1 -
100,
Table I. Variation of Ion Yield with Temperature for Six Elements in Cerrolow 117
1
i 1
Relative ion yield
Bit
In+
PbC
Sn+
Cd+
HgS
54°C 70°C
0.550 0.722
2.07 2.04
0.672 0.622
0.449 0.271
0.798 0.541
2.17
/
~
7 ,&
... .5n
Table I I. Variation of Ion Yields with Temperature for the Elements in GalnSn Relative ion yield
Ga '
In+
Sni
20 " C 45 " C 75 "C
0.942 0.932 0.932
1.75 1.84 1.86
0.332 0.271 0.253
plotted against known atomic concentrations. The Cerrolow 117 data was taken a t 54 "C. The doped and undoped GaInSn data were obtained a t 20 "C. All samples were run at 75 FA of needle current. The elemental concentrations ranged from several hundred ppm atomic for Cd to about 7090 atomic for Ga. Since In+ was used as an internal standard for the doped GaInSn and is known to have a high relative ion yield, the values for Cd, Pb, and Bi appear low for this sample. When these values are corrected for the relative ion yields of In+ in the GaInSn and each element's relative ion yield in Cerrolow 117, the points fall much closer to the ideal 45" line. In the instance of the doped GaInSn. Bil appears high due to a Ga3+ interference at mass 209. Figure 8 shows that all the elements are determined within a factor of two of the known concentrations for the three samples. The use of sensitivity factors for a specific element in a specific matrix would be expected to greatly improve overall accuracy.
- - - __ - I
001
IC 01 CALCULATED ATOMIC PERCENT
'Cr,
Figure 8. Apparent vs. true concentration for elements ( + ) GalnSn, ( 0 ) Cerrolow 117, ( 0 )GalnSn doped with 1% Cerrolow
117, (0) same as ( 0 )but corrected for relative ion yield
Now that a reliable EH ion source has been developed and characterized, experiments are being carried out to determine the more fundamental aspects of the ionization process. Experiments are also being undertaken to evaluate the potential application of this ion source to the mass spectrometric analysis of thermally labile organic compounds. Received for review December 18, 1972. Accepted March 30, 1973. This research was supported in part by National Science Foundation under Grant GH-33634.
Automated Evaluation of Photographically Recorded Mass Spectra Margaret A. Frisch and Wilhad Reuter ISM Thomas J. Watson Research Center, Yorktown Heights, N. Y. 10598
A program has been developed for the semiquantitative evaluation of photographically recorded spark source mass spectra. The densitometer readings are transformed into relative intensities, considering the mass dependence of the photographic response and the instrumental transmission characteristics. Line assignments are made, accurate within 5 to 30 millimass units, depending on the mass. Most interference problems are quantitatively evaluated. Many notations are used to aid the analyst in the interpretation of the data. Finally the concentration for each element is calculated in parts per million.
The photographic plate continues to maintain a strong position as an ion detector in high resolution mass spectroscopy in spite of recent advances in electrical detection systems. The chief attribute of the photographic plate is its ability to int.egrate ion currents simultaneously over a wide mass range. The information is stored permanently
on each plate and can be processed at the analyst's convenience. Such merits are counterbalanced by problems which typically arise in the quantitative evaluation of photographically recorded spectra. The photographic response to the ion beam is a complex function of the ion energy and mass, the silver halide grain size, and the gelatin concentration. In contrast to photon detection, only the top grain layer interacts with an ion beam in the 10 kV range. Any variation in the surface structure of such emulsions, occurring in the manufacturing process, may lead to significant changes in the photographic response to ion detection. In optical spectroscopy, the photographic response to the radiation of interest is usually determined on one plate taken to be representative for all plates of the same emulsion number. For ion sensitive plates, variations in the sensitivity as large as a factor of two were observed by Burlefinger and Ewald ( I ) between plates of the same emulsion number. For precise quantitative work, each plate must be calibrated. For the complete quantita( 1 ) E. Burlefinger a n d H . Ewald. Z.Naturtorsch. A . 18, 116 (1963).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
1889
tive evaluation of a spectrum, up to 100,000 data points have to be recorded and processed. In practice, most analysts employ several more or less justified approximations and limit the evaluation to the specific question raised in materials research. At best semiquantitative information is obtained and often unsuspected important information is missed in this procedure. The obvious solution to this problem is to acquire the data, using a computer controlled optical densitometer, and to analyze these data in a suitable application program. For high resolution gas mass spectroscopy, such automated data acquisition systems and their application programs have been described by Venkataraghavan et al. (2) and Desiderio and Mead ( 3 ) . The major emphasis in these application programs has been placed on the highest attainable accuracy in the mass assignment and enhancing the resolution in regions of overlapping lines. with the aid of deconvolution techniques. In contrast to high resolution gas mass spectroscopy, the requirements in the accuracy of the mass assignment are more relaxed in spark source mass spectrography, operating a t a resolving power lower by one order of magnitude. Instead, for the quantitative evaluation of spark source plates, the reliability of the optical transmissionion intensity transform becomes a major aspect in the application program. Photographic response curves, based on manually recorded transmission data, have been calculated in programs developed by Kennicott ( 4 ) , Woolston et al. (5), Degreve et al. (6), and Franzen and Schuy (7). Automated densitometry has been employed by Bailey et al. (8), using for the plate calibration the rhenium isotopes obtained on each photographic plate used for chemical analysis. Disregarded by Bailey, yet of considerable importance in quantitative spark source mass spectrography, is the mass dependence of the sensitivity of the photographic plate (7). The approach taken by us is similar to the one taken by Bailey et al. but considerably expands on the response characteristic of the emulsion and also includes several routines for the elimination of interference problems a t the same nominal mass. Finally the concentration is calculated for each element detected.
