tensity which are given in the form f & 5. The confidence limits were calculated using
where a t least four measurements were taken. Taking, for example, the peak to background ratio 113/ 800 from the 5.5-ng Cr probe, the minimum detectable quantity (MDQ) can he extrapolated to be 4 ng (0.8 ppm). CONCLUSIONS The work presented here shows results of two elements. Multielement standards are in preparation to investigate the detection limit in this case. The precision of the measurements depends 011 the weighing accuracy, the exact deposition of the probe on the substrate, and the instrumental precision of substrate alignment. The attainable sensitivity may be improved by the use of a semiconductor de-
tector with larger sensitive volume, slightly modified experimental arrangement for more exact positioning of the substrate, improved surface treatment of the substrate to reduce remaining surface defects, and a semiautomatic sample preparation for better reproducibility. LITERATURE CITED (1) D. A. Gedcke, X-Ray Spectrom., 1 136 (1972). (2) IUPAC Comission V4 on Spectrochemical and other optical procedures of Analysis, Appendices on Tentative Nomenclature, Symbols, Units and Standards, No. 26 (Nov. 72) p 17. (3) Y. Yoneda and T. Horiuchi, Rev. Sci. Instrum., 42, 1069 (1971). (4) M. A. Biochin, "Physik der Rontgenstrahien". VeB Verlag Technik, Berlin, 1957. (5) W. W. Weiss, Messtechnik (Braunschweig), 80 (5). 127 (1972). (6) H. Aiginger and P. Wobrauschek, Nucl. Instrum. Methods, 114, 157-158 (1974). (7) E. Storm and H. I. Israel, Nucl. Data Tables, A7, 565-681 (1970). (8)R. W. Fink, R. C. Jopson, H. Mark, and C. D. Swift, Rev. Mod. Phys., 38, 513 (1966).
RECEIVEDfor review September 17, 1974. Accepted January 13, 1975.
Elemental Trace Analysis of Small Samples by Proton Induced X-Ray Emission Thomas B. Johansson,' Rene E. Van Grieken,2 J. William Nelson, and John W. Winchester Depadments of Oceanography and Physics, Florida State University, Tallahassee, FL 32306
Quantitative analysis of 12 to 15 elements simultaneously by proton-induced X-ray emission, PIXE, has been tested for thin small inhomogeneous samples such as impactor collected aerosol size fractions using a low energy, 3.7MeV, proton beam from a Van de Graaff accelerator. The procedure and a number of tests of the performance of the system and the analytical methodology are presented. Detection limits for multielement samples in a 10-min bombardment are in the nanogram range, and standard deviations are as low as 6 % for analyses of homogeneous samples and 15% for heterogeneous samples. The method is calibrated to be absolute in the sense that standards are needed only to check the stability of the system.
Elemental analysis requirements of present environmental quality investigations have indicated the need for ultrasensitive, multielement procedures, capable of ready application to air, water, soil, and biological samples. The recent renewed expansion of scientific literature on X-ray emission spectrometry illustrates that this technique has become an increasingly important tool for elemental analysis, especially since Si(1,i) detectors were introduced a few years ago. Although energy-dispersive X-ray fluorescence, like any other available technique, cannot measure all elements of interest in the concentration ranges of environmental samples, it has the potential of providing a rapid, nondestructive, low-cost, multielement analysis capability needed to investigate and monitor the behavior of many interesting elements in various matrices. X-Ray emission by charged particle excitation offers an attractive alternative to the X-ray tube ( I ) or radioisotope Present address, Department of Environmental Health, University of Lund, S-220 02 Lund 2, Sweden. Present address, Department of Chemistry, ,Antwerp University (UIA), B-2610 Wilrijk, Belgium.
