Determination of Mercaptans by Negative Ion Mass Spectrometry Hans Knof," Robert Large,' and Georg Albers Deutsche
BP Aktiengesellschaff, lnstitut fur Forschung und Entwicklung, 2000 Wedel, W. Germany
Negative Ion Mass Spectrometry using a modified conventional ion source with a hot cathode and a sample pressure of about Torr is shown to provide a viable direct method for the quantitative analysls of indlvlduai mercaptans In hydrocarbon solvents without the need of prior separation. The current limit of detection is 10 ppm of indivlduai mercaptans. The (M - 1)intensities yl related to a mercaptan internal standard are Independent of the hydrocarbon solvent and the gas pressure in the ion source and can be fitted by linear regression to the function log yi = a log xi -t log b, for more than 4 decades, where xl is the concentration of mercaptan i. This is the first reported use of negative ion MS as a quantitative technique.
(b)
pressures above Torr. Under such conditions the hydrocarbon itself gives only a very weak spectrum comprising principally the low mass ions H-, C,- and C,H-, where n = 1-3. In contrast, positive ion mass spectrometry in general shows for saturated hydrocarbons a molecular ion M+ and a series of intense fragment ions. Furthermore most aromatic compounds give a pronounced positive molecular ion peak, whereas benzene exhibits only a weak negative molecular ion. The situation for mercaptans is quite different (5, 6). For such compounds the positive ion spectra comprise a rather strong molecular ion peak and a series of lower mass fragment peaks, whereas the negative ion spectra have an intense (M - 1)- peak, a weak (M - 2)- peak in addition to the peaks of the fragment ions SH-, S-, and H-. A typical comparison is given by the mass spectra of n-heptylmercaptan (Figure 1). Since dialkylsulfides may have the same molecular weight as alkylmercaptans, negative ion mass spectra of these were recorded as well to assess any interference. Figure 2 gives the EA mass spectrum of diallylsulfide in cyclohexane. This spectrum exhibits a strong sulfide anion (CsHsS-) and only a very weak (M - 1)- peak. The same is true for other low molecular weight sulfides. Consequently the molecular peaks of such sulfides may reasonably be neglected in an EA mass spectrum in comparison with those of mercaptans. The qualitative behavior of a synthetic mixture of 6 different mercaptans in cyclohexane is given in Figure 3. The (M - 1)- ions of the individual mercaptans are fully separated and there is no interference with ions arising from the solvent. The objectives of the currently reported work were: (1)to assess the linearity of response of the (M - 1)- ion peak of various mercaptans in a range of hydrocarbon solvents, (2) to establish the limit of detection of the method using a conventional ion source with a hot cathode and a sample pressure of approximately Torr, and (3) to find a suitable internal standard for the analysis.
(C)
EXPERIMENTAL
The quantitative analysis of individual alkyl mercaptans can be performed by using gas-liquid chromatography with a mass spectrometer as a highly specific detector. With such a single ion monitoring technique, 10 ppm is a typical limit of detection. The method, however, is limited by the ready oxidation of mercaptans into disulfides and is rather time-consuming. An alternative approach is to use negative ion mass spectrometry (1-3). The use of a negative ion MS method is based upon the fact that by using a slightly modified conventional ion source with a hot cathode, mercaptans exhibit very simple spectra at particularly high sensitivity. The predominant process for ion formation at a pressure of approximately Torr is one of dissociative secondary electron capture to give abundant (M - 1)- ions ( 4 ) .The secondary electrons e, for this capture process can conveniently come from the positive ion formation of a hydrocarbon solvent involving fast primary electrons ep: ep
+ AB + A + + B + e,,' + e,
+ AB + A + Be, + AB F= (AB-)*
e,
(a)
Whereas dissociative electron capture (b) occurs a t electron energies E , < 10 eV, electron attachment (EA) is possible a t thermal electron energies E , < 2 eV. Process (a) is a particularly efficient one and produces enough low energy secondary electrons to obtain a good yield with processes (b) and (c). Another source of negative ions is pair production: e,,
+ AB
A+
-+
+ B- + e,,,
(dl
Pair production gives mainly low mass negative fragment ions which normally do not interfere with the molecular anions and (M - 1)- ions produced by electron attachment and dissociative electron capture respectively. In addition, instrumental conditions are adjusted in such a way to give a high yield for the processes (b) and (c). This is possible at gas Permanent address; The British Petroleum Company Limited,
BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, England. 2120
0
Negative ion MS measurements were performed on standard solutions of aliphatic mercaptans in a range of hydrocarbon solvents over a maximum concentration range of 10 ppm to 10% v/v using a suitably modified MAT 731 double-focusing mass spectrometer under the following conditions: Acceleration voltage, 4 or 8 kV; primary electron beam voltage, about 100 eV; cathode emission current, 1.6 or 3.2 mA; electron multiplier voltage, 2.25 kV; MS resolution, about 1000 (10% valley); sample pressure, about Torr; and sample amount, 500 11. The samples were injected through a silicone rubber septum into the reservoir of a heated batch inlet system, the temperature of which was maintained at 100 "C. n -Hexane, cyclohexane, isooctane, benzene, and gasoline were used as solvent and isopropanol, tert- butanol, cyclohexanol, n-butyraldehyde, diethyl ketone, and diethyl malonate were investigated as potential internal standards.
