ANALYTICAL CHEMISTRY
1190 (Figure 6). The approximate capacity . . is 0.5 ml.; the approxi_mate weight is 1.5 c a m s . SIZE B. This weighing bottle (Figure 16) should be made from an aluminum allov. and the cau should be lauaed t o fit the body joint. It is descgned for use-with the c o m h s t i o n boat, size B (Figure 2), as shown in the assembly drawing. The manner in which i t is used is described under weighing bottle, pigtype, metal, size A. The approximate capacity of the bottle is 1 ml.; the approximate weight is 4.5 grams. SIZE C. This weighing bottle (Figure 17) should be made of a n aluminum alloy, and the cap should be lapped t o fit the body joint. It is designed for use with the combustion boat, size C (Figure 3), as shown in the assembly drawing. The manner in which it is used is described under weighing bottle, pig-type, metal, size A. The approximate capacity is 2 ml.; the approximate weight is 6 grams. Tare Flasks. Three types of tare flasks (12) are recommended, one with and two without a ground-in stopper (Figure 18). They should be made preferably of soda-lime glass ( I d ) . On all three flasks the serial numbers should be etched in order to avoid rough surfaces. Modified Abderhalden Drying Apparatus. The committee, as well as others in the field, believed that the commonly used Abderhalden pistol dryers needed modification, particularly the desiccator bulb, which must be supported while removed from the main body. There was also danger of breaking the side arm when attaching rubber tubing to it, or on account of the weight of the tubing during actual use. The drying chamber has also been modified so that, in addition to the old practice of drying in vacuo, samples may be dried by the passage of dry air a t reduced pressure, in accordance with the newer methods of drying (18,19). Figure 19 shows the modified Abderhalden drying apparatus intended to eliminate the disadvantages referred t o above. The connection of the desiccator bulb to the vacuum by means of the ball and socket joint minimizes the risk.of breakage and makes the desiccator bulb less unbalanced. The shape of the tube attached t o the ball joint, in the desiccator bulb, is intended t o prevent the desiccant from being carried over into the sample when the vacuum is broken. The tube a t the left of the stopcock and to which the vacuum line is connected may be bent as de-
sired. 4 cap for the ball joint and a stopper for the T40/50 joint may be used to protect the desiccant when the desiccator bulb is temporarily disconnected and stands alone. The end view of part A (Figure 19) shows an upward indentation in the vapor tube located as near as possible t o the reflux return. This prevents cooling of the drying chamber by cold condensate. LITERATURE CITED
(1) Alber, H. K., Mikrochemie, 18, 92 (1935). (2) Alber. H. K., Mikrochemie ver. Mikrochim. Acta, 29, 294-328 (1941). (3) ANAL.CHEM.,21, 651 (1949). (4) Benedetti-Pichler, A. A., private communication. (5) Bromund, W. H., private communication. (6) Friedrich, A , , and Lacourt, A., “La Pratique de la Microanalyse Organique Quantitative,” 2nd (French) ed., p. 335, Paris, Dunod, 1939.
(7) Grant, J., “Quantitative Organic Microanalysis,” based on the methods of Fritz Pregl, 5th English ed., p. 115, Philadelphia,
Blakiston Co., 1951. (8) Hayman, D. F., IND. EKG.CHEM.,ANAL.ED.,10, 55 (1938). (9) Hayman, D. F., private communication. (10) Roth, H., “F. Pregl Quantitative Organische Mikroanalyse,” 5th iluflage, p. 131, Wien, Springer-Verlag, 1947. (11) Steyermark, Al, ANAL.CHEM?., 22, 1228 (1950). (12) Steyermark, 41, “Quantitative Organic Microanalysis,” pp. 30, 31, 34, 108, 179, Philadelphia, Blakiston Co., 1951. (13) Steyermark, Al, Alber, H. K., Aluise, V. A., Huffman, E. W.
D., Jolley, E. L., Kuck, J. A., Moran, J. J., and Willits, C. O., ANAL.CHEM.,23,1689 (1951). (14) Steyermark, Al, Alber, H. K., Aluise, V. A , , Huffman, E. W. D., Kuck, J. A,, &loran, J. J., and Willits, C. O., Ibid., 21,
1283 (1949). (15) Ibid., p. 1555. (16) Ibid., 23, 537 (1951). (17) Van Slske. D. D.. and Folch, J. (with J. Plazin), J . Biol. Chem.. 136,509-41 (1940). (18) Willits, C. O., A x ~ LCHEM., . 23, 1058 (1951). (19) Willits, C. O., and Ogg, C. L., private communication. RECEIVEDfor review April 7, 1954. Accepted April 21, 1954. Presented as a report before the Business Meeting of the Division of Analytical Chemistry a t the 124th Meeting of the AMERICAN CHEMICAL SOCI~TY Chicago, , Ill.