source excited techniques. In 1970, the potential of heavy charged particles as an excitation source was recognized, and an interference-free sensitivity down to the picogram range was demonstrated (2). The excellent capability of accelerator beams for X-ray emission analysis is partially due to the relatively low background radiation associated with the excitation. The main contribution is Bremsstrahlung from secondary electrons in the sample. The very high intensity of particle fluxes obtainable from accelerators also contributes substantially to the power of the method. In a feasibility study, Walter et al. ( 3 ) have reviewed the development of PIXE and give references to the literature. Recently, Gilfrich et al. ( 4 ) have compared several approaches to X-ray elemental analysis. Folkman e t al. (5, 6) and Herman et al. ( 7 ) discussed the optimum choice of heavy particle and beam energy. They concluded that protons of a few MeV energy provide a preferred combination for high sensitivity analysis. As there are a large number of accelerators available that are capable of delivering such beams, the prospects for widespread use are bright. Proton induced X-ray emission analysis (PIXE) has been used in feasibility studies by Johansson et al. ( 8 ) , Campbell e t al. (9),Mandler and Semmler (10) and others and is currently applied to the analysis of aerosols by Cahill and Feeney ( I I ) , Flocchini et al. (12) and Johansson e t al. (13),to malaria infected blood by Barnes et al. (14),and to thick samples such as steel slabs or teeth by Ahlberg et al. (15). I t has also recently been applied to studies of aerosol trace metal transport in the St. Louis Regional Air Pollution Study by Winchester et al. (16) and Akselsson et al. (17).
The present work describes the experimental apparatus designed for routine proton induced X-ray emission analysis of large numbers of samples and discusses the methodology of the analytical procedure. I t presents a thorough ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
855
P S A I3
STAGE I
3 7 MeV pr010ni
Figure 1. Irradiation chamber
CdAhkEL
Figure 3. X-Ray spectrum obtained by proton irradiation of an im-
pactor-collected aeiosol sample
l
i
-
l -
€ b
1
1
1 1
I1
The thickness of the A1 foil used (-9 mg/cm2) and the distance between the A1 foil and last collimator (-1 m) have been chosen to ensure uniform sample irradiation. Using a 1-mm2 piece of Zr as a probe (191, we have mapped the horizontal and vertical beam profile for different diffuser arrangements. One typical diameter for the final experimental arrangement, as shown in Figure 2, demonstrates the beam intensity to be homogeneous to about 5%, quite satisfactory for trace element determinations. The sample holding rods can easily be moved vertically and horizontally to ensure that small heterogeneous samples, such as aerosol deposits from a single-orifice impactor, are completely enveloped by the beam. In the case of large homogeneous samples, the positioning facilities are only manipulated to make sure that no beam hits the frame on which the sample is suspended. A lithium-drifted silicon detector is used for X-ray detection. The detector has a sensitive depth of 5 mm, an area of 80 mm2 and a resolution of 180 eV a t 5.9 keV (FWHM). Figure 3 shows a typical X-ray spectrum recorded during an irradiation in our aerosol analysis program. The weight in grams, G, of an element is given by
where N represents the number of counts in a corresponding peak, W , the atomic weight of the element, n the number of protons per cm2, up the X-ray production cross section, R the solid angle subtended by the collimator in front of the detector, c the detector efficiency including absorption losses in windows between the target and the detector and t the fraction of the produced X-rays that escapes absorption in the target in the direction of the detector. A is Avogadro's number. The proton flux, n, is obtained by dividing the integrated beam current for each run by the beam area. The number of counts in the peaks in the X-ray spectrum is evaluated by fitting a Gaussian with exponential tails to each peak together with a polynomial background that extends over the total interval used in the fitting procedure. Up to 6 peaks can be fitted together in a standard code called SAMPO (20).A smaller and faster code such as those recently developed especially for particle induced X-ray spectrum analysis by Harrison and Eldred (21) or Kaufmann and Akselsson (22),would be advantageous. The X-ray production cross sections in Equation 1 are obtained from an empirical expression for inner shell ionization, aion (23). The X-ray production cross section is then calculated from