RESULTS The results for both mercaptans and prospective internal standards were fitted by linear regression to the equation
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1976
log yi = u log xi
+ log b
(1)
Absolufe Peak Intrnsily (
:=/;E7/
v / 101
n - Bury1 Mercoplon
50
05'
/ $//
Ethyl Mercoplon m / e 61 0=090!
negative 005.
/?
0 01
70 0
10
01
Concentrotron I Ye v / v l
Flgure 4. Concentration dependence of (M - 1)- peak height of +alkyl mercaptans in cyclohexane (1 0-4 Torr ion source pressure)
Ibsolu fr 'oak Intensify
V/lOI
Diefhyimalonate in a gasoline m / e 159
Figure 1. Positive and negative ion mass spectra of n-heptyl mercap-
/ ferf
- Bufanoi
,n
m / e 73
0=0525
0 01
,
y
o Isopropanol in
/4/ 01
Cyclohexane m / e 59 0~06ie
!OC
10
Concentralion i% v/vl
Figure 2. EA mass spectrum of diallyl sulfide in cyclohexane (0.2% v/v,
Torr ion source pressure)
Figure 5. Concentration dependence of (M - 1)- ions of isopropanol
in cyclohexane, diethyl malonate in gasoline, and teff-butanol in isoTorr ion source pressure) octane (
Flgure 3. EA mass spectrum of n-alkyl mercaptans in cyclohexane
(0.1 % v/v each mercaptan,
torr in ion source pressure)
where y Lare the average intensities of the particular (M - 1)peaks, x , the concentrations in % v/v and a and b constants depending on the sample i and the solvent. Some results so obtained for n-mercaptans in cyclohexane are illustrated in Figure 4, where absolute intensities were Torr. Corresponding measured a t a sample pressure of plots for prospective internal standards in various hydrocarbon solvents are given in Figure 5 . Figure 6 illustrates relative intensity results yrelfor various n-mercaptans in cyclohexane, obtained by taking as internal standard n-pentylmercaptan a t 0.1% v/v concentration, the sample pressure for these measurements being lo-* Torr.
117 13
'/ 0.001
0.01
0.1
Concenlralion (% v / v
I
I
Figure 6. Concentration dependence of (M - 1)- peak height of +alkyl
mercaptans in cyclohexane referred to 0.1 YO v/v n-pentyl mercaptan (10-4 Torr ion source pressure)
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
2121
Table I. Relative intensities a of (M - 1)- Peaks Arising From a Mixture of Mercaptans in Cyclohexane
Mercaptan
a
log b
Ethyl n-Propyl n-Butyl n-Hexyl n-Heptyl
0.993 0.93 0.95 1.08 1.10
1.007 1.034 1.008 0.957 0.848
A
fides
Mercaptans
Benzyl2-Propyi-1
I
iert.-8utv/-l
I lllilllll
The intensities are related to mle = 103 of 0.1% vlv n pentylmercaptan. The correspohding regression coefficients a and log b are summarized in Table I, the values of a being between 0.93 and 1.10. Therefore it is not possible to use only one linear calibration curve. Instead different calibration curves must be taken for the various mercaptans.