EXPERIMENTAL The automated microdensitometer used in our study has been described by Segmuller and Cole (9). The instrument is controlled by an IBM 1800 computer, operating under a time sharing system. We operate under a “step and read” mode, rather than the “continuous motion” mode, the former being more accurate than the latter, although the latter is generally preferred where very high data rates are processed ( 3 ) .The stage of a single beam Hilger Watts, Model 486, densitometer is moved in increments of 10 p. This step size yields sufficient data points for the calculation of the line center and for the intensity integration of the relatively broad lines encountered in spark source mass spectrography. The stage can also be moved in 100-p steps along the Y axis, ie., in the direction normal to the mass spectra. The photometer slit is adjusted to give an image 10 p wide by 1 mm high. After focusing, a parallel alignment of the plate is made, such that the slit is centered on each spectral line over the entire mass range. The lamp current, obtained from a constant current power supply, is adjusted to yield for air transmission 3000 mV across a 1 (2) R. Venkataraghavan, F. W. McLafferty, and J. W. A m y , Anal. Chem., 39, 178 (1967). (3) D. M . Desiderio and T. E. Mead, Anal. Chem., 40, 2090 (1968). (4) P. R. Kennicott, Rep. No. 64-RL-3766G, General Electric Research Laboratory, 1964. (5) J . R . Woolston, R . E . Honig. and E . M. Botnick, Rev. Sci. Instrum., 38, 1708 (1 967). (6) F. Degreve and D. Chanpetier de Ribes, Int. J. Mass Specfrom. /on Phys., 4, 125 (1970). (7) J. Franzen and K. D. Schuy, 2. Naturforsch. A, 21, 1479 (1966). (8) C. A. Bailey, R. D . Carver, R. A. Thomas, and R. J . D u p z y k , Develop. Appl. Spectrosc., 7a, 294 (1969). (9) A. Segmuller and H. Cole, Advan. X-Ray Anal., 14, 338 (1971).
1890
M Q resistor for the photomultiplier output current. The optical transmission is read with a Hewlett-Packard 2401C integrating digital voltmeter using a 10-msec integration period. The settling time of the digital voltmeter requires an additional 20 msec. Thus to acquire each data point, a total of 45 msec is used which includes 10 msec for the plate movement. This results in a total scanning time of 20 min for each spectrum. Usually only one spectrum, the longest ion exposure, is scanned and evaluated. The densitometer is operated with a set of instruction cards fixing the range to be measured, the exact location of le02+ as identified by the analyst, two positions in the Y direction for the dark current measurement (light beam blocked by the rim of the plate holder), and the air transmission measurement. At the end of the plate reading process, the data are transmitted uia a high speed channel to an IBM 360/67 computer, operating under TSS, a time sharing system. DATA REDUCTION, PHASE I (a) Background Approximation. In the first step of our data reduction program, the entire range is divided into blocks of 500 data points, corresponding to 0.5-cm segments on the plate. Data blocks much larger than 500 points resulted in a poor definition of the rapidly changing background in the matrix line region. (In this paper, the isotopes of the major constituents, whose identity and concentration are usually known to the analyst, are referred to as the matrix.) Block sizes much smaller than 500 data points may not contain enough data in a signal rich region for a reliable estimation of the background. A five point second order smoothing function is then applied to all data. We have taken two approaches for signal-background discrimination. In our first routine, a second order polynomial fit by the least squares method is calculated, including all data points. Those voltage readings, V, greater than the polynomial fit minus 1.5 u are retained as background data. We selected 1.5 u as a realistic estimate of the background limit (Figure 1).All readings less than the -1.5 u limit are searched for a set of five or more consecutive data points meeting two criteria: V(2) < V(l) and the set may not contain any point V(n)for which V(n 1) < V ( n ) > V(n - I), in order to exclude closely spaced multiple noise spikes. Those points which fail to meet this test are retained as background. Excluding these tentatively identified signal points, a second least square fit is made to all retained points, and again, those points which fall outside -1.5 u are subjected to the above test. This procedure is repeated until either no further decrease in u or in the number of background points is obtained or until the tenth iteration is reached. In practice, u reaches a limiting value after approximately five successive calculations. Much faster and currently used in our program is our second approach, which is identical to the first, except that an initial search is made on all data for a set of five or more consecutive points satisfying the above mentioned criteria. Terminating conditions are usually reached in the second iteration. Background curves obtained by both methods are for all practical purposes identical. The data blocks are overlapped by 50 points. Confusions in the overlapping regions of data blocks are avoided by the inclusion of two criteria. (a) If a data block terminates with points in the background, up to 50 points are transferred to the next data block. However, the overlap may contain less than 50 points, since the transfer ceases a t that position where the data first fall outside of the background limit, indicating the appearance of a spectral line, (b) If a data block terminates with points in a spectral line, all these signal points plus up to 50 preceding background points are included in the next data block, using its background fit for the background correction of the spectral line in the overlap.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
+
T (Tm =O)
I 01
240 1 ' mV 1800!
