Standard in SIMS

One problem, however, associated with the use of bulk standards in SIMS is homogeneity. This is particularly acute for the iron alloys. A recent SIMS study concluded that some of the NBS steel standards are unsuitable as standards in SIMS because all of the elements are not homogeneous within the sampling area of 100a [36, 69]. Therefore, researchers must be careful in both their choice of standard and which elements in each standard are suitable calibrants. 

Catecholamines. The quantitative determination of dopamine and noradrenaline in tissue samples of 0.1-10 mg at levels in the order of 0.5 pmol has been described [84]. These methods are based on extraction, formation of the pentafluorpropionyl derivatives, and the use of the homologues, a-methyidopamine and a-methylnoradrenaline as internal standards in SIM. Higher sensitivity than obtainable with fluorimetric or enzymic assays is reported [462J. Applications have been to amine determination in specific regions of rat brain [84] and to measurement of heart ventricle concentrations [463]. A combination of assays of this type with the use of synthesis inhibitors or radioisotope labelled precursors allows direct estimation of brain amine turnover in animals. 

Ion implants, particularly for semiconductor materials, are widely used as standards in SIMS. Implants can be made with accurate and controllable dopant concentrations. The implant concentration is known in atom/cm. Depth profiling the implant to obtain the integrated ion signal of the implanted dopant and accurately measuring the depth allows for the conversion of atom/cm to atom/cm. This assumes a constant sputtering rate through the depth profile, SIMS ion intensities linear with concentration, and an accurate depth measurement of the analysis crater [28]. 

Standards play a critical role in particular in the realisation of quantitative analysis by SIMS. Only a limited number of materials exist that are homogeneous on the micrometer spatial scale and attain the high level of standardisation appropriate to standard reference materials. It appears that only one certified reference material (CRM) exists for SIMS analysis (for semiconductor materials) [153]. Also, standardisation of SIMS has its main focus on inorganics, such as the quantification of dynamic SIMS [154, 155]. The role of standards in SIMS has been addressed [156]. 

SIMS is one of the most powerful surface and microanalytical techniques for materials characterization. It is primarily used in the analysis of semiconductors, as well as for metallurgical, and geological materials. The advent of a growing number of standards for SIMS has gready enhanced the quantitative accuracy and reliability of the technique in these areas. Future development is expected in the area of small spot analysis, implementation of post-sputtering ionization to SIMS (see the articles on SALI and SNMS), and newer areas of application, such as ceramics, polymers, and biological and pharmaceutical materials. 

Quantification of faecal BAs is carried out in SIM mode by using the internal standard method, and peak areas are obtained from the chromatograms generated by data handling. Component identification is based on fragmentation and comparison of the retention times with those of standards. 

Determinative and confirmatory methods of analysis for PIR residue in bovine milk and liver have been developed, based on HPLC-TS-MS (209). Milk sample preparation consisted of precipitating the milk proteins with acidified MeCN followed by partitioning with a mixture of -butylchloride and hexane, LLE of PIR from aqueous phase into methylene chloride, and SPE cleanup. The dry residue after methylene chloride extraction was dissolved in ammonium hydroxide, and this basic solution was transferred to the top of Cl8 SPE column. The PIR elution was accomplished with TEA in MeOH. For liver, the samples were extracted with trifluoroacetic acid (TFA) in MeCN. The aqueous component was released from the organic solvent with n-butyl chloride. The aqueous solution was reduced in volume by evaporation, basified with ammonium hydroxide, and then extracted with methylene chloride. The organic solvent was evaporated to dryness, and the residue was dissolved in ammonium acetate. The overall recovery of PIR in milk was 94.5%, RSD of 8.7%, for liver 97.6%, RSD of 5.1 %. A chromatographically resolved stereoisomer of PIR with TS-MS response characteristics identical to PIR was used as an internal standard for the quantitative analysis of the ratio of peak areas of PIR and internal standard in the pro-tonated molecular-ion chromatogram at m/z 411.2. The mass spectrometer was set for an 8 min SIM-MS acquisition. Six samples can be processed and analyzed in approximately 3 hours. 

