Abstract:
The excitation of the target nitrocompound with ultraviolet radiation  ress in photodissociation yielding vibrationally excited NO with significant population of the v&#34;=1 and v&#34;=2 levels of the ground electronic state. As the population distribution of ambient NO favors the v&#34;=0 level, discrimination between vibrationally excited NO and ambient NO is possible by probing the NO A-X (0,0),(1,1), and (2,2) bands near 226, 224, and 222 nm, respectively, employing (1+1) resonance-enhanced multiphoton ionization (REMPI). Many complex nitrocompounds cannot be photolyzed near 452 nm since their absorption cross sections are relatively small. Thus, the visible laser radiation is used to facilitate the detection of ambient NO and NO from NO 2  by (2+2) REMPI and to discriminate these species from more complex nitrocompound analytes. The analytical utility of the present invention has been demonstrated at several photolysis/ionization wavelengths for NO/CH 3  NO 2  and NO 2  /CH 3  NO 2  mixtures. Limits of detection have also been determined for NO, NO 2 , nitromethane (CH 3  NO 2 ), nitrobenzene (C 6  H 5  NO 2 ) diethylglycoldinitramine (DEDGN), and trinitrotoluene (TNT), and are in the ppb to ppm range.

Description:
FIELD OF THE INVENTION 
     The invention is directed to a device and process for detecting and discriminating NO and NO 2  by photodissociation with a laser having a two-color output. 
     RELATED APPLICATIONS 
     U.S. Pat. application Nos. 08/744,704 (Attorney Docket No. 96-13) and 08/680,080 (Attorney Docket No. 97-17), filed concurrently herewith and entitled respectively &#34;HAND-HELD PROBE FOR REAL-TIME ANALYSIS FOR TRACE POLLUTANTS IN ATMOSPHERE AND ON SURFACE&#34; and &#34;SENSOR AND METHOD FOR DETECTING TRACE UNDERGROUND ENERGETIC MATERIALS&#34;, disclose related subject matter. The disclosure of these applications is hereby incorporated by reference. 
     DESCRIPTION OF THE RELATED ART 
     Environmental and security concerns have urgently motivated the detection and monitoring of trace amounts of nitrocompounds such as propellants, explosives and nitro-pollutants. 
     Common techniques used for the detection and discrimination of NO and NO 2  from nitrocompounds include infrared absorption, laser infrared differential absorption radar (LIDAR), quadruple mass spectrometry, and ion mobility spectrometry. These techniques are relatively slow (minutes) and the apparatuses are bulky. The infrared techniques also suffer from low sensitivity and interference from other naturally abundant infrared absorbers such as H 2  O. Although ion mobility spectrometry is a sensitive and relatively fast technique (seconds), it is not so accurate as other techniques, since the signal dependence on concentration is nonlinear. Moreover, the technique suffers from clustering of the target molecules with water and competition for protonation with contaminates. 
     U.S. Pat. No. 5,123,274 to Carroll et al describes a device for analyzing explosives. The target molecules are first collected, separated with a gas chromatograph, and then pyrolyzed yielding NO as one of the major endproducts. NO is detected by chemiluminescence. Related to this invention is U.S. Pat. No. 5,094,815 to Conboy and Hotchkiss which discloses an apparatus which uses a photolytic interface for HPLC. NO is cleaved from non volatile N-nitroso compounds by photolysis using a mercury vapor lamp, separated from the solvent through a series of cold traps, and then carried by helium gas into a reaction chamber for chemiluminescence detection. The response time of these inventions is in the order of minutes, making them impractical for large sample operations, i.e. baggage inspection at a busy airport. 
     Recently, Sausa et al of the U.S. Army Research Laboratory developed an apparatus and method for the sensitive detection of nitrocompounds including energetic materials. Selected references include: (1) U.S. Pat. No. 5,364,795 to Sausa, Simeonsson, and Lemire, entitled Laser-based detection of nitro-containing compounds and (2) Simeonsson, Lemire, and Sausa, &#34;Trace Detection of Nitrocompounds&#34; by ArF Laser Photofragmentation/Ionization Spectrometry, Applied Spectroscopy, Vol. 47, No. 11, p. 190, 1993. The technique is based on laser photofragmentation/fragment detection spectrometry and employs one laser to both photofragment the target molecule and facilitate detection of the characteristic NO photofragment. Using a laser operating near 226 nm, NO is detected by resonance-enhanced multiphoton ionization (REMPI) or laser induced fluorescence (LIF) via its A 2  Σ-X 2  II transitions near 226 nm, while using an ArF laser, NO is detected by REMPI processes via the NO A 2  Σ-X 2  II (3,0) B 2  II-X 2  II (7,0), and D 2  Σ-X 2  II (1,0) bands at 193 nm. In both cases the analytical utility is demonstrated for several compounds using time-of-flight mass spectrometry that employs a molecular beam. The use of laser ionization with time-of-flight mass spectrometry allows for the detection and discrimination of NO and NO 2  from nitrocompounds based on the time of arrival of the ions at the detector. Although the sensor has a high sensitivity and fast response time, its utility in the field is limited due to the size of the mass spectrometer. 