I
'
910
1
00:
24.0
9.1
1
1400
1450
cm Figure 1. Densitometer tracing in steps of 10 p and background fit (upper line) in the mass range from 112 to 124 obtained from a l o - - @coulomb exposure of tin metal
The result of a background fit, using the second iterative method described above, is shown in Figure 1 for the region of singly ionized Sn isotopes in a heavily exposed spectrum. The second order polynomial fit, represented by the upper line traversing the background points, gives a good approximation of the background in this nonlinear region. The lower line is the 1.5 u acceptance limit used subsequently for the extraction of spectral lines. (b) Line Extraction. A spectral line is considered detectable provided it contains a sequence of five or more data points lying outside the 1.5 u acceptance limit. In addition, the transmission difference between the background and the line center must exceed 6 u(Bkgd) to discriminate against emulsion noise. The presence of spurious signals near the background (e.g., dust particles, emulsion defects) is effectively eliminated a t the selected 6 CJ level. The line center is determined by a parabolic interpolation of the three points of minimum transmission. An earlier attempt to use either a gaussian or a parabolic fit to all data points on either side of the minimum gave less accurate mass assignments than the interpolation procedure presently used. The variability in the line shapes observed in our spectra did not warrant deconvolution by analytical techniques. Two lines are considered "resolved" if the rise in the transmission between two lines exceeds twice the estimated standard deviation of the noise a t the minimum transmission V(min). The standard deviation u(min) at V(min) is estimated from the empirical relation (min) =
500
OSMIUM TIN CALCIUM
~
-
760
_ _ _ _ -4-B_k _g- d-_) - - - - - 1
+
log [V(Bkgd)/V(min)]
(1)
where cr(Bkgd) is the standard deviation of the background fit obtained for this data block and V(Bkgd) is the estimated background reading at the line center. This relation is based on data obtained by us from heavily but uniformly fogged regions near matrix lines and from the flat portions of broad peaks. From the exact location of the 1 6 0 2 + line identified by the analyst prior to the plate reading process, the massto-charge ratio is calculated for each line using a previously established mass dispersion equation. This is a rough approximation only and is intended to aid the analyst in his evaluation of the first phase of the data reduction program. ( c ) Determination of Maximum Transmission. In optical emission spectroscopy, the accepted procedure is to
I
/€
I
- 6 0 ' -I00
000
-050 LOG 10
I ( 100- T ) / (
0 50
I 00
T-Tm)I
Figure 2. Emulsion calibration curve (Ilford Q2) for calcium, tin, and osmium The charge 0 I S in units of l o - $ COUlOmb
fix the 100% transmission point by the maximum transmission reading, obtained in the exposure-free region between spectra. However, in spark source mass spectrography, the search for a representative 100% transmission point is complicated by an ion induced haze often covering sections of the plate. For this reason, the entire mass range is divided into 25 data blocks containing 1000 readings each. Excluding all data points associated with possible spectral lines, using the criteria described in section b, the center in the frequency distribution is found for each data block. The visual identification of the clearest section (100% transmission) of the plate corresponds in the automated evaluation process to that data block yielding the highest average transmission. This value is selected as the 100% transmission point V(100). By this definition, values exceeding 100% are found in the complete data set. This procedure gives a more realistic estimate of the 100% transmission value for Ilford Q 2 emulsions than the above mentioned technique used in optical emission spectroscopy. (d) Emulsion Calibration. In Figure 2, the Hull function (IO) is plotted against the log of the total accumulated charge, including Q(Bkgd) arising from the ion induced background, for the three calibration elements calcium, tin, and osmium. Fifteen spectra were recorded for each element on Ilford $2 emulsion, covering an exposure range of 10-8 to 10-12 coulomb. The two linear regions at low and high transmission are represented by log [ Q A
+
Q(Bkgd)] = u
+ [(loo -
Ylog TINT - T J 1 (2) where Q = accumulated charge in units of coulomb, A = isotopic abundance, Q(Bkgd) = a hypothetical charge, equivalent to the background measured in this position (see the following paragraph), T = V/V(lOO) = transmission a t the line center, T - = transmission at the saturation. obtained from matrix lines in a 10-7 coulomb (10) C. W. Hull, 10th Annual Conference on Mass Spectrometry and Allied Topics, New Orleans, 1962.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