Terminology for Describing the Performance of Analytical and Other Precise Balances 1954 Report and Recommendations of the Committee on Balances and Weights L. B. MACURDY, Chairman, H. K.-ALBER,A. A. BENEDETTI-PICHLER, HUGH CARMICHAEL, A. H. CORWIN, ROBERT M. FOWLER, E. W. D. HUFFMAN, P. L. KIRK, and T. W. LASHOF
T
HIS is a report and recommendation of the Committee on Balances and Weights in the Division of Analytical Chemist r y of the AMERICAN CHmrIcAL SOCIETY. This committee is a continuation of the earlier Committee on Microchemical Balances (10). The scope and name of the committee were broadened t o include all precision balances and weights of interest t o the chemist. The new Committee on Balances and Weights held its first meeting on January 16, 1953, a t the National Bureau of Standards, Washington, D. C. At this meeting it was decided that the committee would consider terminology for describing the performance of balances, procedures for testing balances, and the requirements of the chemist in the field of precision weighing. .4t ita second meeting, January 22, 1954, also held a t the National Bureau of Standards, the committee adopted the following report on terminology. This report, originally drafted by T. W.Lashof, was approved for publication in ANALYTICAL CHEMISTRYby
the Executive Committee of the Division of Analytical Chemistry on March 29, 1954. T PRESENT
there is no generally accepted terminology for
A describing the performance of analytical and other precise balances. This has caused much confusion in interpreting manufacturers’ literature and purchasers’ needs. The word “sensitivity” is the center of much of this confusion. The word has usually been used without being defined. Sensitivity has been defined or used without definition in the following conflicting ways:
1. As the change in load in one of the pans required to change the swing-i.e., sum of turning points-by one division ( 7 , 8). 2. ils the change in load required t o change the rest point by one division ( 5 , 6). 3. .4s the change in load required t o produce a perceptible change of indication: A perceptible change is sometimes defined t o be division on swing for an analytical and assay balance
V O L U M E 2 6 , NO. 7, J U L Y 1 9 5 4 and l / l o division on swing for a semimicro or microbalance equipped with an index magnifier (I, 9,4). 4. As the angular deflection of the rest position of the beam per unit change in the load. 5 and 6. As divisions deflection, either of rest point or of swing, per unit change in the load. T o avoid confusion it is recommended that the word sensitivity be used in accordance with the definition in the Merriam-Webster Dictionary: “the rate of displacement of the indicating element with respect to the change of the measured quantity.” There are four characteristics of a balance which, because of loose terminology, are often confused. 1. That characteristic or quantity which may be defined as “the rate of displacement of the indicating element with respect to the change of the measured quantity” or the reciprocal or some other function of this. 2. That characteristic or quantity which may be defined as “the least load change which can cause an easily discernible movement of the pointer or indicator.” 3. That charact,eristic or quantity which may be defined as “the ability of the balance t,o give the same result, in two or more weighings of the same thing.” 4. That characteristic or quantity which may be defined as “the agreement between the result of a weighing and the true value of the quantity weighed.”
As all of these have been expressed, rightly or wrongly, as so many “milligrams,” the possible confusion is apparent. A measuring instrument, consists of a sensing element (that part of the instrument which is sensitive to the quantity being measured) and an indicating element (pointer and scale, optical lever, etc.). The angular deflection of the beam per unit change in mass is the measure of the sensitivity of the sensing element in the case of a beam balance. This is sometimes called the “absolute sensitivity” of the balance. The readability of the scale is the measure of the fineness of the indicating element. For theorebical purposes, the atisolute sensitivity in, say, radians per milligram and the readability in, say, centimeters would be satisfactory measures of the first txvo characteristics of a balance. For practical purposes, such as in the manufacturer’s description of the performance of a particular model, the four wordssensitivity, readability, precision, and accuracy-may be used with qualitative or general meanings for the four characteristics of the balance. These words with modifiers (such as sensibility reciprocal) or other ,~vords(such as readable to, standard deviation, error) should be given specific quantitative definitions. The following definitions are recommended. 1 . Sensitivity. Sensitivity is defined as “the ratio of the change in the response to the change of the quaiitit>-measured,” and should be used as the general heading under which is listed this ratio, i t a reciprocal, or any other function of the ratio. Senaitivity and sensibility may be used interchangeably.
I . 1 . SEXSIBILITY RECIPROCAL. The sensibilit,y reciprocal or SR of a balance is the change in load required to change the equilibrium position-i.e., the rest point-by one division at, any load wit’hin the load limitations of the instrument. The SR is expressed in mass units per division. The minimum size of a division must be established. I t is recommended that for a precision balance there be a graduated index scale with at least 20 equal divisions of not less than 1.0 mm. per division, except that if the balance is provided with an index magnifier the latter requirement should be applied t o the magnified image of the graduations. The use of SR as defined above is in agreement with its usage wibh commercial balances (9)and government specifications for analytical balances (3). SR, SR/2, or SR/4 may be applied in the laboratory in computing the mass equivalent of rest,-point differences (see 2.2 below). The sensibility reciprocal may depend upon the load and other factors. Hence the conditions under which it is measured should be described.