     More recently, Simeonsson, Lemire, and Sausa published an article titled &#34;Laser-Induced Photofragmentation/Photoionization Spectrometry: A Method for Detecting Ambient Oxides of Nitrogen,&#34; Analytical Chemistry, Vol. 66, No. 14, p. 2272, 1994. The method utilized a tunable laser operating near 226 nm. The NO ions which were generated subsequent to the photolysis of the target molecule were detected using a pair of miniature electrodes. As the apparatus measures a signal representing a total NO quantity, it cannot discriminate NO and NO 2  from the target nitrocompound. The ability to discriminate these species from more complex nitrocompounds is important, particularly in field use, since NO and NO 2  are atmospheric pollutants which will contribute to the background noise. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a device and method for discrimination of NO and NO 2  from complex nitrocompounds in real time and in situ. 
     Another object is to provide such a device and method which are accurate and reliable, which have a fast response time, and which can be used for monitoring trace nitrocompounds in air or on various surfaces. 
     Yet another object is to provide such a device and method in which the sensor is rugged and capable of field use. 
     To these and other ends, the present invention employs a sensor having a frequency agile laser, such as one with output in the regions of 444-555 and 222-227 nm. The ultraviolet radiation is generated by frequency doubling the output radiation in the visible. The UV-visible laser radiation serves multiple functions. First, it is used to photodissociate the target nitrocompound molecule producing the characteristic NO photofragment. Second, it facilitates the discrimination of ambient NO from fragment NO. Because the NO fragment is formed vibrationally excited, it can be distinguished from ambient NO by probing the NO A-X (0,0), (1,1) and (2,2) bands near 226, 224, and 222 nm, respectively, using (1+1) REMPI. Complex nitrocompounds are fragmented more efficiently by UV radiation compared to visible radiation since their absorption cross-section increases with shorter wavelength. Thus, the visible radiation is used to detect and discriminate possible NO and NO 2  interferents by (2+2) REMPI via the NO A-X (0,0), (1,1) and (2,2) bands. These species have relatively large visible REMPI responses compared to the target nitrocompounds molecules. 
     The present invention has been shown to be useful and has been demonstrated on a number of nitrocompounds such as nitromethane (CH 3  NO 2 ), nitrobenzene (C 6  H 5  NO 2 ), diethylglycoldinitramine (DEDGN), and trinitrotoluene (TNT). 
     The present invention utilizes laser photofragmentation/fragment ionization using UV and visible radiation to discriminate NO and NO 2  from more complex nitrocompounds. Discrimination of the species is achieved by monitoring the difference of UV and visible ionization response of ambient NO, NO produced from the photolysis of NO 2 , and NO generated from the photolysis of the nitrocompounds using the NO A-X (0,0), (1,1), and (2,2) bands. 
     In this way, the present invention can discriminate between species by monitoring their photodissociation efficiencies at various wavelengths as well as their internal energy distributions. 
     The excitation of many energetic material and nitro-pollutants by radiation in the region of 190-250 nm results in the formation of vibrationally excited NO, X 2  II (v&#34;≧1). The population distribution of ambient NO, however, favors the ground vibrational state, X 2  II (v&#34;+0). Thus, both fragment and ambient NO are readily detected with a high degree of sensitivity and selectivity by REMPI via their A-X (0,0), (1,1), or (2,2) bands. The interaction of visible radiation with the target nitrocompound molecules does not result in photofragmentation since most nitrocompounds have relatively low absorptions in the visible. As a result, many nitrocompounds lack a REMPI response using visible radiation. In contrast, ambient NO and NO 2  have large REMPI responses from visible laser radiation. Thus, NO and NO 2  can be discriminated from more complex nitrocompounds because of the difference of nitrocompound photofragmentation efficiency with UV and visible laser radiation and the difference in REMPI response with visible radiation. 
     The UV-visible laser photodissociation and fragment detection technique of the invention is most effective when coupled with ion detection. Ion detection can have nearly 100% collection efficiency and is independent of fluorescence transition strength inherent in the LIF detection method. After ionization, all the ions can be easily directed to the collection plates by an electric field formed from the parallel plates. In contrast, LIF detection suffers from the exited molecules emitting in many frequencies and directions. 
