JOURNAL OF MASS SPECTROMETRY, VOL. 31, 213-215 (1996)

A Rapid Routine to Correct for Skewing in Gas C hromatography/Mass Spectrometry

Wim G. Pool,* and Jan W. de Leeuw Netherlands Institute for Sea Research (NIOZ), Postbus 59, 1790 AB Den Burg, The Netherlands

Bastiaan van de Graaf Technical University Delft, Laboratory of Organic Chemistry and Catalysis, Julianalaan 136,2628 BL Delft, The Netherlands

In gas chromatography/mass spectrometry (GC/MS) with magnetic instruments and quadrupoles, skewed (‘tilted’) spectra are obtained owing to the changes in analyte concentrations in the source of the mass spectrometer during the measurements of one scan. Skewing depends on the ratio of cycle time to GC peak width. The combination of capillary chromatography with magnetic instruments often results in skewing to an extent that both manual and automatic processing of GC/MS data are seriously hampered. An algorithm is described to correct for this effect. The results of the unskew algorithm are evaluated on real GC/MS data.

KEYWORDS: GC/MS; unskewing; spectrum cleanup; deconvolution

INTRODUCTION

In gas chromatographic/mass spectrometric (GC/MS) analyses the eluate from the chromatographic column is led directly into the ion source of a mass spectrometer. The mass spectrometer is scanned continuously to measure the ions formed in the source. However, during the measurements of one scan, the concentration of the analyte in the source is not constant. Magnetic sector instruments generally do not provide enough scan speed over the full mass range of GC-amenable compounds, especially in the combination with capillary chromato- graphy. This is reflected in the mass spectra obtained.’,’ When the mass spectrometer is scanned from high to low mass, low masses will give relatively higher signals with increasing concentration of the analyte in the eluate. In contrast, with decreasing concentrations the higher masses are relatively more abundant. The conse- quence of this is that signals, although belonging to the same component, maximize at different scan positions. In a worst case scenario, for two closely eluting com- ponents, the maxima of signals measured at the end of the scan for the first component can be mixed up with signals measured at the start of the scan for the second component.

This so-called skewing effect can be minimized if the ratio of the cycle time to the GC peak width can be decreased. Unfortunately, this is only possible in a limited number of cases. A fast scan adversely effects both sensitivity and dynamic resolution. Increasing the width of the GC peak adversely effects chromatographic resolution. Scanning over a small mass range will increase the cycle time, but in most cases this will not

* Corresponding author (Email Address POOL@NIOZ.NL, FAX -k 31 222 369569).

supply sufficient information for identification and quantification purposes.

Deconvolution methods to construct pure spectra from GC/MS data suffer from skewing. In this paper a simple and fast algorithm to address this problem is discussed.

THE UNSKEW FUNCTION

The approach used in this work is similar to the one followed by Keller and Massart’ in their evolving factor analysis on liquid chromatographic data with diode- array detection. This method is extended to GC/MS data. All intensities in a GC/MS scan are corrected as if they were measured simultaneously, i.e. at the start of the scan. Assuming that over the period of three suc- cessive scans the chromatographic peak shape can be described by a parabola, one can derive the following equation:

2 I;, s = [+(I , , s + 1 + 1,. s - 1) - I,, ,It, - C$(rm, s + 1 - I m , s- l)Itm + 1,. s (1)

in which la,, is the corrected intensity for mass m at scan s and I is the measured intensity. The value oft, represents the time, in parts of the cycle time, needed to go from the start of the scan to mass m. The calculation oft, depends on the scan law of the mass spectrometer. If an exponentially down scan is used, the correct way to calculate t is

t , = [ 7i + In( $) A]/[ 7i + In( z) s] (2)

where the parameters mh, ml, 7d and 7 i define the high- and low-scan mass, the scan time (s per decade) and the time between two scans (s), respectively.

CCC 1076-5174/96/020213-03 0 1996 by John Wiley & Sons, Ltd.

Received 28 September 1995 Accepted 7 November 1995

214 W. G. POOL ET AL

12.2 -

12.0 - 11.8 -

11.6 -

.

Normal GC peaks are better described with a Gauss- ian function. This can be addressed by using in Eqn (1) the logarithms of the intensities, because a logarithmic Gaussian peak can be represented by a parabola. By this approach, unskewing GC/MS data is fast and therefore it will not be a burden in terms of CPU time.

&--.:-'

RESULTS AND DISCUSSION

The effect of skewing is clearly demonstrated by three mass chromatograms extracted from a GC/MS analysis [Fig. l(A)]. When the data are subjected to the pro- posed unskew algorithm, the mass chromatograms shown in Fig. 1(B) are obtained. In Fig. 2 the calculated maxima for all masses present in the compound selec- ted, gammacerane [Fig. l(C)], are plotted as a function of mass. After subjection to the unskew algorithm an almost horizontal line is obtained, indicating that all

A

191 I00 C $01

B

412 - i

Figure 1. A sample of gammacerane was injected on-column on a 25 m x 0.32 mm i.d. CP SIL 5 CB (film thickness 0.1 2 pm) capil- lary column. The gas chromatograph was coupled to a VG Auto- spec Ultima, scanning exponentially down from 800 to 50 u with 1 s. per decade. The dwell time of the magnet was 0.3 s. With this experimental conditions five scans were measured during the elution of the GC peak. Mass chromatograms of masses 41 2, 191 and 55 show different maxima, as a result of skewing (A). Sub- jection of the data set to the unskew algorithm results in mass chromatograms which maximize at the same position (B). The spectrum and structure of gamrnacerane are given (C).

1 0 . 8 : . 8 , t ' I ' 7

0 100 200 300 400 500

mlz

Figure 2. Calculated maxima of mass chromatograms plotted as a function of mass, (m) before and (0, Gaussian peak profile) after subjection to the unskew algorithm.

masses are now maximizing at the same scan positions. When the logarithms of the intensities are used in the unskew function, slightly better results are obtained (Table 1).

The data system of a GC/MS instrument starts a new scan with the delay time for the magnet. Intensities obtained during a scan are not recorded at the time they were actually measured. The data system shifts them to the time the scan started, resulting in earlier peak maxima. The proposed algorithm corrects these intensities as if they all were measured at the time of the start of the scan. As a result, the peak maxima are shifted to later times, as can be seen from the data in Table 1 and Fig. 2.

The processing of 100 scans with 750 m/z channels on a personal computer with a 60 Mhz entium processor is performed within 1 s.

CONCLUSION

A rapid algorithm is presented to correct for the effect of skewing in GC/MS measurements. Both manual and automatic processing will benefit from it. The method is easy to implement, its demands in terms of CPU time are negligible and it can be carried out as the first step in the evaluation of the data.

Table 1. Range, average position and root mean square (RMS) of the obtained maximizing positions (scans) from the masses present in gammacerane before and after sub- jection to the unskew algorithm

Raw data After unskewing directly Parabolic peak Gaussian peak

Range of position 10.94-1 1.46 11.84-1 1.95 11.89-1 1.95 Average position 11.14 11.91 11.92 RMS with respect to 0.127 0.01 7 0.01 3

average position

UNSKEWING GC/MS DATA 215

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