1794
Anal. Chem. 1984, 56, 1794-1797
Alternative Reagent Ions for Plasma Chromatography C. J. Proctor and J. F. J. Todd* University Chemical Laboratory, University of Kent, Canterbury, Kent CT2 7NH, United Kingdom
Plasma chromatography Is a highly sensltlve method of detectlng and resolving, by means of differing Ionic mobilities in a drM field, traces of volatile compounds present in gaseous mlxtures at atmospheric pressure. Ordinarily, small amounts of substances such as water present in the carrier gas act as “reagent” Ions, playing an Important role In the Ionization of the sample. This does not, In ail cases, result in efficient sample ionization or good resolution in the moblilty spectrum, and lt Is In such cases that we propose the use of alternatlve reagent ions Introduced by the addltion of traces of particular compounds to the carrler stream. Halide ions are found to be partlculariy useful, resulting In a greater specificity of Ionization which may lead both to better sensltivlty to a partlcular compound and to more clearly defined mobility spectra.
Plasma chromatography (I), which is the tradename associated with the system marketed by Franklin GNO,Florida, is very much an analytical development of the type of drift tube apparatus used for some years to obtain reaction rate coefficients for ion/molecule and charge transfer reactions (2). Much of the initial assessment of the analytical potential of the technique was made in the early 1970s by Karasek and co-workers ( I , 3 , 4 ) . In essence the apparatus operates entirely a t atmospheric pressure and may be designed as a compact “hand-held” detector. Samples are best introduced in trace amounts in a carrier stream which may be ambient air, gas chromatography effluent, or an especially pure carrier gas (this being particularly useful in characterizing the system). Ionization, generally brought about by either energetic electrons issuing from a radioactive foil source or corona discharge (5) of the carrier gas molecules, occurs as the first step, these being by far the most abundant species in the source. As the source is at atmospheric pressure, the mean free path is very short and many ion/molecule reactions will occur within the source residence time; in fact, both thermal and chemical equilibrium are reached. This results in efficient ionization of very small quantities of sample and reagent (detection limits as low as 50 fg have been given for atmospheric pressure ionization (6)), the final ionic species being dependent upon the relative ionization energies, electron affinities, acidities, and concentrations of the compounds present. Ionic species are resolved by their mobilities through a countercurrent flow of drift gas while under the influence of a uniform drift field. In order to measure the relative mobilities all the ions must be allowed into the drift region at one time, and there is often an electronic gate between the source and the drift region that may be closed entirely and opened only very briefly (7). The temporal resolution of this initial pulse of ions is crucial to the mobility spectrum. Unfortunately, the mobility of a particular ion cannot be directly correlated with its mass, but instead is dependent upon the collision cross section of the ion with the neutral gas (or the effective size of the ion (8)). The obvious way to assign masses to ions giving particular mobility peaks is to interface the plasma chromatograph (PC) with a quadrupole mass spectrometer (MS), and this path has been followed in several laboratories including our own (8,9). The analytical power of plasma chromatography relies upon the sensitivity and selectivity of ionization, the ability to 0003-2700/84/0358-1794$01.50/0
resolve ionic species by way of their mobilities, and the knowledge to be able to assign peaks in the mobility spectrum to specific ionic species. Sensitivity, in favorable cases, is known to be excellent. However, selectivity of ionization is dependent upon the constitution of the carrier stream; in some cases a very small concentration of pollutant may be ionized at the expense of the sample. We aim to show how such a situation may be put to advantage. Selectivity has been achieved in recent experiments by Lubman and Kronick, who have used laser multiphoton ionization in the source of a plasma chromatograph (10). By use of the “color” of ionic species, that is the wavelength that a photon is absorbed, isomers such as those of xylene may be distinguished. The system is seen to have several advantages over electron ionization PC, but is not of use to those requiring an inexpensive or a hand-portable detector. In the majority of cases, “zero-grade’’air or nitrogen are used as the carrier gas. However, different ionization characteristics may be obtained by using traces of different reagent gases such as isobutane, ammonia, and nitric oxide (5). This results in reactant ions of n-C4H9+,NH4+,and NO+, respectively, and these ions can be used in the same manner a t atmospheric pressure as in conventional chemical ionization mass spectrometry (11). For example, will only ionize compounds with higher proton affinities (such as amines, pyridines, and some ketones). However, the use of specialized gas mixtures is not applicable to ambient air monitoring by plasma chromatograph. We propose the use of alternative reagent compounds introduced into the carrier stream by way of a permeation tube (a convenient method of supplying a continuous low level of a particular compound). Such a tube could be used even in ambient air monitoring simply by mounting it close to the sample inlet. Different reagent ions may then be selected simply by changing the tube. Even trace quantities of volatile compounds have been seen to change the atmospheric pressure ionization mass and mobility spectrum completely, and examples where this may be used to advantage to gain greater selectivity of ionization or better mobility resolution are given here.