     The apparatus described herein is employed in the following fashion. The detection head, composed of a pair of collection plates and lens assembly coupled to one end of a fiber optic, is brought to the point of interest where vapors of the analyte are sampled. UV or visible laser radiation which is transmitted through the fiber optic is then focused in the center of the collection plates. The sample vapor is photofragmented and the characteristic NO fragment subsequently photoionized. An electric field, produced by applying a 40-600 V difference on the collection plates, directs the ions to one of the potential plates. The ion current is converted to voltage by a transimpedance amplifier and then directed into a gated integrator or a digital oscilloscope for real-time display. A laptop computer (or other suitable computing device) interfaced to the boxcar or oscilloscope is used for data acquisition and analysis. In the case of NO and NO 2  discrimination from more complex nitrocompounds, the sample vapor is interrogated at three frequencies in the VU or visible. A simple and fast computation performed on the signals from the three interrogated frequencies determines the concentration of each NO, NO 2 , and target nitrocompound. These calculations can be performed by a programmable microprocessor. (or other suitable computing device). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiment of the invention will now be described in detail with reference to the drawings, in which: 
     FIG. 1 shows a schematic diagram of the experimental apparatus; 
     FIG. 2 shows REMPI spectra of ambient NO, NO generated from NO 2 , and NO generated from CH 3  NO 2  ; and 
     FIGS. 3A-3E show variations of the apparatus shown in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic diagram of trace nitrocompound analyzer 1 within the preferred embodiment of the present invention. Central to analyzer 1 is the UV-visible frequency agile laser system 2. The laser system includes tunable visible laser 3, frequency doubling unit or module 4, and a set of four mirrors 5. An optical parametric oscillator or frequency doubled Ti: sapphire could also be used in place of pulsed dye laser 3. Frequency doubling module 4 includes doubling crystal 6 and chromatic separator 7. Set of mirrors 5 redirects the visible laser radiation away from the frequency doubling module 4 when measurements are made in the visible range. Mirrors 8 and 9 are synchronously rotated to accomplish the redirection. Mirrors 8 and 9 can be replaced by Bragg cells or by an assembly of a polarizer, an electro-optic crystal, and a polarization sensitive reflector. The UV or visible laser radiation is focused into one end of optical fiber 13 by lens system 12. Detection of the nitrocompounds with NO and NO 2  background subtraction is accomplished at the detector head 14 which comprises, at the other end of the optical fiber 13, lens system 15 for focusing the laser radiation between the electrodes 16, heater coil 17, and beam stop 18. For compounds with a relatively high vapor pressure, the laser radiation is used to photolyze the target molecules and ionize the characteristic NO photofragment. For solid compounds which have a low vapor pressure at room temperature, vaporization and/or decomposition of the sample is accomplished with the same laser radiation or a heater coil. In all cases, the ions produced by REMPI are then collected by electrodes 16. One plate of the electrode pair is biased to 40-400 V by power supply 19, while the other plate is connected to transimpedance amplifier 20. Transimpedance amplifier 20 converts the ion current to voltage and the resulting signal is sampled and averaged by boxcar integrator 21 or displayed on digital oscilloscope 22. Laptop computer 23 (or another suitable computing device) interfaced to boxcar 21 and oscilloscope 22 is used for data acquisition and analysis. For hard copies of the data, printer 24 is connected to the computer. 
     FIG. 2 shows a REMPI spectrum in the region of 223.5-227 nm of ambient NO, NO produced from NO 2 , and NO produced from nitromethane. The spectra were recorded at 100 Torr with the laser operating at 10 Hz and a scan speed of 0.005 nm/sec, and with 3 shot averaging. An increase in spectral recording is possible by operating the pump laser at higher repetition rates (100-200 Hz) which are readily attainable from commercially available excimer lasers. The unique features of the spectra and rotational resolution reveal that the present invention can be highly selective based on wavelength excitation. The features near 226 and 224 nm are associated with NO A-X (0,0) and (1,1) transitions. Also observed, but not shown, are NO A-X (2,2) transitions near 222 nm. Similar spectra of nitrobenzene, diethylglycoldinitramine,(DEDGN), and trinitrotoluene (TNT) have been recorded and their REMPI response measured at various wavelengths. 
     The ionization signal dependence on species concentration was investigated for various target molecules at several UV and visible wavelengths, and was observed to be linear over a range of 3-4 decades. Tables 1 and 2 below are tables comparing the responses of NO, NO 2 , nitromethane, nitrobenzene, DEGDN, and TNT for REMPI detection at several UV and visible wavelengths, respectively. As shown in the tables, the complex nitrocompounds, R--NO 2 , have a relatively small REMPI response to visible excitation wavelengths compared to UV excitation wavelengths, while NO and NO 2  have large REMPI responses to both the UV and visible wavelengths. This permits the discrimination of NO and NO 2  from R--NO 2  by comparing the UV and visible ionization response. Thus, the quantitative analysis of NO, NO 2 , and R--NO 2  is accomplished by selecting two visible excitation wavelengths such as 450.7 and 449.2 nm for detecting an discriminating NO and NO 2 , and a third ionization wavelength in the UV such as 226.3 or 223.9 nm for detecting the nitrocompound. The signals from all three ionization wavelength are then interpreted simultaneously to determine the concentration of NO, NO 2 , and more complex nitrocompound. 
     