where w is the fluorescence yield and k the relative intensity of the peak used in the calculation. Values of w and k are obtained from Bambynek et al. ( 2 4 ) .The cross section up depends on the energy of the protons; below 1 MeV, up is approximately proportional to Ep4 ( 2 5 ) ,while for increasing energies a maximum is reached a t about 20 MeV for 2 around 30. The background, on the other hand, increases with increasing proton energy giving an optimal proton energy for high sensitivity analysis of a few MeV, varying with 2 of the element (5, 7). For this investigation, a 4.3-MeV proton beam was chosen, the lowest conveniently available energy from Florida State University's Super F N Tandem Accelerator. After passing through the diffuser foil, the beam energy was reduced to 3.7 MeV with some energy spread introduced by straggling. The effect of the energy spread on the cross sections introduces an error of less than 0.5%. The solid angle subtended by the detector is calculated from the area of the circular 6-mm collimator in front of the detector and its 856
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
2-
,
1 -
5
1s
10
20
I
25
E (keVI We,qht
Figure 4. X-Ray detector efficiency
The points are experimentally determined. The curve is fitted to the experimental points for the low energy region and t h e high energy region is calculated from X-ray absorption in the 5-mm thick Si(Li)detector
+
Figure 5. Results from linearity tests
I
I
I
I
i
I
1
i
0.2
0.5
I
2
1 5
I
I
I0
ZC
1 50
1
0
distance, 10 cm, to the target. The location of the detector is fixed by the use of a specially designed sleeve which slips over the tube on the irradiation chamber which extends toward the detector and over the detector housing. Inside this sleeve, the collimator and suitable absorbers, if needed, are located. The detector efficiency is of primary importance in using Equation l for accurate quantitative analysis. Efficiency curves provided by manufacturers are generally calculated from X-ray absorption coefficients. This is only a rough estimate of the true efficiency as shown by Hansen et al. (26). Also the transmission of the irradiation chamber window must be included in the efficiency curve. Equation 1 can be used to determine t if the amount of an element is known. By using thin metal foils (-50 kg/cm2 f 5%) evaporated on Mylar (27),the efficiency curve in Figure 4 was determined. In the computer code recently published by Kaufmann and Akselsson (22),maximum use of the physics involved in X-ray production using PIXE and X-ray detection have been made. This approach promises spectrum resolution and calculations of amounts of elements present without specific knowledge of the detector efficiency. For quantitative analysis, it is preferable to use thin samples, as in this case no corrections for X-ray absorption in the sample or for the influence of the proton energy loss on the X-ray production cross sections have to be made. In practice, this is achieved by keeping the sample thickness below 1 mg/cm2. In many practical situations, this cannot be fully achieved. For sample thicknesses not too much above the thin sample criterion, the correction is made by using a mean proton energy in the calculations, then taking advantage of the slow variation of the cross sections with proton energy. The X-ray absorption is corrected for by integration in a model where uniform X-ray production is assumed and the sample is approximated by a homogeneous layer. The correction is then /JA
sin P where d is the thickness of the sample in g/cm2, +c the angle between the sample normal and the beam, and 1 the mass absorption coefficient.
RESULTS AND DISCUSSION T h e high sensitivity, multielement analytical method presented has been carefully tested in preparation for routine analytical work. T h e linearity of the X-ray response as a function of the amount of material in the sample was investigated in the case of K, Cu, Zn, and Br (as BrOa-) using Y as an internal standard. T h e samples were prepared from stock solutions by micropipetting onto a polystyrene foil. After evaporation of t h e solvent, the sample is ready for irradiation. As shown in Figure 5, the linearity is very good over the 4 orders of magnitude investigated, as is expected
.
