that the desorption rate in CH 2 CI 2 was too rapid to be observed 
kinetically. The extraction time was fixed at 2.0 minutes. It 
should be noted here that reproducible time course studies were 
not demonstrated until we discovered the necessity of using 
cooling water (30°C) to prevent a temperature rise and sub¬ 
sequent loss problems during the extraction period. 
Effect of Solvent— 
Various solvents were tested for their extractive power. 
The conditions were identical to those used in Table 12. The 
results are shown in Table 12. It is clear that hexane gives 
decidedly poor extractive efficiency, while benzene, dichloro- 
methane and methanol give quite similar extractive efficiency. 
The choice of solvent depends upon more than the rate of 
extraction. The variation in total amounts of extracted organic 
matter was not measured. A simple color comparison of extracts, 
often clarification by centrifugation, showed that hexane exhi¬ 
bited least color, dichloromethane gave a visible light yellow 
color, and methanol showed a substantially deeper color. 
Analyses were performed on the extracts immediately after ex¬ 
traction. After a period of several weeks a qum would form in 
the dichloromethane extract and inconclusive tests suggested 
that dissolved organics might have been lowered in concentration 
as a result. Clearly, methanol would lend to a distinctly 
higher concentration of nonvolatile substances and these would 
be likely to foul the chromatograph. Hexane gave the cleanest 
extract but, of course, it has a poor yield. Benzene is toxic 
so this was ruled out. Therefore, dichloromethane was chosen 
for its high extractability, low toxicity, and ease of 
volatilization. 
TABLE 12. COMPARISON OF ULTRASONIC EXTRACTION EFFICIENCIES 
OF VARIED SOLVENTS MEASURED ON WORKING STANDARD* 
Solvent 
Trials 
Measured DBP 
(ppm) 
Hexane 
3 
0.34 + 0.07 t 
Benzene 
3 
0.65 + 0.02 
Dichloromethane 
3 
0.70 + 0.01 
Methanol 
3 
0.67 + 0.03 
Based on a fixed ultrasonic extraction period of 2.0 minutes 
(Chester River Sample "R"). 
Tolerance is expressed as the standard deviation. 
Internal Standard: D-10 anthracene. 
64 
