Apparatus and method for detection and concentration measurement of trace metals using laser induced breakdown spectroscopy

• US 5,847,825 A
• Filed: 09/25/1997
• Issued: 12/08/1998
• Est. Priority Date: 09/25/1996
• Status: Expired due to Term

• Current Assignee: Board of Regents of the University of Nebraska
• Original Assignee: Board of Regents of the University of Nebraska
• Inventors: Alexander, Dennis R.
• Primary Examiner(s): EVANS, FANNIE L
• Application Number:
  US08/947,449
• Time in Patent Office: 439 Days
• Field of Search: 356/317, 356/318
• US Class Current: 356/318
• CPC Class Codes: B23K 26/0624 using ultrashort pulses, i.e. pulses of 1ns or less
• CPC Class Codes: G01N 21/718 Laser microanalysis, i.e. with formation of sample plasma

Abstract

A method and apparatus for in situ detection and concentration measurement of trace elements in an analysis sample is disclosed. The invention uses laser induced breakdown spectroscopy (LIBS) wherein femtosecond pulsed laser energy is employed to produce the plasma. The femtosecond pulsed laser energy is focused on the analysis sample to produce the plasma and the resulting emission is delivered for spectral analysis. Because the method and apparatus of the present invention allow breakdown of the analysis sample without propagation of energy to the sample-air interface, a plasma is produced that consists essentially of sample materials without being contaminated by air plasma formation. Thus, the background emission is reduced and there is no need to wait for the plasma to cool down over time before detecting the spectral lines of the sample. Because there is no need to wait for cooling before spectral measurement, lower detection limits are possible. Furthermore, concentration measurement accuracy is improved using intensity rationing techniques since a calibration curve produced using the method and apparatus of the present invention is substantially more linear than those using the conventional nanosecond pulsed LIBS.

Claims

1. An apparatus for measuring the contents of an analysis sample, comprising:

(a) a pulsed laser energy emitter wherein the pulse length of said laser energy emitter is from about 1 attosecond to about 1000 femtoseconds in duration;

(b) an optical transmission medium for focusing energy from said pulsed laser energy emitter onto said analysis sample, for generating a plasma state in a portion of said analysis sample; and

(c) a detector for measuring spectral characteristics of energy emitted from said analysis sample when exposed to focused energy.

2. The apparatus of claim 1, wherein the pulse length of said laser energy emitter is from about 5 attoseconds to about 500 femtoseconds in duration.

3. The apparatus of claim 1, wherein said laser energy emitter emits pulses at a frequency of from about 1 Hz to about 82 MHz.

4. The apparatus of claim 1, wherein said optical transmission medium comprises one or more tenses.

5. The apparatus of claim 4, wherein said optical transmission medium further comprises transmission media for delivering pulsed laser energy to said one or more lenses.

6. The apparatus of claim 5, wherein said transmission media for delivering pulsed laser energy to said one or more lenses comprises optical fiber media.

7. The apparatus of claim 1, further comprising transmission media for delivering emitted energy from said analysis sample to said detector for measuring spectral characteristics of energy emitted from said analysis sample.

8. The apparatus of claim 7, wherein said transmission media for delivering emitted energy from said analysis sample to said detector for measuring spectral characteristics of energy emitted from said analysis sample comprises optical fiber media.

9. The apparatus of claim 1, wherein said detector for measuring spectral characteristics comprises a spectrograph and an optical multichannel analyzer.

10. The apparatus of claim 9, further comprising an image intensifier.

11. The apparatus of claim 1, further comprising a computer for storing spectral characteristics of predetermined substances and for comparing measured spectral characteristics to identify substances in said analysis sample.

12. The apparatus of claim 1, wherein trace elements are detected in said analysis sample by the presence of narrow spectral line characteristics.

13. The apparatus of claim 12, wherein the concentration of said trace elements in said analysis sample is determined by spectral line intensity or areas under spectral curves.

14. The apparatus of claim 13 wherein said trace elements comprise heavy metals.

15. The apparatus of claim 14 wherein said heavy metals are selected from the group consisting of chromium, lead, mercury, and zinc.

16. A method for measuring the content of an analysis sample comprising the steps of:

(a) emitting laser pulses from a pulsed laser energy emitter wherein the pulse length of said laser energy emitter is from about 1 attosecond to about 1000 femtoseconds in duration;

(b) focusing energy from said pulsed laser energy emitter on said analysis sample, thereby generating a plasma state in a portion of said analysis sample;

(c) measuring spectral characteristics of energy emitted from said analysis sample when exposed to focused energy; and

(d) detecting elements in said analysis sample by the presence of narrow spectral line characteristics.

17. The method of claim 16, wherein the pulse length of said laser energy emitter is from about 5 attoseconds to about 500 femtoseconds in duration.

18. The method of claim 16 further comprising the step of comparing measured spectral characteristics to entries in a database having predetermined spectral characteristics of known substances stored therein and identifying one or more substances present in said analysis sample based on the comparison.

19. The method of claim 18 wherein said comparison is performed by a processor.

20. The method of claim 16, wherein said laser pulses are emitted at a frequency of from about 1 Hz to about 82 MHz.

21. The method of claim 16, wherein energy from said pulsed laser energy emitter is focused onto said analysis sample using one or more lenses.

22. The method of claim 16, further comprising the step of transmitting said pulsed laser energy using optical fiber media.

23. The method of claim 16, further comprising the step of transmitting emitted energy from said analysis sample to a detector using optical fiber media said detector for measuring spectral characteristics of energy emitted from said analysis sample.

24. The method of claim 16, wherein said detector comprises a spectrograph and an optical multichannel analyzer.

25. The method of claim 24, wherein said detector further comprises an image intensifier.

26. The method of claim 16, further comprising the step of comparing measured spectral characteristics to stored spectral characteristics of predetermined substances to identify substances in said analysis sample.

27. The method of claim 16, wherein trace elements are detected in said analysis sample by the presence of narrow spectral line characteristics.

28. The method of claim 27, further comprising the step of determining the concentration of said trace elements in said analysis sample is determined by spectral line intensity or area tinder the spectrum.

29. The method of claim 16 wherein said trace elements comprise heavy metals.

30. The method of claim 29 wherein said heavy metals are selected from the group consisting of chromium, lead, mercury, and zinc.