0
2
60
Q
80
0.1
I 100
Thickness ( m g /crnz)
Figure 6. X-Ray attenuation in a homogeneous sample as a function of sample thickness
when thin samples are used and the effects of X-ray absorption or proton energy loss in t h e sample are negligible. Inherent in PIXE is the necessity of placing the samples in vacuum and subjecting them t o slightly elevated temperatures caused by proton energy loss in t h e sample. To investigate possible volatility losses for some elements during analysis, several experiments were conducted. Solutions of As (as AsOs3- and AsOd3-), Se (as Se0a2-), Br (as BrOaand Br-), and of salts of all metals which are detectable in aerosol samples were prepared with Y as an internal standard. Drops of these solutions were evaporated on polystyrene foils and analyzed. The internal standard enabled a check that no rapid, initial loss of a volatile species occurred. In these experiments as well as in several aerosol analyses under routine conditions, no significant volatility losses of metallic elements were observed. (Our measurements of Br-, however, were inconclusive, and additional measurements should be made t o rule out the possibility of some loss during analysis.) In most analyses, great pains are taken to ensure t h a t the samples are thin enough t o require only a small absorption correction. In Equation 1, t h e factor t corrects for X-ray absorption in the target. T o quantify this condition, the X-ray absorption has been calculated for homogeneous layers of a “typical” rural aerosol deposit of varying total thickness, consisting of 76% C, 9% 0, 2% Al, 4.2% Si, 5% S, 2.9% Ca, and 0.9% Fe. T h e curves are shown in Figure 6 using X-ray absorption coefficients taken from Storm and Israel (28). No allowances have been made for particle size effects, where appreciable X-ray attenuation may occur for elements contained within very large particles. For low 2 elements, the correction is largest, e.g., 20% for S in a samANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
057
~~
Table I. Results of Analysis of Thin Homogeneous Standard Samples
Table 11. Results from an Interlaboratory Comparison Study
PIXE results Number
Amount Element
given,
Found,
(compound)
UE
lie
C1 (NaC1) C1 (KC1) K (KC1) Ca (CaF) Ti metal Cr metal Fe metal c u (CUS) Ga (Gap) Br (CsBr) Cs (CsBr)
6.8 6.3 7 .O 3.7 10.2 7.2 11.5 7.9 4.6 3.1 5.2
minations
S a m p l e on \*hatman 41
illre.
-
of d e t e r - per deter
Amount
filter
A ~ o u n fto u n d b
91, i'",
5.6 6.6 7.1 3.6 8.9 7.1 13.7 8.1 4.6 3 .O 5.5
3
5.2 6.7 6.5 9.8 9.0 1.0 3.1 7.1 7.8 3.0 6.0
7 7
5 3 5
5 14 2 3 3
(15).
Accuracy and precision were investigated in two different ways. First, the metal foils used for the efficiency determination of the detector were analyzed several times in different accelerator runs over a period of several months. For most of the standards, two different targets were used. Table I presents the data for these experiments. The mean deviation per measurement is 6%, including contributions from sample variations and all steps of the analytical procedure. A second test was conducted using the data from all runs with evaporated drops of solutions containing known amounts of Mo dissolved as (NH&M07024 4H20. The "given" amount was 5.5 pg and the mean value of 30 determinations was 5.0 pg with a standard deviation of 15.5% per measurement calculated from the measurements. The higher standard deviation in this case is, of course, due to the contributions from beam and sample inhomogeneity, micropipetting errors, etc. The mean value is 10%low compared to the given value, su7gesting a systematic error due to an inaccuracy of the pipets or to a small loss of crystallized Mo by flaking off from the specimen. In several runs, other elements were present in the Mo solution, and the ratios obtained between the elements present in these runs confirm this. A test of the ability to perform quantitative analysis was offered in an interlaboratory comparison study conducted during the summer of 1973 (29). About a dozen different laboratories participated. X-Ray excitation using radioactive sources or different types of X-ray tube excitation, proton and alpha-particle excitation and energy-dispersive as well as wavelength-dispersive detectors were represented. Also neutron activation analysis was performed on the samples. All participants analyzed Millipore and Whatman 41 filter paper prepared with a known solution containing several elements frequently measured in X-ray analysis. In Table 11, the PIXE results on these samples as well as the gravimetric values are presented. I t is clear that our results agree very well with the given values considering our quot-
-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
A m o n t ill\ c
nc
K
V Mn
Fe
cu As Pb
Mean
2040 374 1649 1916 300 356 813
1.032 1.071 1.022 1.093 1.110 1.154 1.064 1.078
i
*
imouit
Amount found
qixsn,
~-
noa
i m o m t qir e n
943 233 693 1269 202 244 558
0.981 1.069 0.915 0.988 1.151 1.120 1.083
mination
ple of thickness only 1 mg/cm2, while higher 2 elements may be determined in a much thicker matrix with only a small absorption correction, e.g., 20% for Cu a t 20 mg/cm2. The protons are slowed down when passing through the sample, reducing the effective X-ray production cross section. For samples of a few mg/cm2 thickness, this is an effect on the order of a few per cent and can easily be corrected for. For thicker samples, the varying X-ray production with depth in the sample has to be accounted for by appropriate integration over the energy interval of the protons
858
S a m p l e an \Illlipore
i/l Std deb
1.044
UGiven amount of each element within area of proton beam striking sample. Ratio of PIXE result to given amount for 2 to 4 replicate determinations. The standard deviation for a single determination by PIXE is approximately 15%.