1891
exposure on a plate of the same emulsion number, and y = the slope of the calibration curve. These two linear portions are joined by a spline, which was found to be best fitted by y =
ul
+
y,x
+
e(x
-
xl)'
+
f(x
-
x1)15
(3)
+
where y = log [QA Q(Bkgd)], x = log [(loo - T ) / ( TT - ) ] ,x 1 = the start of the nonlinear section, estimated visually, and ul ylx = y in the high transmission linear section. The exponent of 15 was found by a systematic search, minimizing the standard deviation of data points in the spline region. The abrupt curvature in this region requires the large exponent. The spline chosen follows the experimental data almost perfectly, as can be seen in Figure 2. The coefficients e and f are given by
+
Y1 al
+
ylx2
+ +
2e(x2
-
xl)
+
15f(x,
e(x2
-
xlY
+
f(xz -
-
x1)14
=
Yz
(4)
where x 2 = the end of the nonlinear section, estimated visually, and uz + yzx = y in the low transmission linear section. On the first iteration, the charge contribution Q(Bkgd) is ignored and a least square fit is made between log [QA] and log [(loo - T ) / ( T- 2'-)I. Based on this relation, a first estimate of Q(Bkgd) is made using the background transmission found for this mass position. In the high Q(Bkgd)] transmission region, the linear fit of log [QA to log [(loo - T ) / ( T - T , ) ] yields a new value for the slope of the curve. The procedure is repeated until the change in this slope between two successive calculations is less than 1%. The accumulated charge measured by the ion beam integrator has been used for exposure levels of coulomb and above. For shorter exposures, we observed systematic variations of up to 20% between data sets belonging to different exposures. Such large variations considerably exceed the 5% accuracy found for the integrator circuit upon calibration. We have no satisfactory explanation for this effect except for the pure speculation that the photographic emulsion and/or the monitor collector plate may be sensitive to the arrival rate of ions. For exposures above 10-11 coulomb, fluctuations in the arrival rate are sufficiently averaged so that this effect is not significant a t relatively large exposures. Consequently, we have incorporated an iterative scheme whereby the exposure levels 10-11 coulomb and below are adjusted in small increments, minimizing the standard deviation of the least square fit. This modification of the exposure levels reduced the error to *5%. In the region from 10 to 90% transmission (see Figure 2), the mass sensitivity increases by about a factor of 6 from osmium to calcium. This effect is usually the most important correction required for improving the accuracy of the semiquantitative analysis. Most of this sensitivity increase is due to the transmission characteristics of the analyzer (AEI MS7), yielding shorter (lack of Z focusing) and narrower lines a t the lower masses. The line length and width are both proportional to the square root of the mass ( l l ) ,thus accounting for a factor of 4.5 = (mo,/mc,) in the sensitivity increase over this mass range, because the data in Figure 2 are based on the optical transmission a t the peak center, measured with constant slit width and height. Other contributing factors are the mass depen-
+
(11) J. Mattauch and R. Herzog. 2. P h y s . . 89, 786 (1934)
1892
ANALYTICAL CHEMISTRY, VOL. 45,
NO. 11,
dence of the sensitivity of the emulsion and possible variations in the focusing properties over the mass range. The mass dependence of the instrumental sensitivity of our system is empirically determined to be
In order to find the accumulated charge corresponding to a measured plate transmission at a given mass m, the accumulated charge for this transmission is initially calculated for the three calibration elements. The parameters, p and q, are then determined from the least mean square fit. Finally the accumulated charge is calculated from Equation 6 for the mass rn. This procedure is repeated for each detected line. At this point in the program, the relative intensities are calculated for all points in each spectral line. Background corrections are made using the second order polynomials representing the background in the respective section of the plate. In addition, the intensity of each line is integrated. All routines applied so far are generally applicable for any photographically recorded spectra, except for the estimation of the mass-to-charge