                       TABLE 1______________________________________REMPI RESPONSE (mV/ppm)SPECIES  454.3 nm   450.7 nm  449.2 nm 443.1 nm______________________________________NO     22.57      41.82     0.27     0.15NO.sub.2  19.85      20.01     10.6     8.84CH.sub.3 NO.sub.2  5.0 × 10-4             7.0 × 10-4                       1.0-10-4 --C.sub.6 H.sub.5 NO.sub.2  1.0 × 10-2             7.0 × 10-2                       1.0 × 10-3                                8.0 × 10-4DEDGN  4.7 × 10-3             5.8 × 10-3                       1.8 × 10-3                                --TNT    --         --        --       --______________________________________ 
    
     
                       TABLE 2______________________________________REMPI RESPONSE (mV/ppm)SPECIES  226.3 nm      223.9 nm 222.2 nm______________________________________NO       779           4.5      5.3NO.sub.2 46            19.3     6.5CH.sub.3 NO.sub.2    6.8           4.2      2.4C.sub.6 H.sub.5 NO.sub.2    1.4           1.4      --DEDGN    0.96          0.30     0.20TNT      0.63          --       --______________________________________ 
    
     The total ion signal from a mixture of NO, NO 2 , and nitrocompound, R--NO 2 , at radiation wavelength, λ, is due to the sum of the response from ambient NO, NO produced from NO 2  photolysis, and NO generated from R--NO 2  photolysis. The signal may be expressed as 
     
         S(λ)=R.sub.NO (λ) NO!+R.sub.NO2 (λ) NO.sub.2 !+R.sub.R-NO2 (λ) R--NO.sub.2 !                    (1) 
    
     where R NO  (λ) is the ambient NO ionization response from λ laser ionization, R NO2  (λ) is the response of fragment NO from NO 2  photolysis, and R R-NO2  (λ) is the response of fragment NO from R--NO 2  photolysis. The sample is interrogated at three wavelengths, at least one of which is in the UV range. This generates a set of three equations and three unknowns: 
     
         S(λ.sub.1)=R.sub.NO (λ.sub.1) NO!+R.sub.NO2 (λ.sub.1) NO.sub.2 !+R.sub.R-N02 (λ.sub.1) R--NO.sub.2 ! 
    
     
         S(λ.sub.2)=R.sub.NO (λ.sub.2) NO!+R.sub.NO2 (λ.sub.2) NO.sub.2 !+R.sub.R-NO2 (λ.sub.2) R--NO.sub.2 ! (2) 
    
     
         S(λ.sub.3)=R.sub.NO (λ.sub.3) NO!+R.sub.NO2 (λ.sub.3) NO.sub.2 !+R.sub.R-NO2 (λ.sub.3) R--NO.sub.2 ! 
    