Table 111. Comparison between Heterogeneous and Homogeneous Samples H e t c r o q e n e o u foil
(-1 mm2 plece)
Homogeneous foil
Zr-foil 5.66 i 0.53 mg/cm2 5.01 i 0.35 mg/cm? Fe in Al-foil 21.3 i 2.0 pg/cm2 23.3 f 1.7 pg/cm2 Zn in AI-foil 1.02 i 0.15 pg/cm2 0. 80 i 0.10 &cm2 ed errors. On the whole, the intercomparison study was very successful, with most of the participating laboratories within f10% of the given values, demonstrating the excellent capabilities of the various methods of X-ray analysis to produce high quality analytical data. These results demonstrate the capability to provide good data on homogeneous targets. As described above, arrangements were also made to facilitate the analysis of inhomogeneous samples such as samples of particulate matter collected from the atmosphere with a single orifice cascade impactor. Two different experiments were conducted to test the performance of the system on this type of sample. In the first experiment, thin homogeneous foils of zirconium or aluminum were analyzed. Small pieces with carefully determined area (-1 mm2) were then cut out of these foils, applied to polystyrene foils with a little starch as a binder and analyzed. In the zirconium foils, the K-radiation from zirconium was used for the comparison. In the A1 case, K-radiation from Fe and Zn impurities were compared to avoid the large correction for X-ray absorption of A1 K X-rays in the starch. The results are presented in Table 111. The second test was offered by the solutions used in the accuracy tests discussed above, where heterogeneous residues from evaporated solution droplets were analyzed. I t is clear from these experiments that small inhomogeneous samples can be accurately analyzed. In this work, no direct comparison with standard samples needs to be done to obtain the quantitative result, although thin standard foils were used to establish the detector efficiency curve. This can also be done by using radioactive sources together with theoretical computations for low energy X-ray absorption. Also, as mentioned above, newly developed computer codes may make the determination of detector efficiencies redundant (22). Therefore, the present approach can, in principle, be considered to be absolute.
100-
0 1-
0
20
60
40
, 80
I
2
Figure 7. Detection limits for routine PIXE analysis The curves correspond to our routine conditions of accumulating 10 pC of charge during a 10-min irradiation with the detection system operating at 500 cps. K and L refer to the utilization of K and L X-rays, respectively, in the analysis
The approach to quantitative analysis taken here is, perhaps, most useful in thick sample analysis, when the reduced X-ray production yield with decreased proton energy as well as sample absorptions of the X-rays have to be considered. Figure 7 shows the trend of interference-free detection limits for conditions we use for analysis of aerosol samples collected by cascade impactor. They have been calculated from the standard criterion S = 3 V'E, where S is the signal required to consider that a peak is significantly above the Z(FWHM) background, B. In this calculation, Equation l was used with these routinely used parameters: target-collimator distance 10 cm, detector collimator 0.28 cm2, charge 10 pC, corresponding to a beam of -17 nA in a 10min run, beam collimator 0.17 cm2. Generally, we have used very low count rates, 500 cps, as the available analogto-digital converters (ADC) were not capable of maintaining resolution a t higher count rates. Improved, commercially available ADC's allow considerably higher count rates. The sensitivity for routine operations could thus be improved significantly by simply improving the geometry factor, increasing the beam intensity (suitably treated polystyrene and