1191 2. Readability. Readability, when used in the general sense as a heading, is defined as “the smallest easily discernible change either in the magnitude of the response or in the magnitude of the quantity measured.” 2.1. READABILITY. The readability of a balance is the smallest fraction of a division t o which the index scale can be read with ease either by estimation or by use of a vernier. Readability of a balance should normally be expressed in divisions. The sharpness of the index and of the graduations of the index scale and the maximum distance between the index and the index scale should be established. It is recommended that for a precision balance the width of the index line or pointer and of each of the graduations of the index scale be not more than 0.1 division. If there is a possibility of parallax, it is recommended that the index be a distance of not more than 0.2 division from the index scale. Under these conditions it seems reasonable to state that the balance is “by estimation readable t o 0.1 division.” 2.2. MILLIGRAM EQUIVALEXT O F READABILITY. Knowledge of the sensibility reciprocal and the readability, as defined above, is sufficient for calculating the [‘milligram equivalent of the readability” obtainable by any method of weighing and computation. For example, if the sensibility reciprocal is 8 mg. per division and the readability is 0.1 division, the milligram equivalent is 0.8 mg. when a direct weighing is used and the computations are based upon the position of the rest point. If, however, computationa are based upon the sum of the turning points, a factor of 2 is gained, and if the transposition method of weighing is used another factor of 2 is gained. Hence with these methods of computation and weighing the milligram equivalent of the readability is
41 X
0.8 = 0.2 mg.
(With these methods of computation and weighing, it is convenient to use SR/4 in the computations.) The milligram equivalent of the readability will vary if the sensibility reciprocal varies. 3. Precision. Precision (2)has to do with “the degree of agreement of repeated measurements of the same quantity.” It is usually more convenient to work with a measure of dispersion (such as standard deviation); dispersion increases as precision decreases and vice versa. However, the word precision should be used as the general heading under which measures of both precision and dispersion may be listed.
3.1. STANDARD DEVIATION. The standard deviation of a balance is the standard deviation, S, of repeated weighings of the same mass, a weighing being considered to involve the determination of two rest points (other than those used to determine the sensitivity)-e.g., for a direct weighing, the zero-load rest point and the rest point a t the test load. The equation to be used in computing the standard deviation 1s
where f is the number of degrees of freedom for computing the standard deviation and the d s are the residuals (11). (If ben repeated weighings of the aame mass are made, J = 9, since one degree of freedom is used in computing the average. The d”s are the differences between the individual weighings and their average.) The standard deviation of a balance should be expressed in mass units. Because the precision obtainable with a balance depends not only on the balance but on the skill of the observer, 6he method of weighing, and the surrounding conditions of temperature, vibration, etc., these factors must be established. A statement of precision should include a statement of the procedure and conditions under which the precision was obtained. I t should also be assumed that the test was made by a reasonably experienced observer. The statement of precision should also include a statement of the load at which the test was made. 4. Factors Influencing Accuracy (othen than precision). ACCURACY. Aecuracy ( 8 ) is ‘%he agreement between the resalt
1192
ANALYTICAL CHEMISTRY
Table I.
Definitions of Terms Referring to Performance of Precision Balances Sensitivity. General term to be used as “the ratio of the change in the response t o the change of the quantity measured” SENSIBILITY RECIPROCAL ( S R ) . The SR of a balance is the change in load required to change the equilibrium position-i.e., rest point-by one division at any load within the load limitations of the instrument. Unit. Mass units per division (width of division 1 mm. or more) Readability READABILITY. The readability of a balance is the smallest fraction of a division to which the index scale can be read with ease either by estimation or by use of a vernier. Unit. Divisions Precision. General term to be used as “the degree of agreement of repeated measurements of the same quantity” STANDARD DEVIATION.The standard deviation of a balance is the standard deviation, S , of repeated weighings of the same mass, a weighing being considered to involve the determination of two rest points (other than those used to determine the sensitivity). Unit. Mass units (usually gram, mg., or y) Equation.