ratio. The program output initiated a t this point shows among many other pertinent features the estimated mass for each line detected and the corresponding peak and integrated intensities. DATA REDUCTION, PHASE I1 This part of the program contains special routines for the evaluation of spark source mass spectra. (a) Mass Dispersion. Mass spectra produced in spark source mass spectrography exhibit singly and multiply ionized mass lines of the major constituents in the sample, which are easily assigned to the correct isotope. First, the analyst identifies a number of such lines (usually about 10) which are unambiguous, covering the mass range of interest. Broad lines, such as the lines from singly ionized matrix isotopes, give poorly defined line centers and hence are avoided in this selection. The square roots of the known masses (Handbook of Physics, 1968) of the selected lines, and their respective X positions, are fitted to a third order polynomial, giving the mass dispersion equation. Using this curve, accurate masses are calculated for all unidentified lines. Line centers, calculated from the three points of minimum transmission, give better agreement ( 5 to 30 millimass units) between calculated and known masses, than using line centers calculated from a parabolic fit to all data points excluding the tail ends. In the latter method, line centers are poorly defined when, as often experienced in spark source mass spectrography, heavily skewed lines are encountered. Alternatively the precise 1 6 0 2 + location taken from the phase I output can be used to modify the dispersion equation obtained from another plate. While less accurate (10 to 50 millimass units) than the method based on selected lines covering the entire mass range, we often use this faster procedure with satisfactory results. (b) Ion Assignment. From a stored table containing all isotopes of the periodic system, assignments are made for each line. All singly or multiply ionized isotopes falling within the full width of the line are considered possible assignments (Table I, column 4). However, lines corresponding to all lower ionized states must be present before a higher ionized state of an isotope will be assigned. The full line width is the width between the first and the last data point above background. The deviation of the known mass, as listed in the Handbook of Physics 1968, from the calculated value is computed (Table I, column 5 ) .
SEPTEMBER 1973
Table I. Summary of Ion Assignments Ordered by Increasing Mass Element
Nominal mass
Reference isotope Charge
Calcd and known masses
Notation
Elemenl
-0.036
0.5 0.5
-
Cr
52
+1
Fe
Am
Intensity
Mass
Charge
Cr
52
1+
51.977 51.941
Fe Cr
54 54
1+ 1+
53.962 53.940 53.939
-0.022 -0.024
1 .o 0.9 0.0
*
Cr
56 52
+1 +1
Mn
55
I+
54.928 54.938
0.010
25.5 25.5
-
Mn
55
+1
*
Fe Yb
56 172
+1 +3
Yb
172
+3
Fe
Fe Yb
56 168
1+ 3+
Yb
170
3+
Fe Yb
57 171
3+
Yb
172
3+
Yb
173
3+
Ni
Yb
58 58 174
1+ 1+ 3+
Yb
176
3+
cu
63
1+
Fe
Ni
Te
cu Te
1+
64 128
1+ 2+
65 130
2+
1+
55.960 55.935 55.978 56.641 56.645 56.986 56.935 56.979 57.308 57.312 57.640 57.646 57.969 57.935 57.933 57.980 58.652 58.648 62.955 62.930 63.928 63.928 63.952 64.949 64.928 64.953
0.004
14.7 13.4 1.3 37.6 30.1
-0.051 -0.007
159.6 0.3 142.2
-0.025 0.018
0.004 0.006 -0.034 -0.036 0.011 -0.004 -0.025
-0.000 0.024 -0.021 0.004
Beginning at the high mass end, a first pass (Figure 3, qualitative identification) is made, searching for singly ionized isotopes of the elements. Isotopes found are only tentatively assigned to the elements. Interferences with molecular or multiply ionized ions cannot be resolved a t this point in the program. The objective of the first pass is to establish the absence of elements and to remove all isotopes of these elements, regardless of the ionized state. This search is facilitated by the inclusion in our program of Table 11, listing the most unique isotope for each element. We selected one isotope for each of the first six ionized states, optimizing the criteria of high abundance and low probability of interference by singly or multiply ionized isotopes of other elements. (For example, we selected for zinc the 66Zn1+ isotope as the unique isotope in the first ionized state, rather than the most abundant 64Zn1+ isotope, considering the possible interference of s4Ni1+ with 64Znl+. In the second ionized state, 67Zn2+ was selected, which ranks only fourth in abundance. The uniqueness of 67Zn2+, appearing at the nominal mass of 33.5, more than compensates for the disadvantage of the lower intensity of this isotope in respect to the most abundant G4Zn2+ which is frequently interfered by 3 2 0 2 1 + and/or 3 2 s +
.)