other thin plastic foils withstand PA-beams), and absorbing the count rate contribution from low energy Bremsstrahlung to allow full use of the count rate capability in counting informative events. In some applications, e.g., investigations of microstructures, the beam might be reduced to a several pm diameter, resulting in very high beam intensities ( 3 0 ) . A homogeneous beam can be obtained not only with the diffuser foils used in this experiment but also with beam sweeping (8). Although quite simple, the diffusers produce the required homogeneity but at the expense of increased beam which must be stopped near the detector and must be shielded against. Beam sweeping, as has been used for electron beams in X-ray tubes ( 3 1 ) ,is more complicated. However, this can be coupled to a fast beam control which stops the beam during the processing of a pulse in the analyzing equipment, thus eliminating dead-time correction. Nuclear reactions in the stopping material give rise to a considerably increased background. When a large amount of beam is needed, the diffusers also have the drawback of reducing the beam intensity by over 90%. Although X-ray analysis is basically very specific in identifying the elements present, the use of energy-dispersive detectors with their rather low energy resolution impairs this capability to some extent, and analysis of a sample with too many elements present above their detection limits sometimes offers no unique solution. In the transition element region, for example, the K a X-rays of one element
overlap with the KP X-rays of the preceding element, and the ratio between K a and KP has to be used in resolving such interference. In cases where only one of the two components is present, the assignment of a peak is sometimes uncertain. The analysis of a spectrum would then improve in quality if some more information were available. A possibility may exist in the application of one or two crystal spectrometers to detect L X-ray lines from a few selected elements. X-Ray analysis can thus be successfully employed for the quantitative analysis of elements with 2 2 15. The limitations for low 2 elements are due to increasing X-ray absorption effects in the target. Another approach for the lightest elements showing great promise is the utilization of scattered protons ( 3 2 ) . This technique can be applied to the same samples as P I X E and, using solid state charged particle detectors, it is capable of resolving elements from H to P. The combination of the two techniques offers an even more powerful analytical tool. A characteristic feature of P I X E is the capability to determine simultaneously many elements present in very small absolute amounts in a very small sample. This is many times a useful attribute. For example, in the study of particulate matter in the atmosphere, an analytical task especially addressed in this paper, this permits sampling to be restricted to comparatively short periods of time while maintaining the use of particle size fractionating sampling devices such as cascade impactors. This is of great interest, especially as it permits detailed study of various parameters affecting trace elements in the atmosphere.
ACKNOWLEDGMENT We have benefitted from discussions with Robert H. Davis and Kenneth R. Chapman. LITERATURE CITED (1) R. D. Giauque, F. S. Goulding, J. M. Jaklevic, and R. H. Pehl, Anal. Chem., 45,671-681 (1973). (2) T. B. Johansson, R. Akselsson, and S. A. E. Johansson, Nucl. Instrum. Methods, 84, 141-143 (1970) (3) R. L. Walter, R. D. Willis. W. F. Gutknecht, and J. M. Joyce, Anal. Chem., 46,843-855 (1974). (4) J. V. Gilfrich, P. G. Burkhalter, and L. S. Birks, Anal. Chem., 45, 20022009 (1973). (5) F. Folkmann, C. Gaarde, T. Huus, and K. Kemp. Nucl. Instrum. Methods, 