative data produced are EICs obtained from scanned data. The EICs are of the same ions used for SIM in quadrupole instruments and the calibration external standards are the same. 

The fourth recommendation of the OECD experts is related to appropriate measuring and reporting goodness-of-fit, robustness, and predictivity of the model. The main intention was to clearly distinguish, whether a measure was derived only from the training set, from the internal validation (i.e., cross-validation, where the same chemicals are used for training and validation, but not at the same time) or from validation with use of an external set of compounds, not previously engaged in model optimization and/or calibration (external validation). A widely applied measure of fit is the squared correlation coefficient R2-1 — (RSS/ TSS), where RSS is the residual sum of squares and TSS is the total sum of squares... 

Calibration (external validation) of ecotoxicological (Q)SAR predictions is obviously a major concern for the USEPA because it regulates newly notified chemicals... 

ESMS was perfonned with a Fisons VG Quattro outfitted with a Hsons Electrospray Source. Samples were dissolved in 1.0 mL of 50% methanol-1% acetic acid, then diluted 1 10 with 50% acetonitrile-1.0 mM anmumium acetate to give 25 pmol/pL. A 10 pL aliquot of each sample was injected into a 10 pL/min stream of 50% acetonitrile-1.0 mM ammonium acetate. Data was processed using Fisons MassLynx Software. MALDI-MS was performed with a Vestec Benchtop lit linear dme-of-flight mass spectrometer, opmted in the linear mode with an N2 laser (337 nm). Samples were dissolved in 1.0 mL of 25% acetonitrile-0.1% TFA, then diluted 3 100 to give 5-10 pmol/pL. A 0.5 pL aliquot of each sample solution was added to 0.5 pL of matrix [a-cyano-4-hydroxycinnamic aci saturated solution in 50% acetonitrile-2% TFA]. Samples were dried at ambient temperature and pressure. Each spectrum was the sum of ion intensity from 10-50 larer pulses. Tlie mass axis was calibrated externally. 

Prerequisites for the Calibration Types. It depends on the design of the analytical procedure as to which regression parameters are meaningful and which results are acceptable. In other words, the model to be used for quantitation must be justified. For a singlepoint calibration (external standardization), a linear function, zero intercept, and the homogeneity of variances are required. The prerequisites for a linear multiple-point calibration are a linear function and in case of an unweighted calibration also the homogeneity of variances. A non-linear calibration requires only a continuous function. With respect to the 100%... 

At all stations and laboratories the QA/QC procedures are systematically implemented, such as EU standards, inter-calibration, external audits, validation and verification. At EMEP,... 

Three primary methods are used for calibration external standard calibration, internal standard calibration, and standard addition. Figs. 1-3 show the typical response curves for each method. 

Fig. 10. Mass spectra of from MALDIMS, and from ESIMS. For MALDIMS the mass scale was calibrated with myoglobin ions as an internal standard. For ESIMS the mass scale was calibrated externally using the doubly charged and singly charged ions of gramicidin S. The right hand panel of the ESIMS spectrum shows the deconvoluted spectrum, giving measured molecular weights for the mtgor species, which differ by one hexose. Deviations from the calculated values are shown in parentheses.

Figure 3. Net platelet retention behavior on polyethylene catheters in four goats. Thrombus platelet concentration catheter length was used to calibrate externally detected platelet retention curves. The repeatability of the initial slope ana the variable times to peak are shown. Key I, Goat 32 2, Goat 36 ...

Like all classical quantitative analysis methods, NMR spectroscopy needs calibration, calibration standards and a validation procedure. The standard techniques are used for calibration external calibration, the standard addition method and the internal standard method. A fourth is a special NMR calibration method, the tube-in-tube technique. A small glass tube (capillary) containing a defined amount of standard is put into the normal, larger NMR tube filled with the sample for analysis. In most cases, there are slight differences in the chemical shift of corresponding signals of the same molecule in the inner... 

The most commonly used quantification methods in purge and trap are external standard calibration and internal standard calibration. External standards refer to target compounds of known concentrations that are prepared and analyzed separately from the samples. A calibration curve or calibration factor (CF) is obtained by plotting the peak area count (or height) of the analyte versus its concentration in the standards. The analyte concentration in the sample is equal to its peak area divided by the CF. External standard calibration works well for relatively clean samples where matrix has no or little effects on analyte recovery. 

MS, MS/MS and MS experiments were performed with a LTQ-Orbitrap (Thermo Fisher Scientific Inc., San Jose, CA, USA) mass spectrometer equipped with a Triversa Nanomate (Advion Biosciences Inc., Ithaca, NY, USA). Compound solutions were infused with a nanospray chip. The mass spectrometer was first calibrated externally with a mixture containing caffeine, L-methionyl-arginyl-phenylalanyl-alanine (MRFA) and Ultramark 1621 in ACN, MeOH, HjO, acetic acid. Sub-ppm mass accuracy was finally achieved using an internal calibration (lock mass) in both MS and MS/MS mode. The resolution of Orbitrap MS was set to 100,000 (FWHM) at m/z 400. Electrospray ionization (ESI) was used. For MS/MS experiments, an isolation width of 1.5 Da was used. The normalized collision energy was set to the value when the precursor ion was exhausted. Helium was used as the collision gas. 

The ESI ion source of micrOTOF-Q spectrometer should operate at a voltage of 4.5 kV nebulization with nitrogen should be carried out at 1.6 bar with the dry gas flow of 8.0 L/min at a temperature of 220°C.The system should be calibrated externally using the calibration mixture containing sodium formate clusters. Additional internal calibration should be performed for every run by injection of the calibration mixture using the divert valve during the LC separation. Such a calibration must have at least 5 ppm mass accuracy. MS/MS spectra should be acquired with the frequency of 2 scans/s for the most abundant ions or ions chosen on the basis of preliminary experiments. Collision energy should depend on molecular masses of consecutive compounds and should be set between 10 and 25 eV. 

The great advantage of matrix elimination is the ability to prepare standards in deionized water and to calibrate externally. Kaiser et al. obtained detection limits between 0.2 and Ipg/L for anions such as chloride, sulfate, orthophosphate, and nitrate. A representative chromatogram of a sample spiked with 10 pg/L each of the anions of interest is shown in Figure 10.138. The lonPac ASIO high-capacity anion exchanger used as a stationary phase was operated with a NaOH eluent. 

Since a six-port valve rotates to divert the eluent stream through the filled sample loop, there is a small internal volume (ie. the volume of the valve without the sample loop) which must necessarily be injected. This internal volume can be determined by filling the sample loop with a solution of N,N-dimethyl-p-phenylazoaniline dye dissolved in benzene and injecting the contents into a volumetric flask. The solution is diluted to volume with benzene and the dye concentration determined spectrophotometrically. The sample loop is calibrated externally in a similar marmer. The volume of the small sample loop (4 ul) is subtracted from total volume injected to give the true sample injected volume (2-10 ul). 

Calibration External standard Based on 1L samples from a SUMMA... 

Calibration External standard Range 0.10-4.00 mg/L, matrix spike... 

Calibration External standard Target compounds spiked into blank samples... 

Zhang, L.-S., and Combs, S. M. (1998). Double calibration (external calibration and stable isotope dilution) for determining selenium in plant tissue by hydride generation inductively coupled plasma mass spectrometry.J. Assoc. Off. Anal. Chem. Int. 81(5), 1060. 

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