Elements are rejected if the most abundant isotope is absent. If present and the unique isotope is also present and non-interfered, a search is made for the presence of
216.8 216.8 182.8 160.3 293.0 4.6 0.0 316.4 162.1 126.5 7.4 7.4 74.9 0.1 0.8 4.2 3.3 0.9
*
* *
Yb
56 172
+1 +3
-
Yb
172
+3
Yb
172
+3
Ni Yb
60 56 172
+1 +1 +3
Yb
172
+3
cu
63
+1
60 130
+1 +1
63 130
+1 +1
* * *
-
*
* *
Isotope not observed; RI = P/(Bkgd)/A
-
5
YO
Isotope interferred; Z/(int) < 0.3/(tot) R I = [ / ( t o t ) - Z/(int)]/A Isotope interferred; 0.3/(tot) < B/(int) < /(tot) RI = (0.2 0.7[/(tot) - z/(int)]/l(tot))-
+
/(tot)/A 6
$
8
&
/(tot)
0.2
2.0 1.0 1.0 1.0
5.0 2.0 1.0 1.0 1.0
a Reduced intensity RI(n 1) will be selected if the number in the table isgreater than the ratio RI(n l ) / R l ( n ) . lsotopen 1
RI = O . Z l ( t o t ) / A 7
Sr
Isotope n
RI = I ( t o t ) / A
4
Mo Zr
Table IV. Hierarchy of Selection Rules for the “Best” Isotope of Element Z a
Table I l l . Code Assignments Code Symbol
Ru
Isotope interferred: intensity estimates have not been generated for all interferring ions, upper RI = / ( t o t ) / A For multiply charged ions, a reduced intensity is
calculated from ’/3 the reduced intensity, determined as “best,”for the next lower ionized state a /(tot) is the total line intensity: A is the abundance of a particular isotope; [(Bkgd) is the intensity calculated for the backpround transmission where the line should be observed: Z/(int) is the sum of intensities for interferring ions.
propriate weighting factor (Table IV), the assignment is altered to code 8 and the reduced intensity of this isotope is estimated from 1/3 of its intensity in the next lower ionized state. This code is given a very low priority in the selection rules, accounting for the large uncertainty in the assumption that the intensity decreases by a factor of 3 for the next higher ionized state. However, this “rule of thumb” yields, in some cases, a better estimate for the reduced intensity of multiply charged ions than the other intensity estimates available.
*
+
Several loops are made through the elements in reverse order and over their first to sixth ionized state until no further change occurs in any of the calculated net intensities of all isotopes. Four passes are usually required to resolve all possible interferences, including multiply ionized lines. (d) Quantitative Estimation. Two options are provided in the program for the quantitative estimation of the elemental concentrations. If an isotope of a matrix element is present with a transmission larger than 9% (log [(lo0 T ) / n < 1) in the same spectrum in which the impurity element is detected, the concentration C (atomic) of the latter is calculated from C(atomic) = bQ(imp)/Q(matrix)
(7)
where b = atomic concentration of the matrix element and Q(imp), Q(matrix) = total accumulated charge of the impurity and matrix element. Since we wish to base the concentration of the element only on an isotope in the singly ionized state, Q(imp) is equated in our program to the reduced intensity obtained for the “best” isotope of
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
1895
~~
Table V . Semiquantitative Evaluation Ordered by Elements Relative intensity/abundance Isotope
1
+
2+
3+
4+
Abundance
Element = 26 Fe Interference, Difference 69.2 ppm 56 54 57 58
14.616.4* 7289.2* 88802.3*
4.9< 17.5” 0.0 104.1*
O.O>
0.0
10.2* 0.0 0.0
0.0 0.0 0.0
0.916600 0.058200 0.021900 0.003300
Element = 28 Ni interference, Less Than 32.3 ppm 58
431.7*
60
o.o>
62 61 64
0.0 0.00 6936.0*
0.5 0.0
0.0 0.0
0.0 0.0
0.0
52824.3*
0.0 0.0
0.0 0.0 0.0 0.0
0.0
0.678800 0.262300 0.036600 0.011900 0.010800
Element = 29 Cu Positive identification 50.9 pprn 63 65
10.7= 13.6*
1.0=
0.0
0.0 0.0
0.0 0.0
0.691739 0.308261
Z1+. If the “best” isotope belongs to codes 1 or 2 , the element is reported as “positive identification.” In the case of a code 3 assignment, the concentration is listed as “not detected, less than” the calculated detection sensitivity. The concentration for an element evaluated with a code 4 is designated by “interference, difference.” If the “best” isotope of an element falls under codes 5, 6, or 7 , the concentration is reported as “interference, less than” the calculated detection sensitivity. Alternatively. the concentration can be calculated from C(atomic) = Q(imp)/Q(total) (8) Here Q(tota1) is the total accumulated charge as measured by the beam integrator. In this approach, we make the assumption that photographic plates of the same emulsion number exhibit the same sensitivity for ion detection. While this is not the only error contributing to the accuracy of analysis, this assumption alone may introduce an uncertainty in the results by as much as a factor of2(1). ( e ) Data Output. The output of the data reduction phase I1 is shown in Tables I and V for the analysis of ytterbium sulfide, covering the range from copper to iron. The accumulated charge in this spectrum was 10-9 coulomb. This section of the mass range shows most of the pertinent features of the analysis program. Table I lists all lines detected in this range, the mass assignment (column 4), headed by the mass calculated from our mass dispersion equation and followed by the known masses of all ions falling within the width of the line, the deviation Am from the calculated mass (column 5 ) , the background corrected peak intensity and the intensity of interfering isotopes (column 6), and clues (column 8-11) to the analyst on which reference isotope the interference correction is based. Several notations (column 7 and 11) are used to guide the analyst in the interpretation of the results. Table V gives the estimation of the concentration for each element, arranged by increasing atomic number. Also given are the reduced intensities (relative intensity/abund a m e ) for all isotopes of this element up to the fourth ionized state. Starting at the high mass end, a line is observed, yielding a calculated mass of 64.949 (Table I). Possible assignments within the full line width are 65Cu1+ and 130Te2+. Doubly ionized isotopes of Xe and Ba at the same nominal mass 65 are not considered here, due to the absence of higher abundant singly ionized isotopes of these elements. In the alxence of the unique 125Te2+, the program esti1896