”,+
EXPERIMENTAL SECTION Our plasma chromatograph/mass spectrometer has previously been described in detail (12), and so only a brief sketch of the system will be given here. In our characterization of alternative reagent ions, carrier gases of zero-grade air or zero-grade nitrogen (supplied by British Oxygen Co.) are further purified in the closed inlet system by traps containing activated charcoal and molecular sieve material. The inlet stream is split into two independently controlled flows so that a target sample and an alterntive reagent compound may be introduced simultaneously and the concentrations of each varied separately. Both sample and reagent are introduced in permeation tubes, these providing a continuous low concentration of compound at room temperature of, depending upon the sample, around 100 ppb in the carrier stream. A separate cylinder of zero-grade nitrogen is used for a separate stream which flows countercurrent to the carrier gas, entering the drift tube at the bottom and exiting just before the “gate grids”, as shown in Figure 1. This serves to limit any ion-molecule reactions from occurring in the drift region, and thus spoiling the mobility spectrum. Sample and reagent molecules are transported in the carrier stream into an ionization region where a 63Niradioactive foil source 0 1984 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
carrier stream
I II
1795
h
63Niionisation -source
A
gate grids': Tterf low
=-7
Tii n quadrupole
11
65'
Orifice
spectrometer
channel tron& detector Figure 1. The plasma chromatograph/mass spectrometer.
provides energetic electrons, with an average energy of 20 keV (13))that initiate the series of ion/molecule reactions. The source is operated at 1000 V potential (the sign of which depends upon whether positive or negative ions are to be studied) and the final grid is held at 50 V, so that ions produced in the source travel down toward the collector under the influence of a uniform drift field of ca. 200 V cm-l maintained by a series of electrodes. The sample inlet system is kept at room temperature and the drift tube at a constant 50 O C . The mobilities of ions produced in the source are ascertained by using an electrical two-grid gate system to produce a short burst of ions (of approximately 250 ps in duration). In order to acquire one scan of a mobility spectrum, a multichannel analyzer will at the same time trigger the pulse to open and shut the gate and start a clock while recording the ion count as a function of time. After perhaps 20 ms another pulse will be initiated, the clock started again, and another scan recorded; a mobility spectrum is generally the result of several thousand signal averaged scans. The plasma chromatograph is interfaced to a quadrupole mass spectrometer via a 25-pm pinhole orifice, and detection is effected by channeltron electron multiplier. This arrangement allows three types of information to be acquired. Firstly, a total ion mobility spectrum (TIMS) is obtained by setting the quadrupole to accept a wide window of masses in order that the mobilities of all the species present are observed in one arrival time spectrum. Secondly, the API mass spectrum, where the gate grids are kept open and the quadrupole mass filter scanned in order to accept in turn all masses whatever their mobility. Thirdly, a mass-analyzed ion mobility spectrum (MAIMS) may be acquired by setting the quadrupole to a specific mass and recording the mobility spectrum; this allows the mobilities of individual ionic species to be ascertained.