was employed. Such a list might include ( a ) inequality of arms; ( b ) over-all adjustment, variation with load, and nonlinearity of a direct-reading scale; (c) errors of the chain due t o incorrect setting of the chain knife-edge, nonlinear distribution of mass along the chain, eccentricity or other defect of the drum on which the chain is wound, and the setting error or the “up-down” difference in the effective values of the chain when a position of the chain scale is carefully approached from below and from above; ( d ) errors in the total mass of the rider, in the positions of the rider notches, and in setting the rider in a notch (tilt error of wire rider, eccentricity error of cylindrical rider); ( e ) errors in the masses of the drop-on weights; and (f) errors resulting from change in zero rest point with change in temperature or relative humidity. Some of the errors listed above may under some circumstances be treated not as systematic errors but as random errors, and hence may be included in the “precision” of the balance. This is particularly true of the errors arising from setting the rider in a notch and the “up-down” error of a chain. The recommendations of the committee are summarized in Table I. LITERATURE CITED
Factors Influencing Accuracy (other than precision), Accuracy is to be used as ”the agreement between the result of a measurement and the true value of the quantity measured.” Some of the factors influencing accuracy are listed below. Definitions for these factors will be established in connection with test procedure. Inequality of arms Errors in masses of drop-on weights Position error of rider notch Nonlinearity of chain Error in mass of rider Nonlinearity of direct-reading scale
of a measurement and the true value of the quantity measured.” Unfortunately, we can only “guess” a t the true value of any quantity. This is done b y correcting for all known systematic errors. Thus accuracy is not a very useful performance parameter for the general characterizing of an instrument, It is more appropriate for evaluating a particular technique of measurement, such as weighing with standard and unknown in different pans, use of a “direct-reading” scale without calibration, etc. Under the general heading, “accuracy,” or “factors influencing accuracy,” should be listed the possible systematic errors present in the result of a measurement in which a particular technique
(1) Ainsworth and Sons, Inc., Wm., Denver, Colo., “Laboratory Balances and Weights,” Catalogue AW 12 (1948). (2) Eisenhart, C., Photogrammetric Eng., 18, 542 (1952). (3) Federal Specification for Analytical Balances, AAA-B-92, May 17, 1949, General Services Administration Regional Offices, or General Services Administration Regional Office Building, Seventh and D Sts. S.W., Washington 25, D. C. (4) Fisher Scientific Co., Pittsburgh, Pa., “Modern Laboratory Appliances,” Catalog 90 (1949). (5) Goldstein, S. W., and Mattocks, A. M., J . Am. Pharm. Assoc., 12, 293 (1951). (6) Indiana State Board of Health, Division of Weights and Measures, Indianapolis 7, “Questions Submitted by Members and Answered at Conferences of the Indiana Association of Inspectors of Weights and Measures,” Question S-121. (7) MacNevin, W. >I., “The Analytical Balance, Its Care and Use,” Sandusky, Ohio, Handbook Publishers, Inc., 1951. (8) Meek, R. E., “Information Concerning Prescription Scales and Prescription Weights,” Indiana State Board of Health, Division of Weights and Measures, Indianapolis 7, Ind. (9) Natl. Bur. Standards, Handbook 44, 92 (1949). (10) Rodden, C. J., Kuck, J. 4.,Benedetti-Pichler, A. A., Corwin, A. H., and Huffman, E. W. D., IND.ESG. CHEM.,ANAL. ED., 15, 415 (1943). (11) Youden, W. J., “Statistical Methods for Chemists,” p. 13, New York, John Wiley & Sons, 1951. RECEIVED for review April 7, 1954. .4ccepted M a y 27, 1954
Quantitative Method Using Paper Chromatography For Estimation of Reducing Oligosaccharides W. H. WADMAN, GWEN J. THOMAS, and ARTHUR B. PARDEE Department of Plant Biochemistry a n d Department of Biochemistry, University of California, Berkeley, Calif.
I
E T H E past, separation b y paper chromatography of oligo-
saccharides which contain more than four monosaccharide units has not been practicable oiving t o the low R, values of these substances. Bayly and Bourne ( 2 ) have recently overcome this difficulty by converting the sugars into the corresponding N-benzylglycosylamines. The increase in R, values is such that dextrins containing up to ten units are easily separated. French and Wild have also studied the chromatography of carbohydrates (S). Quantitative separations had been achieved before this b y use of charcoal columns. Whistler and Durso (8) have successfully separated sugars, the quantities used being of the order of 1 gram. Disadvantages of the method are that intermixing of adjacent compounds can beconsiderable and that small amounts of certain compounds may be overlooked.
Another quantitative method, described by Alm (I), involves application of a continuously increasing concentration of ethanol in water as eluting agent applied to a mixture of sugars on a carbon adsorbent which has been pretreated with stearic acid. I n this way, 5-mg. samples of hydrolyzed a-Schardinger-dextrin could be rapidly and quantitatively resolved. I n this report a method is described for rapid separation and estimation of oligosaccharides on a microgram scale, using a technique similar t o that of Bayly and Bourne. The sugars are allowed t o react with AT-( 1-naphthyl)-ethylenediamine and are then separated by paper chromatography. The amount of each sugar may be estimated from the brightness of fluorescence of its derivative under ultraviolet light.
