Mass standard

Other instrumental advantages include its high sensitivity and a linear mass scale to m/z 10,000 at full sensitivity. The linearity of the mass scale means that it is necessary to calibrate the spectrometer using a single or sometimes two known mass standards. Some calibration is necessary because the start of the mass scale is subject to some instrumental zero offset. The digitized accumulation of spectra provides a better signal-to-noise ratio than can be obtained from one spectrum alone. 

Variations in the Force Due to Gravity. The mass of an object is the quantity of matter ia the object. It is a fundamental quantity that is fixed, and does not change with time, temperature, location, etc. The standard for mass is a platinum—iridium cylinder, called the International Kilogram, maintained at the International Bureau of Weights and Measures, ia Snvres, France. The mass of this cylinder is 1 kg by definition (9). AH national mass standards are traceable to this artifact standard. 

ASTM E617, Standard Specijication fiorEaboratoy Weights and Precision Mass Standards, American Society for Testing and Materials, Philadelphia, Pa., 1991, p. 2. 

Because an increase in resolution causes a decrease in sensitivity, it is best to operate at the lowest resolution commensurate with good results. Some instrument data systems will allow calibration with an external reference material such as perfluorokerosene and then use of a secondary reference material for the internal mass reference. Tetraiodothiophene, vaporized using the solids probe inlet, is recommended as the secondary reference. The accurate masses are 79.9721, 127.9045, 162.9045, 206.8765, 253.8090, 293.7950, 333.7810, 460.6855, and 587.5900. For a higher mass standard, use hexaiodobenzene. Because the mass defect for these internal reference ions are so large, a resolution of 2000 is ample to separate these ions from almost any sample ions encountered in GC/MS. 

Structure 2.1 Hz eigenfrequency, 14,200 tons moving mass Standard steel beams, serially produced joints ... 

Figure 2. SDS-PAGE of a six days old culture. Lane 1 and 6 molecular mass standard, lane 2 and 4 apple pectin culture medium, lane 3 and 5 glucose culture medium. Lanes 2 and 3 to isolates ru, lanes 4 and 5 to isolates T2. ...

The values of k (sometimes labeled a) and b are determined using spectra obtained with known mass standards. 

It is important that a measurement made in one laboratory by a particular analyst can be repeated by other analysts in the same laboratory or in another laboratory, even where the other laboratory may be in a different country. We aim to ensure that measurements made in different laboratories are comparable. We are all confident that if we measure the length of a piece of wire, mass of a chemical or the time in any laboratory, we will get, very nearly, the same answer, no matter where we are. The reason for this is that there are international standards of length, mass and time. In order to obtain comparable results, the measuring devices need to be calibrated. For instance, balances are calibrated by using a standard mass, which can be traced to the primary mass standard (see also Chapter 5). The primary standard in chemistry is the amount of substance, i.e. the mole. It is not usually possible to trace all of our measurements back to the mole. We generally trace measurements to other SI units, e.g. mass as in 40 mg kg-1 or trace back to reference materials which are themselves traceable to SI units. 

FIG. 4 SDS gel electrophoresis of insoluble gluten samples from laboratory corn wet milling. Lane 1 Molecular mass standards (250, 150, 100, 75, 50, 37, 25, 15, and lOkDa). Lane 2 Enzymatic milling with commercial protease with added S02 and lactic acid. Lane 3 Enzymatic milling with commercial protease and no added S02. Lane 4 Conventional laboratory milling. Lane 5 Enzymatic milling using Bromelain and no added S02. 

Hofstadler, S.A. Griffey, R.H. Pasa-Tolic, R. Smith, R.D. The Use of a Stable Internal Mass Standard for Accurate Mass Measurements of Oligonucleotide Fragment Ions Using ESI Fourier Transform Ion Cyclotron Resonance-MS With Infrared Multiphoton Dissociation. Rapid Commun. Mass Spectrom. 1998,12, 1400-1404. 

Reference solution (f). Use a solution of molecular mass standards suitable for calibrating SDS-PAGE gels in the range 15-67 kDa. 

Protein molar mass standards covalently labeled with dyes are valuable when the electrophoresis is followed by a Western blot or when electrophoresis has to be monitored. Furthermore, procedures of gel staining alter the geometry, and assignment of bands on blot to marker bands within the gel is sometimes difficult. 

Fig. 1. SDS-PAGE pattern of human IgG heavy chain eluted from PVDF membrane. Human IgG (10 pg) was resolved by SDS-PAGE and transferred to a PVDF membrane. The proteins on the membrane were stained with amido black in water, and the heavy chain was excised and eluted with guanidinium hydrochloride/lysophosphatidylcholine. After precipitation with absolute alcohol, the glycoprotein was subjected to analytical SDS-PAGE, and the gel was stained with Coomassie brilliant blue to ascertain its purity. Lanes 1 and 4 original commercial preparation of human IgG lanes 2 and 3 IgG heavy chain eluted from the PVDF membrane lane 5 mixture of molecular mass standards, from top to bottom phosphorylase b (94 kDa), BSA (67 kDa), ovalbumin (43 kDa), carbonic anhydrase (30 kDa), soybean trypsin inhibitor (20 ldDa), ct-lactalbumin (14.4 kDa).