     The responses of the three compounds at the three excitation wavelengths are measured separately during the calibration of the instrument and stored in RAM. The matrix ##EQU1## can be inverted prior to the measurements and also stored in RAM. The signals: ##EQU2## can be multiplied by R -1  s to determine the concentrations of NO, NO 2 , and R--NO 2 . 
     The choice of ionization wavelengths affects the error in calculating the absolute concentration and in determining the proper scaling of the signals. To demonstrate these effects, we consider two sets of three ionization wavelengths, 450.7, 449.2 and 223.9 nm, and 450.7, 449.2, and 226.3 nm. From tables 1 and 2, a coefficient matrix for the set of excitation wavelength 450.7, 449.2, and 223.9 nm with unit mV/ppm is given by, ##EQU3## The conditional number of R is: 
     
         cond(R)=∥R∥ ∥R.sup.-1 ∥=40.6 (6) 
    
     where ∥R∥ is the norm of the matrix R. In the above case we have used the row norm defined by: ##EQU4## Mathematically, the inverse conditional number is the relative distance of the closest singular (non-invertible) matrix and reveals how the error propagates through the system. For example, if Δs represents the error in measuring the signals, then 
     
         ∥Δx∥/∥x∥≦cond (R) ∥Δ∥/∥s∥         (8) 
    
     is the relative error of determining the concentrations. In other words, the conditional number scales the relative error in the signal to the relative error in calculating and measuring the concentrations. The conditional number is always greater than or equal to unity. It is equal to one only for diagonal matrixes. In the above set of excitation wavelengths, cond(R)=40.6. This value is acceptable, guaranteeing that the relative error in measuring the signal is amplified at most by an order of magnitude. 
     The matrix coefficient of the second set of excitation wavelengths, 450.7, 449.2, and 226.3 nm is given by ##EQU5## and cond(R)=6,258. This value is unacceptably large. The coefficient matrix in equation (9) suffers from poor scaling since the matrix element r 3 ,1 =779 dominates the norm. Scaling R, using diagonal matrixes K and L, yields ##EQU6## As shown in equation (10) matrix R&#39; does not have a dominant element. Calculating cond(R&#39;) yields an acceptable value of 80.5. The absolute concentrations of NO, NO 2  and CH 3  NO 2  can be determined from: 
     
         x=L(KR&#39;L).sup.-1 Ks                                        (11) 
    
     where the scaling compensates for the large response of NO and NO 2  at all frequencies and the large response of all compounds in the UV range. 
     The range of measurable mixtures of CH 3  NO 2  in the presence of NO and NO 2  is determined from the species&#39;s response and signal noise. For CH 3  NO 2  in the presence of NO, we assume that the bulk of the signal is due to the presence of NO. The calculated concentration of CH 3  NO 2  required to exceed three times the signal noise of the system is given by 
     
         R.sub.CH3NO2 (λ)  CH.sub.3 NO.sub.2 !/R.sub.NO (λ)  NO!=ΔS/S≧3N                                 (12) 
    
     where N is the relative noise of the system. Most of the system signal noise is due to variation in laser pulse energy, E. The laser energy varies by approximately 10%, so that (ΔE/E)n -1/2  =0.01, where n is the number of laser pulses for which results are averaged and n=100 (in an illustrative example) Then, 
     
         (1/3N.sub.NO) {R.sub.CH3NO2 (λ)/R.sub.NO (λ)}&gt; NO!/ CH.sub.3 NO.sub.2 !                                                (13) 
    
     determines the maximum  NO!/ CH 3  NO 2  ! ratio. 
     The mixture NO/CH 3  NO 2  can only be determined as long as NO can be detected in the presence of CH 3  NO 2 . Using an analysis similar to that described above, but exchanging the role of NO and CH 3  NO 2  (S=R CH3NO2  (λ)  CH 3  NO 2  ! and ΔS=R NO  (λ)  NO!), yields the minimum  NO!/ CH 3  NO 2  ! ratio that can be measured, 
     
          NO!/ CH.sub.3 NO.sub.2 !≧3N.sub.CH3NO2 {R.sub.CH3N02 (λ)/R.sub.NO(λ)}                            (14 ) 
    