mates the tellurium interference on copper using 113 of the intensity of the most abundant 130Te1+ isotope. The < notation indicates that the )1J intensity argument for successive ionization states usually represents an upper limit for this intensity correction. (Only once have we seen an exception to this rule, when analyzing nonconducting glass samples wrapped in gold foil. Doubly ionized isotopes yielded denser lines than the corresponding singly ionized isotopes.) We are using here the emulsion calibration curve established for singly ionized ions in the intensity correction for the more energetic multiply ionized isotopes. The validity of this approach is supported by the experimental results obtained by Woolston et al. ( 5 ) who found no change in the slope of the calibration curves for Cs ions in the energy range from 5 to 20 kV, recorded on Ilford Q2 plates. From this we make the assumption that the slope remains constant for energies above 20 kV, corresponding to the energy of multiply ionized ions. In Table V, the reduced intensity is calculated, based on the total intensity observed at mass 65. The * notation on 65Cu1+ alerts the analyst to the presence of an interference which can be studied in more detail by reference to Table I. The detected unique 63Cu1+ isotope with no interference by other elements is unambiguous evidence for the presence of copper. The concentration of copper is calculated from Equation 7 , using the 0.135% abundant 168Ybl+ isotope in the same spectrum. The unique 60Ni1+ isotope was not detected. The element is reported as “interference, less than . . .” and an upper concentration level is calculated from twice the background intensity found in this position. At mass 58, a line is found which agrees within 10 millimass units with the known mass of 174Yb3+. Using the noninterfered unique 172Yb3+, an intensity is calculated for 174Yb3+ which is in good agreement with the intensity experimentally found in this position. The upper limit for the contribution of the most abundant 58Nil+ isotope is calculated from twice the background intensity found at the position of the not detected and less abundant 6oiVili isotope. The isotope used for the calculation of the detection sensitivity is given the > notation. The intensity found at mass 64 is in poor agreement with the sum of the intensity contributions of e4Yi1* and 1*8Te2+. The program excluded Zn, because of the absence of a detectable line at mass 68. The signal at mass 64 is caused by 64S21+. While molecular ions are not included in our program, problems rarely arise from this omission due to such internal checking routines built into the program. The unique 56Fe1+ isotope is found with a small interference by ytterbium. The contribution of 168Yb3+ to the total intensity at mass 56 is accurately determined from the intensity of the noninterfered 172Yb3+.The concentration of iron is calculated from the net intensity and is given the - notation, “interference difference.” Also detected was 54Fel+ with negligible interference by 54Crl+. The reduced intensity of this line is slightly larger than the reduced intensity of 56Fe1+; hence the latter isotope was selected for the quantitative evaluation. The other isotopes of iron are heavily interfered by 171Yb3+ and 174Yb3+. The calculated masses shown in Table I, column 4, are based on a third order polynomial fit to 15 unambiguously identified isotopes in the range from 197Au1+ to 1 6 0 2 + . The deviation between calculated and known masses shown in column 5 is smallest at about mass 57 and increases toward higher and lower masses over the short mass range shown in this table. Extending the inspection of the data output beyond the mass range shown here, the
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBER 1973
deviation decreases to 6 a t 16SYb2+ and to 7 millimass units a t 170Yb4+. Such oscillatory behavior in the mass dispersion is also observed by Venkataraghavan et al. (2) and may be attributed to the inhomogeneity of the magnetic field.
CONCLUSION We have developed a program for the complete semiquantitative evaluation of spark source spectra recorded on a photographic plate. Most of the arguments presently available to the analyst in the evaluation procedure are included in this program. Major aspects in our program are the high precision in mass assignments and the quantitative solution to interference problems. We have followed the philosophy that only those application programs
are really useful to the analyst which contain all evaluation criteria presently at his disposal. The considerable commitment of time and effort going into the development of such programs ultimately pays off in the speed, accuracy, and completeness gained by the automated evaluation.