DISCUSSION The results illustrated in Figures 4 and 5 show that the linearity of response on a log/log basis of the mean absolute (M - 1)-peak height of both mercaptans and the prospective internal standards over a maximum concentration range of 100 ppm to 10%v/v is surprisingly good, in view of the problems experienced with the maintenance and accurate measurement of constant sample pressures. Individual mercaptan concentrations may be determined directly from absolute (M - 1)- responses. This approach, however, is very time-consuming since calibration plots must be determined for each species on each occasion. A more realistic and intrinsically more accurate approach is to refer all measurements to an internal standard. The compounds examined in this context during this work, namely isopropanol, tert- butanol, cyclohexanol, n -butyraldehyde, diethyl ketone, and diethyl malonate exhibit a different concentration dependence from the mercaptans. Although all of these compounds showed excellent linear absolute (M - 1)- response (Figure 5 ) , none had the same concentration dependence as the mercaptans. It is thus necessary to predetermine values of a and log b for both this standard compound and the respective mercaptans in which case the subscripts 1 and 2 refer to a particular mercaptan and the internal standard, respectively: logy1 = a1 log x1 + log bl
(2)
+ log bz
(3)
log yz = ~2 log x z
whence, by subtraction and rearrangement:
(4)
Figure 7. Relative intensities of (M (0.1% v/v in cyclohexane each,
- 1)-
for mercaptans and sulfides Torr ion source pressure)
bl
,
,
,
10
,
Pressure I 1 0 . ~Torr)
Figure 8. Pressure dependence of the absolute intensity of the mo-
lecular ion peak (M ane
- 1)-
of l % v/v n-pentyl mercaptan in cyclohex-
Equation 4 may be used for a particular solvent to determine values of x 1, the mercaptan concentration, regardless of the particular ion source sensitivity, since for a particular mixture of mercaptan and internal standard both yJy2 and log (bllbz) remain constant. The applicability of Equation 4 has been assessed using two sets of data, namely that for mercaptans plus isopropanol in cyclohexane over the concentration range 0.01-10% vlv and that for mercaptans plus cyclohexanol in benzene over the
Table 11. Dependence on Ion Source Pressure of the Intensity from the (M - 1)- Ion of 1% v/v Mercaptans in Cyclohexane
Mercaptan
mle
2 x 10-5
4 x 10-5
Pressure, Torr 6X
Ethyl 61 1.27 1.0 1.0 n-Propyl 75 1.45 1.08 , 1.13 n-Butyl 89 1.27 1.04 1.07 n-Pentyl 103 (1.1)c (lac (30) n-Hexyl 117 0.91 0.92 0.90 n-Heptyl 131 0.86 0.75 0.73 Yrei = mean values. SFrel= standard deviation. c ( ) = absolute values (volt). 2122
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
8 X 10-5
10-4
1.0 1.13 1.10 (38)c 0.92 0.78
0.96
1.04
1.11
1.18
1.04
1.10
(45)c
1.00 0.9
0.84 0.68
0.76
0.12 0.15 0.09 0.03 0.07
Table 111. Relative Intensities of Molecular Peak Ions in 1% v/v Mercaptan Mixtures in Low Molecular Weight Hydrocarbons ( T o r r Ion Source Pressure, m/e = 103 Internal Standard Peak) Mercaptan Ethyl n-Propyl n-Butyl n -Pentyl n-Hexyl n-Heptyl a
Yrel =
mle 61 75 89 103 117 131
mean values.
n-Hexane 0.88 1.0 1.0 0.88 0.72
Cyclohexane 1.41 1.17 1.23 (17)' 0.97 0.56
= standard deviation.
Isooctane 1.08 1.15 1.08 (13)c 0.92 0.69
Benzene 1.12 1.2 1.16 (25)'
0.88 0.72
Gasoline 1.0 1.0
1.0 (15)' 0.87 0.66
QEla
1.12 1.10 1.09 (17.6)' 0.898 0.67
SYrel 0.21 0.1 0.1 (4.56)' 0.03 0.06
( ) = absolute values (volt).