RESULTS AND DISCUSSION Both positive and negative ions are readily formed in the ionization source, but the ion/molecule reactions that each are involved in are quite different and so the effect of alternative reagent compounds in each mode will be considered separately. It is well documented, and confirmed by our experiments, that the dominant positively charged ions arising even from very dry zero-grade air or nitrogen are water clusters of the form H+(H,O), (24);the range of n and the most abundant cluster ion depend on the moisture content, but in our case n is 2 to 4 with H+(HzO)3at m / z 55 being the ion of greatest intensity. It is thought that these ions play an important role in the ionization of the sample itself in reactions of the type (15,16)
H+(H20), + M --c M.H+(H20),
+ (n - m)H2O
(1)
Thus ionization in the presence of water clusters leads to both
6
i
4
6
i
Ib
1;
lk TIME,
ms
Figure 2. Some positive ion mass analyzed ion mobility spectra of nitrobenzene in zerograde air are shown in (b) (the lower three traces). The effect of the addition of a trace of methanol on the MAIMS of NB.H+ can be seen in (a).
protonated molecular ions and their protonated water adducts. (It should be noted that because of the internal energies involved practically no fragment positive ions are observed.) The positive ion API mass spectrum of nitrobenzene (NB) (at a concentration of 200 ppb in zero-grade air) shows NB.H+(H20) ion to be the base peak with NB-H+ and NB. H+(H20),of similar intensity to the reagent water cluster ions (15). The mass-analyzed ion mobility spectra of m/z 55 (H+(H2O),),124 (NB.H+),and 142 (NB-H+(H20))are shown in Figure 2a. The water cluster ion gives a sharp, well-resolved mobility peak (of reduced mobility KO 2.14 cm2 V-' s-l) only 0.6 ms wide at half height. This is quite different from the nitrobenzene MAIMS peaks which are very broad and unsymmetrical. This lack of chromatographic resolution is thought to be due to the water molecule attaching to and detaching from the nitrobenzene ion on its passage down the drift tube; if this is the case then the ion does not retain a distinct identity in the drift region and thus a range of mobilities is observed for a single mass reaching the collector. The situation is changed by the addition of a permeation tube containing methanol (releasing 250 ppb) to the carrier stream. The major reagent ions observed are (CH30H),H+ a t m / z 65 and (CH30H)2H+(H20) at m / e 83, while the protonated molecular ion of nitrobenzene becomes the largest sample ion with NB.CH30H.H+ and NB.CH30H.H+(H20) appearing in only slightly lower abundance. (Figure 2b) shows the MAIMS of both NB.H+ and NBCH30H.H+to give much sharper mobility spectrum than was the case when water reagent ions were involved in ionization. Unfortunately the
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984 EGDN.CI-
I THZ0
CI-.H,O
NB.CI-
a
0
100
0
100
(l COT. H20
EGDN.NO;
0
m 100 ass
,
200
,
y7N/’
I
,
EGDN-
,
,
,
100
0
,
,,EiTNO; 200
mass
Figure 3. Negative ion mass spectra of nitrobenzene in zero-grade air: (a) and (b) were acqulred under identical conditions except that a trace of CH2CI2 had been added to the carrier stream for (a).