     At the laser intensities employed in our apparatus, the CH 3  NO 2  ion signal dependence on laser pulse energy is approximately cubic. Therefore, N CH3NO2  =(3ΔE/E) n -1/2 . 
     Inequalities (13) and (14) reveal that the range of  NO!/ CH 3  NO 2  ! that can be calculated is determined solely by the signal noise of the system. It should be noted that the lowest concentration of CH 3  NO 2  in the presence of a very small quantity of NO is not determined by either inequality (13) and (14). Using a similar analysis for CH 3  NO 2  and NO 2  mixtures yields their mixture range. 
     Table 3 below is a table showing the range of measurable NO/CH 3  NO 2  and NO 2  /CH 3  NO 2  mixtures at 223.9 and 226.3 nm determined from inequalities (13) and (14). The range is approximately three decades for both mixtures and wavelengths. In the case of NO/CH 3  NO 2  mixture, the 223.9 nm wavelength is more suited for measuring the larger ratio of  NO!/ CH 3  NO 2  ! while the 226.3 nm wavelength is more suited for measuring the smaller ratio of  NO!/ CH 3  NO 2 . It is noted that below the  NO!/CH 3  NO 2  ! lower limit detection, CH 3  NO 2  can be detected ignoring the NO response. 
     
                                           TABLE 3__________________________________________________________________________RANGE OF MEASURABLE NO/CH.sub.3 NO.sub.2AND NO.sub.2 /CH.sub.3 NO.sub.2 MIXTURESNO                    NO.sub.2R.sub.CH3NO2 /       NO!/ CH.sub.3 NO.sub.2 !                 R.sub.CH3NO2 /                       NO.sub.2 !/ CH.sub.3 NO.sub.2 !R.sub.NO   Maximum           Minimum                 R.sub.NO2                      Maximum                           Minimum__________________________________________________________________________226.3 nm8.7 × 10-3      0.29 7.8 × 10-4                 0.15 2.47 0.013223.9 nm0.92  30.74           8.3 × 10-2                 0.22 3.60 0.020Noise--    0.01 0.03  --   0.02 0.03__________________________________________________________________________ 
    
     Presented in Tables 4 and 5 below are limits of detection of NO, NO 2  and more complex nitrocompounds. The limits of detection are reported as the concentration equal to three times the standard deviation of the background noise evaluated in the absence of the analyte molecule from 20 independent measurements, each the average of 100 laser pulses. As shown in the table, the limits of detection range from low ppb for NO and N0 2  to low ppm for the complex nitrocompounds. 
     
                       TABLE 4______________________________________Visible REMPI Limits of Detection (ppm)454.3 nm     450.7 nm    449.2 nm 443.1 nm______________________________________NO      0.010    0.006       0.75   1.3NO.sub.2   0.011    0.013       0.019  0.023CH.sub.3 NO.sub.2   438      373         2000   --C.sub.6 H.sub.5 NO.sub.2   22       3.7         200    250DEDGN   47       45          113    --TNT     --       --          --     --______________________________________ 
    
     
                       TABLE 5______________________________________  UV REMPI Limits of Detection (ppm)  226.3 nm   223.9 nm 222.2 nm______________________________________NO       0.002        0.120    0.1NO.sub.2 0.032        0.028    0.08CH.sub.3 NO.sub.2    0.20         0.13     0.23C.sub.6 H.sub.5 NO.sub.2    1.1          0.38     --DEDGN    1.5          1.8      2.7TNT      2.3          --       --______________________________________ 
    
     FIGS. 3A-3E show variations in the structure of the analyzer. For example, in detector head 14A shown in FIG. 3A, electrodes 16 may be replaced by multichannel plate 16A. In laser system 2B of FIG. 3B, set of mirrors 5 is replaced by Bragg cells 8B, 9B and fixed mirror 10B, while in laser system 2C of FIG. 3C, set of mirrors 5 is replaced by polarizer 8C-1 for imparting a polarization to the laser beam, electro-optic crystal 8C-2 for selectively rotating the polarization imparted by the polarizer, polarization selective mirror 8C-3 and fixed mirrors 10C, 11C and 9C. In laser system 2D of FIG. 3D, two lasers 3D-1 and 3D-2 output pulses of different frequencies to beam combiner 3D-3; frequency doubling module 4 and set of mirrors 5 are dispensed with. In detector head 14E of FIG. 3E, heater coil 17 is replaced by second laser 17E for volatilizing the sample. Of course, more than one of these variations may be applied to the apparatus. 
     While the preferred embodiment of the invention has been disclosed, those skilled in the art who have reviewed this application will readily appreciate that modifications may be mode within the scope of the invention. For example, the wavelengths may be varied as needed. Therefore, the invention is to be construed as limited only by the appended claims.