28.12.2013 Views

preface to fifteenth edition

preface to fifteenth edition

preface to fifteenth edition

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

7.124 SECTION 7<br />

TABLE 7.75<br />

Iso<strong>to</strong>pic Abundances and Masses of Selected Elements<br />

(a) Abundances of some polyiso<strong>to</strong>pic elements, %<br />

Element Abundance Element Abundance Element Abundance<br />

1<br />

H 99.985<br />

16<br />

O 99.76<br />

33<br />

S 0.76<br />

2<br />

H 0.015<br />

17<br />

O 0.037<br />

34<br />

S 4.22<br />

12<br />

C 98.892<br />

18<br />

O 0.204<br />

35<br />

Cl 75.53<br />

13<br />

C 1.108<br />

28<br />

Si 92.18<br />

37<br />

Cl 24.47<br />

14<br />

N 99.63<br />

29<br />

Si 4.71<br />

79<br />

Br 50.52<br />

15<br />

N 0.37<br />

30<br />

Si 3.12<br />

81<br />

Br 49.48<br />

(b)<br />

Selected iso<strong>to</strong>pe masses<br />

Element Mass Element Mass<br />

1<br />

H 1.0078<br />

31<br />

P 30.9738<br />

12<br />

C 12.0000<br />

32<br />

S 31.9721<br />

14<br />

N 14.0031<br />

35<br />

Cl 34.9689<br />

16<br />

O 15.9949<br />

56<br />

Fe 55.9349<br />

19<br />

F 18.9984<br />

79<br />

Br 78.9184<br />

28<br />

Si 27.9769<br />

127<br />

I 126.9047<br />

Now substituting the percent abundance of each iso<strong>to</strong>pe ( 79 Br and 81 Br) in<strong>to</strong> the expansion,<br />

2 2<br />

(0.505) 2(0.505)(0.495) (0.495)<br />

gives<br />

0.255 0.500 0.250<br />

which are the proportions of P :(P 2):(P 4), a triplet that is slightly dis<strong>to</strong>rted from a 1:2:1<br />

pattern. When two elements with heavy iso<strong>to</strong>pes are present, the binomial expansion (a b) m (c <br />

d) n is used.<br />

Sulfur-34enhances the P 2 peak by 4.2%; silicon-29 enhances the P 1 peak by 4.7% and<br />

the P 2 peak by 3.1%.<br />

7.8.1.3 Exact Mass Differences. If the exact mass of the parent or fragment ions are ascertained<br />

with a high-resolution mass spectrometer, this relationship is often useful for combinations of C, H,<br />

N, and O (Table 7.75b):<br />

Exact mass difference from nearest<br />

integral mass 0.0051z 0.0031y number of hydrogens<br />

0.0078<br />

One substitutes integral numbers (guesses) for z (oxygen) and y (nitrogen) until the divisor becomes<br />

an integral multiple of the numera<strong>to</strong>r within 0.0002 mass unit.<br />

For example, if the exact mass is 177.0426 for a compound containing only C, H, O, and N (note<br />

the odd mass which indicates an odd number of nitrogen a<strong>to</strong>ms), then<br />

0.0426 0.0051z 0.0031y 7 hydrogen a<strong>to</strong>ms<br />

0.0078

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!