Petroleum identification

• US 3,896,312 A
• Filed: 06/07/1974
• Issued: 07/22/1975
• Est. Priority Date: 06/07/1974
• Status: Expired due to Term

• Current Assignee: Christopher W. Brown, Mark Ahmadjian, Patricia F. Lynch
• Original Assignee: Christopher W. Brown, Mark Ahmadjian, Patricia F. Lynch
• Inventors: Brown, Christopher W., Ahmadjian, Mark, Lynch, Patricia F.
• Primary Examiner(s): Lawrence, James W.
• Assistant Examiner(s): Church, C. E.
• Application Number:
  US05/477,414
• Time in Patent Office: 410 Days
• Field of Search: 250/343,344,345,346,373 235/151.35
• US Class Current: 250/343
• CPC Class Codes: G01N 21/3577 for analysing liquids, e.g. polluted water

Abstract

A method for comparing the infrared spectrum of an unidentified oil sample to the infrared spectra of a plurality of identified oil samples is disclosed herein. Also disclosed is a method for taking account of the background interference of testing cells when sodium chloride cells are employed for preparing or measuring the infrared spectra. Also disclosed is a method of using aluminum foil strips to separate oil samples from an aqueous environment and as sample cells in the preparation of infrared spectra.

Claims

1. A method of comparing an unidentified oil sample to a set of reference oil samples which comprises the steps of:

a. preparing infrared spectra of the various oil samples;

b. selecting a plurality of characteristic frequencies in sAid infrared spectra and determining the absorptivity for each spectrum at each characteristic frequency, said absorptivities defining for each reference sample a set of ratios of absorptivities of the unidentified sample to the reference sample at each characteristic frequency;

c. for each reference sample, determining the average of the ratios in said set of ratios and thereafter generating a normalized ratio for each reference sample at each characteristic frequency by dividing each ratio in said set of ratios by said average ratio of the set;

d. for each reference sample and for each characteristic frequency thereof, determining the absolute value of the difference between said normalized ratio and 1.0 and multiplying that difference by 100 to generate a percentdeviation for each characteristic frequency of each reference sample;

e. establishing a plurality of percent-deviation ranges and determining for each set of normalized ratios the number of ratios having a percent-deviation falling within each of said percent-deviation ranges; and

, f. determining on the basis of the largest numerical quantity of characteristic frequencies falling within said percentdeviation ranges the reference sample which the unidentified sample most closely approximates.

2. The method of claim 1 wherein said infrared spectra of the various oil samples are prepared by the steps of:

a. contacting a clean strip of aluminum foil with an aqueous mixture comprising at least in part the oil sample to be tested thereby coating the surface of the aluminum foil with a thin film of said oil and thus separating it from the aqueous mixture; and

, b. employing the surface of the aluminum foil which has been coated with the oil sample to be tested as at least one reflecting surface to reflect infrared light and thereafter measuring the intensity of the light so reflected so as to generate an infrared spectrum over the frequency range of about 600 to 1200 cm. 1.

3. The method of claim 1 wherein the absorptivities of a plurality of reference oil samples at said plurality of characteristic frequencies are determined in advance and are stored in the data bank of a digital computer.

4. The method of claim 3 wherein the infrared spectrum of an unidentified oil sample is introduced into said digital computer for comparison with said reference oil samples.

5. The method of claim 4 wherein the analytical operations are performed by a digital computer.

6. The method of claim 1 wherein said infrared spectra of the various oil samples are prepared by the steps of:

a. depositing an oil sample essentially free of water in a sodium chloride cell comprising two essentially transparent crystals of sodium chloride maintained in a fixed spaced relationship to one another by suitable means; and

, b. passing infrared light through said sodium chloride cell and said oil sample contained therein and measuring the intensity of the infrared light emerging from said cell so as to generate an infrared spectrum over the frequency region of about 600 to 1200 cm. 1.

7. The method of claim 6 wherein the absorptivities of the various oil samples at each of the characteristic frequencies are determined from the actual absorbances of the samples at each of said frequencies and said actual absorbances are determined by the steps comprising:

a. determining the absorptivities of an empty sodium chloride window at a plurality of frequencies between about 610 to 820 cm. 1;

b. generating a cubic equation to define the background absorptivities of each sodium chloride window over the range of about 610 to 820 cm. 1 by fitting said absorptivities determined at a plurality of frequencies between 610 to 820 cm. 1 to a cubic equation by the method of least squares;

c. determining the background absorbances of the sodium chloride cell by means of said Cubic equation for frequencies in the range of about 610 to 820 cm. 1 and by assuming a constant background absorbance in the frequency range of about 820 to 1200 cm. 1; and

, d. determining the actual absorbance of each oil sample at each of said characteristic frequencies from the measured absorbance at each frequency and corresponding background absorbance of the sodium chloride cell.

8. The method of claim 7 wherein the absorptivities of a plurality of reference oil samples at said plurality of characteristic frequencies are determined in advance and are stored in the data bank of a digital computer.

9. The method of claim 8 wherein said cubic equation is determined and stored in the data bank of said digital computer.

10. The method of claim 9 wherein the infrared spectrum of an unidentified oil sample is introduced to said digital computer for comparison with said reference oil samples.

11. The method of claim 10 wherein the analytical operations are performed by a digital computer.