116,487-499 (1974). (6) F. Folkmann, J. Borggren, and A. Kjeidgaard, Nucl. Instrum. Methods, 119, 117-123(1974). (7) A. W. Herman, L. A. McNelles, and i. L. Campbell, ht. J. Appl. Radiat. /sot., 24, 677-688 (1973). (8) T. B. Johansson, R. Akselsson, and S. A. E. Johansson, "Advances in X-ray Analysis," K. F. J. Heinrich, C. S. Barrett. J. B. Newkirk, and C. 0. Ruud, Ed., Vol. 15, Plenum Press, New York, NY, pp 373-387. (9) J. L. Campbell, A . W. Herman, L. A. McNelles, B. H. Orr, and R. A. Willoughby. "Advances in X-ray Analysis," C. L. Grant, C. S.Barrett, J. B. Newkirk, and C. 0. Ruud. Ed., Vol. 17, Plenum Press, New York, NY, 1974, pp 457-466. (10) J. W. Mandler and R. A. Semmier. Proc. Sec. Int. Conf. on Nucl. Meth. in Environ. Res., 29-31 July 1974, Columbia, MO, in press. (1 1) T. A. Cahill and P. I. Feeney, Report UCD-CNL 169, University of Caiifornia, Davis, CA, 1973. 12) R. G. Flocchini, D. J. Shadoan, T. A. Cahill, R. A. Eldred. P. J. Feeney, and G. Wolfe, "Advances in X-ray Analysis," W. L. Pickles, C. S. Barrett, J. B. Newkirk. and C. 0. Ruud. Ed., Vol. 18, Plenum Press, New York, NY, 1975, in press. 13) T. B. Johansson. R. E. Van Grieken, and J. W. Winchester, J. Rech. Atmospheriques, in press. 14) B. K. Barnes, R. M. Coleman, G. H. R. Kegel, P. W. Quinn. and N. J. Rencricca, "Advances in X-ray Analysis," W. L. Pickles, C. S. Barrett, J. B. Newkirk, and C. 0. Ruud, Ed., Vol. 18, Plenum Press, New York, NY, 1975, in press. 15) M. Ahlberg, R. Akselsson, D. Brune, and J. Lorenzon, Nucl. lnstrum. Methods, in press. (16) J. W. Winchester, D. L. Meinert, J. W. Nelson, T. B. Johansson, R. E. Van Grieken, C. Orsini. H. C. Kaufmann, and R. Akselsson, Proc. Sec. Int. Conf. on Nucl. Meth. in Environ. Res., 29-31 July 1974, Columbia, MO, in press. 117) R. Akselsson, C. Orsini. D. L. Meinert, T. B. Johansson, R. E. Van Grieken, H. C. Kaufmann, K. R. Chapman, J. W. Nelson and J. W. Winchester, "Advances in X-ray Analysis," W. L. Pickles, C. S. Barrett, J. B. Newkirk and C. 0. Ruud, Ed., Vol. 18, Plenum Press, New York, NY, 1975. in press. ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1 9 7 5
859
(18)J. K. Kliwer, J. J. Kraushaar. R . A. Ristinen, H. Rudolph, and W. R. Smythe. Bull. Am. Pbys. SOC.,17, 504 (1972). (19)R. Akselsson and T. E. Johansson, Nucl. lnstrum. Methods, 91, 663664 (1971). (20)J. T. Routti and S. G. Prussin, Nucl. lnstrum. Methods, 72, 125-142 (1969). (21)J. Harrison and R. Eldred, "Advances in X-ray Analysis," C. L. Grant, C. S. Barrett, J. B. Newkirk, and C. 0. Ruud, Ed., Vol. 17,Plenum Press, New York, NY, 1974,pp 560-570. (22)H. C. Kaufmann and R . Akselsson, "Advances in X-ray Analysis." W. L. Pickles. C. S. Barrett, J. B. Newkirk, and C. 0. Ruud. Ed., Vol. 18,Plenum Press, New York, NY, 1975,in press. (23)R . Akselsson and T. E. Johansson, Z. fbysik, 266, 245-255 (1974). (24)W. Bambynek, E. Crasemann, R . W. Fink, H.-U. Freund, H. Mark, C. D. Swift, R. E. Price, and P. Venugopala Rao, Rev. Mod. Pbys., 44, 716813 (1972). (25)E. Merzbacher and H. W. Lewis, "Encyclopedia of Physics," S. Flugge. Ed., Springer-Verlag, Berlin, 1957,Vol. 34,pp 166-192. (26)J. S. Hansen, J. C. McGeorge, D. Nix, W. D.Schmidt-Ott, I. Unus. and R . W. Fink, Nucl. lnstrum. Methods, 106, 365-379 (1973). (27) Micro Matter Co., 197 34th St. East, Seattle, WA 98102. (28)E. Storm and H. I. Israel, Nucl. Data Tables, A7, 565-681 (1970). (29)D. C. Camp, J. A. Cooper, and J. R . Rhodes, X-Ray Spectrom., 3, 4750 (1974). (30)J. Cookson and M. Poole, New Sci., 45, 404-406 (1970).
(31)J. M. Jakievic, F. S. Godding, and D. A. Landis, E€€ Trans. Nucl. Sci., NS-19 (3),392-395 (1972). (32)J. W. Nelson, I. Williams, T. B. Johansson, R . E. Van Grieken, K. R . Chapman, and J. W. Winchester, E€€ Trans. Nucl. Sci., NS-21 (l), 618-621 (1974).