ACKNOWLEDGMENT Robert Johnson's contribution, the furnishing of all the mass spectra needed in this study, is gratefully acknowledged. Received for review December 11, 1972. Accepted April 2 , 1973.
Gas-Phase Electron Paramagnetic Resonance Detection of Nitric Oxide and Nitrogen Dioxide in Polluted Air Hiromichi Uehara and Satoshi Arimitsu Sagamt Chemical Research Center, Nishionuma. Sagamihara-shi. Kanagawa, 229 Japan
Gas-phase electron paramagnetic resonance has been applied to the detection of NO and NO2 in polluted air. NO2 in polluted air is detected by Zeeman modulation at 1 atm of sample pressure, without interference from 0 2 and NO, and NO is detected by Stark modulation under the reduced sample pressure of less than 0.1 Torr without any interferences of coexisting substances. The sensitivity is better than 30 ppb for NO and NO2 with a sample volume of 1.5 I . when a novel low-temperature trapping technique is adopted. When the sample is observed without any enrichment, the minimum detectable limit is 10 ppm for N02.
Gas-phase electron paramagnetic resonance (gas-phase EPR) was applied to the detection of NO and NO2 in polluted air. Although NO and NO2 are highly responsible for the photochemical smog ( I ) , accurate determination is not always easy for parts-per-million (ppm) quantities of them. Conventional wet chemical methods seem to be still under discussion (2, 3 ) . Many instrumental detection methods ( 3 ) have been reported such as IR absorption, UV absorption. gas chromatography, chemiluminescent technique ( 3 ) ,IR absorption of tunable laser radiation (5), and a laser method based on the Zeeman modulation of absorption (6). Since any one of these monitors only one species, NO or NOz, complete analysis requires the chemical conversion of NO or NO2 to the other, which introduces inevitable uncertainties to the results. Recently, a carbon monoxide laser was used for a n independent detection of NO and NO2 ( 7). ( 1 ) P. A. Leighton. "Photochemistry of Air Pollution," Academic Press, New York. N. Y . . 1961. ( 2 ) For instance. areport in Chern. Week. 111(3).31 (1972). (3) W. Leithe. "The Analysis of Air Pollutants." Revised English ed,
PJow, we report a new method using gas-phase EPR by which the separate determinations of NO and NO2 are easily and accurately accomplished without using any chemical reactions. The sensitivity is better than 30 partsper-billion (ppb) for NO and NO2 with a sample volume of 1.5 1. when a novel low-temperature trapping technique is adopted. When the sample is observed without any enrichment, the minimum detectable limit is 10 ppm for NO2.
PRINCIPLE Gaseous paramagnetic species in polluted air are only 0 2 , NO, and NO2. Although all of the substances are, in principle, observed by EPR, a detailed consideration shows that separate detection of NO and NO2 is possible. First, the fact is used that among 0 2 , NO, and NOz, those having an electric dipole moment are NO and NO2. The nonpolar oxygen molecule is not observed at all by Stark-modulating gas-phase EPR which was first proposed by Carrington et al. (8, 9). A second base for detecting NO and NO2 independently is that NO is a linear molecule, whereas NO2 is nonlinear. The principle of the gas-phase EPR of NO is shown in Figure 1. Since the NO molecule (211312) has axial symmetry and has also a large spin-orbit interaction, only the molecular-axis components A and Z of L and S, respectively, are quantized (Hund's case a ) ( I O ) . The rotational angular momentum K of the molecular frame is perpendicular to the molecular axis. The quantities A Z ( = 12) and N combine to give the total angular momentum J (the nuclear spin I is ignorable). The magnetic moment ( = A + 22') of NO originates from A and 2 . Since the total angular momentum J is conserved in free space, the applied external magnetic field H interacts with the J component of the
+
Ann Arbor Science Publishers, Mich., 1971. ( 4 ) A. Fontijn, A . J. Sabadell. and R. J. Ronco, Anal. Chern.. 42, 575
(1970). (5) L. B. Kreuzer and C. K. N. Patei, Science, 173,45 (1971). (6) A. Kaldor. Wm. 6. Olson, and A. G. Maki. Science, 176, 508 (1972). (7) L. 6. Kreuzer. N. D. Kenyon, and C. K. N. Patel, Science. 177, 347 (1972).
(8)A. Carrington. D. H. Levy. and T. A . Miller, Rev. Sci. Instrum.. 38. 1183 (1967). (9) A . Carrington. "Molecular Spectroscopy." Elsevier, London, 1968, p 157. (10) G. Herzberg. "Spectra of Diaiomic Molecules," 2nd ed, Van Nostrand, New York. N. y . , 1961.
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