difference in drift time between sample and reagent ions is only 1 ms and so overall resolution is not improved. There is also still a little tailing in these MAIMS peaks, to low mobility for m / z 65 and to high mobility for mlz 124, indicating some detachment and attachment of methanol is still occurring, though to far less an extent than with water as the adduct species. Several other possible alternative reagent species, such as acetic acid, dimethyl sulfide, and acetonitrile, have been investigated though with no complete success. The use of alternative reagent ions for negative ion formation, however, shows great promise. Negatively charged reagent ions in air or nitrogen, in the absence of introduced sample or reagent, are Of, CO,-, and CO, with their monohydrated adducts (17). Ionization may arise from electron attachment (to form M-), dissociative electron attachment (e.g., RX e- R X-) or in addition reactions (e.g., M Of M.02-). Thus, compounds containing halogen atoms (excludingfluorine) or nitro groups give fragment Hal- or NOf ions, respectively. When the alternative reagent compound is introduced alone into the carrier stream, these ions will form addition products with the parent molecules to form M-Halor M.N02-. However, when target samples such as nitrobenzene or ethylene glycol dinitrate are present, then the reagent ions attach specifically to these molecules and no autoionization occurs. The first example involves nitrobenzene in zero-grade air. The negative ion mass spectrum gives no trace of a nitrobenzene anion (though NB- is observed when using nitrogen as carrier gas; this difference is thought to be due to more thermalized electrons being present in nitrogen (17)) or any other sample related peak. The addition of a trace amount of dichloromethane to the carrier stream, though, has a remarkable effect, with the NB-Cl- ion becoming the base peak of the mass spectrum (see Figure 3). Similar results, though not nearly as effective, are found with the introduction of dibromomethane (to give Br- reagent ions) and methyl iodide (for I-). So the detection of nitrobenzene in air is made possible in the negative ion mode simply by the addition of a chloro compound that gives good C1- reagent ions. An improved specificity of ionization leading to increased mobility resolution through the addition of alternative reagents is seen in the case of ethylene glycol dinitrate (EGDN) (18). The lower trace of Figure 4 illustrates the API negative ion mass spectrum of EGDN in zero-grade nitrogen; dissociative electron capture results in the formation of NO2- and NO, which then act as reagent ions and attach to the parent molecule to give EGDN.NOz- ( m / z 198) and EGDN-NOB(m/z 214). A low abundance of molecular anion is also formed.
-+-+
Figure 4. Negative ion mass spectra of EGDN in nitrogen. The upper spectrum was acquired in precisely the same manner as the lower spectrum except for the addltion of a trace of CH,Ci,.
I-‘
EGDN t
+
EGDN
1
0
I
I
4
I
I
I
8 TIME, ms
Figure 5. Total negative ion mobility spectra of EGDN in nitrogen. Again the top spectrum has been acquired under identical conditions to the bottom spectrum, apart from the addltion of CH,Ci,. The total ion mobility spectrum corresponding to this is presented in Figure 5. It is a poorly resolved spectrum because of the overlap of three sample ion peaks. The spectra can be entirely changed by keeping all the experimental conditions constant and simply introducing a trace of CH2Clz into the carrier stream. The changes that his introduction induces can be clearly seen in the top traces of Figures 4 (for the ma88 spectrum) and 5 (for the total ion mobility spectrum). The mass spectrum is much simplified, the only species present being C1-, C1-.H20, and EGDN-C1-. There is an increased sensitivity, presumably because all sample ionization now results in one ionic species being formed, and molecular weight information is much more certain (the EGDNCl- has a single Cl- isotope pattern and so simply subtracting 35 from m / z 187 gives the molecular weight of EGDN). The ap-
1797
Anal. Chem. 1904, 56, 1797-1803
pearance of just one sample related peak, instead of the three seen before addition of the alternative reagent, may be of great advantage in mixture analysis. The character of the total ion mobility spectrum is also changed entirely. MAIMS shows the two sharp peaks to correspond to Cl- and EGDN-C1- (C1-.H20 is seen to have an identical MAIMS to C1-). The tail between the two is common to the mobility spectra of halide addition product ions and is thought to be caused by attachment to and detachment of the halide ion from its host molecule. Even so, addition of CH2C12to provide Cl- as an alternative reagent ion results in a more finely resolved (and more sensitive) mobility spectrum. Again, Br- and I- ions formed by the addition of CH2Br2 or CHJ to the carrier stream give similar increased specificity of ionization, both forming strong EGDN-Hal- ions. If the halogenated compound is introduced at very low concentrations (