KILOGRAM (kg). A unit of mass and is based upon a cylinder of platinum-iridium alloy kept by the International Bureau of Weights and Measures at Paris. A duplicate in the custody of the National Bureau of Standards at Washington is the mass standard for the United States. The kilogram is the only base unit still defined by an artifact. (A kilogram equals (1) 1,000 grams (2) 2.205 pounds (3) 9,842 x 10-4 long tons or (4) 1 102 x 10-3 short tons. 

The samples are prepared by mixing 2 pL of eluted protein, 2 pL H20, and 2 pL of an SDS-based denaturing sample buffer containing [3-mercaptoethanol as well as upper and lower protein mass standards. Samples are heated to 95°C for 2 min and spun briefly (see Note 7). 

MALDI analysis kits (e.g., Sequazyme peptide mass standards kit from Applied Biosystems) are a convenient way to obtain matrix, peptide standards, and solvents however, these items can easily be purchased independently. Any peptides that span a reasonable proportion of the useful mass range can be used for calibration, provided their accurate molecular weights are known. 

Wherever possible, a traceability chain of measured values terminates in an SI unit. When the base unit for mass is appropriate, this relationship is readily achieved through a mass standard, calibrated in terms of the kilogram prototype. The concept of traceability to SI has to be more carefully considered when conformity to SI depends on the SI concepts in the definition of the SI unit itself. 

Hierarchies, such as primary, secondary, and working level, or certified RMs and RMs are extensively used in describing traceability chains. Whilst such terms can be useful in explaining processes and links, they can also be confusing. For this reason their use has been limited in this paper. It is considered preferable to describe hierarchies in terms of the associated uncertainties. It can also be noted that, whereas in physical measurement it is common to have a hierarchy of references of the same basic type (e.g., a series of mass standards), this is rare in chemical measurement where the chain usually contains only one chemical RM, linked to a higher reference by a measurement process. 

In physical measurement, calibration standards are of prime importance, but in chemistry, standards such as mass standards and pure substance reference materials are necessary but not sufficient and often not the most problematic aspect of establishing traceability. As every analytical chemist knows, issues such as sampling, sample stability, contamination, interferences, and incomplete recovery of the analyte are usually the major contributors to measurement uncertainty. It is being increasingly recognized that if we wish to improve the traceability of chemical measurements, then we need to put the effort where the chemical problems are, and not where the problems are in physical measurement. It is a sign of maturity that this is now happening. 

Annual Book of ASTM Standards, ANSI/ASTM E617-97 Standard Specification for Laboratory Weights and Precision Mass Standards, Book of Standards Vol. 14.04, ASTM, 2003. 

Fig. 4. SDS-PAGE analysis of BglA treated with N-glycosidase F. Lane S, protein molecular mass standards lane 1, purified secreted BglA (2.4 pg) lane 2, purified secreted BglA (2.4 pg) treated with N-glycosidase F.

Figure 4.2 GC/MS analysis of methyl esters prepared from a whole cell lipid extract of the YEpOLEX-HzPGDsI-transformed ole1 strain of S. cerevisiae total ion spectrum of fatty acid methyl esters resolved by capillary GLC. The culture was grown in liquid complete medium (YPD) at 30°C to a density of 2 x 107 cells/ml prior to extraction and methylation. The external mass standard nonadecanoic acid methyl ester (19 Me) was added to the cell pellet prior to extraction. Peaks corresponding to Z9-16 Me and Z9-18 Me reflect trace amounts of unsaturated fatty acids in the growth medium. (Reproduced with permission from Rosenfield et at, 2001. 2001 by Insect Biochemistry and Molecular Biology.)...

Figure 7.11. The effects of exposure temperature on protein synthetic patterns of isolated gill tissue from specimens of 13°C-acclimated Tegula funebralis. Autoradiographic images illustrate newly synthesized (35S-labeled) proteins of several size classes (molecular mass standards are shown in the left lane). Two specimens from each temperature of incubation are shown. At temperatures above 24°C, synthesis of heat-shock proteins in the molecular mass ranges of 38, 70, 77, and 90 kDa is induced. Hsp synthesis becomes an increasingly large fraction of protein synthesis as exposure temperature increases, and by 38°C, only synthesis of hsp70 is observed. By 39° C, no protein synthesis takes place. (Figure modified after Tomanek and Somero, 1999.)...

M = weighing result, i.e., the counterbalancing steel mass da - air density at time of weighing1 D = density of mass standard, normally 8.0 g/cm3 d = density of sample... 

There are only two occasions where measured weight equals true mass, when k = 1. This occasion occurs when measurements are made in a vacuum or the density of a sample is equal to the density of the mass standard. Fortunately, the greatest differences only occur when an object s density is particularly low (0.1% for density 1.0 g/cm3 and about 0.3% for density = 0.4 g/cm3). In most situations, the effect of air buoyancy is significantly smaller than the tolerance of the analytical balance. The effects of varying densities (of objects being weighed) and varying air densities are shown in Table 2.22. 

From ASTM Designation E 617 Standard Specification for Laboratory Weights And Precision Mass Standards, reprinted with permission. 

Tolerances The analytical weights meet the tolerances of the American National Standard ANSI/ASTM E617, Laboratory Weights and Precision Mass Standards. This standard is incorporated by reference and should be consulted for full descriptions and information on the tolerances and construction of weights.3 Where quantities of 25 mg or less are to be accurately weighed, any applicable corrections for weights should be used. 

D17. Drozdz, R., and Naskalski, J. W., Lysozyme oligomers as a molecular mass standard for sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Anal. Biochem. 171, 419-422 (1988). 

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