RECEIVEDfor review March 11, 1974. Accepted January 6, 1976. This study, part of a general study of the pollution sources of trace metals in the atmosphere, was supported in part by Grant R802132 from the U.S. Environmental Protection Agency and by Grants NSF-GU-2612 and NSFGP-25974 of the National Science Foundation. One of us (Thomas B. Johansson) is grateful for travel support from the Swedish Board for Technical Development (Grant STU 72-573/U485) and from the Swedish Atomic Research Council (Grant AFR 1213-1). During the course of this investigation, R. E. Van Grieken, on leave from the Institute for Nuclear Sciences, Rijksuniversiteit Gent, Belgium, received NATO and NFWO Fellowships.
Absolute Determination of Phosgene: Pulsed Flow Coulometry Hanwant Bir Singh,' Daniel Lillian,2 and Alan Appleby Department of Environmental Science, Agricultural Experiment Station, Cook College, Rutgers UniversityJersey, New Brunswick, N.J. 08903
Pulsed flow coulometry (PFC) was developed for the absolute analysis of reactive electron absorbing air pollutants, which undergo decomposition in a gas chromatographic column, and was used successfully for the determination of sub-ppb mixtures of phosgene in air. Compared to permeation tube standards, an error of less than 15% was easily achieved when ionization efficiencies were greater than 75 YO.At ionization efficiencies greater than 85 YO, this error could be reduced to 4 % . The electron capture detector response to phosgene was comparable to that to carbon tetrachloride, one of the strongest known electron absorbers. It was demonstrated that ppb mixtures of phosgene in air are quite stable in the presence of moisture and undergo rapid heterogeneous decay on surfaces. PFC should be useful for the analysis of any unstable electron absorber (e.& peroxyacetyl nitrate (PAN), chlorine dioxide, trichloroacetyl chloride) in air and offers a wide scope of application in air pollution and industrial hygiene.
Phosgene finds wide use in industry as an intermediate for the production of a myriad of chemical compounds (11. Accordingly, there are numerous possible industrial hygiene and localized air pollution problems associated with its use. Potential industrial exposure to this compound is enhanced because of the widespread use of chlorinated hydrocarbons, many of which will undergo thermal and photochemical decomposition to form phosgene (2-9). Phosgene may also be important in air pollution work, since evidence exists that i t is synthesized in the lower troposphere during photochemical smog reactions involving halocarPresent address, Stanford Research Institute, Menlo Park,
CA.
94025. Present address, U.S. Army E n v i r o n m e n t a l Hygiene Agency, Edgewood Arsenal, MD 21010.
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
The State University of New
bons (10, 11).An accurate and convenient analytical method accordingly was required for ambient as well as TLV (100 ppb) levels of phosgene in air. The wet chemical procedures reported in the literature for the determination of phosgene vapor have been reviewed by Kolthoff e t al. ( 4 ) and Jeltes e t al. (9). In general, they suffer the typical difficulties associated with wet chemical procedures-namely, lack of specificity, interferences, losses in sampling lines, and the requirement of large samples precluding real time analysis. Furthermore, such methods are often elaborate and require considerable experience for acceptable accuracy. Priestley et al. (12) reported the electron capture (EC) gas chromatographic (GC) determination of phosgene in air. Separation was achieved using an aluminum column packed with 30% didecyl phthalate coated on 100/120 mesh GC 22 Super Support. Dahlberg and Kihlman ( 1 3 ) used a similar GC procedure with a stainless steel column packed with 20% DC 200 on Chromosorb W. The column had to be treated with acetyl chloride to preclude unacceptable phosgene losses. A sensitivity of 1 ppb was achieved by both groups. Jeltes e t al. (9) used an aluminum column packed with 30% diisodecyl phthalate coated on SO/lOO mesh Aero Pak for phosgene analysis and reported a sensitivity of 0.2 ppm with the EC detector. Because of its reactivity and the probelms associated with wet chemical methods, phosgene analysis is best accomplished by on-site GC procedures. The three columns used by the above investigators, based on our experience, require routine calibrations because of the extreme reactivity of phosgene which causes variable column losses, depending on the column history. Clearly, an absolute method not requiring calibration or suffering from changing column characteristics is desirable. Reported here is an extension of absolute coulometric analysis (14, 15) which empirically corrects for column sorption through the use of pulse