The LTE model has been a point of controversy for several years [50] because the values used in the Saha-Egget equation to approximate the temperature and electron density of the assumed plasma are unrealistically high. The model nevertheless continues to be used in SIMS and for some cases the results can be quite good. It is recommended, however, that the model be tested against suitable calibration standards prior to analyzing unknowns [69-71]. 

The mass spectrometer must be operated in SIM mode. The 13C-analogs of isomers may be used as internal standards. The analytes are identified from then-relative retention times and characteristic masses. In low resolution MS, the characteristic masses for 2,3,7,8-TCDD are 320, 322, and 257. Use either 37C14-2,3,7,8-TCDD or 13C12-2.3.7.8-TCDD as an internal standard. The m/z for these two internal standards are 328 and 332, respectively. 

In quantitative mass spectrometry, the signal intensity depends not only on the amount of sample, but also on a number of other variables such as the ionization yield, focusing of the ion beam, and the amplification factor of the detector. As it is very difficult to keep these parameters constant over the whole period of analysis, nearly all quantitative applications of MS are based on a comparison of the ion current obtained from the component of interest, with the ion current obtained from a standard. In quantitative SIM this can be accomplished either by the continuous admission of a reference sample at a constant rate, concurrently with the sample under investigation, or by the use of an internal standard (IS) which is added to the sample prior to MS analysis (Halpern, 1981). The choice of this IS is of primary importance in the design of a new assay and was subject to some controversy in the late 1970s (Claeys et al., 1977 Lee and Millard, 1975 Millard, 1978b Self, 1979). Ideally, an IS should compensate for all possible losses during sample isolation, purification, derivatization, and separation steps and at the same time minimize variances due to the measurement process. In practice, the... 

It seems that in ERD-TOF technique, the coming Glass 0211 can act as a suitable standard for not only the mass and energy calibrations but also for the relative concentrations of several elements. Further work on different Corning Glass samples to explore the feasibility of establishing their use as calibration standards in surface analysis techniques, such as ERD, SIMS, ESCA and AES, are in progress. 

Some of these difficulties may be overcome by employing comparative well-pre-characterized standards. Nowadays SIMS has developed to an important technique in surface analysis - no)... 

GC-MS analysis is performed in SIM mode employing the analytical conditions reported in Table 6.11. Confirmation of the pesticide is established by the retention time of the target ion and the presence of three qualifier-to-target ion ratios (Table 6.12). Quantitative analysis is performed on the peak area ratio of the target ion divided by the peak area of the internal standard versus concentration of the calibration standards. 

Hj-Lorazepam, 3-hydroxy-1,4-benzodiazepine, 45, a sedative hypnotic and antianxiety agent, has been synthesized in a seven-step procedure presented in equation 12 for use as an internal standard in GC-MS-NICI-SIM (NICl-SIM = negative ion chemical ionization-selective ion monitoring) quantitative analysis of this drug in complex matrices encountered in forensic work and for study of its complex kinetics. The procedure involved selective trideuteriation, protection of the amino group and... 

Figure 5.4. The HS-SPME-GC/MS chromatograms recorded in SIM mode in the analysis of compounds reported in Table 5.5 (a) internal standards (33. d6-DMS, m/z 68 34. DPDS, m/z 108 35. MT, m/z 71 36. M ill, m/z 148), (b) analytes (37. EtSH, m/z 62 38. DMS, m/z 62 39. DES, m/z 75 40. MTA, m/z 90 41. DMDS, m/z 94 42. ETA, m/z 104 43. DEDS, m/z 122 44. ME, m/z 78 45. MTU, m/z 92 46. MTP, m/z 106 47. MTB, m/z 120 48. BT, m/z 135 49. IIMT, m/z 122).The SPME conditions are reported in Table 5.4. (Reprinted from Rapid Communications in Mass spectrometry 21, Fedrizzi et al., Concurrent quantification of light and heavy sulphur volatiles in wine by headspace sohd-phase microextraction coupled with gas chromatography/mass spectrometry, p. 710, Copyright 2007, with permission from John Whey Sons, Ltd.)...

Mass spectrum ions used for identification of geosmin are at m/z 111, 168, and 182. Quantification is performed on the ion at m/z 112 recorded in SIM mode. For internal standard 2-undecanone, the signal recorded is... 

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