concentration range 0.1-10% v/v. Although in both cases the sensitivity of the internal standard was significantly lower than that of the mercaptans, use of Equation 4 produced acceptable accuracy. The RMS errors of the calculated x 1 values using individual values of a1 were 11.8% and 9.1%, respectively, for the cyclohexane and the benzene solutions. The necessary calculation is simplified considerably if the mean value for a1 for mercaptans in the particular solvent is employed. Such a procedure, however, increases the above RMS errors to 14.9% and 11.0%, respectively. T o optimize the accuracy of this method, it would be preferable to increase the concentration of the internal standard so that the height of the (M - 1)peaks of mercaptan and standard were comparable and could be measured with comparable precision. One limitation of this potentially very powerful method is that isomeric mercaptans cannot be separately determined. By using suitably weighted mean values of log b l , however, the method will provide reasonably accurate values for the total mercaptan concentration a t each carbon number. The weighting procedure is necessary because highly branched mercaptans, such as tert-butyl and tert-octyl, can have log b 1 values significantly different from the corresponding normal isomers. Another limitation of this method is that the b coefficients in Equations 2 and 3 must be determined separately for each new solvent. The only class of compounds which could be guaranteed as having the same concentration dependence as the mercaptans were isotopically labeled mercaptans, for example R"SH, the use of which would avoid spectral overlap between labeled and unlabeled compounds. Such compounds, however, are not readily available. Therefore n-pentylmercaptan was used as a model internal standard. Under the conditions used, in particular a sample pressure of lop4Torr, the absolute limit of detection of the method is approximately 10 ppm of individual mercaptans in hydrocarbon solvents. The relative intensities yrelof (M - 1)- for a 0.1% v/v solution of various mercaptans and sulfides in cyclohexane which are given in Figure I,indicate a dependence of the limit of detection on chemical structure. In particular the (M - 1)- ions of sulfides are much less intense than those of the mercaptans. Moreover mercaptans with aliphatic groups yield negative (M - 1)-ions which are less stabilized than those with aromatic or unsaturated hydrocarbon groups, and secondary and tertiary mercaptans have slightly higher
sensitivity than their primary counterparts. For n-mercaptans the intensity appears to decrease with increasing hydrocarbon chain length. Since the mercaptan negative ions are produced predominantly by capture processes, there is a very strong dependence of sensitivity upon sample pressure. Such pressure dependence of the (M - 1)- ion peak intensity in the spectrum of 1% vlv n-pentylmercaptan in cyclohexane is given in Figure 8. For this investigation the pressure was measured with an ionization pressure gauge in the conventional position just above the liquid nitrogen trap of the mass spectrometer and not in the ion source, the latter position being impossible. It can be seen that the (M - 1)- intensity increases rapidly at pressures just above the lower practical limit of 2 X Torr. This increase, however, levels off at Torr. The intensities of other mercaptans relative to n-pentylmercaptan, however, are independent of sample pressure within the accuracy of measurement (Table 11). Although the hydrocarbon solvent is an effective source of secondary electrons, it provides itself only weak low mass negative ions, which do not interfere with mercaptan analysis. In effect there is a very useful enhancement of mercaptan response relative to hydrocarbon response. In this respect the negative ion MS method is significantly superior to its positive ion counterpart in which significant overlap occurs between mercaptan and hydrocarbon peaks. In the absence of a prior separation, the direct analysis of trace mercaptans in hydrocarbons by positive ion low resolution MS is not viable. Table I11 represents the influence of the solvent on the relative intensities of molecular ions of mercaptan. There is no detectable dependence on the investigated hydrocarbon solvents.
LITERATURE CITED (1) M. von Ardenne, K. Steinfelder, and R. Tummler, "ElektronenanlagerungsMassenspektrographie organischer Substanzen", Springer-Verlag, Berlin, 1971. (2) H.Knof, "Massenspektrometrie von Kondensationskeimenin der Gasphase", Physik-Verlag,Weinheim, 1974. (3) R . Large and H. Knof, Org. Mass Spectrom., 11, 582 (1976). (4) H.Knof, and D. Krafft, Adv. Mass Spectrom., 6, 303 (1974). (5)H. Knof, R. Large, and G. Albers, Compendium 1975176, p 574, Erdoel und Kohle, Leinfelden, 1975. (6) H. Knof. R. Large, and G. Albers, ErdoelKohle, 29, 77 (1976).
RECEIVEDfor review June 7, 1976. Accepted August 30, 1976.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
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